Aerodynamics Question

[avbug]

Are you a civillian test pilot, then?

Yes, I have used rudder to counter dutch roll, along with aileron in the Learjet. Would you prefer that one simply lean in one direction, instead?

Dutch roll in the Learjet can occur with, or without the yaw damper engaged. Especially in the older 20 series. It is made worse by pilots trying to correct for it. Usually a series of worsening oscillations, and with only a few repitions I have seen pilots roll the airplane right over. I take it you haven't.

Rather than fight it, small inputs on the rudder can stop it, as can slightly crossing the controls, or in some cases, freezing them and letting the osciallations stop. Sometimes, it's easy to stop it simpy with a little aileron input.

Forget the Z axis. Concentrate on the longitudinal axis of the airplane. During a constant bank turn, where does the rolling action come in again? I missed that. Even though you're a test pilot with a z axis kind of understanding and an aero forte, explain to me once more, in kiddie terms, how you get a roll out of a constant banked turn...or any motion whatsoever about the longitudinal axis of the airplane.

 

[172driver]

Now, gravity acting on the CG of the aircraft, and the remaining centrifugal force combine to pull the aircraft outward and down

I will buy this explanation provided we call the force inertia rather than centrifugal force. There is, after all, no such thing as CF.

 

[avbug]

That's centripital, actually. But that's purely semantics; the result is the same. Centrifugal is a common-use explaination of a force vector, where the more correct centripital is the vector in opposition, and in truth exists only in theory.

 

[skiddriver]

Some interesting comments here, not all concerning aerodynamics....

A few notes from an out of practice mil test pilot-

Depending on aircraft characteristics, it may be necessary to hold aileron into the turn or away from the turn to maintain a constant bank angle. This is the product of the individual airplane's spiral mode, and whether the pilot has established a balanced flight condition using rudders. Aircraft designers/manufacturers consider neutral to positive spiral mode desireable, so the aircraft won't keep trying to increase bank angle in a turn. To be a complete geek, the spiral mode is governed by dihedral effect, yaw rate damping, directional stability, and roll due to yaw rate - expressed in coupled pairs. If the product of the first two is greater than the product of the second, spiral mode is stable.

Some factors that effect which wing drops first in an accelerated (steady-state turning) stall are - in no particular order - spiral mode, prop slipstream (for prop airplanes - duh!), lateral CG postioning and CG shift in a turn (interconnected wing fuel tanks), pitch rate due to post-stall pitching characteristics and associated roll and yaw coupling with pitch rate (including gyroscopic coupling), sideslip angle (caused by the turn and rudder positioning), and aircraft rig (is it bent?).

Remember that aerodynamically, stall in level flight is just a special case of stall in a turn (bank angle = 0). As soon as you start the stall phenomenon, a lot of the same things are going on as far as dynamic coupling. Ever have a wing drop during a straight ahead stall? Was it caused by angle of bank, or had you inadvertantly generated a sideslip through rudder miscontrol?

Sparks, there really isn't a rule about which wing will drop in a turning stall. I personally believe that wherever possible, the manufacturer is going to strive to develop flying characteristics that encourage return to controlled flight following a stall. Nose down pitch following stall and wing leveling in a turning stall are two that come to mind immediately, and it may be that the aircraft you're flying have the desireable flight characteristic of dropping the outside wing by design. Or it could be the way that you personally fly the maneuver. Either way, remember that at this point in the aircraft's design life (well past test flight and into customer use) the point of the maneuver is to show you that the aircraft will stall at a higher IAS and with less warning. The post-stall gyrations are just an added value item to reinforce in your mind the undesireability of stalling the airplane in the first place.

 

[profile]

Not logged on for a while, so catching up. Certainly all good stuff in that last post, but I do stand by what I said wrt the differential AoA due to the effective roll rate in a nose-up turn.

