When you kick rudder, the outer wing goes faster, and the aoa becomes less.
That is not entirely true, I am afraid. You turn around the vertical axis in the aircraft coordinate system, and not in the aerodynamic coordinate system. Only the angle of incidence can change the angle off attack in a yawing motion and not the AOA itself.
mh wrote:
Only the angle of incidence can change the angle off attack in a yawing motion and not the AOA itself.
This statement lacks clarity….
The pilot has no control over the angle of incidence.
I compare notes between Alan Cassidy’s Better Aerobatics, Rich Stowell’s Emergency Training Manoeuvres and the RAF AP3456 which is now available. All three sources would suggest that both wings are stalled in a spin, just that the down going wing is more deeply stalled. This is due to the relative airflow component from the roll creating a larger angle of attack, conversely the outside wing the roll is resulting in a reduced angle of attack. If the wings are not stalled this is a form of aerodynamic damping. However, if the wings are stalled the damping becomes a source of lateral instability with the down going wing being more deeply stalled.
In competition aerobatics Cassidy explains that judges expect to see the aircraft nod down in a wings level stall break, before entering the spin. A spin with only one wing stalled would be a flick roll.
Yaw in the stall will provoke the yaw-roll couple which leads to auto rotation. The outside wing is faster when yawing, and this will create additional lift(L=1/2 rho V2 Cl S), leading to roll, which in turn leads to yaw, etc
When inertial forces are balanced by aerodynamic forces the spin reaches its stable phase. The angle of attack in the stable phase is deep in the stall regime for both wings, if critical alpha is around 15 degrees, in a stable spin the wings have an alpha of 30 to 35 degrees, more in the case of a flat spin.
mh wrote:
You turn around the vertical axis in the aircraft coordinate system, and not in the aerodynamic coordinate system. Only the angle of incidence can change the angle off attack in a yawing motion and not the AOA itself.
I’m afraid I don’t quite understand this either, or disagree with. AoA is a vector, with a horizontal component, and a vertical component. If you increase, or decrease the horizontal component (local wing speed) with yaw, it has to lead to a change in AoA, because the direction of incoming airflow changes, relative to the wing. So yaw can change local AoA i.e. per wing.
You have to disagree, because what I wrote was wrong. I will elaborate when I have a keyboard tomorrow.
mh wrote:
Only the angle of incidence can change the angle off attack in a yawing motion and not the AOA itself.
When yawing, the wing rotates. It has a tangential velocity component larger than- and in (often) a different direction to the free stream velocity on the outside wing, and opposite on the inside wing. The aoa is no longer defined only in terms of the free stream vector relative the cord line. It is defined by three vectors; the free stream velocity (the main aircraft velocity vs the air), the velocity of the wing (at any given span) and the velocity vector of the free stream relative to the wing velocity. You get a velocity diagram, the same way you do for propellers and turbines due to three velocity components. In addition you also get the roll component, a velocity component that also will be different for different span, and will help keeping the outside wing unstalled.
Consider that when an aircraft enters a spin, it is yawing. Therefore, the whole airplane is in forward flight (so the wings create lift) turning around a vertical axis. If, in the extreme, that vertical axis around which the plane turned was at a wingtip, it would be obvious that the outboard portion of that wing would have nearly no airspeed, so the apparent AoA at the tip would be silly high. That wing could not produce lift, and would stall first.
I’ve seen some good explanations, here and elsewhere (e.g. Barnes McCormick’s text “Aerodynamics, Aeronautics, and Flight Mechanics”) of a developed spin. It’s clear that both wings are stalled, and that the turn radius is very tight indeed.
However, I haven’t read much I’m confident about on the spin entry, which is Cobalt’s original question. Does one wing reach the critical AOA and stall first, or does the spin develop with both wings stalled? And what causes the departure from wings level? If you could keep the wings level with the ailerons, would the aircraft spin or is some roll rate required to enter the spin?
Wings level will not prevent a spin entry at stall, if the ball is not in the middle. It is the yawing, which induces the spin. Indeed, wings not level stalls are very straight forward up to a point, as long as the ball is in the middle.
Yes, for a spin to develop, one wing will reach a critical AoA before the other, but it need not be much more AoA, or much sooner, for a spin to develop.
Departure from wings level is simply because one wing provides less lift than the other at the wrong time.
A spin can develop from both wings stalled, if the lift being created by both wings at the time is not equal. The fact that wings are stalled, does not mean that they have stopped producing lift, they have just stopped producing enough lift for the G you’re commanding. They’re still lifting, even at 1/2G, so they can still provide different lift left to right, based upon yaw, a glob of ice on the one leaving edge, or other asymmetries.
Both yaw and roll play a role in the spin entry. The effect of roll and its interaction with yaw is well explained in the book “Aerodynamics for Naval Aviators”, so I permit myself to quote it here (p. 307):
In order to visualize the principal effects of an airplane entering a spin, suppose the airplane is subjected to the rolling and yawing velocities shown in figure 4.32. The yawing velocity to the right tends to produce higher local velocities on the left wing than on the right wing. The rolling velocity tends to increase the angle of attack for the downgoing right wing (alpha r) and. decrease the angle of attack for the upgoing left wing (alpha l). At airplane angles of attack below the stall this relationship produces roll due to yaw, damping in roll, etc., and some related motion of the airplane in unstalled flight. However, at angles of attack above the stall, important changes take place in the aerodynamic characteristics.
Figure 4.32 illustrates the aerodynamic characteristics typical of a conventional airplane configuration, i.e., moderate or high aspect ratio and little-if any-sweepback. If this airplane is provided a rolling displacement when at some angle of attack above the stall, the upgoing wing experiences a decrease in angle of attack with a corresponding increase in CL (coefficient of lift), and decrease in CD (coefficient of drag). In other words, the upgoing wing becomes less stalled. Similarly, the downgoing wing experiences an increase in angle of attack with a corresponding decrease in CL and increase in CD. Essentially, the downgoing wing becomes more stalled. Thus, the rolling motion is aided rather than resisted and a yawing moment is produced in the direction of roll. At angles of attack below stall the rolling motion is resisted by damping in roll and adverse yaw is usually present. At angles of attack above the stall, the damping in roll is negative and a rolling motion produces a rolling moment in the direction of the roll. This negative damping in roll is generally referred to as “autorotation.”
When the conventional airplane is stalled and some rolling-yawing displacement takes place, the resulting autotiotation rolling moments and yawing moments start the airplane into a self-sustaining rolling-yawing motion. The autorotation rolling and yawing tendencies of the airplane at high angles of attack are the principal prospin moments of the conventional airplane configuration and these tendencies accelerate the airplane into the spin until some limiting condition exists.
