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Old 04-30-2010, 09:16 AM
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Default Rotor blade design for gyrocopters

The potential advantage of a sharper leading edge.


I believe it is correct to say the the leading edge of rotary-wing airfoils have a relatively large radius so that they can operate over a large range of angle-of-attack [AOA] without experiencing stall.


The leading edge of the F-104 Starfighter had a very sharp leading edge, which was covered when ground crew were working on the craft. This craft was had very high forward speed but was known to stall at relatively low AOA.

I was looking at the blades for a hovercraft yesterday and noticed that they have sharp leading edges. Since the hovercraft operates in ground effect and since the blades have no adjustment for pitch change during 'flight' it is assumed that these blades also experience a very small change in AOA.


Therefore, IF gyrocopter's blades experience a smaller AOA range then those of a helicopter is it not correct to assume that gyrocopter blades will be more efficient if they utilize airfoil profiles that have leading edges with smaller radii than those on a helicopter?


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Old 04-30-2010, 10:27 AM
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Sounds like a bad idea to me - they don't see a small AoA range in fwd flight the way I see it, and lowering the blade stall AoA sounds like a great way to make one even slower & less efficient.

Probably would be "interesting" to get the blades up to speed without a good prerotator too...

Hovercraft blades don't autorotate, nor do they have cyclic control - they have more in common with a prop or fan than a rotor.
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Old 04-30-2010, 11:18 AM
Jean Claude Jean Claude is offline
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Quote:
Originally Posted by Rotor Rooter View Post
Therefore, IF gyrocopter's blades experience a smaller AOA range then those of a helicopter is it not correct to assume that gyrocopter blades will be more efficient if they utilize airfoil profiles that have leading edges with smaller radii than those on a helicopter?
Dave, do not rely on the collective pitch to infer that the A O A range is more large for helicopter relatively to gyrocopter. They are identical for both. ( 3 to 8 at 0,7R)
Consider this: If the same blades, same rpm, same lift then same AOA. Now, if same forward speed, then AOA range is necessarly identical.
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Last edited by Jean Claude; 04-30-2010 at 11:32 AM.
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Old 04-30-2010, 11:22 AM
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No expert!.... But with my Hovercraft, Blade design and pitch is more a factor of Thrust.... Air cushion is provided by the amount of air volume directed for lift.... Usually, 1/3 for lift and 2/3 for thrust, and is manually adjustable. Some due have splitter plates or in-flight adjustable props, so as to remain on cushion with no forward airspeed or for reverse operations. On thrust fans vs props, AoA does change through blade flexing and coning. There fore, blade pitch needs to be adjusted accordingly. Along with keeping blade tip speed under Mach, I believe its something around 550 feet per second. Rotor disk lift principals are a whole different theory.
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Old 04-30-2010, 12:37 PM
Alan_Cheatham Alan_Cheatham is offline
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A sharp leading edge airfoil works if you want to go super sonic, but not so for rotor blades which are sub sonic. Probs or fan blades operating at high tip velocities may find some benefit.
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Old 04-30-2010, 02:36 PM
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The Robinson R22 Beta has a Collective range of +1.5 to +14.5-degrees, a Longitudinal Cyclic range of -9 to +11-degrees and a Lateral Cyclic range of -9.5 to +6-degrees. It appears to me that most of this collective range is used for high rates of climb and autorotation. This 13-degree range of pitch is not obtainable or needed in a gyrocopter.

I am not proposing that the AOA be small, only that the range of AOA be small(er). An under-cambered airfoil can provide a high AOA without being subjected to leading edge stall.

A sharper leading edge will definitely offer an advantage to high-speed flight. However, IMHO it should also provide an advantage to low-speed flight. In other words, the induced drag will be equal but the profile drag should be smaller. Here is an under-cambered airfoil that had an extremely slow velocity, plus a small leading edge radius to chord length ratio. Cornu, Paul, 1907 - First flight.


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Old 04-30-2010, 03:19 PM
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The cyclic & collective pitch range isn't equal to potential blade AoA in flight.
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Old 04-30-2010, 04:34 PM
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Quote:
Originally Posted by brett s View Post
The cyclic & collective pitch range isn't equal to potential blade AoA in flight.
Brett,

Are you saying that "The sum of the cyclic & collective pitch range* is less then the possible blade AoA range in flight, during normal operation?"

* The above assumes that the available pitch change in a helicopter, such as the R22, is the sum of the cyclic and the collective changes.

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Old 04-30-2010, 04:55 PM
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Absolutely - think about what the AoA along the entire retreating blade looks like as airspeed increases.
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Old 04-30-2010, 05:51 PM
C. Beaty C. Beaty is offline
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How could NACA have been so wrong?

