|Centre of gravity||Dynamic balance||Thrust angles||Aileron differential||Mixing|
Trimming - a matter of balance, thrust angles, control throws and mixes - is key in getting the best performance out of an airplane. I learned a lot from Peter Goldsmith's trimming article.
Centre of gravity
Centre of gravity (CG) deals with the longitudinal balance and is the first step in trimming. It's been a real learning experience with the SBach because of confusion between the specified CG and the previous owner's actual CG.
I am the third owner of this plane. When I got it, there was no ballast weight on the motor box; the ignition battery was strapped there and the receiver battery was strapped down behind the wing tube socket. When I checked the CG it was near the centre of the wing tube at 173 mm from the leading edge (LE) of the wing. This is highly typical for aerobatic airplanes. But one of the previous owners had marked the CG on the wing at 150 mm from the LE and when I found a copy of the manual, that was the specified CG. I set out to meet it and the end result was two receiver batteries strapped to the motor box on the opposite side of the ignition battery... plus over half a pound of lead weight. This put the airplane at a portly 19.8 pounds. But it was at the specified CG!
I flew the SBach for the first time at the beginning of 2013 and it seemed fine, but having never flown an aerobatic airplane before I had no basis for comparison. In retrospect it did land rather hot and seemed a little underpowered. Unlimited vertical was out of the question. So early in 2014 I began to think of more power. To the forums I went.
There were lots of response in my quest for a bolt-in replacement for the DA 50. I was considering which engine to chose when a couple of posts changed my mind. The first informed me that in the past Slipstream, the maker of this airframe, has specified their CGs much too far forward. The second informed me that the DA 50 should be plenty of power for this airplane and I was carrying far too much weight. I thought a lot about the situation and asked for opinions on the subject. I was advised by more than one person to remove a little weight, test fly the plane and repeat until I was comfortable with the performance. The test: Establish an inverted 45º upline and let go of the elevator stick. The desired behaviour for an aerobatic airplane: Continue for a beat, then gently start nosing down.
I took the plane to the field that weekend. The first flight was a baseline with no changes from the previous season. In the right attitude I let go of the elevator and the result was an instant and sharp drop of the nose. I landed and removed all of the ballast weight - 258 g (9.1 oz).
The second flight showed a difference. Letting go of the elevator stick still resulted in an instant drop, but the angle was much shallower. I landed and removed the second receiver battery - 188 g (6.6 oz).
Third flight: Very slight delay, then a pull to the nose that was shallower still. That was the final test for the day. I took the plane back to workshop to check the CG and was surprised to find out that a pound of weight removed from the nose only moved the CG back 9 mm to 159 mm from the LE.
I thought back to when I bought the plane - recall that the receiver battery had been mounted behind the wing tube socket. I did that and rechecked the CG - now we were at 170 mm, very close to the original measurement. I took it to the field for a couple of test flights. Now on the 45º inverted upline, letting go of the elevator resulted in a couple of beats along the same path, then the beginnings of a very shallow pull to the nose. However, the plane was flying just a bit tail heavy. When I got in home that night I moved the battery to just in front of the wing tube. The CG is now at 164 mm behind the LE; a recent trip to the field confirms that's just the right place.
Lateral or side-to-side balance. Next step after CG trim is completed.
Checking the amount of down and right thrust needed for the engine.
Making sure the down-going aileron throw matches the up-going aileron throw.
Setting mixes in the transmitter to deal with certain speeds, attitudes and maneuvers.