The following procedures should solve most all of your trim and balancing problems for your rc airplane. It's very important to make sure that the model is correctly balanced prior to first flight. This is essential if you are to make flight training as painless and rewarding as possible. RC helicopters require that you track the rotor blades when you hover for the first time.
There are three methods to trim a model - adjusting the forces so that it will fly hands off.
1. Inflight - Use transmitter levers to correct any anomolies.
This measure is only temporary, and should be followed by the correct
adjustment to the control neutrals once you're on the ground.
2. Between Flights - Adjust the neutrals. Though not a final solution, the model rc plane can be flown temporarily with offset controls.
3. Permanently - To Trim using this method demands a little patience, but is well worth it in flying pleasure. Check the airframe as follows:
1. First, check for unhindered control surface movement (there should be no slop). If the control surfaces won't center properly, the plane will never fly straight and level.
2. Ensure the plane is assembled the same way every time. If rubber bands are used to hold the wing in place, then mark the wing and fuselage to ensure correct positioning each time.
3. Ensure the fin is centered with the stabilizer, and that the stabilizer is level with the wings. If the horizontal stabilizer is low or high on one side, the model will screw in loops.
4. Make sure the trim and balance point of the model, right/left CG, falls on the mid span of the wings. If this is not achieved, the model will only fly straight and level at one speed, and will always attempt to roll out of an inverted flightpath. Do the following to achieve this:
a. Stick a pin in the top of the rudder and attach a hook to the center of the nose, and secure the hook with rubber bands. Let the model hang freely. To get it balanced, add wood screws into the tip of the foam wing, opposite from the low wing.
b. Once the balance is achieved, take the plane into the air and trim it there. While in the air, you'll adjust the control surface throws and trims. You will also further fine tune the balance.
The following chart illustrates faults and subsequent adjustments for high wing models (trainers) without aileron control to check the balance of the model:
|Trim the RC Model|
|Tail is Heavy|
|1. Airplane over responds to elevator commands?||Yes - CG should be moved forward 1/4 inch|
|No - Wing trailing edge should be packed up 1/16 inch.|
|a. Only when you open the throttle?||Downthrust should be increased 1 degree|
|b. Only when you close the throttle?||Downthrust should be reduced 1 degree.|
|Nose is Heavy|
|2. Airplane responds well to elevator inputs?||Yes - Wing leading edge should be Packed up 1/16 inch.|
|No - CG should be moved back 1/4 inch.|
|a. Only when you open the throttle.||Downthrust should be Reduced 1 degree.|
|b. Only when you close the throttle.||Downthrust should be increased 1 degree.|
|3. Flies sharply left when full throttle.||Right thrust should be increased 1 degree.|
|Flies sharply right when full throttle.||Right thrust should be reduced 1 degree.|
|a. Permanent trim needed, constantly goes in the same direction.||possible tailplane warp, wing warp, wing, fin, tailplane or nosewheel misalignment.|
|b. Drops wing in stalls, screws out of loops, but not (3) or (a).||Lateral balance should be checked, and corrected by adding weight to one wing tip.|
If you decide to move up to an aerobatic trainer that's equipped with ailerons, you'll need to do the following trim checks:
Once you're in the air, trim it with aileron and elevator until it will fly straight and level using full throttle. After you've done this, it's a good idea to fly it for a while to make sure you're satisfied with the control throws, i.e. they respond smoothly with no jerky movements but with plenty of 'pep'.
Next thing to do is to check out in the air the balance and trueness that you did earlier on the ground.
|Trim Aerobatic RC Planes|
|1. Fly airplane inverted with down elevator to keep it level.|
|a. Right aileron input needed to fly level?||Yes - Left Wing is heavy. Balance with wood screws.|
|b. Left aileron input needed to fly level?||Yes - Right wing is heavy. Balance with wood screws.|
|2. At full throttle with wings level, pull into steep vertical climb, then neutralize stick.|
|a. Model climbs steeply with noticeable roll and falls off to left?||Yes - Feed in a little right rudder trim and retrim the aileron (if needed).|
|b. Falls off to Right.||Yes - Feed in a little left rudder trim and retrim the aileron (if needed).|
|c. Model climbs straight from level flight?||Yes - Plane should now fly straight using same aileron and rudder trim at all speeds.|
|No - Ensure tailplane is true and engine is pointing to front.|
|d. Plane tries to pull over as it attempts to climb, then climbs fairly straight?||Yes - Check to make sure wings are level to tail. Make these checks before proceeding any further.|
After you trim the plane enabling it to pull up straight, take a look at the position of the rudder while the stick is at neutral. If the rudder is offset quite a bit, and there is a lot of trim on, you'll probably need to adjust the engine thrustline. If the amount of rudder trim is set to yaw the plane to the left, you'll need to shim the engine so that it points to the left. If the amount of rudder trim is set to yaw the plane to the right, you'll need to shim the engine so that it points to the right.
Now you should recheck the forward and aft balance. You're trainer will come to you with the trim set for speed stability. It will probably be nose heavy, and the tailplane will probably be set to give down force. You'll most likely want to set up your new plane the same way starting out. Once you are used to the way it handles, you can make it less speed stable and more neutral.
This is a cinch to do. Just move the CG back a little at a time, until you have it right. Remember, everytime you move it back, elevator power increases. Because of this, you may decide to attach the pushrod clevis out further on the elevator horn. You can easily tell when it's right by rolling the plane inverted at full speed. It shouldn't need much downstick to keep it level. Another thing to remember is that the plane will be more tail-heavy when the fuel tank is empty, so always do your trim and balance checks with the fuel tank near empty.
If these trim steps are followed, you can count on having a model that will fly where you want it to fly.
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