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Faro
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All-Moving Fins For Airliners?

Fri May 08, 2009 8:49 am

How significant can fin drag be at altitude in the cruise with prolonged flight in strong crosswinds? How practical would an all-moving fin be to alleviate the drag caused by the crosswind component acting on the fin taking into account the added weight of the bigger actuation mechanism vs the elimination of the rudder and its related actuators?

Faro
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rwessel
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 9:10 am

There is no crosswind in cruise that the fin sees, except for transitory* effects. If there's a "crosswind" the pilot is flying cross controlled, and needs a good smack.

If the pilot were doing that, everybody in the cabin would be pushed to one side, sort of like what happens when you take a corner in a car.

Coordinated flight (somewhat simplified, keeping the aircraft aligned into the relative wind), is basically the *first* skill you learn as a pilot. It's really fundamental.

The only time the fin will see a much of a side load in normal conditions is with an imbalance in the aircraft (left wing has more fuel, and so you're getting more drag on that side), you have an engine out, you're deliberately slipping to generate extra drag, or you're compensating for some other aerodynamic effect (adverse yaw in a turn, for example).

If there's an actual crosswind (relative to the desired ground track), what happens is the airplane flies a diagonal path through the air mass, but the ground track gets straightened out by the movement of the air mass (the crosswind) in the opposite direction. In all cases the airplane will be flying straight into the relative wind it sees, and there will be no side load on the aircraft.


*For example if you move from one air mass to another that is moving in a different direction, there will be a transient load related to the change in velocity of the wind until the aircraft straightens itself out in the new air mass (which will take a few seconds at most).
 
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Faro
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 9:18 am



Quoting Rwessel (Reply 1):
If there's an actual crosswind (relative to the desired ground track), what happens is the airplane flies a diagonal path through the air mass, but the ground track gets straightened out by the movement of the air mass (the crosswind) in the opposite direction. In all cases the airplane will be flying straight into the relative wind it sees, and there will be no side load on the aircraft.

Otherwise said, the angle of incidence of the air hitting the fin is zero? I can't imagine that this can be the case when a crosswind (relative to desired track) is significant. I am not talking about an impulse imparted to the fin (ie acceleration) but simply a fin which has air hitting it at a constant angle which is not zero, somewhat like the wings in the cruise. In this case, drag is necessarily increased.

Faro
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David L
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 10:02 am



Quoting Faro (Reply 2):
Otherwise said, the angle of incidence of the air hitting the fin is zero? I can't imagine that this can be the case when a crosswind (relative to desired track) is significant.

As I understand it (not being a pilot), since the aircraft is flying in the air around it and not relative to anything on the ground, if you're being blown sideways (at a constant rate) relative to the ground track you want to take, you adjust your heading so that it's not pointing to where you actually want to go.

In other words, as Rwessel says, you simply fly normally but in a slightly different direction. It's the same as changing direction in still air.
 
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Faro
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 10:16 am



Quoting David L (Reply 3):
In other words, as Rwessel says, you simply fly normally but in a slightly different direction. It's the same as changing direction in still air.

Simple question then: what is the angle of the air impinging on the fin in this case? Is it or is it not zero?

Faro
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Jetlagged
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 10:44 am



Quoting Faro (Reply 4):
Simple question then: what is the angle of the air impinging on the fin in this case? Is it or is it not zero?

Simple answer, zero.
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pmk
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 10:46 am

As I read it you are inquiring about a fully moving rudder (vertical stabilizer) assembly. Even in an engine out situation there is sufficient surface area to correct with a normal sized tail area. The weight involved not just in the tail assembly but in the entire aircraft structure to accommodate the increased forces the fully movable rudder could give to the aircraft make it a non starter. As far as I can recall and research the only aircraft with such a configuration are dual tail fighter aircraft. As far as inefficiency, it's only passing through the air at an adjacent angle if the pilot isn't doing their job, the tail should always travel in a straight line, even in cross winds.

As far as fully movable horizontal stabilizers (tails) they are used on everything from small general aviation aircraft and even the L1011 had one as well. Normally the stabilator is more popular on supersonic and trans-sonic aircraft as it's use prevents mach tuck, however it carries a far lower weight penalty.

As a side note Bell Aircraft with the X-1 kind of "Forrest Gumped" their way in to making the flying tail by mistake. The stabilator was not a design element of the X-1 and it experienced mach tuck in testing when the horizontal stabilizer became ineffective at trans sonic speeds, however the pilots discovered that they had control authority when using the trim system, which moved the entire tail assembly, creating a ersatz flying tail.

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David L
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 10:47 am



Quoting Faro (Reply 4):
Simple question then: what is the angle of the air impinging on the fin in this case? Is it or is it not zero?

While you're turning there will be some side-force. Once you're established on the new heading, the angle will be zero.

Forgive me for bringing up the CB phrase but... imagine a car driving at 45o across a stationary conyeyor belt. The side force on the tyres is zero and you will end up at a point on the other side of the conveyor belt that is not directly opposite your starting point. If you start the conveyor belt moving (at a constant speed) there will be a speed of the conveyor belt that will cause your 45o track across the conveyer belt to lead you to the point opposite your starting point. Since the conveyor belt speed is constant, there's no side force imparted to the tyres. If you wanted to travel straight across the moving conveyer belt without "aiming off", then you would need some side force.

Relative to the conveyor belt, your route will be exactly the same as the earlier one. It's only when you look beyond the conveyor belt at your destination that you appear to be being pushed sideways because your destination is not directly in front of the car.

You can drive in any direction on the conveyor belt and there will be no side forces on the car unless the conveyor belt accelerates or decelerates.

Compare this with an aircraft taking off in a cross-wind: During the take-off roll, the aircraft's heading has to match it's direction of motion otherwise it'll veer off the runway or scrape the tyres sideways and there will be side forces acting on it. Once it's airborne, the heading can be changed to neutralise the side forces and allow it to stay on the runway centreline (if that's what is desired).
 
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Jetlagged
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 11:38 am

Quoting Pmk (Reply 6):
As a side note Bell Aircraft with the X-1 kind of "Forrest Gumped" their way in to making the flying tail by mistake. The stabilator was not a design element of the X-1 and it experienced mach tuck in testing when the horizontal stabilizer became ineffective at trans sonic speeds, however the pilots discovered that they had control authority when using the trim system, which moved the entire tail assembly, creating a ersatz flying tail.

Maybe, but they also had access to British design data for the Miles M.52 research aircraft which had such a tail as designed.

http://en.wikipedia.org/wiki/Miles_M.52

Note the similarity to the X-1 configuration ...

I tend to favour the idea that Bell used research data rather than "stumble upon" the answer.

