F li g h t Te s t in g Te c hn i q u e s
Figure 1
Flight Path Stability
Data Reduction
Crunching the numbers
By Ed Kolano
Last month we covered the flight path stability flight test
technique. Now we'll explain how to take the raw data you
recorded during testing and turn them into a flight path
stability curve for your airplane.
The curve shows how changing your airspeed affects
your vertical flight path angle, which is useful to know on
final approach. During each test, you recorded the test
airspeed, start-timing and stop-timing pressure altitudes,
elapsed time, outside air temperature (OAT), and possibly
some remarks. Figure 1 shows our sample completed data
reduction worksheet. So, how did we get there?
Here's the data reduction dance card. You recorded how
long it took to go from one pressure altitude (29.92, remember?) to another. That gives you the vertical flight path rate.
You also recorded OAT, which you'll use along with pressure altitude to determine density altitude. Density altitude
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Vol.2 No.6 / June 2013
and calibrated airspeed are needed to determine true
airspeed. You recorded observed or indicated airspeed,
so you'll have to go back to your airspeed calibration test
work to convert the airspeed you read from the airspeed
indicator to calibrated airspeed. For simplicity, we're going
to assume a perfect correlation between the two and treat
observed airspeed as calibrated.
Altitude Change
During your flight test you noted the pressure altitude
when you started timing (PA1) and when you stopped timing (PA2). Subtract PA1 from PA2, and enter this altitude
change in the "Alt Chg" column of your worksheet. We'll
use the 85-knot test point for the example calculations.
Alt Chg = PA2 – PA1
Alt Chg = 2380 – 2500 = -120 feet