Author Topic: Need help on Thunderbird trim problems (Read 1539 times)

Stan Tyler · « **on:** January 04, 2008, 06:03:37 PM »

I've been having trim issues with a Brodak Thunderbird, upright motor version.
The major problem is that inside loops and squares seem to take more control movement than they probably should, but outsides turn so quick that they're a bit unpredictable and easy to over control.

I've checked thrustline alignment, wing and tail incidence and everything is 0-0, so no unusual incidence problems. The total flap/elev movement is 30 deg. max, setup for equal flap and elevator. Bellcrank is a 3" and handle spacing is also 3" line spacing with the line spacing biased to the UP side by about 1/4".

CG is per the plan. Oh yeah, flaps and elevator are fabric hinged to the hinge lines are sealed.

Any suggestions for the trimming experts?

Thanks,
Stan

Bill Little · « **Reply #1 on:** January 05, 2008, 01:15:32 AM »

Hi Stan,

I would try more bias on the handle to start with. A long time ago, I was told that you used 4" line spacing with a 3" BC, 5" handle with a 4" BC, etc., and it has seemed to be a good starting point. Seems you could go out quite a bit on the up line to get some more bias in the turn. Next step would be biasing the elevators I would think. I rmember Ted's Final Edition had about a 1/4" droop in the elevators, so that is an option, but much harder to do if you are not adjustable on your pushrod length.

Dennis Adamisin · « **Reply #2 on:** January 05, 2008, 07:51:48 AM »

What Bill said makes sense, but I would also look for a roll component that can make the bird harder to turn insides than outsides:

* In the rear view are the elevators aligned? How about the flaps? If not, make it so!

* SLIGHT WING WARP: if in level flight the outboard wing is down then insides will be harder to turn than outsides - and outsides will exhibit loss of line tension too! Should be especially apparent above 45 degrees - try an overhead 8 and compare line tension on the inside versus the outside half. When you do a wingover, when you pass over the top the line tensions usually gts a little softer - does the bird exhibit any tendenciy to roll one way or the other?

* How's the line tensions in level flight? does it "go away" above 45 degrees? is it thes same upright and inverted? Again in level flight vers above the 45? If your line tension goes away above 45, add 1/2 oz tip weight and move leadouts forward 1/8" to 1/4" - see what it gets you!

* Try flying slower for a couple flights - at higher speeds some trim problems (like borderline roll issues) get masked.

* STAB TILT: with all your alignment checks you have probably covered this - in front view, check the stab versus the wing; if the inboard tip of the stab is higher than the outboard, it will generate the kind of issues you described. It can be a chore - but your best fix is to fix it!

Oh yeah KEEP US POSTED on your progress

( or lack of same

)

Crist Rigotti · « **Reply #3 on:** January 05, 2008, 09:29:50 AM »

You need to lengthen the elevator pushrod.

EddyR · « **Reply #4 on:** January 05, 2008, 09:42:14 AM »

Sounds like the pushrod is bending!
Ed

Stan Tyler · « **Reply #5 on:** January 05, 2008, 09:46:24 PM »

Bill,

No problem with increasing the bias for the up line. I usually start with the line spacing matching the bellcrank. IE. 3" bellcrank = 3" line spacing. It seems to be a reasonable starting point.

Ed,

Possible that I'm getting some pushrod flex, but since it's a carbon tube pushrod with ball links I think that the flex would be minimal.

Dennis,

Quote from: Dennis Adamisin on January 05, 2008, 07:51:48 AM

What Bill said makes sense, but I would also look for a roll component that can make the bird harder to turn insides than outsides:

* In the rear view are the elevators aligned? How about the flaps? If not, make it so!

Elevators are aligned with about a 1/16" down bias relative to flaps. Outboard flap had a slight down tweek to compensate for a little outboard tip down in level flight.

* SLIGHT WING WARP: if in level flight the outboard wing is down then insides will be harder to turn than outsides - and outsides will exhibit loss of line tension too! Should be especially apparent above 45 degrees - try an overhead 8 and compare line tension on the inside versus the outside half. When you do a wingover, when you pass over the top the line tensions usually gts a little softer - does the bird exhibit any tendenciy to roll one way or the other?

