choice of airfoil

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if you were to choose the best air foil section for a helicopter what would it be.
4 choices NACA 0012 NACA 23012 NACA 63015 or Boeing VR7. many older helicopters used the tried and true NACA 0012. all Robinsons use a NACA 63015 new modern helicopters started using asymmetrical blades like the 23012 and the vr7. the 0012 and 63015 are easy to find used. not much in the line of small chord 23012 and vr7.

Doug
 

bryancobb

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Hey Doug,

There's a lot of variations in "best." If your goal is to reap the most lift from every gram of fuel is burned, or if you are building an electric or man-powered helicopter, a symmetrical airfoil won't get the job done.

If your goal is an easy-to-manufacture set of blades that is not temperamental to set up, and has no internal twisting moment as the blade twist and feathering changes, stick with the ol' oo12! I'ts been adequate for half a century.
 

C. Beaty

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Doug, the Mosquito helicopter uses DW rotorblades with good results. Installed on a Mini-500, there was a substantial reduction of rotor power.

Chord is 7 inches.

DWs are mildly cambered, similar to a NACA 2312 with trailing edge reflexed for zero pitching moment coefficient. They are internally balanced very near to the aerodynamic center.
 

bryancobb

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Chuck,

Do I understand right, that the asymmetrical blades performed significantly worse on the Mini? I wonder why? I wouldn't have expected that.
 
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Bryan

you hit the nail right on the head. need something that easy to manufacture and set up like the symmetrical blades. but do not want to give up any performance as with the asymmetrical blades. robbies all use a 63015.
so does the mini 500 it was mentioned in another post by the designer of the mini that was a modified 0012 with the CP moved back to 35% to be more efficient. Chuck how does the 63015 compare in efficiency to a 23012 or 2312

Doug
 

C. Beaty

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A 63-015 is a 15% thick, mildly laminar symmetrical airfoil. It was also used on the FH-1100 helicopter. Easy to make and the trailing edge cusp means the TE can be pulled together and riveted.

Symmetrical airfoils have the low drag range center at zero lift, meaning best L/D occurs at say ±4°.

Cambering slides the low drag range to the positive side of zero lift.
 

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bryancobb

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A substantial reduction of rotor power represents “significantly worse performance?”

Here is the original mention of DWs on a mini 500:

http://www.rotaryforum.com/forum/showthread.php?t=2323
Well Chuck, "A substantial reduction of rotor power" COULD have meant "not properly matched to the engine torque curve / gearbox ratio" which would result in poor performance and not have anything to do with how "good" the airfoil selection is for helicopters. ;)

Bryan
 

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Fetters?

Fetters?

...so does the mini 500 it was mentioned in another post by the designer of the mini that was a modified 0012 with the CP moved back to 35% to be more efficient...
You may be right, but I don't remember ever reading about the 35% more efficient aspect. I posted some drawings a few months ago documenting that the feathering axis on a Mini-500 is in fact, at about the 33% to 35% point, but that doesn't move the Aerodynamic Center. The laws of physics are driving the bus there.

I assume Dennis Fetters had valid reasons for having the pivot point aft of the aerodynamic center. It probably has something to do with control forces and what made all the flight controls "feel" like a more substantial helicopter?

If the blade is WANTING to go to increased pitch, as would be the case when the aerodynamic center (25%) is forward of the feathering axis (35%), this would need some mechanical force to keep this from happening, or the pilot would be pushing down on the collective all the time.

The Mini's collective mechanism has two fairly robust springs creating additional up force. They do a good job of making collective forces feel right and with a tiny amount of friction, you can remove your hand from the collective stick for long periods during cruise flight. I guess it's just the right amount to cancel out centrifugal feathering tendency?

The fact that the Mini-500 has a very tame, benign airfoil and has <-8> twist, and has positive pitch force (due to CP being forward of axis) and more positive pitch force by the two springs, probably has something to do with it not having any rotor-related flutter issues like the Helicycle has had? Maybe not?


