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Author Topic: Counter Rotating Propellers for Stunt  (Read 13046 times)

Offline Matt Piatkowski

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Counter Rotating Propellers for Stunt
« on: February 14, 2019, 08:03:54 AM »
Hello,
Three of my F2B colleagues in Poland fly the F2B electric planes with the counter-rotating propellers.

I saw two of them flying in competition, was surprised that the batteries they used survived and started thinking about the efficiency of such "contras".

NASA Technical Memorandum 87656 titled "Low-Speed Wind-Tunnel
Tests of Single-and Counter-Rotation Propellers" contains the following statement:

"Comparisons of the normal and side forces produced by the propeller systems inclined at an angle of attack relative to the free-stream flow show that the counter-rotation propeller produced much lower values of side force and substantially higher values of normal force than the single-rotation propeller."


I would like to start the discussion about this interesting subject. If there are responses, I will move the discussion to the Electric Stunt Segment of the Forum.

Happy Flying,
M

Offline Ken Culbertson

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Re: Counter Rotating Propellers for Stunt
« Reply #1 on: February 14, 2019, 08:32:31 AM »
Hello,
Three of my F2B colleagues in Poland fly the F2B electric planes with the counter-rotating propellers.

I saw two of them flying in competition, was surprised that the batteries they used survived and started thinking about the efficiency of such "contras".

NASA Technical Memorandum 87656 titled "Low-Speed Wind-Tunnel
Tests of Single-and Counter-Rotation Propellers" contains the following statement:

"Comparisons of the normal and side forces produced by the propeller systems inclined at an angle of attack relative to the free-stream flow show that the counter-rotation propeller produced much lower values of side force and substantially higher values of normal force than the single-rotation propeller."


I would like to start the discussion about this interesting subject. If there are responses, I will move the discussion to the Electric Stunt Segment of the Forum.

Happy Flying,
M
My father used to sing this after a few beers:

"Don't give me a P-38.
The props, they counter rotate.
They are scattered and smitten from Burma to Britain,
Don't give me a P-38."

Ken
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Offline John Rist

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Re: Counter Rotating Propellers for Stunt
« Reply #2 on: February 14, 2019, 09:25:07 AM »
Are you talking about twin motor set up or a single motor setup?  If single motor setup what is the source of motor and props?   ???
John Rist
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Offline Peter in Fairfax, VA

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Re: Counter Rotating Propellers for Stunt
« Reply #3 on: February 14, 2019, 11:30:33 AM »
The indoor folks have contra: 



No idea who would make a contra rig for a glow .60

Offline Tim Wescott

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Re: Counter Rotating Propellers for Stunt
« Reply #4 on: February 14, 2019, 12:04:11 PM »
Do it!  See how it works!
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Ken Culbertson

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Re: Counter Rotating Propellers for Stunt
« Reply #5 on: February 14, 2019, 12:07:11 PM »
The indoor folks have contra: 

No idea who would make a contra rig for a glow .60

That was just plain fun to watch.  Indoor PA someday - with electric getting better by the day, why not.  Imagine getting docked for a 3' corner being too big!

Ken
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Offline frank williams

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Re: Counter Rotating Propellers for Stunt
« Reply #6 on: February 14, 2019, 01:08:22 PM »
That is absolutely incredible!!!!!!!  neat little draggies off the wing .... gotta try that

Offline Tim Wescott

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Re: Counter Rotating Propellers for Stunt
« Reply #7 on: February 14, 2019, 01:22:03 PM »
That was just plain fun to watch.  Indoor PA someday - with electric getting better by the day, why not.  Imagine getting docked for a 3' corner being too big!

Someday?

