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By Martyn Gosling 
Chess is an ancient game of skill and strategy with beguiling traps for the unwary. One such trap is known as "the poison pawn" where the queen captures the pawn just ahead of the knight on the opposing king's side. It looks dramatic and great gains such as the capture of a rook and knight look ripe for taking. In reality, the queen has been sent to her death, unable to extricate herself from the corner, while the opposition gathers position and momentum for a usually lethal counter attack.
Aerobatics is also a game of skill of strategy and its poison pawn is the snap roll on the top of a loop. This manoeuvre is a high K factor combination that looks dramatic and great gains are surely ripe for the taking. The judges and our rivals are must surely be impressed by such flying. In reality, it is probably more responsible for low scores and zeroes than just about any other.
The effect of the poison pawn can be judged from our very first national championships in 1985 where virtually every pilot included a snap roll on the top of a loop in their Free sequence. The next year nobody did. Flying Advanced a couple of years ago, one pilot won the Known and the Unknown but his Free included this manoeuvre and it contributed to a score so low that he lost the contest Why? What had gone wrong, and why do sensible pilots avoid the poison pawn? It combines two manoeuvres that are deceptive. We can all fly a loop, right? Okay, now how many can fly a perfectly round loop? And how many of us can time the snap so precisely that we will centre it on the top of the loop every time?
It also takes a long time. And the judges have all the time they need to watch you fly the snap too soon or too late, watch it distort the shape of the loop, and mark you down from a perfect score. The few other manoeuvres that require such timing precision include rolling circles and horizontal eight. And guess what? We'd avoid them too if we could, and for all the same reasons.
However, they are included in the catalogue. Somebody may be bold enough or desperate enough to cite one for the Unknown, and they are included often enough in Known sequences by the International Aerobatic Club designers. It is useful to know how to fly them, where the mistakes will creep in, and how to avoid them. If you cannot resist the position pawn then at least know how to extricate yourself from the corner.
A snap roll rapidly increases the angle of attack of the wing with resultant huge increase in drag. This configuration will rob the aircraft of energy. One of the secrets to the snap roll on the top of the loop is to remember the aircraft is flying far slower than you'd normally initiate a snap, and on completion has so little energy remaining attempts to control the aircraft may be ineffective.
Some people counter this problem by initiating any snap by leading with rudder. Once yaw is induced, then they bring back the stick. This is particularly effective on the top of the loop. But the key to the snap - in any location - is to move the stick hard back, and once the aircraft starts rotating push the stick hard forward. This greatly reduces the angle of attack and subsequent drag. The aircraft emerges from the manoeuvre with higher energy and is more readily controlled. At the top of the loop the forward stick position also means the aircraft emerges from the snap with a nose altitude higher to the horizon, and this helps retain the shape of the overall manoeuvre. (If publishing the stick forward stop the roll too soon - remember to roll erect. NEVER PULL THROUGH). 
In something like a Pitts, it is easier to fly snaps to the right, rather than the left, because the engine torque then helps to stop the snap. Nowhere is more the case than on the top of the loop.
As far as fun flying and displays are concerned, the snap roll on the top of the loop is hard to beat. But bear this in mind. There was a great and wealthy chess player who bequeathed everything to his chess playing son. There was one proviso. The son would forfeit should he ever succumb to temptation and take the poison pawn. So far nobody has made me a similar offer, but the moral of the story is clear - for competition flying snaps on the top of loops are something to avoid. - Reproduced with permission of New Zealand Aerobatic Club Magazine |
