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All-Moving Fins For Airliners?  
User currently offlineFaro From Egypt, joined Aug 2007, 1551 posts, RR: 0
Posted (5 years 4 months 3 weeks 6 days 16 hours ago) and read 5413 times:

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


The chalice not my son
84 replies: All unread, showing first 25:
 
User currently offlineRwessel From United States of America, joined Jan 2007, 2353 posts, RR: 2
Reply 1, posted (5 years 4 months 3 weeks 6 days 16 hours ago) and read 5410 times:
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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).


User currently offlineFaro From Egypt, joined Aug 2007, 1551 posts, RR: 0
Reply 2, posted (5 years 4 months 3 weeks 6 days 16 hours ago) and read 5404 times:



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



The chalice not my son
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 3, posted (5 years 4 months 3 weeks 6 days 15 hours ago) and read 5390 times:



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.


User currently offlineFaro From Egypt, joined Aug 2007, 1551 posts, RR: 0
Reply 4, posted (5 years 4 months 3 weeks 6 days 15 hours ago) and read 5388 times:



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



The chalice not my son
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2556 posts, RR: 24
Reply 5, posted (5 years 4 months 3 weeks 6 days 15 hours ago) and read 5379 times:



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.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlinePmk From United States of America, joined May 1999, 664 posts, RR: 2
Reply 6, posted (5 years 4 months 3 weeks 6 days 14 hours ago) and read 5377 times:

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.

PMK


User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 7, posted (5 years 4 months 3 weeks 6 days 14 hours ago) and read 5377 times:



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).


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2556 posts, RR: 24
Reply 8, posted (5 years 4 months 3 weeks 6 days 14 hours ago) and read 5353 times:

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]


The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineRwessel From United States of America, joined Jan 2007, 2353 posts, RR: 2
Reply 9, posted (5 years 4 months 3 weeks 6 days 3 hours ago) and read 5230 times:
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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.


User currently offlinePlaneWasted From Sweden, joined Jan 2008, 528 posts, RR: 0
Reply 10, posted (5 years 4 months 3 weeks 5 days 13 hours ago) and read 5159 times:



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.


User currently onlineJoeCanuck From Canada, joined Dec 2005, 5472 posts, RR: 30
Reply 11, posted (5 years 4 months 3 weeks 5 days 3 hours ago) and read 5066 times:

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]


What the...?
User currently offlineAirbuske From United States of America, joined Jun 2007, 466 posts, RR: 0
Reply 12, posted (5 years 4 months 3 weeks 5 days 2 hours ago) and read 5049 times:

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.


User currently onlineJoeCanuck From Canada, joined Dec 2005, 5472 posts, RR: 30
Reply 13, posted (5 years 4 months 3 weeks 5 days 1 hour ago) and read 5045 times:



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?



What the...?
User currently offlineAirbuske From United States of America, joined Jun 2007, 466 posts, RR: 0
Reply 14, posted (5 years 4 months 3 weeks 5 days 1 hour ago) and read 5042 times:



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.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 15, posted (5 years 4 months 3 weeks 4 days 22 hours ago) and read 5022 times:



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.


User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 16, posted (5 years 4 months 3 weeks 4 days 21 hours ago) and read 5002 times:

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.

User currently offlineZappbrannigan From Australia, joined exactly 6 years ago today! , 247 posts, RR: 0
Reply 17, posted (5 years 4 months 3 weeks 4 days 20 hours ago) and read 4996 times:



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.


User currently offlineMax Q From United States of America, joined May 2001, 4556 posts, RR: 19
Reply 18, posted (5 years 4 months 3 weeks 4 days 19 hours ago) and read 4985 times:

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 best contribution to safety is a competent Pilot.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17055 posts, RR: 67
Reply 19, posted (5 years 4 months 3 weeks 4 days 18 hours ago) and read 4979 times:

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.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently onlineJoeCanuck From Canada, joined Dec 2005, 5472 posts, RR: 30
Reply 20, posted (5 years 4 months 3 weeks 4 days 18 hours ago) and read 4965 times:

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]


What the...?
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 21, posted (5 years 4 months 3 weeks 4 days 2 hours ago) and read 4863 times:



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.


User currently offlinePGNCS From United States of America, joined Apr 2007, 2830 posts, RR: 45
Reply 22, posted (5 years 4 months 3 weeks 4 days ago) and read 4848 times:



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.


User currently onlineJoeCanuck From Canada, joined Dec 2005, 5472 posts, RR: 30
Reply 23, posted (5 years 4 months 3 weeks 3 days 20 hours ago) and read 4800 times:



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.



What the...?
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 24, posted (5 years 4 months 3 weeks 3 days 19 hours ago) and read 4794 times:



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.


