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Author Topic: Pushme Pullyou- propellor question  (Read 7576 times)

Brian60

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Pushme Pullyou- propellor question
« on: October 30, 2014, 12:50:32 pm »

Sorry but I can't get my head around this question,

Two props on one shaft one are they both the same rotation, ie left handed? or are they opposed one left and one right?

It should be simple to work out but for the life of me I can't do it! My head is saying opposite rotation, but a little devil on my shoulder keeps saying that is wrong. See the drawing to see what I mean..... note the direction of the propellor boss, I want them facing outwards......

Netleyned

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Re: Pushme Pullyou- propellor question
« Reply #1 on: October 30, 2014, 01:53:12 pm »

Bosses outwards gives youonepulling and one pushing turning bow port or starboard
Ned
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Brian60

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Re: Pushme Pullyou- propellor question
« Reply #2 on: October 30, 2014, 02:18:48 pm »

Ned its not port or starboard that is the question.

I was watching a discovery program that showed an airplane with this configuration, one on front of wing one on rear of wing. Its the blade rotation of the prop I wanted explanation of. To my mind the rearward one needs to push the air forward where the front prop pulls it forward.

Is that explained correectly or am I missing something? So the two props need opposite facing blades but turning in the same direction?

mrturbo553

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Re: Pushme Pullyou- propellor question
« Reply #3 on: October 30, 2014, 02:27:04 pm »

This any help
http://en.m.wikipedia.org/wiki/Tupolev_Tu-95
I think they would be contra rotating props and not on the same shaft
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Or Google image search
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Arrow5

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Re: Pushme Pullyou- propellor question
« Reply #4 on: October 30, 2014, 02:51:47 pm »

You are probably confused by the term "Pusher prop" . In aircraft this means that the propeller is at the rear of the engine or aircraft but it still pulls air in and pushes it back just as the propeller at the front of the aircraft does.  The normal anti-clockwise rotation of a model aircraft engine uses the "normal" tractor prop, a model engine facing the rear of the aircraft (same rotation) would need a so called "pusher" prop but both do pull and push. It is to do with the rotation of the engine. In your marine application diagram both props would have to be of the same rotation.  The aircraft you saw may be Dornier flyingboats, their engines could be of  opposite rotation to each other (or geared) to counter "torque reaction" ( another misnomer but it`ll do) or have opposed props as described above.  We have left hand and right hand pitch on boats, same with planes. hope this helps. I thought the question was going to be about the ferries that have props at both ends of the ship. How do they operate ? Are both props working no matter which direction the ship is moving ?
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NFMike

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Re: Pushme Pullyou- propellor question
« Reply #5 on: October 30, 2014, 03:48:54 pm »

The boss is irrelevant (unless it's SWMBO). As arrow5 says both propellers need to be the same 'hand' as they are turning the same direction.

The problem you'll have (unless it's a bow thruster sort of thing) is that most props aren't symmetrical; the blades are shaped (dished) to give best performance in one direction.

imsinking

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Re: Pushme Pullyou- propellor question
« Reply #6 on: October 30, 2014, 04:10:15 pm »

AND just to confuse you further , the Japanese are working on this arrangement . . . .

The lefthand main prop is Diesel driven / variable pitch , the righthand prop is electrically powered fixed pitch , I couldn't see the top but it must be an AZIMUTH POD arrangement which also doubles as the rudder  %%  & contra rotating, the inventor reckons 25% / 30% fuel saving [size=78%] .[/size]
[size=78%]Bill[/size]
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Arrow5

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Re: Pushme Pullyou- propellor question
« Reply #7 on: October 30, 2014, 04:55:42 pm »

A common enough arrangement in aviation, contra-rotating propellers .   Without the steering of course.
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malcolmfrary

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Re: Pushme Pullyou- propellor question
« Reply #8 on: October 31, 2014, 10:11:10 am »

With a prop on each end of the shaft, the blades will need to be pitched the same way when fitted.  With the bosses facing outward, this does mean one left and one right hand prop.  As a previous poster mentioned, blades on most model props are not symmetrical.  Matching the performance of the pair might be a problem.  This will apply whether they are on a single fixed shaft or a coaxial pair, when they can be counter-rotating, but will both then need to be the same hand.
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NFMike

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Re: Pushme Pullyou- propellor question
« Reply #9 on: October 31, 2014, 10:39:58 am »

With the bosses facing outward, this does mean one left and one right hand prop.

No. They will be the same handing.