Regarding all moving vertical stabilisers, the A-5 Vigilante had one, but the idea goes way back to the early days of flight: Wright Flyer, Avro 504, Fokker EIII, etc. After all, ships tended to have rudders without fixed fins so that was the obvious design. However designers soon found a fixed fin aided directional stability. Only the advent of irreversible power controls made the all moving fin viable, albeit rare.

[Edited 2009-05-08 04:42:18]
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rwessel
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RE: All-Moving Fins For Airliners?

Fri May 08, 2009 10:12 pm



Quoting Faro (Reply 4):
Simple question then: what is the angle of the air impinging on the fin in this case? Is it or is it not zero?

As others have mentioned, it's basically zero in all coordinated flight (IOW essentially all the time). Minor exceptions for things like having to compensate for asymmetrical drag (consider, for example, a fighter-bomber with a bomb under one wing, and nothing under the other, or an engine out situation), or the need to deliberately create extra drag with a slip, or in some scenarios of landing in a crosswind (using a slip to maintain your ground track), or to counteract some asymmetrical secondary effects from other controls.

Quoting David L (Reply 7):
While you're turning there will be some side-force. Once you're established on the new heading, the angle will be zero.

Just to clarify, there won't be any side forces from the turn itself. The forces in a correctly flown turn (the forces that make you change direction) come strictly from wing, and sometime a bit from the horizontal stabilizer.

The yaw forces in a turn come from mostly from secondary effects of the change in banking angle used to roll into a turn, the control inputs needed to hold the bank, and from differences in wing tip speeds while in the turn.

To describe two of the biggies, adverse yaw happens because the down-going aileron generates more drag than the up-going aileron, thus generating a yaw force towards the down-going aileron (which is the opposite direction of the term, hence 'adverse' yaw). If the turn is fairly tight and airspeed is low, the differences in wingtip speed also contribute to a significant asymmetry in drag, although that can work wither ways - at moderate differences, the faster (outside) wingtip generates more drag - if the inside wingtip is moving slowly enough that it needs substantial additional *down* aileron to counteract the overbanking tendency (since the faster wingtip is generating more lift), you may get substantial yaw towards the slower side.

Many aircraft move the ailerons differentially (the down aileron moves less than the up aileron) to minimize adverse yaw. And the differences in tip speeds tend not be a major issue for airplanes with conventionally proportioned wings flying at fairly high speeds (which effectively minimizes the possible difference in tip speeds).

Sailplanes suffer seriously from both effects because of their relatively short tail booms, and very long wings, not to mention their habit of flying tight turns at just a hair above stall. You can always tell a power pilot flying a glider for the first time by the way he's flying cock-eyed, because he's used to doing almost nothing with his feet.
 
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RE: All-Moving Fins For Airliners?

Sat May 09, 2009 12:28 pm



Quoting Rwessel (Reply 1):
f the pilot were doing that, everybody in the cabin would be pushed to one side, sort of like what happens when you take a corner in a car.

Interesting for us non-pilots to read your post, didn't know that!

But I find this sentence hard to believe. Because it would mean that the plane was constantly accelerating sideways, as a car does when you turn.
 
JoeCanuck
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RE: All-Moving Fins For Airliners?

Sat May 09, 2009 10:20 pm

Quoting PlaneWasted (Reply 10):
Quoting Rwessel (Reply 1):
f the pilot were doing that, everybody in the cabin would be pushed to one side, sort of like what happens when you take a corner in a car.

Interesting for us non-pilots to read your post, didn't know that!

But I find this sentence hard to believe. Because it would mean that the plane was constantly accelerating sideways, as a car does when you turn.

There is flight heading where the relative wind and the crosswind forces balance out along the flight track. The aircraft, (heading), will be pointed slightly into the crosswind to maintain the ground track.

When the heading is such that the track is maintained, the aerodynamic forces are balanced and the side forces are zero.

Think of a weathervane stuck to the hood of your car. It is completely free wheeling in that it will always point into the relative wind. If you drive on a calm day, the vane will point down the road...in the same direction as the movement of the car...the relative wind is the same as the ground track.

If there is a crosswind, the weathervane will point slightly into the crosswind but still be physically moving in the direction of the car, (ground track). Imagine that weathervane in the shape of a plane.

It is impossible for a weathervane to resist side forces so if it remains pointed in one direction, the forces acting upon it must be balanced.

A plane in flight is basically a weathervane without the stick. It will always fly in a forces balanced condition unless controlled otherwise.

The only time a pilot will fly a plane in an unnaturally balance condition, (side slip, for example), is when the plane physically must be pointed in the same direction as the ground track, which is for take off and landing.

Otherwise, the plane flies perfectly balanced. If you put a weathervane on the plane, it would line up perfectly with the long axis of the plane...even though it is free to move in any direction. Proof is that you don't have to lean when walking to the bathroom.

[Edited 2009-05-09 15:27:53]

[Edited 2009-05-09 15:30:27]
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airbuske
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RE: All-Moving Fins For Airliners?

Sat May 09, 2009 11:43 pm

As other people have already pointed out, in a steady state condition such cruising flight, there is nothing you can do about the drift due to a wind except for flying a heading that when combined with your drift angle, gives you your desired ground track.

Quoting JoeCanuck (Reply 11):
When the heading is such that the track is maintained, the aerodynamic forces are balanced and the side forces are zero.

The airplane doesn't care what heading you are flying. You could be flying your ground track but be slipping in the process. Side forces are only zero if there is no sideslip.
 
JoeCanuck
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RE: All-Moving Fins For Airliners?

Sat May 09, 2009 11:48 pm



Quoting Airbuske (Reply 12):
As other people have already pointed out, in a steady state condition such cruising flight, there is nothing you can do about the drift due to a wind except for flying a heading that when combined with your drift angle, gives you your desired ground track.

Quoting JoeCanuck (Reply 11):
When the heading is such that the track is maintained, the aerodynamic forces are balanced and the side forces are zero.

Not to be picky but how are these two examples different?
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airbuske
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RE: All-Moving Fins For Airliners?

Sat May 09, 2009 11:53 pm



Quoting JoeCanuck (Reply 13):

Not to be picky but how are these two examples different?

I don't disagree with your example. I disagree with you on the fact that in cruising flight, a crosswind is exerting a force on you. It is not. If it were, you would be accelerating in the direction of the wind and would constantly have to adjust your heading.
 
tdscanuck
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RE: All-Moving Fins For Airliners?

Sun May 10, 2009 2:47 am



Quoting Airbuske (Reply 14):

I don't disagree with your example. I disagree with you on the fact that in cruising flight, a crosswind is exerting a force on you. It is not.

