Partially powered rotors from PRA magazine

[RussKing]

Creative Genius

Creative Genius

Dick DeGraw is the creative genius; I’m only a reporter.

Dick is a creative genius. I define it as one who sees what others don't. From Dick's perspective it is no big deal but to us mere mortals it is a wonderment.

I see a genius as one who understands what others only know. A super genius is one who understands and has the ability to articulate things in a way that mere mortals can understand. Chuck is a super genius.

These people need to be venerated. Their class of people make our lives so much better. They are the reason why we don't farm with sticks and live in squalor.

Hats off to Chuck and Dick!

Russ

 

[C. Beaty]

F I was half as smart as Birdy, Russ, I’d also have several million pounds of hamburger on the hoof like he does.

 

[C. Beaty]

Lets get back to parshally powered rotors then... Would it be more efficient to take some engine power, go through an alternator or generator to a motor and gearbox, and then to the rotor, or to route engine power through a prop for thrust to power the rotor with wind?

I expect a rotor could be partially powered electrically with good efficiency, John, but how much extra weight would it add? Electric generators and motors need lots of iron and copper.

Diesel-electric railway locomotives work pretty well and the extra weight comes in handy for improving traction.

Done mechanically, it’s all part of the necessary propeller speed reduction so not much extra weight is added.

 

[groundhog]

my laymens understanding of the degraw systems advantage over go pros or electrical prerotaions included that reaction to resistance goes back to the other side of the differential?

 

[C. Beaty]

Dick DeGraw’s system provides a soft coupling that does not jerk the gyro around the yaw axis when changing power setting.

Adding power to a differential speeds up the propeller and thereby the propeller slipstream blowing on the already deflected rudder. More or less automatic yaw correction without the need of a rudder pedal toe dance.

With direct coupling between engine and rotor, an increase of power would immediately yaw the machine around. And since the rotor wouldn’t speed up very much, neither would the propeller.

 

[Smack]

Can anyone find the demonstrated top speed of DeBird or Gyrhino ?
I saw one reference to where Dick completed a 20 mile course at 'over 100 mph'.
If not a top speed, does anyone know how fast either machine is without the rotors being partially powered vs when they are powered (not sure that they can be NOT partially powered due to the geared design (?).
In other words, how much speed does the partially-powered rotor allow over a non-powered rotor ?
Brian

 

[phantom]

I don't think it would make much difference as to how you power tha rotor you would get roughly the same reduction in drag from the disk being flat, I had a powered rotor with a 253 rotax that drove the rotor through everything that came with it from the snowmobile , a variable ratio transmission, with the head engine off it would go about 85 mph , with the engine on and the disk flat it was about 105, with collective pulled up a bit and the disk tilted forward I could get about 125 with both engines wide open throttle, the 253 didn't like that environment, about ten hours between overhauls on the bottom end.
Norm

 

[Jean Claude]

In other words, how much speed does the partially-powered rotor allow over a non-powered rotor ?

Your question is too blur, Brian.
Partially powered can mean as well 1% or 99%.
Anyway, low difference.
The poor efficiency of the propeller is progressively replaced by the overweight of the gearbox, and the power needed of torque compensation.
Reduce the parasitic drag seems more efficient for increase the top speed.
2/3 of the total power is absorbed there.

 

[Smack]

Thanks, Norm.
So that was about a 20mph (20+%) increase in speed with a flat disk powered by an independent engine.
When you say that the 253 needed frequent rebuild, does that imply that it took some strain due to the rotor vibration? Also, was this one of your 2-rotor with collective or 3-rotor variants?

I've seen one of your snow ski machine take-offs, but would you step through the process? For example, did you run the head (rotor) engine up to full power, THEN take-off, or was it more gradual power input increasing to full power at some higher airspeed?
A Rotax 253 would put out somewhere close to 30Hp?

Thank you,
Brian

 

[phantom]

The 253 is half of a 503 with the fan housing of a 377 , just over 20 hp , these were never a long life even on a snowmobile , the reason that I used it is was more power than the KT 100 Yamaha 15 hp and due to the variable ratio snowmobile drive it was very simple to drive the rotor shaft with a chain as a final drive just like was used on the snowmobile, this was a two blade rotor, everything was done very gently with power changes, the main engine had adjustable offset to help with torque compensation, to start a 0 roll takeoff you set the engine to full offset, get both engines running, open the throttle of the rotor drive to start the rotor and start tilting the rotor back, as the machine started to move backward open the throttle of the main engine to start moving forward and open up the rotor drive to stop it from going forward, the nose would come up and you did this balancing act between the two engines while keeping straight with the peddles until it lifted of in a very nose high attitude, this was a very dangerous place as bad things would happen quickly if either engine failed , worse if main engine failed.
Norm.

 

[C. Beaty]

Some simple arithmetic:

Say you have a gyro with AUW of 600 lb.

Say at 60 mph the rotor disc flies at an angle of 7º

Rotor drag = tan7º*600 = 74 lb.

Power consumed by rotor = (74*60*1.47)/550 = 12 hp (1)

Say pneumatic drive efficiency of rotor = 50% (2)

Say efficiency of mechanical drive = 100 % (3)

You’ve saved 12 hp!