The plot of retreating blade stall- http://www.rotaryforum.com/forum/sho...hlight=Kellett

-in post #5 shows nearly 70% of the retreating blade of a Kellett KD-1 to be stalled at ~
100 mph.
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Old 04-30-2010, 06:02 PM
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Quote:
Originally Posted by brett s View Post
Absolutely - think about what the AoA along the entire retreating blade looks like as airspeed increases.
Yes the AoA on the retreating blade will be greater than that on on the advancing blade, but;
  • is not the pitch of the gyrocopter's retreating blade also greater than the pitch of the advancing blade?
  • does the retreating blade of a gyrocopter ever get near stall during normal flight?


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Old 04-30-2010, 10:44 PM
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Quote:
Originally Posted by C. Beaty View Post
How could NACA have been so wrong?

The plot of retreating blade stall- http://www.rotaryforum.com/forum/sho...hlight=Kellett
I agree, how could NASA have been so wrong?

The plot of mission imposable- http://www.scientificamerican.com/ar...-stealth-plane


Seriously, thanks all for correcting my flawed idea.


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Old 05-01-2010, 12:54 AM
Jean Claude Jean Claude is offline
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Quote:
Originally Posted by Rotor Rooter View Post
The Robinson R22 Beta has a Collective range of +1.5 to +14.5-degrees. It appears to me that most of this collective range is used for high rates of climb and autorotation.
Dave, the pitch is not the AOA. You forget to take into account the speed of airflow through the disc

Quote:
Originally Posted by Rotor Rooter View Post
This 13-degree range of pitch is not obtainable or needed in a gyrocopter.
In vertical flight (from climb to autorotation descent), this pitch range gives an constant AOA (5,5 for example) . Only speed forward gives a AOA range (3 to 7) during the rotation. Same for helicopters for gyrocopters

Last edited by Jean Claude; 05-01-2010 at 01:41 AM.
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Old 05-01-2010, 05:01 AM
C. Beaty C. Beaty is offline
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Sharp leading edge airfoils have no place except on supersonic aircraft with wings of low aspect ratio.

At subsonic speed, a substantial portion of drag at angles of attack near zero is from skin friction rather than form (pressure) drag as long as flow remains attached. Sharp leading edges ensure early separation and a large increase in pressure drag.

The attached graphic shows that flow separates for a sharp leading edge at slightly less than 2 angle of attack. Beyond that, computer plots are meaningless. A simple computer program such as Javafoil can't deal with separated flow.
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  #15  
Old 05-01-2010, 11:43 AM
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Jean,
Quote:
Dave, the pitch is not the AOA.
I understand this.

Using your sketches of a basic vertical climb and vertical autorotation as reference;
The helicopter pilot can change the collective pitch and he can change the RPM of the rotor. Both can change the induced velocity and the AoA. The gyrocopter pilot is unable to change either in a vertical flight situation.

This led me to believe that the gyrocopter operates within a narrower range of AoA then the helicopter even when cyclic changes and forward velocity changes are applied.

Different airfoil profiles stall at different AoA. My thinking (or lack of thinking) was that gyrocopters operate within a narrower range of AoA then helicopters do. If this was true then extending the maximum coefficient of lift to a higher AoA would not be a significant consideration when selecting an optimal profile for the gyrocopter.


Chuck,

The Technical Report 741, which you mentioned earlier, certainly tends to shoot down the idea of utilizing an airfoil that stalls at a lower AoA. The large portion of blade span on the retreating side that is stalled is surprising. Perhaps this is due to the slow rotational speed of the unpowered rotor and the slightly positive blade twist.

Perhaps this presents a reason for evaluating the pros & cons of active blade twist on a gyrocopter's rotor.


Quote:
Sharp leading edge airfoils have no place except on supersonic aircraft with wings of low aspect ratio.
I tend to question this. Not so much in respect to the sharp leading edge but in respect to the effectiveness of undercambered airfoils.

Over the years I have been unable to find any significant amount of information on undercambered airfoils. My primary interest is that of applying it to the root end of airfoils that operate within the Advancing Blade Concept and have active blade twist.

I believe that the application of a relatively thin leading edge combined with undercamber and wide chord was an ideal choice by Paul Cornu. All he initially wanted to do was to hover in ground effect and a slight variation in the pitch or rotational RPM would have given him the change in thrust that was needed.
Perhaps this same concept, with modification, can be applied to low-cost, slow-speed (both rotational and forward) VTOL rotorcraft.


Dave

Last edited by Rotor Rooter; 05-01-2010 at 11:50 AM. Reason: added link
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