As for the Lear cross control thing, I had the opportunity to speak with Pete Reynolds (VP of Flight Test, Bombardier) on this topic last spring when we were both panelists at a NASA Handling Qualities workshop. I asked him about this and he stated that it was certainly not a procedure the manufacturer would endorse and he went on to tell me that there are a number of similar urban myths out there on Learjets.

Don't believe me? Call him and ask. I'll not post the phone number, but it's easy enough to obtain if you are interested in facts.

 

[Timebuilder]

Profile, since you, Avbug, and skiddriver have much more experience and understanding than I do, let me comment from the position of a relative layman.

Perhaps the nose-up attitude mentioned by profile isn't being addressed. Is this attitude considered to be during the pitching moment, before a stable pitch attitude has been reestablished, and thus in a state of constant change? If not, if the pitch angle is stable, how does this affect a rate of roll, or the ascent or descent of a wing which is holding a constant bank angle?

Aside from the peculiarities of different examples of "same model" airplanes as mentioned by Avbug, I was taught that other things being equal (rigging, repairs, fuel tanks) the coordination of the airplane is the major factor that effects a differing critical angle of attack, and determines which wing will drop in a stall while in turning or straight and level flight.

I welcome all of the insights of threads like this. I like to learn, especially if I have believed something that is just plain wrong!!

 

[avbug]

I'm certainly not going to argue with Pete Reynolds, who probably knows more about Lear fringe charactaristics than anyone else alive. However, I will state without question that dutch roll can be stopped with aileron alone, rudder alone, or a combination of the two, including high altitude cruise.

Very often, applying very light rudder control crossed with very light aileron control will steady the airplane, and then gently releasing the control inputs will return the airplane to steady flight.

I had some simulator scientist a few days ago tell me that the airplane is uncontrollable at high altitude cruise without the yaw damper, because a flight safety instructor had shown this to him in the sim. Hogwash. He told me about the dutch roll that takes over and becomes uncontrollable. Perhaps this can be replicated in the sim, but it doesn't duplicate the airplane in flight. The airplane is eminently controllable.

Think about it...following a yaw damper failure, you can let the airplane fly you, or you can fly the airplane. You can do this by applying basic control inputs to make the airplane do what you want. If you want to be a bonehead, you can let yourself get behind the airplane. I have been in the airplane, and watched an individual roll it right over after entering a pilot induced series of divergent osciallations in dutch roll that resulted in his complete loss of control. It can happen. The individual who did that was an idiot, however.

All that aside, I have a good idea of whence I speak, because I maintain currency in the airplane. How about you?

 

[ksu_aviator]

I'm going to agree with AVBUG on this one. He explained it perfectly, I'm just going to offer another view.

The college I went to had a former Royal Air Force, Tornado Pilot as the department head. I remember very specifically him describing stalls as a manuever they used. They would roll into a 90 deg AOB and pull back until the aircraft stalled, if it was cordinated the aircraft would stall parallel to the ground in the exact same manner as an aircraft that was doing a normal straight ahead stall. Neither wing would "dip" or "break" first. He went on to explain that no matter what attitude the aircraft was in that you could recover simply by "pushing forward" or decreasing the angle of attack. Cordinated flight is the key to whether or not a wing stalls first. Also, remember that airspeed has nothing to do with whether or not you are stalled. The AoA is the only factor. Indicated stall speed will vary with weight, etc.

Ok, not as eligant as AVBUG's post, but maybe it will shed some light.

 

[profile]

Actually I find that I agree with most of that last Avbug post.

As for the stall, yes, if your nose in on the horizon at 90 degrees you won't have it break off to one side. The issue is that when the nose is up at at some angle to the horizon the nature of the turn becomes a roll. Take a model airplane and pitch it up, then make it turn, you will see that it is actually rolling as well, the more the nose is up (or down) the more pronounced the effect, until when you get into a vertical climb you are only rolling.

 

[Mickey]

Keep in mind that a lot of trainers are so stable that you need to keep in a lot of aileron to maintain a bank would would cause the raised wing to have a higher AOA due to the lowered aileron at that section of the wing. My $0.02 Have fun.