I am kit bashing a couple different planes into a Skymaster push/pull twin. There are twin booms going back to the horiz. stab with a vertical fin and rudder on each boom.

The fuse is from an LT-40 that I cut off behind the wing saddle and added a rear firewall.

Wing is from a 60 size Ace Bingo. If your not familiar with that---picture a 4*60 semi-symetrical wing with a cord about 10.5"

This is long and complicated--please bear with me:

The center section is 28" wide. The booms are permanently attatched to the center section--which is removable from the fuse via the standard dowel and bolt system. So-when you pull the center section of the wing--the booms and all tail surfaces come with it.

My total wingspan is 84".

I have a removable 28" wingtip--outboard of the booms on each side. 28" center section + 2 wintips at 28" = 84" total

I'm using 3/4" aluminum tubes and phenolics for my removable wingtips.

The problem is that the distance in between the upper and lower spars is about 1/4" WIDER than my phenolics. Thats a lot of area to fill.

Also--the cord is a bit short. I'm using the saddle from the LT-40 and it's about 2" longer cord than my Bingo wing cord.

I was wondering what effect it would have if I made the overall wing THINNER and increased the cord?

If I make the wing thinner---then I can space the upper and lower spars at the right distance to encapsulate my phenolics perfectly--without having to fill a bunch of area between the spars and the tubes. Then increase the overall cord to make it fit in the saddle on my fuse.

So--thinner wing--but longer cord. Semi-symetrical airfoil. What do you think? How will it effect flying characteristics? How will the lift change on thinner wing with a longer cord?

This is NOT a crazy aerobatic plane for 3D. I might do some loops and rolls with this airplane--but no snaps or wild stuff. I just want to build a semi-symetrical wing that will provide some sport aspect--but still be plenty adequate for lifting this plane.

Estimated weight is 8-10 pounds.

Thanks

We need some numbers to help out. What airfoil and what would be the new % thickness. Calculate using thickness/chord x 100=%thick and let us know how what the numbers are.

I don't know what type or number of airfoil it is. As I said. Very similar to a 4*60 shape. Semi-symetrical.

Currently--the thickest part of the rib--at the spars is 1-9/16" thick from the top of the rib to the bottom.

There is 1-1/16" between the spars. My wing tube is only .806" in diam.--so that leaves me with about .240" to fill in--if I install the tube in between the spars; as they are spaced right now.

Cord is 10.5"

I want to decrease the thickness to 1-5/16" and increase cord to 12.25"

That would space my spars so that my phenolic tube will fight tightly between them and not have to do a bunch of filling with balsa and epoxy.

I would be adding approx. 147sq.in. to the wing, via the lengthened cord. Total area would be approx. 1029sq.in.

Decreasing the thickness by approx. 36%--if my calculations are correct.

I don't know much technical stuff about airfoils; but, I would think that a thinner wing would need to fly faster to achieve the same lift. Maybe not--because of the added sq.in.??

At 1029sq.in--and weighing between 8--10 pounds-I don't think I'll have a problem with wing loading. I calculate it at 22.4oz/sq.ft--if the plane weighs 10 pounds.

If it weighs 8 pounds--then wing loading would be 17.9 oz/sq.ft.

Thanks

okay:

1-9/16" thickness = 1.5625"

1.5625" thick at the spars--and a 10.5" cord--currently

1.5625/10.5 x 100 = 14.88

I want to change to:

1-5/16" thickness and 12.25" cord

1-5/16" thickness = 1.3125"

1.3125/12.25 x 100 = 10.71

So- If I did that right--then I'm going from a 14.88% thickness too a 10.71% thickness.

You're thinner section will be fine. Many models work with a 10 to 11% airfoil without issue and the shape you're using will scale decently enough to fit. I guess I was thinking that you were going to thin it to well under 10% and that can have problems.

In any event you need to ensure that the spars are well bonded to the phenolic tubing. I'd make it just a hair larger in thickness, maybe a 1/6 or so, and then pack the resulting 1/32 inch gaps above and below with a mix of epoxy and microballons made thick enough to form a nice putty. You'll need to use saran wrap and some foam rubber to "clamp" the putty in place until cures or it'll run out leaving voids.

Making the wing thinner will also put more load on the spars. I would suggest you take a page from the glider folks and use 1/8 x 1/2 spruce top and bottom caps and then box them front and back between the ribs with 3/32 HARD balsa with the grain running vertical on the webs.

But frankly I don't see what the big deal is. Use the larger thickness, punch the holes for the tubes so they are contacting one spar and then fill in the gap. The wing will have a more gentle stall with the thicker section, won't tend to speed up so quickly when the nose drops and will be stronger thanks to the deeper spar design. As a bonus the 1/4 deep part of the ribs will hold them together while you build in the tubes. In any event you should stick with the webbing of the top and bottom caps. You still need to bond the tube to the spar caps. That's a must do regardless of how you do the final job.

