Thrust or lift?

[Saintsman]

I believe that rotor blades produce lift and 'pull' the aircraft into the air.

Why did they go down this route and not use a type of propellor to generate thrust to push it into the air?

Manouverability maybe?

Thanks,
Saintsman

[the coyote]

Possibly because it would then have to land on its rotor?

[ShyTorque]

Rotors running passenger movements / refuels would be a bit risky, too.

[212man]

Would you like to explain the difference between 'pushing' and 'pulling'? Depends on which side of the propellor/rotor you are, surely?:ok:

[lelebebbel]

because that would violate a number of patents held by lawnmover companies
:ugh:

[Bertie Thruster]

I believe that rotor blades produce lift and 'pull' the aircraft into the air.

Why did they go down this route and not use a type of propellor to generate thrust to push it into the air?


Sounds like you are saying helicopters suck not blow.

[albatross]

LOL

Must be a very slow day on PPrune!

Well if we are going to push then a whole bunch of components will have load forces at 180 degs to what they have now ( compression vs tension ) but why would we want to do it that way? Think of the FOD implications. Controls would be very sensitive what with the mass of the helicopter above the rotor system much like balancing an umbrella on your fingertip - the mind boggles!

Tried to push a rope lately?

Shades of the Cessna 337 and inumerable WW1 aircraft.

Have a dood day everyone and thanks for the chuckle.

[Ian Corrigible]

'Course, you could always pull and push...

Pescara Stoppable Rotor-Wing (http://www.aviastar.org/helicopters_eng/pescara_stoppable.php)

http://www.aviastar.org/foto/pescara_stoppable.jpg

I/C

[6Z3]

Reminds me of a Tee-Shirt I once had with the slogan:

Gravity's a Fallacy...
The Earth Sucks

[Saintsman]

Hmm, perhaps I didn't make it clear enough.

I meant a propellor fitted the same way as the rotor blades, above the fuselage, with the thrust going downwards.

FOD may well be an issue, though I think rotor blades can kick up a fair bit.

Still, I like a chuckle too, so more answers welcome.

[Tractor_Driver]

Maybe something like this?

Unreal Aircraft - Beating Gravity - Convair XFY-1 Pogo (http://www.unrealaircraft.com/gravity/xfy1.php)

TD

[puntosaurus]

Er, Saintsman, think about that one for a moment ! Are you really saying reverse the direction of the forces at the main rotor ? What effect do you think that would have ?

If you wanted to push the helicopter into the air, your blower/rotor would have to be mounted on the ground, and the aerial part would have an umbrella like structure ! Are you sure that's what you want ?

[Saintsman]

If you wanted to push the helicopter into the air, your blower/rotor would have to be mounted on the ground, and the aerial part would have an umbrella like structure !

Not really, after all, you usually find a propellor at the front of a wing. Obviously you would not have a propellor that is only a few feet wide, but say it was half the size of a set of rotor blades. Wouldn't that work?

[puntosaurus]

I think you're going to have to clarify what it is you're proposing. I can see a niche on Dave Jacksons website beckoning !

[Saintsman]

I'm not proposing anything. I would have thought that generating thrust seems like the logical way forward when they first developed helicopters because that would have been what they were familiar with. Designing rotor blades that generated lift would have been radical then.

I'm just curious as to why thrust didn't work.

[traumajunkie]

I must admit I'm having difficulty wrapping my head round how your proposition differs from a normal main rotor :confused:... in which case shrinking it to 50% would, I believe, roughly quarter your thrust/lift/stuff that makes the thing go up.

[puntosaurus]

Saintsman, I think you've got some basic physics to sort out in your head. An aeroplane propeller and a helicopter rotor are functionally identical. They both produce lift and thrust.

For the purposes of this discussion, propellers and rotors only produce a different result because they are mounted in different orientations on an aeroplane and a helicopter.

[Um... lifting...]

Wouldn't that make it into a V-22? Or the Pogo? (The aeroplane that knocks the whole conveyor belt question into a cocked hat...)

http://www.unrealaircraft.com/gravity/thumbs/xfy1_seq.jpg

All propellers generate lift... they're airfoils... it's just a question of the orientation of the thrust vector to the weight vector and other force vectors.
The Wright flyer's propeller generated thrust, and was a pusher. WWI biplanes generated thrust, and were tractor props.
Hovercraft (early ones) generated thrust and used large pushers under the skirts.
Helicopters generate thrust and use very large tractors for the main rotors and sometimes tractors and sometimes pushers for the tail rotors... depending upon design.

If you want other answers why it's a bad idea... read about these two bad boys... and take a look at the horsepower outputs on their engines...

Unreal Aircraft - Beating Gravity - Convair XFY-1 Pogo (http://www.unrealaircraft.com/gravity/xfy1.php)
Unreal Aircraft - Beating Gravity - Lockheed XFV-1 Salmon (http://www.unrealaircraft.com/gravity/xfv1.php)

[Tarq57]

I think Saintsman is asking: why not use a propeller instead of a rotor?
(Identical function and drive; one is just a lot smaller.)

The rotor dynamics experts can probably tell you, but basically to have a much smaller "rotor" (ie: propeller) support the weight of a helicopter it would have to have a lot more power pumped into it, for any given weight, than a rotor designed to support the same weight.

Rotor/propeller...they are both aerofoils. Long and skinny (like a glider) or short and stubby (like an F104.) And with different characteristics.

Too small an aspect ratio, it drops out of the sky at low airspeeds, too high and it's hard to build the required strength into it. (Like that round the world non stop job designed by Bert Rutan..they didn't want to exceed 2G on that wing.) I'd imagine rotors behave in a similar fashion and with similar design limitations.

