up vote 13 down vote favorite

Antennas sticking out of an aircraft obviously increase profile drag, but the folks who design antennas have done a lot to improve aerodynamics: High performance aircraft can have antennas mounted inside of fiberglass components like wingtips, and flush-mounte antennas are available for transponders and DME equipment.

For slower light GA aircraft more aerodynamic antennas are also available (e.g. blade-style transponder antennas are available which produce about 80% less drag than older spike-and-ball style antennas), measured at 250 MPH.

Since the average light GA aircraft has a VNE below 250 MPH and isn't generally covered in antennas, how significant is the antenna drag on a typical light GA airframe? For example, would stripping the ~6 pounds of antenna drag from your average Cessna make an appreciable difference in airspeed?

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  • if you know you have 6 pounds of antenna drag and you know the pounds of thrust of the engine then you can get a rough estimate of how much faster you go faster – ratchet freak Feb 7 '14 at 8:49
up vote 16 down vote accepted

From an aerospace engineering perspective:

The drag coefficient ($C_D$) of a round antenna is can be estimated by around $.5$ depending on shape, while for an streamlined shape it's around $0.045$.

Drag

(Source)

Drag is defined as:

$$D=\frac12C_D{\rho}V^2A$$

Thus, say you have a 0.01 m wide antenna 0.25 m long, for an aircraft moving at 50 m/s. At sea level conditions, air density $\rho=1.225 \mathrm {kg/m^3}$. Say that $C_D=0.5$. If my calculations add up, it's around $1.9 \mathrm{N}$.

Say you have the same antenna, for the same aircraft aircraft moving at 50 m/s. Air density is $1.225 \mathrm{kg/m^3}$, but this time the antenna is streamlined with $C_D=0.04$. This results in a force of only $0.15 \mathrm {N}$.

(Updates: Originally the formula included $\frac12$ twice which was wrong, and that the force was given in kg, rather than newtons; my apologies.)

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  • Does anybody know a link to instructions to find out how to format formulas correctly? – Thunderstrike Feb 7 '14 at 9:15
  • At the moment MathJax is not enabled on this site because of the processing overhead and the related long page loading times. See meta.aviation.stackexchange.com/q/145/19 for the Meta discussion – DeltaLima Feb 7 '14 at 10:51
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    You can do a lot to improve readability of formulae using <sub>, &rho; etc. I took the liberty of adding those. – Marcks Thomas Feb 7 '14 at 12:26
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    I also took the liberty of omitting "/2" from the drag formula, as it was incorrect, but the factor appears to have been edited back in. – Marcks Thomas Feb 7 '14 at 13:09
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    Manfred, what's happening in Delft? Why don't you just use Newtons as a result from your calculations. Now it is confusing because your units don't match up. To express force as mass isn't the way to do AE :-). Also thrust is not defined a power available * velocity. Delivered power is defined as thrust * velocity (TAS). Thus thrust = delivered power / velocity. Note that the rated available power from your engine is more than the delivered power because you loose some in transmission and propeller efficiency. – DeltaLima Feb 7 '14 at 18:09
up vote 5 down vote

Given how abysmal your average GA plane is in the aerodynamics department, the amount of drag added by antennas is almost nothing.

I have some numbers for a Cherokee 160 from an aircraft performance class -although I make no claims that they're 100% accurate- and I'm seeing between 250-360 lbs of drag during level flight depending on the airspeed. Add your 6lbs of antenna drag onto that, and in order to maintain the same airspeed you need at maximum (a density altitude of 10,000 feet) 3 additional horsepower. At sea level it barely registers a change at all.

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  • I think the drag numbers for the Cherokee series are pretty accurate (as is your math on the horsepower at typical GA cruise speeds). Working with about 130KIAS my math came out to between 1 and 4 percent of engine horsepower going to fighting drag (much of the variation from stuff like propeller pitch & cruise power setting) the antenna drag, or roughly 1-3 knots if you completely removed and smoothed the antennas. Obviously that's the mathematical ideal - in the real world I'd expect less substantial gains. – voretaq7 Feb 7 '14 at 17:44
  • It's worth noting though that as lousy as light GA aircraft are in the aerodynamics department some antennas are worse - like these "Flying V" beasts. I don't even want to think about the profile drag on that :-) – voretaq7 Feb 7 '14 at 17:47
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    @voretaq7 That thing looks like it might actually generate lift though! – Lnafziger Feb 8 '14 at 3:16
  • @voretaq7: according to this document, removing the antennas gained 4 knots in cruise speed (probably KTAS), so seems to be accurate. – Qantas 94 Heavy Feb 8 '14 at 4:16

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