Author Topic: Power for counter rotating props (Read 6360 times)

Keith Renecle · « **on:** April 03, 2018, 11:31:32 PM »

Hi All,
I don't want to build a stunter with two props on the nose.....it looks too weird! I would rather do a twin like Bob Hunt with a prop on each side, but I am curious as to the prop and motor size needed for a counter/contra-rotating system. Do you need the same size props as per a single prop, or smaller? If they are indeed the same diameter, and you are using two motors instead of a gearbox, then does this mean that you require each motor to have the same power as the single prop system? This would make such a system impractical because it would be a lot heavier.

In a conventional twin design, you use two smaller motors and props and the overall power requirement is similar to a single engine model. I see the advantages in a counter-rotating prop system as far as torque cancelling goes but practically speaking is it a good idea for a high performance stunt plane? For example would it help to cure the apparent tracking problems that seem to be a problem in electric models?

I would appreciate any help here. Thanks.

Keith R

Trostle · « **Reply #1 on:** April 03, 2018, 11:48:40 PM »

Hi Keith

There are some electric powered RC ships that use counter rotating props. Some are done with gear boxes. Others are done with two electric motors with the shaft of one turning inside the shaft of the other. Maybe a search will show some of these. Packages that accomplish this have been available in the past.

In the past, there have been gear boxes developed for this application for RC, but those mechanisms would tend to be heavy, particularly for the typical size aircraft used for our CL stunt models.

Randy Smith has experimented with counter rotating props where the rear prop free wheeled on the shaft of the front prop driven by the engine. Maybe he can explain his experience with that system.

I think the idea of counter rotating props is a great idea. It will virtually eliminate gyroscopic recession. Also, it will allow being able to utilize the power of these larger engines using smaller diameter propellers. My understanding of how the counter rotating props should be set up, at least for full scale aircraft is that the rear prop should be slightly smaller diameter but with a higher pitch. I think that a lot of experimentation would be required to find out what works best.

It will be interesting to find out what others have tried or know about as far as applying this idea to our CL stunt models.

Keith Trostle (One of the other Keith's)

Keith Renecle · « **Reply #2 on:** April 04, 2018, 07:17:58 AM »

Hi "Other" Keith,

Thanks for that. Yes, I have seen a few attempts at this. Pawel Djuba in Poland made some of the Yatsenko models with gearboxes but I did hear that they are heavy and quite noisy. I've also seen an article some years ago about the rear prop free wheeling on an R/C model. I was wondering if the rear prop in this case would absorb some of the power?

Keith R

Dennis Adamisin · « **Reply #3 on:** April 04, 2018, 07:33:12 AM »

Hello Keith & Keith

Another Keith, (Keith Shaw) who Is a pioneer & innovator for electric power systems in RC models has direct experience with tandem propellers. He built a Bugatti 100 and experimented with different gearboxes and props. He told me that the best solution was two motors (no gearbox) and tweaking the RPM until you get the same watt load on each motor.

Second here's a link to a NACA Technical Report on Tandem Propellers, dated 1-August 1937(!!!)
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090014144.pdf

Skipping to the chase, the conclusions of the report were:

"Those tests have shown that identical, counter rotating, 2-blade, close-spaced, tandem propellers, adjusted in pitch to absorb equal power at maximum efficiency, have from 0.5 percent to 4 percent greater efficiency than that of 4-blade propellers of the same blade form and designed to absorb the same total power. (put another way, the correct diameter for the 2-blade tandem props is the same as a 4-blade prop that replaces a 2-blade, or about ~~81%~~ 84% of 2 blade diameter)

Tandem propellers are inferior in efficiency to single 2-blade propellers for pitch settings at 0.75 R of less than 350. For higher pitch settings, the tandem propellers have an appreciable advantage.

Tandem propellers absorb from 3 percent to 9 percent more power than 4-blade propellers and about twice the power of 2-blade propellers of equal diameter."

Hey Keith R, what you need is a super special version of your excellent timer that will let you set the RPM on one prop, then senses the current load and sets the RPM of the second prop to be the same current load. NO PROBLEM (sez the guy who ain't gonna be doing it..!)

Keith Renecle · « **Reply #4 on:** April 04, 2018, 09:07:07 AM »

Hi Dennis,

Thanks for your input. So it seems that you can use two smaller motors at lower rpm. The props can be bigger diameter at lower rpm than our more conventional props by the sound of things.

I had one customer that played with a foamy model (see pic below) with two motors and he stated categorically that his model performed really well with only one of my timers with the rpm pickup wire connected to the front motor. The governed output throttle pulses were fed via a y-connector to both motors. I told him that I did not think that this would work but I really wasn't sure, and right now after speaking to quite a few folks, I'm still not sure. Maybe if the free-wheeling rear prop works then this idea is also possible.

