Dragon Wings airfoil profile

[jsada]

Does anybody here know what airfoil profile are the Dragon Wings rotorblades?

 

[helcoro]

Gyrocopters has 8H12 airfoil

 

[MikeBoyette]

Not Dragon Wings. They use a proprietary air foil. The closest is a 23012.

 

[Jean Claude]

I believe the DW airfoil is NACA 2312 (different of NACA 23012), but the most important thing is the aerodynamic pitch setting chosen, which I don't know.

 

[Doug Riley]

My (non-expert!) thoughts about DWs:

They have a better L/D than 8H12 blades because of (1) twist that helps keep more of the blade operating at its most efficient AOA, and (2) reduction of the reflex area (reflex produces down-lift, much like adding dead weight).

The different airfoil may help help reduce the NEED for so much reflex -- as does torsional rigidity. That is, you can under-reflex a blade if it's sufficiently stiff in torsion -- but there's a limit to this "cheating" strategy. It's possible to UNDER-reflex a blade so much that it will de-pitch itself by twisting leading-edge down with increased airspeed. The blade becomes unstable, subject to RRPM "runaway."

Of course, Bensen started with wood blades, as limber as overcooked spaghetti. They HAD to have a full, no-compromise reflex. In transitioning to metal, Bensen (and the rest of us) chose not to pull on the tiger's tail by reducing reflex. It can done (a bit), but you have to be really careful.

Oh, and the glue has to be really good.

 

[Aerofoam]

I was wondering if the twist, "wash in" as I understand it, is possibly aeroelastic enough to untwist in gusts and convert the gust to RRPMs...
This would also be an efficiency increase and act a it like a suspension......

Has anyone experimented with zero pitching moment foils like the RS, EH, or MH series?

 

[Jean Claude]

I don't believe that, Doug. A proof that the quality of the DW rotor doesn't lie in its airfoil, is unconsciously provided by C Beaty: For his spreadsheet to predict rrpm correctly, he says to enter an average Cl of 0.6, while for all the others he says to enter an average Cl of 0.5.
Yet Cl is directly related to the aerodynamic angle of attack of an airfoil (0.1/degree) whatever the reflex or airfoil
The lower profile losses alone (proportional to rpm³) then explain the lower rotor drag.

 

[MikeBoyette]

I was wondering if the twist, "wash in" as I understand it, is possibly aeroelastic enough to untwist in gusts and convert the gust to RRPMs...
This would also be an efficiency increase and act a it like a suspension......

Has anyone experimented with zero pitching moment foils like the RS, EH, or MH series?

Has never happened. They hold their twist. Dad tested them extensively on a V-6 powered run up stand. He was able to run them to 600 rpm’s on a set of 22’s. I personally have seen over 500 on a set of 23’s. He built over 3k sets and has an excellent safety record.

 

[raghu]

My sense is that the relatively high pitch and twist are the key to the DW performance and not the airfoil. Adding reflex changes the pitching moment which in turn in torsionally limber blades (which the DW may not be) may increase the pitch and performance.

 

[Doug Riley]

JC, have you observed how narrow the DWs' reflex is? It's quite a bit less than any other blade that I have flown. Whether it is the airfoil, or torsional stiffness, that allows this, I don't know. But reducing trim drag is a standard way to improve the performance of an aircraft.

E.g. sailplanes sometimes often have zero download on their horizontal stabilizers, and tolerate the lack of AOA stability that this creates. Reflex is the essentially same thing as a downloaded horizontal stabilizer.

 

[raghu]

One other thing I could never reconcile with the late brilliant CB was why some blades were more easily hand statable than others. He thought it had to with the airfoil having a sharp stall, but quantitatively this does not work out and the best explanation to me is pitch (and reflex in torsionally limber blades) more than anything else that determines hand starability.

 

[raghu]

E.g. sailplanes sometimes often have zero download on their horizontal stabilizers, and tolerate the lack of AOA stability that this creates.

I guess I digress.... but this is one of the persistent myths- lack of stabilizer download on a sailplane results in lack of AOA stability. A simple calculation of the moments will show this not to be the case. It change the trim or balance of the sailplane but not the underlying AOA stability. The AOA stability in a sailplane, in the first order, is a function of the CG position and relative areas and lift slopes of the wing and stabilizer and not up or down loads. Sure down loads may often result on the stab but it is not what is causing the AOA stability.

Years ago on this forum Udi and I debated this and and the end when he did the moments he was convinced that download had nothing to do with it and was a convert. link to post where this is explained.

 

[Aerofoam]

Has never happened. They hold their twist. Dad tested them extensively on a V-6 powered run up stand. He was able to run them to 600 rpm’s on a set of 22’s. I personally have seen over 500 on a set of 23’s. He built over 3k sets and has an excellent safety record.

