February 1, 1997

By the Numbers

Combat airplane design boils down to basically three things, speed, turning ability, and controllability. A plane that is faster, turns tighter, and goes where it is pointed will fly better and win more matches than one that is slow, turns wide, and flops around in maneuvers. Weíll leave speed and controllability for another day. This article will mainly be about turning ability.

How much lift a plane generates governs how tight it can turn. Lift comes from the airfoil, the wing area, and the speed the plane goes through a maneuver. Of these, the least critical, at least from a lift standpoint, is the airfoil. Looking through literally hundreds of airfoil lift/drag curves, one thing stands out. Symmetrical airfoils like we use are all very similar. They all stall around 12-14 degrees angle of attack, they all generate fairly low drag, up to 7 or 8 degrees. The very best ones might generate as much as 20% more lift than the worst ones. The main practical differences are two. Some of the best airfoils have a reflexed portion aft of the spar and are difficult to build straight. Needless to say, they arenít popular. The shape of the airfoil mainly seems to affect the handling of the plane. The popular combat shape is a very blunt leading edge that delays the stall a bit, and a basically straight line shape aft of the spar that is easy to build and maybe gives a tad better lift than a classic curvy airfoil shape, at least in tight maneuvers.

Wing area is simple. More wing area will turn tighter, at the expense of a slightly lower level flight speed. Combat planes seem to fly best when the plane is trimmed to turn a loop without getting close to a stall. If you look at lift drag curves, you will see that they all show lift going up in virtually a straight line until twelve degrees or so, when the wing starts to stall. However, drag is very low until around 7-8 degrees, when it starts to climb drastically. Letting the plane turn too tight builds excess drag and slows it down drastically. Most flyers find that a disadvantage. Once the motor bogs down, the plane slows and it may take a lap or more for the engine to get back up to speed. In the meantime, youíre a sitting duck.

How fast the plane goes through a maneuver is related to the shape of the wing. Long, skinny wings produce less drag for a given amount of lift. Why do you think gliders have such long wings? It works on combat planes too, up to a point. Longer wings tend to be heavier for a given area, so we have to do a balancing act. The two heaviest pieces of wood in a combat plane are probably the spars. Unless you start using tricky building procedures and materials, every inch of wing span adds a fixed amount of weight from the spars. The best aspect ratio(ratio of span times span divided by wing area), or perhaps I should say, the high practical aspect ratio is 5.5 or so. Longer wings get increasingly hard to keep straight and trim and are morelikely to break. Interestingly, that is the same ratio that most of the prop driven fighters ended up with.

What this all boils down to is that you can easily calculate two numbers for a plane that will give a pretty good idea of how it will perform. The wing loading, or wing area divided by weight, and the span loading, or span divided by weight. Engineers usually reverse those, and put the weight on top, but for easy comparison the numbers make more sense this way. A higher number gives better performance. The table shows several popular designs, including some of my Gotcha series, for comparison. Here are the politically correct, diverse numbers, using square inches of wing area divided by grams of weight!

Plane........................Area.....Wing Load....Span Load....Span......Root......Tip........Weight.......Engine

.................................................................................................Chord....Chord.........gr..............gr.

FAI...........................441..........1.01............0.096...........42.........13..........8...........300............135

Gotcha 550...............540..........0.92............0.092............54.........11.5.......8.5........325............260

Gotcha 500...............492..........0.85............0.082............48.........13..........7.5........320............260

Gotcha 600...............634...........0.95............0.080............54.........16..........7.5........410............260

Arrowplane...............480...........0.83............0.082............48.........12..........8...........320............260

Ready to Fly.............495...........0.80............0.072............45..........14..........8..........360............260

Slow........................480............0.68............0.068............48..........12..........8..........450............260

These numbers were all worked up from actual planes and using similar weights for the engines, so the comparisons relate to each other. Obviously, switching from a Fox to a Nelson and saving half an ounce of weight while getting more power will make any plane fly better. I also observed all of these planes flying in speed limit contests, where they were evenly matched in speed and line length. Obviously, a 15 will not go as fast on 60 foot lines as as most 36ís.

The bottom line here is that a 10% difference in either span loading or wing loading makes a very noticeable difference in performance. The difference between an FAI at 1.01 gr/sq.in. and a Slow at .67 is huge. The FAI ships wildly outperform a typical Slow ship or one of the ready to fly jobs with a 36 on it. Five percent differences, especially if only one number is better, start to get lost in the noise. There isnít a whole lot of practical difference between the three Gotcha series in a match. A slightly better engine setting, a different prop, pilot skill, a different airfoil, and controllability, all start to become equally important.