BMatthews:
Thanks very much for your help. I really appreciate the time you took to answer my questions and the formula you gave me to figure % of thickness.

I hadn't thougth about it before, but:
Since I have almost a 1/4" gap too fill with my current spar spacing--I can just cut 1/8" balsa strips to fit in between each rib section. Glue an 1/8" balsa strip under the top spar and then one on top of the bottom spar---and that will take up my extra void space of 1/4" between the spars and phenolic.[:-] That should make the tubes fit very snug and minimize the use of epoxy and balloons as filler.

I am using 1/2" x 1/4" spruce spars top and bottom. I had planned to drill all the holes in the ribs for the phenolic. Then install the tubes and fill around them with a heavy mix of microballoons and epoxy---just as you suggested.

The rear spars are 1/4" x 1/8" spruce as well.

There are 3--1/8" x 1/8" balsa stringers running along the top side of the wing---from the LE up to the top spar--to kind of make the top LE curve nice and subtle--without all that drooping in between the ribs when you shrink the covering down.

After the phenolics are all glued and epoxied in place--I had planned to encapsulate the whole thing with vertical grain webbing between the ribs--both front and rear of the spars--also as you suggested.

I still want to increase my cord. I've done the calculations.

If I keep the thickness the same at 1.5625" and increase the cord to 12.25"---then my wing thickness will be 12.755%

I don't know if I'll do this--I've already cut 40 balsa ribs and sanded them to shape for the current rib profile. They are all pinned up and set in the jig already. And I've cut 12 lite ply ribs to hold the phenolics in 3 places on each side of the tubes. 3 lite-ply ribs on the left-side center section to hold 12" of tube. Also 3 lite-ply ribs on the right-side center section to hold 12" of tube on that side.

Then 3 more lite-ply ribs for for each side--left and right-- of the 28" long removable wingtips.

If I start over and increase the cord---those ribs that I cut, are all trash. Tough decision. I really want the extra 147sq.in. of the longer cord--but the time and work involved with cutting all those new ribs is not something I'll be looking forward too.

Somebody either talk me into it--or talk me out of it.

It's all up to you in the end. You've got lots of wing area and a nice light projected weight so there's no real need to do new ribs if you don't want to. Or else your next model can use the first rib set.

A hint that's worked well for me doing joined wings. I make the wings all in one peice with the tube in place and solid for now. The two ribs where the joint will be are spaced apart slightly. Doing it this way ensures the tube lines up 100%. Following the glue setting up I run a saw through the joint line to cut the tubes and other wood parts that pass over the joint line. The ends are then dressed up and ply face ribs added to help prevent dings along the joint. By doing it this way if there is any slight misalignment it's not important since the misalignments are complimentarty on each side and the result is a straight wing.

It sounds like you've planned this out well and with some background. Best of luck with the project.

Thanks Bruce:

Your idea of building all the sections together--with the phenolics and a small space between the end ribs is an excellent idea. I had not thought of doing it that way. But, I am going to follow that suggestion and build them like that.

Then--even if my phenolic isn't perfectly aligned--it won't matter. Just cut it all apart and sand the rib ends and phenolics at the saw joint. The wingtips will still fit the center section correctly.

Thats a great idea and I'm definitely going to build the wings and removable wingtips in that fashion. I'll just set the jig up and space the ribs at the joint about 1/8" apart. Run the LE stock and all the stringers, spars, TE, and phenolics right across the open space. Pull out the razor saw and cut it apart after the glue dries.

Can you tell--it's my first scratchbuild??[:-]

Thanks a bunch for all the help and good advice. I'm still torn on the cord. I want the extra sq. in. I don't know what difference 147sq.in will make. I guess I'll do the wing loading calc. with the cord as it is. My previous wing loading calcs. took into account the increased rib size at 12.25" cord.

Oh well, I'm laid-up and off work for several weeks. Back surgery. I got nothing but time and a garage full of balsa. May as well cut the new ribs. I'll never be happy if I keep the cord at 10.5" and it lands like an F-16 on a 100degree day. Then I'll just end up building ANOTHER wing and cussing myself the whole time about how I should have built it with the increased cord in the first place.

I guess that settles it.

Thanks for the help.

Maybe I'll dig this thread up in a couple months and post a few pics and give a flight report. Wish me luck![&:]

I've made up my mind:

I'm going to stick with the 10.5" cord ribs. I've got a lot of hours invested in cutting them all out and then jigging them up in a "bunch" and then using my palm sander to sand them all to a perfect match.

I've got all the spar slots cut and my front end stringer slots cut.

I estimate about 20 hours time invested in these ribs. I'm not willing to spend another 20 hours making new ribs for the increased cord I spoke of earlier.

BTW--the 10.5" cord is only the actual RIB length. Thats not taking into account my aileron cord. Aileron cord is 2.5"

AS I read around here and search for more info--I discovered that the ailerons DO figure into the TOTAL cord and wing area.

So--I'm happy with my sq. in. and wing loading given my current wing size.

Thanks for the help.