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where should I not lighten the plane - 11/2/2012 9:02 PM   
Scratchie



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From: Mendota heights, MN, USA
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Although my next project will be a kit, I thought this forum would be the best for my questions.


I'm considering building a Royal B-17 conversion from glow to electric, the plane would be fully glassed when completed.   I'm going to replace plywood fuselage bulkheads with balsa, and put holes in ribs. Some solid balsa parts used to shape the nacelles will be hollowed out. How far can I go without compromising the structure?  Where shouldn't  I try to lighten the plane?

Thanks



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RE: where should I not lighten the plane - 11/3/2012 3:43 PM   
invertmast


 

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Status: offline 		That is a very open ended question. Especially for those who have no experience with your particular project (myself included).

Your thoughts so far are all good ideas. You can replace the balsa sheeting with contest grade sheets.
You can also do a preliminary calculation of where to put all your components so as to eliminate and nose or tail weight.

As far as where i dont put lightening holes, is directly between wing spars and around the landing gear mounts. Other than that, its pretty much fair game on electric models

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RE: where should I not lighten the plane - 11/4/2012 3:22 PM   
LesUyeda



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Status: offline 		Unless things have changed enormously, Royal/Maratuka are boxes of wood that you carve into the shape of an airplane. If it was me, I would seriously consider some other manufacturer.

Les

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RE: where should I not lighten the plane - 11/4/2012 5:25 PM   
Scratchie



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Status: offline 		I have the royal kit on-hand, got it at a very good price.  I'm oing to do an inventory and see just how many of those blocks I can either hollow out or replace with sheeting

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RE: where should I not lighten the plane - 11/4/2012 5:32 PM   
Live Wire


 

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Status: offline 		As bad as I hate it HK has a B 17 lecky. I have built the Royal kit and they come out heavy. It will take a lot of mods to get it light anough to fly well and also a lot of power

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RE: where should I not lighten the plane - 11/5/2012 3:48 AM   
BMatthews



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Status: offline 		There's so many things you can do and others which you shouldn't.

For starters fully glassed implies fully sheeted, block carved or planked before glass and resin. That means a LOT of balsa. And typically kits do not come with the lightest grades of balsa. You can save a bunch of needless weight by ordering contest sheet and selecting blocks that are lighter than 8 lbs/cu foot.

And instead of glass and resin you will again save a heap of weight by considering using something like silkspan or 1/2 oz glass cloth bonded down with dope or varnish then fill the pores with a sanding sealer followed by high build automotive sandable primers which you sand mostly off in preparation for the sprayed on finish of choice. It's a slow process since you need to give each different product sufficient time to dry that the next non similar one will bond without any issues but it should come out lighter.

Or if you insist on using a finishing resin then study up on the methods for application then blotting away much of the initial coat using paper towel or toilet paper to ensure a minimal amount of the very heavy resin is left to bond the cloth. From there use a second coat of the resin thickened and lightened with glass balloons or phenolic micro balloons or similar filler.

Beyond these general sort of topics it's next to impossible to suggest anything specific without being able to study the plans.

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RE: where should I not lighten the plane - 11/5/2012 5:14 PM   
Scratchie



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Status: offline 		The glassing will be water-based poly over 3/4 oz cloth.  I've never used contest grade balsa, who is a reasonalbly priced source,  Balsa USA?

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RE: where should I not lighten the plane - 11/5/2012 8:11 PM   
sensei



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Status: offline 		You must first set a realistic overall weight target on each component, then you can deal with the details of getting the weight off...

Bob

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RE: where should I not lighten the plane - 11/6/2012 9:46 AM   
saramos



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quote:

ORIGINAL: Scratchie

The glassing will be water-based poly over 3/4 oz cloth.  I've never used contest grade balsa, who is a reasonalbly priced source,  Balsa USA?


Take a look at Lone Star Balsa. They offer 4 lb - 6 lb balsa sheets at some of the best pricing I've seen. A 3" x 1/16" x 36" sheet is 1.51. I checked the Balsa USA website and couldn't even find different grades of balsa.

Scott


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RE: where should I not lighten the plane - 11/6/2012 2:02 PM   
sensei



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Status: offline 		The best advice on this one is "make the fittings big and engineer the hell out of the upholstery".

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RE: where should I not lighten the plane - 11/6/2012 2:57 PM   
Scratchie



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Status: offline 		I was thinking that the aluminum engine cowls are heavy, and I could save a lot of weight by either making them out of balsa or a home-made plastic mold.  I'm not going to spend $50 and buy fiberglass cowls from eBay, I need to save the money for motors/electronics. 

If i made plastic cowls, I could use the aluminum ones as a mold.  The cowls would be only slightly over-sized, but maybe I could get away with it?

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RE: where should I not lighten the plane - 11/18/2012 5:23 PM   
CanDo


 

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Status: offline 		I don't know about all Royal kits, but a friend's P38 was a tank! Since most planes build tail heavy, saving weight in any area behind the CG is critical, including the wing area. Get a scale that can measure grams and weigh the wood. Cooking scales are ~$25. Put the lighter balsa aft of the CG. Replace lite ply ribs, bulkheads, etc with balsa. Lightening holes do not compare with lighter wood selection. Make joints fit properly to avoid excessive glue weight. Details like replacing heavy cowls count when you are trying to save weight. Good luck!

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RE: where should I not lighten the plane - 11/18/2012 7:26 PM   
sensei



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Status: offline 		You should not lighten a plane in the following areas:


Spars and logerons, just about everything else can go on a diet. Really...

Bob

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RE: where should I not lighten the plane - 11/19/2012 2:03 PM   
Scratchie



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Status: offline 		I made a vacuum forming machine over the weekend so I could replace the aluminum cowls with plastic.  It took four try's, but I was able to make perfect reproductions of the cowls, each weighing 0.6 oz. vs. the 2.1 oz. of the aluminum ones.  That's a 6 oz. savings in total, and now I'm considering what other parts I can replace. 



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RE: where should I not lighten the plane - 11/19/2012 8:24 PM   
Edwin


 

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Status: offline 		I dont think glassing would be a good idea since you need to loose weight. I'd go with ultracoat. UNLESS, you've done glassing before and have an idea about how to do a light weight application. My light glass jobs are 1/2oz cloth, and one coat of reson. I fill the weave with light weight spackle thinned to about a consistancy between milk and syrup, and rub it in with my fingers. Sand down to the weave, spot check and redo where needed. Then a coat of primer, sand down to the weave again. Spray the base coat. This process takes a LOT of sanding to get it right but will give you a pretty light glass job, maybe a little more weight than an ultracoat finish.
Edwin

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RE: where should I not lighten the plane - 11/19/2012 8:39 PM   
Scratchie



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Status: offline 		I’m always banging wings/fuselages around and have to fiberglass. I also use 1/2 oz. cloth, but now I use water-based poly over sanding sealer instead of resin.  It's not as strong but strong enough to prevent dings from bumping the parts around and cleans up really easy (can reuse the brush).  The sanding sealer is not compatible with a lot of the more aggressive paints. I’m also going to airbrush this build which I think will save a bit of weight and end paint compatibility problems. It's crazy, but I like sanding and getting a really good shape and finish.  I've gone as far as 600 grit, but this time i may go to 1000 if I use any glossy finishes.

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RE: where should I not lighten the plane - 11/19/2012 8:48 PM   
Edwin


 

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Status: offline 		Good for you. But 1000? Whatever turns your crank. I usually stop around 320 or so, no contest grade here. I dont glass for strength. Only to get a good paintable surface.
Edwin

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