Bensen Aluminum Blades - Questions about design and variations

curtisscholl

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Hi Folks:

I was at the Mentone, Indiana meet in early August and I walked through the museum for a bit, and went back a couple more times. Each time I found a few more interesting tidbits. Glenn, if you are on this board, thank you for the talk we had.

There were Bensen B7 and B8 models of different configurations, some with the plywood blades and others with the metal blades. There were others to be sure, like the Air and Space 18. I also found in the northeast corner parts for a plywood blade laid out on a bench. This was so informative.

What is curious to me is the actual makeup of a Bensen aluminum blade. Chuck has mentioned items of interest about both types. But I have a few questions about how Igor designed the aluminum versions.

The aluminum blade is composed of an aluminum spar that has been milled for the leading edge of the airfoil and to a certain depth of the chord of the blade, the rest aft of that point was aluminum sheet. The one I paid attention to most had rounded rivet heads in a neat line down the blade at the intersection of the sheet metal and the solid spar.

1. Were there two versions of the coupling of the sheet metal to the spar? Round head vs. Flush rivets?
2. Did the rivets go all the way through the milled spar to couple the sheet metal to the spar?
3. Was 6061-T6 aluminum used as the material for the blade spar?

Back in the late 80's I read that Ken Brock used a similar spar, but a different method of applying the skins.

4. Did Ken Brock use bonding rather than rivets and did I remember correctly that he used a wrap around skin?

Chuck also mentioned that the sheet metal was in sections on the Igor Bensen blade, but I did not pay attention well enough to actually see what he meant, but I get what he said that it acted as an air pump.

Thanks in advance..

Curtis Scholl
 
Bensen used a slightly modified version of the NACA 8H12 airfoil with extruded spar of 6061 aluminum.

The biggest deficiency was the unsealed gaps between upper skin segments; the rotor acting as a centrifugal air pump consumed several HP needlessly pumping air (A centrifugal air pump is the heart of your vacuum sweeper).

The rationale of the segmented skins was to reduce stress as the blade flexed during rotation.

No Bensen metal blade used countersunk rivets.

Ken Brock’s copy of Bensen blades was just that; an exact copy with button head rivets and all. Ken did, however, build several sets of wide chord blades that weren’t Bensen copies.

Yes, the rivets went all the way through.

Many gyroplane "designers"” falsely believe the airfoil used by Bensen was a special airfoil designed specifically for gyroplanes but that is not so. The NACA 8H12 airfoil was designed in the late 1940s shortly after laminar flow had been discovered in the then new low turbulence wind tunnels and its intent was to reduce the drag of rotors for the newly emerging helicopters. Unfortunately, it was a failure, never being used in an actual rotor beyond a single set run on NACA’s whirl tower where real world drag was no less than existing helicopter airfoils and maximum lift was substantially less.

Life is different in a low turbulence wind tunnel than it is spinning around on an actual helicopter.
 
Chuck:

Extruded and not milled? OK. the forward part of the blade looked solid I am assuming the extrusion process only gave it a profile for the 8-H-12 foil shape.

I found a paper (and there are probably more from NASA site NTRS) on the XX-H-12 airfoil shapes. The 10-H-12 had a thickest section at around 40% rather than the 30% like 8-H-12. As you say, research is fine, but actual testing is where the rubber meets the road.

And the gap between segments I understand. Lower pressure at the tip (or furthest segment gap where the flow is fastest) than at the root thus air movement toward the low pressure area.
I took pictures of the hub and blade attachment points but did not take pics of the blade profile or the sheet metal trailing edge. Too enthralled with the experience of being at a gathering...

I understand the modified 8-H-12. Where the leading 50 percent (or thereabouts) of the chord is able to be shape controlled by the extrusion, there is only so much you can do with a sheet metal trailing edge, it was close enough for all practical purposes. He did not use the G-2 patented airfoil except on the plywood blades from what I can tell from pictures.

