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jetmech
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RE: Silly Question Regarding Rudder Deflections

Wed Aug 27, 2008 11:56 am



Quoting PhilSquares (Reply 48):
Exactly! While it is possible to change the aerodynamics of a swept wing aircraft, the entailing cost and weight penalties would make it impossible to market.

I see. Thanks for the info!

Quoting NicoEDDF (Reply 49):
And washout is the term for exactly what?

Washout / washin refers to a change in local incidence angle as one moves from the wing root to the wing tip. Washout denotes a wing where the local incidence angle decreases as one moves from root to tip. Washin denotes a wing where the local incidence angle increases as one moves from root to tip.

The 747 ( and apparently, all other swept wing Boeing or Airbus types ) have washout, thus the wing tip appears to be "twisted" down at a lower incidence angle compared to the wing root.


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Nicoeddf
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RE: Silly Question Regarding Rudder Deflections

Wed Aug 27, 2008 1:46 pm

I see, I see. Thanks JetMech
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Blackbird
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RE: Silly Question Regarding Rudder Deflections

Wed Aug 27, 2008 8:04 pm

PhilSquares,

What's an accelerated stall?


Blackbird
 
PhilSquares
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RE: Silly Question Regarding Rudder Deflections

Wed Aug 27, 2008 8:17 pm



Quoting Blackbird (Reply 52):
What's an accelerated stall?

Stalls not associated with a low speed condition, but with wing loading and exceeding the critical angle of attack.
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Blackbird
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 2:41 am

PhilSquares,

Understood.,

BTW: Isn't even a low-speed stall caused by the critical AoA being exceeded as the AoA necessary to maintain level flight exceeds that to stall the plane?
 
rwessel
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 3:21 am



Quoting Blackbird (Reply 54):
BTW: Isn't even a low-speed stall caused by the critical AoA being exceeded as the AoA necessary to maintain level flight exceeds that to stall the plane?

Indeed. The completely artificial distinction between "normal" and "accelerated" stalls has led to more than a few pilots thinking they were going "too fast* to stall," and then being very surprised (and sometimes dead) when the bottom dropped out in a turn. The very notion that stalling is associated with a speed is misleading at best.

Of course since most smaller civilian aircraft are not equipped with AoA indicators (and even many larger ones which have AoA sensors, don't usefully display that information in the cockpit), it's kinda difficult to explain that concept to new pilots. Which is an indictment of how we equip out aircraft and our training, but that's a particular hobby-horse of mine, and I'll shut up now.  Wink

To be complete, the stall warning horns on many smaller aircraft are, in fact, very, very crude AoA sensors.


*no such thing, BTW, except perhaps in the sense that you can be going fast enough that the stalling AoA won't be reached until after you tear the wings off, but that's a structural issue, not an aerodynamic one.
 
Blackbird
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 3:30 am

Rwessel,

Quote:
*no such thing, BTW, except perhaps in the sense that you can be going fast enough that the stalling AoA won't be reached until after you tear the wings off, but that's a structural issue, not an aerodynamic one.

Actually, I thought planes like the F-22 and YF-23 had no alpha limits (at least the F-23 from what I was told)?

Additionally, it's probably possible to have a situation in which you're going so fast (above corner velocity) in which you wouldn't have enough control power to get the alpha high enough to get a stall (up to a point the faster you go the more responsive the plane gets, above a point though the plane becomes less and less responsive as the higher speed makes it harder for the plane to change direction.


Blackbird
 
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Starlionblue
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 4:14 am



Quoting Blackbird (Reply 56):

Actually, I thought planes like the F-22 and YF-23 had no alpha limits (at least the F-23 from what I was told)?

Just speculating but aren't the control systems built so they will just "fix the problem" automagically? Kinda like an Airbus doesn't let you stall, except that you can do aerobatics.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
rwessel
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 4:36 am



Quoting Blackbird (Reply 56):
Actually, I thought planes like the F-22 and YF-23 had no alpha limits (at least the F-23 from what I was told)?

Not quite sure what you mean, but the situation I was describing happens at a (relatively) low alpha. IOW, get your airplane to 500kts indicated, and haul back on the stick, and the wings will come off long before the AoA reaches the typical 14 degree stalling point. Artificial mechanisms preventing excessive G loads excepted, of course.

Quoting Blackbird (Reply 56):
Additionally, it's probably possible to have a situation in which you're going so fast (above corner velocity) in which you wouldn't have enough control power to get the alpha high enough to get a stall (up to a point the faster you go the more responsive the plane gets, above a point though the plane becomes less and less responsive as the higher speed makes it harder for the plane to change direction.

