Flat Spins
Thank you, Surplus1; for your post # 3061 and post #3144.
And thank you, Woodvale; for your post 3155 too.
It seems to me we are pretty close to saying much the same thing, certainly there are many big ifs involved at this stage and maybe always will be. My point was and is to take what seems to be judged as “known” or believed by competent authority and see if it fits anything known in the world of aerodynamics. If there is a fit, and it seems to me that there might be, surely it is interesting and should lead one to wonder about less sure assumptions.
Here, for all you spin skeptics, is a little food for thought and although I may, in the large part, be preaching to the converted, it seems it still needs to be said.
A modern swept wing airliner has somewhat different stall characteristics from straight-wing aircraft that many of us may have stalled when training in the past. The difference is that although straight wings tend and are encouraged to stall at the root first, aft-swept wings tend to want to stall at the wing-tips first, rather than at the root. With a swept wing, the Center-of-Lift may move ahead of the Center-of-Gravity (CG) at the stall causing a pitch-up moment. And so the aircraft may begin descending in a nose up attitude unless positive forward pressure on the controls is applied (Straight-wing aircraft will usually pitch down on their own accord when the stall occurs, as long as they are within aft CG limits.). That is why swept-wing aircraft have stick shakers that give an artificial warning of impending wing stall. Many also have stick pushers to force the aircraft to a lower angle-of-attack before the stall progresses too far. Many types of aircraft will tend to roll or yaw if recovery from the stall is delayed, and swept wing aircraft are particularly prone to becoming laterally unstable as the stall progresses. A spin can be thus inadvertently entered. An aircraft might be very stable in “normal” conditions, but once you find yourself at stall speeds; the rules change.
To further complicate the problem, those aircraft with engines mounted underneath the wings can experience a further pitch up, since the engines' thrust lines are below the aircraft's CG. The pitch-up associated with adding power can cause the stall to worsen, if the controls are not moved forward to counter this tendency, or if emphasis is not placed on lowering angle-of-attack first with forward pressure on the controls.
Swept-wing aircraft do not normally have the luxury of engine or propeller wash over the horizontal tail and elevator, to aid in pitch control. Therefore, if a pilot encounters an impending stall in such an aircraft, he has been taught to hold the pitch attitude and apply maximum power to minimize altitude loss and to "fly" out of the stall. The success of this recovery lies in the fact that a stall has not yet occurred (the stick shaker will typically activate at an airspeed 5-10% above the stall speed.). Thus it is not really a stall recovery at all, since a stall has not occurred.
It also seems that there are a variety of ideas regarding just what a flat spin is, clearly, a number of them are quite wrong. Let me just say that regardless of how it was entered, in a flat spin one wing will have a large angle of attack and the other a smaller angle of attack, in comparison to the relative wind, which is CB activity could be coming from almost anywhere. Both wings might be stalled, but not necessarily so and one will be stalled more than the other. Thus, you can have a doubly stalled flat spin and also a singly stalled flat spin.
In a non-spinning airplane, if one wing were producing more lift than the other, that wing would rise. So the question is; why is a flat spin stable? Or, why doesn’t the outside wing continue to roll to ever-higher bank angles? The answer is centrifugal force. In an airplane spinning about a vertical axis, the high (outside) wing will be centrifuged outward and downward (toward the horizontal), while the low (inside) wing will be centrifuged outward and upward (again toward the horizontal). In a steady flat spin, these centrifugal forces cancel the rolling moment that results from one wing producing a lot more lift than the other.
As has been mentioned previously in this thread, in the 1970s, NASA conducted a series of experiments on spin behavior. They noted that there was “considerable confusion” surrounding the definition of steep versus flat spin modes, and offered the classification scheme shown here.
SPIN MODE:............STEEP;.....Mod’ly Steep;..Mod’ly Flat;...FLAT;
ANGLE OF ATTACK...20 to 30... 30 to 45....... 45 to 65........65 to 90
NOSE ATTITUDE......extreme nose-down....... less nose-down
RATE OF DESCENT...very rapid.....................less rapid
RATE OF ROLLK.......extreme....................... moderate
RATE OF YAW.........moderate..................... extreme
wingtip-to-wingtip
DIFFERENCE in ANGLE
of ATTACK.............modest........................ large
nose-to-tail DIFFERENCE
in SLIP..................large........................... large
The angle of attack that appears in this table is measured in the aircraft’s plane of symmetry; the actual angle of attack at other positions along the span will depend on position. Notice also that in a flat spin the rate of yaw is extreme and angles of attack are quite large. indeed.
In all cases NASA studied, the flat spin had a faster rate of rotation (and a slower rate of descent) than the steep spin. Meanwhile, there are reports of experiments in which the flatter pitch attitudes were associated with slower rates of rotation. This is not a contradiction, because the latter dealt with an unsteady spin (with frequent changes in pitch attitude), rather than a fully stabilized flat spin. A sudden change to a flatter pitch attitude will cause a temporary reduction in spin rate, for the following reason.
In any system where angular momentum is not changing, the system will spin faster when the mass is more concentrated near the axis of rotation (i.e. lesser moment of inertia). By the same token, if the mass of a spinning object is redistributed farther from the axis, the rotation will slow down.
When the spinning airplane pitches up into a flatter attitude, whatever mass is in the nose and tail will move farther from the axis of rotation. Angular momentum doesn’t change in the short run, so the rotation will slow down in the short run.
In the longer run — in a steady flat spin — the aerodynamics of the spin will pump more angular momentum into the system, and the rotation rate will increase quite a lot. The rotation rate of the established flat spin is typically twice that of the steep spin.
Recovering from an established flat spin requires forcing the nose down. This brings the mass in the nose and tail closer to the axis of rotation. Once again using the principle of conservation of angular momentum, you can see that the rotation rate will increase (at least in the short run) as you do so.
As for the point of rotation, it is likely found some way from the inner wingtip following the axis of rotation from outer wingtip via c of g to inner wingtip to point of rotation – the aircraft does not “spin like a top” as some seem to think.
Lastly, in turbulence with a high angle of attack and a stalled wing, you do not need a rudder to induce a spin; adverse yaw brought on by aileron drag in an attempt to level the wings will do just fine. Which is likely why, when learning, we were always taught to use rudder only; at high angles of attack leading up to Insipient Spins. Power will hold the aircraft in the spin; as long as the engines produce thrust and I suspect there would be little if anything the crew could do to recover, regardless of skill, especially on instruments.
Do you think you could recognize a spin in IMC?
The mentioned Russian aircraft that crashed following a “flat spin” entered at altitude, in turbulence, had a Second Officer who was a qualified aerobatics pilot - not that it helped.
So far, although there are those who scoff, no one has yet suggested a solution to better fit what little is known of the end result; damage observed from recovered debris or any of the other evidence, scant though it may be, and I suspect if taken a little more seriously it would be found that other things fit a spin solution too.
Yes, there are many unknowns and much we don’t know but IF AF447 DID fly into moderate to severe turbulence and IF that did cause and inadvertent loss of control and IF that is what the ACARS messages started to try to tell home base and IF yaw and high angle of attack caused loss of power and loss of satellite communication. Would the aircraft likely end up as it did? The idea is to suggest a solution that allows the evidence to fit, not manipulate the evidence to fit a solution.