In the wake of some recent flight testing I also re-visited climb performance data. Last Saturday was a beautiful day for this with nary a twirl in the atmosphere. Winter time and temperatures associated with it make for the possibility to get real sea level performance data at 1000' AGL. So I had to take the opportunity and completed a climb performance series.
Measuring climb performance is a good and simple way to venture into flight testing. All you need in terms of external instrumentation is a stop watch. You need to practice nailing the speed to within 1-2 mph on the climb portion. That's a good piloting exercise all by itself. And the reward for your labor is a hard and fast value for Vx and Vy, two speeds which are eminently useful to know.
To measure climb performance I first loaded the aircraft to 7 kg heavier than MTOW. It takes me 30 minutes to burn 7 kg of fuel. So, if the total duration of one test flight is 1 hour, I have at most 7 kg deviation from MTOW.
The idea is to measure the time to gain 500 feet altitude at a predetermined airspeed. Since zero density altitude corresponded to almost exactly 1000 feet AGL, I determined to time my climb from 1000 to 1500 feet altitude. I started each run at 500 AGL, set full climb power, pitched to the predetermined airspeed and trimmed the gyro. By the time I reached 1000 feet, I had the airspeed nailed to within about 2 km/h. As I passed through 1500 feet, I stopped the stop watch and noted down the elapsed time. I repeated this measurement for speeds from 60 through 150 km/h in increments of 10 km/h.
The vertical speed is calculated from this formula:
VSI (in fpm) = 30000 / time (in sec).
Vy (best rate of climb speed) is the speed at which the greatest VSI is achieved.
To find Vx (steepest climb speed) you simply calculate the climb angle from the data you gathered. Use this formula:
Angle (in degrees) = 16988 / (time * IAS), with time in sec and IAS in kts.
Angle (in degrees) = 19538 / (time * IAS), with time in sec and IAS in mph.
Angle (in degrees) = 31455 / (time * IAS), with time in sec and IAS in km/h.
As an example, consider this:
Time to climb from 1000' to 1500' is 30 seconds at an airspeed of 80 mph.
The vertical speed comes out to 30000 / 30 = 1000 fpm.
The climb angle is 19538 / (30 * 80) = 19538/2400 = 8.14 degrees.
Here's the data a got from last Saturday:
Clearly, Vx = 75 km/h. But Vy can be anything from 100 to 140 km/h. I haven't seen such a broad range of speeds over which the vertical speed remains constant. Could this be attributed to the aerodynamic effect of the stubby wings?
-- Chris.
P.S.: Note that, strictly speaking, I haven't got any data at 75 km/h to determine Vx there. The blue line through the measurement data is a smoothed line, and the hump it shows at 75 km/h is strictly an effect of this. Still, 75 km/h is the proper value to note down for Vx. But it will be necessary next time to do a couple of runs at in-between speeds to verify this.
Measuring climb performance is a good and simple way to venture into flight testing. All you need in terms of external instrumentation is a stop watch. You need to practice nailing the speed to within 1-2 mph on the climb portion. That's a good piloting exercise all by itself. And the reward for your labor is a hard and fast value for Vx and Vy, two speeds which are eminently useful to know.
To measure climb performance I first loaded the aircraft to 7 kg heavier than MTOW. It takes me 30 minutes to burn 7 kg of fuel. So, if the total duration of one test flight is 1 hour, I have at most 7 kg deviation from MTOW.
The idea is to measure the time to gain 500 feet altitude at a predetermined airspeed. Since zero density altitude corresponded to almost exactly 1000 feet AGL, I determined to time my climb from 1000 to 1500 feet altitude. I started each run at 500 AGL, set full climb power, pitched to the predetermined airspeed and trimmed the gyro. By the time I reached 1000 feet, I had the airspeed nailed to within about 2 km/h. As I passed through 1500 feet, I stopped the stop watch and noted down the elapsed time. I repeated this measurement for speeds from 60 through 150 km/h in increments of 10 km/h.
The vertical speed is calculated from this formula:
VSI (in fpm) = 30000 / time (in sec).
Vy (best rate of climb speed) is the speed at which the greatest VSI is achieved.
To find Vx (steepest climb speed) you simply calculate the climb angle from the data you gathered. Use this formula:
Angle (in degrees) = 16988 / (time * IAS), with time in sec and IAS in kts.
Angle (in degrees) = 19538 / (time * IAS), with time in sec and IAS in mph.
Angle (in degrees) = 31455 / (time * IAS), with time in sec and IAS in km/h.
As an example, consider this:
Time to climb from 1000' to 1500' is 30 seconds at an airspeed of 80 mph.
The vertical speed comes out to 30000 / 30 = 1000 fpm.
The climb angle is 19538 / (30 * 80) = 19538/2400 = 8.14 degrees.
Here's the data a got from last Saturday:
Clearly, Vx = 75 km/h. But Vy can be anything from 100 to 140 km/h. I haven't seen such a broad range of speeds over which the vertical speed remains constant. Could this be attributed to the aerodynamic effect of the stubby wings?
-- Chris.
P.S.: Note that, strictly speaking, I haven't got any data at 75 km/h to determine Vx there. The blue line through the measurement data is a smoothed line, and the hump it shows at 75 km/h is strictly an effect of this. Still, 75 km/h is the proper value to note down for Vx. But it will be necessary next time to do a couple of runs at in-between speeds to verify this.
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