Physics of Rudder with regard to lifting a dropped wing.

[SixPapaCharlie]

"Use the rudder to raise the wing"

I have heard this during falling leaf stalls. And in reference to preventing spins, and even heard a reporter mention it this morning in reference to the Trans Asia crash.

So I understand it works and have demonstrated it but I don't understand the "why" or "how" of it.

Lets say I am slow-ish and my left wing is dropping. I kick right rudder which makes the plane yaw to the right. What is happening that this yaw brings the left wing up. In my mind it would make sense if it actually lowers the right wing as it would slow the right wing down but only briefly during the yaw.

I have flown with rudder only and actually in RC planes w/ no ailerons, the rudder banks the plane nearly as efficiently as the ailerons. but I don't understand the physics.

Can anyone sort of explain this without a lot of math?

Thanks.

 

[jsstevens]

"Use the rudder to raise the wing"

I have heard this during falling leaf stalls. And in reference to preventing spins, and even heard a reporter mention it this morning in reference to the Trans Asia crash.

So I understand it works and have demonstrated it but I don't understand the "why" or "how" of it.

Lets say I am slow-ish and my left wing is dropping. I kick right rudder which makes the plane yaw to the right. What is happening that this yaw brings the left wing up. In my mind it would make sense if it actually lowers the right wing as it would slow the right wing down but only briefly during the yaw.

I have flown with rudder only and actually in RC planes w/ no ailerons, the rudder banks the plane nearly as efficiently as the ailerons. but I don't understand the physics.

Can anyone sort of explain this without a lot of math?

Thanks.

The plane will yaw around it's center of mass (which is somewhere close to the center of gravity). Plus effects for aero drag. The effect is to both slow the right wing AND speed up the left wing.

John

 

[dtuuri]

Can anyone sort of explain this without a lot of math?

Thanks.

Sure, I hate math when it comes to explaining aerodynamics. It's a cop out.

One wing speeds up momentarily = momentary increase in lift.

The opposite wing slows down = momentary loss of lift.

The wing (not counting swept wing aircraft) has dihedral, so side slip increases AoA and lift on one side.

The other side decreases AoA and so does the lift.

Side slip causes interference from the fuselage which decreases lift on the lagging side.

You're welcome.

dtuuri

 

[FastEddieB]

One wing speeds up momentarily = momentary increase in lift.

The opposite wing slows down = momentary loss of lift.

dtuuri

And, taken to its logical extreme, you have a spin!

Kind of like a dog chasing its own tail.

 

[RalphInCA]

Interesting comment about RC planes that just use rudder for turning. I haven't thought abott that for a long time.

I guess those airplanes have a lot more dihedral than the full size airplanes do.

 

[nosehair]

And, taken to its logical extreme, you have a spin!

Kind of like a dog chasing its own tail.

And taken to it's other logical extreme, ie., making constant tiny tiny rudder pressure changes, tiny tiny momentary changes of lift on each wing by keeping a religiously constant heading with rudder only.

 

[Checkout_my_Six]

Of course what we are discussing here is an uncoordinated turn....or a skid that lifts a wing.

 

[RoscoeT]

I have flown with rudder only and actually in RC planes w/ no ailerons, the rudder banks the plane nearly as efficiently as the ailerons. but I don't understand the physics.

Stopping a wing drop with rudder during a stall and banking the airplane into a turn with rudder because of dihedral effect are two different things. Yawing a wing with dihedral presents the underside of the outside wing toward the relative wind such that it lifts (banks) in the direction of the rudder input. In this case, left rudder increases the AOA of the right wing.

Things reverse during a stall. Stall the airplane straight ahead, do a falling leaf, and wait for a right wing drop. You can pick up the dropping right wing with left rudder because during a stall, left rudder increases the AOA of the left wing. This causes the left wing to stall more deeply than the right wing. Since the right wing is now less deeply stalled, it is producing more lift than the left wing, and there is now a lift imbalance that produces roll to the left.

In unstalled flight, the airplane will roll in the direction of the wing with lower AOA. In stalled flight, the airplane will roll in the direction of the wing with higher AOA. This is why aileron effects reverse during a stall, and applying aileron opposite stalled yaw rotation will only further stall the down-going wing, contributing to, not stopping the rotation. This is why your CFI will smack you for attempting to use aileron to pick up a wing during a stall.

