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Building Tips and technical articles. => 1/2 A building. => Topic started by: Rob Killick on July 24, 2010, 09:01:21 PM
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Hi ,
I'm nearing completion of my C.G. Lil Jumping Bean and was wondering if I should silkspan the entire model (the Brodak booklet says to just cover the wing) , or leave the fuse and empenage with just a dope brushed on the wood ?
I'm using a Brodak .049 Mk II for power .
Thanks ,
Rob K.
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Lighter is better. Especially with 1/2As.
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I recently did an all-sheet model, and based on a suggestion here I covered it with tissue over a couple of coats of dope.
I've put coats and coats and coats of sanding sealer on open wood and haven't gotten within miles of the finish I got from a total of four coats of dope and one 'coat' of tissue. And I don't think I added much weight with that tissue.
(Moreover, I did the trim with red plyspan, which had to weigh less than the equivalent colored dope would have).
I'd go with silkspan on the wings and tissue on the fuse and tail. Or I'd go with all tissue (maybe two layers, or some heavy plyspan from Sig), and I'd bring a roll of scotch tape to the flying field.
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I feel that its inadequate to silkspan a plane such as the Jumpin Bean. Mine is actually silkspanned and its been nothing but a headache. First day at the field resulted in the battery clip popping off and going through the wing. flipping over in the grass is only going to result in repairs. I have had better luck using Sig Lite Coat as I think its slightly more flexible than the Brodak clear. Many of the planes I've silkspanned and there have been several end up in what I call the potato chip stage. The clear gets very brittle on the silkspan and it takes nearly nothing at all to puncture or tear the covering. For the record, I've never used Dave Brown Flex-All which I'm sure would help. I've used all kinds of covering on my 1/2 a planes Monokote, Ultracote, Sig Koverall and I've had them fly fine. Its pretty obvious when a plane is suffering from obesity. I think if weight is a major concern, Phil Cartier offers SLC covering which is clear and irons on like Monokote. I find this covering to be strong, inexpensive, easy to apply and paintable.I do find the silkspan does indeed strenghthen the structure and is no doubt light but the maintenance and tear factor outweighs the benefits. I'm a true believer in lighter is better but it does become a ritual when using silkspan. Currently, I have a Flite Streak covered in silkspan which is painted in the traditional Flite Streak red,white, black scheme which looks pretty awesome and for some reason has maintained its durability. I feel its the fact that I used Brodak colors with Sig Lite Coat as a top coat. At the time Brodak clear was unavailable so Sig was the choice. I can say it hasn't yellowed in 5 years. I've been using this process for some time now and I know most say stick with the same brand, I'm using it it because it works for me. I guess if silkspan is what you prefer than just put it on. I 'm only offering my experiences which comes from flying over grass essentially and this is typically what happens to my planes. I think for those that haven't tried silkspan its a must to try it. Its a really remarkable product and light but it does have its drawbacks at times. Ken
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I know what you mean by the potato chip stage. The Dave Brown Flex All will keep that from happening. Sig Lite Coat or any other non tautening butyrate will work OK.
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Hi Rob,
I'm a *little* late on this one (that's normal it seems ;D ) but I think it is just a matter of what you are *looking for*. Putting paper on *everything* will give you the ability to put a much nicer finish on the Lil' Jumping Bean, but that's about all. The *extra strength* of silkspan on the wings, etc., is really not needed, IMHO. Keeping the model light is the best idea for flying a 1/2A. Using paper on all surfaces will add more weight than one would think. I use primer and dope on the all wood surfaces, and some grain always shows, but it ain't visible to me when it's in the air! LL~
I am getting ready to do repairs on my son's L.J.B., it has set for close to 20 years with the covering missing on the bottom of one wing. I will have to use a LOT of steam (and probably some cuts on the underside of the LE, TE) to take out the warp! Used a Black Widow on it and it flew pretty good for what it is. ;D I had one, myself in 1964.
Big Bear
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It has been my experience that silkspan will become brittle in a very short time if it is finished with only clear dope...no color. This will happen with either Sig or Brodak dope and seems to be cause by sun exposure. The Flite-Streak mentioned was painted so unlike the clear over silkspan planes, the covering held up. FWIW: I too use Sig Lite-Coat over Brodak colors, no issues yet (fingers crossed). 8)
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Gentlemen ,
Thank you very much for the advice ...
Methinks I will go with a doped fuse and tail feathers and forgo the silkspan .
The last L J B I built was in 1976 , so doing one again has been a learning curve , in miniature :)
The thing I like about it , is that is is fun to build and quick to build y1
Thanks again ,
Rob K.
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Rob,
Just a reminder but it is important to apply the lighter grades of silkspan "spanwise" for better strength and tear resistance on open frameworks like wings.
If you opt for two layers of tissue, apply the first layer spanwise and the second chordwise. This will maximize strength and tear resistance.
Plasticizer is good. A couple drops of castor oil in a 4 oz. jar of dope works if you can't get "store bought".
George