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Building Tips and technical articles. => Building techniques => Topic started by: Motorman on February 17, 2021, 09:58:02 PM
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Motors,
Have not done that. I did try to shift a vibration frequency problem on an already flown and tested Cardinal with an OS .46LA by laminating .040" carbon plate to the outboard (engine mount) side from the spinner to the main spar. It was not an effective mod, and as a add-on of last resort it was also weight that the plane did not need. The nose was already very stiff for a profile, having used a fairly stiff balsa core, and the doublers were real 5-layer birch plywood. The .46 was never happy on that plane. The .40LA actually ran better on it.
The promise of carbon is always that the structure could be lighter if designed from scratch and stressed properly. Mostly what you see though, are "black aluminum" designs that are often heavier. The more stress points or points where loads are introduced, the more likely carbon is to disappoint. Check your design carefully for out of plane loading, point loading and delamination points. You will need hardwood plugs between the carbon wherever those load points are. The usual stuff.
With no plywood at all, I think I would try the .040 since it would be a bit less prone to delam damage from point loads. But if you already have the .025 and have the time to try it.....
The Divot
PS--there is actually one mod that makes the use of light-ply an almost forgivable technique: use post inserts for the engine to sit on. Grind them off slightly above flush so the engine never touches the wood area. Thread the inserts into the engine bearers. It's a common technique used in F2C planes that works.
I
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FWIW .... Hutchinson's Warbirds don't use ply doublers.
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Motors,
I don't recall the construction of the crank. I know I had it apart to replace a peeled liner. That engine was jinxed from the get go. My truck was even totaled on the way to the hobby shop to pick it up.....
I'd dig it out of the engines box to look except I'm afraid to touch it. Bad mojo on it, for sure....
Is one of the two cranks not optimized for dynamic balance? I'd suspect the forged crank might be light.
The Divot
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Do I recall correctly that some of the Mathis profiles from way back also were sans sandwichwood? Such as the Excalibers?
The Divot
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Building a 550 sq. in. profile from a kit for my LA46 so I have to save nose weight. The kit came with two 1/8" lite ply doublers for each side of the nose. I want to replace them with some .025" carbon fiber plate that I have. I estimate it will save 2oz on the nose. Has anyone done this and do you think .025" is thick enough or should I go to .040"?
I have not done that but I would very strongly encourage you to get rid of the lite ply, it is very heavy and quite weak. And it will crush flat under the engine bearers very quickly.
Brett
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I would use the lite ply with carbon fiber veil sandwiched between it and the base material. Apply epoxy, put on the veil and and then the doubler. Cut the ply so it doesn't cover the engine bearers and then use .125" aluminum mounting pads.
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All the "light" in the word doesn't mean squat if you're vibrating the nose and can't get a good engine run.
Unlike Brett, I've found the BUSA and Nosen lite ply to be pretty darned light. Not as light as balsa/CF/balsa sandwich nose method, but close.
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MM,
Have you considered using regular 1/32" plywood with maybe one layer of the CF between it and the balsa. You would epoxy the CF to the balsa fuse then while still workable add the plywood. Use the card scrape to remove excess epoxy and apply weights to hold it in place till cured. Also, add some lighting holes though the back of the hardwood motor mounts will save some weight.
Best, DennisT
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I am VERY weight conscious. My recommendation for a lite weight and robust front end:
I have successfully used balsa sides (1/8 and 3/32) laminated with 1/64 plywood internal doublers.
Also reinforce the outside of the nose with silk, silkspan or carbon mat.
Plywood doublers and external covering extend from the front of the maple motor mounts, back aft to the wing LE, to approx. the high point of the airfoil.
This works well on 60 sized models and smaller, also I use this on E-powered models.
I (and many others) get terrific longevity and light weight from this technique
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Carbon will work very well for adding strength and stiffness to the front end. But you still need some sort of harder material to prevent crushing when the engine and mounting fasteners are tightened. Use a thinner grade of ply and some aluminum pads to help spread the tension..
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....... grade of ply and some aluminum pads to help spread the tension..
Actually: compression
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I can't speak for CF but 4 light ply doublers on a 550 sq profile for a .46? Sounds like unusual construction. What plane is that?
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My comment (reply #12) was for a built up full fuse model.