Propulsion engineering and flight test truly covers the entire spectrum of flight test techniques. For example, consider these questions:
- Performance FTTs - Does the engine perform as expected?
- Flying qualities FTTs – Does a spin or stall or other high angle of attack disrupt airflow into engine inlets enough to cause engine flameout or compressor stall?
- Systems FTTs – Are the controls and displays associated with the engine adequate considering the mission of the aircraft? Does the Digital Engine Control interface appropriately with other aircraft systems?
Today, however, I would like to briefly describe throttle bodies, a flight test technique used, almost exclusively in propulsion engineering applications of flight test. (Bodies is pronounced with a long o, as you would expect to pronounce “bode–ee”.)
Initial setup: trim the aircraft hands off at a given airspeed and altitude and initial power setting.
This allows engine to stabilize. (Often the control room will look at telemetry from the engine to verify desired parameters are in range and stable.)
Maneuver technique: When command of execution is given, test pilot performs a step input to throttle. Test pilot observes engine response and in anticipation of the engine reaching desired setting, performs a second step input to original power setting.
This might be a rapid push of the throttle to maximum power or a throttle chop to idle, for example. The second input–a step input back to max for example–might be made as the engine passes 73% N1, so that the engine “bottoms out” at 65% N1. In other words, this input must be made so that the engine does not pass the desired target parameters. Leading the desired parameter is required because engines do not spool up (or down) instantaneously.
Other considerations: Usually, the aircraft must maintain a constant speed (within some data band like +/-.05 M or +/- 5 knots). Thus at the initial trim condition, the pitch attitude might be nose high. When the throttle is chopped, the test pilot must pitch down to maintain the airspeed within the desired range. Similarly, this entire maneuver (including the pitch up/down) must remain within a certain altitude band (like +/- 1000 feet).
The illustration above from a Test Pilot School course handout (I encourage you to click on it to see it full size) shows the engine response to a throttle body.
On the last Friday of each month, the column What is an FTT describes some of the fundamental maneuver building blocks performed by test pilots to gather data during flight test missions. An FTT is in some sense a description of an experiment. It is a key element of the scientific method applied to aerospace sciences, engineering, and aviation.
You can access all of these posts by clicking on the FTT category hyperlink below the post title.
Each Friday, @FlightTestFact will deliver examples, definitions, and explanations of flight test techniques for the entire day. You can view these tweets by searching for #FTT and #flighttest as depicted below. You can also click on the picture below to be taken to the twitter search results. What FTT would you like to know more about?