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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Doug Moon

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Re: Counter Rotating Propellers for Stunt
« Reply #8 on: February 14, 2019, 01:51:58 PM »
The indoor folks have contra: 



That is awesome!!  Talk about precision aerobatics!!
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Offline John Lindberg

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Re: Counter Rotating Propellers for Stunt
« Reply #9 on: February 14, 2019, 02:12:47 PM »
Randy Smith had one once upon a time:https://www.youtube.com/watch?v=yz3zyDXStOU

Offline pmackenzie

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Re: Counter Rotating Propellers for Stunt
« Reply #10 on: February 14, 2019, 04:18:57 PM »
There was a thread recently on RCG that touched on the efficiency of co-axial contra-rotating props.

https://www.rcgroups.com/forums/showthread.php?3193963-Counter-Rotating-Propellers

A very smart guy , Cristian Lucas, contends that they have higher efficiency.
https://www.rcgroups.com/forums/showpost.php?p=40836673&postcount=16

If you don't recognize the name he is the "L" in LRK. 
If you don't know the acronym , they are the three initials of the guys who basically invented the magnetic design of the outrunners used in F2B (and R/C of course)
Most motors we use are dLRK which means distributed LRK. Original LRK winding scheme ( early AXI motors) had no winding on every other tooth.

The indoor contras, like those posted earlier, are very efficient. A single 120 mAhr cell will fly my 45 gram F3P model for more than 7 minutes.

The usual reason for a contra in full scale has to do with putting maximum power into the available prop diameter given tip speed limitations and minimizing torque reaction.

Pat MacKenzie

P.S. that video is 5 years old. Much heavier model than what is used now.
Here is a recent one, pilot is the 3 time F3P world Champ.
Different sequence, it is changed every two years.

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Offline Trostle

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Re: Counter Rotating Propellers for Stunt
« Reply #11 on: February 14, 2019, 05:44:13 PM »
One of the Polish fliers had a counter rotating prop system at the World Championships in France last July.  It was an integrated electric motor/gearbox in a model similar to size and shape of a Shark.  Performed very well.   Would easily fit in the palm of your hand, really not much larger than a regular electric motor for a model that size.  The take-apart model was very well done.

He was taking orders.  Kind of pricey.  I would have taken one but I do not do electric.  (One of the poles made the finals, but I cannot remember if it was the counter rotating model or not.

I cannot get my photos to post here!

Keith

Offline Dan McEntee

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Re: Counter Rotating Propellers for Stunt
« Reply #12 on: February 14, 2019, 06:50:41 PM »
Randy Smith had one once upon a time:https://www.youtube.com/watch?v=yz3zyDXStOU

      Randy's model has been discussed several times. It was not a counter rotating prop, but instead the rear prop was "driven by the front prop and acted as more or less a stator to help straighten out air flow. I know I'll get something wrong on the description, so I'll let him finish it when he sees this. I was there at the Lawrenceville NATS when he flew it there and it was quite impressive. The corner was really nice, and the sound it made spooling up and winding down was worth the price of admission alone! I think you can see my back in a couple of shots of that video. lts of stories to tell from that NATS!!!
   I saw a guy here locally who flew a twin electric stunt model in electrics very early days, Walt Brownell. Walt's model was sort of a semi scale french bomber with what looked like a Chipmunk wing. Walt settle on counter rotating props turning tips out from the top, if I remember correctly. Walt was an aeronautical engineer with McDonnell-Douglas and he said that this configuration was opposite the norm for full scale operation of twins, but it is what he felt was the best in performance for his model. He felt that the advantage in the twin was that the prop wash blew right over the flaps and made them more effective. He thought the model was a bit heavy but you could not tell when it flew and it was like it was on rails with a very snappy corner. Walt put a whipping on us in the expert class at Paducah that year with it, with no practice flights because he only had two batteries, if I remember correctly. Awesome airplane! If I were to go electric, this is the route I would go. I would still like to try it with some LA.25'w or FP.25's if I could find someone that could make me a left hand crank!
    Type at you later,
  Dan McEntee
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Offline pmackenzie

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Re: Counter Rotating Propellers for Stunt
« Reply #13 on: February 14, 2019, 10:57:33 PM »
One of the poles made the finals, but I cannot remember if it was the counter rotating model or not.



It was the Polish pilot with the contra model that was in the finals.
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Offline Trostle

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Re: Counter Rotating Propellers for Stunt
« Reply #14 on: February 15, 2019, 12:58:25 AM »
It was the Polish pilot with the contra model that was in the finals.