 
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Hi All

When I read that statement by DF I thought it was bull. the airfoil just is a normal 63015 understand why the feathering axis could be at 33%
just like on a 0012 cp is at 30% feathering is at 25% 63015 CP at 35% feathering is at 33% makes sense. but then throw this in. assuming that the blade still needs to be balanced a 25% the blade would need to be installed lagged back to even out the centrifugal pull through the blade. or because the blade has a 35% cp is the blade now balanced at 30 to 35% now not needing to be lagged.I have somewhere have a photo of a FH 1100 rotor head as Chuck said they do use a 63015 but the feathering axis appears to be at 25% chord. holy complicated no wonder why the pioneers used a 0012simplicity.


Doug
 

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?

?

...When I read that statement by DF I thought it was bull...
Giving him the benefit of the doubt, he MAY have meant "more efficient" from the production and manufacturing perspective. I'm pretty sure he had a thorough understanding of the almost mandatory 25% Aerodynamic Center requirement.

His mounting holes at the 35% point are centered in the flat area of the blade root and it all "looks" nice and I know from 1st hand experience, it works?...And they are not lagged back and during assembly and rigging of the helicopter, it doesn't instruct you to even consider the chordwise balance of each blade in relation to feathering axis. The only chordwise balancing that is done is of the entire rotor system AS A UNIT by shifting the entire assembly at the head using shims.
 
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Jean Claude

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Doug, Only the shifting on the chord between gravity center and Aerodynamic Center produces a twisting torque. The position (35% or 50%) of the root hole has no effect on the tendency to nose up the blade..

 
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Hi I figured the feathering axis had no effect aerodynmacly on the blade Just needed someone else to confirm that.What effect the the location of the feathering axis have on the controll of the blade or the controll input feel.
the 63015 has a cp of 35% chord then where should the chord balance be at.
25% as in most blades or or around the 30% chord

Doug
 

Jean Claude

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jean claud take a look at post number 7 in this thread chuck posted a picture of the NACA63015 that I am interested in. looks nothing like the one you posted.


Doug
 

C. Beaty

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JC, I first thought Doug meant the 63015 but he really meant the 63-015.

Doug, without the hyphen, one would assume the NACA 630 meanline; same family as NACA 230 meanline.
 
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Bryan

here is what DF says about the mini airfoil.

The Mini-500 uses an airfoil I designed, based on a NACA0012, where I moved the thicker part of the airfoil back somewhat to help reduce drag and improve lift, similar to what a laminar flow airfoil does, and I modified the trailing edge to improve autorotations. Those mods proved to work out.

There are airfoils that do produce better lift, such as the VR-7 or 8, but I used my modified NACA0012 for several reason to accomplish some specific goals. I wanted a rotor that could be produced to rotate both ways. In this way I could produce aftermarket blades that would work both clockwise or counter clockwise, and use the same molds. My molds were built so I could add twist ether way, evenly or progressively, or no twist at all. That again proved good, because to this day I make custom rotorblades using one set of molds.
sorry about the mix up on the airfoil numbers

Doug
 

Jean Claude

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bryancobb

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Thanks

Thanks

...here is what DF says about the mini airfoil...
Thanks Doug. I have never seen that until now. His thoughts seem pretty unscientific but innovative. It wound up working pretty well.

Bryan
 
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Hi

I may need to re phrase the term. the aerodynamic center is at 35% on the 63-015 where as the A.C. on most other symmetrical blades are at 30%
we know that if the AC is at 30% the blade CG is normally at 25%
now on a 63-015 the AC is at 35% where is the CG of the blade belong. still at 25% or does it need to be at 30%
another example we know the CG of the blade needs to be ahead of the AC to be stable the 23012 has an AC of 25% is it still balanced at 25% or should it be balanced at 20%. Somewhere I read that airfoils with an AC of 35% were lighter due to not needing as much weight in the nose of the blade to get a proper CG. this would indicate that the CG is closer to the AC. I do have two 63-015 blade sections one from a mini and the other R22 both seem to balance at 25% chord

Doug
 

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