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Pusher, Counter Rotate, ARGH!

Old 11-05-2009, 02:35 PM
  #1  
Nitro Blast
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Default Pusher, Counter Rotate, ARGH!

After my recent post of my AC-47 with her crew:


Someone made the statement it would look better with 3 bladed props, painted black.


So... doing a little internet searching, at Tower, I found what I think is a pair of props that I can use. I wanted to try to get one motor turninig the other direction so I figured these props:


Master Airscrew 9x7 3-Blade G/F Nylon Propeller
and
Master Airscrew 9x7 3-Blade Pusher G/F Nylon Propeller


My noob question is I assumed that the pusher was just a prop designed to run the other direction. But... being termed a "Pusher" is it in fact different enough that I cant use it as a counter-rotating prop on my AC-47 twin?

Or is a Pusher just a backwards prop.
Used as pusher, or counter rotate will work the same.


Hircflyer posed this question to me and I honestly could not answer.

Last edited by Nitro Blast; 11-05-2009 at 03:25 PM.
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Old 11-05-2009, 02:58 PM
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JetPlaneFlyer
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If your motors can spin backward then any prop can be used as a pusher.. A pusher prop is really just the same as a tractor except it's designed to spin in the opposite direction. This is required for an IC pusher because IC engines generally only run properly in one direction.

So yes, the pusher will work fine for you if you want the props on your twin to counter rotate. Just make sure you fit them the right way round.

Steve
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Old 11-05-2009, 03:22 PM
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Nitro Blast
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Thanks. For clarification, all brushless motors can spin backwards. They have no designed 'direction'.
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Old 11-09-2009, 04:44 PM
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Sparky Paul
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Merely reversing the direction of rotation on a conventional prop results in all the air being moved forward, instead of aft.
On the AC-47, you will need a prop manufactured as a pusher prop, with the direction of rotation of the motor reversed as well.
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Old 11-12-2009, 11:55 PM
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Nitro Blast
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Default The Props Arrived!

Lookie what Hircflyer suprised me with today!

A Box from TOWER HOBBIES, and inside, a pair of props, one is a pusher! THANKS DAD!





I took a sharpie marker pen to the white control horns, this was the test. Looks ok enough to do the aileron horns too.


I also replaced the chrome screws with all the black ones I could find...
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Last edited by Nitro Blast; 11-13-2009 at 12:36 AM.
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Old 11-13-2009, 12:48 AM
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Sparky Paul
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You might want to swap props.
Having the down-going blade closest to the fuselage is better for single-engine handling, in the real world.
There may be no significant difference with your plane, as the motors are pretty close to the fuselage centerline anyway.
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Old 11-13-2009, 12:49 AM
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gramps2361
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Hey Nitro looks a lot better with the three blade prop looks more scale. Have you had a chance to fly it yet with the new props?
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Old 11-13-2009, 02:37 AM
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crxmanpat
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Originally Posted by Sparky Paul View Post
You might want to swap props.
Having the down-going blade closest to the fuselage is better for single-engine handling, in the real world.
There may be no significant difference with your plane, as the motors are pretty close to the fuselage centerline anyway.
Funny thing is, full-scale P-38s were exactly the opposite. I can't remember why they did it that way on the P-38, but there was a reason.

EDIT - Found the reason. With normal rotation, the P-38 suffered from severe buffeting. Kelly Johnson fixed the problem by making the props rotate opposite of normal (blade closest to the cockpit going up instead of down).


Nitro, Spooky looks sierra hotel now with those black 3-blade props and black screws! Now you should get some OD paint and paint the black screws.
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Old 11-13-2009, 02:49 AM
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Nitro Blast
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Thanks for the kudos guys.

G, I taxied it around today to see that the tailwheel was straight, but I'm saving it for Saturday's Veterans Day Scale event at Miramar.
Gonna display it with the flight crew.
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Old 11-13-2009, 03:14 AM
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A lot of the twins built back then had horrible single-engine handling qualities with the average pilot at the controls.
The design specs today would result in markedly different appearing airplanes built to the same performance requirements, with much larger verticals being the most obvious difference.
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Old 11-14-2009, 12:03 AM
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Default 2/3rds scale P-38

I just got this beauty off the 'net.
Designed and built by Jim O'Hara and his wife Mitzi in San Angelo Tx.
He'a an aeronautical engineer.
Note the prop(er) rotation on the props.
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Old 11-14-2009, 12:46 AM
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Nitro Blast
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Originally Posted by Sparky Paul View Post
Note the prop(er) rotation on the props.
Noted, thanks!

Simple enuf fix!
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Old 11-18-2009, 11:07 PM
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quorneng
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A small scale point but did a C47/DC3 ever have counter rotating props?
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Old 11-18-2009, 11:19 PM
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Originally Posted by quorneng View Post
A small scale point but did a C47/DC3 ever have counter rotating props?
No. But they were each independently controlled, which allows for power adjustments not available to my model.


On a side note...

With the counter prop, and the gyro on the rudder, Spooky is exceptionally solid now.





(on yet another side note, I do however have counter-rotating props on my boat and that made an amazing difference in tracking)
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Old 11-19-2009, 12:15 AM
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Sparky Paul
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Originally Posted by quorneng View Post
A small scale point but did a C47/DC3 ever have counter rotating props?
.
The US Navy Test Pilots School at Patuxent River, MD put a C-47 thru their acceptance tests, and found 13 Safety of Flight items that would have prevented the plane from being certified according to modern standards.
Those things do crash a lot.
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Old 11-19-2009, 11:57 AM
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Originally Posted by Sparky Paul View Post
You might want to swap props.
Having the down-going blade closest to the fuselage is better for single-engine handling, in the real world.
There may be no significant difference with your plane, as the motors are pretty close to the fuselage centerline anyway.
you are correct here. in fact, the advantage of having downgoing blade closer to the fuselage is more important, the closer the props is to the fuse.

this is because when the upgoing blade is closest to the fuse, it "kicks" disturbed air up to the rudder.
the closer the prop is to the fuse, the more of this air gets "kicked" up in the rudder.
having downgoing blade closer to the fuse, of course, kicks this disturbed air downwards, away from the rudder.

the dehavilland mosquito was originally designed with upgoing blade closest to fuse, but rotation direction was changed because of this rudder problem.

