I am confused by my GLIDE SPEED numbers

Aircraft is a -6A, with wheel pants and fairings installed.
Pitot and static system leak free, and seem to be reasonably accurate in level flight.

I am using the "FLIGHT TESTING HOMEBUILT AIRCRAFT" by Askue, and the "ILLUSTRATED GUIDE TO AERODYNAMICS" by Smith as the reference documentation for the performance testing of the aircraft.

I was expecting to see a fairly simple line graph plot of descent velocity, -fpm, (converted to glide ratio) vs airspeed. My plotted numbers do no look anything like I was expecting.

Flight conditions were as follows: max gross weight, Density Altitude = 4,980 ft. Flaps UP. Engine at idle. Slow to 80knots indicated and start the glide test.

As the airspeed stabilizes and the descent rate stabilizes, record the numbers and slow to the next IAS. Repeat...

Here are the numbers:
IAS(knots) Descent Rate (fpm)
85 650
80 700
75 700
70 650
65 700
60 650
58 full stall

When the numbers are plotted out on a graph, I end up with a sine wave sort of plot, with several minimum descent rates at different IAS numbers.

Does this make any sense?? I think I understand how to get Minimum Sink Rate and Max Distance Glide from a smooth plot curve, but not from the data /plots I have captured. Anyone have any words of wisdom here??

I have also performed this flight test at max gross weight, FULL FLAPS, same density altitude. Although the numbers are different, they follow the same general trend of plotting a sine wave of -fpm vs IAS.

Oh, as a comment, I have two different ASI installed. One is a standard Van's round steam gauge, the other is a Dynon D10A. All of the numbers presented are from the steam gauge. It is the one I use because it is easier for me to read. The Dynon unit presents more accurate info at the lower end of the dial, but it to follows the sine wave numbers when plotted out...just different numbers. The actual Stall point according to the D10A is 53 knots w/ flaps up.

58kt vs 53 kt at stall is a big difference! I'd go with they Dynon personally, I'd assume they're more accurate. Does it display altitudes also?

What I understand about glide performance testing, they do a sawtooth-like profile. What I'd do is set your alitmeter to 29.92"HG to get pressure altitude. Climb to a few thousand feet above ground and record the OAT. Start the descent and once your stabilized, time the difference between two altitudes. Say going from 6000' to 5000' for example. That would give you a more accurate vertical speed. Then climb up and do it again at a different IAS. On the ground you can correct for density and temperature variations.

The problem you're having is that the VSI isn't very accurate and 50fpm is too coarse of a measurement to be able to get data from.

The error may be in the sink rate numbers.

Were they taken by a reading a standard VSI?

Timing a descent and reading altitude over xx seconds may be more accurate, or better still, a glider variometer.

And you should be able to pull the altitudes off of the Dynon datalogger. Use the altitudes to obtain your descent rate. Use a running average of descent rates rather than point values. I would trust the Dynon data much more than the steam gauges, and the Dynon data give you many more points which will result in a better average. This should help give you a better curve.

greg

The Dynon unit displays DA in real time. It can completely replace all of the traditional steam gauges and then some. I just find the steam gauge easier to read. In the referenced documentation in my first post, Glide Performance
is not the same a Climb Performance as engine power / cooling issues are not a factor. Then, by calculating the Glide Ratio using the sink rate / speed x 1.689, I still get a funkylooking graph.

When recording the data, I copy both steam gauge and Dynon as a common line item, and then correlate the data on the ground. If I use just the data from Dynon, I get the same basic sine wave shape...the values are shifted by about 8 knots at the low end of the speed range to about 3 knots at the high end, where "low = 55knots & high = 155knots".

The vertical speed indicator is easy to read, the Dynon bar graph not so much so. I may try the stopwatch task to see if it correlates the altimeter / VSI etc. The Dynon altimeter and the steam gauge altimeter are in lock step, just different by about 40 ft.

I have "calibrated" the Dynon vs Steam gauge using a manometer, and according to the manometer, Dynon is very accurate, the steam gauge not so much. Regardless of which instrument is used, the vertical speed profile does not graph into a "smooth" line plot.

One other thing is you are measuring the minimum sink rate, which is not necessarily the maximum glide range speed.

The minimum sink rate glide is very much similar to the best rate of climb speed, they both depend on the power required. In this case, power is the speed * drag force. Minimum power yields the least sink rate. Maximum power yields the highest climb rate.

