[TowerTalk] Wind loading comment misdirected? W7VP

Bill w7vp at comcast.net
Sun Mar 26 19:13:21 EST 2006

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.

----- Original Message ----- 
From: "Jim Lux" <jimlux at earthlink.net>
To: <NPAlex at aol.com>; <towertalk at contesting.com>; <w7vp at 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 at 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 
>>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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