I don't think that using the power and sensing the current load would be a good idea because I reckon that you need to sync the rpm fairly well to avoid differences in torque in the hard corners.

Keith R

Dennis Adamisin · « **Reply #5 on:** April 04, 2018, 01:11:09 PM »

Quote from: Keith Renecle on April 04, 2018, 09:07:07 AM

I don't think that using the power and sensing the current load would be a good idea because I reckon that you need to sync the rpm fairly well to avoid differences in torque in the hard corners.

Keith R

Hi Keith

Actually I think that is why you HAVE to do it that. If the loads on the props are not identical then we are left with a (small) net torque. Even if the front and rear props were EXACTLY opposites (not a given) tandem props influence each other and thus affect the loading which manifest itself as the current draw. The only "tool" in the drawer is to adjust the RPM to level the load. This also assumes that if the current loading is the same then the work done by each of the props should be the same - and that seems reasonable.

It is also probable that the net torque effect will not be exactly zero, but the net torque will be negligible compared to the torque effect of a single prop. I also think the corkscrew of air off the aft prop will induce a certain torque reaction via the flow over the airframe. Small compared to the motor effects but still present. That is why if I ever did this I would start with the LH rotation prop on the back.

I strongly suspect that we need to get the current used in flight and the whole process will be a little tedious: fly, check airspeed & check current used by each motor via the data logs, then adjust and fly again until airspeed and motor loadings all match up. Heaven help us if we decied to speed up the airplane by say 100 RPM and have to go through the whole process again.

I also think the motor mounting will be a challenge; keeping a "stack" of outrunners from excess flexure and unwanted bearing stresses is going to require some TLC.

Ya know, I have been finishing up a Mk 47 Seafire that would REALLY look good with tandem props - but currently plan the easy way out with just a single 2-blader. A traditional twin like Jack Sheeks' "Mossie" is looking better all the time!

Jim Mynes · « **Reply #6 on:** April 04, 2018, 03:40:36 PM »

I saw one on an RC pattern plane. It was impressive.
Only one motor, but it was only slightly smaller than a can of Coke. The magic happens inside the spinner.
The props were not locked to one another, and could be moved either direction independently. It leads me to believe there is a planetary gear involved, and when one prop is driven, it meets air resistance which imparts drive to the other prop.
I don’t remember for sure, but I think the props were different diameters and/or pitches.
600g with motor. You’ll need a wheelbarrow!
https://www.f3aunlimited.com/contrav4

Keith Renecle · « **Reply #7 on:** April 04, 2018, 11:25:09 PM »

Quote from: Dennis Adamisin on April 04, 2018, 01:11:09 PM

Hi Keith

Actually I think that is why you HAVE to do it that. If the loads on the props are not identical then we are left with a (small) net torque. Even if the front and rear props were EXACTLY opposites (not a given) tandem props influence each other and thus affect the loading which manifest itself as the current draw. The only "tool" in the drawer is to adjust the RPM to level the load. This also assumes that if the current loading is the same then the work done by each of the props should be the same - and that seems reasonable.

It is also probable that the net torque effect will not be exactly zero, but the net torque will be negligible compared to the torque effect of a single prop. I also think the corkscrew of air off the aft prop will induce a certain torque reaction via the flow over the airframe. Small compared to the motor effects but still present. That is why if I ever did this I would start with the LH rotation prop on the back.

I strongly suspect that we need to get the current used in flight and the whole process will be a little tedious: fly, check airspeed & check current used by each motor via the data logs, then adjust and fly again until airspeed and motor loadings all match up. Heaven help us if we decied to speed up the airplane by say 100 RPM and have to go through the whole process again.

I also think the motor mounting will be a challenge; keeping a "stack" of outrunners from excess flexure and unwanted bearing stresses is going to require some TLC.

Ya know, I have been finishing up a Mk 47 Seafire that would REALLY look good with tandem props - but currently plan the easy way out with just a single 2-blader. A traditional twin like Jack Sheeks' "Mossie" is looking better all the time!

Maybe this is why the gearbox option is being used so that both props are fixed at the same rpm? Someone needs to build a special esc that syncs two motors and measures the load and all parameters can be kept in step. I hope the Dean Pappas will chime in here. This is his kinda thing!

Looking at the pattern planes with this system, it looks like they did not reduce the prop diameter either. I'm wondering if the rpm is then lower than with a single prop? I'd also like to know how mnuch extra power they use. I do know a few good pattern guys out here so I'll ask a few questions.

Oh.........I agree on a Jack Sheeks Mossie! ........with retracts!

Keith R

Dennis Adamisin · « **Reply #8 on:** April 05, 2018, 06:04:49 AM »

Hi Jim

THANKS for link to the gear drive. I thought the RC pattern flyers had given up on gear drives in favor of dual motors, but that gear drive looks great. That WOULD be a great way to accomplish the tandem prop if we can get one about have the size of the one shown.