The Polish pilot in the finals (11th place) was Krystian Borzecki.

Keith

Offline pat king

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Re: Counter Rotating Propellers for Stunt
« Reply #15 on: February 15, 2019, 10:17:21 AM »
A gearbox for contra rotating props on an IC engine would be substantially heavier than one for an electric motor. The instantaneous torque pulses form the single cylinder IC engine are out of sight higher than the rather constant torque output of the electric motor. That is why props for electric motors are not recommended for IC engines. Small electric props may be usable for 1/2A engines. A 12" electric prop would be in extreme peril for shedding a blade on a .60 engine.

Pat
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Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #16 on: February 15, 2019, 10:24:58 AM »
Hello everybody,
Thank you for replaying.
I am trying to compare the efficiency of the "Contra" electro-mechanical system with the efficiency of the "normal" electro-mechanical system (single prop.) in the C/L stunt. The "Contra" system must be somehow more efficient, otherwise Krystian Borzecki would not be able to use the batteries he is using.

Last season, he was using two Gens Tattu 3S 2300mAh. connected in series. I spoke to Krystian and also to Pawel Dziuba who pioneered the use of "Contra" among the Polish F2B fliers but they never measured the in-flight and after the flight parameters. Both fliers use substantial experience and intuition to set up the "Contra" system and this is working very well for them. Now....80% of 2300 mAh. is 1840 mAh. and this means that Krystian's and Pawel's planes with "Contra", weighting about 65.5 oz. RTF. and flying 5.2-5.3 sec. lap, consume about 22 Amps. average current during the 5 minutes. pattern.

With 22 Amps. in-flight average current, I would not be able to fly the pattern using my MP Bee, weighting about 63 oz. and having the envelope dimensions and lap times similar to Krystian's and Pawel's planes. The flight will be too slow and there will be not enough lines tension for the overhead maneuvers. MP Bee uses about 27 Amps. of average current in the 5 minutes Pattern and lands with the 5S 2800 mAh ThunderPower battery drained 80-82%

This is the reason I become interested in the subject. NASA publication clearly stated that the counter-rotating props. generate more thrust in the direction of the flight because there is no "air swirl". Please see the attached.
In the single prop. configuration, the velocity of the air mass moved by the propeller is at angle w/r to the direction of the flight. Consequently, only the cosine component of it is propelling the plane. In the "Contra" system, the entire air mass behind the propellers moves along the direction of the flight.

Assuming that the air "swirl" generated by a single propeller is at 30 degrees angle, only 86.6 % of the moving air mass momentum is used for thrust and the rest (13.4%) is wasted on uselessly swirling around the fuselage and affecting the aerodynamics of the wings and tail surfaces. This "swirl energy" is wasted but it comes from the battery that is drained 13.4% more.

Please note that 22 Amps./ 0.866 = 25.6 Amps that is quite close to 27 Amps. consumed in the air by the MP Bee power plant. Perhaps the "swirl angle" is larger, 35 or even 40 degrees? For 35 deg. swirl angle the numbers match reasonably well. The swirl cosine is 0.819 and 22/0.819 = 26.86 Amps.

Your further comments regarding the subject are appreciated.

Thank you,
M

 

   







Offline Fred Underwood

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Re: Counter Rotating Propellers for Stunt
« Reply #17 on: February 15, 2019, 11:44:13 AM »
Should the comparison from 6s to 5s be amps used, or watts used?
Fred
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Offline Ian MacNeil

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Re: Counter Rotating Propellers for Stunt
« Reply #18 on: February 15, 2019, 03:05:51 PM »
If the "Contra" system was using a 5s battery rather than a 6s then theoretically the average current flow
would be 26.4 amps. for 5 minutes which is comparable to the standard prop at 27 amp average current flow.

6/5 x 22 = 26.4 amps.
 
As Fred indicated since the voltages are different, use watts not current.

Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #19 on: February 15, 2019, 05:34:51 PM »
Fred, Ian,
My typo. I have written" MP Bee uses about 27 Amps. of average current in the 5 minutes Pattern and lands with the 5S 2800 mAh ThunderPower battery drained 80-82%". I have checked my notes from July - August 2018 and there should be 6S in the quoted sentence (ref: https://stunthanger.com/smf/gettin-all-amp'ed-up!/motors-efficiency-52289/).

Next step is to estimate the efficiency of the "Contra" gear box itself. This little wonder of the CNC machining has four straight teeth gears and the ratio was 1:2.4 for the version I saw in Poland in 2018. Propellers used with it resemble closely 12x6 Graupner 2 blade electric. The lab testing of the "Contra" gear box itself to find its mechanical efficiency was unfortunately never done.

Regards,
M

 

 

Offline Istvan Travnik

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Re: Counter Rotating Propellers for Stunt
« Reply #20 on: February 16, 2019, 02:34:42 PM »
Dear Matt,
As I have heard from Pawel, the next step will be the sloped teethed gearbox, instead of present straigth teeths.
I will love it, since the present version has really terrible noise: the generated noise between the props is quite enough, no need for more...  :)
Istvan

Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #21 on: February 16, 2019, 04:00:17 PM »
Hi Istvan,
The present gear box for "Contra" (with Straight-Cut gears) is the lightest and the machining process is reasonably well known.
The change from Straight-Cut (or Spur) to Helical gears will definitely reduce the noise but will make the CNC machining more expensive.
Helical gears require also bearings that handle better the axial load and the gear box with such gears has lower efficiency because of increased friction between the teeth.

It will be very difficult to make the "Contra" gear box light enough for F2B, quieter, strong enough for 1,000 flights and reasonably priced at the same time. I wish Pawel the best in his quest to improve this interesting design and will see four Polish F2B planes using it this coming season.
Will you be in Wierzawice?
Regards, M 




 

Offline pmackenzie

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Re: Counter Rotating Propellers for Stunt
« Reply #22 on: February 16, 2019, 09:04:08 PM »
Assuming this is a planetary gear set up?  Does the front prop turn the "normal" direction? These motors must have extra long shafts. Would love to see a drawing of this.

If you are asking about the Polish system, it is not planetary.
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Offline pmackenzie

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Re: Counter Rotating Propellers for Stunt
« Reply #23 on: February 16, 2019, 09:26:51 PM »
Just normal spur gears., with a separate reversing shaft.
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Offline Fred Underwood

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Re: Counter Rotating Propellers for Stunt
« Reply #24 on: February 16, 2019, 09:51:11 PM »
MM
Look at the motor wiring(s) on the Ebay one.  Two motors?
Fred
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Offline pat king

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Re: Counter Rotating Propellers for Stunt
« Reply #25 on: February 17, 2019, 07:04:10 AM »
The case construction and the two sets of leads say two motors mated together.
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Offline Istvan Travnik

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Re: Counter Rotating Propellers for Stunt
« Reply #26 on: February 17, 2019, 07:35:54 AM »
Hi Istvan,
...
Will you be in Wierzawice?
Regards, M 

Dear Matt,
Hopefully I will be there, and I targeted 2 or 3 more nice Worldcup contests in MidEurope, too.
I cancelled the European Championships because of distance, climate, costs, and time robbing duration. ("one third pleasure at triple price"... :( )
Regards: Istvan



 

Offline Keith Renecle

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Re: Counter Rotating Propellers for Stunt
« Reply #27 on: February 17, 2019, 08:19:25 AM »
Thanks to Pat for posting the links from the RC Groups. I found this comment from Kuni to be most interesting:

"It is important to tune the prop to the working spot of your plane; even more important then on a normal plane. A coaxial drive system has a "sweetspot" where both motors help each other, where the current sinks and the rpm jumps. That is the spot you need to set to your desired flight regime. It gives you more than sophisticated drives with esotheric windings and such...