Maximum glide ratio or "flattest glide speed" is approximately 1.31* Vmin sink

Do not use the VSI for climb or descent performance testing. As mentioned pick a target altitude say 3000 ft PA, climb to 4500 ft Pa and establish at stabilized descent. Start timing at 4000ft PA and stop timing at 2000 ft ( a 2000 ft block) . Maintain airspeed +- 2 kits ball centered throughout. Note vsi at 3000 ft (your target altitude). Repeat for each airspeed through the range of speeds of interest. You need to be quick to minimize the error due to change in weight (fuel burn) . Post flight simply divide your alt block by the time . This is the average Rate Of Descent at 3000ft PA. Compare this to the VSI and you'll often note a significant diff. To maximize effeciency, stay trimmed at the airspeed add full power and climb back through the same alt block recording the same values to obtain climb rate data. This is in fact called Sawtooth climb and descent testing for obvious reasons. If you have a very accurate drag polar you can negate the effects of fuel burn mathematically but isn't necessary because the change in value isn't significant enough to worry about if you proceed quickly from one airspeed to another.

Static errors

The most likely cause for your 'confusion' is static error, which is causing the inconsistent readings (even if your instruments are perfect). Static readings are notoriously incaccurate at low speeds. The usual result is artificially low airspeed readings - you are actually going faster than the ASI reports, so at least the inaccuracy is on the 'safe' side. The effect on the VSI and Altimeter is moot at a constant airspeed since you are measuring deltas, but turbulence or airspeed variations will cause data variations in them as well of course.

Not much you can do about it unless you want to go to some trouble.

A 'few' years ago I took a college flight test course (I'm an AE) using a Cessna 206 equipped with a long (~100+ ft. maybe 200, it has been a while) plastic tube that was reeled out in flight. A funnel was attached to the end of the tube as a 'drag chute' to keep it stable, and about 10 ft forward of the funnel was a hole to get a static reading. This was a cheap and easy way to get a static reading with minimal effect from the airplane. The plane had a whole set if PS instruments in back for taking readings and comparing them to the aircraft instruments (this was before PC's much less EFIS's ).
You'd have to plumb the trailing line into your static system somehow and do your testing with that static source.

I've never gone to such efforts on my plane.

The problem that I had with V-Speed testing is that it is very difficult to do with other air traffic around or with any kind of turbulence.

Corrective inputs from the flight controls will increase drag and invalidate your measurements.

Best to go up very early in the morning, with no traffic and still air. Always trim to the test speed (no pitch input on the stick).

V

The brainless way, if you happen to have a Garmin x96 GPS:

Use the menu options on the flight parameter page to display "glide ratio".

Vary your airspeed until the displayed glide ratio is maximized

Done deal.

erich

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.

Suggestions

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.

RV8R999 said:

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.

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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

erich weaver said:

...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

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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.

erich weaver said:

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.

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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.

RV8R999 said:

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.

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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

fehdxl said:

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.

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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.

fehdxl said:

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

-Jim

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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.

fehdxl said:

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...

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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.

n5lp said:

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.

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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?

Bob Kuykendall said:

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.

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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

erich weaver said:

The brainless way, if you happen to have a Garmin x96 GPS:

Use the menu options on the flight parameter page to display "glide ratio".

Vary your airspeed until the displayed glide ratio is maximized

Done deal.

erich

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To those that disagree with Erich's statement above - how can this not be true?

The only case where his method could deliver an incorrect IAS for best glide angle is if, while descending and adjusting IAS (to find the minimum gps glide angle), you descend through air with significantly different winds.

The gps is simply telling you the angle through space that the plane is moving. It is, of course, not the angle through the air, unless there is no wind. In any case, hunting for an IAS that minimizes the gps glide angle will give an IAS for minimum glide angle through the air.

OK, I understand now what the glider guys are saying...

Duh. Just needed to draw it out. It can be easily proven with data by simply doing Erich's tests in some headwind, then climb back up and do it the opposite direction. Minimizing the gps glide angle (maximum "glide ratio") will indeed give you different IAS in those two cases.

Minimizing the gps angle (maximizing gps "glide ratio") will give the glider pilot exactly the speed necessary in those wind conditions to maximize horizontal distance.

RV8R999 said:

Do not use the VSI for climb or descent performance testing. As mentioned pick a target altitude say 3000 ft PA, climb to 4500 ft Pa and establish at stabilized descent. Start timing at 4000ft PA and stop timing at 2000 ft ( a 2000 ft block) .

...

Repeat for each airspeed through the range of speeds of interest.

