OK OK thanks but that wasn't really my question.
Excuse me if I wasn't clear enough, what I really meant to ask is "when designing a plane, need I be concerned about the Reynolds number and if so, in what way?"

Yes, you should. But not much (see below, answer to Bob's post).

My first understanding was that it is a property of the airfoil, that seems wrong now.

True. As Richard said, it's more of a property of the air flowing
around the airfoil.

But NOT by the airfoil, then? Does one first determine the (max?) Re the plane will be operating at, and choose an airfoil accordingly?

You should determine the range of Re the plane/airfoil is operating
at. For low speeds/low Re, evaluate cLmax for stall. For higher speeds/
higher Re, evaluate drag in the cL range you're going to encounter.
For example, I'm currently trying to fit winglets to an existing wing.
The wing itself operates (roughly) at Re=1M...5M, the winglets at
300k...5M. Esp. the latter poses a serious challenge for the airfoil
designer/airfoil choice.

So if I'm not wanting the ultimate bit of performance from my DreamBird,
I needn' t bother too much? How then do I go about selecting an airfoil?

Well... cLmax predictions from calculations and wind tunnels don't
always relate well to what you get or seem to get in flight. The so-
called "laminar bucket" (range of cL with low drag) does so much
better. So what you do is choose an airfoil that gets you low drag in
the your cruise speed range, and if you're lucky it has a decent
cLmax. Size the wing accordingly (e.g. to satisfy stall speed
requirements). It all boils down to a trade-off, which is easier with
flaps.
If you don't want to use flaps, the NACA airfoils that have been
proposed (4- and 5-digit-series) do the job pretty well, but don't
expect a high-performance aircraft to come out of it. There's also a
lot of other areas you can work on to reduce drag. The plane I'm
working on has a no-lift-cD of around 0.035, out of which only maybe
0.006 come from the wing (the rest is fuselage, turbulence from the
prop, struts, empennage, landing gear etc.). RVs for example use
countersunk rivets much more than other metal homebuilts it seems (and
maybe larger engines), so they achieve higher performance.

Hmmm. This may be the first negative one. If someone doesn't
understand Re, should they be designing an airplane? I don't think
one can pick up sufficient fluid and structural mechanics on Usenet to
be a reliable aircraft designer.

You don't really have to understand Re. You only have to apply it. I
think that's a difference between somebody who specialized in
aerodynamics (me ;-) opposed to someone who specialized in aircraft
design (me too ;-). And I don't think anyone would design a plane with
knowledge from Usenet. Rather pick up a book and get questions that
remain unanswered in the book answered on Usenet (neither Raymer nor
Roskam _explain_ Re, but they probably mention it somewhere). And
structural design on a (homebuilt) aircraft can be simplified to the
point of only having beams under tension/compression, bending and
torsional loads with maybe some buckling thrown in. Gets you three or
four different equations. The rest is structural testing.
Wasn't there a post about "Designing your homebuilt in 10 equations"
mentioned a while back?

PS2 Oliver, verstehe ich gut du bist Deutsch? Das koennte mal Spass machen, anderes als Englisch zu schreiben...

Absolut richtig, Jan, und so weit ich das verstehe bist Du
Niederländer? Oder Flämisch-Belgier? Ik kann een wenig Platt verstahn,
aber dat is nich dat selbe wie Nedderlannsch ;-)

Oliver