I wasn't implying they lose twist altogether, but that they have enough flex to momentarily lose a little pitch and absorb gusts, which in turn would theoretically reduce turbulence and increase efficiency.
I would be curious to know if the pitch changed at all during a 600 rpm run up.
I would expect a slight difference form the low to high end of the range, but the reflex could prevent that if it is dialed in.

 

[raghu]

Often people talk about an airfoil having more

But reducing trim drag is a standard way to improve the performance of an aircraft.

Yes reflex on the wing helps unload a stabilizer in a sailplane and consequently reduce trim drag but it is not clear to me how this applies to a gyro rotor ? Maybe I am missing something.

 

[Jean Claude]

One other thing I could never reconcile with the late brilliant CB was why some blades were more easily hand statable than others. He thought it had to with the airfoil having a sharp stall, but quantitatively this does not work out and the best explanation to me is pitch (and reflex in torsionally limber blades) more than anything else that determines hand starability.

I agree with you, Raghu.
In my opinion, the ease with which Bensen wooden blades is hand startable comes from their low pitch setting at rest. Correct pitch comes after, with the bending due to the reflex, when the rrpm increases

 

[raghu]

To me the this graph by Bailey (NACA) is one of the most instructive when it comes to profile Drag of a rotor. Of course the surface finish and airfoil characteristics will modify the results but it shows how significant the effect of pitch is.

All you need is the graph to get a reasonable first estimate of the performance of any autogyro rotor.

 

[Aerofoam]

JC, have you observed how narrow the DWs' reflex is? It's quite a bit less than any other blade that I have flown. Whether it is the airfoil, or torsional stiffness, that allows this, I don't know. But reducing trim drag is a standard way to improve the performance of an aircraft.

E.g. sailplanes sometimes often have zero download on their horizontal stabilizers, and tolerate the lack of AOA stability that this creates. Reflex is the essentially same thing as a downloaded horizontal stabilizer.

Let me take this a step farther.
I have built hundreds of variations of flying wings and most foils, even the neutral pitching flying wing foils require reflex, usually at the elevon trim setting.

However, if you move the CG back, you are essentially trimming with weight shift and do not need reflex on the elevons (or H stab).
This is more efficient, but it can get squirrly and pitch divergent.
The way we always test CG/pitch stability is to enter a shallow dive and center the sticks. If the aircraft pulls up by itself, is has up trim and is nose heavy.
If it continues to accelerate but doesn't deviate in pitch, it is completely trimmed and aerodynamically neutral.
(This is a wonderful thing if it is still controllable!!!)
If the dive angle increases, it is tail heavy and needed down trim to fly level. This is causes the plane to be divergent from any pitch condition if the speed changes.

In most cases, you go from the neutral CG and move the CG slightly forward to regain stability.
in the case of a tailed aircraft, the neutral loaded H stab should still be stable because it is still an H stab moving through the air doing it's job. But it will be less effective at low speeds.

I went through lot of flying wing designs trying to get an aircraft that flew level with zero up trim, or reflex and I succeeded on a couple designs.
One of which is one of the most efficient aircraft I have ever seen.
The 8ft. span, 14lb. delta wing aircraft only required 120w. to cruise at 55mph.
this film solar can provide almost twice what it needs to maintain cruise..

The second best swept wing design, didn't need up trim, but I did play with the foil a bit, transitioning from root to tip in a way that aerodynamically created trim thoughout the shape of the wing.
This is what the Hortens did, but they used the Prandtl D lift distribution curve with a lot of twist.
I believe they and Lippisch were the only ones who actually understood the concept until Al Bowers figured it out in the early 2000's.

This all applies to a rotor as the blade is performing much like a flying wing...

 

[Aerofoam]

I agree with you, Raghu.
In my opinion, the ease with which Bensen wooden blades is hand startable comes from their low pitch setting at rest. Correct pitch comes after, with the bending due to the reflex, when the rrpm increases

I think this is correct, a high pitch setting requires a faster speed to bring the "Apparent Wind" (Sailing term) far enough forward to create laminar flow over the top of the foil, and thus, auto rotation.
My model had a thinned clark y foil and neg. 1deg pitch. it could self start from a completely stopped condition by diving, if you had at least 200ft.

 

[raghu]

The reason pitch is critical for handstartability can also be seen from the Bailey graph I posted above. Higher the pitch, the mu at which a significant portion of the blade is stalled (dotted curve) is lower.

 

[Kevin_Richey]

I think this is correct, a high pitch setting requires a faster speed to bring the "Apparent Wind" (Sailing term) far enough forward to create laminar flow over the top of the foil, and thus, auto rotation.
My model had a thinned clark y foil and neg. 1deg pitch. it could self start from a completely stopped condition by diving, if you had at least 200ft.

Mark: From a dead-stop, if @ least 200' of altitude was available to dive into?