Structural Question
 

Hi all. I have a Global Raven 60 kit and was contemplating replacing the lite-ply fuselage sides/bottom with the same-size (1/8th in.) balsa - keeping the lite-ply formers. I want to do this for 2 reasons, 1st the lite-ply seems like it's quite heavy and 2nd it is pretty badly warped.

I also want to replace the inner wing ribs with balsa as they are lite-ply as well - for weight considerations.

My question is will the airframe still be strong enough if I do this? I'm guessing it will be OK, but I would love hear other folks opinions on this as well.

What do you all think?

RE: Structural Question
 

I wouldn't do this.

plywood stands better through stress than balsa.
stress = force / area
means you need much more balsa to stand the same amount of stress as plywood.

who ever designed this airplane didnt choose balsa for a reason.

how many ounces do you think you are gonna win?
while you are risking the whole airplane.

cant you just put a bigger motor?

I dont know the global raven 60

RE: Structural Question
 

The truth is, if you adopt a well thought out plan to place this airplane on a diet during the build as you are already doing, you will be much happier with the overall performance in the end. If you go extreme on your diet you can fly with a smaller, lighter powerplant and still out perform in all aspects of a heavier airplane. JMO ;)

Bob

RE: Structural Question
 

Just my opinion:

Relace the warped sides with home made plywood:

http://www.airfieldmodels.com/inform...wood/index.htm

Keep the ribs, too much work and little weight saving.[sm=drowning.gif]

Global Quality Kits Raven ARF review:

http://www.charlesriverrc.org/articl...ykitsraven.htm

Regards!

RE: Structural Question
 

Sound advise. If the sides are warped, build the fuse with the warps fighting each other. It is almost impossible to get absolutely straight wood, be it sticks and sheets or ply. Build so the warps are in oposition to each other to equal a straight build.

I'm working on an older scratch/plans built pattern plane. It has the old school balsa sides, and it weighs a ton, 13 lb actually, I bought it third hand from an ex club member. Icould probably cut the weight down to around 8 or 9 lbs using a lite ply construction vs the balsa sides and bottom.

When you have to cut a window in the side and put a frame to screw a lit ply sheet to to mount your switch to because the balsa sides are to thick for the screws, lite ply construction sounds better and bettter all the time. I'm talking about a GP Super Aeromater that I built a couple years back. A redesign of the kit using Lite Ply would probably give a model of around 6.5 to 7 lbs vs the 9.5Lbs it weighs today.

Build with what you have and build it square and it will be OK.

Don

RE: Structural Question
 

Don't worry about the warp, it will be straightened out in the building process.

RE: Structural Question
 

Take a look at my recent thread in the scratch build forum entitled "First Scratch Build". I built a small Ugly Stick off some internet plans, and I built the fuse sides and center wing ribs out of balsa (per the plan). In the last couple of posts, you will see the end result. During a relatively minor bad landing the plane experienced pretty serious structural failure. The balsa just didn't provide the strength where it was needed.

http://www.rcuniverse.com/forum/m_87...tm.htm#9141906

This may not apply directly to your question, but it was a good lesson for me.

Good luck

RE: Structural Question
 

So what you are saying is, light plywood airplanes come out lighter than a properly designed or modified balsa airplanes, and if they do come out too heavy, just bolt on more power. This type of thinking is wrong on so many levels, especially on little airplanes. Global model airplanes are cool, but far from an optimum designed airframe in terms of weight consideration. You can build the Global Raven in this size with an all up weight of 6 to 6.5 lb ready to fly. two points to remember, horse power is not an antidote for wing loading, and we build to fly, not crash.

Bob

RE: Structural Question
 

A. I said I wouldn't do it, but I sure would be happy to see a video of the first flight with the mods.
B. "So what you are saying is, light plywood airplanes come out lighter than a properly designed or modified balsa airplanes"
airframe was designed to stand an amount and level of stress not weight, stress is the first consideration here, than you think what kind of material with density[gr/cm^3] will stand this stress.
C. You know decades ago somebody was trying to build a car out of wood instead of steel it ended up being too heavy, they had to use alot of wood to get the mechanical properties of steel.
D. You want to replace 1/8" plywood with 1/8" balsa for a part that carries the whole tail section? why balsa than if you can use cork.
E. maybe the airplane does come heavy, installing weaker parts will make weaker in places were not designed to be weak, safety wise.

