More Roads Not Traveled - He-177 Successfully Reengined

[Stan in YUL]

How does a pair of coupled BMW radials sound?  I think it's possible you could get 2000+ hp out of such a config. Less cooling problems?

Thanks in advance.
Stan

[jcf]

That's the BMW 803, an attempt to make a 28-cylinder engine based on the 801. It was needlessly complex and didn't work very well.

BMW 803

[PR19_Kit]

Needs more cylinders. 

[The Wooksta!]

There were plans for a four engined - conventional that is - He 177B/He 277 with BMW 801s.  I'm trying to figure out how to do the 277B-5 with the revised fuselage and wings.

[Weaver]

How about rearranging the engines in push-pull pairs?

Upsides:

• It'd get around the joint gearbox problem and the cramped, fire-prone engine bay.
• It wouldn't increase the frontal area.
• The pusher props would be more efficient.

Downsides:

• It'd move the CofG aft unless you did something to compensate.
• It'd complicate firing arcs for defensive guns.
• It wouldn't blow as much of the wing as a conventional four (though neither did the coupled engine setup) to reduce landing speed.
• It might need a redesigned landing gear to prevent the rear props grounding.

It doesn't seem too hard to lengthen the 177's forward fuselage to restore the CofG since it's constant section in the relevent area (much like stretching a modern airliner).
The lengthened forward fuselage might then make room for a tricycle undercarriage, which would increase ground clearance for the props.
No good way (with 1940s tech) to deal with the reduced moment arm for the tailplanes except enlarging them.

[jcf]

Wing-mounted pusher props are less efficient because they're always running in dirty air. They're particulary bad in push-pull engine installations like you suggest.

[jcf]

While the DB 606 was finicky most of the problems on the He 177 were because of the poorly conceived engine installation, which exacerbated the problems.

[Stan in YUL]

Thanks guys, this was fun!

[Weaver]

Wing-mounted pusher props are less efficient because they're always running in dirty air. They're particulary bad in push-pull engine installations like you suggest.

Pusher props are inherently more efficient than tractor ones because their wash doesn't scrub down a fuselage. The Do 335 was about 10% faster on it's tail engine alone than it was on it's front engine alone. Wing-mounted pusher props blow over the tailplanes, which makes them a little less efficient than tail-mounted ones, but I'll bet that effect is pretty small because the tailplanes are pretty small.

All pusher props are running in "dirty" air coming in from the front. It's mostly a problem with resonance, i.e. the loads on the rear prop oscillate. It clearly isn't an unsolvable problem though.

[jcf]

Only if mounted in a tail position like the Do 335
they are more inefficient when mounted on a wing
because of the dirty airflow off of the wing. This is
a fact determined by operational use and research. 
Why do you think pusher props never became 
common on multi-engine aircraft? A lot of the prop
problems on the B-36 were caused by them being
installed as pusher rather than tractor, as a pusher
installation is more prone to vibration because of
the uneven airflow. Proposed advanced versions of the B-36 were to be equipped with tractor propeller
installations. The vibration isssues cannot be glossed over and they're not "mostly a result of resonance". 

[PR19_Kit]

But in WhiffWorld pushers are MUCH cooler looking. 

And doubly so when matched with a tractor engine in front of them.................

[jcf]

The BMW 802 could be an option, an 18-cylinder
radial very similar in size to the Wright R-3350.
Various versions were worked on with multi-stage
mechanical superchargers, two-stage mechanical
supercharger plus two turbosuperchargers and
turbo-compound in which the turbosupercharger
supplies mechanical power directly to the engine
as well as compressing the intake air. Single and
contra-prop versions.
With continued development it had possibilities,
of course the early turbosupercharged variants
of the R-3350 had their own issues with fires.

BMW 802 P.8008, supercharger and three turbos.
You cannot view this attachment.

[jcf]

BMW 802 installation proposed for Dornier Do 317.

You cannot view this attachment.

[Weaver]

Only if mounted in a tail position like the Do 335
they are more inefficient when mounted on a wing
because of the dirty airflow off of the wing. This is
a fact determined by operational use and research.
Why do you think pusher props never became
common on multi-engine aircraft? A lot of the prop
problems on the B-36 were caused by them being
installed as pusher rather than tractor, as a pusher
installation is more prone to vibration because of
the uneven airflow. Proposed advanced versions of the B-36 were to be equipped with tractor propeller
installations. The vibration isssues cannot be glossed over and they're not "mostly a result of resonance".

How is the flow behind a wing "dirtier" than the flow behind a fuselage?

Why did pusher props never become common on multi-engined types? Well I suspect a confluence of a few factors: the general move to jets curtailed research and market demand for high-speed multi-props, the landing-speed-reducing effect of tractor props was deemed more important on the large multi-props that were build (mostly military transports), and lastly, that old demon, conventionality: the airline industry and the general aviation market are notoriously conservative.