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I am a pilot, looking for some help (cropdusting spray dispenser questions)

  1. Feb 14, 2017 #1
    My name is Stetson, I am an Agricultural Pilot. AKA crop duster. I am looking for some help to answer a few questions to improve the efficiency, productivity, and safety of my profession.

    I fly an aircraft 6-10 feet above the canopy of a crop and apply various crop protection products. My target speed is 145 MPH indicated airspeed. I have a pressurized spray system with booms that are dropped down below the trailing edge of my wing. I can adjust the PSI on my system. I would like to know, what PSI would I need to use to put my solution into the air so that the relative air flow around the droplets would be 0.

    So, if in a perfect conditions situation, I would want my droplets to fall straight down. So what PSI would propel my droplets at 145MPH.
  2. jcsd
  3. Feb 14, 2017 #2


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    Welcome to the Pf. :smile:

    I'm not able to help directly on your question, but I'm pretty sure the other helpers will need to know the diameter of the dispensing orifices. Can you provide that?
  4. Feb 14, 2017 #3
    it is a .12 flat fan nozzle. X 30 nozzles. I am applying 2 gallons per acre in a 65' swath at 145MPH.

    .12 inch
  5. Feb 15, 2017 #4


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    Do you not have a chart that you use to relate the coverage to the pressure, nozzle diameter, and ground speed?
    Changing anyone of the three will change the coverage.
    Nozzle manufacturers usually have charts. They should be adaptable for speed of transport.

    Not sure why you think a zero stream velocity relative to the air is important for aerial spraying. Can you explain, please?
    Would you happen to know if there would be less evaporation, or more even coverage perhaps?
    What do references and science of aerial spraying say?
    Are the nozzles on the boom pointing straight back or down, or some other angle?
  6. Feb 15, 2017 #5


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    This is an interesting problem. There are several parameters involved.

    For a particular orifice, pressure will primarily control flow rate or droplet size.

    I expect that droplet release velocity will only be important for a big raindrop sized spray. A fine spray or mist will very rapidly reach zero horizontal airspeed, even when released vertically from a moving aircraft.

    By releasing the spray behind and below the wing, the trailing wing vortices are carrying spray in the air, down into and out through the crop.

    If the spray is too fine it will remain aloft long enough to drift significantly with the wind. The critical factor then becomes release of the average droplet size to remain aloft for about half of one turn of the trailing vortex.

    Once you achieve droplet control it should be possible by knowing flow rate and total orifice area to compute the approximate velocity of the spray. But I do not think it will be as important as optimising drop size for an average loft time of half the vortex period.
  7. Feb 15, 2017 #6
    Yes, the primary goal is to control droplet size more consistently. Being able to do this will reduce drift, optimize coverage and essentially make an application more profitable for the farmer. What happens is this, our droplets are atomized by being injected into the airstream and then busted up by that air. Well, this is not very efficient, and what happens is you wind up with some 500 micron droplets, target droplet being 250-300 microns, and then you also wind up with some really fine droplets down to 50 microns. If I can start with the perfect size droplet, and know they will not be affected by air, then I will have much more control. Knowing the math behind it, can give me a starting point and I can adjust some of the other parameters to fit accordingly, speed, nozzle tips, etc.

    We do apply different rates with different nozzles for different applications, such as 4 gallons per acre at 55' swath, or 3 gallons per acre at 60' swath. I thought if I could find a solution to my original question I could use the same math to calculate for the others by adjusting the PSI in my spray system.
  8. Feb 15, 2017 #7
    You are on the right track of what I am trying to solve. My boom span is 65% of my wingspan. It is shorter to help keep the spray droplets out of the wingtip vortex that causes the chemical to lift back into the air. even with this some spray manages to get out to this area. I want to start with a better droplet, and finish with a better droplet. The whole idea is coverage and consistency. I could have 8 250micron droplets for every 500micron droplet that is put out there, and I know that there are droplets much bigger than 500 getting thrown out there. For maximum effectiveness, I need an even distribution. I think the only way to do that is to start with a better droplet and find a way to minimize the effects on that droplet. As it is now, we are starting with the wrong size and hoping we get somewhere close.
  9. Feb 15, 2017 #8
  10. Feb 15, 2017 #9


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    I was looking at these sites just to get a feel of the nozzle selection and droplet size, etc.
    They have some calculators which might be of interest.


    which has a link to,

    Are those not tried and true tested recommendations for the industry for fixed wing aircraft.
    Nevertheless, there is always room for improvement.