I agree. To expound a little, there's no such thing as a crosswind, from the airplane's point of view, when you're in flight. Wind is defined relative to the ground...since the airplane isn't connected to the ground in any way, there's no such thing as "wind" to the airplane. It's just flying in a body of air. If that body of air happens to be moving relative to the ground, the airplane has no (aerodynamic) idea that that's happening.

The navigation system can "see" it by measuring airplane motion relative to the ground, but there's nothing aerodynamic about that.

Tom.
 
411A
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RE: All-Moving Fins For Airliners?

Sun May 10, 2009 4:42 am

An all-movable vertical fin assembly is not suitable for transport aircraft, due to spiral stability certification issues....IE: it would be very costly to meet 14CFR25 requirements.
 
zappbrannigan
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RE: All-Moving Fins For Airliners?

Sun May 10, 2009 5:01 am



Quoting Tdscanuck (Reply 15):
To expound a little, there's no such thing as a crosswind, from the airplane's point of view, when you're in flight

Beat me to it, was going to use those exact words - in flight, as far as the airframe is concerned, there is no such thing as crosswind. Only relative airflow.

In the cruise, in a hypothetical 400 kt crosswind, there will still be zero lateral force on the fin. It's an important concept for people, including pilots completing their initial training, to get their heads around. Wind only becomes "crosswind" when it requires you to point your nose somewhere other than your desired track to maintain that track.

Think of swimming at a 45 degree angle in a strong, steady current of water. Your body feels absolutely no sideways (lateral) force of the strong current, even though you are positioned side-on to it, and if you closed your eyes you wouldn't know you were in a current. It's only when you compare your resultant direction to the surrounding land do you realise you're in a current.
 
Max Q
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RE: All-Moving Fins For Airliners?

Sun May 10, 2009 6:29 am

As an aside, I think there are / were some military Aircraft with 'all moving fins'


The superb RA 5C Vigilante was one, it also had no Ailerons, roll control being accomplished by spoilers alone.


Can't think of any others ?
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Starlionblue
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RE: All-Moving Fins For Airliners?

Sun May 10, 2009 6:47 am

Both the Rockwell B-1B and Tupolev Tu-160 "Blackjack" have an all-moving fin, sorta. The upper part is all moving, while the lower part is fixed.
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JoeCanuck
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RE: All-Moving Fins For Airliners?

Sun May 10, 2009 7:25 am

Quoting Airbuske (Reply 14):

I don't disagree with your example. I disagree with you on the fact that in cruising flight, a crosswind is exerting a force on you. It is not. If it were, you would be accelerating in the direction of the wind and would constantly have to adjust your heading.

Any wind that is not blowing exactly on the same track as the plane is a cross wind.

It is exerting a force which is counteracted by adjusting your heading to maintain your track. If no force was exerted, the plane would always point exactly the same way it is moving.

It's no different than a head wind or a tail wind. Any wind imparts force on the plane.

If flight is coordinated, the forces will be balanced on the whole plane including the fins and the only force the passenger or plane will feel is that of gravity holding passengers in their seats.

[Edited 2009-05-10 00:55:08]
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tdscanuck
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RE: All-Moving Fins For Airliners?

Sun May 10, 2009 11:34 pm



Quoting JoeCanuck (Reply 20):
Any wind that is not blowing exactly on the same track as the plane is a cross wind.

That's true by definition, but it's crucial to realize that "wind" only means anything when referenced to the ground. When you're just talking about the airplane relative to the air around it, "wind" doesn't mean anything.

Quoting JoeCanuck (Reply 20):
It is exerting a force which is counteracted by adjusting your heading to maintain your track. If no force was exerted, the plane would always point exactly the same way it is moving.

A crosswind does *not* extert a force on the aircraft. A *sideslip* does. The plane always points in the same direction it's moving (in yaw) except when you're in a sideslip.

If you adjust your heading to maintain a particular track, the aircraft is not in a sideslip. You're just flying a particular vector through the air and, when you add the vector of that air relative to the ground (the wind), you get the groundtrack you want.

Quoting JoeCanuck (Reply 20):
It's no different than a head wind or a tail wind. Any wind imparts force on the plane.

In the sense that any air moving relative to the aircraft imparts a force, yes. But the motion of the air relative to the airplane and the motion of the air relative to the ground are totally separate and decoupled things.

Quoting JoeCanuck (Reply 20):

If flight is coordinated, the forces will be balanced on the whole plane including the fins and the only force the passenger or plane will feel is that of gravity holding passengers in their seats.

This is true in coordinated straight flight only. In a coordinated turn, the plane and passengers feel both gravity and centripedal acceleration...the turn coordination just makes sure that those two are lined up.

Tom.
 
PGNCS
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 12:51 am



Quoting Max Q (Reply 18):
As an aside, I think there are / were some military Aircraft with 'all moving fins'


The superb RA 5C Vigilante was one, it also had no Ailerons, roll control being accomplished by spoilers alone.


Can't think of any others ?

The RA-5C was a brilliant and beautiful piece of engineering. It also shared its moving vertical stabilizer with the even more brilliant (and spooky) SR-71 family.
 
JoeCanuck
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 5:14 am



Quoting Tdscanuck (Reply 21):

That's true by definition, but it's crucial to realize that "wind" only means anything when referenced to the ground. When you're just talking about the airplane relative to the air around it, "wind" doesn't mean anything.

I do recognize the difference and that's why I mentioned it in relation to track, to take that into consideration.

Quoting Tdscanuck (Reply 21):
A crosswind does *not* extert a force on the aircraft. A *sideslip* does. The plane always points in the same direction it's moving (in yaw) except when you're in a sideslip.

This is only true in relation to the relative wind...not necessarily the track, which is the direction the plane is moving over the earth.

Of course a crosswind, or any other wind, exerts a force on the aircraft. For instance, it takes more energy and time to fly the same route with a headwind than a tailwind. To maintain your track, (track means ground track), a crosswind must be countered otherwise you will drift. By definition, that is a force.

Quoting Tdscanuck (Reply 21):
If you adjust your heading to maintain a particular track, the aircraft is not in a sideslip. You're just flying a particular vector through the air and, when you add the vector of that air relative to the ground (the wind), you get the groundtrack you want.

I didn't say it was a sideslip...in fact I said that it is coordinated flight. It is a heading change that allows the track to be maintained.

Quoting Tdscanuck (Reply 21):
Quoting JoeCanuck (Reply 20):

If flight is coordinated, the forces will be balanced on the whole plane including the fins and the only force the passenger or plane will feel is that of gravity holding passengers in their seats.