 

[ClimbnSink]

Physics fight!!!!!!!!!! P word always gets people here riled up.

 

[JeffDG]

There's another factor as well.

While the rudder primarily acts in the yaw axis, the force applied also creates a moment about the roll axis. It's considerably smaller, because the rudder displacement from the roll axis is much smaller than the distance from the rudder to the yaw axis.

 

[jsstevens]

There's another factor as well.

While the rudder primarily acts in the yaw axis, the force applied also creates a moment about the roll axis. It's considerably smaller, because the rudder displacement from the roll axis is much smaller than the distance from the rudder to the yaw axis.

But it's roll tendency would be the opposite of desired. Right rudder would tend to bank left (i.e. left wing down), for example. And you'd use right rudder to pick up the left wing.

John

 

[JeffDG]

But it's roll tendency would be the opposite of desired. Right rudder would tend to bank left (i.e. left wing down), for example. And you'd use right rudder to pick up the left wing.

John

Right rudder would create an unbalanced force above the roll axis on the right side. As a result, it would raise the right wing and lower the left.

 

[RoscoeT]

There's another factor as well.

While the rudder primarily acts in the yaw axis, the force applied also creates a moment about the roll axis. It's considerably smaller, because the rudder displacement from the roll axis is much smaller than the distance from the rudder to the yaw axis.

So you are saying right rudder applies enough twisting force to the fuselage to apply a noticeable rolling force to the left? Man I think that's a huge stretch. Curious how you've been able to notice rolling force opposite the direction of the rudder input. Does your airplane roll left when you apply right rudder? The vertical fin was not designed to impart such twisting forces to the fuselage, nor would any rolling force be powerful enough to notice, given the...wait for it...physics of it.

 

[JeffDG]

So you are saying right rudder applies enough twisting force to the fuselage to apply a noticeable rolling force to the left? Man I think that's a huge stretch. Curious how you've been able to notice rolling force opposite the direction of the rudder input. Does your airplane roll left when you apply right rudder? The vertical fin was not designed to impart such twisting forces to the fuselage.

That's a function of the arm length of the moment imparted, and you're right, it is much smaller.

I'm just thinking of theoretical forces at play, I'd have to run some numbers to see how much of a moment they would impart vs. an aileron deflection (which have much longer arms to the roll axis).

 

[dtuuri]

Yawing a wing with dihedral presents the underside of the outside wing toward the relative wind such that it lifts (banks) in the direction of the rudder input.
...

Things reverse during a stall. ... You can pick up the dropping right wing with left rudder because during a stall, left rudder increases the AOA of the left wing. This causes the left wing to stall more deeply than the right wing.

I can't buy that. The advancing wing, even if stalled, should get more lift not less because of the area presented to the relative wind and, perhaps, even start to "fly" again because of the effective sweep of the wing. Meanwhile the other side loses lift for the reasons I stated above. IMO.

dtuuri

 

[RoscoeT]

I can't buy that. The advancing wing, even if stalled, should get more lift not less because of the area presented to the relative wind and, perhaps, even start to "fly" again because of the effective sweep of the wing. Meanwhile the other side loses lift for the reasons I stated above. IMO.

You're not really saying anything counter to what I said. When I say the left wing's AOA increases with left rudder application during a stall, that IS a loss of "lift". Increasing AOA after the wing has stalled causes a deeper stall. The deeper a wing is stalled, the less lift it generates. Less lift on the left side will cause a roll to the left.

 

[dtuuri]

Increasing AOA after the wing has stalled causes a deeper stall. The deeper a wing is stalled, the less lift it generates. Less lift on the left side will cause a roll to the left.

In that case, I'd say you're in a left spin and not just picking up a dropped right wing that had stalled "first", so to speak.

dtuuri

 

[exncsurfer]

I accidentally demonstrated this the other day practicing power off stalls, I was erroneously watching the slip/skid indicator ball during the maneuver and when I tried to 'step on the ball' to center it, it dropped that wing quick! Stepped on left rudder, dropped left wing. Oops. So I was told, don't look at the ball, look outside, use rudder to keep level.

 

[jordane93]

So I was told, don't look at the ball, look outside, use rudder to keep level.

Yes. Look outside. You can get just as much if not more information by looking outside.

 

[RoscoeT]

In that case, I'd say you're in a left spin and not just picking up a dropped right wing that had stalled "first", so to speak.

