Weight and Balance

[Jean Claude]

Chuck B. did measure rotor disk angles -- as well as could be done using very simple items.

Yes, Chuck compared the L/D performance of several profiles on rotors of the same diameter, chord and geometric pitch. But he forgot that it's the aerodynamic, not the geometric, pitch angle that's the important parameter to maintain (i.e. rpm).
This led him to believe that the DW section was much better than the others, whereas the others would have performed just as well by adjusting the pitch to obtain the same rpm.

 

[Mayfield]

This led him to believe that the DW section was much better than the others, whereas the others would have performed just as well by adjusting the pitch to obtain the same rpm.

I thought that CB was postulating that a gyroplane of a certain weight, at a certain airspeed, would fly with a lower disc angle and therefore less drag with some brands of rotor blades than with others.

Thanks for your consistent attempts to explain things Jean Claude.

Jim

 

[okikuma]

Yes but when I did that the result was not the same. Kind of close but not exactly. If you really want to know the CG you have to do it right or no point really. Maybe we screwed up something but making the weldment was easy enough for us.

Abid,

I can understand your point. Balancing on the main wheels as a secondary fulcrum point is not the ideal solution since the center of gravity of the gyroplane is well above the main wheel axles.

Wayne

 

[Doug Riley]

I did my "double hang test" based on recollection of a grade-school science fair demonstration.

A purpose-made "second hang point" fixture is an elegant way to do it. But I just ran a stout rope up from the nose of the gyro, and tied it to the mast just under the cheek plates, leaving some slack. The second hang point was a loop in this rope, a foot or so ahead of the mast. Of course, the gyro hung tail-down when suspended from this point. And, of course, the blades were on the gyro during the "second hang."

In the case of my first-generation lowrider Air Command (without a pod), the two hang lines crossed at a point 5-6 inches below the prop thrustline. I.e. the test detected significant HTL. With the 447 making perhaps 250 lb. of thrust, the static PPO moment was about 125 ft.-lb.

 

[Jean Claude]

I thought that CB was postulating that a gyroplane of a certain weight, at a certain airspeed, would fly with a lower disc angle and therefore less drag with some brands of rotor blades than with others.

Of course, a less "dragging" blade profile requires less power for a given rpm, which requires less flow through the rotor, resulting in a lower disc angle and less drag, easier moving forward.
The "Dragon Wing" profile touted by CB did show a higher L/D, but the measured rpm was also lower than the other profiles tested. This means that the comparison was skewed by lower air friction losses (proportional to rpm 3!)

 

[Mayfield]

Of course, a less "dragging" blade profile requires less power for a given rpm, which requires less flow through the rotor, resulting in a lower disc angle and less drag, easier moving forward.
The "Dragon Wing" profile touted by CB did show a higher L/D, but the measured rpm was also lower than the other profiles tested. This means that the comparison was skewed by lower air friction losses (proportional to rpm 3!)

Thank you Jean Claude.

I apologize for being obtuse.

If an aircraft GW is 500 pounds, flying at sea level, at 60 MPH, with a 23-foot rotor diameter, will I require less power to fly with DWs or Bensen blades?

Thanks for your patience.

Jim

 

[Jean Claude]

Jim,
DW are certainly better than Bensen metal blades, but probably no better than the usual NACA 8H12 at the same RPM.
What chord do they have?

 

[Mayfield]

My DWs have a 7 inch chord. My old Bensen segmented top blades were actually delightful to fly. They hand started easily and were easy to track and balance. They were different than my Rotordyne blades and my StanZ blades, but still quite nice.

For recreational flying they were all acceptable. On my Bensen style airframes, I could not tell a lot of difference in day to day flying between any of them, including the DWs.

I certainly appreciate your insights.

Jim

 

[Bob Gregory]

Who ever asked about statements pertaining to Skywheels flying at 8 degrees. This site does not accept my spreadsheets after 13 hours trying.
This from a screenshot.
CBeaty recommends 400 fpm climb, minimum.
Then need more horsepower or less drag.
Clyde Beaty, May 2, 2011.
The angle at which a rotor disc flies reflects its lift/drag ratio.

