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pusher prop weird info ?

pusher prop

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iawnski15/11/2012 14:50:23
1076 forum posts
28 photos

just been told by a supplier that if i need a pusher prop that all i have to do is turn a normal one round ? is this right / mmmm,,, 8x6 is what i have standard prop for icfrown what are u thoughts

Erfolg15/11/2012 14:58:50
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11780 forum posts
1340 photos

I do remember that some people have used a standard prop on a IC in pusher model. This did require the motor to run in the opposite direction, which for most engines, required the timing to change. Again this could be done with some engines by rotating the inlet, be it the front assembly on a shaft induction, with the carb at the front, or te rear assembly on a rear induction. With a reed valve (normally only seen on small engines) start in the opposite direction.

Of course for electric motors, dead simple, just swap wires about.

Peter Beeney15/11/2012 15:11:17
1593 forum posts
59 photos

lawnski - I think you are talking i/c so an engine on the back with a standard prop fitted nornally will just pull the plane backward. Turning the prop round will still pull it backwards, but with much less enthusiasm. However, if you reversed an electric motor in the same circumstances, a reversed standard prop will push it forwards.

PB

Ben B15/11/2012 15:30:57
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1435 forum posts
4 photos

As Peter says, for IC you need a pusher prop because the engine can only rotate one way. With electric you can just swop two motor leads to get it running the other way and use a normal prop backwards.

colin sayer15/11/2012 15:32:29
30 forum posts
3 photos

talking of electric pushers can anyone tell me if you have to offset the motor to counteact the torque effect

Erfolg15/11/2012 16:32:35
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11780 forum posts
1340 photos

Some IC motors can be amended to run in reverse. Granted not all. On a front induction, the assembly needs to be a bolted on assembly. This assembly needs rotating by 90 degrees. The same is done with rear induction. As stated reed valves, will rotate in either direction (small Cox etc).

In the case of a pusher, side thrust and down thrust is no different to any other model in principle. It is a matter of the drag forces on the airframe and where the motor is mounted, hence its thrust line. In most case, the thrust line normally required is a thrust line which is inclined up, where as most tractors with high wing point down. I tend not to bother with side thrust, although most of my pushers tend to want to go left on full power, so a bit of right thrust would help here. In most cases thrust lines area compromise, for a speed, and will not be perfect for all speeds.

Cliff Bastow15/11/2012 16:37:35
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902 forum posts
464 photos

I have two models with pusher props. both electric. One is a nijuhs mig 25 and the other is a canard. both have the motor mounted without any offset and fly perfectly.

iawnski15/11/2012 17:09:03
1076 forum posts
28 photos

sorry forgot to mention tha i am using a electric motor on a flying wing that needs a pusher prop crook so were do we stand now

Peter Beeney15/11/2012 17:31:11
1593 forum posts
59 photos

Change two motor wires over so that the motor runs clockwise, then put the prop on ‘back to front’; so that the front of the prop is against the driver. Then the prop is running the right way, but pushing the motor/plane instead of pulling.

PB

Erfolg15/11/2012 17:34:26
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11780 forum posts
1340 photos

It all depends.

Being more commutative, the thrust line on many flying wings tends to be above the wing. or inline. So it all depends where the wing actually can be considered to pivot. There will normally be a pitch down from the wing itself, pretty much balanced by the reflex. The wings drag, will pretty much be in line with this pivot point, often the thrust line is also arranged to be be in line with this point. So in libne will do as a starting point.

Then again, sometimes it is convenient to have the motor mount effectively above the point, this will introduce a turning moment pitching the model nose down. So we need a vector upwards, yo counter act this moment. Pointing slightly up will pride a vector which can be arranged to balance the force. Yet given the distance is so small on most wings, it probably will be next to nothing.

iawnski16/11/2012 08:26:07
1076 forum posts
28 photos

cheers guys for the infowink

Willy Hardy19/11/2012 14:35:41
2 forum posts

Hi Regards the pusher prop question.I built a Supermarine Walrus a few years ago which has the engine mounted as a "pusher" and bought a left-hand prop for it.Saves all the mcking about with props fitted "backwards"which do not give enough power .

