Flight Trimming Issue:
 

Hi Guys and Guru's,

I have a Typical Viper set up for Q-500 racing with a Thunder Tiger Pro.40. My problem is a trim issue. The Stab, Wing and Engine inciences are all set as per spec (at Zero I believe). When I fly at half throttle all control surfaces are pretty close to nuetral trim and the plane flies OK. When I go to full throttle then the plane wants to dive. A cosiderable amount of up elevator is added via trim, close to 1/8th of an inch of UP elevator. Now the plane flies great, straight and level. When I go back to half throttle the plane wants to climb now. Of course if I take out the up elevator I added to get it to fly straight at full throttle, but it dives again when the throttle is brought to wide opem again. Whats going on here guys any suggestions. Everything looks good. What should I look for? Lets say I was off in my incidence measurements somewhere., where would you suggest I look? Down Thrust, maybe?

Any info, suggestions and help will be much appreciated. We raced up here in Ellington CT. 2 weeks ago and on Sunday and I took first place in the standard class. I could imagine how much better I could do if I was 'nt fighting the plane so much each heat.

Thanks

Ed

RE: Flight Trimming Issue:
 

Did you check the tail incidence per the instructions or via the details here in the forums? There's a known problem with the tail incidence on the Vipers.

Other than that, it sounds like you've got too much down thrust. On a racer like this it should have zero right or down thrust. The tail should also be set with zero incidence.

Good luck,


Chris

RE: Flight Trimming Issue:
 

i thought all i.c. powered models had to have some right engine thrust to counter act engine torque. true or false ?

RE: Flight Trimming Issue:
 

There is no problem with right thrust. We run right thrust in most all the Vortex's built.

For your Viper. First, I'd double check the engine incidence to the wing. Make sure you don't have a ton of down thrust, and be sure there is no left thrust while you have it blocked up on the bench….favor having some right thrust. If you find something way out with the engine, fix that and go fly it again. But I'd bet your stab incidence is still off some. That has a bolt on stab right? If so, Id' shim that back of that stab up (1/64" at a time), and fly it.. Repeat the shimming process until the "up" elevator trim is gone.

Once you do those two things…report back to us and lets us know how it goes.

RE: Flight Trimming Issue:
 

Hi Guys,

I finally got a chance to get back into the basement to check over the incidence, because it was bothering me too. I worked on trying to check the incidence on the Tail, Wing and the engine. I did it several times to be sure I was correct or as close to being correct. I have 2 Robart incidence meters, but one appears to be flaky at some times. It works good as long as you keep it vertical. I tried swapping them around and as I took several readings this way and I ended up with - 0deg. for Wing and the Tail - with possibly up to a tenth of a degree off, but for most of the readings it was dead on ) deg.. The engine has about 1/4deg. down thrust at best. So all in all I say the plane is pretty much dead on 0deg. thruout.

Just as a side note the reason I was switching back and forth with the incidence meters was because they have been accidentally dropped but I have checked them out using other digital levels (Craftman Digital and Craftsman Laser Level) and as long as they stand vertical they work fine. Its just that I have noticed if you go over to much of an angle with them there is some friction due to the needle rubbing on the inside of the gauge.

Are there any other incidence meters made out there by anyone instead of Robart or have they cornered the market on that?

When I checked the Eng. incidence I took the meter off the bar and since the engine is mounted horizontally I sat the meter on the edge of the bolting flange which stands straight up now since its on its side. It appears to be a good way to go, unless this is not. I tried using the attachment to mount to the crankshaft but even with the bar centered ove the engine the plastic seems to have some play in there so i did not think this was as accurate.

I would welcome any other suggestions, tests or checks.

Thanks

Ed

RE: Flight Trimming Issue:
 

Stick a couple THIN shim washers between the mount and the firewall at the two lower holes, assuming the mount pattern in the firewall is a square. Consider there will probably be some crush of the washers into the firewall and compensate as needed.
Or, you have the materials, just cut a shim for the lower holes from brass or aluminum stock (soda can?).
Then test fly it again.

RE: Flight Trimming Issue:
 

Hi!
Only trust the water level meter!

RE: Flight Trimming Issue:
 

Hi Guys,

I finally got the chance to re-adjust the incidence settings. With all of the advice given which I absolutely appreciate, I decided that since my stab was slightly glued down and screwed down I would start there. I removed the rear bolt and tightened down the front bolt a bit more. I was able to raise the rear of the stab ever so slightly and fill it in with some medium zap. I re-measured the stab and I was able to get the stab almost to zero. I took it out and we did some test flights. Now the plane seems to only have about 1/32nd from 1/8th worth of up trim on the elevator and now the plane is flying much better. At full throttle it was running good. I dropped it down to a fast idle and the plane balloons within 1-2 seconds. It goes up about 50 feet within a few seconds and levels off. I understand this is a design characteristic that was designed into the plane by the designer to give more lift at slow speeds. (If the thread I read it in is correct)

Well the next weekend my son and I headed off to another race in Hadley Mass. in which I flew the plane to first place in 424 standard division. So all in all you can say I am a bit happier with the plane now. The plane does seem to have some very good slow flight/landing characteristicsas well. I feel the plane has good control without that feeling that its going to fall out of the sky while flying slow and making turns, as long as your gentle. All of the landings were nice and slow and easy on the planes airframe.

Now I hear that there is a new version of the Viper-500? Is this True? Does anyone know anything about this and what was done or changed? I would love to hear about it or pass on the thread link.



Thanks again to all of the advice.

Ed

RE: Flight Trimming Issue:
 

Hi!
None of my two Vipers balloon if you reduce throttle...!!!

RE: Flight Trimming Issue:
 

Hey There Jaka,

I read in one of these threads that the designer had built that design into the plane, maybe I am wrong, but it does on both of my Vipers and I have talked other guys who have said that theres does as well to some extent or another. Maybe not quite 50 feet, but enough to see that some floating is going on which bacically just seems like a lot of extra lift was added in for slow flight. I may have exagerated the 50 ft but its hard to tell 200 ft up, but I could see it rise and the feel was very much like a trainer. it does it on both of mine. Even when it settles in after a few hundred feet as it comes in to land it slows down very nicely and lands like a trainer, nose high. Great feel.

Ed

RE: Flight Trimming Issue:
 

Ed, i doubt very much that this model design has built in lift for slow flight, just remember when you have a model that has firstly a full type semetrical wing that is set at zero degrees incidence [ 0 ] plus an engine and tail plane that is set to zero [ 0 ] incidence. how would the model get so much lift like trainers do ? what you should be doing with your Viper is to make sure the engine is set to zero incidence, make sure the COG is correct to specs then un glue the tail section and shim the rear by 1/16 th inch. test fly and trim the model for level flight then take note of the elevon positions after the trimming, if there is still up elev trim you need to shim more rear in and vice versa if you have down elev trim. my Viper is new with only 3 flights so far and all is spot on from low to high speeds, my only adjustment was to shim the rear of tail plane by 1/16 inch.

RE: Flight Trimming Issue:
 

Never had this low speed issue with any quickie I have ever built(for any engine).... They take off and fly full throttle which is what it is trimmed for... low speed = shutoff and land only!


Dan