Asymmetrical vs Symmetrical Airfoil Rotor Blades

dwood2066

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I have a question on the Asymmetrical vs Symmetrical Airfoil for gyrocopter rotor blades. I would like to hear everyone's opinion on which design is best.
Does the gyrocopter design prefer one over the other?
 
For those rare gyroplanes with collective control, a symmetrical blade beneficially offers zero lift at flat collective pitch (and is also handy with regard to twisting forces that could arise from motion of the center of pressure as the AoA varies spanwise).

I'll let the folks who fly fixed collective talk about pitching moments and such for their blades.
 
Sorry for the delay
Asymmetry reduces profile drag (and therefore rotor drag) for the same lift.
Sans titre.png
 
To distill the point to it's lowest component:
Any extended curve below the horizontal center line of the foil is creating
negative lift until it is at a positive enough AOA to create positive lift.
But then it is forcing the air around a larger mass than necessary, so the lower curve
can be flattened as much as possible without causing separation of air flow.
Some amount of curvature is needed to transition the air around the mass without causing separation,
so all airfoils are a combination of curvatures that are hopefully most efficient for the task at hand.
At extremely low Reynolds numbers (small hand tossed gliders) a flat bottom is best and the top curve only facilitates
pitching over from the top of the high speed climb. If the speed was constant, a flat sheet would be more efficient....
 
Many of the blade sections used on gyro rotors have a cruder reflex than the 23112 that J.C. has graphed. I suspect the difference between these airfoils and a symmetrical airfoil would be less at a given Cd.

Many years ago, it was common in Florida to use discarded Hughes 269 or 300 blades on gyros. These blades have, IIR, a NACA 0015 symmetrical foil. The blades were built with helicopter twist (same as airplane washout). It was therefore necessary to turn them upside down to make the twist appropriate for a gyro. The measured performance of these blades was better than the blades marketed for gyros at the time.
 
Many of the blade sections used on gyro rotors have a cruder reflex than the 23112 that J.C. has graphed. I suspect the difference between these airfoils and a symmetrical airfoil would be less at a given Cd.
Certainly, Doug. Too much reflex acts unfavorably on the CL max of a blade section.
For this reason, it's not a good idea to choose a more pronounced reflex than is just necessary to conserve Cm0 =0 (such graphed)
Igor Bensen had however, initially chosen a cruder reflex (very positive Cm0), because he could set them at a very low pitch, making them easier to launch by hand. As the speed of rotation increased, they pitch become suitable again in flight, thankswith wooden blades that were very flexible in torsion,
With today's common prelaunchers, and our stiffer blades, the best choice is Cm0 about 0.

Many home made thought they could benefit from the low friction of a laminar boundary layer by set back the maximum thickness of the section on the chord. But the pros of helicopter manufacturers never made this choice (None NACA 8H12)
Below, the latest selection of blade sections by Airbus helicopters: ONERA OA 212
Sans titre.png
 
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That foil looks similar to to the EH series which has no pitching moment and is used for flying wings...
I was reading about using helicopter blade upside down to create a "wash in" twist along the span for gyros and it makes
me wonder if there have been any experiments with transitioning the foil from the root to the tip.
It seems like a wind generator foil with self starting properties could be used at the root and it could transition to
a neutral pitching lift section at the tip.
This is a can of worms because there are so many variables, but conceptually it seems plausible....Eh2010l.jpg
 
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That foil looks similar to to the EH series which has no pitching moment and is used for flying wings...
Don't be fooled by appearances!
These two sections look exactly alike, but the difference in drag is enormous.
Sans titre.png
Fabulous discovery by David R. DAVIS in the late '30s
 
I always thought the Horten series was overly fat, but part of that was making room for the occupant in the nose area and structural concerns...
That being said, a lot of the flying wings I have designed used Eh foils thinned down to 9% and sometimes even 7%.
In the 30's the structural aspects sometimes dictated thicker sections....
 
Certainly, Doug. Too much reflex acts unfavorably on the CL max of a blade section.
For this reason, it's not a good idea to choose a more pronounced reflex than is just necessary to conserve Cm0 =0 (such graphed)
Igor Bensen had however, initially chosen a cruder reflex (very positive Cm0), because he could set them at a very low pitch, making them easier to launch by hand. As the speed of rotation increased, they pitch become suitable again in flight, thankswith wooden blades that were very flexible in torsion,
With today's common prelaunchers, and our stiffer blades, the best choice is Cm0 about 0.

Many home made thought they could benefit from the low friction of a laminar boundary layer by set back the maximum thickness of the section on the chord. But the pros of helicopter manufacturers never made this choice (None NACA 8H12)
Below, the latest selection of blade sections by Airbus helicopters: ONERA OA 212
View attachment 1160272
Hello Jean, There is some very interesting papers written on the net that discuss Aerospatiale experiments with this profile and put it into production! But would the profile work well for gyros?One would think Yes!
 
Some interesting papers and information on the Boeing VR-7 Airfoil used on CH-47D Chinook and the AH-64 Apache HH-02 Airfoil that originally was developed by Hughes Helicopter.

Wayne

1708229728955.png




HH-02 Airfoil
1708231113636.png
Viewing the bottom photo of the AH-64 rotor blade, one can see the 64A006 airfoil is for the swept back tip portion of the rotor blade. The HH-02 is for the constant cord portion.

 
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Juergen,
In 2018, I had pointed the manufacturing difficulties of such a reduced chord at the root, due to excessive bending stress at rest.
Unfortunately, for ten years, no photo of this magic rotor with advanced chord has still come to prove the reality of the improvements alleged by the authors.
 
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I don't doubt that the shapes in plane of their blades could intrinsically reduce the drag rotor for a same diameter, but it is just ignore the mechanical stresses on the roots.

With tapered blades, at the opposite, the more generous root would enable the diameter to be increased significantly before reaching harmful bending/torsional stresses at the roots, and the rotor drag would therefore be much more favourable.

Below is a quantified comparison of three rotors lifting the same load at the same forward speed:

N°1 is with the usual rectangular blades Rotor drag = 382 N
Sans titre.png

N°2 is with tapered blades of the same diameter as N°1 Rotor drag = 388 N
Sans titre1.png

N°3 is with tapered blades of enlarged diameter until obtain the same torsional bending stresses at the root as N°1 Rotor drag = 284 N
Sans titre2.png
 
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