Materials for Large (6 inch by 6 foot) Helicopter Rotors

[BABAR]

Daydreaming about upsizing the Whopper Flopper Chopper. Will have folding rotor design, the 4 inner rotors will be 6" by appproximately 6 feet (looking at an H or I motor.) Cost is definitely an issue. So is "stiffness" (can't have a lot of flex.) So is weight.

Was thinking about 1/4" balsa, I can get 6" by 3 foot sheets and somehow join them (I'm sure there is an appropriate woodworking term for it.) I have never worked with plywood, not sure if I can get equivalent strength at same weight as balsa. Foam-board and other non-organic materials would be possible, but not sure they would have the strength at that size.

Any ideas?

Thanks.

Tom

 

[awseiger]

I would honestly say that this kind of thing is best left to composites....

I would do something like a peice of foam-board covered in fiberglass. It's not hard to implement, and you will end up with a much stronger and lighter rotor blade. It may be worth your while to do the first foot with 3 layers, the second foot with 2, and the rest with 1 or something to minimize weight.

a 6-foot long piece of anything is going to flex under its own weight significantly. And realize that a 6-foot long thing spinning around is going to have *ALOT* of speed and *ALOT* of force. Strength is paramount here....

I can't wait to see this.... It's entirely do-able (look at any helicopter ever) but it will be a significant challenge.

 

[Aksrockets]

Definitely look at Carbon Fiber'd balsa. It should be perfect for this application.

op:

Alex

 

[Zeus-cat]

You also need to consider that long, stiff blades spinning around will be a safety hazard if this rocket comes down near anyone or anything.

 

[JeromeK99]

You also need to consider that long, stiff blades spinning around will be a safety hazard if this rocket comes down near anyone or anything.

ANY rocket that comes down near anyone or anything is a safety hazard! But, ya... spinning 6' foot rotors might get someone's attention!


Jerome

 

[BABAR]

Both fiberglassing and carbon fibering would be new techniques for me, but I am open to them. Would they provide a strength and rigidity comparable to or greater than 1/4" balsa at a 6" x 6 foot length, and what would the weight and thickness be compared to the balsa?
All safety concerns provided are appreciated. My helicopter rotor designs are VERY slow rotators, they are not air foiled, to be honest they work well but they are more like air brake recovery that just happens to spin lazily around than "real" helicopters. In fact , the Spyder Camera series I put out was a relatively simple design with four "blades" that didn't spin at all. They all rely on LOTS of surface area. Since the goal is to make something different, something safe, and something that looks cool to me (I've decided since it is my rocket I am the only one that HAS to like it!) they work fine for me, and seem to provide at least a bit of amusement for my club buddies. My goals are also to keep the rocket in visual site at all times, so enough altitude to give me about a one to two minute launch to touch down time is perfect.

I will draw up some plans and post them. Maybe start with a single hinge rotor and work up to double hinge like the whopper flopper chopper.

Thanks again,

Tom

 

[powderburner]

Sounds to me that what you are describing falls into the category of built-up model airplane structures like the free-flight guys use. Balsa ribs, spruce spars (possibly reinforced with carbon rods, at least on the inbd three or four feet), balsa leading edge and trailing edge inserts, etc. You will probably want a small plywood insert near the root to take the hinge loads. Depending on the rocket configuration you might still be able to skin the wings with a monocote-type product, saving even more weight.

Building with a constant-chord wing shape makes it a bit more simple (all the ribs are the same) but designing with a little taper root-to-tip would probably have some performance advantages.

For multiple rotor blades, you will almost certainly want to make a jig for building each blade in a repeatable, consistent shape. This would (IMHO) be easier than sanding the same airfoil into multiple giant solid balsa rotor blades. The built-up rotors would be just about the lightest way to go.

Possibly you could use a light foam and hot-wire cutter, but I am not sure what consistency or accuracy you would get from a six foot long hot-wire setup. You would need to fiberglass an outer skin on the thing; I would be careful about using too much because the rotors could get heavy in a hurry.

As to people having to duck out of the way of "whirling wings of death," I would expect the rpms on a monster like this would be slower than you might think. Just remember: light weight = low rpm.

 

[stickershock23]

Honeycombed Fiberglass.. light strong... expensive.. but that would be the way to go!

 

[degreaser]

Check out https://www.flyingfoam.com/ . He has CNC hotwire foam cutting capabilities. I think 6' should be within his work envelope. Then put a light fiberglass or carbon skin on it. If you would rather do it yourself, buy the set of DVDs from https://www.tailwindgliders.com/Video.html. Great videos. He covers bagging as well as hot wire foam cutting.

 

[BABAR]

Check out https://www.flyingfoam.com/ . He has CNC hotwire foam cutting capabilities. I think 6' should be within his work envelope. Then put a light fiberglass or carbon skin on it. If you would rather do it yourself, buy the set of DVDs from https://www.tailwindgliders.com/Video.html. Great videos. He covers bagging as well as hot wire foam cutting.

Thanks! This might work, or at least save me a lot of time. Looks like I could get 72" x 6" foam core airfoil constant chord for about $19 each (actually I think it would be two 36" pieces totalling 72 ", the cost quote is for a 72" wing span.) So four rotors for about $80 plus shipping (this is going to be my most expensive rocket, LOL!) Still would be worth it just in the time saved by having the airfoild built in.

