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Electric Stunt => Gettin all AMP'ed up! => Topic started by: John Cralley on April 23, 2012, 08:04:45 PM

Title: Prop rotation on a twin
Post by: John Cralley on April 23, 2012, 08:04:45 PM
OK, I'm building an electric twin and I'm wondering about how to set up the prop rotations. I can do outboard motor tractor and inboard motor pusher (or vise versa) or I suppose I can make both pushers or both tractors.

I'm hoping that wiser folks can advise me on how to go (course I can just do the old trial and error method mw~ HB~>). I have been advised that if I do counter rotating I should have the inboard in pusher mode mainly to smooth take off.

Anyone have experience in this area??

John
Title: Re: Prop rotation on a twin
Post by: Tim Wescott on April 23, 2012, 08:10:36 PM
Try both.  From the history that I have the P-38 was designed with the props rotating with the tips going together at the top, so that if one engine went out the torque and yaw effects would tend to cancel.  Then they found some nasty turbulence effects over the horizontal stab that they solved by swapping the engines.  It guaranteed that the handling would be vicious no matter which engine went out, but when both were going the plane worked just great.
Title: Re: Prop rotation on a twin
Post by: John Rist on April 24, 2012, 01:12:18 AM
With electric twin you are not as concerned with engine stoppage.  I would try two pushers for max line tension.  Should help especially in high flight.
Title: Re: Prop rotation on a twin
Post by: John Cralley on April 24, 2012, 06:16:20 AM
Thanks for the replies:

Tim, Trial and error is tried and true. It is just that after umpteen hours of building I want to avoid an ERROR that re-kits my model!  HB~>

John, I understand the pusher prop use on a single motor model has advantages. However, (and I could be wrong) my understanding is that the line tension in maneuvers is the result of the way the slipstream from the prop impinges on the wings and tail feathers. With a twin and two slipstreams I am confused as to what the effects (if any) are going to be. That is why I am wondering if others who have electric twins have experience in this regard.

Title: Re: Prop rotation on a twin
Post by: John Rist on April 24, 2012, 06:37:30 AM
Thanks for the replies:

Tim, Trial and error is tried and true. It is just that after umpteen hours of building I want to avoid an ERROR that re-kits my model!  HB~>

John, I understand the pusher prop use on a single motor model has advantages. However, (and I could be wrong) my understanding is that the line tension in maneuvers is the result of the way the slipstream from the prop impinges on the wings and tail feathers. With a twin and two slipstreams I am confused as to what the effects (if any) are going to be. That is why I am wondering if others who have electric twins have experience in this regard.


Some one smarter than me will have to address slipstreams.  In my mind the major factor is rolling moment due to engine tork.  However the slipstream may be huge.  The only twin I have ever flown was a DO-335 and it had a tractor up front and a pusher in back.  The pusher was achieved with a left handed crank in a .049 TD.  SO the tork was wrong for both engines but it had good line tention.

With the electric and a throttle testing can be short flights so go for it.  Test - test - and more test.
Title: Re: Prop rotation on a twin
Post by: Dennis Adamisin on April 24, 2012, 06:38:55 AM
Ya know the only combo I would avoid is two RH tractor props!

I read where the original P-82 Twin Mustang was set up "normal" (RH prop on left side, LH prop on right side) with the tops of the props coming together - it would not take off because the props downwash over the center section.  They swapped the motors side to side to fix.  My Brodak F-82 is set up that way to be "scale".

Also consider this: if that carried into your twin, it would suggest that your airplane might turn better one way or the other, and swiching the props would counteract that?

I am also on John R's page, can't say that you will really need it but two LH props should maximize the effect - but will that be too much?

 
Title: Re: Prop rotation on a twin
Post by: Tim Wescott on April 24, 2012, 07:55:54 AM
Tim, Trial and error is tried and true. It is just that after umpteen hours of building I want to avoid an ERROR that re-kits my model!  HB~>
I very much doubt that the effect will be enough to crash.  I kinda doubt that, if I were the guy on the handle, that I'd be able to tell at all.

So I think you're safe playing some games with it, as long as you always make sure that you have the right props on.
Title: Re: Prop rotation on a twin
Post by: John Cralley on April 24, 2012, 08:02:46 AM
I very much doubt that the effect will be enough to crash.  I kinda doubt that, if I were the guy on the handle, that I'd be able to tell at all.

So I think you're safe playing some games with it, as long as you always make sure that you have the right props on.

Tim,  Check my signature below!!!   ;D mw~ HB~>
Title: Re: Prop rotation on a twin
Post by: Tim Wescott on April 24, 2012, 08:27:29 AM
Well, then, should I have said that any crashes that result from playing with rotation direction will be lost in the general noise?

If you're just at an ordinary level of skill, the best thing to do may be to just fly with two conventional rotation props, and be happy.
Title: Re: Prop rotation on a twin
Post by: Howard Rush on April 24, 2012, 01:13:56 PM
Bob Hunt was working on a twin.  I'll give you the same advice I gave him.  Beware of getting the polarity of the props and motors mixed up:

motor 1 rotation   motor 2 rotation   prop 1  prop 2  outcome
        L                       L                  L         L         OK
        L                       L                  L         R         bad
        L                       L                  R         L         bad
        L                       L                  R         R         bad
        L                       R                  L         L         bad
        L                       R                  L         R         good
        L                       R                  R         L         bad
        L                       R                  R         R         bad 
        R                       L                  L         L         bad
        R                       L                  L         R         bad
        R                       L                  R         L         good
        R                       L                  R         R         bad
        R                       R                  L         L         bad
        R                       R                  L         R         bad
        R                       R                  R         L         bad
        R                       R                  R         R         OK

I wouldn't risk it.
Title: Re: Prop rotation on a twin
Post by: John Cralley on April 24, 2012, 07:04:48 PM
Thanks Howard,

If I read your chart correctly, counter rotation of the props is good. Just be sure that the correct prop (to give forward thrust) is mounted on the correct motor.  y1

I sent Bob an email with this question but have received no reply (I thought that he may have finished his twin by now).
Title: Re: Prop rotation on a twin
Post by: Dean Pappas on April 28, 2012, 01:55:45 PM
Hi John,
You will enjoy the same benefit as with a single, with both motors and props as pushers.
The two spiralling airstreams essentially merge into one as long as they are not too far apart.
later,
Dean
Title: Re: Prop rotation on a twin
Post by: John Cralley on April 28, 2012, 04:11:04 PM
Thanks Dean,

That answers my question!! Seems that I can either use counter rotation or, preferably, have both motors spinning pusher props.  ;D
Title: Re: Prop rotation on a twin
Post by: pat king on May 01, 2012, 08:26:42 AM
John,
I don't think rotation direction will make a great deal of difference on a C/L airplane, especially one that is not trying to fly a competition pattern. With your motors it is easy enough to experiment.

Pat