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Date: Mon, 23 Nov 1998 15:26:52 -0600
From: Snowbird <snbird@ibm.net>
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Subject: Re: What could he have done different
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Mike Regish wrote:
 
> I wasn't thinking in terms of how far he could go, but how long he 
> could stay aloft and also if you're lost and going fast, you're just 
> getting lost fast. I think I read that best L/D is the maximum
> endurance speed.  It makes sense that it would also be longest range
> since the drag is lowest and the fuel consumption should also be 
> lowest.  

No.  Maximum range speed (which is close to best L/D or Vbg) is
NOT maximum endurance speed.  Maximum endurance speed will typically
be lower.  You're basically trading off increased lift/increased
drag for decreased thrust (lower fuel burn=more time aloft from the
same fuel).  Fuel consumption is lowest at max. endurance speed, 
not at maximum range speed (best L/D).

Whatever speeds you might choose to fly in that student's situation,
I think it's useful to understand that max range and maximum endurance
are different things.  If your concern is "how long he could stay
aloft", it's best endurance speed you want. 

For a good article on the subject of maximum range and maximum
endurance, and why they are different speeds, try:
  http://www.db.erau.edu/research/cruise/piston.frame.html

There's a less technical article at:
  http://www.avweb.com/articles/lowfuel.html
but I'm a little uncertain about a couple of his points (notably
that best range is near Vy+15%, that's way different than what the
POH for my aircraft has to say).

Snowbird


