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Contra Rotating Propeller Drive for f3a 2m Pattern Planes

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Old 06-13-2012, 08:08 PM
  #801
Zi-Chun Lin
 
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

where can I get Contra Rotating kit?
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Old 06-14-2012, 12:06 AM
  #802
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Dan, when I set up my contra first I cut back the front of the fus so that there was about 2 mm gap behind the spinner ( zero side thrust and approx .5 down thrust), since then I have upgraded my Contra to a Vlll version which added 3mm to this gap and I also decided to use a Hyde mount for Neu motors which added approx 5mmm In total I now had a gap behind spinner of approx 8 mm. I had 2 choices, one was to move my firewall to suit and second was to put a spacer to cover the gap. I decided I was happy with the position of the firewall and didn't want to move it in case I didn't get it exactly right so I went with the second option which as you can see from the photos was to fill the gap with a ring of carbon fibre painted to suit. It's not perfect but I feel it's adequate for this season. When the model is flying you can't see it.
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Old 06-14-2012, 03:03 AM
  #803
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Zi-Chun,

The best way is to send me an email through RCUniverse.

Brenner...
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Old 06-14-2012, 07:28 PM
  #804
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Looks like, when the nose ring is trimmed off as far as it can be without cutting into the lower part of the cowl, the contra spinner diameter is still slightly greater than the nose ring diameter? Your adaptor/spacer also helps match this mis-match?
Thanks, Dan
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Old 06-14-2012, 08:18 PM
  #805
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Here are some pics of the Contra installation in my Nuance. It's not a perfect match but pretty close. It was too windy today to fly it but I hope tomorrow will be better.

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Old 06-15-2012, 12:57 AM
  #806
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Well done Tony for cutting back far enough to have the diameter of the model match the contra. What mounting system are you using? It will fly beautifully!
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Old 06-15-2012, 10:35 AM
  #807
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Here's an interesting effect.

It looks like there may be a potential theoretical noise advantage associated with the Contra Drive when using larger pitch, smaller diameter props. The attached charts show a comparision between a typcical single prop setup using a 21x14 prop, a Contra setup using 22x20 props, and a Contra setup using 20.25x22 props.

The single prop setup and the 22x20 Contra setup both absorb about 2800W statically on the ground, and 2875W max in the air during a vertical climb, but the 20.25x22 absorbs 2650W on the ground for the same 2875W in the air during a vertical climb.

This means that the system is essentially, ATVing the power down to about 95% on the ground when sound tests are taken, and then going back to full power in the air when it's needed, all without using any kind of throttle switch.

Would this be cheating? I tend to think not, because there is no intervention by the pilot. It's all in the setup.

Brenner ...
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Old 06-15-2012, 10:57 AM
  #808
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brenner, this might explain why the APC props are so quet and they don't use as much power. I tried them for a flight with my oldest TP 5000 (20c. 3rd season) in approx 15 mph wind and when I charged today there was still 36% left .
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Old 06-15-2012, 12:01 PM
  #809
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Paul,

I also have the same experience in the standard one prop power system. I guess that we have to called the "APC Magic"

Vicente "Vince" Bortone
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Old 06-15-2012, 01:45 PM
  #810
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Quote:
ORIGINAL: Brenner

Here's an interesting effect.

It looks like there may be a potential theoretical noise advantage associated with the Contra Drive when using larger pitch, smaller diameter props. The attached charts show a comparision between a typcical single prop setup using a 21x14 prop, a Contra setup using 22x20 props, and a Contra setup using 20.25x22 props.

The single prop setup and the 22x20 Contra setup both absorb about 2800W statically on the ground, and 2875W max in the air during a vertical climb, but the 20.25x22 absorbs 2650W on the ground for the same 2875W in the air during a vertical climb.

This means that the system is essentially, ATVing the power down to about 95% on the ground when sound tests are taken, and then going back to full power in the air when it's needed, all without using any kind of throttle switch.

Would this be cheating? I tend to think not, because there is no intervention by the pilot. It's all in the setup.

Brenner ...
Brenner this effect is typical for high pitch props and comes from the blade partially stalling. Power goes down since less air is accelerated through the prop, however the power that still is absorbed by the prop is translated into turbulence, i.e. more noise.

