Cruising speed of agricultural pesticide spraying UAV

What is the typically cruising (during pesticide spraying) speed of agricultural duster UAV? It looks like it flies quite slowly when it’s spraying the pesticide. Why don’t people increase the spraying efficiency by increasing the cruising speed (and the power of the pump at the same time)? 

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  • Ed,
    The only comment I would say is contrary, at least with full sized aircraft, is that the vortices are larger at a slower speed. The wing tip vortices are smaller when the aircraft is going faster with the tail up and "on the step" so to speak. There is a limit to the speed of course and sometimes slowing from working cruise to about 20 mph less will help control droplet shear from the relative wind, but going too slow and letting the airplane come off the step creates larger vortices and weird pattern disturbances. I usually work around 140 mph, so my airplane is comfortable between 120 to 150. Slower than 120 the tail comes down, faster than 150 I start losing control of droplet spectrum for my nozzle setup.
    The vortices also decrease in size as the load comes off and the wing is working less with the lighter load. This also carries true through the overall size of the aircraft, a Piper Pawnee grossing 2500 pounds has a much smaller maximum vortice than an Air Tractor 802 grossing 16,000 pounds.
    Full sized aircraft swath widths tend to run around 110 to 120% of wingspan with boom lengths in the vicinity of 75% of wingspan. Lesser boom will equal narrower swath width, more boom potentially only equals more vortice feeding which means more drift able fines.
    • Thanks George You are correct I often  tie myself in knots a bit when I try to simplify the complex. Vortices are  related to the lift produced by the aerofoil so will often have a higher impact at slow speed or in a high lift flaps down scenario. higher flight speed will reduce the impact of the vortice since it will be effectively dispersed over a longer distance.

      Other complications I spoke off are also due to the interaction of ground effect and the more complex nature of rotary wing aerodynamics.

       I also miss used the term swath when i meant to say boom width. 

      With rotary wing platforms the swath will increase to above the wingspan however any attempt to increase boom width over 75% of wing span will feed spray into the vortices causing both drift and wastage. Higher speeds also cause more spray to be inducted into the vortices but this is most likely due to the added influences of ground effect and transitional lift.

      Regardless the option of increasing speed to increase productivity will be limited by its effects on spray quality and drift control

      Thanks for the input and I happily stand corrected.

  • Ok big question so will start with the simple answer in that most sprayers are set up to suit the characteristic of the air frame on which they are fitted.This means sprayers will be calibrated to apply at the correct volumes at the safe operating speed of the airframe.(ie not to close to stall and not to fast to disturb the spray pattern and droplet formation that is desired.

    With  both fixed and rotary wing platforms increasing airspeed will also increase wing tip (or rotor) vorticies. Allowing chemical spray into these vortices can have dramatic effects on drift and thus the controllablitiy and economy of an application. so if you fly faster you still have to narrow your spray swath (this is normally about 75% of wingspan.) so the extra speed is often counter productive.

    There are other factors specific to certain chemicals but in general a sweet spot is calculated for most aircraft based on the above data. In most western and developed countries The set up must also be approved as suitable for use by various governing bodies and may only be varied by use of permits or by providing substantial data to justify the application method.

    In short you cant just change stuff and fly around mixing chemicals into the air.

    Most ground sprayer air induction nozzels produce droplets that deform dramatically at high speeds so this is also a limitation if you chose to go with these items since you are no longer sparying with the indented droplet sizes.

    the Rmax can spray from 5-20kmh as well as drenching type applications from the hover or slower flight. the pump however imposes other restrictions for certain total spray volumes that may involve using a significant overlap to provide the correct cover.

    Operators are also trained to not maneuver aggressively with sprays on due to the risk of spray being circulated by the tip vorticies.

  • I can't find it here right now but I have seen it advertised around 12 mph (@20 kph) for the RMAX.

    I believe that the block sizes are a bit of a limitation in speed. They aren't in that big of fields and there are a lot of obstacles in many videos that I've seen. The faster you go without FPV, the faster something can get loose when making depth perception judgement calls of field edge and obstacle distance judgement calls.

    Certainly though, more speed is more acres per hour.
    The average full sized ag helicopter runs between 60 and 90 mph, and the average full sized ag airplane runs 115 to 160 mph.

    Did you get your nozzle issue figured out?
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