On the other hand one thing must be clear: a coaxial drivetrain is less efficient thrustwise OR pitchspeed-wise, compared with a "normal" motor of the same power output.
Please let me explain: you have 2 motors of -say-1KW each. You pump the motor energy into the props, who are delivering thrust and airstream of a given speed. But: the second propellor works in accelerated air and has higher friction, needs to re-straight the air, and has to avoid the front-prop-tip-turbulence."

That idea of a "sweet-spot" is really interesting. Maybe one of our resident experts can comment?

Keith R
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Offline IdRatherBeBuilding

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Re: Counter Rotating Propellers for Stunt
« Reply #28 on: February 17, 2019, 12:50:56 PM »
They idea of counter rotating props is to create a more linear airflow over the planes surfaces.
A prop creates a helical flow, usually over the fuselage, under the port wing and on top of the starboard wing. This effects, lift yaw roll and control surface. Barn door style ailerons were chosen by some over the strip ailerons to get those surface out of the propellers wake. Whether or not it  helped is debatable.
Here are some photos to illustrate the flow path of the helical prop wake.

Now, this is theoretical and its been almost 30 years since I was in college listening to this stuff. My interest back then was airfoils, not so much props, so I obviously didn't retain very much.. Either way it's a hoot!

 Click on the last image of the twin.
It shows some pretty cool animation. 


David Miller
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Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #29 on: February 17, 2019, 02:34:47 PM »
Hi IdRatherBeBuilding,
Your comments agree with my present understanding of the "Contra" aerodynamics.
The F2B planes powered by "Contras" do not show the yaw and roll effects caused by the helical wake of a single prop.

Very cool photos and simulation!

Best Regards,
M

 




Offline Brett Buck

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Re: Counter Rotating Propellers for Stunt
« Reply #30 on: February 17, 2019, 02:58:54 PM »
Your comments agree with my present understanding of the "Contra" aerodynamics.
The F2B planes powered by "Contras" do not show the yaw and roll effects caused by the helical wake of a single prop.

  Or the roll and yaw effects caused by the (much stronger) P-factor and precession effects. We have all known from the start that these are undesirable characteristics that we would rather be without. The only issue was how to get rid of it without extreme weight, noise,  and reliability issues of the necessary gearbox. This was essentially beyond solution with IC engines, they would have to be extremely strong to tolerate the pounding (the peak torque during a stroke is something like 25x the average torque, and that's going on at 200 times a second) so they were too heavy and/or unreliable to be practical. Electric removes that, the peak cogging torque is more like 2x the average torque so the peak load on the gears is vastly lower.

   I would point out that your efficiency discussion above, while good as far as it goes, misses the effects of the rear propellor operating in the swirling and accelerated wake of the front propellor. There is every reason to expect that the two props *shouldn't have the same pitch*, for example.

     Brett

Offline Howard Rush

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Re: Counter Rotating Propellers for Stunt
« Reply #31 on: February 17, 2019, 03:18:26 PM »
I would point out that your efficiency discussion above, while good as far as it goes, misses the effects of the rear propellor operating in the swirling and accelerated wake of the front propellor. There is every reason to expect that the two props *shouldn't have the same pitch*, for example.

You'd grind gears to get the desired RPM ratio between props.  You may need a new gear whenever you change props. 
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Offline pmackenzie

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Re: Counter Rotating Propellers for Stunt
« Reply #32 on: February 17, 2019, 03:31:09 PM »
Pretty sure the Polish F2B system is 2:1 for both props.
Even if the pitch of the rear one is not optimized all the torque/pfactor/precession effects if not zero will be much lower than for a single prop.

With the F3P systems front and rear props are the same pitch, and the ratios are as well. They do a pretty good job of eliminating negative effects.

Another reason that the Polish system could well have similar ( or higher) efficiency to normal single props - it uses a geared in-runner.
Outrunners are great, they let you swing big props and are simple to construct ( i.e. cheap).
But they typically don't have the efficiency of higher revving inrunners, even if gear losses are taken into account.