Challenge: I would like to have the option of adding a flip out additional hinge rotor at the lateral/outer aspect of the span (A la "Whooper Flopper Chopper". This would require a very strong spar on the inner rotor AND a fixed hard "plate" at the lateral aspect of the span. I am at work now and can't draw this out, but will add a drawing later.

Start with a 72" x 6" constant chord foam wing, split it lengthwise at the apex/peak of the airfoil (probably a better word than this), and ran a length of, (say the mid airfoil width is 2.5") 2.5" x 1/4" balsa down the length, than added fixed 3"x6"x1/4" balsa plates at the hub and lateral margins to provide attachment points for the central hub hinge and the lateral swing hing (the swing out rotor would be a flat plate of 1/8" by, saay, 5" balsa about 36" long
0---- anyway, the BALSA provides the structural integrity . The foam provides the shape. As for COVERING the foam, since this is not providing any significant structural strength, would MonoKote be adequate and lower weight penalty than carbon or fiberglass?
This would provide a total rotor length of almost 9 feet, little bit less due to the overlap of the hinges. Accounting for dihedral, would be looking at a rotor cone diameter of around 16 feet.

Again for those concerned about injuries from spinning rotors, I believe the air resistance from these large rotors simply will not allow them to spin very quickly. In fact, if it was spinning that fast, the outer 1/8" balsa flat plane rotor would shear off.

Again, continued thanks for the input. You guys are already pretty clear on the concept that I am very much a novice at this stuff.

Tom

 

[fyrwrxz]

Tom-FWIW, Monocote does not have any real flex strength by itself-it's use in aeromodels is usually 1) to provide a 'skin' to bridge the gaps between stringers, ribs, etc. as it's lighter than planking. 2) provide a smooth surface usually lighter than obtained by painting. There are carbon fibre 'veils' relatively light weight and don't forget to investigate the offerings from high performance kite manufacturers for carbon rods. You can also find those on e-bay. Sounds like a very impressive project-carry on!

 

[mbauer]

Rotor blades gain their strength from Centrifugal Force. Think of a bucket spun on the end of a rope. Rope is limp until spun with a weight on the end.

Huey rotor blades have a weight at each end. When not in motion you can bend them easy, once rotating they are incredibly strong.

Unless you plan to use a motor to spin your rotors, you are dealing with three areas of rotation on the rotors for helicopter recovery. Stall area near the hub, Driving Region is rather large area, and finally the Driven Region near the tips. The Driving region needs to overcome the Stall and the Driven regions to work for recovery.

To get everything to work, you're going to have to make sure the blades are balanced perfectly to get the maximum rotation out of them.

One way of making your blades light and strong would be to use a hot knife to form foam into the proper airfoil shape, light weight fiberglass over the foam before disloving the foam to make the blades hollow and light weight. Then add your weights to the end of the rotors to balance them and give them the strength needed for recovery.

Know about rotor blades from being a helicopter pilot.

Hope this helps.
Mike

 

[Zeus-cat]

Centripetal force is the force on the weight at the end of the rope (or rotor blade). Centrifugal force is the force acting on the hub of rotation (you can think of it as balancing the centripetal force). And contrary to what people think, the centripetal force is inwards toward the center of rotation and not outward as people think.

This is relatively simple to prove. Get a bucket and fill it with water. Spin it around a few times. During one of the rotations, just as the bucket reaches the top of the rotation, let it go. Which way does the bucket go? Up or parallel to the ground? If you really released it at the top of the rotation it should travel parallel to the ground. There is no force pulling the bucket "out". Another way to think about it is an orbit. Does a satellite in perfect orbit get accelerated out into space? No, gravity constantly pulls it toward the earth, but its velocity is parallel to the ground at each instant and so it's velocity parallel to the ground is exactly what is needed to keep it at the exact same altitude during its orbit. The acceleration is always towards the earth, never away from it.

 

[BABAR]

Get a bucket and fill it with water. Spin it around a few times. During one of the rotations, just as the bucket reaches the top of the rotation, let it go. Which way does the bucket go? Up or parallel to the ground? If you really released it at the top of the rotation it should travel parallel to the ground.

But the end result is still Inertial Dampening.

 

[mbauer]

Centripetal force is the force on the weight at the end of the rope (or rotor blade). Centrifugal force is the force acting on the hub of rotation (you can think of it as balancing the centripetal force). And contrary to what people think, the centripetal force is inwards toward the center of rotation and not outward as people think.
QUOTE]

I stand corrected, Centripetal Force is correct!! Thank you for the correction, still don't know why I used centrifugal....

Mike

 

[Micromeister]

composite constructions for Large model rotors isn't that difficult.
some years back I made a very large Up Scale Rota-roc Helicopter Demonstration model. It's rotors are 4.5" x 56" constructed of SM Styrofoam air foil with 3/16" diameter carbon fibre Kite tube reinforcements. these Foam airfoils are covered with 3 color adhesive backed vinyls. Works very well.

Aircraft stick and tissue method is also a very doable option.