Originally Posted by crxmanpat View Post
Funny thing is, full-scale P-38s were exactly the opposite. I can't remember why they did it that way on the P-38, but there was a reason.

EDIT - Found the reason. With normal rotation, the P-38 suffered from severe buffeting. Kelly Johnson fixed the problem by making the props rotate opposite of normal (blade closest to the cockpit going up instead of down).


Nitro, Spooky looks sierra hotel now with those black 3-blade props and black screws! Now you should get some OD paint and paint the black screws.
p-38 has twin rudder, wich is directly behind the propeller anyway.
therefore the argument of keeping the rudder out of disturbed air is not valid for this plane.
rudder will be in the propeller slipstream equally much, no matter what direction prop rotates, on this particular plane.
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Old 11-19-2009, 04:41 PM
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Sparky Paul
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I used to talk to Tony LeVier at work and at the retirees get-togethers, but never got the chance to ask him about this.
The 2/3rds scale P-38 has the proper inward rotation, but, it has no radiators behind the wings on the booms.
That could make a difference in single engine handling.
The twin Beeches I've flown in is have both props turning the same way.
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Old 11-19-2009, 05:05 PM
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Multi-engine airplanes have a critical engine, in most cases, the left engine. When the left engine, thrust of the good engine caused an imbalance in yaw which can exceed the ability of the rudder to correct.

When one of the engines on a typical multi-engine aircraft becomes inoperative, a thrust imbalance exists between the operative and inoperative sides of the aircraft. This thrust imbalance causes several negative effects in addition to the loss of one engine's thrust. For reasons listed below, the left engine of a conventional twin-engine propeller-driven aircraft is typically considered critical.

Contra rotating engines allow a twin to be built with no critical engine. On the P-38, the engines were contra rotating, but in the wrong directions, for the reasons Pat stated. That means that on the P-38, both engines are critical engines.

But hey, those pilots were really good, what with their 100 hours of total flight time when the sat their butts down in a P-38, single seat, fighter for the first time. Not all of them made it.
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Old 11-19-2009, 05:12 PM
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By the way, Jim, I suggested the three bladed props and it really looks nice. I also suggested you paint the tips yellow for more realism, since most props in the old days had yellow tips for visibility. People still walked into the prop arc anyway.
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Old 11-20-2009, 12:19 AM
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quorneng
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On high powered twins its not just the thrust imbalance but the torque as well.
Initially the P38 was considered unflyable if either engine failed just after take off. There was insufficient aileron authority at low speed to prevent it rolling in.
The solution was to reduce power on the good engine (I bet that took some nerve to do!) fly level and let the airspeed gradually increase to a point where full power could safely be applied and a climb resumed.
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Old 10-24-2010, 06:12 PM
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pattern14
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Default Trial and error

Initially I was told that I simply had to put my 7x5e APC tractor prop on back to front, and then reverse the rotation to achieve a counter rotating effect. This was to be used in a pusher twin configuration. However, when I did this, the amount of thrust was clearly different. The Tractor prop that was reversed had noticeably less thrust. So I purchased a couple of 7x5p APC dedicated Pusher props and problem solved. I was amazed that so many people had given me the incorrect information, but often times you just find these things out through experimentation
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Old 10-24-2010, 06:39 PM
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Not exactly clear on what you posted, but electrics generally have no need for a pusher prop in a pusher installation.
The prop is mounted for the usual ccw rotation (prop front facing plane front), and the motor rotation reversed.
A pusher prop can be be used in a tractor configuration (turning cw) with the motor rotation reversed.
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Old 10-27-2010, 02:18 AM
  #23  
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Default Try again...

Originally Posted by Sparky Paul View Post
Not exactly clear on what you posted, but electrics generally have no need for a pusher prop in a pusher installation.
The prop is mounted for the usual ccw rotation (prop front facing plane front), and the motor rotation reversed.
A pusher prop can be be used in a tractor configuration (turning cw) with the motor rotation reversed.
What I was trying to explain was the use of counter rotating props in a twin pusher design. I realise from my single engine pusher jets that a tractor prop OR a pusher can be used, but for a counter rotating pusher twin set up I need to use one tractor and one pusher spinning in opposite directions to eliminate torque reaction. Using both tractor props with one facing backwards and the motor reversed didn't work . The thrust was simply quite different.
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Old 10-27-2010, 02:30 AM
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Originally Posted by crxmanpat View Post
Funny thing is, full-scale P-38s were exactly the opposite. I can't remember why they did it that way on the P-38, but there was a reason.

EDIT - Found the reason. With normal rotation, the P-38 suffered from severe buffeting. Kelly Johnson fixed the problem by making the props rotate opposite of normal (blade closest to the cockpit going up instead of down).


Nitro, Spooky looks sierra hotel now with those black 3-blade props and black screws! Now you should get some OD paint and paint the black screws.
Hi
Thats how my Full scale " Forked Tail Devil" is setup
Sorry about the hijack
Take care
Hank

Last edited by kenchiroalpha; 08-15-2011 at 01:45 AM.
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