prop-driven rotor for flying crane?

[slowrotor]

The idea of a prop-driven rotor intrigues me for the obvious reason of simplicity, low weight and torqueless.
It could only be used on a slow turning rotor, such as might be employed on a flying crane, or in my case, for a slow sport copter, because of the prop stresses from rotation.

I found some historical testing was done back in the 1920's by Brennan and Nagler and Flettner.

In 1986 Bill Patterson and students at Cal Poly built and flew the first human powered helicopter and it used tip propellers. www.calpoly.edu/~wpatters/helo.html
Tip jets have been used, but jets are inefficient and the noise is extreme.

Does anyone have comments about tip propellers, why wouldn't it work?
Thanks,
slowrotor

p.s. I did a search of this site and found no mention of prop-driven rotors.

[NickLappos]

Ouch! The typical tip speed for a helicopter is 700 feet per second in a hover, about Mach .6 or so. It then goes faster as you get to higher airspeed, up to as much as Mach .9 at high speed. A propellor becomes a boat anchor at about Mach .5 or .6.

Also, how will you get the power out there? A shaft up the blade spar? How about varying the power - a variable pitch prop?

Not a very practical design. For the manpowered rotor, with a tip speed 5% of a practical helo, it was a fine idea (mostly for structural and anti-torque reasons).

[zeeoo]

Hard to speak after Nick,
but may i also add that centrifugal forces applied to the engine are huge , so , the blade would need an extra reinforcement.

tip jets were inefficient, noisy.. but ..were.. no one really tried to re-visit this simple and wonderful concept.

Thanks

[NickLappos]

The fundamental problem with tip jets is that they apply their thrust at the highest speed point of the aircraft, and since thrust times velocity is power, they waste power by the bucket, even were they at high jet efficiency.

Big probem, even if you solve the noise and complexity, and the fact that you packed a jet where the thinnest part of the blade wants to be.

[teeteringhead]

The fundamental problem with tip jets is that they apply their thrust at the highest speed point of the aircraft, and since thrust times velocity is power, they waste power by the bucket, even were they at high jet efficiency. Would a possible solution then be to mount the jets further inboard, say mid span to avoid reverse airflow problems nearer the roots??

It always seemed an elegant solution to one who (before I got near a real helicopter) just about remembers the Rotodyne and also built a "Jetex" tip powered helicopter......

...edited to add..

in fact, now I remember more clearly, the JETEX model was indeed powered not by tip jets, but by jets on "stub rotors" perpendicular to the two aerofoil blades..

...edited again...

..and I even found a picture! (http://jetex.org/models/kits/kits-other.html) ...isn't Google wonderful!

[slowrotor]

Nick,
You asked, how would the prop be driven? Good question!
How about electric motors, several motors situated along the blade/wing. This conceptual idea is meant for a low tip speed rotor only. That is why I suggested flying crane.A crane does'nt need a fast cruise to be practical and there are other uses for a simple rotorcraft that can hover long term efficiently. In a very large conventional rotor the shaft torque is huge and a tail rotor and long tailboom would be huge as well.

Here is Brennan's huge prop rotor.http://avia.russian.ee/vertigo/brennan-r.html

Multiple electric motors used in the unusual Helios high altitude aircraft.http://www.aerovironment.com/area-telecom/telecom.html

[zeeoo]

About tip-jets.

Don't you think the nozzle doesn't have to be ON the tip but could be about 0.8 R and under the trailing edge , no necessarely round but could be square or rectangle.just wild ideas..

Thanks

[NickLappos]

teeteringhead,

I had entirely forgotten about Jetex! That helicopter is quite something! I note it flips from powereed flight to autorotation by delta 3, as the blades lags forward after the rocket cuts off, neat trick!

slowrotor, the power needs are awesome, and the motors to deliver that power are quite substantial, but it might work...... Uh Oh, you have me thinking!

[Dave_Jackson]

For the fun of thinking outside the box, and provoking thought, the following is submitted for consideration;

Root Jet Rotor (http://www.UniCopter.com/0002.html) :8

[zeeoo]

Dave, nice joke
i dont' understand why you wanna power only adv blade.
If the jet efficiency is based on a reaction air/air the air cushion for the jet reaction is supposed to be more dense ( relative wind) on the retreating blade..no ? if you blow on a jet from behind, you add some thrust..or am i missing something. ?
thanks

[slowrotor]

Nick,
Yes a small disc requires large power... but a large disc requires small power, the extreme being the human powered rotorcraft that flew with between .25 and 1 horsepower.
In between these extremes exists a disc diameter that would require,lets say, for a sport copter about 10 to 20 hp.
This could be done with readily available motors for a few thousand dollars.

I just need to know if there was some reason why the prop drive was abandoned in the past.
Guessing maybe vibration was a problem, but small multiple props might work.

[NickLappos]

slowrotor,
You are right about disk size, but the practical use of rotors with 1/2 lb per sq ft disk loading is nil. Helos that actually fly in the real world need to have disks that fit within counties, and structures that stay together in winds and maneuvers.

Smaller disks that are practical bring tip speeds that make props totally impractical.

[Dave_Jackson]

zeeoo,

No joke. The idea is intended as serious, but it is not thought out.

" the jet reaction is supposed to be more dense ( relative wind) on the retreating blade..no ?"

During forward flight, it appears to me that any force that resists this forward flight is a disadvantage.

This has been expanded on here; Additional Stuff (http://www.UniCopter.com/0002.html#Additional_Stuff)
____________

slowrotor,

Some trivia: The students up here at UBC tried to take off in a man powered coaxial about four months ago. one rotor diameter was about 160 feet and the other about 130 feet. It never came close to lifting off.

If you're interested, there is a 40 to 50 page document at the Seattle Public Library that covers the requirements to achieve manned rotary flight.

[zeeoo]

Dave,
the truth is probably somwhere between that.

IMO you will spin faster and better with jets placed at a higher diameter.

About air distribution, Is the xtra complication worth?
how do you think you could get enough airflow ?
how do you use reverse flow ? it only will tend to orient the jets up, loosing efficiency..

thanks

[Dave_Jackson]

zeeoo,

Earlier you said "how do you think you could get enough airflow ?"

Easy and cheap. The way the US is tossing out new projects, an F-22 Raptor power-train should be coming up on E-bay any day now. :D

Modify the fan to power the rotor blade(s) and drop the whole power-train into the rotorcraft.

Dave

[ShyTorque]

"PERFORMANCE: This Helicopter has been designed so that the Rotor Blades go into auto-rotation the moment the power stops. It will then glide down without damage. Examination of the hinge angle will show that the blades fold UPWARDS, so the angle of attack of each blade decreases. This gives the blade less drag when running free and causes auto-rotation. Centrifugal force keeps the blades in the horizontal position when the rotor is under power."

With regard to the last sentence - can't possibly have worked - There is no such thing as
centrifugal force

I've read it here, on this very website! ;)

Ah, Jetex! I spent days building a Jetex powered dope and tissue aircraft - only to see it burn to ashes in seconds as soon as the fuse was lit!