I found that the outboard tip had about 1 1/2 degree positive incidence compared to the root. The inboard tip had about 2 degrees positive incidence compared to the root. I did the twist and heat with a heat gun to try to remove the twist.
In the wingover, I don't notice a decrease in line tension, but I do notice that line tension gets a bit soft on the third corner of the inside square and the same place in the square eights.

* How's the line tensions in level flight? does it "go away" above 45 degrees? is it thes same upright and inverted? Again in level flight vers above the 45? If your line tension goes away above 45, add 1/2 oz tip weight and move leadouts forward 1/8" to 1/4" - see what it gets you!

Line tension seems to be relatively equal in level flight and above 45 degrees.

* Try flying slower for a couple flights - at higher speeds some trim problems (like borderline roll issues) get masked.

* STAB TILT: with all your alignment checks you have probably covered this - in front view, check the stab versus the wing; if the inboard tip of the stab is higher than the outboard, it will generate the kind of issues you described. It can be a chore - but your best fix is to fix it!

Just eyeballing it, I don't see any stab tilt. I'll measure for any stab tilt because you just can't trust eyeballing it.

Oh yeah KEEP US POSTED on your progress ( or lack of same )

Now that I've got the twist in the wing taken out and tweeked the flaps to be equal I see if that improves anything. I'll take it out to Whittier Narrows tomorrow and see what happens.

Many thanks to everyone. I'll let you know what happens.

Thanks,
Stan

gary tultz · « **Reply #6 on:** January 06, 2008, 07:18:47 PM »

I learned the pattern on a 57 Thunderbird (Veco not Ford)

and they had/have a serious vertical C/G problem. With the upright engine and the somewhat low mounted wing, they have a High vertical C/G which causes the plane to fly outboard wing DOWN in level flight. This will hinder your inside turning rate quit a bit and if you go inverted, your OB wing will be UP.Don't try to DOWN out of inverted. Slowly climb high then UP back to level flight. The CORRECT fix for this is to hang the plane by the lead outs from a pipe in the basement. Viewed from the planes front, the out of plumb hang will be very apparent. Install HEAVY enough wheels until the wing is inline with a plumb line hung on the same pipe. This is a MUST trim procedure on every C/L Stunter, especially the old upright engined ones.This is the main reason the designers soon went to inverted engines.The wheels we used (Back in the Day) were the small hub solid rubber vecos not the pretty spoked ones,(too light). The solid ones are seen often in the wheel boxes at flea markets. Don't try to fix the wing down condition with flap tweak. It'll bring the wing up but of course doesn't fix the real problem, and will open up a new kettle of fish. Hope this helps. Rainman aka Gary Tultz

gary tultz · « **Reply #7 on:** January 07, 2008, 01:52:11 PM »

You're welcome. As I should have said more clearly,this is the first and most important part of Bench Trimming that every C/L plane, Scale, Stunt, whatever, should have before its first trip to the field. Gary T.

Stan Tyler · « **Reply #8 on:** January 08, 2008, 11:51:07 AM »

Gary,

Thanks for the information on getting the vertical CG correct. That's one thing that I didn't even think of to check since this is the first upright engined plane that I've built.

I'll hang the bird by the leadouts, and see how far off it is. I've flown a couple of Smoothies and found that they turn better one way than the other. Similar designs could have similar problems.

Last weekend was too wet and threatening rain, so we just flew the profiles. Next weekend looks better for some serious trimming.

Thanks,
Stan

gary tultz · « **Reply #9 on:** January 08, 2008, 10:34:33 PM »

Glad to help Stan.

Author Topic: Need help on Thunderbird trim problems (Read 1539 times)

Stan Tyler

Need help on Thunderbird trim problems

Bill Little

Re: Need help on Thunderbird trim problems

Dennis Adamisin

Re: Need help on Thunderbird trim problems

Crist Rigotti

Re: Need help on Thunderbird trim problems

EddyR

Re: Need help on Thunderbird trim problems

Stan Tyler

Re: Need help on Thunderbird trim problems

gary tultz

Re: Need help on Thunderbird trim problems

gary tultz

Re: Need help on Thunderbird trim problems

Stan Tyler

Re: Need help on Thunderbird trim problems

gary tultz

Re: Need help on Thunderbird trim problems