From: "Matt Whiting"
Sent: Friday, February 04, 2005 1:57 PM


Ron McKinnon wrote:
From: "Matt Whiting"

Ron McKinnon wrote:


"Matt Whiting" wrote in message

...Why? It is acceptable to "polish the frost smooth" per FAR 91.527.

Interesting. It seems a rather dubious practice to me. It would indeed
appear to be legal in the US - but, FWIW, it isn't legal in Canada.

Why? Smooth frost is less of an aerodynamic issue that a thick layer of
dirt, bugs and dust...


Be that as it may, arguing that its not as bad as something else (that
you
shouldn't fly with either), isn't any argument at all for the
acceptability
of flying with smooth frost.

Frost, smooth or otherwise, or other contaminants arguably affect the
aerodynamic behaviour. How much does it change with a given
level of contamination?, what level of contamination is significant?
or how significant is such a change? how much is too much? how
smooth is smooth enough? in what way will such change affect the
aerodynamic behaviour of the plane? (does it change the critical
angle of attack for the wings? the control surfaces?, what are
the new critical angles (what's the new stall behaviour? is a
tailplane stall more likely? will it spin easier? how does it change
rudder effectiveness?) for instance) are critical questions. But
these are not questions that a typical pilot is in a position to answer
operationally. This a job for computer simulations, and lab analysis
in controlled test conditions, and for test pilots in controlled test
situations.

They only affect the aerodynamic behavior if they significantly affectt he
flow in the boundary layer. On some airfoils this a concern with fairly
small disturbances, but on others it isn't much of a concern at all.
Saying that you should not fly an airplane that has any frost on it is
just as silly as saying you should never take off with less than full
fuel. You have to know your airplane, nobody is arguing that. Most light
airplanes will fly just fine with polished frost on the wings and even the
control surfaces, I know my 182 did.

The issue isn't whether it will fly at all, but what happens to its
flying characteristics, and whether you're still operating
with safe margins for error. A few anecdotal cases where it
'flew', does nothing to answer that. There are many other
cases on record where it didn't.

What, exactly is the expected level-flight stalling speed with
this particular amount of frost, 'polished' to this particalar
smoothness? Is it safe with the current loading, the current
density altitute, the current runway (the destination loading,
density altitude, and runway?) - do you have any safety
margin left? or have you used it all up by leaving frost on
the wings? or have you, in these particular circumstances
not affected it at all? What's the new stalling characteristics
of the stablizers, elevator and rudder effectiveness. Are you
going to be in the neigborhood of a tailplane stall on takeoff?
Is the frost layer uniform - does it affect the whole wing in the
same way?

You don't know. .

And its not just about 'knowing your airplane'. How many
pilots can say with assurance how much contamination
will significantly affect the flow in the boundary layer?, or for
which airfoils its a concern with fairly small disturbances?
There are no doubt some that can, I'll grant you. But this is
not the usual rule. (Off the top of your head - What is the
airfoil on your 182? How sensitive is it to contamination.
How much contamination is too much on your 182.
What thickness of frost is too much for this airfoil? How
'smooth' *does* it have to be? And then what about the
stabilizers, elevators and rudder? Exactly what is the new
level flight stalling speed?)

The proposition isn't that you shouldn't fly an airplane that
has *any* frost on it - it is that you shouldn't fly it with any
frost on the critical surfaces: wings, stabilizers, rudder
(propellers).

It is obviously 'legal' in the US (the original FAR citation
shows this), so in the US you may obviously feel free to
use your discretion in the matter. But that doesn't necessarily
mean it is safe.

In other places (Canada, for instance) it isn't even legal.