Doesn't matter - the dynamics are the same. If the right wing stalls first and drops, left rudder will still increase the AOA of the left wing, and reduce it on the right, negating the lift imbalance that caused the right wing drop in the first place. Of course if you KEEP holding left rudder after correcting that right wing drop, the forces that recovered the initial wing drop will cause a left wing drop/spin entry.

 

[dtuuri]

If the right wing stalls first and drops, left rudder will still increase the AOA of the left wing,

I beg to differ. Dihedral causes the right wing's AoA to increase, stalled or not, and vice versa for the left. That is exactly the purpose for dihedral--to create lateral stability. You can fold a paper airplane to see it easily. (Now, a swept wing with negative dihedral is another thing altogether. )

dtuuri

 

[brian]]

Of course what we are discussing here is an uncoordinated turn....or a skid that lifts a wing.

Well, less so if you have rudder / aileron springs

 

[Blueangel]

But spins are fun!

 

[RoscoeT]

I beg to differ. Dihedral causes the right wing's AoA to increase, stalled or not, and vice versa for the left. That is exactly the purpose for dihedral--to create lateral stability.

I'm losing you. Dihedral effects and rudder dynamics while stalled are two different things. Not sure what you're trying to say. It kinda sounds like you're trying to say that even stalled, left rudder and dihedral will cause the right wing's AOA to increase. If the right wing is already stalled and you increase AOA on the right side, what do you think happens? Hint: you roll to the right. Have you ever done a spin? If so, you might be aware that you cannot produce a spin to the right with left rudder.

I think you're not wrapping your head around the difference between what results from increasing AOA on stalled wings vs. unstalled wings. Aileron effects are reversed between stalled and unstalled wings for the same reasons I've been pointing out. Ailerons and rudder both have the ability to produce AOA differences between the two wings.

Now, a swept wing with negative dihedral is another thing altogether.

BTW, "negative dihedral" is called anhedral.

 

[Checkout_my_Six]

Well, less so if you have rudder / aileron springs

ya but....everyone isn't as special as you.

 

[dtuuri]

It kinda sounds like you're trying to say that even stalled, left rudder and dihedral will cause the right wing's AOA to increase.

Yes.

If the right wing is already stalled and you increase AOA on the right side, what do you think happens? Hint: you roll to the right.

Not so. The wing has more surface area side slipping toward the relative wind, creating more lift.

Have you ever done a spin? If so, you might be aware that you cannot produce a spin to the right with left rudder.

Oh yeah, I've done tons of spins. And you're right about that, which sort of blows your theory right out of the water. If "left rudder" in your scenario causes the right wing to stall more, it ought to be possible to spin to the right. But it can't.

I think you're not wrapping your head around the difference between what results from increasing AOA on stalled wings vs. unstalled wings.

You're entitled to your opinion. FWIW, I think you're confused between negative dihedral effects and positive dihedral effects. I'm discussing postive dihedral, since it's found on non-swept wing airplanes.

BTW, "negative dihedral" is called anhedral.

Touche.

dtuuri

 

[RoscoeT]

Oh yeah, I've done tons of spins. And you're right about that, which sort of blows your theory right out of the water. If "left rudder" in your scenario causes the right wing to stall more, it ought to be possible to spin to the right. But it can't.

None of your responses make any sense, so I'll only quote the one here. Not sure what this theory of mine is you're "blowing out of the water". I never said left rudder causes the right wing to "stall more". YOU said that during a stall, left rudder would cause dihedral effect to produce more lift in the right wing. The only way to get a stalled wing to produce more lift is to reduce AOA. Dihedral effect would produce the opposite...but dihedral effect applies to unstalled wings.

Study and understand this diagram. The more AOA increases beyond the stall (critical AOA), the more lift diminishes. In a spin, rotation occurs because one wing is stalled further (has more AOA) than the other. It doesn't seem you understand what causes an airplane to spin. Not sure what all this dihedral nonsense has to do with spins.

 

[dtuuri]

None of your responses make any sense,

Maybe somebody else can do a better job, I give up.

Study and understand this diagram. The more AOA increases beyond the stall (critical AOA), the more lift diminishes.

