I am confused by my GLIDE SPEED numbers

[AlexPeterson]

Originally Posted by erich weaver

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

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.

[AlexPeterson]

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.

[Kevin Horton]

Originally Posted by RV8R999

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.

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.

[N999BT]

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?

[az_gila]

Originally Posted by N999BT

...
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?

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...

[hevansrv7a]

Originally Posted by az_gila

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...

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.

[az_gila]

Originally Posted by hevansrv7a

....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.

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....