When is a pusher prop not a pusher prop?

[Rob Lewis]

I recently built a model that needed a pusher prop. I brought the required prop, a 10x7P and although i thought something looked strange i thought "it must be right".

Anyhow long story short i haven't managed to get the model in the air yet, with the thought being lack of thrust or too heavy / too much drag.

One thing i decided to change was the prop. I ordered an 11x6P and realised what was wrong with the first prop and maybe why the model hasn't flown....

The top prop although labelled a 10x7P looks to me like a clockwise rotating tractor prop. As i was using it as a pusher, it was effectively rotating backwards, with the trailing edge leading. I'm betting it would be a lot less efficient rotating backwards.

Its not just me is it?

[Chris Bott - Moderator]

It's hard to tell from the pics Rob, but they both look to have pitch in the same direction.

I would suggest though, that the top one needs to be mounted the other way up to how you've placed it in the photo, while the bottom one looks the right way round.

That is, assuming the engine they go on has it's prop shaft pointing towards us. If you see what I mean?

[Rob Lewis]

You know what....I hadn't even thought of turning it over. Doh..... I've always used props the way round as shown, with the writing pointing outwards. The other side of the prop has the cutouts at the hub as usual, i guess theres no reason it can't be mounted upside down so to speak.

You learn something everyday... Cheers Chris

[Peter Miller]

In the old free flight days we used to fit the prop on backwards to reduce the power for trimming.

A lot of the thrust is generated by the airfoil section, not just the angle of the blades.

That should explain your lack ofthrust.

[Mowerman]

The writing (size ) should always face the front of the model. For electric models where the motor can run in either direction the term 'pusher prop' need not apply, however if using IC where the direction of rotation is fixed then a 'pusher' will be required.

[Rob Lewis]

I've never seen IC props being fitted backwards (writing towards engine), but looking at it now that definately seems the only way that prop will work. Strange that they are from the same range of APC pusher props but are different.

Either way at least the lack of thrust should be sorted and the model should fly this time round.

[eflightray]

If you're flying electric why not use an APCe prop, (ones designed for electric power). Those props shown look like the standard IC props, (rather heavy).

If you consider a tractor layout, (prop at the front), then for a 'pusher', imagine the prop just travels down a long shaft to the rear, it doesn't need to turn round.

Electric motors are easy to reverse, so 'reverse pitch ' props are just not needed.

 

Edited By eflightray neath on 16/09/2014 20:03:55

[eflightray]

Depending what the model is even a 'folding' prop can be used at the back. It just folds back with the slipstream similar as at the front.

BUT, it needs a method of stopping it 'over folding', (can explain if required).

[Dave Hopkin]

Surely the writing on the prop is in relation to the direction of intended movement - ie on a tractor prop, the writing is at the front, on a pusher the writing is still on the front, but as its a tractor the front will be facing the engine/motor

[Rob Lewis]

Some interesting points.

efliteray - Its an IC engine so reversing the motor or folding props are out....

Dave - In the pic above both the props are with the writing facing up, and they are both APC pusher props. So the top one would have the writing facing towards the engine and the lower one would have the writing facing away from the engine for them to work. I couldn't say which one is the norm as they are the only 2 pushers i have ever brought, just seems strange to me that there doesn't seem to be a convention. Either that or i have stumbled across a somehow mismoulded prop.

[Chris Bott - Moderator]

Just a though Rob, could the top one be a tractor prop but for opposite rotation to the norm, far scale reasons? (It's a scale-ish sort of shape.) Or indeed for some other reason.

[eflightray]

"Its an IC engine so reversing the motor or folding props are out...."

As this 'Propeller Selection' forum is in the 'Electric Flight' group of forums, I assumed wrongly

[Rob Lewis]

Posted by eflightray neath on 16/09/2014 20:42:28:

As this 'Propeller Selection' forum is in the 'Electric Flight' group of forums, I assumed wrongly

Oops.... When i started the thread i just scrolled down the "select a topic" list and thought prop selection sounds about right. Didn't realise it was in the leccy section. My appologies.

