Snap rolls bring all the components of your set-up into play. It is a very simple maneuver to perform, but the most difficult
maneuver to perform well.
By definition, a snap is a high speed stall with yaw. Use full elevator stick to stall the wing and add full rudder for
the yaw…for insurance, add full aileron too. Simple enough, but the twist is in the subtleties! To begin, there are
“real” snaps - like I just described - and there are competition snaps. Real snaps involve a full stall. The wing
becomes a barn door, and the airspeed drops to nothing. If all you had to do is down-line snaps, then real snaps would work
just fine. The problem is, if we do a real snap on an up-line, we drop our airspeed to zero and we can’t get it back.
Thus comes the necessity for the competition snap.
The competition snap is a creation of the full scale pilots. They have a serious shortage of power and often have to do
multiple snaps on up-lines. They need to maintain their energy…that is, airspeed. They take advantage of the fact that
no spectator - and the judges are spectators - can really tell if an airplane is in a high-speed stall or not. Even the IMAC
rule book states that the only thing you are looking for is nose pitch. In other words, the nose has to move in the up (or
down) direction at least a little. If it does, the judge has to give the benefit of the doubt and consider the plane stalled.
So now, how does it work?
To perform the competition snap, you first pump the stick to make the nose move noticeably. That satisfies the judges.
Once you get the nose pitched, relax the elevator to break the stall and add rudder and full aileron in the direction you
need to snap. The wing only stalls for a second, and then streamlines during the auto rotation. The advantages are:
1) The airspeed, and therefore the energy, is maintained - even on the up lines.
2) Because the wing is not stalled, it is much easier to control the stop.
So when is the competition snap appropriate? For ALL up-line snaps. If you are doing only single snaps on a level line,
it is not needed. If you are doing multiple or partial snaps on a level line, the competition snap is easier to control. And
for down-line snaps, the real snap is actually better, since it helps to control the airspeed. By the way, it is ALWAYS best
to use elevator first, and then add the other controls, whether you are doing a real or competition snap. This ensures the
judges will see the pitch before the wings break from level. That is also why you do not find any unlimited pilots using the
snap switch on the radio.
For those just getting started, lets talk a little about the directions to move the sticks to perform a snap. You have
4 possible snaps. The positive two are pretty common sense. The two negatives take some practice to get the motion down. Here
is how the sticks move for each one:
1) Positive left. The elevator moves back (up) and the rudder and ailerons both move left.
2) Positive right. The elevator moves back and the rudder and the ailerons move right.
3) Negative left. The elevator moves forward (down), then the rudder moves right and ailerons move left. Think of it as
the sticks moving “in”…in other words push them towards each other.
4) Negative right. The elevator moves forward, then the rudder moves left and the ailerons move right. This would be “sticks
out”…or away from each other.
As I mentioned earlier, snaps are very dependant on setup…specifically, your control throws are critical. The difficult
part is that many of the variables overlap in snaps. It is often difficult to figure out which control is not set properly
when you are having trouble. Trial is the primary method of making your plane a good snapper. Here are the variables you are
Center of gravity has a big affect on snaps. A nose heavy plane will never get too deep into a snap. Too nose heavy and
you will not be able to get the nose to pitch at all- but that takes a plane that is way to nose heavy for an aerobatic plane.
In general, nose heavy is good when it comes to snaps.
A tail heavy plane goes deep and won’t come out very easily. So naturally, tail heavy is bad for snaps. In fact,
you may want to try a few snaps before you settle on your final CG, as you may have to adjust it a bit forward if you can’t
stop your snaps consistently - even though it “feels“ right in all other aspects.
You need enough elevator to pitch the nose. The problem here is that on and up-line, the elevator has a lot of airflow
from the prop, so it is very effective in pitching the nose. The nose wants to pitch from vertical, since gravity is also
trying to pull it over for you. If you remember, we don’t need the extra pitch on an up-line. To compensate, be sure
to use low rate elevator for up line snaps.
On down line snaps, you have the opposite problem - with the power back, you may not have enough airflow over the elevator
to pitch the nose. This is especially true since you are fighting gravity while trying to pull the nose up. You may have to
use a higher rate elevator to get the nose to pitch for down-line snaps.
Too much elevator makes your snap very deep and “wallowy”. It is also difficult to stop with any accuracy.
This can be confused with tail heavy, so always keep the big picture in view when trimming - and don’t get locked into
thinking elevator throw is always the problem.
The main thing to watch with rudder travel is to not have too much. Your low rate is normally ample for all snaps. Excessive
rudder has the same symptoms as excess elevator…a deep, “wallowy” snap that is difficult to stop. I have
yet to see a plane with too little rudder for a good snap. If you have your rudder set to hold knife edge, then you have plenty.
It usually takes high rate aileron for snaps - but not with all planes. How‘s that for double talk?. The aileron
is the control that regulates the speed of the snap, especially when doing the competition style snap. If the snap is so brutally
fast that you cannot see the plane, let alone stop on a point, then reduce your aileron rate until you can hit your points
consistently. If the snap spins slower than a typical aileron roll, then increase your aileron travel.
On an up-line snap, gravity is trying to pull the nose to the ground. When the snap is finished, the nose will be off from
vertical, and gravity will try to take it farther off. So, on up-line snaps, you actually have to fly the plane out of the
last ¼ roll. This takes some practice.
The competition snap! As soon as you have master the “real” snap, get to work on the competition version. You
will need it at the higher levels of competition, and it takes considerable time to master without getting zeroed.
On many maneuvers, the snap leaves the nose far to one side when you are finished. You then have to gently bring it back
on track using judicious rudder. This makes an ugly looking snap, even though it is technically correct. Judges are supposed
to give you a break on this, but seldom do. You will frequently get hit for it. How do you fix it? Release the rudder and
elevator ½ roll before you want to stop. This gives the tail of the plane time to realign itself on track AND it is easier
to stop the plane on the point. It stops easier because the stall is completely broken by the time you stop the roll. Once
again, this takes some practice.
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