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All crop dusting aircraft seem to have an externally mounted spray boom for distributing their pesticides, I imagine this significantly increases drag & therefor performance.

What gives? why not integrate the spray system into the wing?

I have read that crop dusters spray booms remain short of the wing tip to prevent interaction with the wingtip vortices, perhaps another aerodynamic interaction along the main span of the wing prevents this integration?

thanks for considering my question!

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    $\begingroup$ That would pigeonhole the plane into a spray-only platform. These planes can tow gliders, carry fire retardant, and fit other types of hoppers to spread different types of material. Removable spray bars are an industry requirement, not a design oversight. $\endgroup$ – acpilot Mar 2 '20 at 22:59
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    $\begingroup$ also looks (but I've not done studies) that placing the nozzles below the flow of air over and under the wing helps in better dispersion of the chemicals, and at least helps avoid contaminating aircraft surfaces with said chemicals. $\endgroup$ – jwenting Mar 3 '20 at 4:54
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    $\begingroup$ Maybe it's because the trailing edge of the wing is an aileron or flap? Mounting sprayers on a moving control surface would require flexible connections, and add stress to the hinges and actuators. $\endgroup$ – Robin Bennett Mar 3 '20 at 11:29
  • $\begingroup$ I think that the drag is negligible for this activity $\endgroup$ – L'aviateur Mar 25 '20 at 12:15
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Different chemicals use different nozzles and delivery systems. Many of the spray agents are very corrosive. Turbulence by the sprayer can be a good thing, as it aids in mixing.

Additionally drag is not a big issue as crop dusters fly slow, and tend to have good power to weight ratios, so the drag of a spray assembly at 70 knots or 60 knots, is not a big concern. Mixing (with air of) chemicals, reduction of airframe corrosion, and ease of removal are larger concerns.

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