/David
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Old 06-15-2012, 09:15 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey David,

No argument with what you say, but the math model that was used to create these charts doesn't take into account stalling of the props. This is just what is supposed to happen assuming that the props don't stall.

If we consider prop stalling effects, then things get quite a bit more interesting, because the props that are used with Contra Rotating Drives have a lot more pitch than the props that are used with single prop systems, which means that a much higher percentage of the propeller blades are stalled when you run a Contra Drive system up on the ground.

When I first started looking into Contra Drive systems for pattern, I was actually quite a bit concerned about this, and consequently I limited the pitch of the first props that we made to 18", but then after we actually flew 18" pitch props, it quickly became obvious that I was worried for nothing, and we moved on to 20", and 22" pitch props.

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Old 06-18-2012, 08:17 PM
  #812
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Brenner

What Hacker motor and contra set-up would you recommend for BJ Craft Nuance?
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Old 06-19-2012, 03:33 AM
  #813
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

I would recommend the following:

Hacker C50-13XL
Budd Mount Soft Mount with soft durometer
10.33:1 gearset
22x22 rear prop and 22x20 front prop for windy conditions
22x20 rear prop and 22x18 front prop for calm conditions

or:

Hacker C50-14XL
Budd Mount Soft Mount with soft durometer
10.33:1 gearset or 10.15:1 gearset
22x22 rear prop and 22x20 front prop for windy conditions
22x20 rear prop and 22x18 front prop for calm conditions

or:

Neu f3a Motor
Budd Mount Soft Mount with soft durometer
9.98:1 gearset or 10.15:1 gearset
22x22 rear prop and 22x20 front prop for windy conditions
22x20 rear prop and 22x18 front prop for calm conditions

Brenner ...
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Old 06-21-2012, 11:19 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

I'm a bit confused, do I have to get the C50 competition version with the gearbox? Or just a C50-13XL?
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Old 06-21-2012, 11:39 AM
  #815
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Comp
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Old 06-21-2012, 12:14 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

So it's the comp version with gearbox and I just have to take off the gearbox?
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Old 06-21-2012, 12:15 PM
  #817
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Yes ...
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Old 06-22-2012, 01:25 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Can't wait to see these at the Nats. I still have a Hacker 14XL Comp here just in case... Seeing/reading about all these changes since first seeing/flying Brenner's Integral is so interesting to watch.
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Old 06-23-2012, 04:24 AM
  #819
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Jason,

We've learned an awful lot since you flew my Integral. At that time we were struggling with yaw stability problems, and the CG was way too far back for using a Contra. Also, the 22x18 props front and rear were very slow, (not a bad thing, just not compatible with North American style pattern flying ..) so I'm guessing that your experience wasn't totally positive.

Since then we've done the following:

1/.. Added rudder strips to flare the trailing edge of the rudder. We've found that this really helps planes with yaw stability issues.
2/.. Moved the CG forward upwards of 25mm. This really helps Contra powered planes because there is very little corresponding loss of rolling performance, probably due to the lack of right thrust in the nose.
3/.. Made 22x20, and 22x22 props availble. (APC also has props for the Contra as well now..) Now the speed range covers anything you can do with a single prop setup.
4/.. Gone to airframes with reduced drag. This is because you get all the speed control you need from the Contra Drive, so parasitic airframe drag only wastes mah for the packs.

Brener ...
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Old 06-23-2012, 04:48 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi,

Sebart new Mythos 2M that he won Italian F3A Championship with in June 2012 would probably be a nice plan to try a Brenner Contra system with.
It seems Sebastiano Silvestri is using a contra system in his Mythos 2M.
Not sure when Mythos 2M will be released by Sebart. One could only guess if it will be all composite.

http://ow.ly/i/G6CP/original


/Bo
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Old 06-28-2012, 10:55 AM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Bem,

I really like my Wind S Pro, so if the Mythos is like the Wind S Pro, then I agree that it should suit the Contra very well.

On another matter, one thing I've learned in the last few week is that Contra Drives seem to be particularly hard on ESCs when ESC braking is used. Just yesterday I was trying 50% braking with my ESC, and my controller went out on temperature overload after exiting a long downline.

