Contra rotating props unit.

[macboffin]

Here you go Munin, article on DIY contra rotation unit with drawing and pics.
Macboffin

[Munin]

Thanks again Macboffin

Looks like nothing is new under the sun.
I am going to let one of my engineering friends have a look at
this as its way out of my technical capability.

For those reading this its a spin of talk of Maxxprods counter rotating
engines that they so gently call "new development".

http://www.maxxprod.com/mpi/newdevelopment.html

Thanks again MacBoffin.

/Munin

[macboffin]

New development heck ; there was an article in, I think, RCM&E about eight years ago about a scale Westland "Wyvern", (British naval fighter) with electric contra rotating props,, back in the days of DEAC power.
As you say, Munin, " Nothing new under the Sun!"
The Wyvern is a good-looking plane, original was a turbo prop.

[clolson]

I'm so tempted to retell my nothing new under the sun story ... :-)

[Unterhausen]

I'm so tempted to retell my nothing new under the sun story ... :-)Heard it:)

[air]

What's the efficiency of a setup like this? I would be thinking not great?
If efficiency was acceptable it might be a nice way of using redundant motors / escs on a uav.

[turbodriven]

I posted this in my XFY-1 Pogo build thread. Basically put together two BP or BM style motors like the article up there does. I spent under $50 for everything including props and shipping for the motors from China. The key is to get the correct bells and being able to find the correct lamp hardware at Ace Hardware. Once you do that it's all just a matter of bolting it together. A drill press does come in handy for setting bearings and boring props though.

Preliminary tests using GWS 9050 3-blade props (matched set 1 regular/1 reverse) have shown to put out about 175watts at 15amps total for the combo. Doesn't feel like it's pulling as hard as some of my other "175watt" setups though. And I don't have a thrust bench set up yet either. For a cheap contra rotating blade setup it seems do well. And with different props I'm sure I could squeeze a lot more out of them. One thing is for sure though... there is near zero rotational torque when throttled up. This would be great for a stubby wing plane that needs to be launched at full throttle (like my pogo). Anyway... here are some pics to show how I built them.

A. Contra-rotating motor setup. Front motor spins the rear prop and the rear motor has a 3mm shaft that runs through everything to run the front prop. The aft motor has a cut up medicine bottle to keep wires from fraying on the outrunner bell (this may only be necessary in my tight spaced XFY-1)
http://static.rcgroups.com/forums/attachments/6/5/7/8/5/a1495459-205-pogo4.jpg

B. Exploded view explaining the parts involved. I forgot to draw on a few "thrust washers" located between the forward bearing and the wheel collar. Note that all the "lamp parts" were found at Ace Hardware in a bin labeled... you guessed it... "lamp parts". For 8mm outer/3mm inner bearing look in the helo section of your LHS. They had all kinds of different ones.
http://static.rcgroups.com/forums/attachments/6/5/7/8/5/a1495842-85-Motor1.jpg

C. Here is the rear bell that drives the front prop. It's got set screws different than the front bell. These set screws hold the drive shaft. The long drive shaft is 3mm and was robbed from a T-rex cross boom parts bag. On the T-rex it's the part that goes perpendicular to the main blades and has small "blades" attached to the ends. It's threaded on both ends and the $5.99 bag comes with two. So essentially enough for 4 motor combos (in case you want to build more down the road). You can also see the wheel collar and the thrust washers in the background.
http://static.rcgroups.com/forums/attachments/6/5/7/8/5/a1498235-231-pogo6.jpg

D. Motor bases screwed together. There are 100 ways to do this but I used two firewalls. Bolted a motor base to each and then both together. Make sure they line up perfect or the drive shaft will be bent. Even a little will cause vibrations.
http://static.rcgroups.com/forums/attachments/6/5/7/8/5/a1498236-8-pogo7.jpg

E. The front bell which drives the rear prop. This is the bell screwed directly into lamp parts with bearings pressed in. Simple really.
http://static.rcgroups.com/forums/attachments/6/5/7/8/5/a1498237-169-pogo8.jpg

F. You can't see the bolt for the front bell because it is hidden under the lamp parts. The front bell has a nut on it like this one.
http://www.happitoy.com/uploadpic/2005112164720308.jpg

[macboffin]

Another full size contra-rotater was the the Fairey "Gannet", another naval aircraft.They used to line them up nose to tail on the flight deck, start the one in front, the next one would start using the propwash from the front plane instead of starter to spin up the turbine, and so on down the line.
Good way to conserve battery life!
Usual reason for contras was because if a single prop, too large, meant long stalky weak undercarriage legs. Fringe benefit was lack of take-off swing since no assymetric disc loading, no side force on tail fin due to spiral flow down the fuselage from single prop.

[John O'Sullivan]

Not so New!!
In 1963 at the British Nationals and at the Belgian Criterium of Aces (European CL Championships), a British Competitor (Randall?,) had a CL Fairey Gannet with glow contra-rotating propellers. A truly wonderful engineering feat from 44 years ago. I still have photos somewhere.
John

[Connexxion]

Hi,

what are the biggest advantages of the counter rotating propellor?

Can't you just mount a 4-blade prop on an engine?Looks like the same to me.

Correct me if I'm wrong.

Btw:it's contra rotating and not counter rotating:

Quote of wikipedia:

The configuration can also be used in helicopter designs, where similar issues and principles of torque apply. Contra-rotating propellers should not be confused with Counter-rotating propellers, a term which describes twin-engined aircraft with the airscrew on one engine turning clockwise and the other counter-clockwise.

[macboffin]

Two advantages.Most important, no torque reactions, no yaw varying with angle of attack due to asymmetric disc loading ,( you know, the way the plane wants to go left when you pull the nose up,) and since the flow is straight down the fuselage, not spiral, slightly less drag, and no take-off swing. Second, smaller prop diameter for given power, allowing shorter lighter undercarriage.As to a four blader looking the same, it is less efficient.
Depends if you want something that flies right, or looks right at first glance.

[Connexxion]

...I got it!