(1) factor of 1.47 converts mph to ft/sec; 1 hp = 550 ft-lb/sec

(2) pneumatic drive efficiency = propeller efficiency*rotor windmill efficiency

(3) efficiency of mechanical drive ~95%

 

[kolibri282]

Very nice back-of-an-envelope calculation Chuck! Do I get it right that you calculate the power consumed by the rotor at a disk angle of 7°? If so you would have to deduce the power you need to drive the rotor at a point where it flies at near zero disk angle of attack, right? How much would that be?
(Wow three question marks in one line...;-) In Germany we have the "Three Question Marks" series of books for youngsters, they are a team of young detectives, sort of "Kalle Blomquist" times three.

 

[Jean Claude]

Yes, with more accurate values, I believe the profit would not be so clear.
But at take-off to low forward speed , a gain of this magnitude is plausible, mainly due to stronger ground effect.

 

[C. Beaty]

Juergen, I measured the rotor disc angle of several gyros a number of years ago and found rotor lift/drag ratios ranging between 5:1 and 8:1.

The measurements were made by use of a straightedge (a long, straight board) pivoted to a post and aligned with the rotor tip plane by the use of draw strings. Angles were measured with an electronic protractor.

The draw string operator could stand back ~8 ft while aligning the straight edge with the rotor tips as the gyro flew past straight and level at a height of 50 ft. or so.

The 8:1 blades were DWs and the 5:1 blades were Bensen.

The poor performance of the Bensens was the result of centrifugal air pumping through unsealed gaps between upper skin segments.

As to rotor power with a partially powered rotor, I expect Dick DeGraw’s scheme is about the best. Dick uses a power split of ~7:1 and as power is increased with speed, more power is added to the rotor. I noticed that on his wife’s gyro (DeBird), the rotor is perfectly flat during high speed cruise flight.

 

[Smack]

Norm,
Was your partially-powered rotor a teetering, 2-blade, rigid mount with a collective?
You mentioned the collective (for fast flying, rotor tilted forward), but you did not mention the collective for the take-off sequence (thrust and rotor engine 'balancing'). So, at take-off, were the blades at a 2.5-4 degree AoA (standard flight setting for gyroplane)?
Regarding the question of just how much power is required, did you vary the power to the rotor engine? For example, if it was not always wide-open, did you slowly back it off during straight and level flight? Could the rotor engine be used to climb/descend in straight and level flight rather than increasing the thrust engine (as is done in normal gyroplanes)?
Please detail this collective a bit more. I've tried to understand Chuck's drawing, but I don't see how it changes blade pitch. It looks to me more like it affects coning angle...
Really great info, thank you.
Brian

 

[Jean Claude]

L/D ratio of a rotor depends of profile Cd but also of the forward speed. With Cd = 0.011
a 24 'x 7' rotor, blades pitch setting 3.5°, 600 lbs lifting, I find the drag is about 105 lbs at 60 mph (397 rrpm)

Reduce the rotor drag at 74 lbs (7°) with the same blade pitch setting requires 8 hp on the rotor shaft. (429 rrpm)
Reduce the rotor drag at 0 lbs (0°) with the same blade pitch setting requires 31 hp on the rotor shaft. (525 rrpm)

 

[phantom]

The machine that had the 253 did have collective but it was automatically changed to try to maintain 325 rpm, it wasn't perfect so I sometimes needed to help out a little but it worked well enough that I didn't need to worry about it on takeoff which was important as I wouldn't have enough hands, the drive system would start at a very low ratio and as the rpm increased it would change ratios , it kept the engine in its happy place and the auto collective kept the rotor close to its happy place, the collective system worked well enough that I will use it on a helicopter that I am building, it will lower the collective if the engine fails without any help from me and will keep the rpm in the green but I still have the ability to fine tune it or override it to cushion a landing after levelling the skids.
Norm

 

[Smack]

'auto collective'......... that's an interesting concept.
Do you have pictures, Norm ?
Was 325 RRPM what the machine flew at ?
Brian

 

[phantom]

The auto collective was my attempt to do what the air and space 18 did with its delta 3 arrangements for a jump takeoff but my rotor couldn't store enough energy for a jump, 325 was the rpm that it should have been but sometimes it was down to 310 and other times it would be up around 340 but it would slowly wander back , it seemed like the govenor that controlled the pitch and the torque sensing pulley system kind of wrestled with each other, this was the most dangerous machine that I have ever flown, I didn't fly it long as I knew that on every takeoff my life was depending on two engines keeping running, far more dangerous than a helicopter, much like if you could take two mosquito helicopters and bolt them to a beam with one nose down and one nose up and fly them it will work as long as they both stay running, if either quits there is no chance of recovery
Norm.

 

[Jean Claude]

If the pitch collective change is possible, then reduce to zero the drag of the rotor (#56) at 60mph requires a pitch setting of 8.2° and 16.5 hp on the shaft (345 rrpm)
So, assuming a propeller efficiency = 0.7, the non-powered rotor will absorb 27 hp on the engine.
And assuming a gear box efficiency = 0.95, the powered rotor will absorb 17.5 hp on the engine (Regardless of the extra weight of the gearbox and collective pitch)

As Chuck said !

Now, in both cases, we must add the power required to ascend, and that required to overcome the parasitic drag.