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In addition to all the great advice that Ken gave above, keep in mind that changes in wind as you descend have the effect of adding or removing energy from the aircraft, and will affect the time required to descend through the altitude block. The effect of the wind is accounted for by doing two runs at each speed, with the heading for each run chosen to put the runs approximately at 90 degrees to the wind. Do one run, then climb back up and turn to do the second run at the same speed, and at 180 degrees to the first run. Use the same headings for all runs. For example, if the wind is from 270, do two descents at each speed, one on a heading of 360 and one on 180.

Best Glide vs. x96

The purpose of flight testing is to find out the aerodynamic characteristics of your individual airplane. My procedure when the engine goes out (if I have some energy) is to immediately zoom to best aerodynamic glide speed. At that point you can then fiddle with locking in best glide distance over the ground with the x96. All this while trying to get the pilot cooling device going again, picking a spot to land, ... . I've done it twice, once in my -4. I didn't evan think of looking at my x96 because there is too much other stuff going on. This is not to say it wouldn't be possible, because in certain situations it might make a big diffference. My main point is you want to get to approximately best glide speed RIGHT NOW, and not try to do an iteration problem with your GPS.

It would be interesting to calculate how much wind speed effects max distance glide speed. Has anyone done this? How much difference does it really make in an RV?

N999BT said:

...
It would be interesting to calculate how much wind speed effects max distance glide speed. Has anyone done this? How much difference does it really make in an RV?

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The rule of thumb I was told - add/subtract half of the wind speed to the glide speed.

If into a strong headwind, then a little faster is usually better than a little slower, so err upwards on your rapid calculation...

UPDATE.... you can work out the change from the glide polar in the old RV-6 plans book.

There is not much change for wind according to Vans diagram.

It isn't well calibrated, but shows about 95 mph as best glide speed. With a 40 mph headwind, it changes less than 10 mph.

So I would use the rule of thumb above, but with 1/4 of the wind - with a higher speed into a head wind.

These charts are the ones you should be creating during that required 25/40 hour test phase...

Please, let me help

az_gila said:

The rule of thumb I was told - add/subtract half of the wind speed to the glide speed.

If into a strong headwind, then a little faster is usually better than a little slower, so err upwards on your rapid calculation...

Click to expand...

OK, first, the best glide speed with wind does not conform to the thumb rule. Please see the sample spreadsheet on my website. There is enough of a difference, IMHO, to warrant revising your technique.

The minimum sink, best glide and Carson's speeds are all related and the ratios are the same for all RV's and all non laminar flow winged aircraft to the best of my knowledge. Thus it is useful to know these numbers when choosing altitudes and speeds.

Also on my website please see my presentation at AirVenture 2010 on this subject and then refer to the spreadsheets. One is specifically for the 6A, using CAFE's numbers. Using the Garmin x96 is covered. Minimum sink versus best glide is covered. The spreadsheet lets you modify your flight profile and see the answers without doing any further math. I'm working on the latest, even simpler version, but the one that's there is good to go.

Last, please remember that while knowing the airplane's actual drag curve is a good thing, it is not the same thing as what it will do with the engine out because stopped or windmilling, there will be additional drag from the prop which is not a part of the airplane's drag curve. In the case of the C-152, the best glide speed changed from 69 to 76 and the drag changed from 130 to 177 pounds. It is not a trivial difference and it will vary with your particular prop-airplane combination.

hevansrv7a said:

....Last, please remember that while knowing the airplane's actual drag curve is a good thing, it is not the same thing as what it will do with the engine out because stopped or windmilling, there will be additional drag from the prop which is not a part of the airplane's drag curve. In the case of the C-152, the best glide speed changed from 69 to 76 and the drag changed from 130 to 177 pounds. It is not a trivial difference and it will vary with your particular prop-airplane combination.

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My corrected update used the Van's curve in the RV-6 instruction book - his test procedure has it as an actual gliding speed/sink measured at minimum engine idle speed, not the zero HP of the CAFE tests.
The plans do note it will be worse with a stopped or windmilling prop.

My 1/2 figure came from higher performance gliders, I had updated it to the 1/4 number based on Vans figure before reading your post...

In a true emergency - no cooling fan up front - you need an airspeed to aim for with the minimum amount of calculation or look up. If you are high, and over fairly uniform terrain, a turn downwind will give you more search area to find a good field, and the slightly slower speed will give you a fraction more time to think...

If you are testing in the Mojave Desert, I think a true engine out glide test directly over a 20 mile long dry lake would be safe and a good thing to perform....