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Old Oct 27, 2008, 05:20 PM
Big Aerial Mission Flyer
Joined Oct 2008
19 Posts
Discussion
Counter rotating propellers

I am designing a twin engine RC airplane, and I just assumed I would use counter rotating props. However, most experts I talk with tell me that counter rotating props on RC aircraft just do not exist. What is the reason behind this?
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Old Oct 27, 2008, 05:48 PM
Baptised Heathen
batman's Avatar
Bundaberg, Qld, Australia
Joined Jun 2002
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http://www.hobbycity.com/hobbycity/s...BL_System_375W

There's one you can buy and I think there are others around. The main problem is procuring left handed props, they don't make every size and in fact only seem to make a couple of sizes. Not a problem if you make your own as some do. I am sure you could find a guide to making your own props here somewher if you did a search.
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Old Oct 27, 2008, 06:02 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
Joined May 2003
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I assume you mean a standard twin engined aircraft with props rotating in opposite direction, not a true contra-rotating motor with normal and reverse pitch props on the same axis.

GWS HD make a series of 3-blade tractor and pusher props in 5x3 --10x6.

Azarr (www.ecubedrc.com) stocks several APC E props which come in tractor and pusher varieties.
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Old Oct 27, 2008, 07:53 PM
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Dayton Intl, Ohio, United States
Joined Jan 2000
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I have APC E props in left hand rotation (aka pusher) from 4.1 x 4.1 to 20 x 13.

If you really mean contra rotating motors, there are three manufacturer's that I'm aware of and their basically a novelty at the moment.

To answer your original question the best I can. Since it's impossible to reverse a glow/gas motor without a new crank and piston liner reverse pitch props were developed for those applications. However, with electric motors all you have to do is reverse the motor wires to run your motor in reverse so there was no real demand. Recently (in the last year or so) modelers have caught on to the idea that twins are really simpler done electric and the demand for "E" style reverse pitch (left hand, pusher) props has increased. APC started with the 20 x 13EP since a prominent modeler was doing a scale project that required props that size. Since then they have added a significant number of sizes.

Blatant add , they're all available on my web site.

Azarr
www.ecubedrc.com
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Old Oct 27, 2008, 08:18 PM
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Minot, North Dakota
Joined Feb 2004
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There is no advantage to contra-rotating props on a twin unless one engine quits. When they are both running contra or not, the torques cancel. Contra-rotating is a great feature for an engine out scenario but electric fliers don't have that problem (as long as we keep our prop adapters tight).

Oh, some gas motors can be reversed by rotating the front housing 90 degrees.
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Old Oct 28, 2008, 02:58 AM
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Azarr's Avatar
Dayton Intl, Ohio, United States
Joined Jan 2000
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Quote:
Originally Posted by chizekaero
When they are both running contra or not, the torques cancel.
Can you elaborate on this? This hasn't been my experience. For example, all last summer I flew a Radical RC Double Trouble and the performance with two props running in the same direction was considerably different than running in the opposite direction. There was a marked torque reaction with both motors running in the same direction.

Azarr
www.ecubedrc.com
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Old Oct 28, 2008, 06:18 AM
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The Netherlands, GE, Nijmegen
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Quote:
Originally Posted by chizekaero
There is no advantage to contra-rotating props on a twin unless one engine quits ...
What's the advantage in that case?

Quote:
Originally Posted by chizekaero
... When they are both running contra or not, the torques cancel. ...
You're wrong there, counter rotating motors do cancel out torque reaction, running in the same direction they don't.

Vriendelijke groeten Ron
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Old Oct 28, 2008, 02:52 PM
Big Aerial Mission Flyer
Joined Oct 2008
19 Posts
By counter rotating, I mean one prop spins clockwise, while the other one spins counter clockwise. This would cancel out the torque and p-factor and make the aircraft more directionally stable.


Azarr, does that mean you have all APC-E props in all sizes that spin opposite directions?

What would I need to do to change the direction the motor spins?

Thanks everyone.
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Old Oct 28, 2008, 03:03 PM
De-Brushed user
Midland, Tx / W. Lafayette, Ind.
Joined Dec 2002
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I was wondering if we would see a flying wing Do you mind if I ask what school you are from?

