How to set your center of gravity

When you have a new plane, you most likely also have a case of new plane anxiety.  Your first job is to trim the plane so you can bring it home safely.  A large part of this is done on the ground.  Center all of your controls, set the center of gravity in the correct place according to the plan or the instructions, and of course you built it with the wing and stabilizer at the correct incidence and the engine with the correct amount of down and side thrust, right?

Assuming that you did everything correctly, you will still need to make fine adjustments due to your engine and propeller choice and the final weight of the plane, and to tailor the flight characteristics to your desired flying style.

Put your plane in the air and trim it according to the How To Flight Trim an Airplane article on this website.  Once you have it flying straight, you need to see if the center of gravity is exactly where you want it.  You may recall that the Flight Trim article tells you to cut the throttle to idle and see what the plane does immediately.  If it immediately veers up or down without slowing down, this is caused by the engine thrust angle being inaccurate.

You’re supposed to set your center of gravity to the specification on the plan, but it’s nice to fine tune it in the air.  To check your center of gravity in the air, repeat the same procedure of going from fast cruise to idle without changing your elevator position, and watch what the plane does after it slows down.  If the nose stays high and the plane goes up, stalls, then picks up speed and then repeats the same pattern again, this is known as porpoising.  It is caused by the plane being significantly tail heavy.  If the plane stays nose high, or drops only a tiny bit, and has a flat glide, the center of gravity is somewhere between neutral and tail heavy.  If the nose drops a moderate amount but the plane doesn’t pick up much speed, the plane is in the slightly nose heavy to neutral range.  If the nose drops a lot and the plane picks up speed, it’s significantly nose heavy.

A nose heavy plane can be managed easily by trimming the elevator up when it’s time to land.  This will cause the landing approach to be steeper and faster, which can actually make landing easier by shortening the glide.  You don’t want your plane to run all the way down the runway without landing.  This is why beginners are generally advised to make their planes a little bit nose heavy.  Just watch out for excessive nose heaviness because it will increase the landing speed and the likelihood of bouncy landings.

A plane with a neutral center of gravity will have a slower and longer landing run, and it will be less likely to bounce, but it will be more sensitive to wind gusts than a nose heavy plane.  This isn’t a big deal once you know what you’re doing, and most competent pilots tend to enjoy a plane the most when it has a neutral center of gravity.

A tail heavy plane will glide way longer than you probably want it to before it comes down, and it will be very sensitive to gusts.  A tail heavy plane also tends to be super floaty when you try to flare, usually resulting in the plane flaring, then stalling, then landing on its nose.

One other point to remember is that all of these generalizations are further modified by wing loading and airfoil shape.  Once you get into heavier, faster planes, this entire spectrum simply moves up the scale so that everything happens faster.  In those cases, the fast nose heavy landing is probably not what you want because if your engine quits you won’t be able to get it back to the field before it dumps itself in the grass.  Tail heaviness is even worse than nose heaviness with a fast, heavy plane, because instead of ballooning when you pull up on the elevator, these planes will suffer a hard stall and spin.  A very experienced pilot can use this tendency for spectacular aerobatics demonstrations and keep the elevator down when the power is off, but unless you know what you’re doing, keep the center of gravity in the middle of the range.