hi: I have a simple question for the piloting physics majors. we all
know that planes have less air resistance to overcome at higher
altitudes, but that normally aspirated planes have less power at higher
altitudes. presumably, both are proportions of what happens at sea
level, and are hopefully not too plane dependent. That is, I would
guess that a 160hp engine would lose about the same proportion of power
as a 320hp engine. for lycomings, at 10,000', this proportion is about
50%. something similar [proportional reduction] may also happen to air
resistance, regardless of whether the plane is a cub or a lancair.

this leads me to a very simple question: on a standard day, without
any winds, what would be the optimal altitude for [cruise] speed in a
normally aspirated airplane? is this best altitude dependent on
aircraft to a first-order, or is it fairly constant across airplanes?

sincerely,

/ivo welch

wrote:
> this leads me to a very simple question: on a standard day, without
> any winds, what would be the optimal altitude for [cruise] speed in a
> normally aspirated airplane? is this best altitude dependent on
> aircraft to a first-order, or is it fairly constant across airplanes?

I pick the "full-throttle" altitude, usually between 6000 and 7000 feet
MSL, depending on temperature and the engine.

"john smith" > wrote in message
.. .
> wrote:
>> this leads me to a very simple question: on a standard day, without
>> any winds, what would be the optimal altitude for [cruise] speed in a
>> normally aspirated airplane? is this best altitude dependent on
>> aircraft to a first-order, or is it fairly constant across airplanes?
>
> I pick the "full-throttle" altitude, usually between 6000 and 7000 feet
> MSL, depending on temperature and the engine.

8,000' is a figure I see quoted frequently, which is <roughly> the highest
altitude a non-turbocharged engine can maintain 75% power, which is normally
considered max cruise.

KB

Yes and no. If you're measuring efficiency in terms of range, once you've
found your best cruising airspeed and engine speed, altitude won't affect
range much, though you'll cover the distance faster the higher you go. If
your best range is at 55% power, then you'll probably do best to climb as
high as 55% will allow (assuming still air) and lean for best economy. See
http://142.26.194.131/aerodynamics1/Performance/Page7.html

That said, the differences between airframe and prop designs are
significant. Light weight, better L/D and a constant speed prop mean better
range at all altitudes. A Mooney is more efficient than a 172 with the same
engine.


> wrote in message
oups.com...
>
> hi: I have a simple question for the piloting physics majors. we all
> know that planes have less air resistance to overcome at higher
> altitudes, but that normally aspirated planes have less power at higher
> altitudes. presumably, both are proportions of what happens at sea
> level, and are hopefully not too plane dependent. That is, I would
> guess that a 160hp engine would lose about the same proportion of power
> as a 320hp engine. for lycomings, at 10,000', this proportion is about
> 50%. something similar [proportional reduction] may also happen to air
> resistance, regardless of whether the plane is a cub or a lancair.
>
> this leads me to a very simple question: on a standard day, without
> any winds, what would be the optimal altitude for [cruise] speed in a
> normally aspirated airplane? is this best altitude dependent on
> aircraft to a first-order, or is it fairly constant across airplanes?
>
> sincerely,
>
> /ivo welch
>

wrote:
>
> this leads me to a very simple question: on a standard day, without
> any winds, what would be the optimal altitude for [cruise] speed in a
> normally aspirated airplane? is this best altitude dependent on
> aircraft to a first-order, or is it fairly constant across airplanes?

It varies from aircraft to aircraft; it's basically the point at which the
engine is putting out 100% rated power at full throttle. It tends to be
somewhere between 6,000' and 9,000'. From memory of the manuals, it's 6,500' for
a Cessna 150J and 8,600' for a Maule MX-7-160.

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.

George Patterson > wrote:
>
>It varies from aircraft to aircraft; it's basically the point at which the
>engine is putting out 100% rated power at full throttle. It tends to be
>somewhere between 6,000' and 9,000'. From memory of the manuals, it's 6,500' for
>a Cessna 150J and 8,600' for a Maule MX-7-160.

Me thinks that full throttle power in the 8000' area is 75%.

Ron Lee

The Bonaza I just got checked out in Burns 13 gah @ 8500 and will cruize
165 knots. If I go to 17500 it will burn 10 gah and cruize @ 160 knots.
This is by memory so it may not be exact.

Ron Lee wrote:
>
> Me thinks that full throttle power in the 8000' area is 75%.

You're right - I should have said 75% power at full throttle.

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.

> this leads me to a very simple question: on a standard
> day, without any winds, what would be the optimal
> altitude for [cruise] speed in a normally aspirated airplane?

That's not really a simple question. You haven't really defined
optimal.

If by optimal you mean highest available cruise speed, then it's sea
level. That's because most normally aspirated engines may be run at
full power continuously.

If by optimal you mean highest available cruise speed at maximum
RECOMMENDED (not allowed) continuous power, it's the highest altitude
at which the engine can develop maximum recommended continuous power.
Depending on the prop, that might be anywhere from 6000 to 8000 ft
(density altitude of course) for 75% power.

If by optimal you mean best fuel efficiency, it's the altitude where
maximum available power produces an indicated airspeed equal to the
best glide airspeed (unless the engine/prop happens to be inefficient
in that condition) - in other words, this will vary with the airplane,
the engine, and the operating weight.

Michael

gentlemen---thank you for your answers. let me mean "highest
continuous cruise speed." 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.

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.

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

regards,

/iaw

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.

ram air, hmmm---I guess this is what I suspected. ram air would have
been too easy. except the part that I would also need more fuel. I
thought there was an efficiency loss due to not enough air, of course
counterbalanced by the fact that I can lean the mixture.

I presume there are no (bicycle or) other pumps that are both
affordable and that could do a decent job (aid efficiency a lot more
than they reduce it). it seems odd for our engines to remain
essentially oxygen starved, or to have a turbo which costs >$20k and
seems to cause all sorts of reliability problems on top of it. one
would think there could be low-priced ways to help the situation at
least a little. but such is life...

thanks for all the info.

regards,

/iaw

> wrote in message
ps.com...

> I presume there are no (bicycle or) other pumps that are both
> affordable and that could do a decent job (aid efficiency a lot more
> than they reduce it).

There are lots of pumps that can pressurize the intake on an engine, they
are generally large and take a lot of power to operate which is what you
would expect when you consider the amount of air being compressed. A small
pump (like a turbo) will have to operate very rapidly (~100,000rpm) and
won't be cheap while a large pump will not have to operate very fast but
will be heavy.

it seems odd for our engines to remain
> essentially oxygen starved, or to have a turbo which costs >$20k and
> seems to cause all sorts of reliability problems on top of it.

All engines can be considered to be "oxygen starved" since an engine
operating on pure O2 will produce about twice the power of one operating on
air. The turbosupercharger is an efficient (over 70%) pump that uses energy
that is normally wasted to operate. The turbo is very reliable, most the
problems attributed to turbos are more a function of trying to operate the
engine at high altitudes and high power where it is difficult to cool the
cylinders.

Mike
MU-2

> 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