Russell Kent wrote:  My hangar neighbor says he's getting 180 HP on 4.2 gal/hr.
How can I

prove he's full of it?

charles.harbert wrote:

Everyone says what great fuel economy they get, but they forget to relate it to
altitude. You lose roughly a .5 psi of air pressure for each 1K', which is
directly related to hp. If you have half the air pressure, you get roughly half
the hp. At 10K', you have a third less air (10/15 psi) and a third less hp. This
is where most normally aspirated engines get best economy/speed.

  Paul is correct that aircraft piston engines have pretty defined fuel burn
rates per hp (BSFC). To acheive maximum power, you must richen the mixture to
around 12:1 air/fuel ratio which in most gasoline piston engines is a BSFC of
around .55 to .6 (rotaries are about 10% higher). If you lean the engine out to
achieve best economy, you can increase the air/fuel ratio to 16-18:1 which is
about a .40-.45 BSFC, but you give up hp.

In your example, he has a maximum of 180 hp (at sea level and high fuel burn),
but at higher altitude, he is able to lean the engine to 4.2 gph, but he's only
putting out about 50 hp. The plane still flys at reasonable speed because the
drag has reduced significantly because of lower air pressure (thin air).

You could probably look his POH (assume certified a/c) and it would show fuel
burn rates at different altitudes and power settings.

The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org