Thorp Air Command - T18.net

I have been in contact with Cubes as to the method of skinning the S-18 outer panels.

There are three different methods/Options to construct the skins:

1) the plans method. This is NOT the way my wings were originally constructed. And cubes reports if very difficult to build.

2) Two skins that "lap". The inboard skin is about 44" wide and the outboard skin is about 28" wide. The lap occurs on a rib 28" from the tip. This is how my original wings were built, and apparently is the most common way the S or folding "C" wings are built.

3) Three skins. The first skin is 48" (+/-) and is full length with the leading edge bend along the total length mid-width. This wrap thus extends around the D-Section and extends back behind the main spar to about the 3rd rivet behind (rearward) of the main spar. The 2nd and 3rd skin are for the remaining top and bottom skin which butt up against the end of the full length skin just described. A 0.032 angle 1.5" x 0.5" bracket is used between each rib and secures the butt joint between each rib. This is how Cubes constructed his "racer" project and reports that it came out great. The only issue is access for the center rib access to rivet the skin on. This can be overcome by two methods. Use of cherry max rivets for the few rivets along the ribs that cannot be reached. I figure a few cherry max rivets on the bottom skin is completely acceptable to me.

As I said, I have been discussing this with Cubes, and I have told him that I like option #3. I really like having the single piece, full length smooth leading edge. Especially since I plan to wet the wing. In addition, since I have to locate the rivets on the existing main spar holes, this allows pretty decent access to the spar with this type of wrap with the edges being only about 8 inches or so rearward of the spar.

Before I fully pulled the trigger on this and Cubes construct the three skins, I thought I would post here and ask if there is ANY reason why I should not go the option #3 method of skinning the wing?

No comments or recommendations?

Your first post absolutely convinced me of the way you are going to go.........and unless Cubes protested number 3, I venture a guess as to the "absolute correct" way to go. Any further considerations posited here I posit will result in analysis paralysis and delay of flying bliss.

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

CM rivets are your friend . Especially if you do not have a helper with the bucking . With the exception of my Fus . my S-18 was constructed almost entirely using CM rivets . Bucking was accomplished only where I could comfortably do it without putting a smiley in the skin . Got the CM rivets off of ebay for about $10.00/100 . A one piece skin would be nice .

Fraser...isn't it time for your psych eval ?

RB

Had one last week - I'm as insane as I ever was ever was ever was ever was.....

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

These wings were built using Sunderlunds drawings and John Waltons mod for 16 gallon of fuel each side. I've built two sets myself and been involved in 3 others all using two piece skins and AN4 rivets. I don't think CM rivets will work here. The more skins you have, more holes to match and error in overlap.

Fraze I knew you were nuttier than a squirrel terd the first time I met you in Durango and had to pick up the tab for lunch(you forgot your wallet)

HAHA - great memory Bill - I did pay you back though....didn't eye?

_________________
Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

If the concern is about cherry max rivets leaking, a dab of sealant on the rivet when it is pulled should offer some extra protection. While I don't have experience building a wet wing, I have repaired a couple on production aircraft. They had copious amounts of sealant inside the wing at the seams and I have seen posts where people talk about coating the interior of the tank with proseal so I am having a hard time envisioning a way to seal a tank that doesn't have access to buck rivets.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Not only did Cubes not protest option #3, it was his idea!

I do not see the need for CM rivets in the wet wing section as there are the inspection holes in each wet wing bay of the D-section forward of the main spar. The entire one peice leading edge will extend back about 22” on the top and bottom of each rib. The remaining skin of about 28” for the top will be able to be bucked with traditional flish rivets from Access through the bottom rear skin which is not yet in place. The bottom rear skin from butt joint to trailing edge, I think can have many of the rivets traditionally bucked, until the skin cannot be rolled back and an arm with bucking bar squeezed behind.

So I THINK, only the last few rivets at the trailing edge of the ribs, and ONLY the bottom rear skin would need CM rivets. The entire top skin should be completely flush AN rivets. As well as a good portion of the bottom of the wing.

The one piece leading edge from root to tip if anything should make wetting the wing slightly easier since it completely avoids the lap joint. Also it should simplify fitting the skin as the lap joint also adds in 0.032 thickness that creates a “wedge” or the need for a joggle at the lap joint. All of that is avoided with the single leading edge wrap and butt joint aft of the main spar (top and bottom).

There will be really no extra holes to match up, other than the butt joint splice. But these can be drilled through and through so alignment should not be an issue.

The downside I see is the added weight of the 0.032 angles and the extra rivets for the splice at the butt joints between each rib. Offset by the pros of a clean one piece leading edge without lap joint. Seem like a worthy trade/balance.

I don't know where I got the notion a cherry max was needed inside the wet area. It was clearly stated the skin would attach behind the spar. I have a lot of pull rivets in my 40 year old wing. I don't know what brand they are but I know they are not cherry max (no lock ring and the stems are soft and can easily be drilled out) and they seem to be holding up okay. In fact, I have only replaced 2 of those rivets and they were holding the hinge on one of the anti-servo tabs. Cherry max rivets would be much stronger although it can be a pain to get them sized so they pull correctly through multiple layers.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Just recieved the ribs and skins from Cubes. Very nicely shipped and packaged by the way!

I think I am glad I decided to use this “new” method of wing skin. Given that I M utilizing existing main spar. This new full length leading edge skin will now allow me to use a strap duplicator to accurately locate the existing holes in the spar caps. Other methods of skinnin would not allow me to locate all the holes with the duplicator.

Unless some one has experience to know that the duplicator will not provide sufficient accuracy!

Hi All,
I've seen reference to drawings by John Walton for fuel tanks in the wings. I sure would like to see those drawings. Are they available? I would be happy to pay for any copying and/or mailing.

Thanks,
Pancho

I have them, I think Lee and Cubes has copies
I need to find them

I should have electronic copies somewhere .. will have to track them down .. new laptop .. if not I have a few copies here I can scan and post on the site. Give me a day or so.

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

Here you go .. I'll post them on http://thorp18.com as well

http://thorp18.com/thorp_files/T-18%20Wet%20Wing%20Plans.pdf

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD