On Fri, 08 Jun 2007 17:26:26 GMT, Dallas
wrote:


Am I reading this graph incorrectly?

http://img.photobucket.com/albums/v1...ofileChart.jpg

At the 45% power setting, the best range for a Cessna 150M would be
achieved by flying anywhere from sea level to a maximum of 1,800 feet?

At 75% power the gain is about 2.5 NM of range by flying at 7,000 feet vs.
1,000 feet.

This pretty much trashes my fundamental belief that higher altitudes give
greater range. What am I missing?

nothing.
your range is not related to speed directly but by fuel use.
(not theoretically but in real flying)
your engine uses about 20 litres per hour. that is the reality.
the size of the fuel tank is what governs the time possible in the
air.

the other factor which plays with the distance achieved over the time
in the air, at the speeds we fly at, more than any other factor, is
the wind direction and strength.

climb settings built in the graph are almost irrelevant because what
you lose in the climb you typically make up in the descent.

your idea that higher altitudes are more efficient is bought undone by
two aspects of your aircraft. the propeller becomes less efficent with
less dense air flowing past it. the engine also becomes less efficient
with less dense air going into it. the reduction in density almost
exactly undoes the benefits of altitude.
turbo charging was developed to remove one of those deficiencies.

another thing that I have accidently tested is that the speed that you
fly at in our aircraft doesnt affect range. flying slower keeps you
aloft for longer but you cover less distance in the time. in my
experience the range is near identical at any speed.

so what the revelation shows you in the graph is that you should run
your engine at max continuous rpm and lean it periodically to max rpm
at the throttle setting and get on with enjoying the scenery.
none of it actually matters! ...except the wind.

what altitude you fly at is actually dictated by the temperature of
the day and the amount of humidity in the air. you climb until the air
cools almost beyond comfortable provided that the humidity of the day
provides clear vision.

have you ever realised that at 105knots your aircraft is *always*
flying into a 105knot headwind. :-)
move the parcel of air that you are flying through across the
landscape and sometimes it can help you. (oh we dream of those days)

welcome to the real world. it is a lovely, far simpler place than you
may have imagined :-)
Stealth Pilot