Looking for tips: unwarping a glider body

[aeppel_cpm]

I’m working on a 2.28x Tercel, and I’ve noticed the body has picked up a bend.

https://farm5.staticflickr.com/4813/44558877040_3a64d092d6_z.jpg (https://flic.kr/p/2aTvTJG)

It’s a 3mm bend.

The construction is basswood over foamcore. I’m planning on capping the laminate with balsa. I’ve only done the nose so far.

Any suggestions for straightening this thing out?

[Jerry Irvine]

Moisture and a press/books. If it turns in flight that is a good thing and requires less wing clay.

Just Jerry

[LeeR]

Another option is a heat gun. It works well, with moisture, to get parts straight. If clamping is difficult, you may even be able to hold it for a few minutes in the correct position long enough for cooling to cause it to set in its new position.

If you don’t have a heat gun, you can get them for as little as $10 at Harbor Freight, using a 20% off coupon.

[Ez2cDave]

Moisture and a press/books. If it turns in flight that is a good thing and requires less wing clay.

Just Jerry


I'm concerned that it might "spiral" or "corkscrew", during boost.

Dave F.

[astronwolf]

Moisture and a press/books. If it turns in flight that is a good thing and requires less wing clay.
This. I'll add patience to that.

Maybe if you used something to anchor the glider to your work table in the position shown in the photo, like a book or somekind of weight. Use a rubber band to deflect the boom aft of the wing's trailing edge so that it's straight. You'll have to rig something up to do this. Moisten the boom in the area just aft of the trailing edge of the wing. Don't get the wing wet. Allow to dry overnight, and check for straightness the next day. Repeat if necessary.

[aeppel_cpm]

I'm concerned that it might "spiral" or "corkscrew", during boost.

Dave F.

I'm also more concerned about boost than glide.

Thanks for the suggestions, gents. I'm hoping the capping - turning the laminate into a box - will help stabilize it - but I'd like to get it straight first. I'll try heat before moisture, as I suspect moisture got me into this pickle in the first place. Laminated with TBII.

How I did the body, clamped to a launch rail.
https://farm5.staticflickr.com/4857/44558877080_c2106da3fe_z.jpg (https://flic.kr/p/2aTvTKo)

And the capping around the nose. 1/32” balsa, ammonia shaped.
https://farm5.staticflickr.com/4890/31436489897_9f4759ef35_z.jpg (https://flic.kr/p/PTWedv)

[ghrocketman]

If you moisten wood with ammonia water (Ajax, BoPeep, etc) the wood flexes much easier than with plain water.

[Ez2cDave]

If you moisten wood with ammonia water (Ajax, BoPeep, etc) the wood flexes much easier than with plain water.



The use of Ammonia & Water would work on an all wood fuselage, but he used a foam "core" for the fuselage, with Basswood sides bonded to the foam.

On an all-wood fuselage, Steam could also be used, but I am concerned about how the foam might react to the heat.

I'm wondering if the warping was either caused by a reaction between the foam and the adhesive used or if the fuselage was un-clamped too soon, before it had fully dried / cured .

Dave F.

[ghrocketman]

I don't think ammonia would attack any foam.

[Ez2cDave]

I don't think ammonia would attack any foam.

I wasn't thinking about the Ammonia attacking the foam. I was thinking that since the wood was bonded to the foam, the Ammonia would only penetrate the wood and, as a result, not allow the wood to remain straightened if the foam is causing the warping.

Dave F.

[aeppel_cpm]

I didn't remove the paper from the foamcore, some in some sense it's basswood-paper-foam-paper-basswood.

It's bowed so the middle arches away from the side against the rail during clamping. So the outer side is effectively slightly longer than the side against the rail. I'm wondering if I ran into some differential drying issue - the outer side drier faster?

I switched to laminating wing cores while I ponder the next step. For the wing cores, I'm using BSI 30 min epoxy and vacuum bagging - doing the bottom in one pass and the top in another. I left the paper on the lower side of the wing to help stabilize it - I find the foam warps all over if I take off both sides.

So far, they are behaving.