Big Ada

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Re: Pushme Pullyou- propellor question
« Reply #10 on: October 31, 2014, 05:27:38 pm »

I am with Plague.
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Umi_Ryuzuki

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Re: Pushme Pullyou- propellor question
« Reply #11 on: October 31, 2014, 08:24:21 pm »

No. They will be the same handing.


+1... Same handing.
The rear propeller rotating will be pushing, say clockwise
The front propeller will be pulling(in reverse) or anticlockwise.


 O0

hammer

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Re: Pushme Pullyou- propellor question
« Reply #12 on: October 31, 2014, 10:34:54 pm »

If you call them screws not propellers you can see they will be the same if rotating in the same direction. Never mind about push & pull both screwing through the water.  This reminds me of my aero modelling days. Needing to drive a 16"prop with a 60 glow plug motor, it would have to be geared down. Problem this would then be rotating the wrong way  for a commercial prop. The solution turn the motor around, mount a shaft from the gears along the side of the motor, and mount the prop on the back of the motor. The 6ft Sopwith Camel  flew quite successfully for a few weeks. That was the time (69-72) of 27megs & CB radio, not a good combination.
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Brian60

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Re: Pushme Pullyou- propellor question
« Reply #13 on: November 01, 2014, 08:40:14 am »

Thanks for all the replies, lots of food for thought and can I say that to me its all still clear as mud %% {-)

Arrow5

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Re: Pushme Pullyou- propellor question
« Reply #14 on: November 01, 2014, 09:08:44 am »

Ah, now a prop designed for operating in mud....... %) :D :embarrassed:... :}
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Netleyned

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Re: Pushme Pullyou- propellor question
« Reply #15 on: November 01, 2014, 09:19:05 am »

Archimedes is the guy to blame for all this confusion >>:-( >>:-(
Ned
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sparkey

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Re: Pushme Pullyou- propellor question
« Reply #16 on: November 01, 2014, 09:29:52 am »

 :-)) Screw him! :-))
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jaymac

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Re: Pushme Pullyou- propellor question
« Reply #17 on: November 01, 2014, 10:08:03 am »

On an aircraft I was taught back in the day a rear prop was a puller as it pulled  Through the wing as opposed to a front one pushing it   its creating lift . There is no similarity with boats 
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malcolmfrary

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Re: Pushme Pullyou- propellor question
« Reply #18 on: November 01, 2014, 10:31:24 am »

Hand help up and an admission of too much thinking under the influence of thinking the brain was working properly.
Both the same hand.  Yes.   :embarrassed:
The problem will remain that with most commercially available props, the blades are asymmetrical, so it might be a case of needing to remove the pointy end of the boss to fit the front one with the blades not only pitched correctly, but not running in permanent reverse, which is likely to do nothing for efficiency.
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Arrow5

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Re: Pushme Pullyou- propellor question
« Reply #19 on: November 01, 2014, 10:58:43 am »

On an aircraft I was taught back in the day a rear prop was a puller as it pulled  Through the wing as opposed to a front one pushing it   its creating lift . There is no similarity with boats
   Correct term for an aircraft propeller at the front is "tractor"  at the rear it is a "pusher" . I don't think it was the wing but the whole aircraft that was being pushed or pulled.   The screws(water) and airscrews(air) both work in liquids( one of which is a gas) and use the same principle of pulling the medium in, accelerating it and pushing it rearwards.  Or have I been wrong all these years {:-{  A prop at the front of a ship (harbour ferry for example with props at both ends) will pull the ship, the prop at rear  will push the ship.  My question in post#4 , any Sydney Harbour ferry experts, I believe they have props at both ends.
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derekwarner

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Re: Pushme Pullyou- propellor question
« Reply #20 on: November 01, 2014, 12:26:46 pm »

arrow5....... we have our own OZ member John......"South Steyne2" who has modelled & steams the same namesake vessel  :-))....so lets wait & hear his comments .... Derek
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Arrow5

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Re: Pushme Pullyou- propellor question
« Reply #21 on: November 01, 2014, 12:36:27 pm »

Great Derek, trouble is he might explain it all in Strine and we will be non-the-wiser {-)
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rickles23

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Re: Pushme Pullyou- propellor question
« Reply #22 on: November 01, 2014, 12:38:54 pm »

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jaymac

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Re: Pushme Pullyou- propellor question
« Reply #23 on: November 01, 2014, 12:48:47 pm »

Was not talking about the wing being pushed  but about airflow  through the wing ie over and under it rear prop is pulling it  front prop is pushing it.
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Arrow5

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Re: Pushme Pullyou- propellor question
« Reply #24 on: November 01, 2014, 12:57:17 pm »

How about pylon mounted engines then ?
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