This is true in coordinated straight flight only. In a coordinated turn, the plane and passengers feel both gravity and centripedal acceleration...the turn coordination just makes sure that those two are lined up.

Right...so I should have used 'g's' instead of gravity...otherwise we said exactly the same thing. In coordinated flight, (turns included), passengers will only feel gravity, plus in turns a force which feels like gravity, holding them in their seats.

Quoting Tdscanuck (Reply 21):
Quoting JoeCanuck (Reply 20):
It's no different than a head wind or a tail wind. Any wind imparts force on the plane.

In the sense that any air moving relative to the aircraft imparts a force, yes. But the motion of the air relative to the airplane and the motion of the air relative to the ground are totally separate and decoupled things.

Yes...which I deal with every time I pilot a plane. I especially enjoy the challenge of crosswind landings and practice them often, for entertainment and self preservation.
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tdscanuck
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 6:33 am



Quoting JoeCanuck (Reply 23):
Quoting Tdscanuck (Reply 21):
A crosswind does *not* extert a force on the aircraft. A *sideslip* does. The plane always points in the same direction it's moving (in yaw) except when you're in a sideslip.

This is only true in relation to the relative wind...not necessarily the track, which is the direction the plane is moving over the earth.

It's true relative to the track too. If you're not in a sideslip, there's not side force on the aircraft. And you can only hold groundtrack in a sideslip if you've got assymetric thrust (or some other yaw imbalance).

Quoting JoeCanuck (Reply 23):
Of course a crosswind, or any other wind, exerts a force on the aircraft.

Wind, defined as air motion relative to the aircraft, does exert a force on the aircraft. A crosswind only has definition relative to the ground, not the aircraft. A crosswind exerts no force on the aircraft.

Quoting JoeCanuck (Reply 23):
For instance, it takes more energy and time to fly the same route with a headwind than a tailwind.

Yes, because you have to fly farther, not because you've got an extra force on the aircraft.

Quoting JoeCanuck (Reply 23):
To maintain your track, (track means ground track), a crosswind must be countered otherwise you will drift.

Yes. You counter if by flying a different heading, not by sideslipping.

Quoting JoeCanuck (Reply 23):
By definition, that is a force.

No, it's not. It's just relative motion. If you've got a split between heading and track (i.e. you're flying in a crosswind) then the sideforce on the aircraft is *zero*...it's just that the vector sum of the wind and your motion in the air add up to your track.

Tom.
 
JoeCanuck
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 10:22 am

Quoting Tdscanuck (Reply 24):

This is only true in relation to the relative wind...not necessarily the track, which is the direction the plane is moving over the earth.

It's true relative to the track too. If you're not in a sideslip, there's not side force on the aircraft. And you can only hold groundtrack in a sideslip if you've got assymetric thrust (or some other yaw imbalance).

Reread...I've been talking about heading not sideslip. The only reason to sideslip with a crosswind is during landing where the aircraft must physically point in the same direction as the track as to not damage the plane by excessive side loading of the landing gear.

If there is no side force on the aircraft in flight, why would one have to counter it to maintain their track? One would simply point down their track and not have to worry about drift. The aerodynamic forces are balanced in coordinated flight but that doesn't eliminate the force which caused a heading change to maintain track.

Quoting Tdscanuck (Reply 24):
Quoting JoeCanuck (Reply 23):
To maintain your track, (track means ground track), a crosswind must be countered otherwise you will drift.

Yes. You counter if by flying a different heading, not by sideslipping.

Dude...that's exactly what I have repeatedly said. I can quote me if you wish.

Quoting Tdscanuck (Reply 24):
Quoting JoeCanuck (Reply 23):
By definition, that is a force.

No, it's not. It's just relative motion. If you've got a split between heading and track (i.e. you're flying in a crosswind) then the sideforce on the aircraft is *zero*...it's just that the vector sum of the wind and your motion in the air add up to your track.

It's not merely a split between force vectors. Relative to the ground track, the forward vector is smaller and flight time is increased. Since the goal of the flight is to travel along the track, energy is being used by being applied to a vector not along the track. If it was a direct headwind along the track, the ground velocity would be smaller still.

If there was no force, there would be nothing to be countered, for example, by changing heading or flying longer. It's the same force that pushes around a plane while it's on the ground. It doesn't magically disappear when the plane flies. The plane reacts to it differently in the air but it still must be dealt with.

A strong enough wind and the plane would hover.

This is relative to track. Relative wind is all well and good but planes don't get to their destination if they don't fly over the ground.

Anyway, perhaps our disagreement is merely a matter of semantics...perhaps not. In any case, I don't see much benefit to pursuing this discussion any further. It has been interesting and you are certainly free to continue to disagree with my points as I am to disagree with yours.

Have a nice day.

[Edited 2009-05-11 03:28:37]
What the...?
 
zappbrannigan
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 10:28 am



Quoting Tdscanuck (Reply 24):
Wind, defined as air motion relative to the aircraft, does exert a force on the aircraft. A crosswind only has definition relative to the ground, not the aircraft. A crosswind exerts no force on the aircraft.



Quoting Tdscanuck (Reply 24):
Yes, because you have to fly farther, not because you've got an extra force on the aircraft.



Quoting Tdscanuck (Reply 24):
No, it's not. It's just relative motion. If you've got a split between heading and track (i.e. you're flying in a crosswind) then the sideforce on the aircraft is *zero*...it's just that the vector sum of the wind and your motion in the air add up to your track.

All very well put, and I agree.

When examining any aerodynamic property or behaviour of an aircraft in flight, the ground *does not exist*. And the relative movement of the ground underneath a body of air cannot possibly have any effect on the forces acting from any direction on an aircraft flying within this body of air.

JoeC, think of the scenario of an increasing wind velocity in-flight - if crosswind exerted a force on the fin, then by definition an increase in crosswind velocity would increase this force - and the aircraft would yaw toward the wind and its heading would change. Same as booting in some rudder.

But what actually happens in-flight when wind velocity increases? Heading does NOT change - only track. The airframe is "unaware" that the crosswind has increased - only the navigation systems care. And if heading doesn't change, then the aircraft hasn't experienced an increased force on the fin, or it would have yawed.
 
David L
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 11:39 am

It might be easier for the OP to think of an aircraft flying in still air while the destination drifts slowly sideways in the distance. The aircraft's heading in the still air would have to be altered to anticipate where the destination will have moved to when the aircraft arrives. As far as the aircraft is concerned, it's the same as the real-world drifting-air/stationary-destination scenario because the aircraft only "feels" the air around it and not the ground.
 
