P-63 (?) Airacobra/Kingcobra power question

Ricky wrote:
A question for the physics-minded among us, or for anyone who just has
the answer.

I have heard the Airacobra was underpowered and that got me to
thinking;
Does having an engine at the middle of the fuselage (in any plane for
that matter, like the XP-58) and connected by a long shaft contribute
to a loss of power delivered to the propeller? In other words; would
there be more power delivered the closer the engine is to the prop?
Does the shaft "eat up" power in any way? I am a mechanic and pilot
and fairly knowlegable about a/c physics & aerodynamics but this has
me stumped.

The shaft itself won't eat up power, but the various gearboxes and shaft
supports required will. Every bearing has a bit of friction; every
gearbox has a bit of drag. A long drive shaft, and the gearbox required
to let the cannon shoot through the hub, would cost more power than the
straight installation.

Whether it was enough to matter, in the case of the P-39, is another
thing. ISTR the P-39's problem was the lack of a turbocharger rather
than overall low power... the Airacobra started losing oommmmph above
12,000 feet, and it turned out that most of the combat was higher than that.

Remember, Tex Johnston won the Thompson Trophy in a P-39, and set a
speed record, besides.

Ron Wanttaja

In article ,
Ron Wanttaja wrote:

Ricky wrote:
A question for the physics-minded among us, or for anyone who just has
the answer.

I have heard the Airacobra was underpowered and that got me to
thinking;
Does having an engine at the middle of the fuselage (in any plane for
that matter, like the XP-58) and connected by a long shaft contribute
to a loss of power delivered to the propeller? In other words; would
there be more power delivered the closer the engine is to the prop?
Does the shaft "eat up" power in any way? I am a mechanic and pilot
and fairly knowlegable about a/c physics & aerodynamics but this has
me stumped.

[snip]

Whether it was enough to matter, in the case of the P-39, is another
thing. ISTR the P-39's problem was the lack of a turbocharger rather
than overall low power... the Airacobra started losing oommmmph above
12,000 feet, and it turned out that most of the combat was higher than that.

Combat in the ETO tended to be higher than other theaters, which, since
the USAAC pulled the turbocharger from the P-39, hamstrung it in that
arena. The Airacobra was also a small aircraft, with limited fuel, as it
was designed as a point defense fighter, and lack of range hurt it, too.

That said, the Russians liked it pretty well, since air combat on the
eastern front tended to remain below 15,000', and it performed well down
there against the Luftwaffe. (In spite of the 37mm cannon, the P-39 was
used much more in the anti-air role, and not against German armor; the
Il-2 was much better down in the mud.)

The P-39/P-400 didn't exactly shine, not that it embarrassed itself, in
the Pacific, partly due to the long distances that were common there.

Ricky wrote:
A question for the physics-minded among us, or for anyone who just has
the answer.

I have heard the Airacobra was underpowered and that got me to
thinking;
Does having an engine at the middle of the fuselage (in any plane for
that matter, like the XP-58) and connected by a long shaft contribute
to a loss of power delivered to the propeller? In other words; would
there be more power delivered the closer the engine is to the prop?
Does the shaft "eat up" power in any way? I am a mechanic and pilot
and fairly knowlegable about a/c physics & aerodynamics but this has
me stumped.

Thanks in advance for your ponderings and/or solution!

Ricky

It's a question often asked in connection with long shafts.
The elastic angular compliance can be a positive help with vibrations,
which are absorbed by a quill shaft. But a shaft drive train that's
curved takes pillow blocks to support the curve, and these bearings take
some (small) power on their own account. Otherwise, air drag, and
bearing drag apart, there's no loss in a long quill shaft.

Brian W

On Nov 3, 8:51*pm, brian whatcott wrote:
Ricky wrote:
A question for the physics-minded among us, or for anyone who just has
the answer.

I have heard the Airacobra was underpowered and that got me to
thinking;
Does having an engine at the middle of the fuselage (in any plane for
that matter, like the XP-58) and connected by a long shaft contribute
to a loss of power delivered to the propeller? In other words; would
there be more power delivered the closer the engine is to the prop?
Does the shaft "eat up" power in any way? I am a mechanic and pilot
and fairly knowlegable about a/c physics & aerodynamics but this has
me stumped.

Thanks in advance for your ponderings and/or solution!

Ricky

It's a question often asked in connection with long shafts.
The elastic angular compliance can be a positive help with vibrations,
which are absorbed by a quill shaft. But a shaft drive train that's
curved takes pillow blocks to support the curve, and these bearings take
some (small) power on their own account. Otherwise, air drag, and
bearing drag apart, there's no loss in a long quill shaft.

Brian W

I had a ME design a centrifuge application with the shaft running well
above its critical speed a bunch of years ago, but don't remember the
tradeoffs that led me to accept that embodiment. It was not an
aviation application in any event. I did find the observation about
wind profiles around a pusher imposing design constraints -- I'd have
thought that far aft winds in the disk would be fairly uniform except
at high angles of attack.

Have you a sense of the improvement of thrust, if any, a given engine
might have wing mounted vs nose mounted? There's less to blow against
but the wind does extend to well past the prop disk so some of that
air near the outside diameter is compromised by the wing. I'm thinking
of an application where one wants the maximum endurance at fairly low
speeds. That around the world airplane that came out of Scaled
Composites might hold the answer.

On Nov 3, 9:52*pm, a wrote:
On Nov 3, 8:51*pm, brian whatcott wrote:



Ricky wrote:
A question for the physics-minded among us, or for anyone who just has
the answer.

I have heard the Airacobra was underpowered and that got me to
thinking;
Does having an engine at the middle of the fuselage (in any plane for
that matter, like the XP-58) and connected by a long shaft contribute
to a loss of power delivered to the propeller? In other words; would
there be more power delivered the closer the engine is to the prop?
Does the shaft "eat up" power in any way? I am a mechanic and pilot
and fairly knowlegable about a/c physics & aerodynamics but this has
me stumped.

Thanks in advance for your ponderings and/or solution!

Ricky

It's a question often asked in connection with long shafts.
The elastic angular compliance can be a positive help with vibrations,
which are absorbed by a quill shaft. But a shaft drive train that's
curved takes pillow blocks to support the curve, and these bearings take
some (small) power on their own account. Otherwise, air drag, and
bearing drag apart, there's no loss in a long quill shaft.

Brian W

I had a ME design a centrifuge application with the shaft running well
above its critical speed a bunch of years ago, but don't remember the
tradeoffs that led me to accept that embodiment. *It was not an
aviation application in any event. I did find the observation about
wind profiles around a pusher imposing design constraints -- I'd have
thought that far aft winds in the disk would be fairly uniform except
at high angles of attack.

Have you a sense of the improvement of thrust, if any, *a given engine
might have wing mounted vs nose mounted? There's less to blow against
but the wind does extend to well past the prop disk so some of that
air near the outside diameter is compromised by the wing. I'm thinking
of an application where one wants the maximum endurance at fairly low
speeds. That around the world airplane that came out of Scaled
Composites might hold the answer.

Opps, not built by scaled composites after all. Interesting that the
rear engine was the one that ran all of the time, so that was the more
efficient location (but what does Rutan know?).