Could well be Chris. Thats what it looks like. I don't have the packaging anymore but the writing on the prop just says 10x7P which is the same format as on the 11x6P. Just checked the site i brought from and theres no special info attached to the prop either. Who knows....

[Peter Beeney]

Rob,

From the photo it looks to me as though the top prop, the 10 x 7P, actually is a pusher. Assuming this is all standard counter-clockwise rotation from the front, which I’m sure it is, then a standard tractor prop would have the leading edge away from the engine and the trailing edge closer. A pusher will have the leading edge closer to the engine and the trailing edge away; also the airfoil section is on the back, but this is now the front anyway, so to speak, just as in the picture. It’s of little use putting a standard prop on back to front, it’s still a tractor but it simply reduces the efficiency by about 90%; however, if you could now run the engine clockwise at this point it would indeed be the perfect pusher; … and by the same token, of course, if you bolted a pusher on back to front and reversed the engine direction again it would be a tractor. As it so happens, the lower 11 x 6P also appears be the same, too.

So maybe it comes down to what size of engine and model this is? What sort of power is produced and crucially perhaps, at what revs? Does this have some sort of bearing on the matter? To me the prop seems ok, is there another factor at work here?

Good luck with it all…

PB

[reg shaw]

Just a thunk, does the manufacturer make opposite rotation tractor props, say for twin engine use where one goes clockwise and the other goes anticlockwise, P38 Lightning stylee?

Ian.

[Chris Bott - Moderator]

That's pretty much what I was thinking Ian, but then, it's marked 10x7P. (P for pusher)

[Rob Lewis]

Thanks for the detail Peter. When the top prop was bolted to the engine i had the writing pointing away from the engine as i normally would, so when the engine was running counter-clockwise (viewed from front) in effect the prop was running backwards with the trailing edge leading into the airflow. If i flip the prop over and have the writing facing the engine then it becomes the perfect pusher as you described with the leading edge closer to the engine.

The second prop by contrast would need to be bolted to the engine with the writing facing outwards for the leading edge to be closer to the engine. This was what caused the confusion.

Incidently this is for a Flexifoil windbag that i have built

The plans call for a 0.40cu engine. I have a Super Tigre 45 on it that i had in my spares box. With the 10x7P on (running backwards) i was getting around 11,000 RPM at full throttle, so should be more than enough power for the model, if the prop is correct.

Tonight i will try the 11x6P or maybe the 10x7P backwards and see if it makes any difference. Thinking about it logically, it makes sense that power would be down with the prop effectively running backwards, so hopefully all problems will be resolved tonight.

I brought 2 of the 10x7P's but from the same supplier at the same time (always carry a spare), they are both the same, but i guess they could always have been a bad batch?

As with Ian and Chris I assume clockwise tractor props are available, will have to do some research and see if it sheds some light.

[Peter Beeney]

Thanks for your reply, Rob, but I’m now somewhat confused. I’m by no means convinced about all this, though, I have to say, but without being able to see it all I’ll wait till you get some results. 11k on the 10 x 7 should be plenty of power, I’d have thought, but a Flexifoil may be slightly tricky to get sorted for a first flight, perhaps. Something I’ve never had any experience of as yet.

At the end of the day I still think your best success with those props will come with the front of the prop boss next to the nut, as in the photo - but as always, I do stand to be corrected.

PB

[reg shaw]

Hi rob, thats a familiar looking machine you have there!! Its exactly the same as the old 'Windbag' from I'm guessing the late 80's early 90's. I remember mine fondly but sold it on a few years ago.

Are they still available?

Ian.

[Rob Lewis]

Hi Peter. Just going back to your first post, you said that a pusher prop will have the leading edge closer to the engine. I'll repost the pic so that we don't have to keep scrolling to the top.

Both of these props are pusher props, and both are placed the same way up. My understanding is that the top prop, the 10x7P, if mounted so the surface we can see is next to the prop washer / nut, will have the props aerodynamic leading edge furthest from the engine and hence will be leading with the trailing edge first when the engine is running counter-clockwise.

