Trimming a model for flight

supercruiser · Post by **supercruiser** » Fri Mar 25, 2011 8:50 pm

ADW asked about trimming his Arrow for flying. Since that was in the "What paint?" thread, I thought it best to start a new thread to answer his question.

I am by no means an expert on trimming rubber powered models.
My first suggestion is Don Ross's book available in the Building Supplies section at www.guillow.com

Here are the basics I use for my models:
1. balance them as per the location on the plans.

2. test glide them over tall grass. add or remove temporary weight such
as clay to the model to get a nice glide. Some of the Guillow kits come
with trimming instructions. They have helpful information. I
recommend adjustable rudder and elevators using grocery tie wires.
I think it is phugoid, on this forum, that has pictures of this method.

3. Once the model is trimmed for glide; mark the location on the model
where it balances. This is your permanent balance position.

4. Install the rubber motor and propeller. Then remove
or add weight to get the model back to it's original balance position
you found in step 3.

5. Wind the rubber motor about 50 turns and launch the airplane. Most
airplanes will climb steeply, stall, and head for the
ground. Add a small downthrust shim. Here is photo of such a shim
at www.virtuaerodrome.com http://www.virtualaerodrome.com/image_d ... 2&offset=0

Keep increasing the amount of winds of the rubber motor and adjusting the shim thickness. This is where an adjustable nose block
is helpful. Obviously, you want the the airplane to climb without stalling.
Once you get it to fly well with maximum winds on the motor you are
finished with trimming.
5. Wind it up and let it fly.

ADW 123 · Post by **ADW 123** » Fri Mar 25, 2011 9:00 pm

Thanks. I'll post pics of my arrow when I get the chance. Unfortunatly, the basement where I build is off limits because of excessive amounts of fiber glass for the weekend. I'd appreciate if you followed along

StefanJ · Post by **StefanJ** » Fri Mar 25, 2011 10:44 pm

i just started building an Arrow. Specifically, just punched out the stab and rudder pieces.

I'm wondering if it would be worth it to make the rudder adjustable. Add an extra strip of balsa and some hinge wires so the rear 1/3 or so can pivot.

The horizontal stab would be a lot more difficult to modify, since it is a built-up airfoil.

ADW 123 · Post by **ADW 123** » Sat Mar 26, 2011 11:11 am

I'm looking for just adding some washout for a left turn. Any idea about how much I'll need on each wing? I pin down my parts for a jig, but how do I go about shrinking with the washout shim in place? I was thinking I just shrink on side at a time, but keep in mind that this plane has quite a bit of dihedral

supercruiser · Post by **supercruiser** » Sat Mar 26, 2011 4:46 pm

StefanJ wrote:i just started building an Arrow.

I'm wondering if it would be worth it to make the rudder adjustable.
The horizontal stab would be a lot more difficult to modify, since it is a built-up airfoil.

I think an adjustable rudder and elevator is well worth the trouble.

adw123 wrote:
PostPosted: Sat Mar 26, 2011 11:11 am Post subject:
I'm looking for just adding some washout for a left turn. Any idea about how much I'll need on each wing?

I suggest a small amount. A few degrees. A tiny bit does a lot on such a long wingspan.

Phugoid · Post by **Phugoid** » Sat Mar 26, 2011 6:07 pm

The washout adds stability by deacreasing the lift at the tips.

Differential washout can be used to add stability in the turn, it is not a way of inducing a turn unless you want to spiral out of control. Normally for a left turn you put less washout in the left tip. This wants to make the model turn right as it makes more lift on the left. However the the rudder set left will still make the model turn left, but with the differential washout set in the wings it will make the turn nice and flat, without the left wing spiraling in.

Scigs uses another technique which is rather more interesting. He uses tabs which induce drag on the side in which you want to turn, this is like holding the model on an imaginary piece of cord. The tab also makes more lift locally which stops the left wing from dipping also keeping the turn flat.

Try both ways, I'm yet to try Scigs's method myself but I will do soon....

ADW 123 · Post by **ADW 123** » Sat Mar 26, 2011 6:29 pm

i have yet to hear from the man himself on details of that tab. ive seen it before. maybe scigs could also tell a little of that when he gets the chance.