Bob M. wrote:

: I have read that the usefulness of the Bell P-39 was greatly decreased
: by certain decisions made by the USAAF before it went into production
: in the 1930s.

The air force was not the only party responsible.
The installation of the supercharger was criticised
by NACA, which suggested a number of modifications.
And Bell did not protest; the turbocharger was
troublesome and the company urgently needed to sell
some aircraft.

: The question is, just how much more effective would this plane have
: been had these changes not been made? Would it have a much greater
: climb rate and been more effective at high altitudes? Or would it
: still have been pretty much of a bust as a fighter/interceptor?

A P-39 with a turbosupercharged engine (in a better
installation than available on the prototype) would
have retained the basic P-39 problem, that it was a
small fighter with most internal space taken up by
the engine installation, and its handling sensitive
to correct loading. Performance at altitude would
have been improved, that at low altitude could have
suffered because of the extra drag and weight.
Other disadvantages -- such as the eccentric cockpit
design and the rather unsuitable armament -- would
also have stayed. Overall, however, the P-39 might
have been a more useful aircraft, as its altitude
performance was one of the biggest complaints about
the type (at least in the USAAF).

That the concept held promise was proven by the P-63,
with a V-1710 with a two-stage mechanical supercharger
and laminar flow wings; the Kingcobra was an excellent
fighter, though handicapped by the small range inherent
in the basic design (i.e., the engine was were the fuel
tanks ought to have been.)

--
Emmanuel Gustin