Backpack helicopter

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Zippiot
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re: Backpack helicopter

Post by Zippiot » Thu Feb 16, 2006 3:07 pm

500 for cf work!?!?

i would say expect to pay 1500 for a precision application of that size...possibly even more. the world is out of carbon fiber, only a few sites on the internet still have it, and prices have doubled in the last 2 years...
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re: Backpack helicopter

Post by Mike Everman » Thu Feb 16, 2006 3:21 pm

If you're going to put this on your back, any sentence that starts with "it would be nice" can't end with "but it adds weight"! ;-) ha

Another significant thing you're fighting with rotating ducts is gyroscopic precession; adding moment of inertia is a bad thing here. If you pitch forward, one rotor will roll right, one left, crash. The nice thing about the system as it stands is that this precession is cancelled out by having two that counter-rotate, with added stress on the bearings when manouevers are made. It's this precession on single rotor craft that makes them so hard to control, I think, have to look that up.

He's got it as simple as it gets. The small diameter he's got is likely to limit all of these deflections. Of course it wouldn't be much of a discussion if we all agreed that it's perfect already!

I'd add tip winglets to limit tip vortices (a major source of losses, and one reason that ducts work so much better) to the rotors at slightly different diameters, with big skinny rollers so you'll have a non-destructive warning when they start to snuggle up to each other.
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re: Backpack helicopter

Post by dynajetjerry » Thu Feb 16, 2006 8:23 pm

Hi, Guys,

I am moved to add a little info to what you have been discussing.

The problem of varying lift on a heli's blade as it advances in the direction of flight and recedes has been addressed in several ways. Sikorsky developed a complex but relatively obvious solution, at least for his larger machines. Using the concept first espoused by de la Cierva, his blades were permitted to flex up and down as loads changed and also fore and aft. Pitch of individual blades are also adjusted during rotation.

Igor's system provides a hydraulic cylinder at the root of each blade that extends and retracts as necessary, to swing the blade forward (as it moves opposite the direction of travel,) and back (as it moves with the direction of travel.) In this manner, the "airspeed" of each blade varies less than if it did not swing. To further compensate for the variations in a lift blade's air speed as the heli moves, the pitch of each blade changes, depending on its movement relative to the direction of movement of the craft.

De la Cierva and Pitcairn worked with autogyros but their solutions to the problem of cyclical changes in effective lift of each blade was also applied to helicopters.

So-called "rigid rotor" helis employ different concepts and I am not conversant in any of them.

Jerry

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Re: re: Backpack helicopter

Post by Bruno Ogorelec » Fri Feb 17, 2006 10:03 am

Anders Troberg wrote:
One is BIG -- you don't need cyclical pitch variation.
You lost me there. Why is that?
Because the blades no longer travel in line with the air stream. The blades of a free rotor normally advance into the airstream on one side and regress on the opposite side. The duct shields the rotor from the air stream. It forces the air to travel roughly in axial direction, perpendicular to blade travel. (This is a vast oversimplification, of course, but valid enough.)

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re: Backpack helicopter

Post by Anders Troberg » Sat Feb 18, 2006 6:22 am

Ah, but hasn't we already taken care of that with the dual rotor configuration? That should more or less even things out.

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re: Backpack helicopter

Post by Zippiot » Sat Feb 18, 2006 6:32 am

any idea how he controls the little yellow fin in the back? its moving like crazy...
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re: Backpack helicopter

Post by Jim Berquist » Sat Feb 18, 2006 7:31 pm

Rotors contacting each other could be a none issue as they are so short.

All I could see was simplicity.

Did the rear stablizer just provide means to rotate dirrection using prop wash?

All turns looked as though they were exicuted by weight shift.

The blue bottle could had been a NOX thing.

Look spooky to me, but I still want one.

The drive system is most compact.

Like you said he may have been able to fly! Looked a little under powered to me.......Jim
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re: Backpack helicopter

Post by Anders Troberg » Sun Feb 19, 2006 6:28 am

I have looked at the video a bit more and have a new theory about the strap between the front supports. Very simple, it may be that he put it there to have some place to rest his feet in flight.

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Re: re: Backpack helicopter

Post by Bruno Ogorelec » Sun Feb 19, 2006 12:34 pm

Anders Troberg wrote:Ah, but hasn't we already taken care of that with the dual rotor configuration? That should more or less even things out.
No, that only takes care of the torque reaction, so he doesn't have to have a tail rotor to prevent the helicopter from spinning.

There's very little you can do about unequal lift of the advancing and retreating blades except employ some kind of mechanism that changes blade pitch or (as Dynajet Jerry has pointed out) sweep angle.

Or you can put the rotor inside a duct.

Or you can ignore the problem and only fly slowly. The lower the forward airspeed, the smaller the magnitude of the problem.

I have not seen it mentioned anywhere, but a weight-shift helicopter or gyrocopter will probably have teh pilot correct for thus automatically by tilting the rotor slightly, so that the retreating side of the rotor disc would be horizontal and the advancing side drooping a little. Lift vectors might be equalized that way.

Not terribly efficient, but very little about a backpack-size weight shift helicopter will be very efficient, I guess.

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re: Backpack helicopter

Post by skyfrog » Tue Feb 21, 2006 2:33 am

Hi,

Found this site, good for beginners who wish to learn more about helicopter aerodynamics.

http://www.copters.com/helo_aero.html
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re: Backpack helicopter

Post by Cita » Thu Feb 23, 2006 9:12 pm

The designer/builder is not German but Austrian,Franz Schoeffman.
The "gearbox" is located between the two rotors (IVO Ultra light propellers).
It are just 4 conical gears not unlike the car differential setup.
The lower rotor is driven via chain and sprocket,the conical gears reverse thu upper direction.

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re: Backpack helicopter

Post by Mike Everman » Thu Feb 23, 2006 9:57 pm

Welcome, and Thank you so much for the detail!
Do you have anything else? Is it in fact underpowered? Does he plan on a bit larger rotors or a different ratio?
This thing is very compelling.
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re: Backpack helicopter

Post by Cita » Thu Feb 23, 2006 10:50 pm

Rotax 125cc water cooled kart engine-28hp
Apparently there are cooling problems so the blue container is probably water.
Power is seemingly marginal but being filmed i Austria I dont know at what altitude this test was.
Could well be that at sea levell or on a cold winter day the power is just sufficient to fly in ground effect.
Rotor diameter is 2.13m.
Rudder vibration is due to sloppy connection.Two cables activate the rudder.

Picture is my own project.

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re: Backpack helicopter

Post by Bruno Ogorelec » Thu Feb 23, 2006 11:15 pm

Thanks Cita and welcome to our group here. It's a good place to be, though most people here are ever so slightly mad.

Your own backpack helicopter project looks great! Very exciting!

Are those two engines 125 cc each?

Also, I see the makings of another project in the picture, a single rotor with a big single-cylinder engine. Did you finish that one? Did it work?

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re: Backpack helicopter

Post by Dang911 » Thu Feb 23, 2006 11:17 pm

I race witha rotax 125, you should be getting MUCH more out of it than 28 hp. I get around 36 on mine..... What fuel are you running. Try alcy.... Much cooler
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