In all fairness, I was testing the braking setting, so I had it on from the start of the flight, which is something that I wouldn't do during a normal flight, but from now on I think I'll limit my ESC braking to something like 35% max.

In hindsight, something like this seems to be fairly easy to explain, because if you consider that my gear ratio is 9.89 vs 6.7 for a single prop setup, and I'm using two props instead of a single prop, then the increase in braking effectiveness is the ratio of the number of props times the ratio of the gear ratios which is:

2*(9.89/6.7) = 2.95

So, at 50% ESC braking, I'm using the equivalent of a 150% ESC setting with a single prop setup.

Obviously this is a highly simplified way of estimating braking effectiveness, but the basic idea is that it's a lot more for an ESC to handle than with a single prop setup.

Also, on another competely separate issue, I've discovered that Contra Drives work much better if the ESC is set up to apply power gradually over a finite period of time. I've set my ESC up to go from zero power to maximum power linearly over the span of one second, and it makes power transitions in the air much smoother.

Brenner ...
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Old 06-28-2012, 01:09 PM
  #822
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Brenner,

Last weekend I witnessed one of your contra drive units in action. It looked very well engineered and performed quite well in calm conditions. The owner said that he was waiting on the arrival of new props to suit windy conditions.

I was quite surprised by the noise footprint having heard about the funny games played by the other contra drive manufacturer with noise tests. Your unit sounds great and is much quieter than I expected. It had everyone's attention.

Attached is a photo of the model.

Cheers,
Jason.
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Old 06-28-2012, 06:34 PM
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hey Jason,

That looks like Gavin Dark's Nuance. The plane looks very sharp with the red Contra spinner. I heard that he now has it in the air. He's obviously done a very meticulous installation. It looks like he's modified the nose for zero degrees right thrust, and to fit the 82mm Contra Spinner diameter, and he's also blended the colors into the rudder strips on his rudder.

I agree with your comments about noise. Right now when we fly at contests, people don't distinguish the sound of the Contra from any of the other planes. In fact, there are inrunners that are louder than the Contra due to gearbox noise. We happen to have an advantage in this regard because our pinion gear has 18 teeth, and this is more than enough to create gear teeth with fully formed involute profiles, which reduces gearbox noise significantly.

As far as prop noise is concerned, we (with the help of many others...) have discovered the following basic design rules:

1/.. There is a minumum separation distance that is necessary to avoid significant noise coming from the blade passing frequency of the two props passing each other in opposite directions. More distance than this minimum doesn't help much, but less distance creates a very large noise.

2/.. There needs to be a mismatch in speed between the front and rear props in order to avoid syncopation noise. This can be done with mismatched pitches, or mismatched diameters.

3/.. The diameter of the rear prop needs to smaller than the diameter of the front prop so that vortices shed from the tips of the front props don't interfere with the rear prop.

Brenner ...
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Old 06-28-2012, 07:44 PM
  #824
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hi Brenner,

You're right, it's Gavman's Nuance and yes, he's done a nice job on the install and finishing the model off.

Cheers,
Jason.
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Old 06-29-2012, 02:49 AM
  #825
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Default RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes

Hello Brenner,

Thank you for a terrific system to power my Nuance, I originally powered the model with a Plettie Advance but decided to change as I could see a definate advantage with the contra system. Out with the dremel and a dustie 2 hours later gone was the old mount and about 10mm (1/2 inch) was missing from the nose, the rest was just simple installation of the NEU motor.
The flying is brillant and with a hanger of several electric OXAI models plus others the combination of Nuance and Contra Drive is far ahead of anything that I have previously flown (30 years of pattern flying).
My set up is as follows, NEU motor, Contra drive, all futaba servos, YGE speedie, zero side thrust. .5 upthrust, I fly currently with the 22x20 rear and 22x18 front with the 9x89x1 gear set, great for calm conditions, pulls like a bull and is slow and predictable, ready to fly with Zippy compact 5000 25c the weight is 4930 grams about 3200 batt for a F3A P13 schedule.
I have no idea how to tell the gear sets apart, I understand that you cannot mix match gear sets for obovious reasons, could you please reply with an answer.
Thank you again

Gavin (The Gavanator)
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