How to simply determine the L/D of your glider

On Mon, 10 Jan 2011 10:07:54 -0800 (PST), jan
wrote:


If you haven't done these seat of the pants measurements with your
own gliders I would suggest you all do them. It takes a little
practice and time to do the averaging over many flights. Interestingly
after a time and with practice you will find the measurements become
quite consistent which suggests that they might represent something
close to reality.

Hi Dave,

as others already pointed out, you are measuring an L/D that is
typical for your flying style, but not the performance of your glider.

In my AS 22-2 I usually get an L/D of 75 (!) on long flights, although
in reality its maximum L/D is only about 55.


Andreas

The purpose to me in knowing the real L/D is not for bragging rights
but to have the correct value in the flight computer.

On Tue, 11 Jan 2011 08:55:45 -0800 (PST), Gary Evans
wrote:

The purpose to me in knowing the real L/D is not for bragging rights
but to have the correct value in the flight computer.

The "correct value" at which speed?
I suspect that you don't want the correct value, but the correct polar
in your flight computer - which, obviously, cannot be measured in the
way described.


Andreas

On Jan 11, 10:46*am, Andreas Maurer wrote:
On Tue, 11 Jan 2011 08:55:45 -0800 (PST), Gary Evans

wrote:
The purpose to me in knowing the real L/D is not for bragging rights
but to have the correct value in the flight computer.

The "correct value" at which speed?
I suspect that you don't want the correct value, but the correct polar
in your flight computer - which, obviously, cannot be measured in the
way described.

Andreas

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.

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

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

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.

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.