[Jerry Irvine]

I think we should overthink the problem. :D

[aeppel_cpm]

Anything worth doing is worth overdoing.

[ghrocketman]

Always use WAYYYYY more Epoxy than you think is required.
I prefer to build FLYING TANKS.
They almost never break upon landing.

[Ez2cDave]

I think we should overthink the problem. :D

Hmm . . . Let's "underthink" it . . . Build a new fuselage !

Dave F.

[Jerry Irvine]

Correct.

[aeppel_cpm]

If I must, but I’d like to have a better idea of what caused it, first.

Trying a counterbend tonight.

[Ez2cDave]

How I did the body, clamped to a launch rail.
https://farm5.staticflickr.com/4857/44558877080_c2106da3fe_z.jpg (https://flic.kr/p/2aTvTKo)

I have a theory as to what caused the warp . . .

In your pic, the fuselage is completely supported by the launch rail, on ONE side. HOWEVER, the clamps are providing uneven pressure ( at their contact points ).

As a result, they "pinch" the fuselage, likely compressing the foam core, and introduce a "pre-load" to one side of it.

I suspect that the 3rd clamp ( the Black one, 3rd from the left ) may be the "culprit", due to its location, although all uneven clamping pressure is the prime suspect.

Solution : When you build the next fuselage, use even support and clamping pressure on BOTH sides of it and make sure it is FULLY dried / cured before touching it.

Dave F.

[aeppel_cpm]

I think you’re on the right track - though the third clamp actually has the least grip. It must have a weaker spring. But the one sided part likely was the root cause. Since this was paper to wood, I used Titebond II. I suspect the moisture left at different rates on the two sides. If I ever do this again, I’ll try epoxy.

I tried an overnight bend. Wet with ammonia water, then clamp against the rail with a 1/8th spacer on the nose and a clamp on the tail and nothing between.

It worked. I’m putting the edge balsa on now.

[Ez2cDave]

I think you’re on the right track - though the third clamp actually has the least grip. It must have a weaker spring. But the one sided part likely was the root cause. Since this was paper to wood, I used Titebond II. I suspect the moisture left at different rates on the two sides. If I ever do this again, I’ll try epoxy.

I tried an overnight bend. Wet with ammonia water, then clamp against the rail with a 1/8th spacer on the nose and a clamp on the tail and nothing between.

It worked. I’m putting the edge balsa on now.

Glad to hear you got it straightened out . . . Hope the glider flies well .

Merry Christmas !

Dave F.

[aeppel_cpm]

Do you fly at Bong? It will likely fly there.

[aeppel_cpm]

Some update pictures. I will countersink and cover the nose weights eventually.

https://farm5.staticflickr.com/4879/31975874677_6b81346c0c_z.jpg (https://flic.kr/p/QHAHw6)

https://farm8.staticflickr.com/7837/46864949252_32a159c1bd_z.jpg (https://flic.kr/p/2epi7Eq)

[Ez2cDave]

Looking good . . . When is the first test flight going to happen ?

Dave F.

[aeppel_cpm]

I guess I built too heavy. I can't get her to glide in hand-toss testing. She just flops. Doesn't seem to be unbalanced - just doesn't fly.

[Ez2cDave]

I guess I built too heavy. I can't get her to glide in hand-toss testing. She just flops. Doesn't seem to be unbalanced - just doesn't fly.

Can you shoot some video of what's going on ?

It could be diving, stalling, or "mushing" . . . There might be a wing loading problem ( too heavy) or an "incidence" ( alignment ) problem involved.

Dave F.

[aeppel_cpm]

I'll think about it Dave, maybe I won't give up just yet. I went looking for my wing loading notes, but I can't find them. I'll re-estimate and re-weight and post. Because it may be too heavy to be worth the effort. I thought I was ending up in Estes Sweet V territory, but I might have miscalculated.

I think it's neither diving, nor stalling. (Had to think about the grammar I wanted for a second.) It looks like it has a left yaw that I wasn't able to trim out. Could be that blasted warp. And a left roll. I'll have to revisit the left-right balance (I was focused on the Cg previously) or maybe use clay (eek, more weight) if there is a dLift/dY. And, now that I've looked it up, I think 'mushing' is a good description of what it was doing.

