twin prop pusher

[rotordisc50]

Has anyone ever built or considered building a gyro with 2 (smaller diameter) horizontally adjascent, counter rotating pusher props, and powered by the same centrally located motor? How would this affect dynamic stability, centerline thrust, angle of attack, and other dynamics involved? :rolleyes:


flying low and flying slow, OGE - on ground effect
Jim Bania
jbania@juno.com

[PW_Plack]

Jim,

I'm aware of one such design, never built, with a couple of unresolved questions.

First, knowing that small props spinning fast produce less thrust per horsepower than large props turning slowly, would the gains made in getting clean air to the props, and the increased efficiency in using two of them, gain back what's lost?

Second, did we just invent the worst of both worlds? A twin-engine aircraft can lose and engine and still fly, but not this one. If one prop fails, (which becomes more likely due to the transmission complexity,) you still have the difficult control scenario of a twin with one engine out.

If you're counter-rotating to interlace the prop arcs, thereby reducing the separation required between them, failure of one prop drive could be catastrophic, since loss of synchronization would allow the prop still being driven to strike the idle one.

The only advantage which comes to mind is the potential to achieve centerline thrust without the long legs.

Since the highest points of the two prop discs would be farther from the mast, you might see reduced rotorblade clearance despite the smaller props, which might require a longer mast.

[barnstorm2]

I have heard some tails but the old-salt's here can tell you better.


The N-Number on this one might reveal some info..

I think several have been built.

[PW_Plack]

Tim, he's talking about twin props but a single engine.

[rotordisc50]

THANKS Paul, and barnstorm2
to Paul's suggestions
1) Clean air and efficiency... 2-33" 3 blade props vrs. 1-66" prop
2) transmission - 2 v belt drives per prop, utilizing a simple planetary gear mounted directly to the crankshaft, with a power disengagement
3) If one prop should fail, you would cut off power to the the other prop and descend
4) The counter rotation cancels torque and vibrations transmitted to the frame-smaller props less vibration
5) no synchronization, 6" distance between both prop archs
6) potential to alter prop rpm to enhance maneuverability with large HS
Questions;
1) In seperating your thrust line horizontally is there an advantage, more leverage in controling the yawing, or side to side moment?
2) Paul, you mentioned "achieving center line thrust without long legs, can you explain this? Are the numbers there for 2 props vrs.1?

[barnstorm2]

Yep, sorry. Should have read closer :(

[Screw]

Screw-In

Iether my eyes are bad...I did an "N" number search. The tail looks like N23GE. I couldn't find anything.

Screw-Out

[Dean_Dolph]

I can't tell for sure but this looks like the three seat machine designed and built by Gary Goldsberry and Art Evans. What do I win if I'm right! :D

[rfi]

How about a Yankee Dime, or do you know what that is? You are right, that is the machine that Goldsberry & Evans built which explains the suffix, "GE" on the tail number. The props are overlapped and staggered. That is the second configuration that they came up with. The first had the engines "over and under" with counter-rotating props which didn't work very well because the V-belts slipped too much.

Twin props can be more efficient than a single prop but not at half the diameter. If you consider twin 56 inch diameter props the total disk area is a little over 17 square feet per prop for a total of 34.2 sq.ft. That is equal to a single prop of 79 inches diameter which should provide additional thrust over a single 68 inch prop which is pretty much the norm for most gyros.

Another way to look at it is that a 130 Hp engine (Subaru EJ22) will develop around 575 pounds of thrust with a 68 inch prop. A single 65 Hp (1/2 of 130 Hp) direct drive EA81 will develop 300+ pounds of thrust. Double that and you have 600+ pounds which is more than the single 68 inch prop will produce.

I built a twin prop EJ22 system and it flew but the belts slipped. I used two flat ribbed industrial belts with 3/16 groove spacing and two inches wide. The belts started slipping at around 400 pounds of thrust. The props were overlapped and staggered fore and aft which put the overlapped disk areas (about 30%) in counter-rotating configuration.

[barnstorm2]

Don,

Do you still have it? What about a 'toothed' belt like used on HD motorcycles?

[PW_Plack]

Jim,

Don P's numbers seem to confirm an advantage for two props vs. one with comparable total disc area, and I guess the difference could be considerably greater if the two smaller pros benefitted from cleaner airflow. Does that make sense, Don?

