Push-Pull propeller combinations.

Euno...
There are counterpoints to your counterpoints etc. especially true in
engineering, so I'll cut to the chase, more below...

On Sep 3, 10:04 pm, Eunometic wrote:
On Sep 3, 10:20 pm, "Ken S. Tucker" wrote:



On Sep 2, 7:08 am, Eunometic wrote:

Several aircraft have been built with both a pull (or tractor)
propeller and a push (or rear) propeller aligned with each other.

Supposedly the arrangment is inefficient, yet the some of the Dornier
aircraft were record breakers.

Given that some aircraft were ruined or delayed by the problem of
combining gearboxes (He 177, Northrop XB-35 and if it ever came to it
the B-29 with its backup V-3420) it looks like an attractive
proposition.

What's going on here?

Overall the Puller-Pusher is complicated.
1) the Pusher needs serious clearance for landing and takeoff.

Overall I agree with what you say, however the conventional arrangment
also has some problems.

Also in the case of only having a limited horsepower the push-pull
arrangment does have a record of outperforming the side by side wing
arrangment it seems to me.

Yes, but no so bad with a tricycle undercarriage.

2) if it's a twin engine, a strong structure is required to connect
the engines, (wing based twins use the existing spar).

But the by mounting engines on the wing the spar exeperiences twist,
also a problem

3) a single engine P-P needs a long horizontal shaft.

Yes, but can be reduced with a snall engine.

4) the inertial moment is greater if a passenger bay is between
the engines.

Definit stabillity issues but countered by the intrinsic contra
rotation and gyroscopic stabalistation of the props.

5) there is an unpredictable airstream for the Pusher.
...and more.

Pushers can be slightly more efficient IF the structure ahead of the
pusher is kept small and streamlined.

Ken

I was thinking more along the lines of wing mounting of inline tandem
push-pull pairs in the fashion of this aircraft:http://en.wikipedia.org/wiki/Dornier...pages/Dornier-...

An arrangment that had been used succesfully for decades on Dornier
aircraft and these aircraft seemed to give nothing away in terms of
speed or range.

At one point it looked like being transfered to land based:http://www.luft46.com/prototyp/me264.html
(Right at the bottom, youy see an illustraion of an Me P.1075 )

Nice pics, (me likes Do-26 too), I think the science of operating a
Pusher prop in the the prop wash of a Puller is difficult mainly at
slow speeds such as landing and take-off, that's beyond theory.
Additionally is the Pusher hub suction and cooling, given that one
can work those problems to near irrelevence then it looks like a
heck of good design to me.
Ken