[Ez2cDave]

Charles,

Here is more about "mushing" . . .

https://aviation.stackexchange.com/questions/26932/what-is-a-mushing-glide

Dave F.

[aeppel_cpm]

Yes, I found that article, too.

[jetlag]

Charles,
I balance all my airplanes a bit nose heavy; slight nose heavy is your friend!
R/C, glider, sailplane, boost glider all get the following treatment:

I find the recommended cg location and screw in a small screw eye dead center (left-right) at that point. Then, I hang the model from a string. This way, one can see left/right as well as front/rear balance in one easy step. Adjust counterbalance as needed.
Never fails.
With the warpage, you may then need to build in some aileron influence. Sometimes, a stiff piece of tape to one wing to make an aileron is all that's needed. Just trim the tape to get desired effect/influence.
I suspect your glider to be tail heavy, too. I'd bet a $1 on it!
Do the balance first, then see how she does. You might be pleasantly surprised!

Pretty glider! Looks like a great finish!
Allen

[ghrocketman]

+1 for placing a bet on "tail heavy".
Roll can be balance or warp related.
Yaw is usually caused by fin/rudder misalignment, but could be due to an extreme tail heavy condition or not enough rudder/fin area.

[aeppel_cpm]

Okay:
Mass = 200g
Wing area ~110 sq. in.

Here's the fore-aft balance, suspended from the CG in the Tercel instructions. It's upside down, but still a little tail heavy.

https://live.staticflickr.com/7912/32632463947_ecfb24228f_c.jpg (https://flic.kr/p/RHBUCK)

And here's the left right balance after a repair to the wing attachment. It looks to me like the wing isn't on the fuselage square. I'm going to redo that.

https://live.staticflickr.com/7803/46851052494_be615e3517_c.jpg (https://flic.kr/p/2eo4TD9)

[ghrocketman]

That's quite a bit tail heavy.
Always error on the opposite of that. If it was nose-down that much it would probably glide, albeit fast.

[Ez2cDave]

Useful information . . .

[aeppel_cpm]

I'm at ~9 oz.sq ft, when it looks like hand toss gliders are under 2. I wonder if it's worth trying to save.

[jetlag]

Absolutely!
It will just set up in a fast glide, but you can still get an exceptional sink rate. Fix the tail heavy state, go slightly nose heavy, and it will fly.

Allen

[tbzep]

+1 for placing a bet on "tail heavy".
Roll can be balance or warp related.
Yaw is usually caused by fin/rudder misalignment, but could be due to an extreme tail heavy condition or not enough rudder/fin area.
Pitch/Yaw can be due to off axis thrust with regular rockets, and once I got my hands on a good DSLR I found that off axis thrust is extremely common. It might be a factor on rear engine boost gliders also, but probably not as much on a front mounted motor. After burnout, it would cease to have any effect anyway, and that's the majority of the ascent with most models.

[ghrocketman]

The OP stated these were hand tosses.
It wasn't yawing under power.

[tbzep]

The OP stated these were hand tosses.
It wasn't yawing under power.
10-4. I think I mention it would really affect his model anyway. ;)

[aeppel_cpm]

I did check to see if the earlier warp had returned, and it hasn't. The fuselage is still straight.

We're in for a few days of crappy weather, so I won't be able to make much progress for a while.

I'm thinking that I'll go out to the range on an off weekend and do a test flight before flying at a club launch.

[aeppel_cpm]

Ok,
12g of additional nose weight and 7g on the right wing tip and I can get level, if not great, glides.

Hanging from the frame, she looks left-right balanced. I must have accidentally made the aerofoil unsymmetrical to have such a large left roll.

On to test launches.

[aeppel_cpm]

Okay glider fliers, I would like opinions:

It took 7g of wingtip clay to trim the left roll out of my glider. Would I be better off with the lump of clay on the wingtip, or making an aileron?

My thought is that they would behave nominally the same during glide, but may behave quite differently during boost.

