Tractor/Pusher

[NARSES2]

My interest in aircraft has always been more based on historical interest rather than on the engineering/aeronautical aspect of the things but occasionally something makes me think. So please forgive my ignorance but can anyone explain this to me in fairly simple terms ?

In the current SAM there is an interesting conversion of the Airfix Be 2c into the Be 9 "pulpit" fighter. In it the author says "the pusher would never be as aerodynamically efficient as the tractor type of airplane". It's the aerodynamically bit that piqued my interest and I just wonder why ? Was it simply that a tractor engine forces air over the flying surfaces thus increasing lift ? Or am I being silly ?

Also if the tractor was that superior how come designers were producing designs and indeed building pusher types in the 30's through to the late 40's ? Was that simply a case of trying to get the armament concentrated in the nose ?

Cheers

Chris

[Captain Canada]

That`s an interesting query Chris. Looking forward to following this thread for the answers. Also think I should hit up the book store for a look.

[PR19_Kit]

Perhaps with the prop at the back it doesn't get such a 'clean' air supply with a pusher?

I don't know that for sure, but it seems possible to me.

[jcf]

Here's a place to start:
http://www.flyingmag.com/technicalities-2

[KJ_Lesnick]

I'm curious how a contra-prop compares with a pusher/puller

[Green Dragon]

seem to remember reading the Do335 flew with just the pusher prop in tests and they found the pusher was more efficient.

Paul Harrison

[wuzak]

The Do 335 did fly faster on its front engine than it did on the rear.

One theory is that the propeller wake interacts with the fuselage and wings and causes extra drag.

[NARSES2]

Here's a place to start:
http://www.flyingmag.com/technicalities-2

Right where do I sit my exam ? 

No, seriously, thank you Jon. That's really interesting and I think I have a slightly better understanding of it now, it's also interesting to see what a heated subject it is even in aeronautical circles.

Chris

[PR19_Kit]

For the same reason pushers create less torque. Theory works with R/C, not sure if it scales up...

Really?

Torque is a characteristic of the engine fitted, so I find it difficult to understand why that should be different depending which end of the airframe it's fitted to.

[JayBee]

Torque is the effect of the turning power of the engine.
Torque Effect is to do with the airflow from the propeller over the air frame, and it's effect on the aerodynamics of the whole.
Pushers avoid the latter points, mostly. 
Contra-props avoid this by avoiding both problems but they tend to be heavy units, with their own problems.

[PR19_Kit]

I'm not convinced.

[wuzak]

Torque is the effect of the turning power of the engine.
Torque Effect is to do with the airflow from the propeller over the air frame, and it's effect on the aerodynamics of the whole.
Pushers avoid the latter points, mostly.  
Contra-props avoid this by avoiding both problems but they tend to be heavy units, with their own problems.

The airframe experiences a torque reaction from the engine regardless of whether it is a pusher or tractor design.

The reaction is opposite the propeller rotation.

The pilot will need to account for this on take-off, usually with the rudder.

The airflow from the propeller does cause effects on the airframe of a tractor aeroplane due to the spiral wake.

The Do 335 was faster on rear engine alone, but this is not always the case.

The Cessna Skymaster/O-2 was, from what I have been told, faster on its front engine than its rear engine.

https://en.wikipedia.org/wiki/Cessna_O-2_Skymaster
https://en.wikipedia.org/wiki/Cessna_Skymaster



The shape of the airframe near the propeller is vital. On the Skymaster the fuselage is quite bulky ahead of the prop, unlike on the Do 335.

The Northrop XP-56 was a pusher, but didn't go anywhere near as fast as expected.



With the portly fuselage housing the radial engine there was a lot of turbulent flow off the fuselage, into which the engine's exhaust was pumped, exacerbating the problem. The propeller suffered badly from vibrations, and its efficiency was well down.

The Do 335 also experienced instability when the oil cooler ducts opened, as they were just in front of the pusher prop.

[PR19_Kit]

IIRC the Rutan Voyager flew most of its round the world flight on its rear engine, but the engine/prop combinations on the aircraft were different front to rear for that flight. They were the same originally but were changed after some initial trials.

[kitnut617]

Personally, I like the prop to be at both ends --- 

[kitnut617]

No mate, it was built before I discovered forums   

I have always wondered just how fast you could make a piston-engined aircraft fly (in the level) and whether swept wings would have been discovered if turbo-jets proved to be a flop.  I've got the Rolls Royce Crecy book and one design piqued my interest, it was an X-24, 2-Stroke, sleeve-valve engine which the book says theoretically, could put out nearly 8000 hp. So what I came up with was what you see, and the idea is it has two engine in the rear fuselage (one in front of the other), the front one driving the front contra-prop via a drive shaft, while the rear one drives the rear props and is attached directly to the engine.  I then needed an airframe which could handle the power.

Here's a couple of others of it.  I wonder if you can identify what parts came from other aircraft though ----