gyroscopic precession

[RandySmith]

The "counter-rotating" prop I saw in a video had the second prop just free wheeling.. It certainly looked cool, and seemed practical, but I don't remember it being overly noisy.

However, how about the efficiency of such props, free wheeling or driven? I know we tend to not be terribly concerned with propellor efficiency in CL Stunt (exception for electrics, right). But it seems like a lot of energy would be lost just correcting the airstream.

Of course, with a driven version, you have the gears associated with the reverse drive, which means mechanical loss of efficiency not even considering the aerodynamics of the system.

Interesting thread.

L.

"Sometimes you get shown the light in the strangest of places if you look at it right." -Jerry Garcia

Hi Larry

Your not correct about the contra system freewheeling, It was a fluid drive system, the same system that runs your car , automatic transmissions in Cars are not direct coupled, they are driven by fluid , air for all intents and purposes in this use is a fluid
 It does ,and will drive props, the prop went from 0 RPMs to about 10,000 RPMs in a matter of about 1.5 seconds, If you think it is just freewheeling and not driven ,stick your finger in the "freewheeling blade" only, It will take a good chunk of it off.
I have held the blade (on the bench)still while the engine was running and there is a hard force trying to drive that rear blade, as soon as you let go the blade will shoot to 10 grand immediately...doesn't sound like free wheeling to me.
The only time it free wheels is when the front drive prop stops, then it spins down.
And the only thing that is important is the blade spin, it does not matter how it spins, GP forces have NO idea how a prop is being turned

As far as efficient, the Homemade .64 driving the props had more than enough power to do so, with much in reserve if needed, and if that didn't work I had 2 74s that would have :-)

As I stated earlier the plane flew tremendously better with the contra props

Just for info , there is/ was a geared engine system, a belt drive system, a fluid drive, and a traction drive, props used ranged from 12 inch to 15 inch

regards
Randy

[Bill Little]

One thing that I remembered from some long lost Physics class is that air is a "fluid".  WOW!  I didn't realize that I could actually understand some of this "technical" stuff!

BIG Bear
RNMM/AMM

[Larry Cunningham]

Sorry, poor description on my part. Of course, the flowing air, a fluid, rotated the prop..

I'm wondering if one "problem" might be finding an matching counter-rotating replica of a stunt prop of choice. Or maybe that is not extremely critical.

And yes, someone would have to be a complete fool to stick his bare finger into the reverse prop at virtually any RPM.

L.

"Pity the meek, for they shall inherit the earth." -Don Marquis

"You don't get to be old, bein' no fool." -Redbone, Richard Pryor character

[RandySmith]

Hi Larry
The system is driven, and it drives the back coupled prop very hard, but has the huge advantage of being the easiest one to crank.
I am working on another Contra system that is much less expensive and has fewer parts and is lighter, to maybe market. The others are all doable but are very expensive...The props were very special ones , They were , a tractor in the front and a custom made Pusher in the rear, the rear prop I carved and pitched, It was a pusher prop that was extremely undercambered on the front side to couple with the front as close as possible. It didn't look strange unless you really took a good look. When I get some time I will get back to the T DRive Contra system that I got from Scott Bair a few years back.

Randy

[proparc]

The Contras in F3A while technically perfected, have not been delivering the results at the scoreboard as much as hoped for. For some reason, it seems to just come down to who practised more. They certainly don't seem to hurt you but, they don't seem to help you either. My theory is that the"mixing" capabilities in modern radios are so advanced, they can program out just about anything reasonable. Hence, the VERY expensive Contra front-ends just get canceled out. We don't do violent snapping manuevers-they do, but they can program the rate right AND left.

In short, the radios seemed to have outpaced the powerplants.  

Of possible interest to us, is Gerhard Mayr's I.C. contra setup. The front prop pulls and the much smaller rear prop freewheels.

[Bruce Shipp]

Dale,
Traditionally configured airplanes like our stunt ships, with standard tractor props and large rudders above the centerline of the fuselage, have four left-turning tendencies.  They are:

     Torque:  the left rolling tendency resulting from the resistance to the turning of the propeller.

     P-Factor:  Asymmetrical thrust resulting from the descending blade having a greater angle of attack.

     Gyroscopic Precession:  the reactant force on a rotating mass, 90 degrees ahead of an applied force in the direction of rotation.

     Spiraling Slipstream:  The airflow behind the propeller spirals in the direction of rotation, and exerts a greater force on the left side of the fuselage and rudder than on the right side, causing the airplane to turn left.

These tendencies all cause a left turn when the airplane is under positive G, and some are exaggerated in a hard corner.  When flown at a negative AOA, as in outside corners, P-Factor and GP change to right turning tendencies.  Torque and Spiraling slipstream remain left turning tendencies.

When we use a reverse rotation prop (pusher prop) these four phenomenon become right turning tendencies.  Under positive AOA, they turn the airplane to the outside of the circle.  In this situation, when flown at negative AOA in outside maneuvers, P-Factor and GP become left turning tendencies while torque and spiraling slipstream remain right turning tendencies.
 
With a tractor prop, the four forces work against us on insides and level flight and we are 2v2 on the outsides and inverted flight.  With a pusher prop, they all four work for us on the insides and level flight while again we are 2 v 2 on outsides and inverted flight.
 
Reference the improved performance at the top of the hourglass that many pusher prop users experience, I would conclude that the advantages gained by increased line tension in the climb to the top of the hourglass and the advantage of adding torque and especially spiraling slipstream in the two outside corners offset the losses due to P-factor and GP in the same corners. 

Not tested, just thought out…fire away…

[RandySmith]

I'm wondering if one "problem" might be finding an matching counter-rotating replica of a stunt prop of choice. Or maybe that is not extremely critical.


"You don't get to be old, bein' no fool." -Redbone, Richard Pryor character

Hi Larry
 WE had little problems, The props were not a problem for us with any system except the fluid coupled, Pusher props were not hard to find or alter, The other would be a problem to anyone who does not know how to carve or modify "greatly" some props.
Making parts is a huge problem for many people, that is the reasons you do not see many of them, Price to make them is another

Regards
Randy

[Mike Lauerman]

<<Of course, with a driven version, you have the gears associated with the reverse drive, which means mechanical loss of efficiency not even considering the aerodynamics of the system.>>

Not to mention lubricating gears and bearings...

[RandySmith]

Lots of driven system have no gears, i have seen a belt drive model airplane engine in a contra prop sytem, traction drive, and fluid drive. lubrication is not a lrge problem at all because of the large amount you have access to from the fuel

Regards
Randy