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optimal altitude calculations?



 
 
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  #1  
Old August 26th 05, 12:17 AM
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Default optimal altitude calculations?


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

  #4  
Old August 26th 05, 03:48 AM
Seth Masia
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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/...nce/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.


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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



  #6  
Old August 26th 05, 05:03 AM
Ron Lee
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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
  #7  
Old August 26th 05, 05:10 AM
Aluckyguess
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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.


  #8  
Old August 26th 05, 05:14 AM
George Patterson
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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.
  #9  
Old August 26th 05, 03:55 PM
Michael
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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

  #10  
Old August 27th 05, 01:11 AM
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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

 




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