As for the "CLT without long legs," if you keep one big prop and move its center off to one side, it can be lower before it hits the keel. Use smaller pros, you can mount them lower still without keel interference. Taking it to an extreme, if two 56-inch props were offset 30 inches either side of center, they could be mounted at keel level and still not strike the keel.

[Dean_Dolph]

I knew who owned the gyro (obviously!) when I posted my guess. I don't know all the details but it appears that Gary and Art spent a lot of time on this machine with several mods. In the end they put it up for sale. Apparently they weren't satisfied with the performance or possibly the complexity. I don't know if it sold or who bought it if it did.

I posted once before about watching this machine during a thrust test at the 1990 PRA convention. The props were installed with one in front of the other and they overlapped slightly as Don has mentioned. During the thrust test the rear prop appeared to distort very badly. This is a different installation than what I think is being purposed.

[rotordisc50]

WOW! THANKS FOR YOUR KNOWLEDGE AND INPUT GUYS! This really clears up a lot of questions I had about the idea. I see what you mean Paul about the CLT issue and clearance for the keel, thanks. RFI...do you absolutely need 130hp for adequate thrust, for 2-56" props, and also is it necessary to overlap? Is this a clearance issue for the rotor if you seperate them vrs. overlap or is it related to something else? I understand the idea now of using a cog type, positive drive belt. I know there is a challenge invovled here in simplifying the complex drive train issues also, but I think it can be done. Thanks again.

Jim

[quadrirotor]

May be there is something there!

[rfi]

I no longer have the twin prop drive. It has been disassembled and scrapped because the flat ribbed belts just wouldn't absorb the power and I didn't want to spend the money to have custom sprockets developed, especially since I was going out of business.
BTW, I am not going totally out of the drive business. I have a verbal agreement with a fellow in California who wants to buy the business and give me a distributorship for the reduction drives. He wants the drive design mostly for a new airplane kit that he is developing.
The props were overlapped. The original idea was to provide a reduction drive that would allow centerline thrust on a Bensen type machine without dropping the keel. That was achieved and it did put the props out in cleaner air, so it should perform okay and probably even better than a single 68 inch prop.
The unit was test flown by Maxie Wildes on his "Mad Max 2000" which was a sise-by-side Mad Max with an RAF 2000 enclosure on it. He said it flew pretty well but that the belts were slipping which was true.
I don't think there would be a problem with the props being overlapped, but I may be wrong. I watched another twin engine gyro (single seat) flying with two Rotax 503 engines turning 60 inch props that were staggered and overlapped. It flew well with one engine shut down. It did make an odd sound, very much like a Cessna 337 push-pull twin.
You don't need 130 Hp for two 56 inch props. I was just using the direct drive EA81 as an example since it makes about half the power. The original plan was to use an EA81 with twin props for Bensen and Bensen type machines.
The twin prop design does add more weight--probably about 40 pounds, but the increased efficiency of two props would probably be worth it.

[Dean_Dolph]

Don, you have the experience so I certainly wouldn't dispute anything you have said. However the overlapped props that Gary G. and Art E. were using showed a definite warping of the rear prop to the point that it was a little scary to watch. How, or if, it affected performance isn't something I would know about but I suspect that it would have an effect on component wear and tear.

[Jon B]

Jim-

I had been thinking of something similar to many posted here. My idea was to use a single engine with dual side by side proprs with the Harley-Davicson belts, but the twist I was considureing was using a duct around the propeller. Supposedly, this greatly improves effeciency, but only at low speeds (all gyros are slow anyhow).

I thought the advantages would be.
-Greater prop effeciency and in cleaner air.
-More flexibility in how to acheive centerline thust by moving either CG or thrust line
- Easing of the whole prop clearance problems.
- Finally, the duct could be a structural providing for some interesting design options.

But alas, I am a long ways from that now. My current plan is to order a gyrobee kit (probably next month), get training at AAI as I near completion and then after some experience with that, maybe look at designing something, with some help from several of my friends who are Aeronatical and Mechanical Engineers.

Quite a few years from now. 'course, by then I hope to be smarter, and maybe this will all seem like foolishness.

Am I rambling? :)
OK, just my 2 cents.
Jon