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How to simply determine the L/D of your glider



 
 
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  #1  
Old January 12th 11, 03:56 AM posted to rec.aviation.soaring
mattm[_2_]
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Posts: 167
Default How to simply determine the L/D of your glider

On Jan 11, 9:12*pm, Andreas Maurer wrote:
On Tue, 11 Jan 2011 13:56:32 -0800 (PST), Gary Evans

wrote:
This flight computer I used required 3 pieces of speed/sink rate data
to represent the polar. Best L/D, speed at best L/d and V2.


This what all flight computers do - they convert these three points
into an approximated polar curve.

But how do you get these three points by the method described by Dave?

Andreas


Well, I'll bite. Here's what I do for the planes I fly:

First, I've been fortunate enough to fly planes that have Johnson
reports
available. I know there are probably Akaflieg reports as well, but I
haven't
seen those. I carefully extract the data points from Dick's polar
charts and
correct them for my flying weight (unfortunately always considerably
higher
than Dick!). I input the adjusted values into my PDA (which just
wants
the sink rate at 3 airspeeds, rather than the numbers listed above).
Finally,
I set the Polar Potential via experiment. Typically I'll set it to
90% and then
see how well my final glides work out. If I have a bunch of altitude
left over
on a glide then I'm doing better, and I'll increase the potential. If
I tend to fall
below glideslope a lot then I'll decrease the potential. For the most
part I've
wound up with values around 90% or 92% (which probably means I need to
work harder at tuning up the planes I fly).

Essentially this is a refinement of the beginner approach to glide
slopes:
take the published value and divide by 2 as a safety factor. I divide
by something
closer to 1.1 and usually make it home just fine. The times I've had
to
break off have been because I was below glideslope to begin with.

-- Matt
  #2  
Old January 12th 11, 10:36 AM posted to rec.aviation.soaring
Bruce Hoult
external usenet poster
 
Posts: 961
Default How to simply determine the L/D of your glider

On Jan 12, 4:56*pm, mattm wrote:
Essentially this is a refinement of the beginner approach to glide
slopes: take the published value and divide by 2 as a safety factor.
I divide by something closer to 1.1 and usually make it home just fine.


2 is a bit harsh!!

I use 32.8:1 plus circuit height no matter what I'm in :-)

It's about right for a PW5 (or K6), about your factor of 1.1 for a
Libelle or Grob Twin, and maybe 25%-30% pessimistic for a Janus or
DG1000.

Those are also in order (and I think reasonable proportion to) the
cost and embarrassment and inconvenience of breaking each of those
gliders!

It also happens to be 100 ft per km, which doesn't require a computer
to calculate.
  #3  
Old January 12th 11, 02:26 PM posted to rec.aviation.soaring
jcarlyle
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Posts: 522
Default How to simply determine the L/D of your glider

That certainly works, Matt. I've found it easier, though, just to go
to the set-up page on the SN-10 and select the polar values from
there. grin But you can also go to Paul Remde's site; he has polar
values for a large number of planes listed he http://www.cumulus-soaring.com/polars.htm

-John


On Jan 11, 10:56 pm, mattm wrote:
Well, I'll bite. Here's what I do for the planes I fly:

First, I've been fortunate enough to fly planes that have Johnson
reports
available. I know there are probably Akaflieg reports as well, but I
haven't
seen those. I carefully extract the data points from Dick's polar
charts and
correct them for my flying weight (unfortunately always considerably
higher
than Dick!). I input the adjusted values into my PDA (which just
wants
the sink rate at 3 airspeeds, rather than the numbers listed above).
Finally,
I set the Polar Potential via experiment. Typically I'll set it to
90% and then
see how well my final glides work out. If I have a bunch of altitude
left over
on a glide then I'm doing better, and I'll increase the potential. If
I tend to fall
below glideslope a lot then I'll decrease the potential. For the most
part I've
wound up with values around 90% or 92% (which probably means I need to
work harder at tuning up the planes I fly).

Essentially this is a refinement of the beginner approach to glide
slopes:
take the published value and divide by 2 as a safety factor. I divide
by something
closer to 1.1 and usually make it home just fine. The times I've had
to
break off have been because I was below glideslope to begin with.


 




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