[jetlag]

ALWAYS achieve the balance of the aircraft first.
Then can use a little aileron. You may find a little rudder is all that is needed.
If you use only aileron to correct roll, the effect magnifies as the craft increases airspeed. Increasing your dihedral is another way to make the aircraft seek level flight. If you need more lift, either slide the wing forward a little (if possible) or increase the wing leading edge angle of attack slightly. It's important to do only one adjustment at a time, so you can tell immediately whether the one adjustment you performed worked.
Balance first. Then test fly.

Allen

[aeppel_cpm]

Thanks for the guidance, jetlag.

[ghrocketman]

+1 to what jetlag just said.
Balance first, aileron/rudder second.
That thing sure is requiring a LOT of weight though.

[mojo1986]

+1 to what jetlag just said.
Balance first, aileron/rudder second.
That thing sure is requiring a LOT of weight though.


Yeah, that was my first thought. 19g...........almost 3/4 oz. Yikes! Any suggestions as to how to correct the problems without adding so much weight? I'm particularly interested in this thread because I built a couple of gliders last year and have done test glides, but I still haven't gotten up the nerve to boost them.


Joe

[aeppel_cpm]

Recall, it started off at 200g.

Given that I'm upscaling a Tercel, I don't know that I could have done much to make the fore-aft balance better. I don't think the tail is -that- overbuilt, or that the RC horns are adding that much weight. I think it's part of the design and upscaling it just amplified the noseweight needed. That said, I noticed after testing that I had left a little up elevator trim in the servos from the last test. So it probably doesn't need -quite- 12g up front.

The 7g on the wingtip bothers me more. I sanded my own airfoils out of foamcore and then laminated 1/32" balsa over them. I suspect that I just ended up unsymmetrical in either the original sanding, or in the lamination (it was vacuum bagged) and that the angle of attack is different left vs right. And that's just poor construction skills on my part. To my defense, it -is- my first attempt at a glider this size, or using these techniques. I didn't want to spend the money on a kit, so I scratch built, and am suffering the consequences. Though I did enjoy most of the build itself.

[ghrocketman]

How about "lightening" the heavy wing tip ?
Punch holes in the solid material near the tip then cover with Monokote ?

[aeppel_cpm]

From the hang tests, I don't think the other wingtip is actually heavy - I think I've got a differential lift problem. Which will vary with airspeed.

I don't know that I can take out 7g and leave much of the other wingtip remaining :-)

[jetlag]

You might be surprised.
7g is just 1/4 ounce. GH is right. You can lighten the heavy side, then use less weight on the light side. Your weight difference most probably is due to variation in the balsa density. 1/4 ounce could easily be due to this issue.
Correct one thing at a time. Get that right. Then see what else can be done.
Please let us know how it goes!
I'd be happy to trim it for you, but you'd have to send it to me.
Good Luck!
Allen

[blackshire]

Another option is a heat gun. It works well, with moisture, to get parts straight. If clamping is difficult, you may even be able to hold it for a few minutes in the correct position long enough for cooling to cause it to set in its new position.

If you don’t have a heat gun, you can get them for as little as $10 at Harbor Freight, using a 20% off coupon.Yes--model HLG (Hand-Launched Glider) and catapult glider books recommend using the hot steam (which is really hot water vapor--the true steam is the narrow, invisible portion of the plume closest to a spout) from a tea kettle's spout, combined with gently bending (or twisting, if that's the problem) a warped balsa or basswood part in the direction opposite the warp, until it is straight. (*Deliberately* steam-warping fins has also been recommended for clustered-motor models, to induce a little spin--which evens out slight thrust line misalignments and/or thrust variations between the motors--during powered ascent.) Also:

Stove-top--for gas or electric stoves--tea kettles with narrow-aperture, whistling spout lids (which are opened for pouring out the hot water using an included finger "trigger" on the kettle handle) provide a narrow stream of steam and (above the steam) hot water vapor, which is ideal for such work. If the area of warpage is larger, the stream from the kettle is wider farther from the whistle aperture; one can use whatever height above the aperture that "blankets" the warped area. If the warp is very long, it can be 'steamed and straightened' in sections, until the entire warp is straightened out.

[Blushingmule]

Disregard...