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Faro
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 1:19 pm

What I have learned from this thread -correct me if I am wrong- is that sideslip is zero in coordinated flight regardless of enroute winds and vector diagrams (which can introduce a level of confusion here). I may be flying a due north heading at 200 kts groundspeed and experiencing an easterly wind component of 200 kts, my sideslip is still zero and the airflow hitting my rudder remains at zero degrees.

Faro

[Edited 2009-05-11 06:56:09]
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JoeCanuck
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 11:06 pm



Quoting Zappbrannigan (Reply 26):
When examining any aerodynamic property or behaviour of an aircraft in flight, the ground *does not exist*. And the relative movement of the ground underneath a body of air cannot possibly have any effect on the forces acting from any direction on an aircraft flying within this body of air.

My position has been that wind exerts a force on an airplane. This force effects the entire aircraft movement over the ground. That's it.

Quoting Zappbrannigan (Reply 26):
JoeC, think of the scenario of an increasing wind velocity in-flight - if crosswind exerted a force on the fin, then by definition an increase in crosswind velocity would increase this force - and the aircraft would yaw toward the wind and its heading would change. Same as booting in some rudder.

I never mentioned forces acting on a fin in isolation, but the plane as a whole. A crosswind exerts a force pushing the entire plane off track unless the heading is changed to compensate.

Quoting Zappbrannigan (Reply 26):
But what actually happens in-flight when wind velocity increases? Heading does NOT change - only track. The airframe is "unaware" that the crosswind has increased - only the navigation systems care. And if heading doesn't change, then the aircraft hasn't experienced an increased force on the fin, or it would have yawed.

If you go back and read what I wrote, you will notice I never said that crosswind causes heading to change. What I did say is that heading must be adjusted for crosswinds to maintain track.
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Jetlagged
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RE: All-Moving Fins For Airliners?

Mon May 11, 2009 11:31 pm



Quoting JoeCanuck (Reply 25):
Reread...I've been talking about heading not sideslip. The only reason to sideslip with a crosswind is during landing where the aircraft must physically point in the same direction as the track as to not damage the plane by excessive side loading of the landing gear.

If there is no side force on the aircraft in flight, why would one have to counter it to maintain their track? One would simply point down their track and not have to worry about drift. The aerodynamic forces are balanced in coordinated flight but that doesn't eliminate the force which caused a heading change to maintain track.

The only way you'll get a sideforce on the aircraft is if it has sideslip. No sideslip, no sideforce. The wind is moving the entire air-mass, through which the aircraft is travelling. It's the moving air-mass which causes the aircraft to drift sideways.

Quoting JoeCanuck (Reply 25):
A strong enough wind and the plane would hover.

So you think an aircraft in a headwind needs more power to maintain the same airspeed? I think not. The force being exerted on the aircraft by the air is that generated by the relative wind, i.e. the airspeed. Groundspeed has nothing to do with it.

Quoting Joecanuck (Reply 29):
My position has been that wind exerts a force on an airplane. This force effects the entire aircraft movement over the ground. That's it.

The effect of wind is only noticeable if the wind is changing velocity or direction, as in windshear situations. In a steady wind there is no effect on the aircraft's flight.
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rwessel
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 12:11 am



Quoting PlaneWasted (Reply 10):
Quoting Rwessel (Reply 1):
f the pilot were doing that, everybody in the cabin would be pushed to one side, sort of like what happens when you take a corner in a car.

Interesting for us non-pilots to read your post, didn't know that!

But I find this sentence hard to believe. Because it would mean that the plane was constantly accelerating sideways, as a car does when you turn.

You may believe it.  Wink

If you're in a slip, the plane *is* accelerating sideways. So for as long as you're in the slip, there's a side force. Now normally an airplane will have a tendency to turn into the relative wind (which then end the slip), or eventually be accelerated to match the sideways wind (which also effectively end the slip), but you can hold crossed controls to keep it there. And this continuous side force does generate a "turning" component for your ground track (although you can counter that with the exact way you set the controls).

Perhaps one of the bigger insights the Wright's had was that you *didn't* want to skid around turns like a boat does, rather you wanted to bank into them.

Quoting Faro (Reply 28):
What I have learned from this thread -correct me if I am wrong- is that sideslip is zero in coordinated flight regardless of enroute winds and vector diagrams (which can introduce a level of confusion here). I may be flying a due north heading at 200 kts groundspeed and experiencing an easterly wind component of 200 kts, my sideslip is still zero and the airflow hitting my rudder remains at zero degrees.

You've got it. And note that you ground track in that scenario will be north east, and your ground speed 280kts. The wind obvious impacts your trip to the destination (if you forget to take it into account you're going to end up way east of where you want to be), but does not impact how the airplane flies. And in practical terms, with a 200kt airplane, and a 200kt (east or west) crosswind, you can't make any progress at all if your destination is due north (or south), or anywhere towards the opposite side of the compass from the wind.
 
JoeCanuck
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 5:19 am



Quoting Jetlagged (Reply 30):

The only way you'll get a sideforce on the aircraft is if it has sideslip. No sideslip, no sideforce. The wind is moving the entire air-mass, through which the aircraft is travelling. It's the moving air-mass which causes the aircraft to drift sideways.

The wind isn't moving the air mass...the wind IS the moving air mass. It moves as it may and for a plane to travel on its desired track, it has to adjust to the wind. It is indeed the moving air-mass, (otherwise known as wind), which causes the aircraft to drift from its track, which has been my assertion all along.

Quoting Jetlagged (Reply 30):

Quoting JoeCanuck (Reply 25):
A strong enough wind and the plane would hover.

So you think an aircraft in a headwind needs more power to maintain the same airspeed? I think not. The force being exerted on the aircraft by the air is that generated by the relative wind, i.e. the airspeed. Groundspeed has nothing to do with it.

What I said was, a strong enough wind and the plane could hover. Hovering is generally accepted as being in relation to the ground, so by definition, groundspeed has everything to do with it. For example, if the wind is moving at cruising speed and cruising power is being applied, it would hover, if flying directly into that wind.

I never said that an aircraft in a headwind needs more power to maintain the same airspeed...not once. Ground speed, though, has very much to do with moving an airplane from one physical location on the planet to another.

Quoting Jetlagged (Reply 30):
Quoting Joecanuck (Reply 29):
My position has been that wind exerts a force on an airplane. This force effects the entire aircraft movement over the ground. That's it.

The effect of wind is only noticeable if the wind is changing velocity or direction, as in windshear situations. In a steady wind there is no effect on the aircraft's flight.

...except in its path over the ground...which is all I've been saying.

The effect of the wind, is to effect the aircraft in relation to the ground. It does effect the aircraft's flight in that the aircraft must adjust to it in order to maintain it's track, groundspeed, eta, etc. It is very noticeable if you happen to be looking at the ground while flying.

