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Subject: Re: L/D Max
Date: Sun, 26 Aug 2001 09:25:02 -0500
From: Dane & Cindy White <brammage@earthlink.net>
To: "Lancair List" <lancair.list@olsusa.com>
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          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
          <<  Lancair Builders' Mail List  >>
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>>
> Determining maximum range would
>require graphing fuel burn against airspeed, and somewhere along that curve
>you'd find the best speed to fly to go the farthest based on available fuel.
>
>  <Marv>     ]

Marv,

You are correct about using a graph of fuel burn versus airspeed to 
determine the speed to fly for max range or endurance.

Recall from aerodynamics that if you plot total drag (thrust required for 
level, unaccelerated flight) versus angle of attack, you get a "U" shaped 
curve. The bottom of that curve defines the angle of attack for minimum 
drag - along with a corresponding airspeed at a given weight. This 
defines the angle of attack and airspeed (for a given weight) for maximum 
endurance since this is the minimum thrust required for level flight.

A line drawn tangent from the graph's origin to the same total drag curve 
defines the angle of attack and total drag for max L/D. Obviously, this 
will be a faster airspeed, at a lower AOA, with more total drag, but 
you'll maximize your air range flown.

Once you've determined the angles-of-attack for max L/D and min drag 
you're done. These angles don't change unless you change the 
configuration (lower flaps or gear). The airspeed to achieve these AOA's 
changes with weight. 

So in addition to avoiding aerodynamic stalls, an AOA gauge can help you 
quickly determine the airspeed to fly for maximum performance.

Disclaimer: This discussion ignores the effects of changes in efficiency 
with airspeed changes of piston powered aircraft. 

Dane White
Lancair IV-P
Murfreesboro TN


{Thanks for the backup, Dane.  My point in trying to respond to Fili's
post about max range vs endurance, is that endurance is a measure of time,
without regard to distance.  Range adds distance to the equation and is
a separate issue.  The two are not necessarily the same in the context
of an AOA discussion.

  <Marv>       ]

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