Belt Drive PreRotator

ElJay

Member
Joined
Sep 12, 2004
Messages
27
Location
Big Spring, TX
Please bear with me, I'm still new. I have seen pictures (not very clear ones)of the Bensens that have a small gas engine/v-belt pre rotator. This sounds a little interesting, and since I am still building mine, I have thought about a chain saw engine coupled via hand powered idler pulley to a larger pulley mounted to the main rotor. Just thinking out loud....I'd think there would be plenty of torque and you could/would just ease the idler in as the rotor gains speed......whutcha tink?
Thanks
JT
 
Al Hammer recently said that a small amount of power (up to about 15 hp) to the rotor will flatten the coning and increase performance.

Use your search engine and you will find it.

Jim
 
I am bidding on a 49cc gas scooter motor on e bay that has a ele. starter, 5 to 1 gearbox and clutch. If it is what they say it is I will see how it works as a pre-rotoator like the Bensens.
 
Here is a pic of a Benson gasoline motor pre-rotor that was sold on ebay.

Mike
 

Attachments

  • [RotaryForum.com] - Belt Drive PreRotator
    bensenprerotor.webp
    14.2 KB · Views: 0
engine pre rotor

engine pre rotor

I have a engine driven prerotor using a Homelite go cart engine that I built in the 50's (about 1954) It used a belt drive to a jack shaft and then a chain drive to the head. Worked very well . Don't know the rotor RPM but when I used it to train pilots it would get the gyro up as soon as the car could accelerate to 20 MPH. Used it a few times when in the air and the climb rate was fantastic . By todays standards it would need upgrading. If there is any interest I will dig it out and post pictures. Was powerful enough to skid the nose gear if you engaged it too fast. Seems like it took about 30 seconds to bring the wooen blades up to speed. Was great for tracking. Had a sprag clutch to prevent the rotor from driving the engine. Ray
 
Very nice!!! Ray could you post the pictures or drawings, I would luv to see it.

David
 
I won the scooter motor on ebay and paid for it. He said it will be here weds. next week. I have seem this small CVT. gearbox that fits this 49cc motor. I think this same CVT would work on any pre-rotator with some mods. this will give a soft start. Here it is.
 

Attachments

  • [RotaryForum.com] - Belt Drive PreRotator
    cvt.webp
    24.9 KB · Views: 0
Prerotator: the french connection.

Prerotator: the french connection.

Has anybody infos or photo on the french prerotator with a strap which goes from the main engine to the rotor head?
 
Brent_Brown said:
Yes I want the same thing but completely different.


Wow.........I'm havin truble with this one.......I'll turn me compewta off, and try again :rolleyes:
 
Averso Guépard from France.

Averso Guépard from France.

Any more photos or infos?
 

Attachments

  • [RotaryForum.com] - Belt Drive PreRotator
    aversoguepard2.webp
    21.8 KB · Views: 0
Jim said:
... a small amount of power (up to about 15 hp) to the rotor will flatten the coning and increase performance.

Good point, Jim, but the coning angle isn't what flattens, its the disk angle.

Adding partial power to the rotor should actually lower rotor rpm, interestingly, and the coning remains the same.

Think of it this way: If you were to prerotate fast enough, you would lift off.
Lets say this happens at 320 rpm. You would need a lot of rudder, probably more than what's available, but pretend you are hovering at 320 rpm.

Now , as you move into forward flight, you turn off the prerotator. Power to the rotor has to come from somewhere, so the rotor tilts back about 10 degrees to drive air through the rotor. The rotor must now make additional lift to maintain altitude, because rotor thrust is at a 10 deg angle to vertical.
This additional loading drives rpm up to maybe 350, the normal flight rpm fpr this machine.

If you engage partial power from the prerotator, the disk can fly flatter and the rpm will decrease back towards the 320 rpm that you hovered with, settling at something greater than 320, but less than 350. I'm just guessing at these numbers.

Coning stays the same oddly enough, due to the way centrifugal force relates to rpm change.

It is more efficient to power the rotor meachanically than it is to power it aerodynamically. The rotor is lousy at converting wind energy to usable thrust. If you add too much power, then you need a tail rotor for ant-torque, (and the rotor will need more pitch and won't be in autorotation)so the best combo is a gyro with some power coming from autorotation and some coming from prerotation.
 
Last edited:
You're right. For a moment I did not think straight. The original posting was:
"If you are looking to improve efficiency, I suspect that number of blades won't have as much effect as going to a full time partial power drive on the rotor.
Supplying up to 15HP to the rotor allows it to fly at a reduced angle of attack, thus reducing drag considerably. Any more than about 15Hp and torque is a problem." At https://www.rotaryforum.com/forum/showthread.php?t=961&page=2&pp=15&highlight=15hp
 
an other one!

an other one!

very simple and effective!
[RotaryForum.com] - Belt Drive PreRotator
 
Johnny Hay or Frank Black had a v-belt prerotor that ran off the front of the engine up to a pair of small pulleys and then wraped around the larger pre rotor wheel. This was a continuous belt some what long and a pair of guides on each side. This worked very well and did not weigh much at all. Tom Milton where are you? Help me!
Bud O,Neal
 
Rotorcraft Magazine June-July 1995, page 22, has an article with pictures of a similar design by Allen Cox.
He said he took caliper measurements out of an earlier Rotorcraft mag article about a French Prerotator( maybe the one in the pic above?) and copied the design. Said it was good for 200 rpm with 24 ft Sky Wheels. (503 engine)
 
Johnny Hay had one for sure and I suspect that Frank did also since they both are, or were, members of Chapter 40 and worked closely together. I believe that Johnny used this pre-rotator to finish 2nd in the jump takeoff contest that Dick DeGraw won. He also had a turbo on his 2443 cc VW at that time.
 
Back
Top