That's exactly backwards. Changes in alpha require rotating the aircraft around its CG. That's done with the horizontal stabilizer and elevator. The faster you're going, the more force those can produce, and the more rotation they can create around the CG. It's at low speeds that you can run out of elevator authority (that, for example, is where the forward CG limit comes from - too far forward, and you can no longer hold the nose up at landing speeds).

Changing direction is *not* the same as pitching. The rate at which you can change direction is dependent on how much lift you can produce (which depends on alpha, indicated airspeed and the structural capabilities of the aircraft), and your true velocity. The time needed to turn is linearly dependent on the first, and inversely dependent on the second. In terms of space, it’s related to the square and inverse square of the two parameters.

Pitching requires none of that and can be very, very fast assuming sufficient elevator authority, which is primarily dependent on indicated airspeed. At high speeds some mach effects (like the aft movement of the center of lift and the general blanking of the elevator by the shockwave from the structure in front of it) will reduce the effectiveness of the elevator. Obviously stalling the horizontal stabilizer and elevator will also (drastically) reduce its effectiveness.

At high speeds most aircraft can aerodynamically pitch well into the stall regime in a very, very short time. Likewise into the wings-come-off regime. That's why most aircraft (with conventional control systems) are flown much more gingerly at high speeds. That near full stick deflection that you can (and often do) use at low speeds is fatal at high speeds. That's the structural limit I was mentioning. At low speeds you may not be able to do a 4G turn period, because your wings will stall. At high speed you might be able to pull 4G with only a 3 degree alpha (obviously well below the stall). That high speed 4G turn will require only a very small elevator deflection (although that might** require a substantial amount of force - but not movement - on the stick).


*Assuming acceleration in the plane of the turn - the actual G load will be higher in a typical level turn since you also have to hold the aircraft up.

**Assuming typically balanced flight controls.
 
SlamClick
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 4:37 am



Quoting Blackbird (Reply 56):
Actually, I thought planes like the F-22 and YF-23 had no alpha limits

I think what they really do is kind of blur the definitions a bit. When thrust exceeds weight and control can be maintained independant of airflow the word "stall" does not necessarily have much meaning anymore.

I saw one of Wayne Handley's last (complete) shows with his Turbo Raven. He would pull it straight up just after takeoff, and go straight up (zero angle of attack) but he would then reduce power and back right straight down. (180 degree AOA Smile) He could get aileron control with the propwash alone and he could add power and start climbing right back up again. For all that, when the prop pitch stuck he was unable to avoid striking the ground very hard.

So take a plane like you cite and from level flight do a maneuver like Pougachev's Cobra and you might, indeed have a very high AOA but only until the forward (horizontal, rather) motion stops. AOA is not what we've been conditioned to think it is in such a case. It is not carrying the airplane, vectored thrust is.
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tdscanuck
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 6:02 am



Quoting PhilSquares (Reply 46):
However, I can assure you the 744 stalls just like any other swept wing aircraft, from the wing tip inwards.

You're going to have to help me understand why you believe this is inherent in swept wing design, rather than particular aircraft. For *any* wing you can design an airfoil/washout setup that will prevent this.

I kknow you already said:

Quoting PhilSquares (Reply 21):
The sweep creates an induced upwash angle of the relative wind at the leading edge that increases from wing root to tip. This means that the wingtip is flying at a higher angle of attack than the root. Thus, it will reach its critical angle of attack first.

The forst part is true (lower angle of relative wind at the root). However, the angle of attack is between the local wind and the chord line. If you have washout, you can hold any arbitrary angle of attack distribution you want.

Quoting JetMech (Reply 47):
In this case then, what is the general purpose of washout in relation to swept wings? What does it do specifically on Boeing and Airbus commercial types?

The other major reason to do it is to shift the load inboard...this reduces the strength you have to have in the wing and lightens the overall structure.

Tom.
 
PhilSquares
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 7:12 am



Quoting Blackbird (Reply 54):
BTW: Isn't even a low-speed stall caused by the critical AoA being exceeded as the AoA necessary to maintain level flight exceeds that to stall the plane

All stalls are caused by exceeding the Critical AoA, which is always constant no matter what the speed. However, most pilots associate a stall with a low speed/low energy condition. I can assure you, I used to be able to get into my F-4 and have the aircraft fully loaded and be doing 400 KIAS and get into an accelerated stall with no problem. In each condition, both high speed and low, the critical AoA has been exceeded.

Quoting Tdscanuck (Reply 60):
You're going to have to help me understand why you believe this is inherent in swept wing design, rather than particular aircraft. For *any* wing you can design an airfoil/washout setup that will prevent this.

This is a pretty good explanation www.erau.edu/er/newsmedia/articles/wp3.html
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ALexeu
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 9:46 am

Is rudder trim only used in case of engine failure?
 
