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.
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.