On Wed, 01 Jan 2020 14:33:36 -0800, Eric Greenwell wrote:

Martin Gregorie wrote on 1/1/2020 1:56 PM:
On Wed, 01 Jan 2020 06:32:46 -0800, India November wrote:

I am thinking of the theoretical advantage in drag coefficient of a
blade vs rod antenna which holds for any speed.

I found this: https://www.grc.nasa.gov/www/k-12/airplane/shaped.html
which may help sort things out for you, though these numbers are for 3D
shapes while what you really want is drag comparisons for 2D shapes
with the shape extending right across the wind tunnel or (better) with
one end attached to a side wall that's in the airstream.

Other, less well written web pages, suggest that a teardrop shape with
a blunt LE and max thickness at around 30% chord and slightly convex
surfaces behind max thickness that terminate at a sharp TE should have
about half the drag of a cylinder with the same frontal area.

As others have said, drag will rise sharply if the AOA of the
streamlined shape isn't zero and this will get worse as the max
thickness moves back toward 50% and/or the LE becomes sharper.

If you're really worried, find a University with an aerodynamics dept
and try to persuade a student to write a term paper based on wind
tunnel tests on a rod antenna and the best shaped blade antenna you can
find.

Now, you can calculate your glider's drag at, say, best glide and max
cruise (easy to do from its flying weight and glide polar) and then
calculate how much each adding each antenna would affect the glide
angle at those speeds.

We'll all be interested to see just how much effect the antenna has.

A 50:1 900 lb glider at best L/D has 18 lb of drag; the rod antenna at
62 knots has a drag of 0.41lb/16 = 0.025, or 0.14%. That's about 0.07
L/D loss.

The rod drag quadruples at 125 knots, but the drag of the glider would
increase substantially, also, so maybe the percentage doubles, which is
still only 0.3% of total drag.

I thought it was likely to be a pretty small effect. Thanks for
confirming.


--
Martin | martin at
Gregorie | gregorie dot org