Help please with basic rudder aeronautics

[aussiepax]

Could the experts please explain to me the necessity of the rudder in regard to staying airborne ? I appreciate the wings / ailerons, and the elevators in regard to maintaining roll and pitch respectively. The rudder - I can see its use in adjusting yaw, coordinated turns and crosswind landings - but why did the snapped rudder on the AA 587 (and the loss of the vertical fin) ensure it would crash ? I presume the part of the tail with the elevators was still attached / working .
Thank you.

[SNS3Guppy]

Oz,

That's a valid question. The answer isn't so much in what the rudder does, but what it doesn't do. Specifically, it doesn't withstand loads for which it wasn't designed.

In airplanes, we do nearly everything based on speeds. We lower flaps at certain speeds, raise them at certain speeds, bring the nose off the ground at certain speeds, penetrate turbulence at certain speeds, land at certain speeds. It's all about the airspeed.

There's a speed known as "maneuvering speed," at which pilots are taught from their early days of flying, that a particular control can be given a full deflection without harming the airplane. That is, if one gives full left aileron, for example one should stall the control surface or the flying surface before one can build up enough force of loading to damage the airplane. If one is at or below that speed, the theory goes, one can't really hurt the airplane. The problem is, this is an extreme oversimplification, and therefore a fallacy when applied to many operating conditions.

The crew of flight 587 were taught the same as every other crew had been, and were not aware that the certification standards for making a full control deflection were for ONE control deflection. What they did was use the rudder to counter some wake turbulence through which they flew...something that's commonly done in a light airplane, but not so much in a big airplane. Specifically, they performed a control reversal...full rudder one way, then full rudder the other way.

The easiest way to imagine this is getting punched in the stomach. Get punched in the stomach, you double over. That's the first control deflection. However, if while you're doubling over, someone were to punch you in the face, you get hit not only with the force of someone's fist in the face, but he force of you bending into it...a double whammy, if you will.

Same for the airplane. Deflecting the rudder fully one way applies a bending and torsional force on the vertical stabilizer; the stresses it's experiencing are high, and are in effect, bending one way. Suddenly reversing it not only applies forces in the opposite direction, but increases them. Simply put, the airplane was designed for one full rudder deflection in one direction, but not a second one in the opposite direction. The truth is, it is possible to break an airplane at lower speeds than many previously believed.

Since that time, there has gone on considerable informing of the industry, educating, and passing along of information that should have been made clear before. This doesn't help the crew and passengers of Flight 587, of course. An additional wrinkle in the issue is that Airbus aircraft are supposed to be written with "laws" in their computer logic that prevent this sort of thing. In that case, it wasnt' applicable, and shouldn't have been a problem...it was an unforseen combination of control inputs that broke the airplane in flight.

I used to fly a large four engine WWII airplane on wildland fires. We threw it around the sky like a supercub (little single engine general aviation airplane). That included constant full deflection control inputs, full slips down canyons, etc. That airplane has a very tall, big vertical stabilizer. That means very large loads are put on the vertical stabilizer. What we began to find during deep maintenance checks was that the attach brackets and bolts for the vertical stabilizer (the tail) were cracking or breaking. I quit doing full slips in the airplane.

I'm sure the big question in your mind is how this applies to you. You shouldn't be worried. For a number of reasons, there's no reason this should ever happen again. I've flown a couple of airplanes which broke up in flight, losing their wings...not when I was flying them, obviously. In one of those airplanes, a wing did break when I was flying it. Those were flown on fires, and were subject to considerable stresses that airline aircraft are not. They were also much older than any airline aircraft, with a lot more exposure to a violent environment. You won't see this sort of damage, stress, or wear in an airliner.

Last week I encountered severe turbulence during a flight over the ocean. When we arrived at our destination, we were greeted in the cockpit by mechanics who took our report, and immediately went to work taking care of any necessary inspections. This takes place after each landing in an airliner. The aircraft tend to be newer; new design materials, newer engineering, newer powerplants...and most importantly...what's designed today is often based on learning made from errors of the past. Aviation has come a long way. The Airbus crash was a tragedy, as is every loss. It's important to understand that it wasn't a design flaw, but an operational error...not one remotely likely to be repeated again.

