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HeliTorque :: View topic - How does a helicopter create lift?
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HeliTorque Forum Index » Flight Dynamics

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How does a helicopter create lift?
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Hannan
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PostPosted: Fri Nov 26, 2010 3:32 pm    Post subject: How does a helicopter create lift? Reply with quote

If we add more blades to the rotor, does it increase lift?
If we rotate the rotor faster does it increase lift?
Is it possible to put 4 rotors (coaxiel), one on top of the other?
Does it make any sense? Will it create double the lift of 2 coaxiel rotors?
Will it require half the diameter of 2 coaxiel rotors?
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tombeeston
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PostPosted: Fri Nov 26, 2010 7:17 pm    Post subject: Reply with quote

i am not an aerodynamicist, i'm just a student ppl, but i do like fizicks....


"If we add more blades to the rotor, does it increase lift?"

I would say yes, according to
lift =0.5 x Cl x rho x v^2 x S

lift is directly proportional to the surface area, S, so increasing the number of blades increases the surface area, thus increases lift. An absurdity seems to be reached when so many blades are added that the rotor disk becomes a solid circle - difficult to see that producing any lift at all by spinning. I assume the relationship breaks down sooner than this. The above formula is theoretically for 2d (infinite wings) and is probably torn apart by helicopter aerodynamics in the real world.



"If we rotate the rotor faster does it increase lift?"

Yes, same formula/argument applies as above, lift varies directly with the square of the speed of the airflow over the airfoil in a 2d wing. I believe the response is too slow to be used as a method of controlling lift in flight - the disk won't change speed quick enough - seems to slow down bloody fast enough in an autorotation though...



"Is it possible to put 4 rotors (coaxiel), one on top of the other?"

I don't know


"Does it make any sense? Will it create double the lift of 2 coaxiel rotors?"

If lift is created by the rotors imparting momentum to the air (forcing it down), there must be a point where the air is moving down so fast that the rotors below are unable to impart any more momentum to it and won't produce lift. Would they in fact be in a vortex ring like state?



"Will it require half the diameter of 2 coaxiel rotors?"

Don't think it would work, see above. But surface area (of a circle) is equal to pi x radius^2, ie varies with the square of the radius. Halving the diameter (and so radius) reduces the surface area by a factor of 4, so there may not be a linear relationship. I don't even know if area is relevant to this argument

Just my limited understanding, feel free to alter/amend etc
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ALFA8C
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PostPosted: Fri Nov 26, 2010 8:14 pm    Post subject: Reply with quote

Tom,

Have YOU been on the 'pop'?

Or maybe I need to go on the 'pop'!

J.
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haggishunter
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PostPosted: Fri Nov 26, 2010 8:59 pm    Post subject: Reply with quote

I am not going to go into formula for aerodynamics but...

If we add more blades to the rotor, does it increase lift?

Yes it will increase lift as you increase the lifting surface, however you also open a huge can of worms. More blades means a larger rotor hub most likely to be heavier, with the extra weight and the significant increase in rotor drag you will probably need a more powerful engine, which will be bigger, and therefor heavier also. To deal with the increase in engine power you will need to beef up the drivetrain and gear boxes, adding more weight to the aircraft.

So you are simply adding power to lift the extra weight, back to square one (in simple terms). Although a multi blade head has advantages over two-bladed heads, but lets not get into that argument.

If we rotate the rotor faster does it increase lift?

To a point it will, before you start seeing the effects of advancing tips going sonic, blade stall on the retreating side of the disc, eccessive blade flapping and power and drivetrain limts. Manufacturers have already tested the rotor system and it's normal operating range and Vne will be towards the limits of it's efficient and safe range.

Is it possible to put 4 rotors (coaxiel), one on top of the other?

I have not seen it done, but that doesn't mean it's not possible. 2 rotors, on top of eachother, is the normal. The reason is probably due to the huge volume of downwash from all the systems, you'll probably find the rotor at the bottom is producing little effect due to a mass inturrupted airflow above it. Controlabillity will be a big issue! In general it sounds way too messy.

Does it make any sense? Will it create double the lift of 2 coaxiel rotors?

I see your thinking but...no.

Will it require half the diameter of 2 coaxiel rotors?

No, because you would have encountered so many problems before, you wouldn't get to this stage.

So, why the questions? Building something fun?

HH Shocked
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tombeeston
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PostPosted: Fri Nov 26, 2010 9:49 pm    Post subject: Reply with quote

A8C,

Unfortunately not - just trying to post more! Need to do something helicopter related each day in view of all these cancelled lessons! Well and truly outclassed by a real pilot in the shape of HaggisHunter though! Smile

Tom
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Hannan
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PostPosted: Fri Nov 26, 2010 10:25 pm    Post subject: Helicopter lift Reply with quote

I was looking at this:
http://www.newlaunches.com/archives/the_worlds_smallest_one_person_helicopter.php
Japanese made world's smallest personal helicopter.
The diameter of the rotor is quite large at 4 meters.
I was just thinking if there is a way to cut the length by half (to 2 meters) and compensate by adding more blades (4, 6, 8?) , increase the rotor rpm (since now the radius is cut by half, you can safely double the rpm ???), or by adding more rotors (possibly under it). Of course a bigger engine would also be required.
I guess that there is a point by which all of those factors are optimized.
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Hannan
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PostPosted: Fri Nov 26, 2010 10:28 pm    Post subject: Reply with quote

BTW, the contra rotating rotor actually doubles the lift or is just used instead of tail rotor? Does it add lift at all to the main rotor?
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PostPosted: Sat Nov 27, 2010 10:28 am    Post subject: Reply with quote

Contra-rotating rotors cancel out eachothers torque forces that would normally require a TR on a single rotor helicopter.

Because power is not needed to drive a TR then power can be put directly to the MR. A google search will bring up a lot of info about these rotor systems and probably explain it better then me.

Now you little jap helicopter (dare I call it such a thing) look like it has a fixed pitch rotor, as there seems to be not control on changing pitch. Therefore life is generated by by increasing RPM of the rotors, this works to an extent as I mentioned before however, it probably won't be able to turn fast enough to produce sufficient lift to get to a decent height.

Directional control is achieved by tilting the full rotor system with the handlebars as you dangle dangerously below... let's just hope the engine never fails. However it does look fun, we are helicopter pilots, risk is just a part of the game.

HH
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PracticeEngineFAILURE
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PostPosted: Tue Feb 15, 2011 3:30 pm    Post subject: teehee Reply with quote

Huggis I just had a wee giggle at the poor lad who referred to you as a 'real pilot' and who also believed he had been 'outclassed' by you!

Well young Tom, our courageous 'hunter hasn't outclassed anyone since his days trawling the mean streets of Aberdeen Harbour and in fact a 'real' pilot would never fly one of those yankee vans! Razz

PEF


Ps 3 weeks til carnage Nel! Laughing
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haggishunter
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PostPosted: Mon Apr 11, 2011 5:02 pm    Post subject: Reply with quote

I just noticed you post Mr Failure...

Leave my american beefburger of a helicopter out of it and go back to your french designer handbag of a helicopter. Good thing there's a median line between us.

And yes the carnage was good, my turn next.

HH
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PostPosted: Tue Apr 12, 2011 7:57 am    Post subject: Reply with quote

Now kids, play nice.

Mr Failure, leave the Hunter alone. Just cause he flys one of those inferior Yank Tanks, you know the sort... can't take corners, sway all over the place with soft suspension, drunk overweight drivers etc etc, doesn't mean that he himself is inferior.
Just a little lower class......
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