Mike James
Posts: 2438
Joined: 1/19/2002
From: Anchorage,
AK, USA
Status: offline
|
Hi everybody,
This is an interesting conversation.
(Yes, my number examples, like "6+4" were in ounces.... i.e. 6 oz cloth + 4 oz cloth)
William:
I never really thought about this in terms of "weight vs. length", but that's one way to look at it, and you're right of course that each plane deserves it's own plan. I don't keep figures on this, because most of the fuselages I've built have been so similar in size (say, 60 to 90 inches length) that I can almost always use the same formula.
I tend to overbuild the first fuselage, as a safery concern, then determine through flight testng if I can build it lighter. Using a pretty standard flight testing plan, going from "gentle" to "aggressive", I want to know that the plane can take whatever I'm going to do with it without a failure. If the first one can, then I consider building the next one lighter, until a failure occurs. Then I'll use the previous, stronger one as the "production" version. Last year on the Berkut project, I used the "6 to 8" method. on that fuselage, which was about 12 inches in diameter and about 70 inches long, and it was fine. The plane we rung out the most in test flights ( a "3.5 + 3.5" layup ) was the lightest one, and it was plenty strong.
I think the biggest determining factor is where the loads go. Whenever possible, I try and think of a fiberglass fuselage as simply a fairing around the structure. Some loads will be transmitted to the skin of course, but since every plane is different, and the shape affects how loads get transmitted, it would be hard for me to put a precise "formula" on it. Aside from crash damage, I've never had a fiberglass fuselage fail from flight-induced stress. This includes Pattern and other aerobatic types. If you're doing a pylon racer, turbine-powered jet, or a plane designed for heavy cargo lifting, my ideas may be inappropriate. You would need more reinforcement, whether it's in the internal structure, or the composite shell itself.
One other thing...
All composite structures don't necessarily have to be super rigid. A small amount of flex at certain points in the structure can be tolerated, as long as it doesn't cause flutter or some other divergent flight problem. Some very high quality aerobatic planes I've looked at felt flimsy in my hands, but were plenty strong enough for flight loads.
Lots of trial and error in composites...
How the thing is shaped, how it's built, how the internal structure takes flight and ground loads, and how it's maintained... All have an effect, best learned through a combination of studying what everybody else does, and then creating your own experiences to prove or disprove the idea.
_____________________________
Mike James
RC Design and Building - www.nextcraft.com
|
Printable Version
All Forums >> RC Airplanes >> Composites Fabrication And Repair >> thickness of the aeroplane body 
thickness of the aeroplane body - 
