Original Bensen Blades Good to use or ?

FlyingFish

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Apr 29, 2004
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Winnipeg, MB
I picked up an original Bensen B8M it was put together in 1964. The the fellow that used to own passed away some 10 years ago so I am unable to ask him. I was told that he did fly it quite abit and did love it.

It has wood blades which his wife said he liked better. The aluminum blades were never used as he did not like them. I am not sure why.

Would these have come balanced? How was the design of these for lift etc. ?
Any help to these questions and other thoughts would be greatly appreciated.

I am in the process of taking pictures before I start the overhaul. I was asked to post some up. Should I start a new thread somewhere?

I am like a kid with a new toy .... weeeeeeee.

Peter
 

Sonnyj

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Hiya Peter I have had all kinds of pros and cons about plywood blades.But I realy dont know why nobody uses them exept nobody sell's them.I've heard they hand start better than most other blades,and very smooth running if set up properly,and are lighter than most.I built a set from planes just to see if I could,out of cabenet grade birtch.They looked very good and it only took me about 8hr. of work to finish.Ocourse I never tryed to fly them.Now please dont take me for an expert on the subject,having no gyro flying under my belt.You can talk Monte Hoskins he sell's planes to build the plywood blades and can answear all your guestions.Hope this helps Sonny
 

raghu

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Sonny,

Are the plywood rotor plans that monty sells the same as the bensen plans? Could you give a brief discription of the construction. Thanks.
 

Sonnyj

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Hiya raghu I realy dont know if there is any simularity,Mitch from the land of oz has beem kind enough too send me some info on the oem Benson blades.I will be glade to share when the snail arives.The plans that Monty sells seem to be very complete,materals,cutting,assymbly,ballancing,and how too build a blade binch.The plans are only $25.00 so I felt I coulnd go wrong even if for just the knowledge.I plan on using the blades on my Gyrobee project. Regards Sonny
 

raghu

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J3 cub/ schweizer 2-33/ a few flights on twinstar
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Thanks sonny! Do let me know when you hear from mitch. I have a mate who would like to have a peak at the plywood rotor plans.
 

Doug Riley

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Jan 11, 2004
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I have the plans for wood blades that came with my Bensen plans in 1969. I never built my own, but bought a nice used pair of woodies and flew them a fair amount.

They are pretty to look at (as you'd expect a mahogany object would be). They are very easy to hand-start -- partly because they are so limber that, at low RPM, the nose weights twist them nose-down into a distinctly negative pitch. Once they pick up RPM (that is, airspeed), the trailing edge reflex gains power and cranks the nose of the blade back up.

They are, of course, delicate. I just about cried when a bolt on my trailer (a junker boat trailer) ate a hole into my blade box and put a notch in the trailing edge of one blade. It was, however, easy to patch using a bit of plywood left over from a model airplane project.

Some people complain about wood blades' warping with changes in humidity. I did not have a problem with that. The blades should not be left drooping on the gyro, however. Take them off and keep them in a place with normal humidity.

Chuck Beaty has calculated that Bensen wood blades are over-reflexed. This means that they automatically twist in more pitch as you load them up, making them more-or-less constant-RPM blades. IOW, you can't store energy in them by overspeeding them in a hard bank. This will cramp some people's style of flying.

You might get away with hitting a bird in flight with metal blades, and survive. I think wood blades would shatter. That alone is enough to steer me clear of them these days.
 
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FlyingFish

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Apr 29, 2004
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Location
Winnipeg, MB
Yes I have to agree with not using the wood blades. I do have an original pair of all alluminium that also came with it. I have not been able to find out any aero dynamics on these those or how well the perform. They have only been used a few times as he did not really like them. I am wondering about the balancing of them also.

Any help would be great.
 

Doug Riley

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Genuine Bensen metal blades are well-balanced and otherwise well-made. They fly very smoothly. Their design involves some compromises that make them rather inefficient: the upper skins were made in small pieces that were butted together. The joints between the pieces add drag, as do the many non-flush rivets. The blades are quite light, which means that they don't store as much energy as heavier blades. This means that they are a bit weak in a flare; your timing has to be good to achieve a soft touchdown. Bensen blades can made slightly more efficient by pitching them up to 2 degrees, as contrasted with the original factory scribed setting (more like 1.5 deg.).

Bensen metal blades are distinguished by their use of rivets and a "dog bone" hub. The latter has no coning angle and does not use steel straps to join the blades to the hub bar. Instead, a set of four aluminum blocks at each blade root form a pitch-adjustable joint. To change pitch, you loosen four 3/8" bolts that are normally in tension during flight. You can them rotate the blade in the pitch (feathering) axis around a central pin.

Other metal blades are constructed differently.
 
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