Smoke On Go

Stalling, Autorotation and Spinning – Part 5

SNAP ROLLS

Part 5 deals with what earlier generations of aviators called “flick rolls”, but what are now called “snap rolls”. These manoeuvres are directly linked to the concept of autorotation as described in Part 1 of the “Stalling, Autorotation and Spinning” series. Read part 1 here.

So…Imagine that a Pitts Special S2 aerobatic biplane at its maximum aerobatic weight is flying straight and level in one “g” flight. If the engine is throttled back, the airspeed will start decaying and the angle of attack of the wings will need to be increased so as to provide sufficient lift to maintain the aircraft’s altitude. Eventually, the angle of attack of the wings will reach a critical value of about sixteen degrees or so, and at 65 miles per hour, the aircraft will stall. This speed will correspond to the manufacturer’s published stalling speed.

If the aircraft was then put into a level, sixty degree banked steep turn, the loading on its wings would be increased. The aircraft would be subjected to a 2 “g” load factor and its stalling speed would increase from 65 to 92 miles per hour.  At that speed, the angle of attack of the wings would have reached their critical angle of attack of 15 to 16 degrees. The relative airflow would no longer be able to follow the contour of the top of the wing and the aircraft would stall.

The wings could also be brought to their stalling angle of attack at speeds that are a lot higher than the 92 miles per hour experienced in the steep turn. At 130 miles per hour, which is exactly DOUBLE the aircraft’s published stalling speed, the stick could be brought back firmly and almost aggressively, so as to achieve at least a 4 “g” pull-up. The wings would  then reach their stalling angle of attack. This particular stall would have happened at a speed very close to 130 miles per hour!

Unless it is stall training that is specifically being done, an aircraft will seldom stall at the published speed. There are indeed an infinite number of different speeds that it can stall at. The vital thing to know and to understand is that the aircraft stalls when the angle of attack of its wings reach their critical angle of attack! 

We now proceed with the execution of a snap roll. The aircraft is being flown with its wings level and with the engine  developing aerobatic power. The indicated airspeed is being contained between 125 and 135 miles per hour by making small power adjustments.  These speeds are close enough to being DOUBLE that of the published stalling speed of the aircraft. The stick is brought backwards in a very deliberate and positive manner towards the pit of the pilot’s stomach and the aircraft pitches up, almost violently. The upper and lower wings are rigged with a slight difference in their angles of incidence, so, almost simultaneously, but not at exactly at the same time, both wings will reach their stalling angle of attack.

In that very instant, full right rudder is applied. This happens to be the more appropriate rudder to use because the Pitts Special’s propeller turns clockwise as viewed from the cockpit. The abrupt pitch-up will cause a huge force to be APPLIED at the front top of the propeller’s disc. This force precesses clockwise through 90 degrees and ACTS at the front “three o’clock position” of the propeller disc, thereby augmenting the yawing force provided by the rudder. 

The aircraft is now in a situation where it is both stalling and yawing simultaneously. Virtually all of the aerodynamic moments associated with the classic onset of autorotation will have been brought into play. The aircraft will rotate clockwise very quickly. Instead of the nose dropping and a huge loss of height occurring as it would in a classic autorotation, the inertia possessed by the aircraft because of its mass and its speed will continue to carry it forwards close to the extended flightpath that existed at the time that the manoeuvre was initiated.

In the early stages of learning to accomplish the snap roll, the manoeuvre should be stopped and recovered from at no more than one rotation of roll. This is enough excitement to have to cope with.

The aerodynamic moments required to stop the snap roll are opposite rudder to the direction of rotation in order to stop the yaw and forward stick to un-stall the aircraft and to get the wings generating lift again.

Snap rolling an aircraft in such a manner that a crisp and neat manoeuvre results and so that the manoeuvre is stopped in precisely the desired attitude, is not easy. Aileron application, in either direction might help to get a faster roll rate. Also a reduction in the angle of attack of the wings could reduce the drag and therefore curb the decay of the speed. This could help in achieving a cleaner and more accurate recovery from the roll. The ideal placement and movement of controls will eventually come to the pilot after lots of experimentation and judicious practice. Snap roll characteristics are extremely dependant on aircraft type and no two aircraft behave in the same manner.

In conclusion, snap rolls feature prominently in the sport of competition aerobatics as they are a sure manifestation of a pilot’s abilities. They may be performed as positive “g” manoeuvres where the stick is pulled back to stall the aircraft or as negative “g” manoeuvres where the stick is pushed forward towards the instrument panel. 

Apart from being performed in horizontal flight, positive to positive, or negative to negative, they can be flown on 45 degree upward or downward lines, vertically upwards or downwards, positively through one and a half turns to inverted flight or from an erect attitude through one and a quarter turns to knife edge flight. Then too, from knife edge flight through one and a quarter turns back to positive flight. The amount of permutations blow the mind!

Three notes of caution come to mind. Firstly, aircraft having propellers that turn anti-clockwise as viewed from the cockpit should be snap rolled to the left. Secondly, an over ambitious approach to snap rolls can put enormous stress on the airframe and its  engine mounts, crankshaft and propeller. Thirdly, until you can really claim to have mastered the snap roll, never, ever perform one unless you have plenty of altitude. If any manoeuvre’s outcome has ever been unpredictable, it has been so with a snap roll. If it is indeed going to be blotched, this will happen at low level!