Partially powered rotors from PRA magazine

C. Beaty

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Dick DeGraw
And his partially powered rotors

Magic?


No, it’s not magic, but it’s a scheme that at the same time is outrageously clever and dirt simple. Dick has interposed a differential gear set between propeller and rotor. It’s the sort of arrangement that leaves people wondering to themselves; “Why didn’t I think of that?”

Background


The concept of partially powered rotors has been around almost as long as have been functional rotorcraft. Cierva referred to it as a Gyrodyne.

The Lockheed AH-56 Cheyenne was a helicopter that permitted a split of power between the main rotor and a pusher propeller, the split being controlled by the respective collective pitch ratios. The Cheyenne was an extraordinarily complex helicopter with a host of other advanced features.

How does it work?


Let’s take an ordinary automotive differential, the sort that can be found today at the rear end of pickup trucks and similar vehicles and hang a propeller on one wheel hub and a rotor on the other.

I suppose most people understand there is equal division of torque between axle shafts and the purpose of a differential is to permit a vehicle to negotiate a curve without dragging the wheels. Also that if one wheel is locked and the other is free, the free wheel will spin at twice the speed of the pinion carrier.

Now what happens with our rotor/prop rear end if 200 ft-lb. of torque is applied to the pinion carrier? Both will accelerate until a speed is reached where each absorbs 100 ft-lb. of torque. It could but need not be at 300 rpm for the rotor and 3000 rpm for the prop, dependant upon the diameter and pitch of each.

The power absorbed by the rotor is: 300*100/5252¹ = 5.7 hp.

The power absorbed by the prop is: 3000*100/5252 = 57.1 hp.

The speed of the pinion carrier is: (3000+300)/2 = 1650 rpm.

The power applied to the pinion carrier is: 200*1650/5252 = 62.8 hp so everything balances.

With the hub that is connected to the rotor locked, each axle shaft would still be subjected to 100 ft-lb. of torque with 200 ft-lb. applied to the pinion carrier but the locked hub would consume no power. The propeller would still turn at 3000 rpm and consume 57.1 hp as before. The pinion carrier would turn at 1500 rpm and also receive 57.1 hp.

The actual implementation

The scheme just described wouldn’t be very durable because automotive differentials aren’t designed for continuous operation at high differential ratios. Even running wheels of different diameters on the rear end of a pickup truck will fry the differential in short order.

If 10:1 reduction gearing was used on the rotor side of the differential of the previous example, 1,000 foot-lb. of torque would be applied to the rotor and to hold its rpm at 300 would require a substantial increase of collective pitch. Then the pinion carrier as well as both axle shafts would turn at 3,000 rpm and there would be no relative motion of the differential gears. The power split would be 1:1 with 114.2 hp being applied to the differential if the torque input was still 200 ft-lb.

It should be noted that if the gearing following the differential is selected such that there is no or minimal rotation of the differential gears, the power split is identical with the torque split but any power split imaginable can be obtained if differential gear rotation is permissible.

Automatic transmission planetary gearsets can also be used as differentials but the torque split will be something other than 1:1. Refer to Fig. 1.

On both the Gyrhino and DeBird, Dick has used automatic transmission compound planetary sets that produce torque splits of around 7:1. Subsequent gearing is selected to present a load to the differential that eliminates relative motion of the planetary gears, thereby providing a 7:1 power split. Ought to last 100 years.

I believe the latest project, Gyro X, which is a joint undertaking of Dick DeGraw and Ernie Boyette uses a single planetary set to produce the necessary torque split. Since automatic transmission planetary sets aren’t available with the required ratio in a single stage, Dick cut and heat treated the gears himself. It should have flown by the time this appears in print.

Why not a hard connection between engine and rotor?

That would eliminate much of the plunder but it would be impossible to fly. The slightest change of throttle setting would immediately yaw the machine around. I’ve flown Karol’s DeBird and yaw vs. throttle change is hardly detectable.

A considerable amount of power applied continuously to the rotor also requires a cyclic control system that isolates rotor torque from the control system. A Bensen style tilt head cyclic control won’t work; most anyone with a strong prerotator has observed that a hard engagement slams the stick over to one side.

What does it do?

It improves the efficiency. A rotor driven pneumatically eats a considerable amount of power, its overall efficiency being the product of propeller and windmill efficiencies.

The rotor rpm doesn’t change dramatically between fully autorotational and partially powered, 20 rpm or so. Partially powered, the rotor flies at a flatter angle since it doesn’t have to extract as much power from the airstream.

Karol’s gyro flies with the rotor almost flat at top speed.

A gyro flies through a homogeneous body of air and has no more tendency to “dig in” or “trip” than a fixed wing aircraft, contrary to the expectation of some who may be confused by water skis and racing hydroplanes. Water skis operate at an air/water interface; a gyro doesn’t.

