Rubber Powered Free Flight Airplane Models: How to Trim for the Perfect Flight
Free flight airplanes can be powered by rubber, electric motors, glow or diesel, even CO2 motors. Since they don't have a radio for control, they are generally easier to build and lower cost than most RC airplanes.
However, they can be just as challenging and fun. The airframes must be built well and adjusted (through a process known as trimming) to be able to fly predictably on their own. This part of the hobby also tends to give us a bit more exercise, and is great for bonding with kids.
Building
The airplane must be built straight, or it will not fly straight or efficiently. Follow the instructions and plans carefully. Make sure that the wing, fuselage and tail surfaces are not warped. Hold the plane at eye level and compare the wings, make sure there is no twist as you go from one wingtip to the other. A twist can be undone by holding the wing over a steaming pot of water (use oven mitts!), and gently bending in the opposite direction. Hold the opposite bend as you remove from the steam and let it cool for a few seconds. In a pinch, warps can be removed at the field, by using the same technique but using your breath to heat the affected area.
Also make sure that the parts are aligned correctly. The vertical fin and the horizontal stab should be at 90 degrees to each other. Use a stationary triangle to check this while you're gluing. Also the stab and wing centerlines should be at 90 degrees to the fuselage centerline. It's also helpful to measure from each wingtip to the nearest horizontal stab tip: the distances on each side should be equal.
If you follow the plans correctly, you will build in the correct angle of incidence. This is the angle of the horizontal stab relative to the wing. For most free flight planes, the angle will be zero or 1 to 2 degrees negative (i.e. front of horizontal stab angle downwards slightly).
Glide
The free-flight plane really performs two types of flight on every trip: (1) the powered phase and (2) the gliding phase. These must be adjusted for separately.
The angle of incidence needs to be correct, per the plans. If it's not, add some shims to the front or rear of the horizontal stab to get it correct. Then, check that the center of gravity (CG) is correct. Balance the plane by putting one finger under each wing, near the fuselage. The fuse should hang level. Note the position of your fingers. Add weight to the nose (common) or tail (not as common) till it balances at the correct point. If there is no balance point on the plans, as a starting point try 30% of the wing width (chord), measured from the leading edge of the wing.
Now for the glide tests. Pick a day and time with very little wind. Toss the plane forward, at the horizon, into the wind. Don't throw upwards! Just straight forward at the horizon.
Observe the plane's behavior carefully. If it dives to the ground sharply, it's nose heavy. Add a small amount of up elevator trim and try again. This is done by bending the rear of the elevator tabs upwards. If your plane has no adjustable tabs, you can add some from stiff paper or aluminum sheet from a can. If the up trim is not effective, reduce the nose-weight or add a bit of tail-weight.
If the plane zooms up and slows, then drops the nose suddenly (and repeats), it's stalling. This means it's tail heavy. Add a bit of down trim. If down trim is not effective, add some nose weight till a nice long glide is obtained.
If the plane tries to turn while gliding, you may not get a good feel for how well the glide is adjusted. So add some left or right rudder trim to get a straight line glide during the test phase.
Power
After you get a nice even glide, you will need to adjust for the powered portion of the flight. Start by winding in a few turns on the rubber motor, then test fly. Increase the number of turns and fly again. You will probably find that the plane has a tendency to "zoom" up somewhat under power, leading to a stall and a "porpoising" flight. To fix this, add a shim between the nose-plug and the front of the plane, at the top of the nose-plug. This may be a piece of wood of only 1/32 to 1/16 inch thickness, but it could have a major effect on the flight performance. See the Pilatus Porter plan for a drawing showing the shim position.
If the plane zooms down suddenly under power (less likely), add a shim at the bottom of the nose-plug.
Turning
The plane will almost certainly turn while under power (very often to the left). This happens because the plane tends to roll in reaction to the turning propeller. The turning circle is actually a good thing for two reasons: (1) you'll need to walk less to pick up your airplane! and (2) if you launch into a thermal, a circling plane can stay centered in the thermal, and you will get a free ride from Mother Nature.
Adjust the rudder trim as needed to get a nice wide circle under power.
Now you're ready to crank in the turns and go for the long flights!