One of my early electric experiments was with an inrunner motor and an Astro gear drive. Worked great - for ten flights - till the gears wore out!

Howard Rush · « **Reply #9 on:** April 05, 2018, 03:38:41 PM »

Quote from: Dennis Adamisin on April 04, 2018, 01:11:09 PM

I also think the corkscrew of air off the aft prop will induce a certain torque reaction via the flow over the airframe. Small compared to the motor effects but still present.

Remember when I tried to get you to have APC make mirror image props so I could compare rotations? I eventually did the comparison with carbon quadrotor props (as I remember). The torque difference was evident on takeoff, but at cruise the corkscrew air was the dominant rolling effect.

None of these effects is very big, so, as Igor said, you only need to come close in matching the props and RPM. You would probably use differential RPM as a fine trimming tool.

Keith Renecle · « **Reply #10 on:** April 05, 2018, 11:05:05 PM »

I'm pretty sure that Howard is dead right about using differential rpm as a trimming tool. I would still like to know about how much power this system needs. Can you use smaller motors and props, or do you use the same size motor for each prop as the single prop system? Maybe you can use smaller motors with standard size props turning at lower rpm to get the same flying speed? Is the overall thrust a lot more with the two props?

One more thing..........what about the number of blades. Is there any advantage in using more than two blades or is that just a waste of power?

Keith R

Paul Smith · « **Reply #11 on:** April 07, 2018, 03:49:17 PM »

There were plenty of off-the-shelf counter-rotating electrics at Toledo.

No big whoop.

Keith Renecle · « **Reply #12 on:** April 10, 2018, 11:47:08 PM »

Hi Bob,
Thanks for that. Old, weird and forgetful?? Damn........I thought it was the only one!

Mind you the "weird" part has been with me since birth. Like the upside down bumper sticker says......Why be normal?

I soaked up your articles on the Second Wind, so thanks for doing such an adventurous project. How much battery power did you end up with? It would be nice if Dean chimed in, but in the meantime I did speak to one of our local R/C pattern guys here and it does seem like the rpm for similar lap times will be a lot lower for each prop that with a single prop setup. From what I hear it could even come close to half of the normal rpm but I guess that this figure will be a bit higher than half.

I see that the indoor F3P pattern guys have developed a very light contra prop setup that uses two big balsa 13 x 4.7 props. Here is the thread on RC Groups https://www.rcgroups.com/forums/showthread.php?2361556-New-single-motor-contra-rotating-Kimmo-system

The models weigh around 56 grams all up. They have a couple of video's but they have been slowed down a bit......I think. Either way, the guys are no longer using air brake things with this prop setup. The models are made from carbon rods and covered with Mylar. I love those big balsa props!

Keith R

pmackenzie · « **Reply #13 on:** April 11, 2018, 04:57:00 AM »

FWIW, with the switch to single cell setups F3P models are often in the low to mid 40s.
My own weighs 43 grams RTF, uses 11" carbon props.
Kimmo's system now uses 15" balsa props, most others use 16" foam core props.

https://www.rcgroups.com/forums/showthread.php?2860944-Rane-2017

Pat MacKenzie

Igor Burger · « **Reply #14 on:** April 11, 2018, 05:29:35 AM »

Quote from: Keith Renecle on April 10, 2018, 11:47:08 PM

I see that the indoor F3P pattern guys have developed a very light contra prop setup that uses two big balsa 13 x 4.7 props. Here is the thread on RC Groups https://www.rcgroups.com/forums/showthread.php?2361556-New-single-motor-contra-rotating-Kimmo-system

May be you do not know, but one our friend, Zdeno Bajer, may be you remember him, he flew F2B few years ago. He makes and flies such F3P contraprops :- ))

https://www.rcportal.sk/f3p-contaprop-1s-am7418

However it is usually not usefull for us, because they do not use it 100% of flight time. The combo from Pawel Dziuba is designes especially for F2B long hard work and so far probably best available (picture) but life time is also not comparable to classic outrunner. May be those coaxial motors look better, but they have also micro bearings, so forget about long realiable work (and plus they need 2 ESCs). So for now I see best solution are two independent motors, but I do not know how it influence stab / wing coincidence .. looks like question for Bob.

