Bill Freeman wrote:

          SNIP

The issue is that the airfoil accelerates the air

as it passes over the

top (DUH), which increases the velocity enough to

generate a

local shock when freestream is .7 to .8 Mach.

Notice that no aircraft

cruise above about .82 Mach, for the same reason

(I mean subsonic A/C,

of course).

                       SNIP

Bill
Long EZ builder & pilot
mechanical engineer

DENIS SMITH wrote:

Lockheed L1011 and Boeing 747-400 aircraft routinely
cruise at .85 Mach and higher.

OK, I have no problem with being off by 0.03 Mach.  My point
still stands.  Airfoils locally accelerate the air and as you approach
Mach 1.0 you reach local shocks well before the A/C (0r prop tip) speed
is Mach 1.0.  Airfoils which have moderate/low camber and very thin
sections delay local shocking longer.

For very high tip speeds, the Clark Y airfoil is not good, thinner
aifoil sections with lower camber help a lot, like the NACA
16-series.  Clark Y is very good for fixed pitch wood props
with moderate tip speeds due to an exceptionally wide useable
range of angle of attack (alpha), high CL max but it unfortunately
does have an unusually high drag coeff, which is not as important
with props as it is with wings.

Bill
Long EZ builder & pilot
mechanical engineer

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