Conversely the lower prop, an 11x6P when mounted the same way will have the aerodynamic leading edge closest to the engine as you described, and will be leading with the leading edge when the engine is running.

If as Chris suggested the 10x7P is turned over, so the face we see is against the prop driver, then the prop will lead with the aerodynamic leading edge as required.

I've probably over described a lot of this, but it just about makes sense to me.....

Have we crossed paths or am i not understanding your explaination fully?

Cheers Rob

[Rob Lewis]

Posted by reg shaw on 17/09/2014 14:23:03:

Hi rob, thats a familiar looking machine you have there!! Its exactly the same as the old 'Windbag' from I'm guessing the late 80's early 90's. I remember mine fondly but sold it on a few years ago.

Are they still available?

Ian.

Yep its the same machine Ian. I stumbled across the old plans and have always been keen on things that are different. I brought a 2nd hand Flexifoil Stacker 6 kite from ebay and built the rest.

I've still got the plans saved on my laptop if you fancy a copy. I had to print it out on loads of A4 sheets and tile them together but it got the job done. PM me your email address and i'll forward them on if you like.

[Rob Lewis]

Well the good news is, the Windbag flys. Used the 11x6P and she was airborne within about 10 meters. Can't believe something so simple kept it grounded for so long. At least it was a cheap fix. Bad news was it rolled over on landing and broke the canard, only a couple of hours work to repair though. All in all a sucessful outing.

[Peter Beeney]

Nice one, Rob, very pleased you’ve now had a good success with it. They are certainly rather different looking little beasties, and I suppose that Flexifoil wing has a fair amount of drag, too. I’m still not sure why the 10 x 7P doesn’t develop enough thrust though, and I’m not yet quite convinced it’s a clockwise tractor, but there does appear to be something odd about it. But, as you’ve now found exactly the right answer, that’s brilliant.

Well done indeed.

PB

[Rob Lewis]

Hi Peter, I was having a think about our discussion and think i may have realised where we crossed wires.

When you were talking about the leading edge of the prop being closest to the engine on a pusher, i think you were refering only to the pitch of the propellor?

I was talking more about the aerofoil shape of the prop. Obviously the prop to all intent is just a rotating wing and has a leading and trailing edge. My thoughts are that although the pitch of the prop was okay and produced thrust in the correct direction, the aerofoil section was opposite to that needed for the pusher prop. So when rotating the prop was leading with its aerodynamic trailing edge, like trying to fly an aircraft with the wing on backwards.

Maybe that will clear things up? (Or open an even bigger can of worms?)

Either way, its great to get the Windbag in the air. There is definately a lot of drag from the Flexifoil, with full throttle applied it climbs at about a 45 degree angle and i think that is mostly because of the wing holding it back. The guys at the club loved seeing it fly as well.

Rob

[Peter Beeney]

Rob, I’m sure you’re right. I assumed, without thinking too much about it, that both props were anti-clock rotating. However, if I also now consider that the printed size figures near the hub are always on the leading edge then as you say that would appear to definitely make the top prop clockwise rotating. Thus it becomes a tractor, a mirror image of an anti-clock tractor, but from the photo the top camber doesn’t look quite right for this, it certainly gives me the impression it’s now upside down. … But I could be wrong again… Perhaps you can see better if you examine it. However, if the above is true it is indeed going backwards on a normal engine….

Also I’d agree the prop is a rotating wing, but because it’s rotating it has a twist in the blade to keep the pitch constant along it’s length. The angle sharpest at the root, flattening out to the tip. The aerodynamic shape or airfoil camber on the front of the blade gives it a degree of lift to assist with the angle of attack. I guess props can be manufactured to suit any situation, that’s one of four, I think. But I’m sure you already know all this too…

With regard to the photo and constant referring back to this, I just save it to the desktop, and then I can leave it in view, plus to some extent I can manipulate it as well; and I don’t have to have the browser open, either.

We sometimes have a local lad flying nearby with a full size Flexifoil wing, powered by the motor and prop strapped on his back. He was flying today, near the strip but out of model range. Mostly at around treetop height or lower; occasionally he goes for height, which takes him a fair old climb up.

PB