Best Range or Glide in Wind

[hevansrv7a]

I used an image instead of text in order to keep the table readable.

Please note that this uses CAS but wind speeds are "true". So if you are at 10,000 feet, your L/D max is 100 CAS but your TAS is 116.4 and that 20 mph wind is 17%, not 20%.

[kentb]

I look forward to playing with it tomorrow.

Kent

[the_other_dougreeves]

The exact answer requires what is sailplane folks commonly called a "polar" plot, which is simply a plot of sink rate vs IAS (obviously under zero thrust conditions). Here is one for the common 2-place ASK-21 trainer:



If you have such a plot, finding best L/D is easy - just shift the wind axis the appropriate number of knots.

The simple rule of thumb we teach students is to use half the wind value. It's a fairly good rule for sailplanes, but not sure how good it is for RVs.

TODR

[hevansrv7a]

Originally Posted by the_other_dougreeves

The exact answer requires what is sailplane folks commonly called a "polar" plot, which is simply a plot of sink rate vs IAS (obviously under zero thrust conditions). Here is one for the common 2-place ASK-21 trainer:



If you have such a plot, finding best L/D is easy - just shift the wind axis the appropriate number of knots.

The simple rule of thumb we teach students is to use half the wind value. It's a fairly good rule for sailplanes, but not sure how good it is for RVs.

TODR

I did use the drag curve to compute those values. The spreadsheet is in increments of 1 mph instead of a graph. Then, through some other calculations, that gives a sink rate. For a given CAS, (and weight) the sink rate is the same. The ground speed changes with wind. That's how to find the ground speed and thus the CAS that has the best ratio of sink to forward progress. The ratio gets worse as the HW increases and better as the TW increases, but the percentage change from L/D max is easily observed on the "sheet".

The data I posted shows that the "rule" is not as good as it could be. There should be no difference, for this, between an RV and a sailplane because the shape of the curve is the same and the numbers I gave are useable as percentages.

[flyboy1963]

ah, for us guys who sniffed a little too much avgas.....

so, if you are gliding into a headwind, go faster? ..to maximize distance and time aloft?

I love the test-pilot stuff and charts & graphs, just want something I can retain and perhaps actually use in the cockpit when things get quiet!

...should this be added to the POH, and emergency procedures, or is it just for fun?

[hevansrv7a]

Originally Posted by flyboy1963

ah, for us guys who sniffed a little too much avgas.....

so, if you are gliding into a headwind, go faster? ..to maximize distance and time aloft? YES for distance, not time. SEE BELOW

I love the test-pilot stuff and charts & graphs, just want something I can retain and perhaps actually use in the cockpit when things get quiet!

...should this be added to the POH, and emergency procedures, or is it just for fun? EITHER/BOTH. SEE BELOW.

In other words, as usually recommended, fly faster into a HW and slower with a TW. BUT not as much as usually recommended. I gave examples at 5% intervals. That's the wind component on the nose or tail (not the wind itself, usually) as a percentage of your L/D max airspeed.

For instance, if your L/D max is 100 and the head wind is 10, then fly 103.

Fly-in-ointment: if your L/D max is 100 at sea-level (CAS/IAS), then your L/D max at, for example, 10,000 feet is 116.4 (TAS) so fly 102 (CAS/IAS), roughly. You have to do the percentage in TAS because we don't know wind any other way. If you want 1 mph increments, download the spreadsheet and insert the wind where it wants it.

You asked about fun vs. POH. That's a very important point.

Your L/D max with a stopped engine is not the same as your real L/D max speed and it's going to be slower than the real one.

You can only find out what the engine-out best glide or L/D max speed is by experimentation. So use the real L/D max for best range, but if the fan gets quiet, your numbers will be very different. This is a critical point for safety or I would not make such a fuss about it.

The best data I have to prove this point is for the C-152. The real best L/D speed is 76.3 mph and the engine out & windmilling speed is 69 mph. So if you were in a C-152 and trying for range, use 76.3 plus or minus wind correction and if you were looking for an emergency landing site, use 69. Big difference! YNWV! (Your numbers will vary: the CAFE RV6A L/D max for range is 106 mph.)

The method for finding your best L/D max for level flight is in the presentation I gave at Oshkosh in 2010 for which the PowerPoint slides are found at the link below. I am always happy to discuss that off-line at length.