Push-Pull propeller combinations.

On Thu, 2 Sep 2010 18:22:49 -0700 (PDT), Eunometic
wrote:

On Sep 3, 10:48*am, John wrote:
On Sep 2, 10:08*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?

List of such aircraft:

Cessna 337 Skymaster
Rutan Model 76 Vogager
Adam A500

Dornier Wal
Dornier Do X
Dornier Do 18 Seaplane, the German PPY Catalina.
Dornier Do 26K Seaplane, possibly the longer ranged seaplane ever
built.
Dornier Do 335 Pfeil (arrow) *perhaps one of the fastest piston
engined aircraft ever built.
Dornier Seawings Seastar, modern Seaplane of composites.

Savoia-Marchetti S.55
LeO H-242

I don't know about the others but my understanding is that the
Skymaster was an attempt to design a twin engined plane that would
have the same overall dimensions as a single engine aircraft and be
safer to fly since it would eliminate asymetric thrust during engine
failure. *The ironic thing is that the Skymaster safety record is no
better than a conventional twin.- Hide quoted text -

- Show quoted text -

Possibly psychological "a single engined failure isn't dangerous so I
won't maintain the aircraft factor" and ofcourse perhaps engine
failure isn't the major cause of light plane crashes.

A single engined turbo prop is safer than a twin engine piston and
possibly even safer than a twin turbo since the pilots inabiility to
handle asymetrical thrust may be worse than his abillity to handle a
glide/crash landing.

Famously some singles have crashed and smashed through brick walls
and the pilot walked free unscathed, the engine acting as a battering
ram and protection.

That a single piston is safer than a twin is a classic of
probability theory, taugh in junior high around here (not back in
the day though).

Peter Skelton

On Sep 3, 8:24*am, Peter Skelton wrote:
On Thu, 2 Sep 2010 18:22:49 -0700 (PDT), Eunometic





wrote:
On Sep 3, 10:48*am, John wrote:
On Sep 2, 10:08*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?

List of such aircraft:

Cessna 337 Skymaster
Rutan Model 76 Vogager
Adam A500

Dornier Wal
Dornier Do X
Dornier Do 18 Seaplane, the German PPY Catalina.
Dornier Do 26K Seaplane, possibly the longer ranged seaplane ever
built.
Dornier Do 335 Pfeil (arrow) *perhaps one of the fastest piston
engined aircraft ever built.
Dornier Seawings Seastar, modern Seaplane of composites.

Savoia-Marchetti S.55
LeO H-242

I don't know about the others but my understanding is that the
Skymaster was an attempt to design a twin engined plane that would
have the same overall dimensions as a single engine aircraft and be
safer to fly since it would eliminate asymetric thrust during engine
failure. *The ironic thing is that the Skymaster safety record is no
better than a conventional twin.- Hide quoted text -

- Show quoted text -

Possibly psychological "a single engined failure isn't dangerous so I
won't maintain the aircraft factor" and ofcourse perhaps engine
failure isn't the major cause of light plane crashes.

A single engined turbo prop is safer than a twin engine piston and
possibly even safer than a twin turbo since the pilots inabiility to
handle asymetrical thrust may be worse than his abillity to handle a
glide/crash landing.

Famously some singles *have crashed and smashed through brick walls
and the pilot walked free unscathed, the engine acting as a battering
ram and protection.

That a single piston is safer than a twin is a classic of
probability theory, taugh in junior high around here (not back in
the day though).

Peter Skelton

Twice the number of things to go wrong?

On Fri, 3 Sep 2010 12:05:31 -0700 (PDT), "David E. Powell"
wrote:

On Sep 3, 8:24*am, Peter Skelton wrote:
On Thu, 2 Sep 2010 18:22:49 -0700 (PDT), Eunometic





wrote:
On Sep 3, 10:48*am, John wrote:
On Sep 2, 10:08*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?

List of such aircraft:

Cessna 337 Skymaster
Rutan Model 76 Vogager
Adam A500

Dornier Wal
Dornier Do X
Dornier Do 18 Seaplane, the German PPY Catalina.
Dornier Do 26K Seaplane, possibly the longer ranged seaplane ever
built.
Dornier Do 335 Pfeil (arrow) *perhaps one of the fastest piston
engined aircraft ever built.
Dornier Seawings Seastar, modern Seaplane of composites.

Savoia-Marchetti S.55
LeO H-242

I don't know about the others but my understanding is that the
Skymaster was an attempt to design a twin engined plane that would
have the same overall dimensions as a single engine aircraft and be
safer to fly since it would eliminate asymetric thrust during engine
failure. *The ironic thing is that the Skymaster safety record is no
better than a conventional twin.- Hide quoted text -

- Show quoted text -

Possibly psychological "a single engined failure isn't dangerous so I
won't maintain the aircraft factor" and ofcourse perhaps engine
failure isn't the major cause of light plane crashes.

