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Test-Flight Card, Best Rate of Climb, Best Angle of Climb

By Jack Dueck, EAA Homebuilt Aircraft Council, EAA 337912

So far in this series, we've covered everything from planning for the first flight through its safe execution to last month's engine break-in article. View all of these articles.

The next item to explore in our flight envelope is the establishment of the best rate of climb (Vy) and the best angle of climb (Vx).

Vy represents the airspeed that will give us the greatest gain in altitude with time. For instance, if we are cruising at 6,000 feet ASL in bumpy air and we have received a clearance to 10,000 (seeking a smoother ride), this is the airspeed that will get us to our new altitude in the shortest time. Vx airspeed will take longer to gain altitude, but it will do so in the shortest distance over the ground. We would use this airspeed to get over obstacles on a short field takeoff. Both airspeeds are important and can easily be established with a simple flight test.

The data that we seek in our flight test is the altitude climbed in a set time frame at differing airspeeds. For instrumentation, we need an airspeed indicator, an altimeter, and a stop watch.

Methodology

We start with determining a series of airspeeds at equal intervals starting at about 1.1 Vs (stall speed) to about 1.5 Vs. (In the case of our RV-9A, our Vs with flaps up is in the order of 60 knots indicated airspeed (KIAS), so a good starting airspeed would be about 70 KIAS; and a final airspeed of about 90 KIAS should suffice.) We chose the following airspeeds for our test runs: 70, 80, 90, and 100 KIAS. (We could have chosen intervals of 5 KIAS for additional data.)

We chose 6,000 feet ASL as our base altitude from which to start our timed runs. We also started with timed periods of 30 seconds for our runs but found it difficult to target accurate airspeeds over a short 30-second time frame, so we changed to a full one-minute time frame for more accurate data.

Starting at an altitude of about 500 feet below our starting base, we established a climb airspeed of our initial target speed of 70 KIAS for our first timed run. When we climbed through the 6,000 feet base, we initiated the time clock and then continued to climb at a constant 70 KIAS through a full minute. At this instant we noted our new altitude and recorded it for this first test run. We repeated this same process for our remaining three airspeeds. The data is shown below.

Airspeed Altitude Gained
70 KIAS 810 feet
80 KIAS 860 feet
90 KIAS 840 feet
100 KIAS 780 feet

With this data, we could graph the curve below, showing the altitude gained (in one minute) on the Y-axis against the KIAS on the X-axis.

Airspeed Graph

Results

From the curve, the maximum height (read altitude gained in one minute) achieved represents the Vy, 82 KIAS. By extending a line from the 0-0 origin to the tangent of the curve, we establish the Vx, 68 KIAS. These two values will be entered into our pilot's operating handbook for future reference.

If you normally use partial flaps for your takeoff climb, you may want to introduce an additional value for best angle of climb (Vx, takeoff flaps) by repeating the tests. You would not normally use flaps for your Vy.

It should be noted that these airspeeds in KIAS apply for all conditions of weight and pressure density. On a hot day with a heavy aircraft, you will gain less altitude and cover more distance in the same time as you would on a cold morning in a lighter aircraft, but the values of Vy and Vx would remain 82 and 68 KIAS respectively. Remember, we are not measuring distance traveled but rather "rate of climb" airspeeds as indicated on our airspeed indicator.

Reference: Advisory Circular 90-89A

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