Hi all
I realy need help with this one
i have a wing design for open b combat, 64 in like the falcon,
this is the 3rd day i am trying to fly it with no results , bad ones, on lunch it will dip in or go up and do a loop, tryed all moving the cg aft and forward, forvard makes it better but toooo sensitive with almost no input. it will fly level and will dip or go up for no reason,
i read a bnunch on cg location i am lost
some help will be great and thank you

The CG on a flying wing should start out at around 10% back from the LE. The problem you describe sound a lot like a WAY too far aft CG.

HTH,
D

I agree. Sounds like the CG is way too far back.

Also, wings are sensitive to CG adjustment, so once you get close, move it in very small increments, like 1/8" or so.

ohh so its only 10% form the LE. i will try that in the morning, let you know what happes,
so the secitivity can be coused form that??

Yes, that kind of pitch instability is exactly what a rearwards CG results in.

10-15% of MAC is typical for flying wings, start around 10 as Darin said.

Be sure to figure out what your MAC is. If the wing has a straight leading edge, and a swept forward TE, then the CG will be farther forward than if you have a swept back LE, for example.

THank you guys for your input

CG is definitely it but also you need to see what your engine incidence is and what your elevon reflex is. As a very general rule your elevons should have about 1/32" or 1/16" up reflex. Your engine should be inline with the wing with no up or downthrust. Wings are highly sensitive to CG changes so once you get it flying, spend an hour or so dialing it in. With every flight things will change. I like my wings on the ragged edge and use airfoils that are not so violent when it comes to snapping out.

Good luck and post pics. Let us know how things turn out.

HI all
went out an hour ago and same story, I have the engine at 0 degrees, the elevons are big about 2 inches wide so they are sensitive but have a 60% expo and only 25% throw. today it just made a big loop, outside loop and will not come out of the loop and hit the ground on the downside of the loop. let me know what you think because i am getting upset.
thanks for the info hayk

If you could post some pics of your plane it give someone an idea. Don't give up sometime it a little tougher than others.

i will post some pics as soon as i get my camera back, other than that i went to fly it same story, the plane looks just like the falcon 72, i have cg at 12.5% from root, the elevon reflex as i understand is the elevon offset to up 1/64 inch( is it have to be taken out after the plane is going) it loops inside or outside at lunch every time, i dont know what to do anymore.
thanks guys

Test launch it without the engine running.

Move the CG forward until is always dives on launch. Then, add up reflex to the elevons until it will launch into a stable glide.

Or:
Set the elevons at 3/16" up from 0. Then, move the CG forward until it launches into a stable glide.

Either method will give you a starting point from which to adjust the CG and reflex to suit your flying style.

HTH,
D

Also:
That's not back far enough.

That's not enough up reflex.

The reflex must stay the same for all flight modes. The trick is to set the CG and reflex so the airplane is stable at any speed, flying just like an airplane with a standard tail.

HTH,
D

i did test fly it without the motor and it glides ok, but on the lunch it will loop up like crazy and its uncntrolable, a have been flying for 5 years so i think it is not all my skills fault but could be,
12.5% FPRM LE IS NOT ENOGHT, HOW FAR THAN.
tHANK YOU

Don't get testy. We're trying to help you.

The first suggestions you got for the CG is to start at 10% aft (back from the front). To find this location, you'll have to find your mean airfoil chord (MAC). If your wing is swept forward or aft, the MAC is swept as well requiring you to find the correct measurement for the CG location.

OK,
If it glides stable on its own WITHOUT ANY INPUT FROM YOU, but won't fly stable with the engine running (at least I think that's what you're trying to say it is doing), then the engine is not set at 0°. -> Set the enine at exactly 0° (no left/right, no up/down) and try again.

What airfoil did you use for this design?

D

Hi Demon
thank you for your input,
i am using a selig, like they use on Avengers, it's semisimetrical, dont remember the number. the plane will glide in a straight path without the engine but will climb like crazy HI it running, now i measured the it and its +/_ 0.5 degrees. i even tried to put 1 degree down trust.
the thing is when i lunch it and get up there it feels like it is stalling every time i make an input. it will flop over,
the only thing i had the cg at 12.5% instead of 10 i will try 10 and i calculated the (MAC) using on line calculator
i am gono take it to school with me and see if i can get it in a tunnel.
thanks hayk

The Selig 8036 used on the Avenger is notorious for being hyper sensitive to reflex and CG location - both are critical on a flying wing as it is. If the 10% CG doesn't solve the problem, you might want to scap that wing and start again with a more stable airfoil.

D

WHat will be a good airfoil?

Clark-Y is a good one. That's what's used on the real Falcon . It's also what I use for the Cobra kit (SSC Flying Wing) that I sell.

i looked at the clark y before, but how do you fit the blladers inside the wing because it is not very thick, i will play with clark y this weekend see what happens, you use full depth spar in your wing??? i have read about the cobra constuction and overall Lees planes, i am using s10 high strenght fiberglass rope which makes a great spar after cured with balsa caps, the wings are very strong, i hit a tree and nothing, i have one covered with rip-stop nylon and it is lot stronger. covering takes much longer and it is lot more messy than type so i dont know which is better. I am planing to go to Paris meet so i am under little pressure to get something to take with me. i have another one that has a tail, i will see how that does.
anyway if i dont make it i will just stop by in Paris and just watch and maybe meet some people that been helping me for so long, thanks guys
Hayk