Push-Pull propeller combinations.

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

"Peter Skelton" wrote in message
...
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 -

Yes. One of the classic crash modes of the Skymaster is when the pilot attempts
a takeoff after failing to notice that the rear prop is not turning. The
procedure developed to detect that simple condition is to lead with the rear
engine throttle. If you push in the throttle and the noise level remains
unchanges, it is time to abort the takeoff and investigate!

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.

Of course, the biggest danger is loss of an engine on takeoff, but even a
single-engine *landing* can come to grief in various ways. We had a twin
fatally crash into our neighborhood from exactly that event.

One situation where twins are considered inherently safer is night IFR. The
classic advice for a forced landing at night is to maintain control of the
airplane; in particular maintain a safe airspeed. At 300 feet, turn on the
landing lights. If you don't like what you see, turn them off!

Vaughn