If you have carefully read all five parts of the “Stalling, Autorotation and Spinning” articles and you have a reasonable understanding of the phenomena of spinning, then now is the time to read up on the subject of flat spins. Our starting point will be from when the aircraft has already settled into a steady-state spin and all the aerodynamic and inertial forces acting on it are in equilibrium. If you missed part 5, click here.
For simplicity and consistency, this article pertains to monoplanes, which only have one set of ailerons and that are fitted with American engines, where the propeller rotates clockwise as viewed from the cockpit. The principles about to be discussed apply to all aerobatic aircraft, both biplanes and monoplanes.
If you happen to have a small piston-engined model aircraft handy, then hold it in one of your hands so that you can move it around and have its movement in all three planes corresponding to what you are reading. By so doing, you will find it fairly easy to understand the flat spin.
We have already decided that since we have chosen an aircraft that is fitted with an American engine, the flat spin that we are going to describe MUST BE TO THE LEFT. We will soon see why.
The stick is being held back into the pit of the stomach. Full LEFT rudder has been applied and the ailerons are being held in the neutral position. THE THROTTLE IS CLOSED, WHICH IS WHERE IT SHOULD BE.
Once the spin has stabilized, the stick is moved smoothly over to the right. This is opposite to the direction of the spin. The aileron on the left wing will move downwards, with respect to the aircraft, and the aileron on the right wing will move upwards.
The overall effect will be to reduce the difference in the lift components of the two wings. There will now be an increase in the amount of lift being generated by the down-going left wing, and a decrease in lift from the up going right wing. This causes the lower left wing to rise, and the higher right wing to fall, thereby reducing the aircraft’s bank angle.
At the same time, the left aileron, which is deflected downwards into a high pressure area, will cause more drag than the right aileron which is deflected upwards into a lower pressure area. This difference in drag of the wings because of the oppositely deflected ailerons, is enough to cause an increase in the rate of rotation of the spin.
The increase in the rate of rotation causes, in turn, an increase in the longitudinal inertial moments of mass. The front-mounted engine is being “thrown” upwards and outwards, and the mass of the rear fuselage which might contain the additional weight of a second pilot sitting in tandem or a full smoke-oil tank, is being thrown outwards and downwards. The nose therefore moves upwards into a somewhat flatter attitude in relation to the horizon.
Within a second or two, the spin motion of the aircraft has changed, and it has now settled into a new and different steady state spin. This spin is still somewhat insipid compared to what is about to come, but it is certainly a flatter than normal spin that also exhibits a higher rate of rotation than before. The first tentative steps towards performing a flat spin have now been made…
So, how does this “insipid” flat spin develop into the dreaded and feared flat spin? We now start adding power, more and more until we have full power. The spin rotation is to the left and the propeller rotation is to the right. Work the gyroscopic precessional effects out for yourself and you will see that the application of power will cause the nose to rise. A significant change in pitch attitude will occur. The aircraft will now achieve another steady state with the aircraft’s attitude within a few degrees of the horizontal. The nose would be almost, if not on, the horizon.
This is the real deal! The power on flat spin!
When the throttle is eventually brought back, the precessional force on the top of the propeller disc will reduce and the nose will drop away from the horizontal attitude of the “wicked and disconcerting” flat spin, into the “insipid” flat spin. Once the aileron is moved back into the neutral position, the bank angle will increase from being almost level, back towards the direction of the spin and the rate of spin rotation will diminish.
The final recovery from the spin can then be effected in the tried and tested, time-honoured, manner.
Instructors that have the ability and experience to teach flat spinning are very mindful of the fact that the noise, vibration, buffeting and discomfort that pilots are subjected to when doing this manoeuvre could cause elevated feelings of fear, confusion and disorientation. Such instructors always teach that it is of vital importance before doing a flat spin to go through the sequence of actions that must be followed so as to effect a prompt and clean recovery.
Getting the sequence of control inputs wrong could lead to another variation of the spin. This is the ACCELERATED FLAT SPIN and it is a manoeuvre that is flown intentionally by only advanced and unlimited class competition aerobatic pilots. So, with a lot to chew on, this is where we leave the flat spin!
In next month’s issue, we will talk about inverted spins and maybe even, briefly, how inverted flat spins can happen!