Glide ratio emperical data

[jnmeade]

Let's suppose I wanted to test a glider to determine it's glide ratio (or to see if the published glide ratio is accurate).
Given that I don't know how to do this. The following equipment is on board:
Dynon 100 EFIS
Dynon 120 EMS
Garmin 496
TruTrack 2 axis AP
What ways do you know of that would let me get close in a couple of tests - not a dozen rigorously done in ideal weather but a couple that were done with care. Precision important but not vital.
One way is to start at an altitude, maintain a guess at and carefully maintain the best glide speed, and measure the time to another altitude. Gives you A & C, Solve for B.
Other ways of measuring A&B would require one to make good estimates on wind, I'd think. Ideas?

 

[CnBeta]

Well, given you said the following....

What ways do you know of that would let me get close in a couple of tests - not a dozen rigorously done in ideal weather but a couple that were done with care. Precision important but not vital.

Why bother? Just trust the data in the manual.

The test required to determine Vbg are fairly rigorous and require ideal air quality (smooth air, no temp inversion). Wind at altitude is ok as long as its at a fairly consistent direction and speed and the test are conducted normal to the wind direction.

 

[jnmeade]

1. I want to do it anyway.
2. I don't trust the manual, for reason.

 

[CnBeta]

Ok, then how about this. Try a series of descents starting at an airspeed just above stall ~1.1Vs. Be sure to do the descent in very smooth air and hold your target airspeed +\- 2 kts for at least one minute. Note your altitude at the start of the minute and at the end then average your rate of descent for that minute. Then, increase your target airspeed 5 kts and repeat. Do at least 5 speeds or until you reach 1.5 Vs. Then take a look at the average minute ROD values and plot them on an X-Y chart of ROD vs IAS. Select the airspeed that has the smallest ROD.

It's essential the test be done is smooth air. Yeah, not the ideal weather for flyng gliders. Any lift during your one minute timed descent will skew the data. Best of luck.

 

[denverpilot]

1. I want to do it anyway.
2. I don't trust the manual, for reason.

Might be easier to talk to Tony. He's an expert at landing out.

(Loved the photo in the field on FB today, Tony!)

 

[jnmeade]

Might be easier to talk to Tony. He's an expert at landing out.

(Loved the photo in the field on FB today, Tony!)

That's way harsh!.

Actually, I emailed Tony off-line and he sent back a very good plan as used by Dick Johnson of glider fame who has done a lot of these. I'm kind of trolling here for other good ideas.