RE: Structural Question
 

which and who's?
Replacing a part of the airframe with a weaker material is an engineering decision.

RE: Structural Question
 

Bob, Iread my post again, just to make sure, and I didn't mention the word engine at all.

Don

RE: Structural Question
 

I recommended a bigger motor, seems to me its a better idea than just remove parts of the airframe.
if the problem is in the design that comes too heavy putting a bigger motor will make the airplane fly, taking parts of the airplane will make it not air worthy.

Alex

RE: Structural Question
 

I'd go and buy a decent bit of lite ply and use the originals as a template to cut your own non-warped fuselage side/top or whatever.
In the past, I've made fuselage formers by cross-graining balsa sheet and forming a ply construction, it's always worked well, but if the design originally used lite-ply, I'd stick with it, even if the kit manufacturer might have used the wrong grade of wood.

There are a couple of specialist balsa suppliers in the UK, luckily one near where I live in Essex. They will supply graded balsa and lite-ply to a cutting list; if you say what the balsa will be used for, invariably you get the right grade with matched pieces for each side. Surely there must be loads of them over there in the land of the free?

RE: Structural Question
 

Thanks for the replies folks. I like the idea of making my own balsa plywood for this task. The sides are warped in the same direction, unfortunately, and the bottom is quite bad. Funny enough, the balsa in this kit is actually quite nice, but the ply is garbage.

RE: Structural Question
 

The hardest part about asking questions on these forums is to decide who's advice is BS. I will say this, lighter fly's better especially on smaller platforms. You can pull alot of weight from these kits and gain performance without sacrificing safety as some may have you believe simply because they don't know how to get there. I have been posting airframes on a diet build threads for years and have helped hundreds of builders enjoy the benefits of lighter airframes. Here is a short video of just one of my 40 % airplanes, it weighs 28 lbs. so that tells you that I left nearly 10 lbs of wood and garbage out of the build, this airplane is now 3 years old with over 500 hard flights, you be the judge...

PS This airplane should have a 150cc powerplant on the nose, but I installed a 100cc powerplant because of the lighter all up weight.

http://icanflyrc.com/JRFlyin/slides/Bob260.html

Bob

RE: Structural Question
 

Just for what it's worth, making your own balsa ply is unlikely to give you straighter wood than you now have. My first build getting back into the hobby after a 40 year absense required laminating 1/16 ply onto 3 /32" balsa for the sides from firewall back behind the wings. Iused tighbond for the glue. I glued up the pieces then seperating with wax paper, I clamped them using some 4quarter x 4 stock and let them dry for a couple days. When I pulled them out of the stack, Ihad two badly warped pieces. Idecided to go with them and I had to use a couple 8" C clamps with verticlal boards to try to straighten the wood to the two main formers. What the end result was that I still had warped sides, not length wise, but from top to bottom. This was really appearant when I tried to cover it. Iwas trying to cover the inside of a pipe sawed in half. The big clamps let me get a good glue joint on the formers, but inbetween the warp was still there. The fuselage ended up like a bannana. After all this, the plane still flew quite well until I dumb thumbed it into the ground and killed it three years later.

Don't sweat the warped ply sides. When you get everything together, it will be straight as long as you follow directions well.

You probably don't want to know how warped the wood was that your house was built with. It all works out if the build is good.

Don

RE: Structural Question
 

A friend of mine built this kit (not the ARF) back in the 90's. The thing was way overbuilt and had way too much heavy plywood in it. It flew like the proverbial lead-sled. If you ever wanted to see what a high speed stall looked like, this was the plane to do it with. With it being so overbuilt, I'm sure that it could be lightened a whole lot without sacrificing any strength. You might even consider using 1/4 square balsa sticks to build up the sides, using the heavy ply sides as a template (Look at the SIG Kadets to see what I mean). A couple of 1/8 sheet balsa doublers at critical areas and you are in business. Good luck with it.

Cheers!

Jollidude