    As Balancore stated, it is an interesting problem
    Just to summarize to what he has stated considering the down draft produced by the aircraft:

    If there was no down draft at all:
    to reach a zero speed wrt the air, one would have to produce a stream velocity exactly equal to the air speed of the aircraft in the rearward direction so that forward air speed and rearward stream velocity cancel to give the anticipated zero velocity of the droplets. The droplets will hang in the air, having only the force of gravity to pull them to the ground resulting in what would be a mist with smaller droplets drifting and evaporating before they reach the ground would be undesirable as coverage would not be as anticipated.

    Thus the stream must have some downward velocity so as to propel the droplets to the ground in a reasonable amount of time. To do so the nozzle would have to have some angle tilt downwards. What that angle is, and the time to reach the ground, of that I am not too sure. But, the velocity components of the stream at any angle can be calculated by basic vector algebra.

    With a downdraft:
    If it would be desirable to have the stream take advantage of the downdraft velocity, or part thereof, depending upon boom location, rather than compete against it, then that changes the whole angle of the nozzle and stream. And this is where some complications come into play as @Baluncore alluded to in his post.

    Certainly some testing would be necessary.
    You could for example do test runs with water on a non crop area with collector plates and obtain a data set of information.
    A good research project.
  11. Feb 15, 2017 #10


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    Because the total mass of air is so much greater than the mass of droplets, the air will take control of the droplets immediately they are released. I believe that the direction and velocity of the jet is largely irrelevant.
    The best way a droplet is going to reach the ground quickly is to inject it into the downward moving air behind the wings. That is why the droplets are released below and behind the wings.

    I very much doubt that droplets will break up when released at any speed backwards into the slipstream downwash. Drops big enough to become unstable and break up would not give good crop coverage anyway. The wing loading pressure of up to about 10 pounds per square foot is significantly less than the pressures used to atomise the spray. The downwash is a much lower energy environment than the nozzle environment.

    Evaporation is unlikely to be a problem as the active agents will probably not evaporate with the water. The remaining agent will still be wet and sticky when it enters the crop.

    Finding a nozzle geometry and pressure that produces consistent sized droplets is the first task.
    Maybe you should consider alternative spray generation technology such as spinning disks.
  12. Feb 15, 2017 #11
    So... do you know what the math would be? To find that PSI?
  13. Feb 15, 2017 #12


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    Nozzle pressure will vary greatly depending on nozzle design, droplet size and fluid characteristics.
    See; Spray Performance Considerations. http://www.spray.com.au/cat70/cat70pdf_m/ssco_cat70_a06.pdf

    Here is a rotary atomiser for use on aircraft; http://www.microngroup.com/aerial
    Download the au400 manual; http://www.microngroup.com/files/au4000_manual_web.pdf
    Start by looking at page 22.

    Selecting a design to produce a well controlled droplet size is more important than changing the pressure.
  14. Feb 16, 2017 #13
    I already know about the rotary atomiser systems. I really just want to figure out what PSI I would need with the parameters stated. Planning to do some real world tests and would like to have a concrete number to start with
  15. Feb 17, 2017 #14


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    I am sorry, but it is just not that simple. You need a valid physical model before you can put numbers on parameters. The psi will determine droplet size from the nozzles. Changing psi will not change spray velocity in the air once the spray has been released into the airstream.

