Spin aerodynamics

6 Answers

1. 
   

   
   

   Skyfox on Nov 02, 2016

   In a spin, both wings of the airplane are stalled, but one is more stalled than the other. The one that’s more stalled will drop, and that side is the direction the spin will go with. The reason the plane spins rather than simply stalls evenly is because of uncoordinated flight. For example, if you’re doing a hard side slip to the left, you’ll have low left wing and high right wing with hard right rudder to counteract the turning tendency of the bank. In that attitude, the high right wing will be in a sort of aerodynamic shadow created by the fuselage blocking the airflow from hitting the wing root and down the wing some distance (more pronounced with low wing aircraft). To maintain a level altitude that side slip will require high pitch; if altitude was being lost it would be like a controlled forward slip to a landing. That loss of airflow will cause the wing to lose lift and stall, and the very high drag on that wing plus the hard right rudder will yank the nose around to the right as the spin begins.

   It would be slightly different if you were in a wings-level nose-high stall situation and kicked the rudder one direction or the other to cause a spin right at the moment of stall. In that case, the wing on the outside of the turn would have slightly greater airflow than the one on the inside of the turn, causing the inside wing to stall more than the outside wing and combining with the rudder forces again to get the spin going.

   From the Airplane Flying Handbook [FAA-H-8083-3B]:

   “A spin is caused when the airplane’s wing exceeds its
   critical angle of attack (stall) with a sideslip or yaw
   acting on the airplane at, or beyond, the actual stall.
   During this uncoordinated maneuver, a pilot may not
   be aware that a critical angle of attack has been
   exceeded until the airplane yaws out of control toward
   the lowering wing. If stall recovery is not initiated
   immediately, the airplane may enter a spin.
   If this stall occurs while the airplane is in a slipping or
   skidding turn, this can result in a spin entry and
   rotation in the direction that the rudder is being
   applied, regardless of which wingtip is raised.”

   and

   “Often a wing will drop at the beginning of a stall.
   When this happens, the nose will attempt to move
   (yaw) in the direction of the low wing. This is where
   use of the rudder is important during a stall. The
   correct amount of opposite rudder must be applied to
   keep the nose from yawing toward the low wing. By
   maintaining directional control and not allowing the
   nose to yaw toward the low wing, before stall recovery
   is initiated, a spin will be averted. If the nose is allowed
   to yaw during the stall, the airplane will begin to slip in
   the direction of the lowered wing, and will enter a spin.”

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2. 
   

   
   

   connor on Nov 03, 2016

   So, in order to induce the spin, I need to intentionally raise the wing and move the aileron down so it has greater AOA and more drag?

   Also, Why do we use less of aileron than rudder when turning during slow flight?

   Connor.

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3. 
   

   
   

   Skyfox on Nov 03, 2016

   Raising the wing can contribute to a spin because of those factors but it’s not guaranteed, and it’s not guaranteed that wings-level flight won’t spin in a stall. The main factor as far as I know is that it’s uncoordinated flight (ie. the ball in the turn coordinator isn’t centered).

   To answer your question about slow flight, one of the effects of slow flight is that the rudder induces a rolling motion. In a slow flight turn, for a given amount of rudder the aircraft needs much less aileron to put in the needed amount of roll, possibly even requiring opposite aileron to prevent excessive roll and resulting in a cross-controlled situation.

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4. 
   

   
   

   connor on Nov 08, 2016

   You mentioned “In that case, the wing on the outside of the turn would have slightly greater airflow than the one on the inside of the turn, causing the inside wing to stall more than the outside wing and combining with the rudder forces again to get the spin going.”

   If the outside wing has slightly higher airflow, than it will rise and how come low wing exceed teh critical angle of attack?

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5. Best Answer
   
   

   
   

   Brian on Nov 09, 2016

   There PHAK and AFH only gloss the surface. Their definitions are comparable to saying a toaster works by being plugged in allowing metal wires retain heat, generated by your electricity, used to toast your food. Well any curious kid who shoved a paper clip into an outlet while bored in science class knows there has to be more to this story.

   The same goes for spin dynamics. You can gain a bit more if you check out 307-311 of Aerodynamics for Naval Aviators. If you really want to dig in Sammy Mason writes an incredible book on the subject called ‘Stalls, Spins and Safety.’ It’s far less technical and quite an enjoyable read.

   That said here is a bit more information:

   Roll: When an aircraft rolls the wing going downward is experiencing a component of upward airflow that increases its angle of attack (AOA); the wing going upward encounters a component of downward airflow reducing its AOA. In normal flight this action dampens the rolling moment the pilot is trying to produce. You often hear this referred to as posative stability.

   However, in stalled flight this action is quite the opposite. The increase in AOA on the downgoing wing deepens its stall, decreasing its lift coefficient (Cl) and increasing its drag coefficient (Cd). The opposite occurs on the up going wing. What is important here is the drag because the downward wing has more drag imposed on it than the upward wing the aircraft experiences a yawing moment in the same direction as the roll. (This is exactly the opposite of the adverse yaw we would normally experience if the aircraft were not in stalled flight when rolled.)

   Yaw: The yaw created causes the downward wing to retract from the relative wind. This action reduces that wings velocity and results in less lift being produced. The upward wing is being thrust into the relative wind increasing its velocity and its lift. In similar fashion these actions also impact both wings AOA as well. This is a condition experienced in both stalled and unstalled flight. Step on a rudder in flight and hold it, you’ll notice the aircraft rolls toward the direction of rudder input; termed pro verse roll.

   Important connection: The link one needs to make here is with the rolling component and how a roll impacts our aircraft in stalled flight. When an aircraft is not stalled the rolling and yawing tendencies are in competition with one another. Any roll produces adverse yaw and that yaw produces a rolling moment that is opposite the roll: stabilizing. The opposite is the case when stalled: when stalled a roll will cause yaw in the same direction and that yaw will cause more roll. (Ref to the paragraphs on roll above).

   The actions of the roll and yaw working together result in the aircraft stabilizing itself in autorotation (a spin).

   —–

   As for your questions on aileron input I will leave the dynamics aside as it will take another page to explain in detail. Use what is above to analyze the why yourself and feel free to ask more question!

   With that said, ailerons in the direction of the spin tend to tighten the spin where as ailerons in the direction opposite the spin tend to flatten it. However, aileron input is not needed to put an aircraft into a spin and during entry they will help a stable airplane like a 172 enter a tight spiral instead of a spin. In most cases aileron input should be withheld till a stable spin is established and then you can experiment with how they work in a spin.

   —-

   On a final note this information is generic to a typical stalled aircraft. With the right airfoil, and an aircraft that is deeply stalled, increases in AOA can result in an increase in Cl. If one were to draw a Cl curve for certain Airfoil’s you would find that it typically peaks twice; once at the first stall and a second time when deeply stalled. Often times the second peak is higher than the first, but the drag at that point is much too high for it to be a feasible AOA for the pilot to fly.

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6. 
   

   
   

   Brian on Nov 09, 2016

   Skyfox: Rudder input produces a rolling motion in both slow and normal cruise flights. The important factor is that the rudder, when at min controllable speed (Vmca), can bring the inside (down going) wing to stall. The inside wing reaching stall is what causes the need for opposite aileron. The correct pilot input is to apply opposite rudder to stop the yawing motion.

   It is most easily recognized by performing a 5-10 degree bank turn in normal cruise and watching how the horizon moves past the airplane and then compare it to the same maneuver at slow flight. Often times pilots input the entry controls but forget to reestablish the correct rudder after the turn has begun; particularly in left turns. The result is a nose that appears to be sweeping through, versus slowly tracing, the turn.

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