practical best range application?

I understand the theory vis a vis lowest L/D TAS and the power
available/power required chart, but let's say you're in flight in a normally
aspirated prop plane, your destination is blown up, your alternate is
unservicable for some reason or other and you need to stretch the fuel you
have the farthest distance possible.

What are the quick and dirty steps to working out the optimum alt and speed?
I mean in the cockpit, not in the comfort of home.

Thanks in advance.

"xerj" wrote in message
...
I understand the theory vis a vis lowest L/D TAS and the power
available/power required chart, but let's say you're in flight in a
normally aspirated prop plane, your destination is blown up, your
alternate is unservicable for some reason or other and you need to
stretch the fuel you have the farthest distance possible.

What are the quick and dirty steps to working out the optimum alt and
speed? I mean in the cockpit, not in the comfort of home.

Thanks in advance.



Unfortunately, there's no quick & dirty way. The usual (Bruguet) theory
is deficient in its assumptions, and the usual cruise table is deficient
in that the wrong independent variables were selected by the GAMA
committee. At those lower speeds, you can't depend on propeller
efficiency being independent of air speed. For details and an example of
what to do, see the cruise chapter of Performance of Light Aircraft.

John Lowry
Flight Physics

If you have a general idea of the best altitude for your aircraft (fuel
economy wise), and you can pull power and lean until you are able to
keep the RPM in the green and maintain altitude, then all you need do
is turn so that the wind is behind you. Remember, the more angle of
attack, the more drag you will have. So you may need to keep more
power, say to 60%. I would guess in my PA28-180 that I would need to
cut power down to about 2000RPM (well above bottom of green). This
would probably put me down in the 4x% range, but with a large angle of
attack. However, if I brought the power up to 2200 and was at ~5000
MSL...

Note that at this point I have not referred to the POH. I'm doing this
sitting here by seat of pants outside the plane.

Without going into lots of physics, I would guess that this would give
you your best time in air and distance.

If you MUST deal with any kind of headwind, then you will have to run
with enough power to overcome the wind. And now you must start doing
some serious figuring as to what altitude gives you your best chance of
getting to where you need to go.

And in the situation you have just given, forget GPS and LORAN. Hope
you have an ADF, because you probably won't pickup VORs either.
Later,
Steve.T
PP ASEL/Instrument

On Mon, 31 Jan 2005, xerj wrote:

let's say you're in flight in a normally aspirated prop plane, your
destination is blown up, your alternate is unservicable for some reason
or other and you need to stretch the fuel you have the farthest distance
possible.

Wouldn't it simply be your airplane's best glide speed? Or am I missing
something?

Of course if you can't avoid fighting a headwind, you'd have to do some
math with ground speed vs. fuel consumption at some higher airspeed to
find max range. Or is that what you're looking for?

-Dan

Wouldn't it simply be your airplane's best glide speed?

It would be the same AoA, but not necessarily the same speed. This speed
also varies with density altitude and weight.

What prompted these questions is thinking "O.K. what if this actually
happened in the cockpit without the luxury of power curve charts and a place
to go work it all out for a ten minutes".

Thanks John. I'll look for your book. Probably pretty pricey here in
Australia, but seems like a good one from all the mentions.

"John T Lowry" wrote in message
ink.net...

"xerj" wrote in message
...
I understand the theory vis a vis lowest L/D TAS and the power
available/power required chart, but let's say you're in flight in a
normally aspirated prop plane, your destination is blown up, your
alternate is unservicable for some reason or other and you need to stretch
the fuel you have the farthest distance possible.

What are the quick and dirty steps to working out the optimum alt and
speed? I mean in the cockpit, not in the comfort of home.

Thanks in advance.



Unfortunately, there's no quick & dirty way. The usual (Bruguet) theory is
deficient in its assumptions, and the usual cruise table is deficient in
that the wrong independent variables were selected by the GAMA committee.
At those lower speeds, you can't depend on propeller efficiency being
independent of air speed. For details and an example of what to do, see
the cruise chapter of Performance of Light Aircraft.

John Lowry
Flight Physics

I do not know anyway to determine this from inside the cockpit unless
you have ground distance covered and fuel flow data on the panel...
Best thing is to work this out before hand... Use your GPS (wonderful
research instrument) for distance measurements... Take off with one
test tank carefully filled to a specific mark, using the non test
tank... Pick the best range setting from the book, pick a start point
on the ground, switch to the filled tank at the start point, and make a
square pattern run (4 sides) for five to ten minutes per side, switch
off the test tank when again at the start point... Land... Fill the
test tank... Do your calculations... Now you have a data point to start
from... Pick a slightly higher or lower power setting and repeat the
test... Now you will know whether you are improving or not, and adjust
your next run accordingly... After three or four runs you will have a
good idea where the peak of the range curve occurs... Of course, a
different altitude and temperature and wing loading will change it a
few percent, but you will be as close as you can get in the real world
if you ever have to do it for real... Besides, this testing gives you
a reason to go flying other than the $100 hamburger run...

xerj wrote:

Thanks John. I'll look for your book. Probably pretty pricey here in
Australia, but seems like a good one from all the mentions.

I found that it was available through inter-library loan from one of our
colleges. That book has more equations in it than my Calculus textbook did. :-)

George Patterson
He who marries for money earns every penny of it.

"steve.t" wrote in message
oups.com...
If you have a general idea of the best altitude for your aircraft (fuel
economy wise), and you can pull power and lean until you are able to
keep the RPM in the green and maintain altitude, then all you need do
is turn so that the wind is behind you. Remember, the more angle of
attack, the more drag you will have. So you may need to keep more
power, say to 60%. I would guess in my PA28-180 that I would need to
cut power down to about 2000RPM (well above bottom of green). This
would probably put me down in the 4x% range, but with a large angle of
attack. However, if I brought the power up to 2200 and was at ~5000
MSL...

Note that at this point I have not referred to the POH. I'm doing this
sitting here by seat of pants outside the plane.

Without going into lots of physics, I would guess that this would give
you your best time in air and distance.

If you MUST deal with any kind of headwind, then you will have to run
with enough power to overcome the wind. And now you must start doing
some serious figuring as to what altitude gives you your best chance of
getting to where you need to go.

And in the situation you have just given, forget GPS and LORAN. Hope
you have an ADF, because you probably won't pickup VORs either.
Later,
Steve.T
PP ASEL/Instrument

Why would you forget GPS?

On Mon, 31 Jan 2005, xerj wrote:

Wouldn't it simply be your airplane's best glide speed?

It would be the same AoA, but not necessarily the same speed. This speed
also varies with density altitude and weight.

hmm, AoA... Well just off the top of my head here, someone please correct
me if I'm wrong...

Stall speed, of course, also depends on AoA, not speed. So, if you have
an idea what stall speed does at different weights & density altitudes,
best glide speed should follow the same curve. Best glide is the point at
which overall drag is lowest, so it stands to reason it's also where max
range would be. If you can't avoid a headwind you may need to speed up a
bit to get the best ground speed per fuel burn, but that's a simple
calculation using ground speed & a close estimate of fuel burn.

So it seems, in practice, one should be able to get really close to max
range speed very quickly without any complicated calculations.

-Dan