ETLB Squawk Forums — Archive

I had asked this in another thread but probably that was a wrong thread as the question is of a general nature, Hence I am repeating the question here.

I apologize for asking this,  I realize I have asking too many questions.
Thanks.

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Hi all, I have a question on fabric covering of the wings..

I noticed that after shrinking, the top side fabric seems to rest on the top of the rear spar due to curvature of the wing surface. The fabric is touching the top of the spar all along , except for a couple inches on either side of each rib. The construction manual recommends that only ribs be glued to the fabric, ( besides the leading and trailing edges, of course.)

Is it a good idea to glue the fabric with a touch of fabric cement at this location - on top of rear spar ? I was thinking to apply cement for a length of about 8 inches towards the center of each section of the spar between adjacent ribs.

Thanks for any suggestions / tips.

I did not on my old Airbike.

Keith,
Are you sure you have the fabric taunt enough? I know on my wing the fabric doesn't touch the rear spar unless I push on it and then it might be 1/8" away from the spar cap. If you do try to shrink the fabric some more be careful that you don't get it so tight that it pulls the trailing edge strip off of the rib. To answer your question.....no I would not glue to the spar.
Dick

The standard practice is not to glue at the rear spar and there should be an air gap between the ribs. I put a 1/4' cap strip between all ribs on the upper and lower rear spar. I did this to effectively "seal" the wings off. That small gap allows dirt dobbers and wasps the opportunity to build nests inside your wings! I know because I bought a project that had some previously flying wings and had been stored for a while. When we loaded the wings up you could hear dirt clobs rattling around inside the wings, so I opened them up and verified. I do not plan on gluing the fabric to the rear spar cap strips either. I also glued doublers to the trailing edges at each rib. The previously owned wings from above reference also had the wing trailing edge pealed up by the fabric being overly shrunk. This effectively triples the glue bonding surface area where the trailing edge is attached to the rib. this may be overkill, but my trailing edges will not separate from the ribs now!

Charlie, Dick Rake and Scott, Thanks for responding to my question. Your suggestions and opinion are very much appreciated.

I have uploaded a video of the wing after covering / painting. If something is wrong in general, please do let me know, so I can correct it before I cover the other wing.

https://www.youtube.com/watch?v=ANQHRt-MSR8&feature=youtu.be

Thanks

Looks Good to me! I can't believe your wife actually lets you work on your plane in the house! Does she have a sister?!!  

Quoted from ScottLuckman Looks Good to me! I can't believe your wife actually lets you work on your plane in the house! Does she have a sister?!!  

I started the build in the garage, but the garage was not heated and was too cold to work out of from Dec to April, so I moved the project indoors. I promised my wife I will move the project out by Thanksgiving this year. That is also the reason I am trying to speed up things. My present plan is to make the first flight by end of Oct this year. I was lucky to find a room that had no carpeting ( this is our only room with vinyl flooring ), and which could easily take a wing or a 12 ft by 4 ft table, and still have some place to walk around it. The kitchen is still usable, just that we need to eat our food in the living room to the front.

Your wing looks great.

This is not a trick question.
Just trying to analyze forces acting on the fabric as we increase tension / tautness.
The fabric is glued all the way around both rods , making it a cylindrical tube joint made of fabric.

On my Max the fabric on the top does not touch the top of the rear spar.I did pull the fabric titter than I probably should have and did pop a couple of the trailing edge to rib joints lose and I did as suggested and put doublers on the ribs. I think that part of the problem comes from putting too much pressure on the the trailing edge to rib joint when making the glue joint. I used spring clamps and had an awful lot of epoxy squeeze out of the joint. If you look at the top wing surface while flying it is amazing how much the fabric bows up between the ribs.  

Closer.  The reason your fabric is touching the rear spar cap is the chordwise tension of the fabric. Trying to shrink the fabric further will increase the chordwise tension along with the spanwise tension.  Your wing looks fine as is and the fabric will lift in flight like aeronut said.

Keith,
Your wing looks real good. From your drawing I had envisioned the fabric just laying on top of the spar cap. Bob and Aeronut are right about the fabric lifting in flight. I was a little surprised when I saw mine. Here's a picture.

Quoted from radfordc Your wing looks great.

Thank you, Charlie.

Quoted from aeronut On my Max the fabric on the top does not touch the top of the rear spar.I did pull the fabric titter than I probably should have and did pop a couple of the trailing edge to rib joints lose and I did as suggested and put doublers on the ribs. I think that part of the problem comes from putting too much pressure on the the trailing edge to rib joint when making the glue joint. I used spring clamps and had an awful lot of epoxy squeeze out of the joint. If you look at the top wing surface while flying it is amazing how much the fabric bows up between the ribs.  

Aeronut, thank you.  I was lucky not to encounter any of the issues of warping of the trailing edge. I just followed the construction manual which clearly states, being an ultralight, there is really no need to shrink the fabric as tight as a production airplane. So I was taking care not to over-tighten. But thanks for letting me know how the fabric lifts upwards in flight.

Quoted from Dick Rake Keith,
Your wing looks real good. From your drawing I had envisioned the fabric just laying on top of the spar cap. Bob and Aeronut are right about the fabric lifting in flight. I was a little surprised when I saw mine. Here's a picture.

Dick Rake, Thank you for the picture. That illustrates gluing the fabric to the rear spar would have actually harmed the aerodynamics of the wing by not allowing the fabric to lift up evenly chord-wise, front to back..

Quoted from Bob Daly Closer.  The reason your fabric is touching the rear spar cap is the chordwise tension of the fabric. Trying to shrink the fabric further will increase the chordwise tension along with the spanwise tension.  Your wing looks fine as is and the fabric will lift in flight like aeronut said.

Bob, totally agree with your analysis. As usual you hit the nail on the head when it comes to technical questions.