25 JoeCanuck : Reread...I've been talking about heading not sideslip. The only reason to sideslip with a crosswind is during landing where the aircraft must physica
26 Zappbrannigan : 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 r
27 David L : 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
28 Faro : 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
29 Joecanuck : 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. I never men
30 Jetlagged : 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
31 Rwessel : You may believe it. 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 normal
32 JoeCanuck : 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 ad
33 Tdscanuck : 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: What I, and sever
34 JoeCanuck : 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 defini
35 Airbuske : 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 ple
36 JoeCanuck : 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 tr
37 Jetlagged : 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
38 2H4 : 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, cru
39 JoeCanuck : Quite the opposite, I have attempted to always say exactly what I mean and not imply anything. I believe I clearly differentiated between aerodynamic
40 Jetlagged : How about this? 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
41 Faro : 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 air
42 Post contains images JoeCanuck : 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 l
43 Post contains images VirginFlyer : 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 definit
44 Airbuske : Adding to what VirginFlyer has already said, aside from the theoretical world, winds are never ever constant. At any instant, there is always some sma
45 Zappbrannigan : 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. JoeCan
46 Rwessel : 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 coordinat
47 Tdscanuck : Absolutely not true. Excepting momentary transients, neatly described by Zappbrannigan in Reply 45 (and vectored thrust nozzles), there is no compone
48 Jetlagged : Is wiki where you get all your aerodynamic knowledge? Are you deliberately ignoring what I actually said? You have asserted a crosswind exerts a forc
49 David L : 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
50 JoeCanuck : Absolutely true if one has been talking about motion relative to the ground which I have been doing and clearly stated a number of times. I never onc
51 2H4 : But wouldn't gusts or shear within an air mass act upon an aircraft in such a manner? A gust from the left, for example, would push the aircraft to t
52 Airbuske : We aren't disputing that you must fly a new heading in order to maintain the same track. We aren't disputing that there is no sideslip. We are disput
53 Pihero : Funny how people keep on their own argumentation and disregard others'. IMO, the best -and the only - way to look at an airplane in flight is to say t
54 Jetlagged : You may be a pilot but with respect that doesn't make you infallible, even on A.net. I was trying to be civil, but you made it very hard by avoiding
55 David L : With all due respect, if you look back I think you'll find that you've introduced some complications that are not really going to help the OP underst
56 2H4 : Ah, that makes sense. Sorry about that....my mind keeps jumping from the theoretical world to the real one, and my constant multitasking isn't helpin
57 JoeCanuck : This kind of comment should beneath you. So why does a crosswind have to be countered if it has no force? Can it effect ground speed? Can it effect t
58 Zappbrannigan : I don't think a single person on here will try and dispute this. But this entire thread was about whether a *steady* crosswind (i.e. no gusts/shear e
59 Rwessel : Yes it would. But a gust within an airmass is, pretty much be definition, a transitory effect. Either you fly out of the gust, or the gust accelerate
60 David L : Exactly!
61 Pihero : . To start : In flight one doesn't counter a crosswind ; One only compensates for it by adjusting one's heading inside a moving air mass. On a glider
62 Zappbrannigan : Exactly. The argument is not that we don't have to counter a crosswind to maintain track (this is ridiculous) - it's that there is absolutely no chan
63 Tdscanuck : Then I don't understand this statement: During the transient period (e.g. when hit by a gust), then yes. But in steady coordinated flight, none of th
64 JoeCanuck : I didn't disregard his remark. I didn't feel it answered my questions, hence my response. I don't think there's any reason to be less than civil. So
65 Tdscanuck : That is not what I'm saying. I'm saying it's moving through the air at exactly the same speed relative to the air (that's all airspeed means), regard
66 Zappbrannigan : Mate, I think this quote sums up why we're having this discussion in the first place. With all due respect, it doesn't make much sense - not with any
67 JoeCanuck : No problem with this at all. Let's deal with going from one steady state to another, 100 to 80 works for me. Isn't that just a change in a force, not
68 Faro : To try to simplify as much as possible, this is how I now see things: i) suppose the aircraft is flying in co-ordinated flight in a totally static air
69 David L : Correct. There's no difference from the aircraft's point of view between (i) and (ii) once the aircraft has stablised within the moving air mass.
70 Jetlagged : I guess you mean me. I thought I was being as clear as possible in showing why no force was being resisted. Apparently I wasn't the only one who thou
71 Tdscanuck : In the specific case of a headwind, yes, it's just a change in an existing force (drag). The change in force only lasts as long as the speed transien
72 JoeCanuck : ...which has been my point all along. Hopefully we can also agree that with the aerodynamic forces aligned through the axes of the plane, this is not
73 Tdscanuck : [ Agreed. Yes. Yes. That force is drag, which has nothing to do with the crosswind. No. The force countering thrust is drag. At constant airspeed, the
74 JoeCanuck : So what is creating the drag which must be countered, specifically by the thrust component perpendicular to the track?
75 Tdscanuck : Airspeed. You're moving forward, so you've got drag. Drag is balanced by thrust. If you change heading (with or without a wind), drag and thrust rota
76 JoeCanuck : We're just going in circles, Tom. I do know that when I file a flight plan, I need to know the wind speed and direction so I can calculate the proper
77 Tdscanuck : Yep. Your understand of how the airplane behaves is dead-on, as you'd expect for a pilot. I'm not questioning, in any way, your understanding of how
78 Pihero : Airline pilots think that way, too... "heading correction" would be a far better name to it. If you think "navigation", you have to consider the wind
79 David L : The way I see it is that the "force" for which you change heading is the force of "not going where you want to be going". It would be the same as fly
80 Tdscanuck : Thank you, David L! That's exactly what I've been trying to express, but you did it *much* more succinctly. Tom.
81 David L : Thanks. It was certainly better than my earlier attempt with the car on the conveyor belt... especially since so many A.netters are now suffering con
82 Pihero : David, you eaten an eagle raw with the feathers ?
83 Post contains links and images David L : Armed with new information about that expression, I can confirm that I'm not sure. Had the question been meant literally, on the other hand, my respo
84 Glom : This comes down to the all important concept of Newtonian frames of reference. We have a frame A, which is the air. We have a frame B, which is the gr
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