I've been flying airplanes, large and small, into conditions of severe to extreme turbulence for many years...conditions that are very, very rarely, if ever encountered in airline flying...without damaging an airplane so far. I've flown aircraft intentionally during research into thunderstorms that broke headsets and even my personal computer on board in turbulence, but through which the airplane flew without incident (thunderstorms are dangerous places, and I don't advocate ever going in one, incidentally...and airline operations avoid them like the plague).

I suppose what I'm really saying is that you have a very valid question, but no reason to worry.

[barit1]

Actually if AA587's rudder simply failed - parted company from the fin - the airplane would have survived, as the Air Transat (http://en.wikipedia.org/wiki/Air_Transat_Flight_236) A310 demonstrated a few years later. If there were no crosswind on landing, and no assymetric thrust, it would be quite easy to handle.

But the fact that the rudder cyclic loads, combined with the plane's yaw inertia, caused the fin to fail, and AA587 lost directional stability.

[aussiepax]

Thanks for that. I appreciate the range of movements in that AA accident which caused the rudder to fail. The crux of my question is mostly as to why loss of the tail fin was so bad for directional stability that couldn't be alleviated by the remaining control surfaces. Is it that at that velocity, the aircraft is put in a position that the other surfaces stall? or does it lead to a roll over or some other catastrophic yaw position from which there is no maneovure possible to rectify.

Thanks again.

[aussiepax]

The air Transat hyperlink should I think be this one Air Transat Flight 961 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Air_Transat_Flight_961) as the one previously quoted leads to the same airline's fuel starvation incident.

[SNS3Guppy]

Oz,

A conventionally configured airplane without a vertical stab lacks directional stability and authority. Particularly in a swept wing airplane, lacking a rudder leaves the airplane open to spiral and dutch rolling instability. This can quickly compound (particularly in the case of an airplane that was engaging full rudder deflections to counteract a wake turbulence encounter).

While sideloads on the fuselage contribute somewhat to directional stability, without a significantly large enough surface with which the airplane can "weathervane" (align itself with the slipstream) it quickly becomes directionally unstable. Depending on the loading of the airplane, this divergence in some cases could easily roll the airplane over in one or two osciallations, or it could continue for a time before control is lost...or it might theoretically be possible under the right calm conditions to actually return and land.

The results of the loss, however, were what made the papers, and are self-evident.

[Groundloop]

An aircraft will rotate in all three axes around its centre of gravity. The horizontal stabiliser/elevators provide balancing forces in pitch, the ailerons/spoilers in roll and the fin/rudder in yaw.

As the fuselage of an aircraft is not equal in length each side of the centre of gravity any forces on the side of the fuselage (x-wind, etc) will cause the fuselage to yaw. The vertical fin provides a large surface to resist this yaw and the rudder provides final control to control the yaw.

However the rudder/fin probably has one of its most important function in the event of an engine failure. When an engine fails on one side there is an immediate imbalance of forces on each side of the aircraft. This will cause the aircraft to yaw (often quite violently). The fin and rudder are used to counteract these imbalances. In this situation you could not attempt to counteract yaw with roll via the ailerons as you could not roll the wings if the aircraft is still on the runway or just airborne.

[aussiepax]

thanks 'guppy, and groundloop, that's the info I was after. Many thanks fo your help.

[chornedsnorkack]

But there are planes which do not have vertical surfaces at all. The classic Etrich Taube (which has tail with tailplane and elevator, but neither fin nor rudder - turns are made solely by wingtip warping/aileron), Northrop B-2 (no tail at all!)...

How will B-2 handle asymmetric thrust of engines out?

[Groundloop]

How will B-2 handle asymmetric thrust of engines out?

B-2s engines are very close to the fuselage centreline. Therefore the imbalanced forces with one engine out will produce a turning force but significantly lower than for an aircraft with engines much further out. Turning force is a product of thrust X moment arm and on the B-2 the moment arm is quite short. Therefore the B-2 fly-by-wire system will have some very clever combinations of spoiler/elevon movement to handle this.

To look at fin/rudder size with engine position compare the size of fin and rudder on a BAC 111 with fuselage mounted engines and a 737 with wing mounted engines. Fin and rudder on the 111 is tiny compared with 737 because moment arm of engines is very small.

[barit1]

A flying wing sans vertical surfaces uses "spoilerons" for yaw control, but needs artificial (closed-loop) stability augmentation. It has no natural yaw stability.

