Reflex Airfoils

curtisscholl

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Hello the Forum

There was a thread where Chuck Beaty was told "that Airfoil won't work" because it was not "Flat Bottomed". And in the same thread stating that a symmetric Airfoil stalled quickly thus not allowing an "armstrong spin up" but flew just fine. The 8H12 profile is a reflexed foil an so are the Bensen G2 foils.

The VR7 Airfoil was experimented with, the "B" version with a tab, thus making it a Reflex Airfoil. I think Chuck said it was successful.

The NACA 25112 is a Reflex Airfoil. The Munk M15 is a reflex Airfoil but is not listed in the UUIC database as one.

The Clark YS and YH could be considered reflex foils. they have a relatively flat under camber.

My question is whether or not a symmetric Airfoil could become a reflex Airfoil by adding a tab following the lower curve upward past the tail end of the foil or gently sweeping the trailing edge up,?

And

What other Airfoils would be considered Reflex?

Inquiring minds want to know..
 
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Jean Claude

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Symmetrical profile works already well without tabs . Adding the tab would reduce the CL max and require more weight in the nose.
 

curtisscholl

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Hello the Forum

There was a thread where Chuck Beaty was told "that Airfoils won't work" because it was not "Flat Bottomed". And in the same thread stating that a symmetric Airfoil stalled quickly but flew just fine. The 8H12 profile is avreflexed foil an so are the Bensen G2 foils.


The VR7 Airfoil was experimented with, the "B" version with a tab, thus making it a Reflex Airfoil. I think Chuck said it was successful.

The NACA 25112 is a Reflex Airfoil. The Munk M15 is a reflex Airfoil but is not listed in the UUIC database as one.

The Clark YS could be considered a reflex foil. It has a relatively flat under camber.

My question is whether or not a symmetric Airfoil could become a reflex Airfoil by adding a tab following the lower curve upward past the tail end of the foil or gently sweeping the trailing edge up,?

And

What other Airfoils would be considered Reflex?

Inquiring minds want to know..

Symmetrical profile works already well without tabs . Adding the tab would reduce the CL max and require more weight in the nose.
Jean Claude

Good...the there is no need to concern myself with reflex only design.

Thanks.

Curtis
 

Doug Riley

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Curtis: Various gyro blade designers have used cambered airfoils, but intentionally omitted reflex -- or at least have under-reflexed them, leaving some pitching moment in place. They have relied on torsionally-stiff construction to prevent the blade from twisting node-down and over-speeding. As JC said, reflex reduces efficiency by creating an aerodynamic down-load on the blade.

While the under-reflexed blades do perform a bit better, owing to the reduced down-load, this type of design can be a dangerous game. We had quite a serious gyro crash some years ago, traceable to RPM runaway from an under-reflexed blade airfoil.

Some of the 1930's autogiro blades used an un-reflexed, cambered airfoil, but with an intentionally aft blade CG. This also is perhaps not the most wholesome approach, compared to the use of a zero-moment airfoil...
 

curtisscholl

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Hi all

"Zero Moment" ... Zero or near Zero Cm..
That is what I should be looking for..

So what I am understanding is that there is a tendency for a blade to "turn turtle" given the situation that would allow it to happen and ruin everyone's day.

So the questions are 1. Zero Moment or more positive than negative moment at all angles of attack and airflow? And 2. To insure that Blade CG or CoM remains at 25% of chord and CP does not travel much around CG? And 3. That the blade spec has enough reflex to keep the blade from nosing down, but does not have too much reflex...like comparing Clark YH to Clark YS...more pronounced reflex on Clark YS? Thanks in advance...
 
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Jean Claude

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1. Zero moment or moment more positive than negative at all angles of attack and airflow?
Yes: Zero or slightly positive moment

2. Make sure that the CG or CoM of the blade remains at 25% of the chord?
Yes: The point at 25-26% of the chord is called the Aerodynamic Center. This is where the additional forces due to the additional angle of attack of the profiles are applied.
If CG is in front of this point, then torsional stability is ensured because an accidental increase in angle tends to reduce itself.
If CG is behind it, then torsional instability occurs because an accidental increase in angle tends to worsen by itself.

2.bis That the CP does not move much around the CG?
A point exists where the moment of the Center of Pressure is constant, despite variations in Cl . It is the Aerodynamic Center
Thus the displacement of Center of Pressure is already determined by the Moment Coefficient.
For example with Moment is - 0.025 (- is for nose down) then at Cl= 0.1, CP is at 50% of the chord, and moves at 30% when Cl = 0.5
Now, choosing a profile that has a moment of + 0.02 means that when Cl =0.5 for example, then CP is at 0.21 chord.
and when CL = 0.1 then CP goes to 0.05 chord

3. That the blade specification has enough reflex to prevent the blade from pricking down, but does not have too much reflex ?
Choices 1 and 2 solve 3 by themselves
 
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curtisscholl

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Jean Claude:

Excellent explanation.