Pat MacKenzie
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Offline IdRatherBeBuilding

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Re: Counter Rotating Propellers for Stunt
« Reply #33 on: February 17, 2019, 04:19:55 PM »
They are used in full scale. Not too often though. With the added forces on the structure from the prop wash, trim is needed. A few Reno racers have been using them for years. I think like over 20? I could be wrong about that though. Some guys sware by them, others say it really doesn't matter. Especially once you add in the drag of the added prop. But that's racing.
I would definitely go for it on an electron powered stunt. Heck with the prices today on them, it's worth it just as a conversational piece   n~

David Miller
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Offline Brett Buck

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Re: Counter Rotating Propellers for Stunt
« Reply #34 on: February 17, 2019, 06:26:48 PM »
Especially once you add in the drag of the added prop. But that's racing.

  I think Matt's point is that he was surprised that the extra prop might have been expected to be less efficient (more draggy), but in fact, it doesn't appear to be. His premise is that the counter-rotation has the effect of straightening the flow, so less or no energy goes into the swirl, compensating for the fact that the rear prop is less efficient running in the wake of the front one. I am sure there is something to that but that's not likely the only issue involved here - for instance the rear prop is running at a greatly reduced effective pitch, maybe even negative in spots along the span (and therefore sucking up power less effectively).

    Brett

Offline IdRatherBeBuilding

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Re: Counter Rotating Propellers for Stunt
« Reply #35 on: February 17, 2019, 10:16:00 PM »
Wow..
I just googled "Reno Racing Counter Rotating Propellers". There's a lot guys out running these now. I guess it just takes a little time to convince some people. But the debates really gets you thinking of the pros and cons. Not to mention the conception itself.

I also found some info on a plane I remeber reading about in high school. Yup, I was hiding the magazine in my text book at the time. It's called the "XB 35" by Northrop. The cool thing was John Northrop was alive and very active on the flying wing concept. The article I was reading in 1980 was about his life. Sadly it was written due to his passing. What a great engineer.

Oh yeah, the XB 35 used contra props. That was developed during ww2!! The plane was flying in 1946.
David Miller

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Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #36 on: February 19, 2019, 06:42:22 AM »
Hi Patrick,
The latest design of the "Polish Contra" gearbox has the 1:2.4 ratio. Two 12"x6" two blade carbon propellers (based on Graupner electric 12x6 propeller geometry) are used with the rear one cut to 11.5 diameter.
I will meet again with Stefan Galdynski, who custom builds the 8 magnets poles in-runners used by the top polish F2B fliers, to measure the Kv of this motor and other parameters. Last year he told me that he did not know Kv for his motor (!) and did not have one for me to test.

The in-runners v/s out-runners efficiency is another interesting subject but first I have to know the basic "Galdynski motor" parameters.

Thanks,
M


   

Offline Doug Moon

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Re: Counter Rotating Propellers for Stunt
« Reply #37 on: February 19, 2019, 09:54:57 AM »
The one for sale on ebay is two separate motors.  One outrunner with the prop mounted on the hub and one inrunner with the shaft running through the outrunner and the prope shaft mounted. With two sets of motor leads this would require a 2 ESC 2 battery setup no?? 

Anyone have pics of the Polish setup from the worlds?
Doug Moon
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Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #38 on: March 02, 2019, 08:44:38 AM »
Hello Doug,
No pictures yet but I am working on it.

For now, I can provide the following information: the gear box has four spur gears packed inside the aluminum gear box. The "Gaudynski motor" powers the main shaft with the first gear (gear#1). This gear powers the gears #2 and #3 that are positioned in the same plane as gear#1. Gear #3 is thicker and this allows for its engagement with gear #4 that powers a hollow shaft rotating in the opposite direction to the main shaft.

12x6 carbon composite two blade propellers are attached to both shafts.

The gear box ratio was 1:2.4 last year but I know that a new gear box is being developed, with different ratio. This ratio is TBD at this moment.

I still do not know the Kv of the "Gaudynski motor" and the allowable current but I estimated both. Please see below:

12x6 two blade carbon composite propellers must turn within 9,000 - 10,000 RPM range, otherwise the full size (60" wings span and ~64 oz. RTF weight) F2B plane would not maintain the required lap time around 5 seconds on 64-65' long lines.

