How to flight trim an airplane

A plane that’s correctly trimmed is a pleasure to fly, but if you don’t know how to do it the process can be a bit of a mystery because there are several factors that influence the flight trim of a model plane.  These factors also influence each other, so it’s important to work them out in the proper order.

STEP ONE: CHOOSE YOUR PROPELLER

Your first goal is to choose the right propeller.  Install the propeller you think you want to use, but remember that this may change.  If your plane is slow and draggy, no amount of propeller is going to make it go fast, so don’t try.  A slow, draggy plane such as a biplane needs a long, low pitch propeller so it will at least have good acceleration.  A racing plane will need a short, high pitch propeller for speed, which will come at the expense of acceleration.  But this won’t be a great loss because the sleekness of the plane will allow it to accelerate faster than a draggy one anyway.

You will also need to make sure your total propeller load is right for the engine.  For instance, if both the length and pitch are too much, the engine won’t be able to run fast enough and will not produce its full power.  Likewise, an inadequate propeller load will have your engine spinning at full power with nothing to show for it.

Here’s an example for the inexperienced.  Conventional wisdom says that a .40 size engine runs best with a 10×6 propeller, and most of the time you can just throw one on and go.  Let’s say you’ve built two planes with high powered .46 engines.  One of them is a biplane and the other is a slick pattern plane.  10×6 is not enough load for a hot .46, so your engine will be racing but your plane won’t be doing much.  Engines such as these can handle 10×7, 10×8, 11×6, 11×7, 12×6, etc.  For the biplane you’ll probably want to start with the 12×6.  Maybe you’ll end up with a 12×5, depending on the exact characteristics of the plane.  At any rate, this kind of propeller will get the plane up to speed immediately, and you won’t miss the high speed performance because the drag of this plane won’t allow it anyway.  For the pattern plane you’ll probably start with a 10×8, which will produce more speed.  Acceleration isn’t such a problem with this plane anyway because of the low drag, so the high pitch isn’t much of a handicap.

I can’t stress this point enough because propeller selection is a topic that a lot of guys don’t even think about.  Most of the time when performance is unsatisfying a larger engine will be installed, which adds more weight, which makes a plane not fly as well.  So always be willing to try different propellers.  For instance, a few years back a lot of planes were equipped with the 40 FP engine made by OS.  This engine has been out of production for a while, but I have a lot of them, as do a lot of other guys, and you can still get some comparable engines, so this is a good example.  Let’s say you have one of these lower powered engines, and you fit it with a 10×6.  Usually this will be a pretty good match for an average plane such as a 40 size trainer.  But I put one on a biplane once and I found that it had better acceleration with a 10×5.  Top speed didn’t change a bit one way or the other.  I found that an 11×4 was the same load as the 10×5, producing similar engine performance, but the plane had more acceleration with the 11×4 and greater speed with the 10×5.  I stuck with the 10×5.

After you build your airplane, propeller choice is the greatest factor in your flying satisfaction, so do what it takes to get it right.  The reason why you get this out of the way first is because your choice will influence the side thrust angle setting.

STEP TWO: VERTICAL THRUST ANGLE

This is the easiest step in the trimming process.  Most planes have a vertical thrust angle specified on the plans, but usually a bit of fine tuning is required.  Fly your plane at full speed, trim for level flight, then cut the throttle.  If the plane surges downward when you cut the throttle, adjust the thrust angle downward.  Of course the plane should be expected to drop off a bit when you cut power, but it shouldn’t surge downward.  On the other hand, if the plane seems to float up when you cut power, you have too much down thrust and should shim the engine upwards a bit.

STEP THREE:  EVERYTHING ELSE

The next step is to find the correct side thrust angle.  Set the throttle at full speed and trim the controls for level flight.  Then cut the engine to idle.  If the plane turns left when you cut the power, you have too much right thrust.  If it turns right when you cut the power, you need more right thrust.  Shim the engine mount accordingly, re-trim the plane in flight, conduct the test again, adjust shims again, test again, etc, until your plane doesn’t turn when you cut the throttle.

Your next step is to find the correct balance between rudder and aileron trim.  Let’s say the plane was always turning left, so you want to trim to the right.  Should you trim rudder or ailerons?  If you trim the wrong one, you’ll notice unusual performance whenever you do anything other than fly straight and level.  Take your plane to a comfortable altitude at high speed.  Cut the throttle to idle and then pull the elevator back to make the nose of the plane go straight up.  Pretend you’re going to do a loop, but you cut the power before you pull up, and you never go over the top.  Just go up.  If your plane goes up and yaws to one side, you need to trim the rudder the other way.  When you return to straight and level flight you will probably need to trim the ailerons to compensate for you rudder trim change.  Repeat this test until the plane goes straight when you cut the throttle and pull the nose vertical.

The reason for this maneuver is to remove the engine torque, P-factor, and spiral slipstream from the equation as much as possible.  Once you get the rudder straightened out and the ailerons trimmed for level flight, it’s a good idea to go back and make sure you really have the thrust angle right.  Fly fast and cut the throttle, and see if your plane turns.  If it still acts funny, go through the complete process from the beginning, always proceeding in the correct order, until your plane will fly straight at any throttle setting.  If your plane goes out of trim when you change the throttle, you know you still have work to do.

Now that you have all of that worked out, the only thing left is throttle and elevator trim. The throttle is easy enough.  Set it so it idles with the stick down and the trim up, and it dies when you move the trim tab down.  The elevator is simply a matter of taste.  Maybe you like to just cruise around.  Whether you cruise at low speeds or high, you’ll probably want to trim for level flight at your preferred speed.  If you like to do constant loops, rolls, stalls and spins so your little pilot loses his lunch, you’ll probably want to set the elevator for a very slight climb.  If you like to fly inverted a lot, this gets a bit more complicated.  You’ll want to set the center of gravity in just the right place so the plane will fly level while upright or inverted with the least required change in elevator trim.

Landing is another maneuver that involves elevator trim.  I always try to emulate the planes I see at small airports.  I set my trim tab so I can adjust a few clicks for cruising, then simply reduce throttle for a slightly nose-down descent, and use the stick to flare.  With a no-throttle engine such as a Cox 049, on some planes you trim down a few clicks when the engine is running, then trim up a few clicks when it runs out of fuel.