Erfolg19/11/2012 15:23:44
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11780 forum posts
1340 photos

Willy

A conventional prop is not actually put on backwards relative to the direction of travel. It is put on backwards relative to how it would be fitted if a tractor model.

The issue then is with a conventional prop, is that it will not work properly. that is unless the rotation of the engine or motor is reversed.

With your Walrus you avoided the issue.

Unfortunately there is a limited range and availability of pusher props.

On this basis, reversing the rotation of the motor is the solution. Not an issue for electric motors, requires some work if an IC engine, and not all IC engines can be arranged to run clockwise.

Peter Beeney19/11/2012 15:43:19
1593 forum posts
59 photos

Willy, - When you fit a standard prop on an electric motor that is turning clockwise and is at the back of a model facing backwards, provided that prop has it’s front against the driver, for the same amount of revs it will provide exactly the thrust as if it’s on the front turning anti-clockwise. It’s exactly the same situation. If you consider an i/c engine, looking at it from the back, the crankshaft is turning clockwise. If you could fit a standard prop on this extended crankshaft, with the front of the prop to the back of the engine, that’s now a pusher prop!

Any prop that that is put on back-to-front in any circumstance will severely reduce the thrust. Once upon a time this was recommended with first flights, but I suspect was soon to be seen as a really big mistake; and in some circumstances the prop could be incredibly noisy!
I assume by left-hand prop you mean one designed for i/c engines that run clockwise from the front? I’m not quite sure how you fit this facing the other way, and still make the plane go forwards?

As I see it, provided the prop is turning in the direction that it’s designed to to turn, it doesn’t matter if it’s being driven from the front or the back.

PB

Simon Chaddock19/11/2012 16:34:42
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5730 forum posts
3034 photos

Its amazing how many people build planes that had Gipsy engines and do not realise it rotated the opposite way - clockwise viewed from the front!

Like this. Scale to the nth degree but which way round does the prop go?

Peter Beeney19/11/2012 20:45:38
1593 forum posts
59 photos

Simon, - Maybe the engine rotation direction reasoning goes back a long way. As I remember, according to The Tiger Moth Story, the de Havilland engines were designed by Major Frank Halford, who was really exceptionally good at it. All motor vehicle engines turn clockwise, facing from the front, and as he also designed motor car engines, such as Aston Martin, maybe this was the reason. This is a guess, but I’d tend to think that there isn’t a really outstanding reason why the engine shouldn’t turn in either direction, or at least back in those times.

When road vehicles first appeared, for many years they were generally started with a crank handle in the front, and this is again all supposition, but for a right handed person this is easiest if the engine is turning clockwise from the front. So is it possible it’s as simple as that? All orientation is taken from the drivers seat though, so if you spoke about the right hand side of the engine it’s looking at it from the back; and this follows from sitting on a horse, nearside and offside, which are now nearside and offside from the back or the front. In fact, I guess in some quarters (!) cars still have a nearside and offside, as looked at from any direction.
Closer to the present, little diesels started to appear in droves after World War II, and they were all started by flicking the prop. No starters. It seems to me it’s naturally easiest for a right handed person to flick it backwards to the left. So is this the reason model engines turn anti-clock?

One of the big advantages of electric motors must be the ease at which the motor direction can be reversed, in some circumstances this makes them very flexible. For an i/c engine you have to have a pusher prop if it’s installed on the back of the wing, for an electric motor you can just reverse the direction!

PB

Shaunie21/11/2012 22:43:54
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951 forum posts
78 photos

It appears to be a common convention for engines to rotate anti-clockwise looking into the drive end. So with a conventional north-south car installation that would be anti-clock going into the gearbox and thus clockwise at the front end. Whereas aircraft engines would be anti-clock at the front where the prop is. But it is only a convention so there are exceptions as we see. God only knows why Rolls Royce chose to rotate the Griffon engine in the opposite direction to the Merlin.

Shaun

Simon Chambers21/11/2012 22:52:21
789 forum posts
42 photos

One thing to point out is that if you reverse a electric motor as a pusher, the prop adapter threads are not reversed! So they have a tendancy to want to throw off their prop nuts when you throttle up hard...

I usually double nut them and if a lot of power is going through, to locktite it too.

Si.

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