That is way off my friend
tin man said:
Here's another piece of interesting but useless information. It takes approx 5 hp to drag a round comm antenna thru the air at 200 mph. Jim Ayers who lurks on this list has done all the math.
Tom
Jim Ayers is math impaired I am sorry to say.
5 HP Really.
That is off by a factor 20. I guess if you
had 10 antennas the airplane fall out the sky? Silliness
Here are some numbers (conservative)
Fluid Dynamic Drag, by Hoerner, assume a drag coefficient (Cd)
of approximately 0.5 for a whip antenna
(test data in figure 13 on page 3-9).
Drag is equal to the dynamic pressure times frontal area times the Cd.
Dynamic pressure (lb per square foot) at sea level equals the speed
in mph squared, divided by 391 (from Hoerner, eqn. 16 on page 1-10).
Frontal area of the antenna in square inches, we get:
20 inches long, diameter of 3/16 inch, frontal area of 20 * 0.1875 = 3.75
square inches. (actual area really 2.88 in sq, bent whip including base)
drag = speed squared times frontal area times Cd divided by 56,304
(drag in pounds, speed in mph, frontal area in square inches)
At 200 mph the drag would be: 200 x 200 x 3.75 x 0.5 / 56,304 = 1.3 lb.
Power required = speed x force.
200 mph x 5280 ft/mile x 3600 sec/hour = 293 ft/sec.
So, the power in ft-lb/sec = 293 x 1.3 = 381.
One hp = 550 ft-lb/sec, so it takes 381/550 = 0.69 hp.
Prop efficiency assumed as 0.8,
that means we need 0.69/0.8 =
0.87 engine horse power
to drag that antenna.
Assume it takes 160 hp to go 201 mph (RV-6)
Speed loss, 1/3^(159.31/160)*201=200.71
So it takes .29 MPH.
The above is conservative. It is closer to 0.50hp /.25 mph.
Jim sells a Comm antenna fairing that goes on the top of the vert stab.
Jim was on another list with this claim that one comm antenna was
worth 1/3 the drag of the wing! He got it down to 5 hp, which is still
too high of a number. Like Jim, but he tends to exaggerate or get the
math wrong. At least he tries. Now all we have to do is get him to
admit MT props are slower than Hartzell and Sensenich props.
George
As long as you go with one Comm, one Transponder and a VOR antenna
the drag of antennas is small about, 0.81 mph total! that's it, about 2.1hp.
I can see hiding the VOR antenna since it is like two comms, but still
its not a big loss. However wing tip VOR's work better than comms, but
if you want to fly long range IFR NAV with a VOR I would go with an
external VOR antenna. Any antenna jammed inside a metal plane is
compromised. When I raced I took my tail VOR antenna off in about 2
minutes (to get that 1/3 mph). Picture below, click.