The "Gaudynski motor", like I wrote before, is the 8 pole in-runner, therefore the RPM of this motor must be in 22,500-23,000 range (9,500x2.4) on 6S battery (the average voltage for 6S is assumed to be 24 Volts). Now...22,500 RPM must be "upgraded" by at least 20% to account for the gear box friction and the electromagnetic losses in the motor. 22,500 / 0.8 =28,125 RPM.

Finally, the Kv of the motor = 28,125 / 24 = 1172.

Please note that this Kv is almost as twice as high as the Kv of the out-runners used by the top F2B fliers using the single prop. configuration.

Happy flying,
M


 
« Last Edit: March 02, 2019, 12:24:14 PM by Matt Piatkowski »

Offline Brett Buck

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Re: Counter Rotating Propellers for Stunt
« Reply #39 on: March 02, 2019, 11:07:56 AM »
They idea of counter rotating props is to create a more linear airflow over the planes surfaces.
A prop creates a helical flow, usually over the fuselage, under the port wing and on top of the starboard wing. This effects, lift yaw roll and control surface.

     Of course, the pictures do not show helical flow, they show the path of the prop blades through the mostly fixed air. The condensation trails stay nearly stationary WRT to the atmosphere.

    There is substantial angular momentum transferred from the prop to the air, because drag exists, but these helix patterns do not show that.

     Brett

Offline Trostle

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Re: Counter Rotating Propellers for Stunt
« Reply #40 on: March 02, 2019, 03:37:35 PM »
One of the reasons for counter rotating propellers on full scale aircraft is to utilize the power available from the engines without resorting to large diameter propellers where tip speed approaches the speed of sound with the attendant losses and complications as well as not requiring otherwise lengthened landing gear.  That is one of the reasons we saw some late model Spitfires with counter rotating propellers as well as the Spiteful, the Shackelton and the MB-5 all with Griffon engines.  (There is or has been a Griffon powered racing Mustang with counter rotating props.)  The counter rotating propellers significantly reduced take-off torque problems.  There are other aircraft with counter rotating propellers as well including the North American XA2J-1 and the Douglas XA2D-1.  The Russian Tu95/-142 aircraft with their huge turboprop engines are another example.

This comment does not really relate to advantages or disadvantages of counter rotating propellers for our CL stunt models but I thought was worthwhile mentioning.  However, the near elimination of torque would seem to be a significant factor for our stunt models.

Keith


Offline Dan McEntee

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Re: Counter Rotating Propellers for Stunt
« Reply #41 on: March 02, 2019, 09:05:28 PM »
   Would not going the twin motor route still give you what you are looking for in a counter rotating set up and eliminate the complexity of the gear boxes?  You have a large variety of motors available along with props, so you could make the resulting model any size you could want.I have posted before about a local guy, Walt Brownell, building a series of twin motor stunt models that flew exceptionally well. I remember him saying that he thought having the prop wash going right over the wing made the flaps work better and had no affect like it would if swirling around the fuselage. he experimented with prop tips turning in and turning out at the top and settled on turning out. i don't know if Walt is still among us but sure would like to talk with him again about this. I have been wanting to build a twin I/C powered model with counter rotating props but finding crank shafts is too difficult.
   Type at you later,
    Dan McEntee
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Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #42 on: March 15, 2019, 10:28:39 AM »
Hello,
I have just received two CR3516-1030 Contra Rotating HiMax motors with all necessary hardware and props.
These systems will be tested with different props. and batteries in Poland this spring and summer.

