Tuesday, August 17, 2010

High-speed taxi tests


A line in a news story about the death of RV-6 builder Alan Clark caught my attention today.

Nampa Fire says Clark was high-speed taxiing and testing his plane when wind lifted it and sent him hopping up and down before it flipped over.
I wrote an article -- or rather pieced together submissions from pilots -- on the practice of high-speed taxi tests for RV Builder's Hotline a couple of years ago.

Van doesn't like them:
We expect that the motivation for such testing is often
the eagerness to “see how it works” while waiting
weeks for that final inspection. We assume that there
are many successful, thus unreported, high speed taxi
tests and “down-the-runway” lift off flights made in new
RVs. But still we wonder…what do pilots hope to learn
from fast taxi tests and brief lift-offs that they cannot
learn from sedate taxi speeds and actual take-offs?

Well, there’s theories and there’s facts:

THEORY: It is desirable – even safer -- to perform
high speed taxi tests during the pre-test flight phase of
homebuilt aircraft development because nothing can go
wrong at speeds less than stall/take off speed.

FACT: There is little to be learned from high speed
taxi tests, other than that RVs accelerate faster than
expected, and may take flight at lower speeds than expected.

An RV is capable of flying, particularly in
ground effect, at very low throttle settings. Even at far
less than full throttle, an RV can quickly accelerate to,
maybe, 40 mph. The pilot then pulls the throttle back a
bit to hold that speed while he exercises the ailerons
and elevator a bit, to "feel it out". But that retarded
throttle position, maybe only 1/3 open, is still too much
and has, within seconds, accelerated the plane to 60+
mph – enough that in the hands of an inexperienced (in
RVs) pilot, unanticipated flight is probable.

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