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How can glide ratio be accurately measured with typical GA airplane instruments in real conditions?

What, if any, specialty instruments are used for professional glide ratio testing (such as a manufacturer developing a new model)?

I would like to know the actual glide ratio of my airplane at several combinations of configuration and airspeed. The POH only gives the best glide distance in the clean condition.

I know there are a lot of guesses and assumptions in the pilot community, often applied far too broadly even if they can be traced to some legitimate source, but I want some real data; e.g., what is the true effect of flaps 10? Just how much is the glide ratio harmed by a 5-kt or 10-kt speed change? What is the glide angle with gear down?

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  • $\begingroup$ GPS measures position and time. What more would you need? $\endgroup$ Feb 25, 2023 at 23:42
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    $\begingroup$ GPS is completely blind to actual wind speed and direction. $\endgroup$
    – Max Power
    Feb 26, 2023 at 0:01
  • $\begingroup$ There definitely are specialized instruments for measuring drag in flight, which is directly relevant. Google "drag rake flight testing". (Leaving off the last two words will lead to a lot of hits on lawn tools!) I'm not completely certain how applicable this device is to powered aircraft, but I don't see why it wouldn't be. $\endgroup$ Feb 26, 2023 at 8:42

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It is tricky to do well.

First, GPS isn't helpful because it can't account for the effects of wind. You can fly upwind and downwind legs to try to cancel it out, but it will still be there. We usually use airspeed and altitude instead.

The next challenge is that you want to eliminate the effects of thrust. In a glider, this is easy. In a powered aircraft, even when you pull back to idle, the propeller is either producing thrust or contributing to drag.

For a moment, let's assume you're in a glider -- just to get the procedure down.

Get to altitude, then pitch for some speed. Let the phugoid damp out until you are in a stabilized descent. Measure and record speed and rate of descent. You'll also need to know your weight and the temperature and density of the air (OAT and PA will do). Repeat this process for a bunch of different airspeeds.

You can process this into a 'glide polar' -- which can then be processed into a drag polar for the aircraft. This will give L/D vs. CL.

Many approaches have been taken to eliminate the effects of thrust. They took the propeller off of the P-51 and towed it aloft to have it glide down as one way.

The CAFE Foundation developed a sensor sensitive to the play in the propshaft -- in thrust, it is forward (one light is lit) -- in drag, it is aft (another light is lit). The pilot would adjust throttle and/or propeller pitch until the lights were flickering back and forth. Dig around in the CAFE Foundation's website to learn more about all that they did.

You may be interested in Lowry's 'Performance of Light Aircraft'. It details approaches that a GA pilot can take to measure the performance of their aircraft. Unfortunately, it is out of print and used paper copies can be pretty pricy and hard to find. Set up an eBay search and be patient. Or, the electronic copy is more reasonable https://arc.aiaa.org/doi/book/10.2514/4.103704

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  • $\begingroup$ The missing prop scenario is not relevant because the aircraft has a prop. However, being at idle, power off windmilling, or dead stick might be a significant consideration, but I will just lump those into "configuration". $\endgroup$
    – Max Power
    Feb 26, 2023 at 0:14
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    $\begingroup$ In a glider, this is easy. Actually, polar measuring is a contested topic in the gliding community, and historically it has happened that different measurements for the same glider do not agree as much as one would want them to. $\endgroup$ Feb 26, 2023 at 13:17
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You want the propeller stopped for best glide in a power plane with a fixed pitch prop. A windmilling prop is a powerful speed brake. On the other hand, if an engine quits at low altitude, trying to get the propeller stopped might be too dangerous. But it would be useful to know the values for both cases.

The propeller spinning with the engine at idle will be reasonably close to the drag of a stopped prop, which is significantly less than a windmilling one, and more than one turning at a "zero thrust" speed (somewhere off idle, but no way to know exactly where that is without the ability to measure thrust at the engine)

Just get it stabilized and trimmed hands off in a glide in the configuration you want, in smooth air, engine idling, and note VS and TAS. Convert the TAS to Ft/Min, or the VS to knots, divide one into the other, and there's your glide ratio. If you want more precision, do a timed descent using the altimeter instead of trying to discern the VSI.

If you're feeling brave, you can get to a safe altitude over your airport or some other uncontrolled airport with a decent long runway, and shut the thing right down at altitude, and take your measurements with the propeller windmilling and stopped, restarting as soon as you're done, and allowing for the engine to warm up.

You might have to slow down close to stall to get it to stop, depending on how good the engine's compression is (a newer, tight engine may stop turning at or above best glide speed). If you had an engine failure at high altitude and needed to plan a long glide and needed every foot of distance because your engine quit over a glacier or a field of boulders, it might be worth the effort to get the prop stopped.

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  • $\begingroup$ I suppose IAS along with GPS for true vertical might be the best bet. Still doesn't account for updrafts/downdrafts but picking good calm days and averaging a few runs should result in reasonable accuracy. Though I'd rather limit the number of flight hours spent on it. $\endgroup$
    – Max Power
    Feb 26, 2023 at 23:43
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    $\begingroup$ Go out at 8 am when things are dead calm, and just set up a glide and time it to go from 5000 ft to 4000 ft off the altimeter to get the VS, and convert IAS to TAS and carry on from there. You will get a value good enough for any normal use. For practical application, work out the distance covered per thousand feet of altitude and memorize that. $\endgroup$
    – John K
    Feb 27, 2023 at 5:56

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