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Old 02-20-2012, 06:59 PM   #259 (permalink)
aerohead
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Quote:
Originally Posted by ERTW View Post
so...21 hours later...I realized how to drastically cut calculation time.

Results for a sharp nosed revolved template - 295 N total drag => Cd = 0.178
The frontal area is 3.0656 m^2 (60" tall, 5" ground clearance)
air speed is 30 m/s (108 kph or 67 mph)
air density at 20C is 1.204 kg/m^3

I realised that from 70% to 80% of the template is a straight section, whereas I use a curve. Phil, if you were expecting a lower Cd, I strayed from the template, as you can see, and this is what I got. This software has good correlation. I didn't want to go beyond solid models and get into surfaces, so I'll leave it at that.

It's expected, and still pleasant confirmation, to see the surface pressure *increases* near the very tip of the tail. I made the nose sharper because there was a large high pressure area on the blunt nose.

Tomorrow will be an interesting day at the University of Toronto
The template is modeled from the aft-body of a 2.5:1 streamline body of revolution.
In free flight is has a coefficient of aerodynamic drag of Cd 0.04
In ground proximity Cd 0.08.
At 70% Cd 0.09
At 80% Cd 0.083
I allowed Cd 0.13 with wheels @ 100%
Maybe Cd 0.12 or less with wheel fairings.
I made the nose more blunt,as all my fluid mechanics text regarded a convex hemispherical nose to be plenty good below transonic flow ( about 250 mph for automobiles).
All of Hucho's tables for crosswind drag coefficients show the bulbous nose to have the lowest Cd.
If the nose is pushed out any at all,the opportunity for separation is increased,as now, the corner radius is reduced,and with it, stability of the turbulent boundary layer which cannot feed energy around the sharper corner.
Hucho's emphasis for drag reduction is in aft-body streamlining,hence the tail.
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