7º = 8.1:1 Dragon Wings
8º = 7.1:1 Skywheels
9º = 6.34:1
10º = 5.7:1 Bensen blades
Nine degrees is a fairly typical 50 mph number. As airspeed increases, the rotor flies at a shallower angle.

 

[Jean Claude]

Once again, without knowing the rpm obtained with these rotors, comparisons of discs A.0.A do not allow us to conclude on the quality of their profiles.

 

[MikeBoyette]

Jim,
DW are certainly better than Bensen metal blades, but probably no better than the usual NACA 8H12 at the same RPM.
What chord do they have?

Not what we found to be true. When most tried DW’s from a standard 8H12 they noticed a significant difference in performance. This was due to the reduced drag. The airfoil and the twist in the blade was responsible for this.

Many who tried them when we first brought them to a fly-in couldn’t get their engine out simulation landings where they wanted. They almost always went beyond their intended target. Fred Taggart proclaimed to not like them because they had too much lift. What he was describing was less drag since all blades lift the same “one gyro”.

 

[Doug Riley]

I noticed a remarkable performance increase when I first flew McCutchen blades after starting with Bensen wood and metal blades. Bensen blades had flat bottoms AND flat upper skins from the spar to the trailing edge. The Bensens also had bolts and non-flush rivets, along with segmented skins. The woodies had hollow external noseweights and trim tabs to boot. Lots of draggy items in the breeze.

OTOH, some of the performance increase may have been an illusion, brought on by the greater weight of the McC's. They held RRPM during a landing flare. This resulted in a bit of landing "float," as Mike describes. Bensen blades did not allow landing float!

Also, I imagine that reflex plays a role in blade efficiency. Reflex produces "down lift" (if I may use an odd expression). Moreover, reflex adds to induced drag in creating its down-lift.

Bensen blades had relatively large reflex areas, perhaps due to the large pitching moment generated by their non-standard airfoil. Dragon Wings appear to have a smaller reflex area. This may mean either that (a) their airfoil has a lower pitching moment than Bensens, or (b) they are a bit under-reflexed, relying partly on torsional stiffness to prevent nose-down twisting caused by pitching moment. Or both.

Reducing reflex and relying on torsional stiffness can be a dangerous game if carried too far.

 

[Aerofoam]

Also, I imagine that reflex plays a role in blade efficiency. Reflex produces "down lift" (if I may use an odd expression). Moreover, reflex adds to induced drag in creating its down-lift.

We refer to it as "Negative Lift".
I spent more than 20 years chasing "Flying Wing" designs and the best foils are derived from original Horten foils, but thinned and optimized for lighter stiffer construction.
The EH series is excellent for general purpose flying wings, almost completely neutral pitching moment.
The RS series is better for speed.
I would think either series would be good for rotor blades if they were thinned a bit.
Here's a 12% version, they are similar to Eppler 635 series....
I would think a rotor blade would be around 9 or 10%

 

[Jean Claude]

Not what we found to be true. When most tried DW’s from a standard 8H12 they noticed a significant difference in performance. This was due to the reduced drag. The airfoil and the twist in the blade was responsible for this.

I agree with that, Mike.
My point is that it's not the profile itself that's responsible for the observed improvement, but only the aerodynamic pitch setting, which is higher for the same geometric pitch due to low reflex
This is clearly reflected in the lower rpm. Chuck Beaty shows in his spreadsheets CL of 0.6 for DW blades instead of 0.5 for the others.
This means that the other rotors would reduce their drag as well if we set the CL of the blades to the same value of 0.6 (pitch setting higher).

