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Lighter structures - 8/7/2002 12:50:39 AM   
Mike James



Posts: 2465
Joined: 1/19/2002
From: Anchorage, AK, USA
Status: offline 		Hi everybody!

Building to FLY, not to crash...

A few years ago I went through a "hot dog" aerobatic phase that resulted in the building of a bunch of typical .60-size kits.... Plywood sides with doublers and stringers, 1/4" plywood firewalls, 3/32" shear webs, spruce spars. These kits can take a beating, but are too heavy! Today, with better training methods and buddy boxes, I don't even think this kind of construction is necessary for a trainer.

Re-visiting some "old fashioned" methods has reminded me about smart construction, and a recent "3D" ARF I built for a kit review has reinforced it. This ARF was built of sticks mostly. Not only that, but it was built of "contest grade" balsa... VERY light. The firewall was 1/8th ply, not 1/4. The shear webs were 1/16th balsa. It was fragile to handle around the shop. BUT...

This plane now has about 70 flights on it, with a YS .91 4-stroke, and is the type of plane I usually "wring out" for most of the flight. It's absolutely strong enough, is NOT showing signs of wear, does NOT contain expensive composites, and will slow to a crawl, due to it's light wing loading. Of course, it wouldn't take a crash very well, but it was built to FLY. (I'm not trying to sell a model here, so I'll leave the name out.)

I'm interested in hearing others opinions on keeping structures light, yet strong. I know there are many ways we can achieve this, so why don't we start a thread here that can become a good source for ways to build 'em to fly? I have some ideas to contribute too, but wanted to get things started first.

I'd especially like to hear from you engineer-types, who have specific ideas, backed up by some facts. A good starter question might be, "When you build a structure using alternating diagonal sticks, is the angle of those sticks important?"


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RC Design and Building - www.nextcraft.com
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Lighter structures - 8/7/2002 1:48:15 AM   
AKMac



Posts: 135
Joined: 6/18/2002
From: Tooele, UT,
Status: offline 		This is exactly why I still think the Kadet Seniorita is one of the best trainer kits on the market, I just wish the plans called for ailerons because most everybody does or eventually make them with it. If you look at the fuselage the entire structure is make out of 3/8 balsa sticks in the shape of triangles. Kind of like a bridge. My entire Seniorita probably weights only about 5-6 lbs total, maybe less. Really the only argument you can make against it is that for some people it's hard to build because you have to make a lot of 90 and 45 degree cuts. Oh well though. I thought the experience helped my building skills. Ohh and it doesn't like flying in the wind much

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Lighter structures - 8/7/2002 2:01:15 AM   
Cactus.



Posts: 6138
Joined: 12/19/2001
From: CamborneCornwall, UNITED KINGDOM
Status: offline 		take a look at the ED3, its anorexic
The firewall size and things are pretty much standard or over kill for my sport planes lol
The fun flys here have 1/16 light grade ribs and 1/4x1/4 lg spars.
i did build a FF15 with 1/32 ply sides, light and it took a 50' dive and only broke the prop, a rolled 1/62 fuz is also amazingly strong

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Good point! - 8/7/2002 2:56:38 AM   
Mike James



Posts: 2465
Joined: 1/19/2002
From: Anchorage, AK, USA
Status: offline 		That's another good point about building light... a lighter structure has less mass, which can actually minimize crash damage...especially if it's properly engineered to break at easily-repairable places. We've all seen trainers survive mishaps because the wing separated on landing, absorbing some of the energy.

This is gonna be a good thread...


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RC Design and Building - www.nextcraft.com

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Lighter structures - 8/7/2002 3:17:42 AM   
Thomasr


 

Posts: 66
Joined: 7/8/2002
From: Tonsberg, NORWAY
Status: offline 		To make stiff fuselage sides, I make them with triangles (as described above), because it is very light, and extremely stiff, if you take the weight in consideration. Wherever I have to use heavy wood, I make as much lightening holes as possible. I try to use single beams in stead of covering an entire surface with balsa.

