Wind Tunnel Tests Overview

It’s possible to design and build an airplane without doing wind tunnel tests and get a reasonable result.  It’s also possible to design and build an airplane without doing wind tunnel tests with catastrophic results.  This is true.  It all depends on how sure you want to be about the results before you commit time, bucks, energy, etc, to your favorite design.  Now, the “Big Boys” (Boeing, Lockheed, Airbus, etc) wouldn’t dream of pursuing a new airplane without first doing wind tunnel testing, even though wind tunnel testing is very expensive.  Why is that?  It’s because, even with all the technology at our disposal, there’s only so much that you can do with a computer and “canned” analytical programs and a lot that you can’t do.  Think about wind tunnel results, properly used, as “Truth Data”.  What you see, in the tunnel, is pretty close to what you’re going to get when the bird flies.  That’s very, very important.  Imagine Boeing, not having done their wind tunnel homework on the 787, discovering that the wing had to be moved two feet to get a stable airplane AFTER the prototype flew. Bad news, indeed, particularly for the bottom line.

Now, to the best of our knowledge, small kit airplane manufacturers don’t do wind tunnel tests.  The rationale, supposedly, is that these are small “simple” airplanes and so wind tunnel tests are not needed.  Nothing could be further from the truth (unless you’re building an airplane which is identical to an airplane that is already flying and is known to have good characteristics).  The way these “simple” airplanes fly is governed by the same rules that apply to the Big Guys and there’s no way to determine, from looking at a drawing of your favorite “Belchfire Special” whether the tail is just right, too big or too small.  Even Boeing can’t do that, it’s just too complicated.  If the airplane is “unconventional”, in some sense, there’s definitely no way to do it.  Consider Phoenix and Varieze, for a moment.  Both aircraft are Pushers.  Both aircraft use a canard.  Both aircraft use an aft mounted wing.  We had available to us NASA wind tunnel results on the Varieze and, in fact, had flown the Varieze; but, in many other subtle ways there were differences.  We decided that we couldn’t rely on the Varieze test results to justify the Phoenix geometry and so we went ahead with our own tunnel tests (interestingly, the Varieze wind tunnel data helped us in understanding our own results).

One other thing, and it’s an important one.  Most small kit airplane manufacturers don’t have Mega-Bucks in the bank and, therefore, don’t have Mega-Bucks to spend “fixing” problems that show up during flight tests, that should be fixed.  Human beings, being what we are, are nothing if not rational critters and we’re very good at rationalizing problems away.  So, if the ol’ “Belchfire Special” doesn’t fly quite the way it was intended, well, we can live with that, it’s not so bad.  The end result is that an airplane is put on the market with problems that shouldn’t be there.  When we initiated the Phoenix project, it was a foregone conclusion that we were going to do wind tunnel tests because (1) the goals were set very high, (2) the design was “unconventional” and our ability to analyze on paper and in the computer was not sufficient and (3) we didn’t want to be put in the position where rationalizing away big mistakes was necessary when the bird flew, in other words no big unpleasant surprises (there are always small surprises in any flight test program and they’re OK).  For us, the wind tunnel tests were worth every cent that we spent on them.  We know that Phoenix will work when it flies.