Whatever your profile, there are unfortunately a few latent risks and drawbacks to increasing the aerodynamic angle:
- slower rpm recovery after a low g
- More difficult to launch by hand
- more sensitive to flapping during the take-off run
- lower mu max

 

[MikeBoyette]

Doug the original DW’s had no reflex. Chuck ran the numbers and it was acceptable for the airfoil. It also made it easier to make them by hand. Later a reflex was added and rivets eliminated.

 

[Abid]

I agree with that, Mike.
My point is that it's not the profile itself that's responsible for the observed improvement, but only the aerodynamic pitch setting, which is higher for the same geometric pitch due to low reflex
This is clearly reflected in the lower rpm. Chuck Beaty shows in his spreadsheets CL of 0.6 for DW blades instead of 0.5 for the others.
This means that the other rotors would reduce their drag as well if we set the CL of the blades to the same value of 0.6 (pitch setting higher).

Whatever your profile, there are unfortunately a few latent risks and drawbacks to increasing the aerodynamic angle:
- slower rpm recovery after a low g
- More difficult to cast by hand
- more sensitive to flapping during the take-off run
- lower mu max

Hi JC
I agree with you on all this but I think unless you are doing a way older design and you are young, no one is really doing pre-rotation by hand anymore.
My guess is that you are saying that it is more sensitive to flapping because the Aerodynamic effective pich is higher meaning it is already closer to its critical angle of attack than one which isn't as close. That makes sense but I would be interested if you have thought about how much of an effect it really is. If someone is going to flap the rotor, is it say a second or 5 seconds difference because people flap when they have really lost the plot. Wondered if you have any thoughts or any approximate analysis on this.
In my experience although I did not personally fly the DW on the AR-1, Greg did. His assessment was that Stellas held the float etc. better initially but he thought he needed to do more testing. These were the high inertia versions of DW blades. I have about 12 hours in a Dominator with the standard DW and their inertia is significantly less than Stellas, Magni, AG and ELA blades that all seem to use 8H12. I think most of these are set to a geometric pitch of 2.5 degrees.

 

[Jean Claude]

Abid, I agree that hand pre-rotation is no longer an option, but with the current immediate full-throttle procedure, the limit of flapping divergence is not far from being reached at some point in the run, and setting the rotor pitch only 1° higher would be much less tolerant, at rotor equal inertia (for example when the rear seat is unoccupied, or the stick is on the side where the wind comes, or the runway is downhill or prerotator wear precaution).

 

[Jazzenjohn]

On my first gyro i switched between Bensen blades, Dragon Wings, and Sport rotors. The Dragon wings and Sport rotors required substantially more trim spring force than Bensens. Both Dragon Wings and Sport rotors were dramatically more efficient than Bensen blades. The both gave me double the altitude at the end of the runway than Bensen blades. That was a big difference to me at that time because I was flying on a Rotax 447. I tried different AOA's on the Bensens and didn't see a whole lot of difference in the range i experimented in.

 

[okikuma]

The biggest issue with the Bensen metal rotor blades are the gaps between each upper skin segement. As the blades rotate, each would become a centrifugal air pump consuming several HP of energy as opposed to non-segmented (i.e. smooth skin) rotor blades with the same airfoil. Because of this construction flaw, the Bensen metal rotor blades should not be considered the reference point to compare all other rotor blades against.

The only reason why many do compare against the Bensen metal rotor blades, outside of hand build wooden rotor blades, the metal blades were the only production rotor blades available for recreational gyroplanes.

Wayne

 

[MikeBoyette]

The biggest issue with the Bensen metal rotor blades are the gaps between each upper skin segement. As the blades rotate, each would become a centrifugal air pump consuming several HP of energy as opposed to non-segmented (i.e. smooth skin) rotor blades with the same airfoil. Because of this construction flaw, the Bensen metal rotor blades should not be considered the reference point to compare all other rotor blades against.

The only reason why many do compare against the Bensen metal rotor blades, outside of hand build wooden rotor blades, the metal blades were the only production rotor blades available for recreational gyroplanes.

Wayne

The segmented skins were done because Igor only had a small sheet metal brake to form the top skins.