I am confused by my GLIDE SPEED numbers

[RV8R999]

Unless the x96 knows airspeed the glide ratio calc will not be accurate as it will be using groundspeed which is subjected to errors due to wind. Don't do this.

[N999BT]

Interesting data, but not that surprising. Some of these have already been mentioned, but here is what I would suggest:

1. You should do at least 3 data runs and use the average numbers. This is a pain, but a single data point may have errors in it that you did not notice when you were doing the flight, like thermal activity.
2. My best glide is faster than your highest reading, so you may want to go up to 100 knots or so.
3. As was mentioned earlier, measure the time it takes to descend between two altitudes. This is more accurate than any other method.
4. Since evidently either the steam gauge or the dynon or both have some sort of calibration error, you should figure out what the error is. I suspect that your steam gauge is not accurate at lower speeds and the dynon is better, so I would use the dynon. You can calibrate your ASI's using three leg (or 4) GPS runs. This is described elsewhere on the forums.
5. All other things being equal, you will want to use the best glide ratio that is the fastest IAS becuase there will be less effect in a real live engine out situation due to wind. (Been there, done that, it happens.)

You probably know this already but Glide ratio is [(TAS (NM/hr)) x (6076 ft/NM) / (60 min/hr)] / descent rate (ft/min)

I hope this helps.

[erich weaver]

Originally Posted by RV8R999

Unless the x96 knows airspeed the glide ratio calc will not be accurate as it will be using groundspeed which is subjected to errors due to wind. Don't do this.

Huh? I think you may be missing the point. Glide ratio is the horizontal distance traveled compared to the vertical distance. When you have an engine out, you typically will want to fly at the airspeed that gives you the best glide ratio until you have your landing spot made. That speed may vary somewhat depending upon your flight direction relative to the wind, and the wind speed. All that is necessary is to adjust your airspeed until the glide ratio, as indicated on your x96 GPS, is maximized. You have then by definition achieved best glide speed.

erich

[n5lp]

Originally Posted by erich weaver

...All that is necessary is to adjust your airspeed until the glide ratio, as indicated on your x96 GPS, is maximized. You have then by definition achieved best glide speed.

erich

Yes, that would work nicely for an actual engine out situation where you have to go in a specific direction to a landing spot. It does not find the best glide speed through the air, it finds the best glide speed over the ground for one specific situation. The best speed for maximum distance over the ground does vary greatly with wind direction and speed.

[Bob Kuykendall]

Originally Posted by erich weaver

Huh? I think you may be missing the point. Glide ratio is the horizontal distance traveled compared to the vertical distance. When you have an engine out, you typically will want to fly at the airspeed that gives you the best glide ratio until you have your landing spot made. That speed may vary somewhat depending upon your flight direction relative to the wind, and the wind speed. All that is necessary is to adjust your airspeed until the glide ratio, as indicated on your x96 GPS, is maximized. You have then by definition achieved best glide speed.

I think that what they're trying to do is record the airspeed that gives the best no-wind L/D. Just reading it from the GPS will give the best L/D speed for whatever wind prevails at the time, but that speed is not necessarily applicable to other wind conditions.

And, if you propose to just twiddle up the Best L/D feature on the GPS when the engine quits, I'm thinking you might be a bit busy just then to be heads-down doing the black box menu dance. Better perhaps to know what the no-wind best L/D speed is, capture it, and then set about working other aspects of the problem.

Thanks, Bob K.

[fehdxl]

Originally Posted by RV8R999

Unless the x96 knows airspeed the glide ratio calc will not be accurate as it will be using groundspeed which is subjected to errors due to wind. Don't do this.

True, the actual glide ratio number will be incorrect because of wind, but the speed at which relative maximum glide is achieved will be correct. Or are you considering the winds are changing during the descent and therefore even the relative ratios are skewed?

-Jim

[n5lp]

Originally Posted by fehdxl

True, the actual glide ratio number will be incorrect because of wind, but the speed at which relative maximum glide is achieved will be correct.

No it will not be. The best speed to fly for maximum distance over the ground will vary above and below best aerodynamic glide speed.

Originally Posted by fehdxl

Or are you considering the winds are changing during the descent and therefore even the relative ratios are skewed?

-Jim

No, with steady winds, the method still does not work to find best aerodynamic glide speed.

I had to chuckle to myself that the first two people to answer you are glider pilots. These principles become ingrained very quickly when you are trying to cross inhospitable terrain and there is no more lift. With a tailwind you need to fly slower than best glide speed and with a headwind you need to fly faster. Student glider pilots learn this very early in training.

[Bob Kuykendall]

Originally Posted by fehdxl

True, the actual glide ratio number will be incorrect because of wind, but the speed at which relative maximum glide is achieved will be correct...

Actually, that's not necessarily correct. As every glider pilot knows, the speed for covering the most ground definitely varies depending on the wind.

Consider the corner case where your no-wind best L/D speed is 70 knots, and you are flying into a 70-knot headwind. Your effective glidepath will be straight down. If you increase your speed to 100 knots, your sink rate goes way up, but at least you're going 30 knots forward and getting somewhere.

Likewise, when you have a tailwind, the speed to fly to cover the most ground is generally somewhere between the best L/D speed and the minimum sink speed.

Thanks, Bob K.

[WhiskeyMike]

Originally Posted by n5lp

With a tailwind you need to fly slower than best glide speed and with a headwind you need to fly faster. Student glider pilots learn this very early in training.

This of course is true. However, in order to accurately modify your airspeed to optimize the headwind/tailwind conditions you first need to know the "still air" best L/D (glide) speed of the aircraft. I assume that this is what the OP was attempting to ascertain through flight tests.

I do not have experience with the Garmin x96 best glide calculation function. But if using this product/feature actually derives an accurate best L/D speed for the then-present conditions it should be relatively straight forward to develop best L/D data for still air. Simply employ the Gx96 function during an early morning test flight (no convection) when there is zero wind. This would be substantially simpler and less costly than making multiple-climb test flights and timed descents.

Can those with experience comment on the accuracy of the Gx96 best glide calculation function?

[erich weaver]

Originally Posted by Bob Kuykendall

And, if you propose to just twiddle up the Best L/D feature on the GPS when the engine quits, I'm thinking you might be a bit busy just then to be heads-down doing the black box menu dance. Better perhaps to know what the no-wind best L/D speed is, capture it, and then set about working other aspects of the problem.

No, I do NOT propose to go heads-down into the black box menu dance, because I have already set up the GPS to have this on the screen (flight parameters page) so I dont have to.

In response to some of the other posts - the only glide speed I care about is the one that applies to the conditions in place when my engine quits.
Go up and test out the GPS function a few times at idle speed when conditions are relatively calm and you will very quickly know the approximate airspeed to shoot for. When the big engine out event actually happens you can fine tune your speed with the GPS display function as time/mental state permits. As with many things, a little bit of practice/experience can make things go a lot better when you have a bad day. Certainly no harm in trying out this tool.

erich