New rotors and rotor blades

C. Beaty

Gold Supporter
NACA 9H12:

http://naca.central.cranfield.ac.uk/reports/1946/naca-rb-l5k02.pdf

NACA developed a series of 4 rotorblade airfoils in the early 1940s in the hope of improving the performance of helicopters of the time.

Unfortunately, they were designed with too great an extent of laminar flow for use in helicopter blades.

Rapid changes of angle of attack, a consequence of forward flight, as well as centrifugal force acting on the boundary layer caused early flow separation.

They looked good in NACA’s low turbulence wind tunnel.
 

C. Beaty

Gold Supporter
Had Bensen picked the 9H12 rather than the 8H12, it would have become the “standard” gyro airfoil. Its hand startability looks about equal to the 8H12, perhaps a little better.

The abruptness of stall determines ease of starting.
 

okikuma

Member
I notice in the comparison photo, the AAI 9H12 airfoil has a more dramatic leading edge rise in curve as compared to the 8H12.

Chuck, it makes sense that there's a connection to the more abrupt the stall the airfoil possess, the ease in hand starting. When the highest part of the leading edge curve (the greatest part of thickness) of the airfoil is at around 24% of the cord, there will be a more abrupt stall at the critical angle of attack as compared to an airfoil with the highest part of the curve at around 30% of the cord.

The one time I met Dr. Bensen around the 1994 time period, I asked why he chose the 8H12 airfoil for his metal blades and he said it was a good airfoil from manufacturing stand point in respect to ease of construction; it was easy to duplicate. He also said the airfoil will autorotate very easily in respect to hand starting. I wish I had the foresight back then to know there was a difference in the airfoil Bensen used for his original wood blades compared to the later metal blades. The Bensen wood blades had a more dramatic leading edge curve compared to the metal blades. I know from many accounts from all the senior gyroplane builders and pilots that the early Bensen wood rotor blades were extremely easy to hand start and autorotate in the lightest of breeze.

Chuck, am I correct in remebering that most Bensen owners who flew with the wood rotor blades had set 0.5 to 1 degree of pitch, and with the later metal blades 1 to 1.5 degree pitch?

Wayne
 

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C. Beaty

Gold Supporter
I made a poor choice of words when mentioning hand starting, Wayne. The more abrupt the stall, the more difficult to hand start. Rotors using the NACA 0015, with its very abrupt stall are quite difficult to hand start.

Bensen wood blades were normally set with the bottom surface parallel to the hub, there being no pitch adjustment.

They hand started easily because they were ever reflexed and quite flexible in torsion. With a fairly flat incidence setting, they would start in the slightest breeze and as they came up to speed, the excess reflex twisted them nose up to proper flying incidence.

When I first showed at the airport with a set of Hughes 269 blades on my gyro, –NACA 0015 airfoil- my good friend Willie said; “Chuck, them things won’t autorotate.”

I said; “Why is that, Willie?”

Willie replied; “No ducktail.” (The wood blade’s reflex was refereed to as a “ducktail.”)

It seems that a SRC member before my time had built a set of wood blades but had omitted the reflex. They in fact wouldn’t autorotate; as they began to come up to speed, the negative pitching moment twisted the blades nose down, depitching them. My understanding is that they ticked over slowly, 50 rpm or so.

I don’t believe that’s a Bensen wood blade in your photo, Wayne or else someone has taken some liberties with the plans. The TE reflex of Bensen blades was accomplished by beveling the lower skin and the nose wasn’t nearly as blunt. The first 20% or so of wood blades quite closely followed a Clark Y.

Well, maybe so, I went back and looked again; the second photo looks more like it, except the steel spar was exposed. Bensen factory wood blades had a concealed spar.
 
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okikuma

Member
You know Chuck, I took your mistake as gospel without thinking it through. I know better! I was cognitively being lazy! Shame on me! I retract my previous statement since I said everything in reverse! LOL

In fact, a symmetrical airfoil as the NACA 0012 does have an abrupt stall whereas a Clark Y and USA 35B are very docile. That statement makes sense.

I do understand about the torsional flexibility of the old wood rotor blades in respect to the outward portion of the rotor blade that would increase in pitch (wash-in) for better "autorotatability." The same reason why you operated the torsionally stiff Hughes 269 blades up-side-down.

The first photo are original Bensen riveted metal rotor blades. The second photo are not Bensen metal blades but somewhat a copy without the rivets into the extruded solid D spar and a one piece upper and lower skin. Most likely the skin is bonded however the trailing edge is riveted. The third photo are factory manufactured Bensen wood rotor blades with the interior metal spar.

I have no doubts that the Clark Y or USA 35B with an added reflex "ducktail" from the earlier vernacular, would hand start easily and spool up quickly, but in the long run be a very draggy airfoil.

Wayne
 

okikuma

Member
The more I look at the ILT-LT-11.0 airfoil, the more the airfoil closely resembles the Bensen wood rotor blade airfoil but with less thickness, and with less dramatic reflex.

Please keep us all updated on your test results!

Wayne
 

Trendak&Son

Gyroplane Manufacturer
...

Please keep us all updated on your test results!

....
We will publish the results as soon as they are available.

In the meantime visit us at PRA convention 2014.

We invite you to Mentone Indiana to meet us from 5 to 9 of August. We will be there with our dealer to answer all your questions and to present you the newest gyrocopters.
 

SkyGreg

Newbie
We will publish the results as soon as they are available.

In the meantime visit us at PRA convention 2014.

We invite you to Mentone Indiana to meet us from 5 to 9 of August. We will be there with our dealer to answer all your questions and to present you the newest gyrocopters.
What is the latest news? Any developments?
 

Mike G

Junior Member
Note the use of Smart Avionics PB3 dynamic balancer.

Sorry I couldn't resist a little plug for my favourite balancer:whoo:

Mike G
 
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