Profile Twist

[Motorman]

I just built a profile "Kell". I put CF laminate on both sides, painted with gloss white Rustolium and it was very stiff. When I glued the stab on it was so parallel with the wing we were writing songs about it.

I took it out for the maiden flight and it flew good. As I was putting it away, I sighted the stab with the wing and dog gone it the inboard tip of the stab was up about 1/4".

I weighed it down for a couple of days but it just springs right back. Do you think if I make a razor slit in the fuselage under the stab then twist and glue it would straighten it out. Maybe if I cut the whole thing off glue joint and all and re-glue? How do you usually fix this?

The stab is covered with bias fiberglass and has CF laminate top and bottom spanwise and there is allot of epoxy in the joint with epoxy/ micro balloon filler and all. I really don't want to dig into all that.

So, if the inboard tip of the stab is tilted up where will that show in the pattern. I'd like to see a flight problem before I do anything.

Thanks,
Motorman

[Ken Culbertson]

I just built a profile "Kell". I put CF laminate on both sides, painted with gloss white Rustolium and it was very stiff. When I glued the stab on it was so parallel with the wing we were writing songs about it.

I took it out for the maiden flight and it flew good. As I was putting it away, I sighted the stab with the wing and dog gone it the inboard tip of the stab was up about 1/4".

I weighed it down for a couple of days but it just springs right back. Do you think if I make a razor slit in the fuselage under the stab then twist and glue it would straighten it out. Maybe if I cut the whole thing off glue joint and all and re-glue? How do you usually fix this?

The stab is covered with bias fiberglass and has CF laminate top and bottom spanwise and there is allot of epoxy in the joint with epoxy/ micro balloon filler and all. I really don't want to dig into all that.

So, if the inboard tip of the stab is tilted up where will that show in the pattern. I'd like to see a flight problem before I do anything.

Thanks,
Motorman

With a 1/4" you are going to have problems on all insides.  Good news is that the top of the hourglass will look nice.  I like the slit.  Chances of warping the fuselage back and have it stay aren't good.  Better to let it settle then slice it open.  I have done several by slicing one side and inserting a wedge.  Good luck - ken

This may sound silly but where was the sun?  Until they get all gassed out sun on one side for a long time can start twisting them.

[Tim Wescott]

I hate to disagree with Ken, but he's a long way away from here, so I'm probably at least physically safe.

Try fixing that sucker with heat.  Something made it bend -- see if you can bend it back; if that doesn't work, then whack it up and glue it together.

Get it really warm (until the Rustoleum blisters, then back it off 10 degrees!), twist it past straight (i.e, 1/4" bias the other way) let it cool, and see if it did any good.  If it makes no difference then yes -- get out the saw.  If it does make a difference, it should be apparent whether you can fix it.

My experience with twists like this is twofold: first, you want long slow heating -- you're not in it to bubble the paint, you're in it to get everything under the paint as warm as you dare; second, if you get it perfect for this month's contest, then by next month's contest it'll have relaxed a bit and you'll have to do it again.  Eventually it'll settle out -- especially if you start by judiciously overshooting the mark, so that the morning after it's a bit twisted in the opposite direction -- do that, and if you're lucky then by the week after it'll be just about straight.

Actually, it's threefold: I did have one plane that I just couldn't straighten.  I have no clue what that thing is finished with, but it does not budge with heat.  That one I ended up cutting a wedge out from under the stabilizer and straightening it Ken's way.

[Motorman]

Thanks guys, that's the stuff I was looking for.

In flight it acts like too much tip weight even after I took all the tip weight out. It came out pretty light at 45oz for a 575 sq in plane. Does this mean the side engine and tank would have a bigger influence? I see a little bottom wing on insides and a little top wing on outsides. It has good line tension, and I moved the slider forward during the 3-4 times I flew it.

Had to stop flying because the inboard wheel kept sliding up the wire and locking on the bend. I soldered a couple of washers to keep the wheels straight so I'm ready for the wind to die down.

Do you ever have to add weight to the inboard wing on a light profile?

Motorman

[Dennis Adamisin]

Do you ever have to add weight to the inboard wing on a light profile?

Motorman

If that's what it needs, that's what you do!  I am encountering that on some of my Spectra changeovers.  I have (had) a habit of building in some tip weight and using the weight box to fine tune.  However with Spectra the amount of tip weight needed is tiny.  Some of the built-in weight is virtually inaccessible and adding a slug to the I/B is the easiest "weigh" out.  BTW, have you weighed the outboard wingtip so you can see exactly where you stand?

I also vote for heat before resorting to surgery.  However, I remember a piece Bob Zambelli wrote for CLW magazine where he was able to twist the fuselage on his Argus using hot towels.  He propped up the fuselage and preloaded opposite twist with a little overshoot.  He applied the hot towels and let them dry  If you are getting that hot weather we got last week you could do it outdoors and the towel would dry very fast.  I think he did a couple cycles of that and was able to get it done.  I also think it would be good to apply the towel on one side, then for the next cycle towel it on the other side.  Best news is that you will not blister the paint this way.  Of course a serious heat gun is hotter & faster, but a wee bit risky for blistering paint.


