Flap spring help

I am setting up the flap bellcrank on my 2+2 wing; everything will be stock PA-18 set up. I looked at the wing drawings and they show a spring with a cable. It seems this is done just with a spring, no cable. Univair shows just the spring, U14384-000HD; is this the one that I need; no cable? Any other supplier for the springs? Also, how is the spring attached to the bellcrank and to the spar; just a simple metal tab?

Thanks,
Marty57

The spring is attached to the bellcrank under the bolt head (shows a cable shackle). For attaching to spar, I made a little tab. I didn't use a cable but it would be nice to be able to tension the flap springs when needed. They can go slack over time. I also bought my springs from Dakota Cub.

Marty, just a spring according to the info I have. However it attaches to the bellcrank at the same point the flap actuating cable attaches, so maybe that's what you're seeing??? The other end hooks onto a tab which is bolted to the spar web. My IA had me preload the spring a little bit more than the drawing showed -I think he said an inch extra initial stretch? That is because the flaps tend to droop a little bit otherwise. If you want, I can dig out my STC drawing and scan that part for you.

Gordon,
I just pulled up the picture you sent me and I see that's all it is, just the spring. The Super Cub drawing shows a cable and spring but that didn't agree with the picture you sent me, spring looks better. Thanks Bugs, I'll check Dakota Cubs. Any reason I can't extend the thread on an AN 4 bolt, cut and weld into a 3/8" X .035 tube to make the push-pull tube? The drawing shows 1" of thread using a AN490 ($14 X 4, cheaper to buy tube from Univair at $29).
Thanks,
Marty

Dunno how well AN bolts take to welding - if TIG welding I'd do a 400 deg pre-heat, then try destroying a couple of test pieces just to be sure it is plenty ductile.

Marty, I have 2+2 built in 1986 and recommend you preload the spring as Geezer recommends. My flaps are a bit slack on the ground but operate fine when flying and they almost fully retract when operating at the tie-down. It's seen twenty-five years of operating now so it isn't so bad, but a slight retract spring preload would have kept them tighter. I've owned it for twelve years and it's a great fat-body cub, bush workhorse and very capable at hauling a load, bulk and weight.

Phil

Thanks guys, preload makes sense. I guess I'll order the springs from Univair unless a better source comes across. I'm not worried about welding the push tube, that's what's shown in Bingelis book; just cut off the head of a bolt and weld into 3/8" tube. The problem is not enough thread on standard AN bolt. I'll have to get the AN490 unless I can thread more of the bolt. I need 1" of thread to get enough adjustment on the rod.
Marty57

(real cub/-12)
old style spring had a cable.... (not used in decades????)

new style univair is one whole piece spring and strait section...

dakota had a 2 piece spring with a beehive end... all made of wires

I always put 2 springs on each side(no mater what brand/type) to keep flaps from banging in the wind from behind so easy...

using 2 of the the dakota 2 piece beehive end springs..

PA18 spring hooks a hole on the fuel tank strap. This is a 1971 PA18-150.

All info here is good. On my donor wings, they had a little spring with a cable. I tossed 'em and purchased the univair one-piece long springs attached to the fuel strap. Works great. I think either will work just fine.

Thanks for the pictures; great help. I like the idea of the double springs, any harder to deploy flaps with two? Looking at the bellcrank, I have to drill mine for bushings. The stock bellcrank looks like it has bushings at all three bolt locations, is that correct or just the bushing at the pivot point at the attach point at the spar?
Thanks,
Marty

I don't remember bushings in the double ear part of bellcrank, just the main pivot and the accuating side. The double ear has a countersink for flush AN bolt, but not sure that is needed for your application.

mike mcs repair said:

(real cub/-12)
I always put 2 springs on each side(no mater what brand/type) to keep flaps from banging in the wind from behind so easy...

Click to expand...

Yeah, wish I had done that. Mine flap even though I have tensioned good.

The double spring set-up is good.

Marty,No problems applying flaps against the extra spring.

The Dakota "beehive" end seems more durable than the original parts. I have not replaced a Dakota flap spring, yet, ever!

PS. Consider what access would be required to replace a spring and install access at covering time. You'll be glad you did. I am always grateful when the mechanic-guy ahead of me who covered the wing used some fore-thought about this.

PPS. STILL...... USE GUST LOCKS . A pair of 8 inch square pieces of 3/8" plywood with 2 holes in each, faced with carpet, and held together in the holes and tensioned by some rope make great, simple, cheap, secure gust locks. If the rope is left long, it can be tied to the jury struts to hold the aileron/flap junction so it WILL NOT come off. When used for rudder and elevator, they do not seem to work themselves off like the aileron/flaps taper shape allows. These are truly more secure and kinder to the airplane and it's finish/covering/paint than any manufactured commercial product I have seen yet! The carpet doesn't seem to freeze down like some commercial foam or 'felt' padding does. There is no metal to scratch, or scrape paint. The ropes are adjusted once, then these simply "slip" in place. ....whew.....where'd that come from? ......I didn't use gust locks for a time (wing covers had always saved the flaps/ailerons before, in my experience), and had a flap blow down far enough to take out a rear window this winter!

Looks like time to order 4 springs and a few other pieces to get the flap install done. Next will be the false spar; going to use wood with same cove the entire length of wing like Bugs did. I think I'm getting there, slow but sure. Thanks for the help guys.
Marty57

I ordered 4 of the springs from Dakota Cub, thanks Mike. The springs will be about a week or so, temporally out of stock so that gives me time to fabricate some parts.
Marty57

Marty, your 2+2 project pictures are awesome!!!! What a great job you are doing . Your link is a sleeper but I would heartly suggest to all to view it. Keep up the good work. I have bookmarked your site and will follow progress. Greg

I think I have the mounting location for the bellcrank and springs figured out; different from the PA18 just a bit. Here is what I have come up with so far:

I took the bellcrank and mounting bracket and positioned them on the wood spar. The J3 style hangers I am using (stock plans built 2+2) mount much higher than the PA18 so there would be interference with the stock PA18 bellcrank positions. The bellcrank location on the '18 has the fork for the push tube outboard of the flap hanger about an inch or so. That won't work with my hangers, they would hit. I plan on locating my bellcrank so the fork for the push tube is inboard of the hanger the same amount as the stock set up is out board. From the looks of it, I think the geometry of the push tube to the flap hinge is the same. This also places the mounting bracket fully on the existing ply reinforcement on the spar for easy mounting. I located my hangers first, than made my flaps and ailerons accordingly so everything fits nicely. I remember how much time I spent trying to figure out the placement of everything before I glued up the ribs and drilled for the hangers. I guess I tried to take all this into account back then but I just don't remember exactly, it was over a year ago. Any way, does any one see any gross error that I am missing here regarding the geometry? I will need to hang the flap and make up a push tube to see how it all works before drilling any holes in the spar. Thanks for looking over my shoulder here, it's a big help.
Marty57

Marty, make 2 of everything, and I'll be out. Looks real good, you have the patience of a saint.

Thanks Tim. I have CAD drawings of everything so your conversion should be pretty easy. You have the same hangers as mine so I think your addition of flaps should go pretty smooth. My flaps might have some measurements that are different than stock flaps but easily handled when building up a set of flaps in either wood or metal.
Marty