Fully sheeted built-up wings

[Bob Hunt]

On another thread here on the Open Forum the subject morphed from foam vs built-up wings to a discussion of fully sheeted built-up wings.
Over the years I’ve had occasion to build several built-up fully sheeted wings for some of my RVP customers. Until just recently I didn’t have the need of such a wing for my own use, but that has all changed. A new twin-motored design that Buddy Wieder and I have been working on would have been too heavy if done in foam, and we didn’t want to use a cap stripped, open-bay wing due to the lack of good covering tissue being available, and because we simply didn’t want the appearance of a tissued open-bay wing.

Our new twin features double tapered panels with a “cuff” at the root (much like a Mustang wing) and also a swept-forward hinge line. To make four panels in balsa covered foam and then spar them together would just add too much weight. We decided to go with a fully sheeted built-up wing and build the wings in my Lost-Foam process. The nature of the Lost-Foam process would make the assembly and sheeting process a snap.
I called Dennis Adamisin for a consult on fully sheeted wings. He has used such wings many times over the years and so I figured that a call to him would yield some wisdom. It did. Dennis suggested that the rib spacing (center-to-center) should be no more than 1 ¾ inches. This close spacing helps prevent any “dipping” of the sheeting between the ribs. You know, that “starving horse” look…

I designed the wing to have a straight spar from tip to tip. I opted for ¼-inch square spars on top and bottom to form a classic C-tube structure. I chose ¼-inch square spars to allow for a “land’ on which to make the joint between the molded leading edge shells and the “plate” that would cover from the back of shells to the back of the wing. Joining them over the spar would yield a “platen” on which to sand smooth the joint between the two pieces without sanding a “dish” into the wing.

The wing halves, with the cuffs, were assembled separately. The molded leading edge shells were installed onto each of the half frames and then 1/8-inch balsa shear webs (with vertical grain) were cut and fit between the ¼-inch square spars out three rib bays past the area where the cuff and the outer panel join. These shear webs were actually cut and dry-fitted before the molded leading edge shells were installed. They were re-fitted and glued in place after the shells were installed.
 
The panel halves were joined in the Lost-Foam fixture and then basswood tension and compression plates were fitted and gusseted. The control system was installed during this process.

The four (top left and right and bottom left and right) rear balsa sheeting panels were joined and detail surface sanded. The inner face of the four balsa panels were treated with an application of Z-Poxy finishing resin. We poured the resin onto the inner surface of each of the four panels and then used a MonoKote heat gun to thin the epoxy enough to allow us to coat the entire panel by rubbing the glue around with a folded paper towel. Once the skin was entirely saturated with the epoxy resin, we used dry paper towels to wipe off all the excess glue, and kept wiping until the panels were just damp with epoxy. We let that coat cure and then block sanded the epoxy smooth. We added less than 1/3 ounce of epoxy in total, but the balsa panels were now extremely rigid and sealed.

The bottom panels were attached to the wing frames one at a time. We put a very thin coat of Z-Poxy onto the surface of each rib and onto the front edge of the panel. We then positioned the front of the balsa panel against the rear of the molded leading edge shell and taped it tightly in place. The top balsa panel was positioned in place, without glue, and the top cradle piece was put in place atop the wing. Weights were placed on the top of the foam cradle piece to put even pressure onto the entire plate and it was left to cure. The process was repeated to glue the other bottom balsa plate to the opposite lower wing frame.

At this point we added 1/16-inch shear webs near the back of the wing for strength. Then we covered the tops of the panels using the technique described above. The result was one flat, straight, light and rigid wing that is easy to finish. This is how I intend to build most of my wings from now on!
The photos below should answer most questions. I’m in the process of preparing a detailed, step-by-step manual for those who wish to give this a try.
Later – Bob Hunt
 

[MarcusCordeiro]

Nice work as usual Bob.
Looking forward to another fine step-by-step from you...

Marcus

[Bill Morell]

Looks incredibly strong. What is the span of this twin?

[steve bittner]

Hey Bob, looks great, What thickness are the ribs and wing sheeting are you using? Thanks, take care

[Bill Hummel]

We recently spent a day at Bob's incredible shop.  I THOUGHT I was a pretty good builder, but after seeing the level of detail and accuracy Bob works at, I realize I'm just putzing around.  To fly at Bob's level, and the level of all the top stunt flyers, you must be willing to build to Bob's level as well!  I'm not just talking a little bit better building, but exponentially better!  Fortunately, Bob is totally willing to share every bit of his expertise.  This wing is just another example of incredibly outstanding construction skills!

[Bill Morell]

Hi Bill:

It's 62 1/2 inches with the tips (which are not pictured here installed). The inboard panel is 1/2-inch longer than the outboard panel. And, yes, it is extremely rigid and strong!

Later - Bob

Thanks Bob, beautiful construction, almost a crime to cover it all up!

[Allan Perret]

How does this compare weight wise with sheeted foam cores and covered open structures ?

[peabody]

Neat stuff!
A quick question: did you make the lightening holes in the horn retainers or buy them?

[john e. holliday]

Bob, that construction is awesome.  A few years ago I built the Sweet Pea of Dennis Adimisin.  Wish I had seen this before hand.   Would you use this construction on one of your prior designs?   Also do you ever sleep?

