Hi. I'm building a P39 Airacobra kit. The directions call for the aileron servo to be centered in the wing. In order to do this, I will need to cut a square in the center rib deep enough to accommodate the servo. I'd prefer to keep the rib intact, because that seems to be a lot of rib to remove at such a critical location.

The ailerons move with torque rods. Does the servo have to be perfectly centered in order for the ailerons to move correctly (ie equal throws)? If I install the servo just to the left or right of the center rib, can I still get it to work correctly?

I have an Alpha 40 RTF trainer, which was shipped with the servos installed, and the wing in two pieces. Obviously, they couldn't center the servo in the wing, so they installed it in the first bay of one side. The two pushrods are not perfectly symmetrical, but it does not seem to affect anything.

On the other hand, you can center your servo, and then reinforce the cutout box with some lite ply. It shouldn't weaken your wing. I did this on my Four Star 40.

Good luck.

Put it in the center.

Cutting out that big chunk in the rib will not damage thw wing structurally at all.

I know it SEEMS like it would, but if you could see the forces at work, you'd realize that that area of the rib does nothing in regards to the wing's strength

What Minnflyer said.

Ribs are not a critical strength component in the wing. The purpose of the ribs is to give the wing it's shape. You can cut out the area for the servo without worrying too much.

Ken

With torque rods and rigid connections, yes. You'll build in odd differential in if you use rigid rods and offset them. Now, you can instead actuate the torque rods using lengths of flexible NyRod anchored in channels and move the servo off center, using a "T" fitting they both connect to or attaching the connectors on opposite sides of the servo. The lengths of NyRod don't even have to be equal as long as the last few inches of leade-out is perpendicular to the ailerons and they "aim" at the servo arms/wheel at the same approach angle.

Easier to hack into the center wing rib.

Thanks guys. So where does the wing get its strength? The plans called for shear webs to be placed between the ribs on the main spar. Do these give the wing strength? If I wanted to really beef up a wing to take a ton of abuse, where should I concentrate?

1) Spars and stringers
2) Shear webs (make the spars stronger)
3) Sheeting
4) Dihedral brace(s)

A great deal. I believe this was one of the reasons the Platz/Fokker Dr-1 & D-VII didn't need interplane rigging. Remember the grain must go up and down. I've seen a lot of builds where these were installed with the grain horizontal - usually in post-crash trash heaps.


And fiberglass the wing root where the fuselage covers it.

Bingo!!!! What he said.

Those are the main structural components of a wing. Like I said, the ribs are there to keep the airfoil shape of the wing. If you look at most good kits you will see that the ribs will have huge holes cut out of them. Those are called lightening holes. Because the ribs aren't main structural components you can remove a large amount of the wood and the rib will still function in maintaining the shape of the wing, the wood is removed the lighten the overall weight of the plane.

Ken

Well, my wing should be able to take quite a bit. It has shear webs and sheeting in addition to the dihedral brace and spars. Good info. Thanks.

A off center servo will cause a corkscrew roll that is most likely more noticeable in one direction than the other? Or at least noticeably different between directions?

In all planes I built I added 2” fiberglass tape with epoxy on top and bottom of the wing at the center.
Not instructed, probably not necessary, but gives a piece of mind and strength.

Foosball, lots of good info up there above, however as an RCer for 37 years, after many CL/FF competition years, I say it doesn't make a small rodent's posterior if you place the servo off center, lay it down, angle it or whatever.
From your post you are doing a relatively small model, with ONE aileron servo. Back in the days when 1/4 midget Pylon was a popular racing event, limited to .15 engines, and there WERE NO SMALL SERVOS THAT WOULD DO THE JOB, No small receivers, etc, we found all manner of ways to snug in the radio. Back in the '70s almost any servo -- and they were almost all alike -- cost 40 Yankee Dolla', (at least $100 of today's money) so unlike today we didn't have 'umpteen servos laying around on the work bench. I flew Q-500 a number of years with one wing servo, laying on its side, at about 20* angle from wing centerline, and hooked to torque rods via pushrods each with one or more line-up bends in them.

You're obviously not flying a 3D gas burner, or a 70" plus Pattern machine. Your airplane will fly and fly well. Many pylon fliers, especially in FAI and the old Formula One (near 200 mph), used only ONE aileron. So what if there is a tad of differential between the two ailerons? Makes no never-mind and I'm betting you will never know the difference. BTDT many times.

BTW, in flight all weight is resting on the wing center area. I like my ribs & center sections to be strong.

One major way to strengthen the wing was mentioned (as an addition to all that was said by several others) is to glass the joint of the two wings. Our club 'pro' had a show-and-tell demo at one of our meetings a while back and demonstrated the proper procedure to glass (add fiberglass strengthening) to the wing joint. He did it in several steps, depending on how strong you want that joint to be. And, of course, there is a practical limit to that. It can be so strong that it will never break, but why would you go to that extent, anyway?

He puts on a wide strip (perhaps six inches wide so there is three inches on either side of the joint) of light-weight glass on the joint allowing it to overlap on the front (LE) and back (TE) of the wing You will trim it off later with a sharp (read new) exacto knife or one-sided razor. He first mixes the long setting fiberglass resin (epoxy) and puts a coat on one side (top or bottom) the wing. Then he adds the glass by slowly.. well, not so slow that it takes a half hour.. but lay it on the epoxy and he uses a playing card to press the epoxy into the glass. You want to completely soak the glass. Then, slowly scrape the excess resin. You work from the center to the outside left and right, then center to the bottom, scraping the epoxy off. When done, the glass will look almost like it does not have any epoxy on it, but is totally soaked and the resin has had a chance to work into the wood at and around the joint on that side of the wing. Once you are happy with the way it comes out, you can then remove the excess using the razor from the TE and LE. By the way, the width can, if you do it right, extend beyond the edge of where the wing would sit on the saddle. If you do it right, it will come out smooth enough and can be sanded to be very attractive and acceptive to either paint or covering.

Let it set overnight then repeat it on the other side.

You can then add another layer of glass, that is more narrow than the first one, to add strength to the joint.

One thing I forgot to mention was that once the first layer of glass and resin has set up, if you did it right, the tape will cut very easily where the servo would go using an exacto knife. It will hold the servo screw very securely, that's for sure. It will be both strong and secure. Make sure you do not get resin in the torque rod holes or you wll be adding problems with torque rod binds. You can put some grease, vasiline, whatever, in the torque rod tubes to keep the epoxy from seeping into the guide tubes.

I've done this on a few planes and it comes out very nice. Perhaps on a lot of trainer type aircraft, or sport aircraft that do not do extreme maneuvers, this is probably not necessary, but it is up to the builder.

And, if done right, and if you scrape enough of the epoxy, it will not be heavy at all, and will add considerable strength to the joint.

Good luck no matter what method you choose. I'm sure it will fly just fine, though, with the method you have chosen.

CGr

Thank you CG
now i know to use a playing card.
Alex