¹) A winch with a drum of one foot radius, hoisting a weight of 1 lb. (1 ft-lb. of torque), would have to spin at 5252 rpm to deliver one hp: 33,000/2*pi. One hp = 33,000 ft-lb/min.
 

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kolibri282

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Great writeup, Chuck! There is nothing half as enlightening as a sample calculation. Thanks for sharing your extensive gyro and engineering know how!
 
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Smack

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Fabulous information. Thank you for posting this, Chuck.
Brian
 

C. Beaty

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Here’s a diagrammatic sketch of the rotorhead that Dick DeGraw used on his wife’s partially powered seesaw rotor gyro.

It is identical in principle to the rotorhead of the Bell-47 helicopter but using a cyclic control system passing through the center of the rotorhead instead of a swashplate.
 

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RotoPlane

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I can now better understand how Dick's differential system works. Thank you Chuck!

I see that you are still one sharp dude that continues to give out useful information to this group.
What sucks is that I on the other hand, will head out to the kitchen or wherever and when I get there, I'm at a loss as to why I'm standing there.....a daily freaky occurrence.
 

C. Beaty

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During the brief time that I spent flying Karol’s gyro, propeller torque roll of the geared Subaru was far more objectionable than rotor torque-yaw coupling when blipping the throttle.

Rotor torque compensation takes care of itself; an increase of power speeds up the propeller and thereby speeds up the slipstream acting on the rudder.

Propeller torque roll was something else.
 

Smack

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So, Chuck, if the vertical stabilizer/rudder helps to eliminate the objectionable yaw, wouldn't a sufficient horizontal stabilizer help to damp out the prop roll?
Maybe it is more the moveable rudder that allows the damping of yaw, thus there would need to be moveable (independent?) 'elevator' type devices on the horizontal stabilizer?
Brian
 

C. Beaty

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So, Chuck, if the vertical stabilizer/rudder helps to eliminate the objectionable yaw, wouldn't a sufficient horizontal stabilizer help to damp out the prop roll?
Maybe it is more the moveable rudder that allows the damping of yaw, thus there would need to be moveable (independent?) 'elevator' type devices on the horizontal stabilizer?
Brian
Cierva patented propeller torque compensation via differential aerodynamic surfaces in the propeller slip stream in the 1930s.

http://www.google.com/patents/US2098230?printsec=abstract#v=onepage&q&f=false
 
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gyromike

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Chuck,

Do you have any actual photos of Dick's setup?
I thought I had some of the Dick's jump takeoff gyro, but I couldn't find them last time I looked.
 

C. Beaty

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Sorry Mike; I don’t have a thing.

I often wish that I was more into photography; I would love to have pictures of Gary Yanson crashing my 1-blade gyro or a picture of my rotor blades after my gyro was pulled out of the Gulf of Mexico.
 

birdy

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Coupla years ago i was all for maken a partial powered gyro with active collective and JTO ability.
Got all the info i needed off CB.
Alltho i found i was go,n to be 200 years old before id get around to it, and even aquired a heli mast, frame, 30/70 bias soob center diff gearset, and many sleepless nites, i still am very greatful for your help CB.
 

groundhog

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Birdy, if memory serves you also looked at goped partially powered. Did you ever try it ? Would seem to be easily attainable and fitting for a ferel ?I would love to hear your impressions.
 

birdy

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Iv met sum reporters CB, never wanted to share a rum or three with any of them.
 

JAL

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Here’s a diagrammatic sketch of the rotorhead that Dick DeGraw used on his wife’s partially powered seesaw rotor gyro.

It is identical in principle to the rotorhead of the Bell-47 helicopter but using a cyclic control system passing through the center of the rotorhead instead of a swashplate.
I am not sure I understand how this rotorhead works.

Is the rotor bearing located inside the rotor head which then the stern passes through it? I am thinking that the stern is what locks down the rotor (it's the "Jesus Bolt") through a bearing located in the rotor head so obviously the stern doesn't turn. Does that require s different type of bearing? .

Also in this set-up am I right in saying that each blade has its own teeter bolt?

Does the universal joint allow 360 degree movement of the rotor head and how is the clearance achieved so that the head can pivot in pitch? I thought the reason for on offset gimbal head was to reduce the stick force required to tilt the rotor, so why does this setup work or is the leverage provided by extending the stern "arms"

And finally why aren't there more heads like this? It seems it would work well even for unpowered blades.
 
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C. Beaty

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Maybe I should have said, Birdy; “I’m only the messenger” since reporters aren’t universally loved. Then someone would say; “shoot the messenger.”
 

All_In

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Not sure over 10,000+ logged FW, 260+ ultralights, sailplane, hang-gliders
Boy Chuck you cannot win either way with those two jobs.

When I read reporter. My first thought was "OK but it's the first reporter who knew what he was reporting and told the TRUTH!" Much of what most reporters write is imagination or just plain wrong.
 
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