I did say that in coordinated flight, the only force that passengers would feel is that of gravity, (and the addition of 'g's' in a coordinated turn), holding them in their seats...coordinated flight, not slips of any sort.
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tdscanuck
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 5:49 am



Quoting JoeCanuck (Reply 32):
It is indeed the moving air-mass, (otherwise known as wind), which causes the aircraft to drift from its track, which has been my assertion all along.



Quoting JoeCanuck (Reply 32):
For example, if the wind is moving at cruising speed and cruising power is being applied, it would hover, if flying directly into that wind.



Quoting JoeCanuck (Reply 32):
Ground speed, though, has very much to do with moving an airplane from one physical location on the planet to another.



Quoting JoeCanuck (Reply 32):
The effect of the wind, is to effect the aircraft in relation to the ground. It does effect the aircraft's flight in that the aircraft must adjust to it in order to maintain it's track, groundspeed, eta, etc

The above are all correct, and I don't think anybody on the thread is arguing any of the above with you.

The bone of contention is:

Quoting JoeCanuck (Reply 20):
Any wind that is not blowing exactly on the same track as the plane is a cross wind. It is exerting a force



Quoting JoeCanuck (Reply 23):
Of course a crosswind, or any other wind, exerts a force on the aircraft.

What I, and several of the other posters as far as I can tell, disagree with you on is whether the crosswind (or any other wind) is exerting a force on the aircraft. You believe it is, based on this:

Quoting JoeCanuck (Reply 25):
The aerodynamic forces are balanced in coordinated flight but that doesn't eliminate the force which caused a heading change to maintain track.

You appear to be saying that the wind is exerting a force on the airplane, which must be balanced by another force on the airplane (presumably from the control surfaces), so that the net force is zero. This is wrong.

There is no force from the wind on the aircraft...the net force is zero because the wind force and the control force are both zero. When cruising in a crosswind, there is no force from the wind on the aircraft and there is no balancing force from the aircraft control surfaces. This is the disagreement and, as far as I can tell, the only disagreement.

Tom.
 
JoeCanuck
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 7:16 am



Quoting Tdscanuck (Reply 33):
Quoting JoeCanuck (Reply 25):
The aerodynamic forces are balanced in coordinated flight but that doesn't eliminate the force which caused a heading change to maintain track.

You appear to be saying that the wind is exerting a force on the airplane, which must be balanced by another force on the airplane (presumably from the control surfaces), so that the net force is zero. This is wrong.

There is no force from the wind on the aircraft...the net force is zero because the wind force and the control force are both zero. When cruising in a crosswind, there is no force from the wind on the aircraft and there is no balancing force from the aircraft control surfaces. This is the disagreement and, as far as I can tell, the only disagreement.

The force of wind isn't countered by control surfaces, per se...it's done by flying in a different direction. We might be having problems with definitions.

Relative wind is in relation to the aircraft. A crosswind is in relation to track, not the aircraft.

In coordinated flight, relative wind is always directly on the nose. A slip points the nose away from directly into the relative wind. To maintain track, a crosswind must be countered. It is done with coordinated flight. The plane isn't slipping in any way. It is a heading change caused by turning the aircraft in the same way any other turn is made...bank, turn, level out on new heading...no slipping required.

This new heading will also be directly into the relative wind...the plane will merely be pointed in a different direction than the previous relative wind but countering the crosswind allowing the aircraft to maintain its intended track.

The aerodynamic forces, lift, gravity, thrust and drag are perfectly balanced the whole time.

You can do a 360 degree coordinated turn and the relative wind will be directly on the nose through the entire circle. During the turn, the plane would have drifted downwind.

In coordinated flight, a heading different from the track isn't a side slip...it is merely where the aircraft is pointed, perhaps to counter a wind which is across the track, a crosswind.
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airbuske
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 1:29 pm



Quoting JoeCanuck (Reply 34):
The force of wind isn't countered by control surfaces, per se...it's done by flying in a different direction.

For the airplane to counter a force due to wind so that it's ground track remains unchanged, it must exert an equal and opposite force. Could you please explain how flying a heading corrected for wind drift exerts a force on the wind?

While you come up with an answer, recalling Newton's 1st law : F = m * a
where F = force,
m = mass,
a = acceleration.

Therefore in order for the airplane to be exerting a force, it must be accelerating in the direction against the wind.

Quoting JoeCanuck (Reply 34):
We might be having problems with definitions.

This might be the case. It is clear that you have the right idea, but I think you have your terminologies slightly mixed up. (No offense meant)
 
JoeCanuck
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 5:30 pm

The equal and opposite force is created by the thrust vector being adjusted to counter the wind. The results are the aircraft remains on its ground track and is slowed on its ground track.

The aircraft accelerates into the wind while at the same time decelerating along its track. When it gets back on its track, the flight forces are once again balanced and the net acceleration is zero. The aircraft flies along its track at a constant slower speed while applying a constant opposing force to the wind.

[Edited 2009-05-12 10:47:31]
What the...?
 
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Jetlagged
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 6:21 pm



Quoting JoeCanuck (Reply 32):
What I said was, a strong enough wind and the plane could hover. Hovering is generally accepted as being in relation to the ground, so by definition, groundspeed has everything to do with it. For example, if the wind is moving at cruising speed and cruising power is being applied, it would hover, if flying directly into that wind.

I never said that an aircraft in a headwind needs more power to maintain the same airspeed...not once. Ground speed, though, has very much to do with moving an airplane from one physical location on the planet to another.

You implied it by saying the wind applies a force on the aircraft. If it were, in a headwind the aircraft would need more power to maintain speed. In considering aerodynamic forces one must always deal with the forces generated relative to the air in which the aircraft is travelling. If this air mass is moving it does not affect the aerodynamic forces, but it does affect inertial velocities.

Quoting JoeCanuck (Reply 32):
The wind isn't moving the air mass...the wind IS the moving air mass. It moves as it may and for a plane to travel on its desired track, it has to adjust to the wind. It is indeed the moving air-mass, (otherwise known as wind), which causes the aircraft to drift from its track, which has been my assertion all along.

My mistake of course about wind and the air but that error is not the point. If an aircraft is drifting we are all agreed that it affects its track. The aircraft does not counter this with a sideforce (which requires sideslip), but by changing heading to correct for drift.
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2H4
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 6:25 pm



Quoting Tdscanuck (Reply 24):
Wind, defined as air motion relative to the aircraft, does exert a force on the aircraft. A crosswind only has definition relative to the ground, not the aircraft. A crosswind exerts no force on the aircraft.



