Stall recovery

[KL-666]

In flightgear i have identified two implementations of stall.

1) High sink rate stall (777): This stall consists of getting only a high sink rate, no other effects like nose dip or whatever happening. This stall is recoverable.

2) Game over stall (747-8i): This stall has nice stall effects, but it is unrecoverable. You can dive and pick up lots of speed, but the plane never flies again. As if someone flipped the lift-switch off.

Due to that i find recoverability more realistic, i prefer the first type of stall even though it has no stall effects.

Kind regards, Vincent

[Richard]

In the LevelD sims that I've flown the B737-2/3/4/5, B757, B767, B747, all you need to do is to keep pulling the stick to maintain the stall. The A320 doesn't stall in normal law, you can pull back on the stick as much as you want and just get a maximum rate climb; in alternate law I believe you can stall but I've never experienced that. The MD-83 is a bit different as it has a stick pusher that will force the stick forwards to avoid a deep stall. The warnings (klaxon, stick shaker), buffet (through the motion) all let you know that you're stalling.

On the 737 I spent a lot of time trying different combinations of stick and rudder to get an interesting result and it was always recoverable. Now this may be due to the aerodynamic package as I believe it doesn't go about 25degrees alpha, so some of the separation and adverse asymmetric affects that may be there may just not be modelled. However you're really not supposed to get an airliner into this region at all, as structural limits will also come into play and things might fall off.

So I'd expect warnings, but full recovery providing you have sufficient altitude.

Any aircraft that has hardwired aero parameters to cause a stall effect is wrong; the aerodynamic data should be modelled correctly and let physics do the rest; simplistically this is gradually descending Cl with rising Cd. You can see this in the F-15, F-14 and F-16 Cl/Cd curves. The F-15 has a high alpha yaw moment (caused by vortices from the nose / gun vent asymmetry) which usually results in a handling problem if you're not quick with the rudder. So the experienced aero modeller, with sufficient data and/or evidence can usually get something that is close enough for a desktop simulator.

[bomber]

Any aircraft that has hardwired aero parameters to cause a stall effect is wrong; the aerodynamic data should be modelled correctly and let physics do the rest;

This we agree on, it makes me cringe when I see these hardwired attempts..

The physics however is another issue.... well actually it's not so much the physics but just what the hell is happening and how JSBsim parameters reflect it..

simplistically this is gradually descending Cl with rising Cd.

Er.... what ?
angle of attack and the asymmetry of it on your major lift surfaces is the key to understanding spin...

You can see this in the F-15, F-14 and F-16 Cl/Cd curves. The F-15 has a high alpha yaw moment (caused by vortices from the nose / gun vent asymmetry) which usually results in a handling problem if you're not quick with the rudder. So the experienced aero modeller, with sufficient data and/or evidence can usually get something that is close enough for a desktop simulator.

Good I'm sure it has some effect..... but it's tiny compared to the effects asymmetry of forces generated by the wings... Get these correct and spin would be modelled.

Simon

[Richard]

This we agree on, it makes me cringe when I see these hardwired attempts..

The physics however is another issue.... well actually it's not so much the physics but just what the hell is happening and how JSBsim parameters reflect it..

Going back to basics the forces affect the position of the model and the moments affect the orientation; Cl and Cd are forces.

There is of course side force generated as a result of high alpha when there is beta; but usually not until there is beta. The same is true for the moments. Often this is a deliberate so that high alpha effects can be fed in by separate tables.

Good I'm sure it has some effect..... but it's tiny compared to the effects asymmetry of forces generated by the wings... Get these correct and spin would be modelled.

Asymmetric lift will only usually happen when there is beta; and this is true whether or not you are stalled, so this is usually in the CNbase(alpha,beta) function. There may well be a CNextra(alpha) which has values at say >30 (as in the F-15).

When I said "simplistically this is gradually descending Cl with rising Cd" I meant that stall is really all about high drag compared to lift, there are other effects but a lot of the time these will come out as a result of sideslip via the FN(alpha,beta). To take an example (and with good piloting -) you can balance the F-15 (and F-14) above 20 degrees alpha with the thrust and have vertical speed of zero, but it takes a lot of power compared to unstalled surfaces in level flight, directional control is difficult, any rolling will probably not end well and rudder will have to be used to counter the yaw moment.

[bomber]

Stall and Spin need to be looked at differently...

Stall you're correct in saying it's a rise in drag and reduction in lift.

The problem comes with the introduction of spin....ie asymmetrical lift or drag....

But all of it comes from angle of attack.... caused by the linear u velocity against w velocity in tandem with rotational velocities....

Yes beta is important as it influences the u velocity.

[Richard]

Stall and Spin need to be looked at differently...

The problem comes with the introduction of spin....ie asymmetrical lift or drag....

I'm not sure it's right to say that stall and spin need to be looked at differently; I've always understood one to be an effect of the other; i.e. spin to be rotations that happen when a craft is fully stalled; maybe my understanding is wrong.

The rotations are either going to be a result of kinematic/inertial coupling (which I think the mass model in JSBSim will take care of), or aerodynamic moments which is what needs to go into the datatables as a function of alpha and/or beta in the appropriate axis, in the way I do aerodata these will be components in roll moment (CL) and yaw moment (CN).

[Octal450]

I think the Stall in my MD-80s and MD-90 is pretty good. It is recoverable, the nose drops slightly, but it will fly again if you let it gain speed, and add thrust.

[bomber]

The reason I say they should be treated separately is so as to keep their effects clear in the mind, the thought process of what's actually occurring.

Stall as you've said is the rapid reduction in lift, and vortex drag. Whilst at the same time increase in form drag. If we consider no torque effect from the engine or props, like a glider. There's no non-pilot reason for any wing to drop and enter into a spin. The plane's sink rate will increase and with the increase wing form drag being now more at the wings centre of area, as opposed to a working airfoils centre of pressure at 1/4 cord, ie now further aft, q rotation increases and the nose will sink.

Nothing dramatic should happen, and you're well into the stall regime.... if you do nothing whilst the nose is dropping the plane will pick up speed the AoA will reduce and the wings will start to generate sufficient lift, the decalage with the h-stab will start to work and the nose will rise. If the planes longitudinally stable after a few of iterations the plane will level itself. If it's not longitudinally stable then the dips and climbs become worse until the plane enters a stall turn, a spin starts and the plane spirals down to the ground.

The problem with the stall regime is what it's doing to the lateral stability of the plane...

Whilst in the normal flight regime if the right wing was to drop, and the left wing rise. Then the AoA on the right wing increase and decrease on the left, increasing lift on the right decreasing on the left. This is the natural damping of the roll and helps give the plane its natural lateral stability..

Simply understood it's wing rolling downwards increases AoA.

When you're in the stall regime the wing rolling downwards puts that wing further into stall, reducing lift and increasing the lift on the rising (reducing AoA) wing, there is no damping taking place, in fact the opposite is happening, positive feedback.

Anyway I'll leave you to think on it.

Regards

Simon