Wednesday, February 16, 2011

An. Petrovich post about SPINS


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Translated by TX-BroWright

About spin recovery.

It was always a mystery for me: why the f* they often write in aircraft manuals, that in case of unanticipated spin, at first (after pilot has had identified the direction of rotation and the type of spin) one has to set controls in the direction of the spin, and only after that - in the opposite direction, for recovery. I considered it a "military by the book". It turned out, I was wrong. Detailed study of the physics of a spin gave me a very definite answer, why it is necessary to set controls in the direction of a spin ( at first). In a spin, pilot has 2 main enemies: high (past critical) angles of attack, and high slip/skid angles. And at that, the high slip angles are even more dangerous than high angles of attack. For due to various kinds of interactions between longitudinal and lateral motions of the airplane (so called inertial, aerodynamic, kinematic interactions while airplane performs complex motion in space) angles of attack and slip/skid angles in established phase of spin "work" in tandem, complimenting each other: if the angle of attack is on the increase, then the slip/skid angle will decrease, and vice versa: if the AOA decreases, the slip/skid will increase. So much so that even at a low AOA but under high slip/skid angles the airplane can continue to stay in established spin. 

This is exactly why every method of spin recovery (which under standard classification are 5 for regular spin and 3 for the inverted) prescribes to first remove the slip/skid, and only after that to correct high angle of attack situation. If, instead, while in spin, one immediately sets controls to recovery - then - for some types of stall and for some airplane types - it oftentimes leads to the situation where the angle of attack indeed decreases, but the slip/skid increases! And an airplane would stay in the spin, and with more enthusiasm at that, with increased rates of turn (spin up). 

But if one at first sets the controls in the direction of the spin (i.e. stick aft, and rudder position - regardless, because if airplane responds well to the rudder, it will recover from the spin without any voodoo, but if it does not respond well - then at this moment one can push rudder pedal in the same direction as spin, airplane will not be likely to respond anyway), then the angle of attack will increase, but the slip/skid? right, it will decrease. And the airplane will slow its rate of rotation (get closer to a flat spin). Only now lets begin with rudder set to recovery (and it is desirable to set control column in the direction of bank and spin direction - in order to assist in removing the slip/skid), and after a small delay (0.5- 1.0 seconds), when slip/skid angle decreases even more - push control stick forward. Airplane leaves high angles of attack with ease and appreciation of the pilot, who has just mastered the airplane's "practical aerodynamics".

By the way, sometimes it is not even necessary to push stick forward - for those "strange" cases where slip/skid in the spin is predominant factor over angle of attack. Airplane is simply slows down its rate of rotation, and either leaves high angles of attack (with stick still aft), or transitions to the spin in the opposite direction (exactly the occurrence described by RB). In practice, this way of spin recovery is rarely used, because it has potential risk of a secondary spin. However for modern airplanes with a one-piece stabilator (the whole vertical fin turns as one piece) this method works fabulously (so called 5N or "Fedotovsky").
An. Petrovich

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