Backpack helicopter

[Zippiot]

i wonder if:
1. make the top blade out of kevlar, kevlar is very very strong but flexible. if done properly it is 10 times stronger than steel (not sure if it is tensile or yeild...or really the difference between the 2...)
2. what if an adjsutable fixed compressor blade was placed in between. if can change pitch to adjust wind flow and control. it might help the little motor out at low speeds, like variable valve timing on a car.


ducting as in a piece of material connecting the blades, like we see in those little toy helicopters where you pull the rip-chord to make them fly off?

and are you guys pretty sure the blu canister is an air filter? seems a little big...

[Anders Troberg]

As to ducting, even advanced materials wouldn't be practical. The weight is one issue, but the main killer is that the tips need to be so close to the inside of the duct for it to be worth doing, it would have to be unreasonably round for it's size, and stay that way under all conditions.

I wanted it mainly for safety, not for perfomance. If it's for safety, the gap is actually good.

ducting as in a piece of material connecting the blades, like we see in those little toy helicopters where you pull the rip-chord to make them fly off?

That would probably be enough for a huge safety gain.

and are you guys pretty sure the blu canister is an air filter? seems a little big...

I thought the same thing. I've even speculated in it being a huge fuel pump.

What's the black bend on the other side? Exhaust? Protection?

[Zippiot]

black thing is definately exhaust, look up a scooter or go ped, they have those types of exhaust. the little pipe expand sinto a big pipe then contracts again...

[El-Kablooey]

The black pipe is a 2-stroke's tuned exhaust pipe, but it looks like it is way oversized for that little engine. The blue canister may be a boost bottle.

[Bruno Ogorelec]

Hi Bruno,

I enjoy this video very much, thank you. Here's another instance of contra-rotating coaxial rotors :

http://www.gyrodynehelicopters.com/qh-50e.htm

You can find a drawing of the transmission system in that link. This is no more a secret as it has been 40 years old, or more. I am more interested in this one because it was powered by a turbo-shaft engine, the Allison.

Of course it is no secret, Horace. It is not even particularly complex. I am just saying it is complex for an amateur to engineer, especially if the assembly is to be exposed to bending loads, which the shaft 'mast' of such a helicopter certainly is. You can always make it massive very easily. But, doing it properly light for miniature helicopter use -- and bombproof-reliable -- is something else.

[Bruno Ogorelec]

Brick is right! I think the rule of this thing is, "don't fly higher than you are willing to fall, under a motor and shrapnel bomb"
As to ducting, even advanced materials wouldn't be practical. The weight is one issue, but the main killer is that the tips need to be so close to the inside of the duct for it to be worth doing, it would have to be unreasonably round for it's size, and stay that way under all conditions.

Mike, it need not turn within a duct. The duct could be integral with prop tips. You'd just have to keep the two rotors separated, so that they do not rub together even under peak loads.

[Bruno Ogorelec]

MORE DATA! Some of it confusing.

Here's a response i received to one of my queries:

----------
That's a 125 cc, single cyl motor hooked up to a coaxail drive, 21KW (28 HP) at 12,000 rpm. Built by a certain Mr. Schöfman in Germany.

It's the new version, since the previous one (a year back) used two
100 cc motors, each driving one rotor.

What I find the most impressive is getting that amount of thrust out
of a mere 125 cc!!!! Guess the prop diameter does some of that and
the fact that both props must be running at a very low AoA (pitch).
Close to best glide and without a problem since that angle will not
be reduced by aircraft forward speed.
No idea if the wash from the one prop makes the second prop's work
easier but I wouldn't think so. Possibly even the opposite.

That guy does not look particularly slight, so say 75 - 80 kg, the
contraption itself maybe 35 - 40 kg including rotor blades and gas.
He's getting 110 - 120 kg thrust out of 125 cc??!!!!WOW!!!!! The
prop's efficiency numbers must be right up there with the best.

Or does the math change completely when it comes to rotorcraft?
----------

The 28 Hp figure is not too bad in my eyes. I am less surprised at the thrust figure, too. The 120 cc Radne Raket engine developing 15 HP and spinning a 2-bladed 1360-mm prop is claimed to generate roughly 50kg of thrust in level flight. Double the power and two props might indeed produce double the thrust.

What it all says is that we need a bit more -- perhaps 150 kg thrust, meaning perhaps a 40-50 HP engine. Maybe less with a ducted fan, which would be more efficient.

What I'm thinking about is this: The engine on Herr Schöfman's little spinner must weigh at least 40 pounds without fuel. More with the reduction drive.

