Yes Jim and I wish I could get my aerodynamics students to see it beyond
that level of simplicity. Indeed the coefficient of drag will increase
rapidly when there is boundary layer separation, which occurs within a range
of Reynolds Numbers. After separation the flow behind the round object (or
for that matter any object) becomes turbulent, changing the pressure
distribution behind the object hence increasing the drag. In teaching
pilots about this it is often a chore to get them to appreciate the
significance of the Reynolds Number to the flow situation since most of them
do not have the math necessary to do the evaluations using the compressible
flow fluid calculations. I introduce them to compressible flow with the
Bernoulli Equation for compressible flow to show them how the standard
atmosphere is determined .That is usually the limit of their capability.
They are not aeronautical engineering students)and so we tend to use
aggregate parasite drag coefficients for airspeeds below about 250 knots
since it can be proven that the compressibility of air below that airspeed
is minimally significant and can be disregarded.
Best regards.
Bill
 Original Message 
From: "Jim Lux" <jimlux@earthlink.net>
To: <NPAlex@aol.com>; <towertalk@contesting.com>; <w7vp@comcast.net>
Sent: Sunday, March 26, 2006 3:40 PM
Subject: Re: [TowerTalk] Wind loading comment misdirected? W7VP
> At 02:44 PM 3/26/2006, NPAlex@aol.com wrote:
>
>>===========================================
>>I don't think I said a thing that contridicted your comments and Rohn's
>>requirements. I believe it is IO (intuitively Obvious) that a Flat
>>surface
>>drag/profile factor would be larger then a round surface.
>>============================
>
> While intuitively obvious, it's also wrong.
>
> Oddly, there are lots of situations where the Cd of a round member is >1.0
> (i.e. the drag area is larger than the projected flat plate area)..By the
> way, a flat plate usually has a higher Cd than a long body with a flat
> front of the same cross sectional area; think about the flow behind the
> plate. It all depends on the Reynolds number and all those things about
> laminar and turbulent flow.
>
> You can play with the numbers at:
> http://www.fluidmech.net/jscalc/cdre01.htm
>
> Try putting in different diameters of the cylinder like 0.1, 0.01, and
> 0.001 ft
> with airflows of 10, 50, 100 ft/sec (88 ft/sec = 60 mi/hr)
>
> 0.1 ft = 1.2 inches..
>
> The classic area where this comes up is in biplanes, where the bracing
> wires, although small in cross section, have a drag area that's larger
> than
> that of a substantially larger streamlined strut.
>
>
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