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Jetlagged
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 12:46 pm



Quoting PhilSquares (Reply 61):
This is a pretty good explanation www.erau.edu/er/newsmedia/articles/w....html

That article makes some rather generalised comments, like swept wing aircraft don't create natural stall buffet, which as you know is certainly not always true. As Tdscanuck says, the increased AOA at the tip can be compensated for somewhat by washout. Also the aerofoil can be adjusted to reduce critical AOA at the tip.

LE devices are often less effective designs inboard (Kruegers or droops rather than slats) to ensure stall occurs at the root first with LE devices deployed.

Quoting AlexEU (Reply 62):
Is rudder trim only used in case of engine failure?

For jet aircraft that is generally true. Propeller driven aircraft need rudder trim to compensate for propeller slipstream effects, which vary with power.
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PhilSquares
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 1:42 pm



Quoting Jetlagged (Reply 63):
That article makes some rather generalised comments, like swept wing aircraft don't create natural stall buffet, which as you know is certainly not always true. As Tdscanuck says, the increased AOA at the tip can be compensated for somewhat by washout. Also the aerofoil can be adjusted to reduce critical AOA at the tip.

Sorry, but I was only trying to provide a simple non-technical reference. If you want a more technical explaniation try here. http://ntrs.nasa.gov/archive/nasa/ca...asa.gov/19640014908_1964014908.pdf

As I tried to point out, there is a balance on structure weight/cost and an economic aircraft. In reality, transport aircraft, such as the 744, don't operate anywhere near the speeds or AoA Crit we're talking about.

Remember, I'm just the messenger. But, I do think it's funny that all these people who have never flown a swept wing aircraft let alone a 744, seem to think the wing stalls just like a straight wing aircraft.

But then again, what do I know.
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Jetlagged
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RE: Silly Question Regarding Rudder Deflections

Thu Aug 28, 2008 10:19 pm



Quoting PhilSquares (Reply 64):
Sorry, but I was only trying to provide a simple non-technical reference. If you want a more technical explaniation try here. http://ntrs.nasa.gov/archive/nasa/ca...asa.gov/19640014908_1964014908.pdf

As I tried to point out, there is a balance on structure weight/cost and an economic aircraft. In reality, transport aircraft, such as the 744, don't operate anywhere near the speeds or AoA Crit we're talking about.

Remember, I'm just the messenger. But, I do think it's funny that all these people who have never flown a swept wing aircraft let alone a 744, seem to think the wing stalls just like a straight wing aircraft.

But then again, what do I know.

Phil,

I don't need a more technical explanation, but others might not know the difference and be misinformed.

I'm certainly not trying to tell you how the 744 flies, but as an aero engineer I do have a technical understanding and as a simulator designer I have access to information about aircraft handling characteristics most non-pilots don't have. I often meet and discuss such issues with pilots on an equal techincal level. If that's a problem for you then I'm sorry. It's not just pilots who understand these things.

I only posted because I thought Tom a valid point about the ability to compensate for the ill effects of swept wings. Of course, all things being equal, a straight wing will have "friendlier" characteristics.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
tdscanuck
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RE: Silly Question Regarding Rudder Deflections

Fri Aug 29, 2008 12:59 am



Quoting PhilSquares (Reply 61):
This is a pretty good explanation www.erau.edu/er/newsmedia/articles/w....html

Actually, beyond the generalizations that Jetlagged pointed out, there are some flat out inaccuracies. The only part of the article that addresses swept wing stalls is what you posted earlier, and it doesn't do anything to explain why it has to be that way because it ignores washout. It also makes some, at best, dodgy explanations regarding upwash.

Quoting PhilSquares (Reply 64):
If you want a more technical explaniation try here. http://ntrs.nasa.gov/archive/nasa/ca...8.pdf

This one is much more thorough, but it doesn't seem to support your contention...there are several examples in this paper of swept wings that don't stall at the tip. There certainly are *some* configurations that do (I don't think anybody disagrees with that in this thread) but there are many configurations that don't. Although I can't prove it at the moment, I strongly suspect that most current commercial airliners are of the variety that don't.

Tom.
 
PhilSquares
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RE: Silly Question Regarding Rudder Deflections

Fri Aug 29, 2008 5:11 am



Quoting Tdscanuck (Reply 66):
I strongly suspect that most current commercial airliners are of the variety that don't

The only modern airliner I haven't flown is the 777 and I know for a fact, that wing has the same basic stall characteristics as any other swept wing aircraft. So, the A-3xx family, the 744, the 747, the 757 all exhibit the same basic stall characteristics as any other swept wing aircraft. I think your suspicions might be wrong.
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