I am looking at the Cm curves for many airfoils that are related to the 8H12...and depending on Reynolds Number (dimensionless, that I recall) this higher Reynolds number curves dip well into the -.08 region for negative angles, and near .01 at 15 deg AoA.
The Clark YS however, flat bottom, with a well developed reflex has a Cm of +.04 at 15 Deg AoA and a -.06 at 0 deg depending on which Reynolds number curve one references. Whereas the 8H12 has a max Cm of +.023 at 15 deg AoA and dips to -.08 again depending on which curve referenced. The Cl/Cd numbers are lower than the 8H12...presumably because of the well developed reflex...reflecting what Jean Claude stated earlier about the lower lift.

IF I selected the Clark YS, I think that would be a better choice...is it more positive than an 8H12. Cl is equivalent..
The aircraft is an ultralight gyro, and draggy is fine,,,not going past 65 statute MPH anyway.
Am I on the right track?

Curtis Scholl
 
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Jean Claude

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According NACA TN 1998, we see that Cm = 0 when Cl = 0, and Cm = -0.020 when Cl = 1
This variation is due to the usual measurement relative to the point 0.25 c, instead of the accurate Aerodynamic Center.
It is easy to obtain the precise Aerodynamic Center for which Cm is constant from Cl = 0 to ClMax : If Cl =1 gives a difference in Cm of -0.020 with respect to Cl =0, then the difference between accurate AC and 0.25 c is 0.020. Therefore AC is exactly at 0.25 + 0.02 = 0.27 c
So, for the NACA 8H12, Cm = 0 relatively to the Aerodynamic Center (This was certainly intended by NACA !)

Can show you the curves for Clark YS ?Sans titre.png
 
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curtisscholl

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Jean Claude:

I see this is a NACA plot. I do not have a reference on Clark YS from NACA.
What Reynolds number was this graph based upon?
The airfoil database I am using shows many curves based on Reynolds Number. I can isolate a curve if I know which RN was used.

Apologies for seeming a bit dense...

Thanks ...

Curtis Scholl
 
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curtisscholl

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Yes, that is the one of the many curves on that page and I am glad you set me straight on the Reynolds number. I should have paid more attention to the URL and downloaded the file myself <<l’étudiant est embarrassé>> Sorry to make you do double duty.

So am going to now use what you wrote:
" Cm of -0.020 with respect to Cl =0, then the difference between accurate AC and 0.25 c is 0.020. Therefore AC is exactly at 0.25 + 0.02 = 0.27 c"

In the above The Cm of -.020 was converted to a positive number to increase the distance from leading edge to past AC...to make .27 chord.

The difference of the YS values was positive .003 which to me says the AC is .247 x chord...
and the positive value of .034 means that the leading edge wants to fly up rather than turn turtle? OR am I reading the sign incorrectly?

And if it does want to fly up, then I would hope that the tip would not apply forces that would twist the blade to high. ((more lead in the nose??))

Curtis
 
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Jean Claude

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"In the above The Cm of -.020 was converted to a positive number to increase the distance from leading edge to past AC...to make .27 chord."
In the above the Cm discrepancy of -.020 shows the AC is not at 0.25 chord, but at 0.27 chord. So, around this point, the Cm is constant whatever the Cl (as the definition of AC wants) and it keep zero for 8H12

"The difference of the YS values was positive .003 which to me says the AC is .247 x chord."
Exactly. So, around this point, the Cm is constant whatever the Cl, and it keep 0.034

" the positive value of .034 means that the leading edge wants to fly up rather than turn turtle?"
Yes, the leading edge wants to fly up around the AC

"And if it does want to fly up, then I would hope that the tip would not apply forces that would twist the blade to high. ((more lead in the nose?"
The blade produces an elastic reaction to the aerodynamic moment, and because of its flexibility the profiles do not rotate around AC , but around the Center of Mass .
If CM is in front of AC (or on), then the moment around CM decreases (or remains constant) when A.o.A increases, and all is well.
 
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curtisscholl

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Jean Claude:

Thank you for being so patient. This was a very enlightening experience. Now I understand more of what the curves are trying to tell me when I see them.

Sincerely

Curtis Scholl
 
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