Peter Dohm wrote:
What diameter of prop can you turn at 4000 rpm?

A Volkswagen is NOT a good direct drive aircraft engine because it
produces good power at 3200 to 3800 rpm. But it's a hell of a lot
easier to put a redrive on a VW than an O-200.

Actually, 46 to 48"

The VW 1600 was used that way with considerable success in the early KR-2
aircraft. However, that only meant 60 to 65 HP with the small displacement,
and the modestly oversized cylinders which would be fitted without fairly
radical case machining provided only a very modest addition of take-off
power. Cruising speed was only about 115 kts within the thermal capacity of
the stock heads--which has been discussed previously in this NG--so pilots
who were heavier or wanted to fly faster sought more oomph.


Since VW engines in race cars and hot rods (sand rails, etc) operate
under continuous power at higher power settings than this (I have
driven Bugs up tall mountains in 90 degree weather at 25+ inches Hg at
3000-3700 rpm for as long as the mountain lasted, which was longer than
enough to heat the head all the way through) this thermal analysis
theory is flawed. The 356/912 Porsche has a head not much bigger and
they run for hours on the Autobahn flat out.

The difference is these engines have a cooling blower, where most
aircraft installations run them as free cooled engines.

Free air cooling and direct drive are simple. In the old A-65 and the
airframes it went into that worked okay. But the time has come to
recognize that for an airplane to not be something looked on as an
antique, it needs a liquid cooled engine with a flywneel, redrive,
single lever power control and enough power to haul fat people and lots
of crap out of high and hot fields with healthy margins to spare.