Happy Flying,
Matt




Offline Lauri Malila

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Re: Counter Rotating Propellers for Stunt
« Reply #43 on: March 16, 2019, 10:17:36 AM »

Dan,

Propably not a big deal with good IC engines, but as Fox .35 was mentioned (by MM) I have to remind that running asymmetries tend to reverse when engine’s running direction is reversed. That may cause some quite interesting trim issues.
I’ve seen the Polish contraprop flying, and apart from the terrible sound I was positively surprised of its performance.
Maybe a better way would be to use 2 coaxial motors, front one with a hollow shaft. Yuriy Yatsenko has been working with something along those lines, it’ll be interesting to see it once he gets it ready. L

Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #44 on: March 16, 2019, 10:25:55 AM »
Hi Lauri,
Like I wrote in this thread yesterday, I bought two CR3516-1030 HiMax system.
They will be extensively tested in Poland with different propellers soon.
Regards,
Matt

Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #45 on: March 18, 2019, 03:37:36 PM »
Hello,
I still do not have the photographs of the "Gaudynski Gear Box" but I have more numbers. 
On March 02, 2019, 08:44:38 AM I wrote in this thread: "12x6 two blade carbon composite propellers must turn within 9,000 - 10,000 RPM range, otherwise the full size (60" wings span and ~64 oz. RTF weight) F2B plane would not maintain the required lap time around 5 seconds on 64-65' long lines.

The "Gaudynski motor", like I wrote before, is the 8 pole in-runner, therefore the RPM of this motor must be in 22,500-23,000 range (9,500x2.4) on 6S battery (the average voltage for 6S is assumed to be 24 Volts). Now...22,500 RPM must be "upgraded" by at least 20% to account for the gear box friction and the electromagnetic losses in the motor. 22,500 / 0.8 =28,125 RPM.

Finally, the Kv of the motor = 28,125 / 24 = 1172.
."

The CR3516-1030 contra system has Kv=1030, uses 12x6 and 11x8 APC E propellers and runs on 3S battery having 4000-5000 mAh.capacity. 3S battery has the average voltage of 11.85 Volts.
1030 x 11.85 = 12205 RPM. The CR3516 system drives both propellers directly - there is no gear box. Assuming 20% RPM loss due to propellers air resistance, 12205 no load RPM drops to 9764 Rpm.

9764 RPM is close to 9500 RPM assumed for the propellers powered by "Gaudynski Gear Box". Kv=1172 for the Gaudynski motor is also close to Kv=1030 ( 12% difference) for the HiMax CR3516-1030 contra system.

The main difference between these two systems is the battery: 6S having around 2300 mAh. (2 x 3S 2300 mAh. GensTattu) for the geared system v/s 3S having between 4,000-5,000 mAh. for the direct system (for example: https://hobbyking.com/en_us/turnigy-5000mah-3s-20c-lipo-pack). The weights of these packs is the same: ~360 grams (12.7 oz.).

Both systems seem to be able to deliver similar power but the direct system is more efficient mechanically and five times cheaper (yes...FIVE times!) than the geared system.

If there is no major quality flaw in CR3516-1030, this system looks very promising for F2B.

Happy Flying,
M




Offline Air Ministry .

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Re: Counter Rotating Propellers for Stunt
« Reply #46 on: March 18, 2019, 05:50:16 PM »
With TWO co - axial motors , you could start to have some fun , with programing etc .





Blame the Westland Wyvern , heres another one , both filched from R C Groups .





A black & decker , a bit of wire , some sandpaper , and your there .  :-X


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Re: Counter Rotating Propellers for Stunt
« Reply #47 on: March 18, 2019, 05:53:25 PM »


Two high-density YASA electric motors are planned to provide the power for the P1e.

https://www.aerosociety.com/news/bright-sparks-the-quest-for-electric-speed/

Offline Matt Piatkowski

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Re: Counter Rotating Propellers for Stunt
« Reply #48 on: March 19, 2019, 07:18:11 AM »
Hi Matt,
It is always good to know what other people did and are doing.
Happy (electric) Flying,
Matt

Offline John Rist

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Re: Counter Rotating Propellers for Stunt
« Reply #49 on: March 19, 2019, 12:24:59 PM »


Two high-density YASA electric motors are planned to provide the power for the P1e.

https://www.aerosociety.com/news/bright-sparks-the-quest-for-electric-speed/

Now there's a candidate for an electric scale project.  Won't need a sound generator.
John Rist
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