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-- own & others' writings, and links

 

A perspective that applies well to aviation decision-making:

"Those who will not reason
Perish in the act;
Those who will not act
Perish for that reason."
                       -- W.H. Auden, poet

 

Page index:

1. Thoughts on flight testing homebuilt aircraft

2. References and links to resources

 

Thoughts on flight testing homebuilt aircraft

In the last decade the amount of information available to assist the homebuilt aircraft pilot in understanding and carrying out flight testing seems to have grown greatly. Information is available on the web, and frequently in EAA's Sport Aviation magazine. A pilot needs to find the level of detail that is sufficient for testing their own airplane.

Flight test procedures for homebuilt aircraft tend to fall into three categories:

  1. Testing for basic safety
  2. Testing for performance
  3. Testing for handling characteristics

The first is the most vital, designed to make sure the aircraft is flying properly on the first few flights -- the aircraft is controllable, the engine isn't overheating, and other major systems function. The test plans are designed to limit exposure to risk and incrementally test new situations. Numbers have limited importance at this stage, but some need to be recorded or calculated, such as indicated speeds for approach and stall.

The safety issue naturally can't be forgotten in the rest of the testing. In performance testing, measurements can be made of parameters such as rates of climb, speeds, engine temperatures and pressures. Number crunching becomes useful at this stage to make the data more comparible across different conditions -- different density altitudes and different aircraft weights. Handling characteristics can be a little more difficult to quantify, as gauges for stick force or to measure a yaw angle are not available on the panel.

The three categories are listed by order of importance -- safety first, performance second, and handling third. Yet there may be critical issues in all three categories, especially for a complex, high speed aircraft, or one of a new design. For a slower and simpler bird, testing to a professional level of detail is just not necessary, and isn't done by most people. For a pilot being checked out on a homebuilt, it is important for them to know some basic numbers and general handling characteristics, particularly if different from other airplanes of the same class or different from what the pilot is used to. It may be good for the pilot to know, let's say, that stick forces are light in pitch, that at X rpm it tends to cruise at about Y mph indicated using under Z gallons per hour, or that a little power should be carried into the flare because of a relatively high rate of descent at idle.

But assuming that the plane is is known to basically be safe, then the pilot does not absolutely need to know the exact stick force per g at a given c.g. location, nor the size of the stick's lateral centering band, nor the number of seconds for the stick-free long period (phugoid) pitch stability cycle. Qualitative information is sufficient for many light aircraft. Numbers can be difficult to interpret anyway, unless the characteristics are compared to another aircraft the pilot is familiar with.

Even if a lot of flight test detail is not that important for a simple aircraft, understanding flight test techniques is useful. It allows a pilot to better understand why an airplane does what it does, and enables them to better convey information about that behaviour.. In the early days of flight testing, test pilots were not necessarily selected for their ability to provide clear feedback to designers. I've heard of pilots making notes like, "Moving the stick is like churning butter." It may be a good quote, and might be interpreted that stick forces are high. But it doesn't make clear what the real issues are -- force vs. stick deflection, centering bands, centering forces, breakout forces, roll inertia, roll rates, or control harmony?

The flight test resources discussed below emphasize different aspects of the three categories I have mentioned. Some techniques therefore will be important for the first few flights of a homebuilt aircraft, while others may be fun to experiment with but don't have to be done..

Flight test resources (on and off the web)

My own writing:

The absolutely best site which I know, through which to find flight test information for homebuilts:

Intended for builders of Van Grunsven designs, it includes links and documents on all aspects of flight testing, including ones such as:

-- Various info of use for performance flight testing (eg, pitot-static calibration, using GPS data to determine TAS)
-- FAA Advisory Circular AC-90-89 "Amateur-Built Aircraft Flight Testing Handbook".
-- A straightforward Chris Heintz (Zenair/Zenith) article on first flights
-- Articles on the general philosophy of flight testing

Other sources follow, some of which are not on Kevin Horton's site. Links are current as of September 2002.

Sources that tend to concentrate on "the basics", on organizing a flight test program, on making the first flights safely:

Going beyond that one can get into all sorts of detailed stability, handling, and performance tests:

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