The Hughes 500 has a fairly compact rotor for its mass, makes it ideal for getting into small spots, and very maneuverable. Is a more extreme example of that what you are thinking?

One problem I can see immediately with that configuration (conventional prop above, acting as a rotor) would be the auto-rotational capabilities of it, should the powerplant go all quiet. It'd probably glide in a more brick-like manner, and the window for successful flaring and touchdown would be nanoseconds.

[Saintsman]

Thanks guys.

Although they perform a similar function, propellors and rotor blades are different - one is 'straight' and the other twisted, which is sort of why I was asking the question.

Auto rotation is a pretty good reason not to use a prop though.

[puntosaurus]

Rotor blades are twisted, and for the same reason as propellers.

Propellers autorotate; ever tried restarting an engine in a twin engined aeroplane ?

The differences between propellers and rotors exist because they are optimised for different purposes, but their modus operandi and basic function are the same, ie chucking air from above the blade to below.

[Um... lifting...]

I was going to agree with puntosaurus... rotor blades are indeed twisted, aerodynamically as well as geometrically. One aspect of rotor blades that is a bit interesting is that the centripetal acceleration acting upon the mass of the blade provides tension along the blade axis... and stiffness. A blade rotating is much stiffer than one at rest.

[Tarq57]

Propellers autorotate; ever tried restarting an engine in a twin engined aeroplane ?
Of course they do. (No, I haven't. Only windmilled/zero-thrusted/feathered.)

But whether they'd store enough energy while autorotating to allow the helicopter fitted with such a lift source to successfully flare and land at a reasonably survivable speed might be another story.

[Um... lifting...]

But whether they'd store enough energy while autorotating to allow the helicopter fitted with such a lift source to successfully flare and land at a reasonably survivable speed might be another story.

And there again, you touch upon an issue in the V-22... though it does have wings. There are numerous hotheads over on the V-22 thread discussing this and other issues.
You can hover an airplane with enough power (Sean Tucker and other aerobatic pilots prove it often enough).
You can even hover a Harrier (though not for very long... you can see the fuel gauges moving).

[puntosaurus]

But whether they'd store enough energy while autorotating to allow the helicopter fitted with such a lift source to successfully flare and land at a reasonably survivable speed might be another story.

AhHa. Well I think that would be down to the certification authorities :).

[[email protected]]

So Sainstman, you want to put a propeller on top of a helicopter - you will have to make it big enough to produce enough lift/thrust within the constraints of the engine power and then find a way of controlling the direction of that thrust.

Now you could either tilt the whole propeller/engine/gearbox combination (V22) or use a cunning method of adusting the pitch on each blade individually and hinging the blades so they can move - thus giving a means of tilting the thrust vector without moving the shaft/gearbox/engine.

If you get yourself a good book on helicopters (Shawn Coyle's for example) you will discover how a helicopter works - read it before posting here again pleas:)

[Davey Emcee]

Leave him alone, there's nothing on the telly.

[feathering tickles]

Seems to me that Saints' original post might not be suggesting any particular fundamental change to a "normal" helicopter layout. (please correct me if I'm wrong Saints').

Rather he might be grappling with the oft misused/misunderstood words LIFT and THRUST and what they actually mean within the context of a "rotating air movement propulsion system" (of whatever name it's given).

In essence does it "suck", "blow", both or neither? And whichever you decide it does, what do you want to call it?

[Dave_Jackson]

Saintsman,

I think that the words 'push' and 'pull' relate to the location of the thrust in respect to the object to which this thrust is being applied.

In this context these two rotorcraft would be construed as having push.
Hiller Flying Platform (http://www.aviastar.org/helicopters_eng/hiller_platform.php)
de Lackner HZ-1 Aerocycle (http://blog.modernmechanix.com/mags/MechanixIllustrated/8-1955/stand_up_fly.jpg)

__________________________

If you are talking about using propellers as an alternative to rotors, you may find this page of interest. (http://www.unicopter.com/1764.html)


Dave

[slowrotor]

Saintsman,

A propeller is device that produces thrust to propel an aircraft forward and is designed for a particular forward speed.

A rotor is a propeller optimized to produce static thrust.
Other than that, the two are very similar.
Any propeller can be used to produce static thrust, but the larger rotor is more efficient for static thrust or lift (thrust is the same as lift, we often use either word when talking about rotors).

The early helicopters had large rotors and heavy engines. As engines get lighter, rotors get smaller. Eventually rotors will look like propellers.

[Dave_Jackson]

Hi slowrotor, Have you started any project?

[slowrotor]

Hello Dave,

All of my vertical flight ideas have been determined to be impractical.
Instead I am building an ultralight airplane. This aircraft might be used in the future as a testbed for some ideas. But right now, the hope for vertical takeoff has dimmed.

I check here once in a while, not much design stuff anymore.
How about you, any projects?

[Dave_Jackson]

slowrotor,
All of my vertical flight ideas have been determined to be impractical. .................. But right now, the hope for vertical takeoff has dimmed. Your not alone. This is typical throughout Rotorland.

I check here once in a while, not much design stuff anymore.
http://www.unicopter.com/Tape.gif

Instead I am building an ultralight airplane. This aircraft might be used in the future as a testbed for some ideas.
How about you, any projects?

Yup. Two of us got tired of theorizing about concepts that also have limitations. One limitation being the lack of a few billion dollars.

We have started at the bottom and are building an extremely light electric rotorcraft (http://www.unicopter.com/ElectrotorMicrolite.html). It is also intended to be the testbed for developing advances in electric VTOL propulsion.


Dave