How significant is antenna drag on light GA aircraft?

voretaq7
  • How significant is antenna drag on light GA aircraft? voretaq7

    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?

  • 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, $air~density~\rho=1.225kg/m^3$. Say that $C_D=0.5$. If my calculations add up, it's around $1.9N$.

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

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

  • 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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