Is the Benson B-8 a CLT?

steve5248

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Jul 19, 2011
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Sunset, Utah
I have heard that both the Benson B-8 and the Ken Brock KB-2 are pretty close to CLT already. All you have to do is raise the engine mount and add a Rotax with the PRSU rotated to the bottom position. Then add the Benson T-tail modification and it's done. No PPO or PIO problems.

This has got to be way to simplistic. I was hoping to get comments from those of you who have actual experience with these two designs.

Steve
 
Steve- I did a double hang test on several gyros at Bensen Days a few years ago. Of all the gyros....Mike Gaspards Mac 90 Bensen came in the closest with his thrustline only 0.5 inches above the CG.

Stan
 
The KB2 and B-8 are pretty much dead-on CLT with lightweight wheels and a seat tank (standard on the KB2).

But that is using a Mac or VW engine swinging a 52" prop. Using a Rotax with a 60" prop will raise the thrustline up ~4" or so.

If you want a Bensen style machine with a Rotax, put an 8" step in the keel. That raises the front half of the machine (~ half the gross weight) 8", thereby raising the CG 4" and matching the thrustline again.

Mark Carmouche, "Friendly" on the Forum, did this to a Rotax-powered KB-3 and made a stable flyer out of it. He also used a Dominator tail on it. I have flown it and it flies like it is on rails.
 
The Bensen B8/Brock KB-2 were designed for short prop, direct drive engine. If you put a rotax on it without making a lot of changes to the airframe you won't be able to use a prop larger than 52". For a prop this short you'll have to use 2:1 gears in the gearbox. If you want to use a larger prop then you'll have to do some redesigning, use a taller mast, change the seat location, change the engine location, etc. Here is the thread on my Rotax conversion on my Brock KB-2.
http://www.rotaryforum.com/forum/showthread.php?t=16758

The Gyrobee was designed to be a simple gyro using a Rotax engine. If you want to use a Rotax engine on a simple gyro that was designed for that type of engine it would be a good choice.
 
is the bensen B 8 a CLT

is the bensen B 8 a CLT

the way I made a bensen type gyro CLT was to lean the mast forward, extend the nose and main gear and change the seat angle, I did this so my rotax 640 fan cooled engine could turn a 62 inch prop with a chain redrive. the first flight of this machine was march of 1977, after all the changes I could no longer call it a bensen.
Norm
 
The main problem for those that want a Bensen look but also want to use an engine other than a Mac with a larger prop and still have NCLT….is the portion of the keel that runs directly under the prop.

To keep the tail surfaces within the prop stream, this portion of the keel could be split and run on each side of the prop….perhaps in a large enough diamond shape to clear the prop tip swing. Gear struts could angle down to the wheels to help provide adequate ground clearance for the prop. The higher engine CG (Rotax with gearbox swung down) would probably offset the added frame weight. I think it would still look like a low-rider with NCLT.
 
The Mac 72 I had with an Oswego prop would pull 280lbs while my friends 90 and a Ken Brock prop would pull 305lbs. I don't remember the size or pitch.

If putting a 503 on with a 52 inch prop and inverted reduction (2:1) will give 270lbs is it worth the difference to go to a larger prop and have to change the whole Gyro around?

What thrust will you get from a 503 with a larger prop? How much weight does changing the Bensen to a drop keel add?

JD
 
I was actually thinking of using a 90 HP Rotax 583 off a snowmobile. It has been drilled to accept a Rotax 3.0 C-box PRSU. I don't know what size of prop it can turn. I looked at the Gyrobee, but on the site Mr. Taggart says it won't handle anything larger than a 48 HP 503. That's why I started looking at the B-8 and KB-2. What do you think?

Steve
 
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