Quoting Tdscanuck (Reply 33):
What I, and several of the other posters as far as I can tell, disagree with you on is whether the crosswind (or any other wind) is exerting a force on the aircraft.



Quoting Tdscanuck (Reply 33):
There is no force from the wind on the aircraft...the net force is zero because the wind force and the control force are both zero. When cruising in a crosswind, there is no force from the wind on the aircraft

The above bolded/italicised sentences seem to contradict each other. Can you clarify a bit?

I am reminded of my time in an open cockpit aircraft, cruising on a north heading in coordinated flight. The winds were out of the west, and I felt it on the left side of my face.

2H4
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JoeCanuck
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 7:05 pm



Quoting Jetlagged (Reply 37):
You implied it by saying the wind applies a force on the aircraft. If it were, in a headwind the aircraft would need more power to maintain speed. In considering aerodynamic forces one must always deal with the forces generated relative to the air in which the aircraft is travelling. If this air mass is moving it does not affect the aerodynamic forces, but it does affect inertial velocities.

Quite the opposite, I have attempted to always say exactly what I mean and not imply anything. I believe I clearly differentiated between aerodynamic forces and the movement of a plane in relation to the ground...at least that was my goal.

Quoting Jetlagged (Reply 37):
My mistake of course about wind and the air but that error is not the point. If an aircraft is drifting we are all agreed that it affects its track. The aircraft does not counter this with a sideforce (which requires sideslip), but by changing heading to correct for drift.

I don't think I ever said that a crosswind was to be countered by slipping, except when landing. I'm sure if you read my posts you will find that I mention at every turn that a crosswind is countered by a heading change to remain on track.
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Jetlagged
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 7:20 pm



Quoting JoeCanuck (Reply 39):
I don't think I ever said that a crosswind was to be countered by slipping, except when landing. I'm sure if you read my posts you will find that I mention at every turn that a crosswind is countered by a heading change to remain on track.

How about this?

Quoting Joecanuck (Reply 29):
My position has been that wind exerts a force on an airplane. This force effects the entire aircraft movement over the ground. That's it.

That would be the part I'm disagreeing with. If the wind exerts a force on the aircraft, then the aircraft must react to this with a sideforce, which requires sideslip.
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Faro
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 7:22 pm



Quoting 2H4 (Reply 38):
I am reminded of my time in an open cockpit aircraft, cruising on a north heading in coordinated flight. The winds were out of the west, and I felt it on the left side of my face.

Bingo! This was my initial lietmotiv in creating this topic. Now I am thoroughly confused; if there is a side-component of the wind acting on the aircraft, then the air is not hitting the fin at zero degrees...

Faro
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JoeCanuck
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RE: All-Moving Fins For Airliners?

Tue May 12, 2009 11:58 pm



Quoting Jetlagged (Reply 40):
Quoting Joecanuck (Reply 29):
My position has been that wind exerts a force on an airplane. This force effects the entire aircraft movement over the ground. That's it.

That would be the part I'm disagreeing with. If the wind exerts a force on the aircraft, then the aircraft must react to this with a sideforce, which requires sideslip.

It doesn't. A plane merely has to change its heading. It will be pointed in a new direction, not slipping to adjust to the wind.

Here's a diagram I lifted from wiki;

What the...?
 
VirginFlyer
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RE: All-Moving Fins For Airliners?

Wed May 13, 2009 12:20 am

Quoting 2H4 (Reply 38):
The above bolded/italicised sentences seem to contradict each other. Can you clarify a bit?

Without wanting to take words out of Tdscanuck's mouth, the key phrase is "air motion relative to the aircraft".

Maybe I could throw in a few definitions here (I've written these off the top of my head - always dangerous I know!)
Crosswind - movement of air relative to the surface of the earth, in a direction perpendicular to the track of the aircraft.
Heading - the direction in which the nose of the aircraft is pointing.
Relative air flow - the vector defining the direction of the air flow arriving the aeroplane. It may be considered to be opposite to the direction of the flight path.
Track - the path an aircraft makes across the surface of the earth.
Wind - the movement of air across the surface of the earth.

Now, let me try and explain the situation, and see if I can clear some of the confusion.
[Please not I have deliberately ignored vertical movement here - when I refer to vectors, I am referring to horizontal motion only, as you would see if you looked down at the situation from on top]

Let's start at the beginning, and first consider a condition of nil wind. If the aircraft is flown in such a fashion that the heading is in line with (and opposite to unless we're going backwards!) the relative airflow, then there should be no net sideforce on the aircraft. This is what is termed (by pilots anyway) as balanced flight. If control input is now made such that the heading is not in line with the relative airflow, there will be a sideforce on the aircraft, and this is considered unbalanced flight. This is purely to do with the difference between the heading vector and the relative air flow vector. Note that I can point the aircraft in any direction whatsoever, and fly it either in balanced or unbalanced flight.

Now lets consider a different situation, where there is a constant wind blowing. To take 2H4's example, we have a wind from the west, and an aircraft travelling to the north. Like before, I can point the aircraft in any direction and set it up such that the heading vector is in line with and opposite to the relative air flow and balanced flight is maintained. There is no net sideforce on the aircraft. The vector of the aircraft moving across the ground will be equal to the vector of the aircraft's movement through the air plus the vector of the air's movement relative to the ground. If I take up a heading of exactly north, maintaining balanced flight, then the track of the aircraft will be to the north-east. Likewise, if I wanted to maintain a track of exactly north, I would have to maintain a heading in a north-westerly direction. Again, we have established that this can be done with no net sideforce.

If the wind is not constant, then yes, at the instant the wind changes strength or direction, there will be a net sideforce on the aircraft. This is a transitory effect, and will return to zero once the wind returns to a constant strength (any constant strength).

If you are still having difficulty following, let me use two analogies.

1) Imagine a gold fish in a fish bowl which is sitting on top of a table. The fish swims in such a way that it travels straight through the water. Now imagine I set the whole bowl moving (maybe using a conveyor belt    ). After the transitory effect of starting the bowl moving, the fish can continue to swim in the bowl unaffected. Lets say I am pushing the bowl perpendicular to the direction the fish is swimming. So far as the fish is concerned, it is still swimming straight ahead. If someone were to look at the position of the fish relative to the table however, they would see it moving in a sideways direction.

2) Take a hose or a tap and turn it on. Push your finger through the stream. You will feel the stream exerting a force on your finger. Now try to get your finger moving sideways at the same speed as the water (this is difficult!) and then introduce it to the stream. If your finger is maintaining the same speed sideways as the stream, it will feel like you are plunging your finger into stationary (albeit turbulent) water.