[bob whitney]

Bob ,are u saying that u covered the inside of the sheeting with epoxy  if so ,what if u did the same thing to the outside at the same time ,would make for easy sanding  and a hard base to start with   Bob Whitney not Whitely 

[Dennis Adamisin]

Hi Bob - thanks for the shout-out!

We started building sheeted wings in the late 1960's.  Was easierto get a good finish and that finish was invariably lighter.  We were following Don Bambrick who used a sheeted wing on his Fury, the 39 pointer from the 1966 NATs.  The system evolved from to where we joined the complet skin off the wing, finish sanded it then installed it.  Used to use ribs every 2" with a spar and half ribs - then evolved to a full rib every inch with no spar.  Could build just about as fast as a foamie and they were stronger & lighter.  Used to coat the inside of the sheet with HobbyPoxy #2 but I was not properly disciplined about scraping off the excess so my wings tended to be heavier than Dad's or the others in the shop.

Over time I spaced the ribs back out ot 1.5" (Orange Crate) and omited the glue from the entire surface, using epoxy on just the ribs.  I still built heavy!

Why epoxy? because it absolutely does not shrink or warp.  Used judiciously, per your method, it also does not have to be a significant weight issue.

You have taken it to another level with the lost foam methods - the new wings are gorgeous!

[M Spencer]

  these Adaminson wings were akin to the blue foam core carbon / kevlar Americers Cup / Aviation stuff .

In that the Balsa forms a filler / packer/ core for a dual epoxy skin / laminate - optimiseing strength-weight .

Though Ive concerns with the seperation of the skins , should the vertical rib members be seperate vertical grain
pieces , to maintain wing depth under duress / deflection ( such as an unspared foam core sheeted wing ) .
or be vertical geodesic or cross braced girders constructed from 1/32 Sq. balsa . 

This is a bit along the line of wether shear webs are vertical or horizontal grain , pobably boths best .
Well Braced / securely located center section stucture - to maintain dimensional integredy at the area of greatest load concentration ,
while outboard the load concentration is proportionally less , so progressively less structure is necessary outboard . Though one can get carried away.
Then Again - some have more time than others , and less wood ( 1/32 sq. built up rib supports  )

Id think one could consider both fully ridgid and lateraly ( along the span ) or both. As in a combination . The trick being progressive absorbtion of loads.
And the Olde dreaded fokker wing flutter / control reversal , where if torsional strength is incorrect flying loads alter the incidance through flexure .
Grabbibg a I beam wing over your knee ( detroiter ) you can test the ' Bow ' , but ' D ' tube wings have considerably less dimensional displacement tolerance
before they snap . IF we're talking about DIMENSIONAL rather than LOAD . Obviously less displacement / Bow per LOAD on the 'D' tube or fully sheeted type .
 

This means That Looks About Right ,  there . 

[Paul Wescott]

Bob (Mr. Hunt), I really enjoyed what you have written thus far and the pics.  I'm very much looking forward to the full how-to manual.  Along the lines of Bob (Mr. Whitney) I stumbled over your statement that the wing skins were coated with epoxy on the inner surface (and therefore "sealed"), but your follow-on explanation cleared it right up.  Thank you for being so patient and thorough, and sleepless...

Now, can anyone explain what Matt Spencer just said?  They're basically toy airplanes right?  Do I have to take some engineering classes to understand the insides of a built-up wing?  Or should I plan on sticking to solid balsa wings, solid foam wings, or resign myself to fiberglassing the outside of all future wings?  Just kidding, that's my 5am humor.  I'm sure Matt's essay was cogent and made sense to some, just not to me.

Paul Wescott

[john e. holliday]

Yes Matt is from down under.   I great guy when you can figure out what he is trying to get thru to you.   Yes Matt, I will be starting on the plane you supplied plans for several years ago.  Just need to get in the right place in the shop and the mood.   You should see some of his remarks on the Barton site. 

[Bill Little]

Hi Guys,

Yep, Matt S. is "from the land down under".  His dialect is sometimes difficult, but I always seem to get the jist of his messages.  Matt is a great guy and is beyond helpful!  I suspect it might be difficult for him to understand some of our writings! 

BIG Bear
RNMM/AMM

[RC Storick]

So what is the weight of the wing in this state? Flaps -tips etc.?

[MarcusCordeiro]

You know

I'm totally in for fully-sheeted wings....
Looking forward to that PDF....

Marcus

[Allan Perret]

Put me down for the .pdf manual also.  If fact I am ready to order a new lost foam jig with full sheeting option soon as you are ready.  You have templates for the Novar ?

[Douglas Ames]

The Z-poxy impregnated "thin" ribs and 1/16 inner wing skin is an excellent idea!

I'm curious what the weight and strength of your ribs vs. say, conventional 1/8 bare balsa ribs of the same profile.

How did you do the seams of the skins. I always get a ridge of CA no matter how careful I sand.

I've had good luck thinning Z-poxy with Alcohol, never tried heat?

 

[Richard Oliver]

another way to clean up the ca joints after sanding is to wipe with nitro or acetone.