Item 1597

DESIGN: Electrotor-Simplex ~ Gimbal - Assembly - Gyrocopters in General

Gregg's Glossary

Offset:

By Doug Riley on Rotary Wing Forum:

Duncan, the correct offset is a function of the rotor blowback or flapping angle. That's the angle between the tip-path plane and a plane that is perpendicular to the mechanical axis of the spindle. This angle is zero in a vertical descent and, with a given rotor and aircraft, it gets larger as your airspeed increases.
The nominally correct offset of the pitch pivot will lie on an extension downward of the tip-path-plane axis. For instance, if the blowback angle of your rotor at cruise is 3 degrees, then the pitch pivot will lie along a line that passes through the teeter bolt and is inclined aft 3 degrees relative to the spindle axis.
In practice, we use extra offset and balance the controls with a spring.

http://www.autogyro.com/technic/offsetg.htm Have hard copy

Question to Rotary Wing Forum:

What is the maximum structural teetering angle for gyrocopter rotors, before there is contact with the teeter stops?

I assume that this angle will be less than that of a teetering head on a helicopter. This is because the teetering stand on a helicopter is fixed to the mast axis where as the teetering stand on a gyrocopter is fixed to the control axis (stub mast axis).

Prouty has mentioned that the tip path axis of a helicopter rotor aligns with the control axis within a single revolution. This would appear to imply that the gyrocopter's maximum teetering angle can be quite small, since it may only have to accept flap-back and perturbations.

Replies:

From Passin' Thru

That angle really depends on several variables, so I'll answer this way:
If you hand start your blades, that's about the most flap angle you're apt to use unless you do a bunt over or something like that. Way back in the olden days of yore, Bensen started using 9 degrees fore and aft, total 18 degrees, and everyone pretty much followed suit. I personally use 10 degrees at the stop plate and insert a couple of small button head rubber bumpers each side which reduces the actual angle to somewhere between 7 and 8 degrees each side. Except in ground handling when the blades ain't turning, I doubt I ever use more than half of that. It's for ground handling when head and blades are flopping around that I use the rubber bumpers.

I suspect the number you're looking for is the angle of difference between the spindle axis and the rotor tip plane axis when in flight. That angle varies with blade efficiencies, blade weight and tip speed and probably some other things I can't remember right now!

From Birdy:

Max angle would depend on AS and blade load, but iv been told its usually never more than 3* in flight.
Min angle can be 0*, with 0 AS. [ vertical decent]

Found this on Web:

C. Beaty

John, you really need the ±9º that Bensen standardized on.

The greatest flapping angle occurs at the instant of liftoff before the rotor has reached full flying speed. You never get the last 30 or 40 rpm until the machine is hanging on the rotor.

On my first gyro, I had interference between hub and something or the other so drilled the striker plate and installed short 1/4" bolts so the heads would act as elevated stops.

I don't remember how much the teetering angle had been reduced but on every liftoff, the hub would bang the bolthead stops. The stick was like holding a jackhammer until the rotor had acquired the last few rpm and then it would fly OK.

If you need to restrict travel during periods of low rotor speed, the centrifugal stops used by Ron Heron and others work fine.

My thought: Since the Electrotor is not intended to do a rolling gyrocopter takeoff (IE. the rotor must be up to RRPM) the above may not be applicable.

Notes:

Gyrobee:

The Gyrobee web site: http://taggart.glg.msu.edu/gyro/gbee.htm

From automan 1223 on Rotary Wing Forum:

http://www.geocities.com/CapeCanaveral/Station/6122/ Have hard copy.

http://twistairclub.narod.ru/rhead/assemblyman.htm Then open English links at top of page. It has the Wobbly Flyer drawings.

Flight-Control Hinges and Vertical Spacing:

A question.

Is there any operational reason why the pitch hinge, the roll hinge, and main bearing need be at different elevations?

Dave

No. The best is to have all hinges and main bearing at the same elevations and, as close as possible from the teetering hinges, to minimize efforts and transmission of the vibrations...It's easier to make the derived Bensen type rotorheads as they are made today. But the Barnett spindle rotorhead is made with all hinges and bearing at the same elevation...
Remains the teetering hinge which is, anyway, the most troublesome, with the height of the tower!...
But may be there is a way to minimize the height of the tower:

Andre

André

Thanks.

Your Barnett photo appears to show the ability to make a custom universal joint that surrounds the main bearing and also incorporates the pitch hinge offset, but as you say "It's easier to make the derived Bensen type rotorheads"

I wonder if the vertical separation of the hinges, combined with gravity, create desired moments that attempt to center the cyclic stick?

Dave

"I wonder if the vertical separation of the hinges, combined with gravity, creates moments that attempt to center the cyclic stick?"
Yes, It does.

Passin' Thru,

Outside Stuff:

• Gyrobee:

No Gyrocopter is complete without a hub bar to mount the rotorblades. This hub bar is made of 6061 Aluminum. The coning and blade pitch angles are machine into the hub bar. \$250.00