My modified Parsons

Foam, I'm not sure that the phenom we're discussing is flutter as such, though the mechanism here is related. I suspect that the McCutchen flare is not oscillatory, just statically divergent.

In an object as torsionally limber as a fiberglass rotor blade, any attempt to constrain the blade against twisting by merely holding onto it at the root won't work beyond a rather low value of torque.

Being so torsionally limber, the blade will tend to twist in reaction to an applied torque around its CG. If the CG is aft of the center of pressure, then the blade's own lift is continually applying a pitch-increasing twist to the blade. At higher aircraft airspeeds, the advancing blade experiences greater airspeed than at slower aircraft airspeeds. The advancing blade may overcome the twisting stiffness built into the blade, twisting nose-up about its CG. This, of course, will work the same way as back stick, tilting the rotor aft and causing a mid-air flare.

On my light Air Command with 23-foot McCutchens, I never experienced an uncommanded mid-air flare. In thermally conditions, though, it seemed to exaggerate the updrafts, ballooning up with an audible whoosh. This may have been a mild example of this same AOA instability.
I think we said the same thing, you stated it with less ambiguity.
Just as a flying wing with reflex in the foil and as slightly aft CG will diverge UP at a certain speed, the rotor blade behaves the same way.
There is no way for it to be rigid enough to overcome the twisting. Even with a super stiff carbon blade with lots of bias plies, the phenom will only be delayed to a higher speed and when it occurs, it will be catastrophic.
 
Mark,

The Dyke Delta is unique whereas the wing spars and ribs are built of 4130 tubing, truss style and thus very rigid.

My modified Parsons


The sailplane and C-141 in the video are a prime example of what you have explained.

Wayne
I am very familiar with delta wings, I love them and have built more than 50 iterations of delta wing flying wings.
My business logo is a delta wing, the pic below was a demo at a baseball game at Kino Stadium in Tucson back in about 2003....
At lower wing loading, they are incredibly efficient, which most aerodynamics experts will deny until they are foaming at the mouth....
One series was initially a sailplane and it was incredibly efficient. I ended up building an electric powered UAV that had an 8ft. span, 3.5ft. chord at the root and at 14lbs., could fly at 50kts consuming only 120watts. This plane could fly all day and about 5 hours into the night with thin film solar panels.

deltastadium2.jpg
 

Attachments

  • My modified Parsons
    deltaoverstadium1.jpg
    14.8 KB · Views: 12
One way to look at it might be to say that the McCs' ballooning behavior is the first half of a flutter cycle. The difference between this and regular flutter is that there's no elastic mechanism that quickly un-twists the blade -- the blade is too torsionally limber for that. Instead, the teeter hinge and the rotation of the blade away from the advancing position somewhat limit the upward excursion of the blade. Good thing, too.
 
The simplest way in checking the cord wise balance of a McCutchens or any other rotor blade is by using the two opposite 45 degree fulcrum balance point method. Hopefully, someone on this forum with McCutchens rotor blades is willing to perform this test and report the results to all of us who are interested. This could be a PRA chapter meeting project. Through evidence based analysis we will finally possess a definitive answer.

In the mean time, I've started a new thread on rotor blade balancing and tracking.


Wayne
 
The simplest way in checking the cord wise balance of a McCutchens or any other rotor blade is by using the two opposite 45 degree fulcrum balance point method. Hopefully, someone on this forum with McCutchens rotor blades is willing to perform this test and report the results to all of us who are interested. This could be a PRA chapter meeting project. Through evidence based analysis we will finally possess a definitive answer.

In the mean time, I've started a new thread on rotor blade balancing and tracking.


Wayne
Sure do miss Chuck. He had done that very thing when the phenomenon was first noticed. He suspected that was the cause and if memory serves me correctly he either did something similar to or exactly what you’re talking about. He used a good section of a wrecked set of SW’s. I want to say the section was a foot or two long.
 
Sure do miss Chuck. He had done that very thing when the phenomenon was first noticed. He suspected that was the cause and if memory serves me correctly he either did something similar to or exactly what you’re talking about. He used a good section of a wrecked set of SW’s. I want to say the section was a foot or two long.
Mike,

I was unaware that Chuck had performed a cord wise balance check on a McCutchen rotor blade. I certainly would like to see a cutaway profile section of a Skywheel.

Wayne
 
Last edited:
Top