If you were at the competition two years ago, we (Purdue) used counter-rotating propellors on our BWB design with some success. We came in third, so our report may be available online. At the time, there were not many left-hand propellors available, I believe we only found APC E series in 9" and 10" diameters. Because of our short span (22.5") at a 4' length, the counter-rotating propellors were beneficial. VarioProp (spelling?) may offer left-hand blades as well, so you might look into them.

To change the direction any brusless motor spins, you just have to reverse any two wires. Leave the third as it is. Some speed controllers (Castle Phoenix line, for example) allow you to reverse rotation in the controller software, if you have already soldered you motor leads to the controller. A brushed motor, you have to reverse the two wires.

Good luck,

Jonathan
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Old Oct 29, 2008, 08:18 PM
Registered User
Minot, North Dakota
Joined Feb 2004
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Quote:
Originally Posted by Ron van Sommeren
What's the advantage in that case?

You're wrong there, counter rotating motors do cancel out torque reaction, running in the same direction they don't.

Vriendelijke groeten Ron
Ron, I must respectfully retract both claims. Many years ago, both points were demonstrated to me on paper but I cannot recreate the force diagrams. Intuitively, I must agree with all on this thread.

Nevertheless, not all full scale twins incorporate contra-rotating props so I will state that the benefit must not be so great to overcome the complexity of manufacturing a reverse rotating drivetrain. Of course, R/C electric fliers have it easy if they can find matching tractor and pusher props.

Jeff
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Old Oct 29, 2008, 10:57 PM
Big Aerial Mission Flyer
Joined Oct 2008
19 Posts
Thanks for the advice CAFplanekid. I knew it would be pretty easy. I could never understand why everyone told me to avoid counter rotating props.


Just to clarify,
All I need is one opposite oriented prop, and I can either switch the wires, OR change the programming in the speed controller to make it counter rotating?


I am from a school in CA. I don't want to give away too much.


I am not sure how much p-factor is generated from two props spinning the same way (I would guess it is not much), but I figure it does not hurt to have counter rotating props, and it will only help.

Thanks everyone.
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Old Oct 29, 2008, 11:16 PM
De-Brushed user
Midland, Tx / W. Lafayette, Ind.
Joined Dec 2002
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Quote:
Originally Posted by aerospacepilot
Thanks for the advice CAFplanekid. I knew it would be pretty easy. I could never understand why everyone told me to avoid counter rotating props.


Just to clarify,
All I need is one opposite oriented prop, and I can either switch the wires, OR change the programming in the speed controller to make it counter rotating?


I am from a school in CA. I don't want to give away too much.


Thanks everyone.
Correct. But not every speed controller has the ability to change rotation.


You do have to be careful sharing information on this forum if you don't want it being common knowledge. I spend study breaks on here and have come across a few other recently joined members asking interesting questions for 'school projects' It's still early in the game so I won't say anything other than we are registered as the Astronots
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Old Oct 30, 2008, 03:40 AM
Old age is not for sissies
Azarr's Avatar
Dayton Intl, Ohio, United States
Joined Jan 2000
8,032 Posts
Quote:
Originally Posted by aerospacepilot
By counter rotating, I mean one prop spins clockwise, while the other one spins counter clockwise. This would cancel out the torque and p-factor and make the aircraft more directionally stable.


Azarr, does that mean you have all APC-E props in all sizes that spin opposite directions?

What would I need to do to change the direction the motor spins?

Thanks everyone.
APC doesn't make them in all sizes for the "E" props, but there are 12 sizes in the "EP" series and 2 in the "SFEP" series. All have a matching "E" size and all are in stock.

http://ecubedrc.com/propsandadaptors...tating%20props

Azarr
www.ecubedrc.com
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Old Oct 30, 2008, 04:15 AM
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vintage1's Avatar
East Anglia, UK
Joined Sep 2002
29,705 Posts
Quote:
Originally Posted by chizekaero
Nevertheless, not all full scale twins incorporate contra-rotating props so I will state that the benefit must not be so great to overcome the complexity of manufacturing a reverse rotating drivetrain.
In a nutshell, yes.

It wold be more trouble than its worth to make reversible engines, and props so most contra rotating designs are done with a single shaft and gears.

Its not so hard with electrics
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Old Dec 01, 2008, 10:14 AM
Registered User
United States, TX, Montgomery
Joined Aug 2006
1,263 Posts
reversing rotation

on three wire [brushless] motors simply switch two wires. if standered brushed motors reverse the two wires. donnie
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