The thought struck me when replying to the AOA indicator thread, and i wondered how such a synthetic AOA calculator (ingenious!) would behave in an uncoordinated turn or spin…
So here it is – when you enter a spin, the usual technique is a combination of, just before the stall, applying full-back elevator to quickly stall the wing, and rudder to start the rotation to the desired side. The inside wing loses a lot of lift in an instance, and off you go – wheeeeeee!
Now, both wings initially have the same angle of attack, unless the aircraft is bent.
As soon as one wing is going down, it has a higher angle of attack than when flying level (something that is useful to know if you are trying to recover from a stall using pitch and power, and failing, say, over the middle of the atlantic) so once stalled, the downgoing wing remains stalled.
The question is – what does cause the soon-to-be downgoing wing to stall earlier in the first place? The yaw alone induced by the rudder does not change the angle of attack on either side… Is it the roll induced by the yaw?
When yawing, the leading wing does provide more lift than the trailing wing, even in a steady yaw. This is why you have to cancel the roll moment with the aileron if you fly with a yawing angle, level wings and constant heading. You can test this.
The other effect is during rotation around the vertical axis. Since the aircraft turns around his center of gravity, the outer wing get’s a bit airspeed extra and thus the transition to the seperated flow on the wing surface is located a bit aft of the separation point on the inboard wing. This results in a higher lift on the outboard wing, starting the roll into the spin. This rudder-roll moment can be demonstrated at higher airspeeds (lower angles of attack), although not the difference in separation points, but the differential speed of the wings cause the differential lift on the wings.
There is a very good german film about stalling / spinning here:
Spins can be happen from any flight attitude with sufficient yaw at the stall point.
In a normal spin, the wing on the inside of the turn is stalled while the outside wing remains flying; it is possible for both wings to be stalled but the angle of attack of each wing, and consequently its lift and drag, will be different. Either situation causes the aircraft to autorotate (yaw) toward the stalled wing due to its higher drag and loss of lift.
The best way to avoid entering a spin is to keep the ball centered when approaching stall (nose up high angle of attack) when climbing. Always try to make coordinated turns with the ball centered. The load factor increases the steeper the turn and produces higher stall speeds which can be a disaster at slow speeds and low to the ground. The base to final turn is the best example of this situation.
Cobalt wrote:
The question is – what does cause the soon-to-be downgoing wing to stall earlier in the first place?
Different aoa due to different relative wind component due to the rotational component of the yawing motion. You are flying fwd with a high aoa. When you kick rudder, the outer wing goes faster, and the aoa becomes less. The opposite happens to the inner wing, and causes it to stall.
As with propellers, in a yawing motion, it’s the relative wind components you have to look at.
Cobalt wrote:
The question is – what does cause the soon-to-be downgoing wing to stall earlier in the first place?
Is it not the slip-roll coupling, from dihedral or vertical wing position? It was your friend in the normal flight envelope, but has suddenly become your enemy around the stall. There’s also a similar reversal of roll damping, which is why, traditionally, being heavy handed with the ailerons around the stall is discouraged.
bookworm wrote:
Is it not the slip-roll coupling, from dihedral or vertical wing position?
A (initial) spin and a snap roll is the same thing (more or less). You cause the inner wing to stall by rotating in yaw. It’s all about relative velocities and its affect on aoa.
LeSving wrote:
It’s all about relative velocities and its affect on aoa.
Can you explain that please? I don’t see why a rate of yaw should affect the relative AOA of each side of the wing.
bookworm wrote:
Can you explain that please? I don’t see why a rate of yaw should affect the relative AOA of each side of the wing.
It is because the aircraft is pitched up – the yaw is not in the plane of the movement of the aircraft.
Airborne_Again wrote:
It is because the aircraft is pitched up – the yaw is not in the plane of the movement of the aircraft.
Exactly. What is important is the velocity and direction of the air (the vector) relative to the wing. At steady coordinated flight, this is trivial, because the wing moves in the same direction as the flight path. The relative vector is equal to the absolute vector in that case. When yawing, the wing suddenly starts moving relative to the flight path, and in a different direction, because the aircraft rotates, the difference is higher at larger aoa.
LeSving wrote:
different relative wind component
There would be a relative airflow component, which could differ between the two wings, based upon yaw. Wind is relative to the earth, and will be uniform over the entire aircraft.
It is because the aircraft is pitched up – the yaw is not in the plane of the movement of the aircraft. Quote
Pitched up relative to the vertical direction of flight, which could be quite down, so pitched down, but yes, the yaw axis is no longer perpendicular to the pitch plane of normal flight.
It is very interesting how little dis symmetry of lift left to right it takes to induce a spin, if the yaw is not well controlled during a stall. There are aircraft which will spin vigorously out of a well coordinated stall entry, because they are poorly rigged. Those are the ones I will not sign for after a maintenance check flight!