Of course, there is always surgery...

GOOD LUCK!

[Brett Buck]

I weighed it down for a couple of days but it just springs right back. Do you think if I make a razor slit in the fuselage under the stab then twist and glue it would straighten it out. Maybe if I cut the whole thing off glue joint and all and re-glue? How do you usually fix this?

  I would use the boiling water/towel trick, and if that did not work, cutting it is the obvious alternative. I have straightened full-fuse models built to my standards (i.e. like a bridge girder) using the boiling water and towels, it's pretty hard to beat.

     Brett

[Motorman]

I Fixed it! It was a little disturbing how little it took to move it. I just weighted it down and warmed it with a monokote heat gun. Didn't take long and Rustolium was ok.

After doing that, I know what happened. I took it out on a hot day and the twisting forces while flying put a warp in it.

It's two 1/4" slabs epoxied together, carved to an oval cross section and it has two strips of CF laminate in a cross pattern between the wing and tail on both sides. The CF was glued with CA if that matters. If feels rather stiff if you try to twist it.

Don't understand why it's so vunerable. Does CA glue soften that easy with heat?

Thanks,
Motorman

[Tim Wescott]

Dunno if it was the CA or the laminate.  CA lets go with heat, epoxy will let go with heat (possibly at different temperatures depending on the resin).  I know that at least some CF arrow shafts are made of CF and polyester, because I could smell it when I cut them down for pushrods -- I also know that cheap polyester resin can have an incredibly low (barely above boiling water) glass transition temperature.

But something gave a little bit, and isn't that a good thing?

[MikeyPratt]

I Fixed it! It was a little disturbing how little it took to move it. I just weighted it down and warmed it with a monokote heat gun. Didn't take long and Rustolium was ok.

After doing that, I know what happened. I took it out on a hot day and the twisting forces while flying put a warp in it.

It's two 1/4" slabs epoxied together, carved to an oval cross section and it has two strips of CF laminate in a cross pattern between the wing and tail on both sides. The CF was glued with CA if that matters. If feels rather stiff if you try to twist it.

MM,
I don’t think it was the CA for this reason, CA glue soaks deep into the wood for its bond.  If the wood was heated to the point of burning the wood, then maybe.  I think it was the epoxy and glass covering that was applied, then the heat could affect it and the paint maybe.  I’ve never seen that happen to any of the models I’ve built over the years.  One of the things I do is once the stab is installed and level, reinforce the the bottom of the joint with 1/4” triangle stock to increase the foot print area of the glue joint.  Sand and couture the triangle stock fore a better look (almost fillet style).

Later,
Mikey 

[Ken Culbertson]

I Fixed it! It was a little disturbing how little it took to move it. I just weighted it down and warmed it with a monokote heat gun. Didn't take long and Rustolium was ok.

After doing that, I know what happened. I took it out on a hot day and the twisting forces while flying put a warp in it.

It's two 1/4" slabs epoxied together, carved to an oval cross section and it has two strips of CF laminate in a cross pattern between the wing and tail on both sides. The CF was glued with CA if that matters. If feels rather stiff if you try to twist it.

MM,
I don’t think it was the CA for this reason, CA glue soaks deep into the wood for its bond.  If the wood was heated to the point of burning the wood, then maybe.  I think it was the epoxy and glass covering that was applied, then the heat could affect it and the paint maybe.  I’ve never seen that happen to any of the models I’ve built over the years.  One of the things I do is once the stab is installed and level, reinforce the the bottom of the joint with 1/4” triangle stock to increase the foot print area of the glue joint.  Sand and couture the triangle stock fore a better look (almost fillet style).

Later,
Mikey

Nobody does structural fillets much anymore.  They are a lot of work.  1/8" bead of micro-balloons or SuperFil and that is it for most of today's ships.  Virtually zero additional strength.  I am a bit more old school, maybe because I am old.  I put huge (by today's standards) balsa filets on both the stab and wing. 1/2" on the wing, 3/8" on the stab.  Sand to fit then glue.  Round them with a sandpaper dowel, then some superfil to blend them in.  As Mickey says, it adds to the base that the stab sits on.  Big Base, less twisting in flight.  Another place to put fillets on a profile is the wing TE.  I use a large horn and add as large a fillet next to the fuselage as I can get away with, usually about an inch.  That moves the first place that the fuselage can flex past the wing instead of the the flap horn hole.  If nothing else put the blocks under the stab.

Ken

[Steve Helmick]

It would be tempting to make profiles with a bolt-on stabilizer. A simple rectangular plywood platform and 4 each 2-56 socket head screws should do it just fine if you remember to put in all the screws and tighten them. Then, you could apply shims as needed to remove stab tilt or change incidence to that magical +.001 degree. I think I'd put them in from the bottom, into the stab, and not through the stab and into the platform. 2-56 T-nuts are available. Put those big fillets under the plywood stuff.  Steve

[kevin king]

Hopefully it doesn't throw
rudder off center.