I have a little fun-fly plane ready to be covered (if i ever bother to do), it has a wingspan around 90cm, and weighs very little (will find out exactly how much tomorrow). Almost all of the plane is built of balsa, wherever it is possible without making the structure weak there is holes.

When I make foam wings, and want them to be stiff, but extremely light, I remove very much of the foam, and insert carbon fiber rods to stiffen up the structure, works great! I`ll make a drawing of it tomorrow, to give you the idea of it.

I`m no engineer, so I can`t answer your questions regarding angles, and I have no facts to back up what I`m saying either, just my own personal experience.

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Lighter structures - 8/7/2002 6:47:18 AM   
jtech



Posts: 150
Joined: 12/10/2001
From: Las Vegas, NV,
Status: offline 		Yes the stick truss type fuselage construction is very light and very strong as well; I like the stick fus!

However those very light wing designs bother me. A wing should not be constructed so light ( and weak) that iron-on covering can warp and twist the wing. What happens as the outside air temperature changes or your airplane sets in direct sunlight?

Airplanes should be built on a near perfect flat surface and hold their shape or rigging until the aircraft expires.

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jim

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Lighter structures - 8/7/2002 10:11:53 PM   
Cactus.



Posts: 6138
Joined: 12/19/2001
From: CamborneCornwall, UNITED KINGDOM
Status: offline 		use a diffrent covering, i used Easycoat on my new Diamond Dust unlimited rebirth type thing, it only srinks to the heat set, ie, a hot day isnt enough to change it, and if it did warp over time, you turn up the heat a fraction and redo.
a covering like this can also add strenght to a light structure.
i did a foam wing once with the center cut out as well, that was light.
also i crashed a light glider a few days ago, unlike any of my power planes with the same wing bolts, this bolt snapped and the plane and OHHHHHH SOOOOOO light wing where left 100% undamaged

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Lighter structures - 8/7/2002 11:18:41 PM   
Al Stein



Posts: 968
Joined: 1/7/2002
From: Johnstown, PA, USA
Status: offline 		The Divided Warren Truss type fuselage structure that we use a lot is based (at least in balsa stick construction) on all cross members (both spreaders and diagonals) acting under compression. That lets you rely a lot less on the strength of glue joints to hold the structure together.

The truss is way way lighter than a solid sheet part, but if you'd like to lighten the structure even more, all you have to do is reengineer the diagonals to work in tension insted of in compression -- then you can substitute much lighter materials.

In the Divided Warren design, the verticals maintain the spacing between the longerons to keep them from bending toward or away from each other, and the zig-zagging diagonals keep the truss from shearing -- i.e. they keep the longerons from moving inopposite directions from each other. The top-right to bottom-left ones prevent the upper longeron from moving left with respect to the lower, and the bottom-right to top-left ones prevent shearing in the opposite direction.

So, what I do is remove the diagonals (that leaves a horizontal ladder shaped structure) and add criss-crossed kevlar threads in each "cell" of the "ladder." Instead of preventing topright shear with a balsa compression resiting member from top-left to bottom-right, I prevent it with a tension resisting piece of braided kevlar from bottom-left to top-right. I do both diagonals in each cell of the ladder because the Kevlar weighs practically nothing... and at 35 to 70 pounds test strength, it ought to be stronger than the eliminated pieces.

...Ooh, did I say this was my idea? Actually it's a pretty common feature in very early airplanes... WWI vintage and before. They used lots of rectilinear sticks held straight by diagonal tension wires.

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Lighter structures - 8/8/2002 2:11:53 AM   
DICKEYBIRD



Posts: 2671
Joined: 4/8/2002
From: Collierville, TN, USA
Status: offline 		Great post Al! Just as I was scratching out a sport park flyer for GWS power and was looking for ways to "add lightness."