Quoting 2H4 (Reply 38):
I am reminded of my time in an open cockpit aircraft, cruising on a north heading in coordinated flight. The winds were out of the west, and I felt it on the left side of my face.

This is either the result of unbalanced flight (which may have come about as a result of the pilot trying to keep the heading and track aligned, and thus flying out of balance - it is a very common error when flying close to the ground in strong wind conditions), or more likely the result of the slipstream of a clockwise rotating propeller (as you see it from the cockpit) striking you from the left. Did you happen to take note of where you felt the wind coming striking you when you were cruising on a south heading (assuming you were)?

V/F

[Edited 2009-05-12 17:24:20]

[Edited 2009-05-12 17:28:10]
"So powerful is the light of unity that it can illuminate the whole earth." - Bahá'u'lláh
 
airbuske
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RE: All-Moving Fins For Airliners?

Wed May 13, 2009 4:46 am

Adding to what VirginFlyer has already said, aside from the theoretical world, winds are never ever constant. At any instant, there is always some small deviation which is why you could feel a wind blowing.
 
zappbrannigan
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RE: All-Moving Fins For Airliners?

Wed May 13, 2009 6:42 am



Quoting 2H4 (Reply 38):
I am reminded of my time in an open cockpit aircraft, cruising on a north heading in coordinated flight. The winds were out of the west, and I felt it on the left side of my face.

I believe there must have been others factors at play here. If it was constant, my first guess would be propwash, as mentioned by VirginFlyer.

JoeCanuck, are you saying that when an aircraft experiences a crosswind that it wasn't previously experiencing, a force is imparted on the fin? Cause that's a big yes, and I don't think anybody here will disagree. But once it weathercocks into the wind, it re-balances and gets back to coordinated flight - no more force on the fin.

I mean, think of a model aircraft in a wind tunnel. If the model was yawed off-centre then released, it would immediately weathercock back directly into the airflow, thanks to the fin - and the final force on the fin would be zero once it was stable. There is absolutely no difference between this and the introduction of crosswind - the position of the ground is meaningless with regard to forces acting on the aircraft.
 
rwessel
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RE: All-Moving Fins For Airliners?

Wed May 13, 2009 6:45 am



Quoting 2H4 (Reply 38):
I am reminded of my time in an open cockpit aircraft, cruising on a north heading in coordinated flight. The winds were out of the west, and I felt it on the left side of my face.

I'm sorry. That is simply impossible. Other than transitory effect (you fly into an air mass moving at a different speed), you cannot be in coordinated flight and feel a wind from the side. You could have been in a slip to counter the crosswind, and then felt a side wind, but that's not coordinated flight. Or perhaps there was some aerodynamic artifact of the airplane that caused you to feel a wind from the left (perhaps the right side of the windshield was longer, a caused a low pressure area to the right of the cockpit).

Consider the forces that make a turn and bank indicator indicate a slip, or the same reaction (but in the opposite direction) from a yaw string. If there's a side wind, it'll be pushing the airplane, and the ball will roll off to one side. A yaw string is even more obvious - you're in coordinated flight if it's pointing straight back. By definition. If there were a wind from the side, it wouldn't be pointing straight back.
 
tdscanuck
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RE: All-Moving Fins For Airliners?

Wed May 13, 2009 7:40 am



Quoting JoeCanuck (Reply 36):
The equal and opposite force is created by the thrust vector being adjusted to counter the wind.

Absolutely not true. Excepting momentary transients, neatly described by Zappbrannigan in Reply 45 (and vectored thrust nozzles), there is no component of thrust that is perpendicular to the relative wind. Unless you're sideslipping, which both of us seem to agree isn't happening, thrust is directly aligned (and opposite) to relative wind.

Quoting JoeCanuck (Reply 36):
The results are the aircraft remains on its ground track and is slowed on its ground track.

It's slowed because the air you're flying in is moving relative to the ground, not because it's eating any of your thrust. Motion over the ground is the vector sum of your motion relative to the air plus the air's motion relative to the ground. The latter vector is wind...changes in wind will change ground track because you changed one of the vectors, but forces on the airplane are entirely determined by the first vector. If you don't change your motion relative to the air, it's physically impossible to have a change in force on the aircraft. And if you're not in a sideslip and going, say, 300 knots, your motion relative to the air is exactly the same regardless of heading.

Quoting JoeCanuck (Reply 36):
The aircraft accelerates into the wind while at the same time decelerating along its track.

I think you're mixing acceleration and velocity here. An aircraft in straight flight isn't accelerating at all, in any direction.

Quoting 2H4 (Reply 38):

The above bolded/italicised sentences seem to contradict each other. Can you clarify a bit?

VirginFlyer caught it:

Quoting VirginFlyer (Reply 43):
Without wanting to take words out of Tdscanuck's mouth, the key phrase is "air motion relative to the aircraft".

*If* you define wind as "air motion relative to the aircraft", then (and only then) will it exert a force on the aircraft. However, that's not the normal definition of wind, and it's certainly not the definition of wind that applies when you're talking about crosswinds and heading vs. track.

I thought we all had the same physical picture and were just in a definition problem (hinging around "force"), but bringing thrust into the discussion has me thinking that we don't all have the same mental picture.

Tom.
 
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Jetlagged
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RE: All-Moving Fins For Airliners?

Wed May 13, 2009 12:00 pm



Quoting JoeCanuck (Reply 42):
It doesn't. A plane merely has to change its heading. It will be pointed in a new direction, not slipping to adjust to the wind.

Here's a diagram I lifted from wiki;

Is wiki where you get all your aerodynamic knowledge? Are you deliberately ignoring what I actually said?

You have asserted a crosswind exerts a force on the aircraft, and that the aircraft resists this force by altering heading. I said that to resist a sideforce the aircraft must generate a sideslip angle, which you agree is not the case in cruise, therefore no sideforce exists on the aircraft in steady state.

There is no force being applied by a steady crosswind and the aircraft is not resisting the wind effect by altering heading. It is compensating for the effect of the wind, no more.
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David L
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RE: All-Moving Fins For Airliners?

Wed May 13, 2009 1:09 pm



Quoting Jetlagged (Reply 48):
and the aircraft is not resisting the wind effect by altering heading.

That's the way I see it. I'd tend to agree that there's no extra force to overcome unless you want to achieve the same ground track and arrive at the destination in the same time, rather than at the same airspeed but that's not the scenario we're looking at. Simply changing heading without changing airspeed should involve exactly the same forces on the aircraft as doing the same in still air.

Let's face it: in what we call "still air" the air and destination are both moving, due to the rotation of the earth, but at the same speed and in the same direction as each other. Whether or not the air and the destination are moving in sync with each other shouldn't affect the forces of the air on the aircraft.

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