optimal altitude calculations?

wrote
are there some rough equations that can show that 6000' to 8000' is
about optimal?

I think that you are misleading most of the group by using
the word "optimal". This leads most to infer "max range", or
"best economy".
If you had said "maximum true air speed" we would have had a
better idea of what you are seeking.

Bob Moore

wrote:
I think most people are recommending 6000' to 8000', because it gives
75% power. is this just "experience" or "rule of thumb that I learned
somewhere" ? in other words, why is 6000-8000' where the air
resistance vs. power curves cross? or, why is 4000' not better? there
is more power. why is 10000' not better? there is less air density.

Keep in mind that this is "density" altitude, not "pressure" altitude.
To understand why, you have to remember what the throttle controls.

wrote:

I think most people are recommending 6000' to 8000', because it gives
75% power. is this just "experience" or "rule of thumb that I learned
somewhere" ?

You have several factors here. As you ascend, the air density decreases, so drag
decreases. A plane with the engine producing 75% power at 2000' will fly faster
than a plane producing 75% power at sea level.

The figure of 75% power is important because with most of the engines used in
light aircraft you need to use a rich fuel mixture if the engine is set to
produce more than this. That uses significantly more gas per hour.

As you ascend, the air pressure also decreases. This makes the normally
aspirated engine work harder for each slug of air/fuel mix. The pilot must keep
increasing the throttle as he climbs to get the same amount of power. At some
point, the plane will be producing 75% power at full throttle.

Many of us have assumed that by "optimal" you mean the best combination of fuel
economy and speed. That point is the highest density altitude that the aircraft
can reach and still produce 75% power.

That altitude is usually published in the operating manual for the aircraft or
otherwise available from the manufacturer. In the case of my Cessna 150, the
value was in the operations section of the owner's manual. In the case of my
Maule, I had to call the manufacturer and ask.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

thank you, george. I am a relatively new pilot, and am just beginning
to find the physics here interesting. I also had not thought about the
fact that it is the power, rather than the throttle setting, that
determines the fuel mixture. but it makes perfect sense.

(it still leaves my question of whether there is an altitude that
maximizes GS [TAS on a no-wind day] and why, but I guess this is not as
constant as I had thought and/or also in this 6000-8000' vicinity.)

here is another dumb question, and this is almost off topic. presume
we have an experimental, so I can experiment ;-). making air denser
should not be a big problem. well, I can't have my passengers blow
into a tube, but presumably any air pump increases air density. Even a
funnel shaped cowling should create more air to be breathed by an
engine flying at speed. Would relying on such cost more power in added
drag than it would create through making the engine breathe better? [I
looked at the prices of turbo normalizers and they seem upward of
$25,000---about the price of a Honda Accord. Ouhh! Maybe this is
because they use exhaust heat as the source of power?!]

hope I am not taking up too much airtime, and I am not the only one
curious.

/iaw

What you are talking about is "ram air". And it works, but only
slightly. The problem is the air drag created by a large funnel will
negate any increase in power. Also, to use that much air, you have to
use more fuel. But ram air is for real and most planes intake their air
from the front, right behind the prop, right where it is optimum. You
get a slight increase in air pressure from such an arrangement. But you
can't do it if you increase drag as well. A very large funnel (larger
than the nose of the aircraft) would create so much drag, the plane
would not go as fast. It would make more power, and consume more fuel,
but the power created could not overcome the drag created.

michael--I find it hard to believe that even
highest available cruise for short periods would be ground level. the
air resistance is pretty high down there.

Nevertheless, it is so. The highest available speed is always at the
highest altitude where the engine can make full power. For an engine
that is air-limited (which is almost ever normally aspirated engine)
that is sea level.

For engines that are power limited - meaning turbocharged with a flat
rating - it's the maximum altitude where the maximum allowable manifold
pressure can just be attained.

Michael

"Michael" wrote in message
oups.com...
michael--I find it hard to believe that even
highest available cruise for short periods would be ground level. the
air resistance is pretty high down there.

Nevertheless, it is so. The highest available speed is always at the
highest altitude where the engine can make full power. For an engine
that is air-limited (which is almost ever normally aspirated engine)
that is sea level.

For engines that are power limited - meaning turbocharged with a flat
rating - it's the maximum altitude where the maximum allowable manifold
pressure can just be attained.

Michael



This isn't always true. The MU-2 reaches maximium TAS at the altitude where
the engines can produce approximately 90% power. No idea of why.

Mike
MU-2