Adjusting glide ratio

[Yanosik]

Hello,
I am trying to adjust a proper glide ratio and I have few questions.

1. How do you measure glide ratio during testing? My way: I get latitude, longitude and altitude data from simvars and simply glide for about a minute to get a good measure. Then I compare distance traveled to height loss and thats it. I conduct these tests at a relatively small altitude to reduce air density error (about 1000-2000 feet). I found this method reliable but it takes a lot of time. Also you must fly all these glides and have your airspeed stabilised very well and that is exhaustive. Maybe there is a easier way to measure L/D?

2. Achieved speed polar (measured using above method):
   Airspeed / Achieved L/D / Proper L/D
   85km/h / 51 / 31
   120km/h / 25 / 23
   150km/h / 16 / 17

So at speed of 120 and 150 there is not that much difference and i could live with that but at 85km/h (where actual best 31 L/D should be achieved) thats obviously too much an error.
I tried fixing this with induced and parasitic drag scalars with an understanding that reducing lift-induced drag scalar would decrease lift at low speeds and wont change much at higher speeds. After many tries of trying with different combinations of these two scalars i couldnt find proper one.
Then I tried fixing the problem using drag_coef_zero_lift_mach table (I read this topic Understand tables and formatting? - #3 by JacobW_xblms). I tried this way: convert airspeed to mach feg. 80km/h is around 0.07m and than play with different coeficient values (like 0.001, 0.1, 1) for this speed and also add table rows with different speeds (150, 120kph) at the same time or only for one speed - anyway I was getting no change in performance or a drag-machine that sinks 15m/s at best L/D I think there is no point in explaing exact way i wrote these numbers as far as someone says that could be actually the way to fix the L/D problem but maybe that’s a wrong method.

3. And the last one - That is not a question but a project reveal to get you interested in
   Here are few shots of my SZD-30 Pirat!
   
   
   
   
   
   
   
   
   
   
   

Thanks for you help!

[B21]

If you look in the other freeware gliders (e.g. LS4?) you’ll see there’s a XML component that computes glide ratio, so you can look at that var in the Tools / Behaviors / LocalVars SDK window. Note that ANY instantaneous display of glide ratio MUST use TE-corrected sink rate plus some smoothing otherwise there will be too much error in any readings. Only use that as a GUIDE, don’t rely on it.

After you’ve got the induced/parasitic drag giving you something approx right, the next thing to tweak adjusting the polar is the airfoil AoA/lift table as that allows you to tweak the relative efficiency of high speed flight (low or negative AoA) and low speed flight (high AoA).

When you think the L/D is in the right zone then there is no substitute for flying over the sea with Clear weather with the wind layer deleted in straight line for 10km from maybe 3000 feet and record a GPX tracklog using SimFlightPath or Little Navmap. You can look at that tracklog in B21 Task Planner (google) and it will calculate the glide ratio for any section of the tracklog you zoom in to.

When you’ve got the polar curve mostly right with zeroes in the drag mach table, you can then tune the drag at speeds using the mach table as you’ve tried. The number in the mach table simply gets added to the overall drag figure. It’s a very useful capability for tuning gliders where the L/D matters.

There are parameters that control when the stall happens, and how quickly the stall kicks in - these actually give you a little bit of control at the low speed end of the polar and mitigates the default MSFS behavior that L/D improves as you slow down all the way to the stall.

You’ll need to extend the manufacturer polar to guess the sink at (say) 140 knots / Vne, and then try and get the L/D right at (say) 40 knots, 90 knots, 140 knots. I’m assuming the Pirat has no flaps & no water ballast, so this process is nowhere near as complicated as it would be with those.