Thrust or lift?
 

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

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

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

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

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

Sounds like you are saying helicopters suck not blow.

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.

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

Pescara Stoppable Rotor-Wing

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

I/C

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

Gravity's a Fallacy...
The Earth Sucks

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.

Pogo?
 

Maybe something like this?

Unreal Aircraft - Beating Gravity - Convair XFY-1 Pogo

TD

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 ?

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?

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 !

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.

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.

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.

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/gravit...s/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
Unreal Aircraft - Beating Gravity - Lockheed XFV-1 Salmon

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.

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.