Do you use one long thread and tie it at each junction with a Sots knot and then "freeze" the knot with Cya? How do you keep from inducing warpage, leave the thread very slightly loose and let the structure give a little? Does Kevlar "grip" well enough to hold a knot? I've used Spectra before and it knots like a greased noodle. So many questions....sorry!

Milton Dickey

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Composite Materials - 8/8/2002 5:52:52 AM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		The home built aircraft community has used some interesting unidirectional pulltruded carbon rods. See:
http://www.continuo.com/marske/carbon/carbon.htm
This material is available to model builders in small quantities from Composite Structures Technology and Model Research Labs.

Compared to 11 pound per cubic foot balsa, the pultruded carbon rod only needs 1/150 the crossection for the same strength. It has about fifteen times the strength to weight ratio of 11 pound balsa. This could lead to serious weight savings but we would have to give up our razor blade carpentry and learn how to use it effectively in structures.

Although most of you are not interested in R/C gliders, Mark Drela has designed some interesting structures for these types that have incredible strength to weight ratios. These structures are worth studying. One of them, the Allegro Lite, has a 2-meter span, weighs 18 ounces ready to fly and is capable of withstanding over 100 G maneuvers withot complaint. See:
http://www.charlesriverrc.org/articles/allegrolite2m/markdrela_allegrolite2m.htm

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Now we're getting somewhere... - 8/8/2002 5:55:57 AM   
Mike James



Posts: 2465
Joined: 1/19/2002
From: Anchorage, AK, USA
Status: offline 		Welcome Ollie!

I knew you'd have some good information to share on this subject.
Thanks!


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Mike James
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Lighter structures - 8/9/2002 2:10:37 AM   
Joe Petro


 

Posts: 264
Joined: 2/24/2002
From: Utica, NY, USA
Status: offline 		I enjoy building light!

One problem that I encounter is that when I sell one of my own designs, or a plans built design, the new owner usually has a few "hard" landings, and looses some important pieces of the aircraft (gear, stab, fuse, etc).

The excuses are usually "you didn't use enough glue", or "you built it too light" (my favorite).
When I tell people that I build to fly, not crash, they look at me funny.

What's funny is that I can fly and land these planes for hundreds of flights with no problems at all, and when someone else flies them and bounces them in, I don't know how to build...

I feel better now!

Joe Petro

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Lighter structures - 8/9/2002 2:13:47 AM   
Cactus.



Posts: 6138
Joined: 12/19/2001
From: CamborneCornwall, UNITED KINGDOM
Status: offline 		the ED3 gets the same thing.
tho i think this is the point of this thread, how can we do this, and still have the strenght

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Fly or Crash - 8/9/2002 3:26:47 AM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		It's not just a choice between designing light to fly or strong to crash.

For example the practise is to use dowels at the leading edge and 1/4-20 nylon screws at the trailing edge to mount and align the wing.

In my opinion 1/4-20 nylon wing mounting screws will destroy the trailing edge and pull out the mounting blocks before the screws shear. The screws should be no larger than necessary to carry the worst case flight loads and not big enough to propigate failure into the structure in a crash. For a 1.20 size model two number eight nylon screws are plenty big enough. A pair of number 10 nylon screws are more than adequate for flight loads on models up to the 55 pound limit. In a crash they should shear and minimize the damage. For the nylon screws to shear cleanly the screw has to be supported closely by hard material on both sides of the junction between the fuselage and wing. To minimize the stretch of smaller screws the heads have to be deeply countersunk to limit the length of unsupported threads. The bulkheads ahead of and behind the wing should slope so that the wing can come off easier when the bolts shear.

Similarly for landing gear. It should fulfill its normal function but break away cleanly in a crash. Engines should be equipped with prop screws that will break away before the crankshaft or bearings fail.

None of these measure cost any appreciable weight and may actually save a bit by reducing the structural reinforcement required. A little creative design thought can ease many problems of this type. There should be a thorough failure analysis after every crash and the structural design modified to reduce failure propigation without increasing weight. Structural design will then evolve in a better direction.

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Lighter structures - 8/10/2002 1:43:42 AM