That diagram only applies to the wing before sideslip changes its characteristics. Afterwards, the leading wing has more AoA, more cross-sectional surface area exposed to the relative wind and more induced drag than before, while the lagging wing has less of all three (whether they're stalled or not). The net lift advantage goes to the wing ahead.

dtuuri

 

[RoscoeT]

That diagram only applies to the wing before sideslip changes its characteristics. Afterwards, the leading wing has more AoA, more cross-sectional surface area exposed to the relative wind and more induced drag than before, while the lagging wing has less of all three (whether they're stalled or not). The net lift advantage goes to the wing ahead.

That diagram shows the relationship between lift and AOA. There are no exceptions. You are saying that in a spin, the leading (outside) wing has more AOA than the "lagging" (inside) wing. Incorrect.

In a spin, the inside wing has higher AOA. You are still operating off flawed fundamentals. The diagram below shows the AOA of the two wings during a spin. I give up trying to convince you, maybe others can:

http://www.copanational.org/pilotsprimerdec09.cfm

 

[dtuuri]

That diagram shows the relationship between lift and AOA. There are no exceptions.

Never say "never".

You are saying that in a spin, the leading (outside) wing has more AOA than the "lagging" (inside) wing.

I didn't say that. In a sideslip (one that occurs to prevent a spin) the leading (outside) wing has more AoA than the "lagging" (inside) wing.

In a spin, the inside wing has higher AOA.

Of course, that's why it spins--it's stalled. The outside wing isn't. Around you go.

You are still operating off flawed fundamentals. The diagram below shows the AOA of the two wings during a spin. I give up trying to convince you, maybe others can: <snip>

I don't care if you confuse dihedral with anhedral or slips with spins as long as you can recover when the nose drops. :wink2:

dtuuri

 

[RoscoeT]

I didn't say that.

Dude...I'm quoting you here -

...the leading wing has more AoA, more cross-sectional surface area exposed to the relative wind and more induced drag than before, while the lagging wing has less of all three (whether they're stalled or not)

Of course, that's why it spins--it's stalled. The outside wing isn't.

Outside wing IS stalled. I'd refer you back to the article and diagram I posted, but I'm going to cut my losses and not waste more time.

 

[txflyer]

It's real simple.

You're just moving the trailing wing forward which creates more lift on it.

 

[dtuuri]

Dude...I'm quoting you here -



Outside wing IS stalled. I'd refer you back to the article and diagram I posted, but I'm going to cut my losses and not waste more time.

You apply what I say to circumstances other than that of which I spoke. If you want to pick a specific point in the genesis of a spin I'll be happy to tell you what my opinion is.

What happens to AoA prior to a stall or right after due to the effect of dihedral is one thing and different from the damping effects and rotational effects encountered in a spin. It is conceivable to me that in a fully developed spin the outside wing may be at least partially stalled, but it's a moot point if it is.

What's happening is the inside wing is at a higher angle of attack because of the rolling motion coupled with the yaw. This induces a higher AoA on the inside wing, yet reduces the AoA on the outside wing--so who cares if it's stalled too or not? The imbalance spins you like crazy toward the earth, and the recovery is the same anyhow.

dtuuri

 

[dtuuri]

The good rudder lessons start at about 5:15 but the whole video is worth watching.
https://www.youtube.com/watch?v=xwrfEsCiltc

I watched the section you mention and see a traditional explanation for why one wing stalls first, namely because the lowered aileron increases the angle of attack on the "lagging" wing in sideslip.

HOWEVER, true that it all is, the same wing will stall first even if you neutralize the ailerons just prior to the break! At that moment (one that takes careful practice to achieve, btw), dihedral continues to render the main part of the wing that had the "up" aileron a moment before to remain at a higher AoA than the wing that had the "down". Yet the higher AoA obviously does not cause it to stall first; the result is the same as when the ailerons were deflected--when the downward-deflected one took all the blame.

So, something other than AoA and downward deflected aileron is at play. Don't take my word for it though, you can find out for yourself: Put your plane in a stable, slipping bank in one direction, then just before it "breaks" neutralize the ailerons. The bank will immediately begin to shallow out, but if you've timed it right it will stall before reaching "wings level", and rapidly roll over the top just the same as if ailerons had been deflected--when the downward deflected one took the rap as in the video. But in this case it has an alibi--it was held neutral at the break. Something else is clearly going on then. One explanation would be the rolling effect which raises the AoA on one wing while decreasing it on the other. But then, why wouldn't the roll stop? My pet theory is that the sideslip causes airflow disturbances that cause one wing to stall first, since it's always the wing following the fuselage through the air sideways that goes first at the stall, ailerons or not. YMMV.