A single engined turbo prop is safer than a twin engine piston and
possibly even safer than a twin turbo since the pilots inabiility to
handle asymetrical thrust may be worse than his abillity to handle a
glide/crash landing.

Famously some singles *have crashed and smashed through brick walls
and the pilot walked free unscathed, the engine acting as a battering
ram and protection.

That a single piston is safer than a twin is a classic of
probability theory, taugh in junior high around here (not back in
the day though).

Peter Skelton

Twice the number of things to go wrong?

That and a non zero probability of crash as the result of a
single failure. The number was worked out sometime around the end
or WWII, aircraft with higher power reserves might show different
results.


Peter Skelton

On Fri, 03 Sep 2010 08:24:47 -0400, Peter Skelton
wrote:

That a single piston is safer than a twin is a classic of
probability theory, taugh in junior high around here (not back in
the day though).

Peter Skelton

As a general rule you put multiple engines on an aircraft 'cause you
want to haul a bigger load. The safety issue is more complex because
just how you add the additional engines can have significant
consequences if one (or more) fail.

There's also, however, a question of environment. Aircraft that
operate almost exclusively over land are going to have some different
considerations than those that operate almost exclusively over water.
There the that junior high principle might not be so clear. :-)

On my one flight in a Skymaster the "rear engine first" technique was
used. Of course if you watch the gauges you can also tell if the rear
engine is producing rated power.

One place that the push-pull system might just work out well is the
seaplane/amphibian. I note that Dornier says it's going to produce a
new one in Canada starting later this year (IIRC). It looks like an
interesting machine. We'll see how well it does in a very depressed
market.

On Sep 4, 4:36 am, Bill Kambic wrote:
On Fri, 03 Sep 2010 08:24:47 -0400, Peter Skelton
wrote:

That a single piston is safer than a twin is a classic of
probability theory, taugh in junior high around here (not back in
the day though).

Peter Skelton

As a general rule you put multiple engines on an aircraft 'cause you
want to haul a bigger load. The safety issue is more complex because
just how you add the additional engines can have significant
consequences if one (or more) fail.

There's also, however, a question of environment. Aircraft that
operate almost exclusively over land are going to have some different
considerations than those that operate almost exclusively over water.
There the that junior high principle might not be so clear. :-)

On my one flight in a Skymaster the "rear engine first" technique was
used. Of course if you watch the gauges you can also tell if the rear
engine is producing rated power.

One place that the push-pull system might just work out well is the
seaplane/amphibian. I note that Dornier says it's going to produce a
new one in Canada starting later this year (IIRC). It looks like an
interesting machine. We'll see how well it does in a very depressed
market.

Good, thanks for the notice...
http://en.wikipedia.org/wiki/Dornier_Seastar

The economy is always cyclic, canucks will need that type of thing
for commodity developement, of course the Twin Otter is a good deal,
we'll need to see the price.
Ken

On Thu, 2 Sep 2010 17:48:37 -0700 (PDT), John
wrote:

I don't know about the others but my understanding is that the
Skymaster was an attempt to design a twin engined plane that would
have the same overall dimensions as a single engine aircraft and be
safer to fly since it would eliminate asymetric thrust during engine
failure. The ironic thing is that the Skymaster safety record is no
better than a conventional twin.

I flew a Skymaster once. It was the pressurized version. I was
unimpressed by the single engine performance. While there was no
asymetric thrust there was a distince lack of climb capability, too
(with either front or rear out).

Your understanding is correct on the reasoning for the arrangement.
But sometimes there's no substitute for horsepower. ;-)

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.
2) if it's a twin engine, a strong structure is required to connect
the engines, (wing based twins use the existing spar).
3) a single engine P-P needs a long horizontal shaft.
4) the inertial moment is greater if a passenger bay is between
the engines.
5) there is an unpredictable airstream for the Pusher.
....and more.
Ken

List of such aircraft:

Cessna 337 Skymaster
Rutan Model 76 Vogager
Adam A500

Dornier Wal
Dornier Do X
Dornier Do 18 Seaplane, the German PPY Catalina.
Dornier Do 26K Seaplane, possibly the longer ranged seaplane ever
built.
Dornier Do 335 Pfeil (arrow) perhaps one of the fastest piston
engined aircraft ever built.
Dornier Seawings Seastar, modern Seaplane of composites.

Savoia-Marchetti S.55
LeO H-242

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_Do_26
http://www.historicaircraft.org/Othe...nier-26-2.html

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 )

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