Sidewayskiwi
What is the thrust difference between a front mounted "pull" application and a parkjet prop in slot "push" application
As power tables for some of my motors fall well short of what's advertised
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Wildthing
I think you would have a slight more thrust on a pull setup just because you don't have anything obstructing air flow to the prop
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solentlife
AS WT says - I would assume tractor prop has more 'pull' .... thrust on the other hand ? How much does a 'body' behind the prop hinder thrust ?

I know a pal of mine with the big Fokker Triplane had to increase prop size dramatically to overcome that cowl.

Most prop in slot blades have air flow directed to them via open fuselage ... and minimal behind ... 

I would then suggest that difference is minimal.

I have a Schumach F15 as tractor and it flies just the same as the prop in slot I had before ... same prop / motor etc. Only difference being where battery sits.
I'll fly anything if I can launch it ! Youtube : solentlifeuk
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DualDesertEagle
I think it all depends on how much stuff is in the way. A Gee Bee R2 with its fat fuselage and the prop barely protruding is obviously gonna have alot less thrust than a plane with a very slim fuselage spinning the same prop at the same RPM.

Same goes for profile vs full fuselage. With a profile plane there's barely anything in the way beside the motor itself while a scale fuselage does obstruct the airflow a little. And of course the shape matters too. Quite obviously the front of an I-16 is gonna hinder the airflow a bunch more than the sleek nose of a Spitfire.

And provided the obstruction narrows down towards the prop at a reasonable angle one should still get away with most of the thrust even with fuselages as big as that of the Gee Bee R2.
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solentlife
Luckily its the outer part of the blade that does the work ... 
I'll fly anything if I can launch it ! Youtube : solentlifeuk
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quorneng
The conventional view is that it is not so much "obstruction" that reduces the efficiency of a pusher prop as the disturbed airflow. This means the airflow at the prop leading edge is locally continually changing direction reducing the prop efficiency. With a tractor layout the prop is always facing undisturbed air at a constant angle.
An advantage of a pusher is that it can reduce the drag caused by the high speed prop wash flowing over the fuselage. By appropriate prop layout this 'prop wash' benefit can substantially compensate for the efficiency loss from it being a pusher.
A pusher prop in slot is likely to be the worst case as the front edge of the slot has a dramatic effect on the airflow to the prop and it also makes a good siren!
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Sidewayskiwi
quorneng wrote:
The conventional view is that it is not so much "obstruction" that reduces the efficiency of a pusher prop as the disturbed airflow. This means the airflow at the prop leading edge is locally continually changing direction reducing the prop efficiency. With a tractor layout the prop is always facing undisturbed air at a constant angle.
An advantage of a pusher is that it can reduce the drag caused by the high speed prop wash flowing over the fuselage. By appropriate prop layout this 'prop wash' benefit can substantially compensate for the efficiency loss from it being a pusher.
A pusher prop in slot is likely to be the worst case as the front edge of the slot has a dramatic effect on the airflow to the prop and it also makes a good siren!

true 
but iam asking because iam down quite a bit of thrust on some engine brands 
and i have a big build progressing that i require at least 1.8 kg thrust
so been doing a tun of testing and prop testing
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solentlife
I have pondered long and hard on various own designs / modifications .... pusher or tractor.

What has actually become obvious to me in my ponderings ... is the layout of gear is biggest decider.

Lets take Steve Schumachs F15 as example. A very good design and well worth considering. Here he has used a pusher and the battery needs to be well forward to compensate. This leads to a stress moment at the wing leading edge to fuselage. Luckily the F15 having the intakes helps to reduce the breakages there.
I modified the plan to have a tractor prop and that meant I could place the battery into the main fuselage near the rear where greatest strength was.
It also reduces risk of prop strike to anyone launching.

Now lets take my Hawker Hunter OD ... where I elected to go for pusher prop ... (prop in slot is not feasible on this) .... already 3x I have broken the fuselage for exact reason I give for the F15 ... battery weight up front has caused fuselage break at the leading edge joint. It would have been much better to have made her tractor and carry the battery in a better location.

Prop in slot ... pusher props are best where fuselage has wide or considerable section to them .... the stresses of battery location etc. can be sorted. As we see with the F16 ... Eurofighter ... Mig29 style models - the battery is well catered for inside the main underbody. If you change that design to tractor - then the battery is going to hangng of the back end !! 

What is my point ?

Choice of prop position is not only a matter of 'clear air', aesthetics ... but also structural.
I'll fly anything if I can launch it ! Youtube : solentlifeuk
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Sidewayskiwi
solentlife wrote:
I have pondered long and hard on various own designs / modifications .... pusher or tractor.

What has actually become obvious to me in my ponderings ... is the layout of gear is biggest decider.

Lets take Steve Schumachs F15 as example. A very good design and well worth considering. Here he has used a pusher and the battery needs to be well forward to compensate. This leads to a stress moment at the wing leading edge to fuselage. Luckily the F15 having the intakes helps to reduce the breakages there.
I modified the plan to have a tractor prop and that meant I could place the battery into the main fuselage near the rear where greatest strength was.
It also reduces risk of prop strike to anyone launching.

Now lets take my Hawker Hunter OD ... where I elected to go for pusher prop ... (prop in slot is not feasible on this) .... already 3x I have broken the fuselage for exact reason I give for the F15 ... battery weight up front has caused fuselage break at the leading edge joint. It would have been much better to have made her tractor and carry the battery in a better location.

Prop in slot ... pusher props are best where fuselage has wide or considerable section to them .... the stresses of battery location etc. can be sorted. As we see with the F16 ... Eurofighter ... Mig29 style models - the battery is well catered for inside the main underbody. If you change that design to tractor - then the battery is going to hangng of the back end !! 

What is my point ?

Choice of prop position is not only a matter of 'clear air', aesthetics ... but also structural.

Yes i see your point
But my other point remains we rely on the the specs supplied from the manufacturers
Some of the time these "Specs" are generous in the extreme leaving us the builder scratching our heads and WTF???
wheres this 2kg of thrust---????
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solentlife
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But my other point remains we rely on the the specs supplied from the manufacturers
Some of the time these "Specs" are generous in the extreme leaving us the builder scratching our heads and WTF???
wheres this 2kg of thrust---????


Totally agree ... thats why I only rely on my Wattmeter and Thrust tests to decide best setup.

I think most people get happy with a combo of motor / ESC / Prop / LiPo and stick with that as tried and tested.
I'll fly anything if I can launch it ! Youtube : solentlifeuk
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Wildthing
Specs from a manufacturer have nothing to do with layout. Some exaggerate their specs to sell the product so the testing before it goes into a plane is a big plus that way you know what you are getting or should I say not getting. 
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F1wanabe
In my opinion, the prop slot is the most inefficient layout for thrust numbers and I've found the best performance from a power set up from the rear prop set up. Our rear prop planes fly with more thrust than a prop slot set up despite having the tv vanes mounted just in front of the prop. The sound coming off the prop tells a lot- very quiet from a pusher. I really think clean air leaving the prop is important. I did try a front mounted parkjet and the obstruction from the fuselage made for worse performance than a prop slot. However, I think having a twin puller configuration mid wing may be pretty ideal from a thrust prospective as we have on the Mosquito and Lancaster. Pretty clean air coming to and leaving the prop.
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