Control Line: Aerobatics

Author: F. McMillan

Edition: Model Aviation - 1997/03
Page Numbers: 139, 140

Control Line Aerobatics

Frank McMillan 12106 Gunter Grove, San Antonio TX 78231

Introduction

I'm going to tackle a difficult subject: trimming to make the best-flying airplane.

I've discussed almost every variable in a control line stunter, but there is one more useful area in evaluating the trimming process. The real question is how to define that area—the ideal airplane. This is certainly a matter of opinion, and I know we'll have responses to what follows, but here it goes.

At the end of the flying season I had the opportunity to test several fliers' models. The striking differences in the way the aircraft were set up prompted me to reflect on trimming and a situation from my own experience of making tuned pipes work.

In each of these cases, and I suspect many more out there in stunt land, a baseline was missing. How do you know what your goal is unless you've experienced it or had it defined for you?

Trimming is relevant not only to the competition-minded flier but to the sport flier as well. Improving your combination improves your flying enjoyment and provides positive problem-solving feedback.

What the Best Models Feel Like

During the last few years I can recall conversations with Bob Hunt about what the really good models felt like when you flew them (instead of just watching them). Watching a good flier can be deceptive when you're trying to evaluate how good a particular airplane is, because a good flier can make a marginal airplane look superior when it isn't.

A heavy or inaccurately constructed model will never be outstanding, even though we have seen some heavy aircraft fly acceptably. But what is it like to get your hands on an exceptional combination?

Bob said that during flights he is never able to see a flying surface. This may sound strange, but it makes sense—the wing was adjusted to ride level at the optimum line rake. That means the wing was aligned and leveled, and the tip weight was measured to compensate for the lines. Throughout the pattern, the airplane tracked.

Well-adjusted models track where they are pointed, and they stay there. In fact, you can tell if an airplane is going to be really good by how well it flies level, upright and inverted—given that the center of gravity (CG) is acceptable.

Each model has an acceptable CG range, just as it has ranges for tip weight, line adjustment, rudder adjustment, and so on. A good airplane will lock into a nice groove and require no effort to maintain, whether the model is upright, inverted, or in heavy wind and turbulence.

I want an aircraft to track in the rounds, entries, and exits; to do this, the model must go where I point or guide it. Now I am into an area determined by personal preference and/or the individual airplane.

Control Pressure and CG

To define how a model should turn, use the term "control pressure." In level flight, control pressure maintains track. To be consistent in maneuvers, a model should be adjusted so that the inside turn and outside turn require equal control pressure.

Equal turn-rates and balanced pressure, which are interrelated, are also important. Personal preference determines how much pressure is exerted.

A forward CG will generate heavy control pressure.
A rearward CG will generate relatively lighter control pressure.

(I'm ignoring control system ratios here, even though they do have an effect.) My preference is to set up the airplane for light-to-moderate pressure—but to retain that pressure throughout the maneuvering hemisphere is difficult.

There are many airplanes with heavy line tension and control pressure below 45°. The pressure diminishes drastically in the vertical and overhands, especially in the wind. Some of the large variances can be balanced to trade heavy tension for low tension/pressure, and equal pressure can be achieved by adjusting the leadouts forward.

The ideal airplane is neutralized in turn-control pressure, and with minimum effort the tension goes where you place it; however, I haven't yet talked about cornering maneuvers.

Cornering and Maneuver Precision

You can get aircraft to groove, track, and fly well by moving the CG forward, but the hard part is to also get an acceptable corner. A really good airplane will maintain the same positive turn while showing a sharp corner. Bob Hunt's comment about Bill Werwage's models not showing the wings anywhere indicates that in the cornering maneuvers, the wing is not throwing a tip or rolling.

Furthermore, the model turns and stops quickly to establish the required flats in the squares. This is not easy because rapid turns establish momentum that must be stopped and pinned instantly. This requires the pilot's ability and an excellent, properly trimmed aircraft.

When considering the cornering requirement, think back to the neutral, equal-pressure airplane. It's going to be much easier, as well as more precise and more consistent, to fly the inside corners than the outside corners.

Practical Advice on Trimming

Everything I've discussed is determined by feel and personal preference; however, every model has a range of adjustments set up to individual tastes to make it perform well. You need to explore the available adjustments and find the extremes so you can optimize your model's performance. For the airplane to go exactly where you want, the neutral feel is important.

Key adjustment areas to explore:

Center of gravity (CG)
Tip weight
Line adjustment and line rake
Rudder adjustment
Leadout position