Igor Burger · « **Reply #15 on:** April 11, 2018, 05:45:39 AM »

Quote from: Bob Hunt on April 10, 2018, 06:33:59 AM

I did a bit of experimentation with the prop rotation direction on my twin at the suggestion of both Dean Pappas and Howard Rush (Howard even supplied a flow chart for this test program...). Hey, I was going to do that anyway... The results were interesting. At first I had the tops of the props spinning towards the canopy (okay, the "painted-on" canopy...). This worked very well. At the Nats in 2015 I opted to try it with the top of the outboard prop turning turning towards the canopy (reverse rotation to what we normally have on a single glow engine model), and the inboard prop turning in the same manner. There wasn't too much difference, but I did perceive a bit more vertical tension (it was very slight). I flew the Nats with the rotation in that manner. After the Nats I did an adjustment to the area of the inboard flap (adding more area near the tip of the flap with a long tapered piece of balsa) and that cured the vertical tension problem I had been having through the top of the vertical eight. I flew it that way with the prop rotation as per at the Nats, and then with the counter rotation variation with the tops of the props again turning towards the canopy. That proved the best of the permutations tried. I'm sorry to say that I did not continue the test program to include the other two variations (the tops of the props turning away from the canopy, and both props spinning in the normal glow direction.) I recently sold the airplane, so further testing will have to wait until I test fly Frank Imbriaco's new Turning Point twin that I designed. Hey, better to test on someone else's plane anyway!

Perfect Bob, but I am missing one thing. I guess, if props are running down front of nose or up front of nose, it must somehow influence the effective AoA at tail. So I GUESS, it will need some correction for that - means permenent AoA at stab. Did you find some permanent pitching moment when you changed rotations? Was it necessary to readjust neutral? There must be clearly some moment, but flow can be easily "strightened" by wing front of stab, so I am on doubt about it.

Igor Burger · « **Reply #16 on:** April 11, 2018, 09:41:21 AM »

Quote from: Bob Hunt on April 11, 2018, 09:12:01 AM

The neutral never changed, even with the gear up/down. I had expected that it would effect that for sure!

Good news, thanks for saving us time, at least it means there are no tricks with stab AoA necessary :- )))) ... may be one day I come with twin as well :- ))

... Double Bee? :- ))))))))))

Dan McEntee · « **Reply #17 on:** April 15, 2018, 10:55:51 AM »

Quote from: Bob Hunt on April 11, 2018, 09:12:01 AM

Hi Igor:

Actually I didn't notice any pitch changes at all with the twin in any of the various configurations of the prop rotations. Perhaps I'm just not that sensitive at the handle...

The neutral never changed, even with the gear up/down. I had expected that it would affect that for sure!

Keith R: I used a 4,000 mAh 4S Hyperion pack to run both motor and the landing gear. I forgot just how much of the battery I used, but it was way within acceptable limits. Here's a neat fact: On the test bed twin we flew the first flight with the gear left down. I recharged the battery and put 2,400 MAh back in. Then I flew the next flight with the gear up.When I recharged the battery I found that I only put 1,800 mAh back in! That's 25 percent less power usage with the gear up! Amazing...

Later - Bob

Hi Bob;
I believe you were introduced to the electric twin configuration several years ago by a local guy here, Walt Brownell, and a Muncie NATS? I remember Walt building two. The first one was a sorta scale French bomber and seemed about Chipmunk size. He would come into the hobby shop I worked at part time and we talked about props for it. I asked about the rotation issue and he had experimented with it, and I think he preferred the tip to turn out at the top. I believe he said full scale practice was to have the tops turn in but he felt that the model handled a bit better the other way. I only got to see the first model fly at a Paducah contest. Walt complained that the model was heavy but it sure didn't look that way when flying! It had an excellent turn and flat pull outs. As I recall (not really sure of the year) Walt won expert and was surprised by the out come! I tried to comfort him by explaining that he did indeed put a hurt on all of us that day from what I could see outside the circle! I never got to have the chance to fly it. Walt soon moved on to C/L scale pretty heavy, and has since been dealing with some health issues. I don't know for sure because he hasn't been around the field in several years. Walt is a super nice, super intelligent and outstanding modeler and one of the earliest proponents of the electric model.
Type at you later,
Dan McEntee

Author Topic: Power for counter rotating props (Read 6360 times)

Keith Renecle

Power for counter rotating props

Trostle

Re: Power for counter rotating props

Keith Renecle

Re: Power for counter rotating props

Dennis Adamisin

Re: Power for counter rotating props

Keith Renecle

Re: Power for counter rotating props

Dennis Adamisin

Re: Power for counter rotating props

Jim Mynes

Re: Power for counter rotating props

Keith Renecle

Re: Power for counter rotating props

Dennis Adamisin

Re: Power for counter rotating props

Howard Rush

Re: Power for counter rotating props

Keith Renecle

Re: Power for counter rotating props

Paul Smith

Re: Power for counter rotating props

Keith Renecle

Re: Power for counter rotating props

pmackenzie

Re: Power for counter rotating props

Igor Burger

Re: Power for counter rotating props

Igor Burger

Re: Power for counter rotating props

Igor Burger

Re: Power for counter rotating props

Dan McEntee

Re: Power for counter rotating props