dtuuri

 

[coloradobluesky]

When the airplane stalls, it starts falling. When you push on the left rudder right at stall, that throws the empennage up and to the right, so the nose is pushed down and to the left. The airplane is falling so it falls where its nose is because that is the way its weighted. And it spins, because of some REALLY COMPLICATED dynamics, that I don't understand. It IS an airplane and has wings that have aerodynamics even stalled and spinning, this is true by virtue that it spins and doesn't just fall like a rock or tumble randomly down. Does anyone really understand it? I don't know. The wings are each doing different things, there are some uneven forces being evened out by the spinning.

Take a Dynamics class in and engineering program and you will encounter simpler versions of bodies in motion. And believe me, even the simple ones can get really hard to understand. Conquer the math of a spinning top before you try and really understand something complicated as a spin....But spin they do.

 

[RoscoeT]

HOWEVER, true that it all is, the same wing will stall first even if you neutralize the ailerons just prior to the break! At that moment (one that takes careful practice to achieve, btw), dihedral continues to render the main part of the wing that had the "up" aileron a moment before to remain at a higher AoA than the wing that had the "down". Yet the higher AoA obviously does not cause it to stall first; the result is the same as when the ailerons were deflected--when the downward-deflected one took all the blame.

Well I guess I have a few minutes to waste here....

You're still hung up on dihedral. Did you know that airplanes with zero dihedral will stall, spin, and snap roll the same as airplanes with dihedral? Forget your dihedral nonsense. Just because a downward deflected aileron can exacerbate a stall and roll doesn't mean anyone's trying to claim the aileron deflection is the sole reason why one wing stalls more or before the other.

In your example of removing slipped aileron input so that the airplane stalls just at the moment the ailerons are neutralized is just you turning a slip into a skid, with the expected results.

I guess you never read this article. No need for all your homegrown pet theories. None of what we're talking about here is uncharted territory for aerodynamicists.

http://www.copanational.org/pilotsprimerdec09.cfm

 

[dtuuri]

You're still hung up on dihedral. Did you know that airplanes with zero dihedral will stall, spin, and snap roll the same as airplanes with dihedral? Forget your dihedral nonsense.

Dihedral has everything to do with AoA in sideslip and you seem to have it all backwards.

Just because a downward deflected aileron can exacerbate a stall and roll doesn't mean anyone's trying to claim the aileron deflection is the sole reason why one wing stalls more or before the other.

That's the message in the video.

In your example of removing slipped aileron input so that the airplane stalls just at the moment the ailerons are neutralized is just you turning a slip into a skid, with the expected results.

Nope. If it was a slip it will remain a slip and the top wing will still stall first despite its having a lower AoA than the other wing.

I guess you never read this article.

Actually, I did waste my time reading it. Nothing new there and it doesn't contradict anything I've stated or believe.

No need for all your homegrown pet theories. None of what we're talking about here is uncharted territory for aerodynamicists.

Are you an aerodynamicist?

dtuuri

 

[RoscoeT]

If it was a slip it will remain a slip and the top wing will still stall first despite its having a lower AoA than the other wing.

Uncle uncle! I cannot follow your circular incoherence. Now it sounds like you're saying stall is unrelated to AOA? I don't know...post after post I cannot identify any clear points because your writing style is inarticulate to the point of befuddlement. I do like aerodynamics discussions, though - just not this one. File this one away...I think everyone has long been groaning at this one.

 

[dtuuri]

Uncle uncle! I cannot follow your circular incoherence. Now it sounds like you're saying stall is unrelated to AOA? I don't know...post after post I cannot identify any clear points because your writing style is inarticulate to the point of befuddlement. I do like aerodynamics discussions, though - just not this one. File this one away...I think everyone has long been groaning at this one.

I get it. You're NOT an aerodynamicist. Are you a pilot?

dtuuri

 

[BillTIZ]

Instructing in gliders, the aileron has much more adverse yaw effect. A wing drops during a stall and use the aileron and the down dropped aileron creates more drag, adverse yaw, plus further stalling the wing or delaying the wing returning to a "creating lift" situation.

Use of aileron fights the effect of the rudder to pick up that low wing. Turning or cross controlled Stall recovery, same as spin recovery. Stick forward and centered, opposite rudder to pick up the low wing.