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Test-Flight Card, Best Glide Speed

Glide Graph
Graph of Best Glide Speed

By Jack Dueck, Chairman - EAA Canadian Council, EAA 337912

Our last test flight allowed us to develop the best rate of climb (Vy) and the best angle of climb (Vx) for our RV-9A. And the previous test flight provided our indicated airspeed to calibrated airspeed correction figures. Now with our newborn knowledge of these data, we can establish the best glide speed for our aircraft.

The best glide speed can be described as the least altitude lost per distance traveled. If we lose power, then determining our best glide speed will give us the greatest horizontal distance that we can travel and hopefully our best chance of selecting an appropriate off-airport landing site should it become necessary.

We will work with three parameters - time, distance, and knots indicated airspeed (KIAS) - and determine the loss in altitude over a set distance in a set time period. The test is simple, and the reduction of the data involves several simple steps to provide a useful single airspeed for our best glide distance.

Methodology

We start with three target airspeeds. We use Vy, Vy minus 10 knots, and Vy plus 10 knots. (We could use Vy, VY - 10%, and Vy + 10% for an aircraft with a narrower speed range.) We have established Vy at 82 KIAS from our previous test. So our target speeds will be 72, 82, and 92 KIAS.

To fly the test, we climb to a safe height of 5,000 feet AGL, and after establishing slow flight (cooling the engine after our climb), we apply carburetor heat and close the throttle. As our airspeed bleeds off, we establish a glide speed pegged at 72 KIAS. Once established, we mark off the altitude at the start of one minute and again after one minute of glide. Then we immediately increase the downward pitch angle to give us a pegged 82 KIAS glide speed and repeat the altitude lost over an exact minute. Finally we peg 92 KIAS airspeed, again recording the altitude lost in one minute.

To improve the accuracy of our tests, we repeat them, but this time we start with 92, then the 82, and finally the 72 KIAS. Because our glide rate (read altitude lost over distance traveled) will depend upon the weight of our aircraft, we take into account the fuel burn. So if we match the first of a test run with the last, and the last with the first in the second test run, we mitigate this weight-introduced error to a significant degree.

The flight-test data collected is as follows:

Test Run #1 Test Run #2
 
KIAS Altitude lost KIAS Altitude lost
72 650 feet 92 900 feet
82 700 feet 82 750 feet
92 800 feet 72 675 feet

When we average the altitude loss for the target airspeeds, we get:

KIAS Altitude loss (average)
72 662.5 feet
82 725 feet
92 850 feet

But wait a minute; these airspeeds include instrument errors as we found in our IAS calibration flight test. So from data obtained in this previous "calibration" test, we can develop the corrected (KCAS) values for our current test. Also with the corrected airspeed values, we can determine the exact distance (in feet) traveled at the target airspeeds in one minute. Since one nautical mile per hour equals 101.269 feet per minute, we can develop the following spreadsheet:

KIAS Correction factor KCAS Distance (feet/minute)
72 +4 76 7,694
82 +2 85 8,608
92 +1 93 9,418

From this we calculate the altitude loss per mile of distance traveled.

KIAS Altitude Lost/Nautical Mile  
76 523 (662.5 x 6,076/7,694)
85 512 (725 x 6,076/8,608)
93 548 (850 x 6,076/9,418)

We are left with two parameters that we can graph: altitude lost versus airspeed. The graph turns out like this:

From the graph on this page, we can see that the least altitude lost (520 feet) in a given distance of one nautical mile occurs at 83.5 KCAS. We convert this back to our KIAS from our previous test-flight chart, where the error of 3 knots is now subtracted to give us our best glide speed of 80.5 KIAS. We will record this number in our pilot's operating handbook. This is almost the same number as our best rate of climb (Vy) of 82 KIAS. (These values should be very nearly the same since this would indicate the most efficient wing L/D airspeed.)

We can gain one additional important piece of information. Since at 80 KIAS we will lose 444 feet of altitude for each mile of distance traveled, for every 1,000 feet of altitude we drop, we can glide 2.25 (1000/444) statute miles. We also know that we have measured our altitude loss over one minute, 512 feet in this instance, so we can calculate that we have approximately two minutes per thousand feet to respond to whatever caused us to be losing altitude. Remember, however, that these figures are based on ambient temperature and pressure conditions, as well as the aircraft's weight at the time of the test. Keeping this in mind, though, they can still be a useful guide.

It would be interesting to run these tests again, under 25 percent power, simulating a feathered or stopped propeller. (You will glide farther in this instance than with a turning propeller acting as an air brake.) In an emergency, perhaps any added advantage gained by a stopped or feathered propeller might simply be considered a mitigating plus to the pilot's "pucker factor".

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