Ken Brock blades had no differentiation? So no bonding involved. I guess I misread the article, but that was 20 years ago.

Thanks. this gives me a lot to think about.

Curtis S.
 
Bensen CLAIMED that the bottom skins were both riveted and bonded. Occasionally, Igor offered "alternative facts," though.
 
Bensen’s claim about bonding was sorta factual, Doug. He used contact cement to hold the pieces in place during drilling and riveting.
 
Oh my goodness. LOL. DAP Weldwood Contact Cement or DAP 107 <<snicker>>. I guess it worked well enough. Improvisation, as necessity is the mother of invention.
 
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Well, Chuck, that powerful redundancy (using shelf-paper glue) certainly makes me feel better.

Igor's "alternative fact" about bonding the lower skins came from his article in the PRA mag about bird collisions. The article points out that Bensen metal blades survived simulated bird collisions quite well, but (brief commercial message) not all blades are built like Bensens, so not all will do so well.
 
Both Stanzee and early Rotordyne blades were bonded as you know, Doug. But the glue job wasn’t all that great but didn’t have to be with wrap around skins.

My little hole in the wall company had proper structural bonding facilities; chemical pretreatment tanks, etc., that were required for some military contracts.

I used those facilities to bond a set of blades with skins lap jointed rather than wraparound that were crashed by a character named Red Iron Ron. Bensen, who observed the crash and the condition of the blades afterwards, asked me to bond a set of his blades.

I agreed and Bensen sent me an unassembled blade kit. Unfortunately, I got sidetracked by other pressing matters and never got around to it.
 
Interesting. I smacked up a set of Dragon Wings one time and was impressed with the tenacity of the glue joints. However crumpled the blade skins, the glue joints held without exception. Top and bottom skins crumpled as a unit.

OTOH, the simple riveted structure of the Bensen and Rotorhawk blades worked out remarkably well. Stan Dzik, in his Helicopter Design and Data Book, relates that early metal helo blades with riveted construction had a fatigue life of about 100 hours. Bonding wasn't just a nice new touch, it was indispensable to reasonable blade life. Clearly, variable collective pitch and the comings and goings of drive torque beat up blades much worse on a helo than on a gyro.
 
Doug, are you referencing early RotorHawk blades? Was their technique similar to Bensen? I do not have a reference picture. Gyroplane history since Bensen seems to be undocumented.
 
Curtis, I don't have any firsthand experience with RotorHawk blades.

Here in Vermont, we have the concept of "rock skis." Rock skis are your old beaters that you use when there are rocks poking up through the snow and you're going to gouge up your skis. You don't use your nice new ones in those conditions.

Some gyro people spoke of Rotor Hawks as rock skis.
 
While we're on this subject, why did Bensen use small sections on the skin vs one large skin?
 
I have a set of riveted Brock blades and while flying with a Mac heard 3 loud pops over the sound of the Mac. I set it down and found 3 rivets missing. I will add that I was going about 70mph and did a very hard turn, probably overstressed the blade. I think it would have been better with a little more shelf paper glue. I don't fly Bensen blades anymore.
 
AirCommandPilot:

Chuck Beaty had this to say about segmented Bensen blades:

The rationale of the segmented skins was to reduce stress as the blade flexed during rotation.

The biggest deficiency was the unsealed gaps between upper skin segments; the rotor acting as a centrifugal air pump consumed several HP needlessly pumping air (A centrifugal air pump is the heart of your vacuum sweeper).

Curtis Scholl
 
Doug Riley:

So, Rotor Hawks were the go to blades for knocking around in...Not bad. So, I will keep my eye out for examples.

Thanks

Curtis S.
 
Dakb2:

When I heard about the "bonding" with contact cement I had a laughing fit. The 3M bonding process is not an easy one to do for a homebuilder, but Chuck Beaty has done it.

Curtis S.
 
No Title

I just got a set of those Bensen factory aluminium blades model G-4 serial 7474...anyone got any info on them
 

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