Can we build a 150-kg (300 lbs) thrust pulsejet (or two 150 lbs ones) and have them weigh less than 40-45 pounds? An iffy proposition. And it would be a very bulky assembly. Damn.

[Greg O'Bryant]

Bruno:
ArroProp has stacked 4 blade airboat props that have the second 2 blade prop with more pitch than the first one. That may give an answere to the wash helping the second one. They claim it is a more eficient.

[skyfrog]

Of course it is no secret, Horace. It is not even particularly complex. I am just saying it is complex for an amateur to engineer, especially if the assembly is to be exposed to bending loads, which the shaft 'mast' of such a helicopter certainly is. You can always make it massive very easily. But, doing it properly light for miniature helicopter use -- and bombproof-reliable -- is something else.

Bruno, when I said that I was thinking of a Taiwanese company producing coaxial drive helicopters for personal use, and obviously they treat it as secret. They might have a patent on it, really don't know what's new inside the gearbox, however it is powered by two seperate engines, hence safer than single engine configuaration. Since it cannot provide autorotate capability, twin engine might be a good choice.

Check this site for info of the Taiwanese personal helicopters :

http://www.yoshine.com.tw/index.htm

[Anders Troberg]

Mike, it need not turn within a duct. The duct could be integral with prop tips. You'd just have to keep the two rotors separated, so that they do not rub together even under peak loads.

The problem of not rubbing together must be handled anyway, with or without ducts. In fact, a duct probably makes this safer, as it will only rub together instead of clashing together.

This is a very real problem. Look at the Kamov helicopters, it's not a coincidence that there is such a large space between the rotors. A hard landing can cause the blades to flex quite a bit. In fact, when my father had an accident with a helicopter (engine failure with hanging load and no good landing space), the blades flexed so much that they bashed the tail boom clean off the helicopter. This caused the only injury he recieved in a crash (four crashes total), as the stick struck his hand and he lost a small piece of skin on one finger.

[Bruno Ogorelec]

I was thinking of a Taiwanese company producing coaxial drive helicopters for personal use

Horace, thanks for the link! That machine looks like e very serious piece of kit. That rotorhead in itself is a piece of work. I'm sure Mike will enjoy looking at it. It looks horribly expensive to me, though.

[skyfrog]

Just found this English website, same company,

http://yoshine.com/home.php

I've been tracking the development of this company for a while. Let's see what will happen in the next few years.

[Bruno Ogorelec]

The problem of not rubbing together must be handled anyway, with or without ducts. In fact, a duct probably makes this safer, as it will only rub together instead of clashing together.

Uh-huh. You are right.

There are two bonuses about the ducted prop. One is BIG -- you don't need cyclical pitch variation. That's a huge saving in cost, mass and complexity.

Another is perhaps smaller but still very nice -- the duct itself generates lift passively, like a wing, when moving through the air. The ducted-fan Hiller Flying Platform reportedly generated 40 percent of its lift at full forward speed that way. That's a LOT.

I have looked at multi-blade ducted fans and it turns out that a carbon fiber one of some 50" diameter generating some 200lbs of thrust will set you back some $ 500. We are talking of a custom design, which will inevitably cost more. (And I would certainly leave this one to professionals.) Two smaller ones for the same total amount of thrust would set you back some $ 250 each, so we're back to the same figure. Probably double that for a custom-designed prototype.

But I must say that two ducted props stacked one upon another would look either mindboggling or totally silly or a mixture of the two.

[larry cottrill]

But I must say that two ducted props stacked one upon another would look either mindboggling or totally silly or a mixture of the two.

Oh, I don't know - how about having a small one fire into the center 70% area of a big one, and that into a bigger one. Ducted-fan boosted multi-stage ejector action. Of course, if you like, fit a big afterburner to the tail end, for motorjet fun ;-)

Nuts! Sorry - thought I was still in the "Far-fetched Idea" thread!

L Cottrill

[Anders Troberg]

One is BIG -- you don't need cyclical pitch variation.

You lost me there. Why is that?

But I must say that two ducted props stacked one upon another would look either mindboggling or totally silly or a mixture of the two.

As the rotors are on the same axle, perhaps the duct could be put on one of them and be made high enough to cover both? Attach it to the top rotor, then through three supports to the rotor axle below the lower rotor. Should give the needed precision as long as it's sturdy enough to remain circular. In fact, when I think of it, the lower supports could be rotor blades driven by the rotating duct, giving us a three rotor aircraft! I hope I don't have to test fly the prototype...