Trimming Flying Wings

The goal in trimming a flying wing is to get the center of gravity as far aft as possible and still maintain stable control over pitch. Since the flying wing has very little tail moment there is a tendency for the wing to be very pitch sensitive.

I think some sort of exponential control is necessary to smoothly control most flying wings in the pitch axis, while retaining the ability to have large control surface deflection without using dual rates.

I typically set up my wings with 100% neg. expo on both axis. Then I set the deflection to at least 45 Deg. pos./neg. or 90 Deg. total deflection.

The roll axis is not as sensitive. Differential aileron mixing is not recommended because it usually results in pitch input while rolling.

The tail moment on a flying wing is the distance from the elevon to the center of gravity. This is usually not very far so the elevons don't have much leverage, but they are usually very large compared to a horizontal stabilizer. This is why the CG is so critical. If the wing is too nose heavy, it will require a lot of upward deflection of the elevons to fly level.

This results in a lot of unnecessary drag and sometimes a nose high attitude in level flight. It also makes the wing far more stable in pitch response so it is best to start the trimming procedure with some extra weight on the nose and a little reflex in the elevons.

I have used the following procedure for trimming R/C aircraft for some time and it is quite painless. You start by finding a neutral trim for the wing. This is best done by setting up everything as per the instructions (or your eyeball) and putting some additional weight on the nose. I use tape and lead. Then add a click or two of up
trim and either run with the plane or ride a bike (mobile wind tunnel) while holding the plane level.

This will let you know if it is close to its flying trim. At this point I like to use a bungee to simulate a hard throw. I tape a hook a couple inches in front of the C.G. so the plane will take off straight without trying to rotate as if it were on a hi-start.

This will produce a manageable launch that is high enough for trimming. On first launches I hold a little bit of up elevon just to make sure it doesn't slam into the ground. I usually have to put some down input to the wing after it comes off the hook because I always set them up with too much up trim the first time. At this point you will probably add some down trim and start launching harder. Once you have the wing flying level in trim, you can move the nose weight back an inch or two. This will cause it to have too much up trim so the process starts all over again.

I generally keep moving the weight back and re-trimming until I can barely control the pitch, then I move it forward until I like it.
This determines the C.G. and the balance point you want for controllability. Once you are sure about the C.G. you can dial in the wing for It's best glide ratio and speed. This is very simple. When the wing is trimmed and flying level you simply add a click of down trim and watch the wing. It should nose down a bit and speed up. If it pulls out of the shallow dive by itself add another click of down trim and watch it again.

You keep adding a click of down until it stops pulling out of the dive by itself, and then add one click of up trim and you are finished. Most gliders actually have a normal oscillation cycle where they are going through the sky in repetitions of shallow diving
and pulling out, but it is so shallow that it isn't noticeable if the plane is in trim. Get to know the speed that the wing likes to fly at when in level trim. It will probably be much faster than you thought it would be. It is important to let it fly at this speed
if you are trying to maintain a good glide ratio. If you pull back on the stick and try to "float" it, it will sink much faster than if it were at it’s cruise speed.