Note: the Northrop B-35 (http://www.military.cz/usa/air/war/bomber/b35/b35_en.htm) (prop powered) had enough lateral area in its pusher props to provide yaw stability. When the props were replaced w/ jets on the XB-49, it was necessary to add small vertical fins.

The air Transat hyperlink should I think be this one Air Transat Flight 961 - Wikipedia, the free encyclopedia as the one previously quoted leads to the same airline's fuel starvation incident.

Roger that. I copied the link before checking. :O

[forget]

But the fact that the rudder cyclic loads, combined with the plane's yaw inertia, caused the fin to fail, and AA587 lost directional stability.

Surely what really brought the aircraft down (before directional stability played any part) was the CG upset. What's a fin and rudder weigh?

[SNS3Guppy]

But there are planes which do not have vertical surfaces at all. The classic Etrich Taube (which has tail with tailplane and elevator, but neither fin nor rudder - turns are made solely by wingtip warping/aileron), Northrop B-2 (no tail at all!)...



None of which have any relation nor bearing upon a swept wing jet transport category airplane, as pertains to the question at hand. Also the reason the answer was given "conventionally configured airplanes."

Issues regarding engine-out operations are likewise irrelevant, as flight 587 didn't have an engine problem. It had a missing major flying surface problem.

Surely what really brought the aircraft down (before directional stability played any part) was the CG upset.


Surely not. Center of gravity was the least of their worries, as they lost directional control due to a missing vertical stabilizer.

[barit1]

If anything, the loss of the vertical tail moved the CG fwd, which should have made it MORE stable - but not enough to overcome the loss of the aero stabilization of the tail.

[forget]

What I was trying to say is that the aircraft will have pitched nose down - beyond control - before it (may have) yawed - also beyond control. End result's the same.

[SNS3Guppy]

What I was trying to say is that the aircraft will have pitched nose down - beyond control - before it (may have) yawed - also beyond control.


That didn't happen. The CG wasn't the issue.

[drivez]

AA587 lost it's vertical stabalizer due to the inaddequate training of the flight crew as prior stated by guppy, but the fight crew didn't have to worry about CG problems the aircraft just went into a spin due to loss of the vrtical stabalizer.

[chornedsnorkack]

None of which have any relation nor bearing upon a swept wing jet transport category airplane, as pertains to the question at hand. Also the reason the answer was given "conventionally configured airplanes."

All planes, conventionally or unconventionally configured, have the same freedoms of movement. 3 translational and 3 rotational ones, incl. yaw. And stabilities for all six freedoms.

What are fin and rudder doing in a conventionally configured plane? It should be instructive to see how unconventionally configured planes do without.

Jet or propeller should be completely irrelevant to the stabilities of the plane (those should be the same for unpowered glider). B-2 may have artificial stability augmentation and spoilers, but Taube has wing warping (close to aileron in effects) and nothing else. Is Taube stable in yaw?

[SNS3Guppy]

Jet or propeller should be completely irrelevant to the stabilities of the plane (those should be the same for unpowered glider).


Hardly. Considerable influence is brought to bear by the use of a propeller and center line thrust vs. underslung turbojet engines, for example. Spiraling slipstream, the influence of the propeller on the vertical and horizontal stabilizer, location of the powerplant, or even the use of a propeller moving airflow over a wing make large differences in how each may be considered and compared.

These are quite relevant to the powered stability of the airplane.

More on point in a discussion including the Taube is that it did have vertical stabilizing surfaces. The issue in this thread isn't the loss of a rudder, or event the presence of a rudder, but the complete loss of a vertical stabilizer in an aircraft that required it. The Taube was introduced as an aircraft that didn't have a vertical stab...but it did...with surfaces rising above and below the fuselage terminus, or empennage proper.

The aerodynamic need for, and requirements of the vertical stabilizing surfaces on the Taube, however, are very different from that on the Airbus, as are it's basic stability characteristics. Even more on point, the question concerns the loss of the vertical stab on an aircraft that needed it, and was designed with a large vertical stabilizer.

but Taube has wing warping (close to aileron in effects)


Not really close to ailerons, especially in effects, other than both have some degee or relative influence regarding movement about the longitudinal axis.

Regardless, lacking a swept wing, a comparison between the use of the rudder and vertical stabilizer on the Airbus, and the use of warped wings and elevators on the Taube, is non-sequitor and meaningless.