    What psi do you now use to get those application parameters ?
    Is the 0.12 inch the diameter of a round hole, or the area of the flat fan ?
    Is one of your gallons = 3.785 or 4.546 litre ?
    How do you measure airspeed, 145MPH, or nautical miles, 145 knots ?
  16. Feb 17, 2017 #15


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    Injecting into the wing air downflow is what I am not sure about since in that vortex some of the drops will never reach the ground before evaporating but be swept up into the subsequent upflow and turbulence , resulting in more evaporation and drift. I would tend to think that it would be advisable for most of the drops to be able to reach the ground before the downwash has a chance to affect them. One of the reasons of the use of the 65 % boom length is to avoid the wing tip vortex with liquid applications. For solid application the dust will settle eventually, but with liquid application, the factors of humidity, temperature, cross winds, height of boom from the ground, droplet size, aircraft air speed, all part for the coverage so that the correct size reaches the foliage.

    Depending upon the nozzle and psi, the droplet size and distribution output can be determined from the manufacturer. Entry into the air, with the added air speed breaks up the larger droplets into more of a distribution favouring the recommended 200 micron size. Smaller drop don't suffer the same effect to the same degree. Angle of nozzle backwards will result in less airspeed- breakup of drops, which can be preferable depending upon the makeup of the application of which include, surfactants, and differences in viscosity, as well as those listed before.

    What I have not seen from Stetson is a justification that a relative zero airflow around the drop is preferable, or how that can ever be achieved, for fixed wing application, other than sending the stream from the orifice straight back at the same speed as the aircraft. Fan out gives different velocities of the stream, so in that respect no two parts of the stream at differing angles from the centre line are both at the same time at the zero reference.

    A quick Bernouilli manipulation should give a ballpark stream exit velocity from the orifce.
  17. Feb 17, 2017 #16
    Ha, I am a pilot. And the discussion came up the other day amongst myself and a few colleagues, also pilots, about this discussion. The theory that the PSI on the fluid moving straight back, that if you could essentially push it out at 145 MPH and you were flying 145 MPH the opposite direction, it would have less things effecting it. Basically it boils down to this, some think lower PSI is preferable to minimize drift, and some argue higher pressure reduces drift. Because the lower pressure spray, is getting more wind shear on the droplets and causing more fines. The higher PSI spray has less wind shear and keeps the droplets more consistent. I was just looking for a ballpark number that we could start at 0 and make some real world applications and decide. Aerial application has come a long way and it is still not a perfect science and I don't think it ever will be, because we work around the weather and it's never exactly the same. Temp, humidity, wind speed /direction, high pressure low pressure, figure in density altitude and performance of the aircraft with a load and as the load comes off the plane gets lighter, so your downward force should be less and less after every pass. everything is constantly changing.
  18. Feb 17, 2017 #17


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    Higher pressure NEVER reduces drift. Higher pressure always equates to smaller droplet size and smaller droplet size always equates to higher probability of drift. Smaller droplets also equates to higher probability of evaporation which means a larger percent of the product does not make it to the target.
    It also depends on what you are spraying for. If you are spraying an insecticide often times this requires smaller droplet sizes. Herbicides vary. Some herbicides only require contact on a small portion of the leaf to be effective and others require a large coverage. This also varies from plant to plant. Some weeds are very susceptible to a particular herbicide and others are not at all. Also, one chemical may require X gallons per acre and the next chemical will require 3X. This means a change in nozzles.
  19. Feb 17, 2017 #18


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    I have my feet on the ground as an engineer who, amongst other things for over 30 years now, have been designing and manufacturing herbicide application control equipment.

    The two competing effects are.
    1. Lower psi makes bigger droplets that fall faster so drift less, while higher psi makes smaller droplets that drift for longer.
    2. Smaller droplets are less effected by wind shear on deceleration to zero airspeed. Big droplets will break up.

    You are assuming that these effects overlap and that there is only a narrow optimum droplet size range. In that case, release at zero speed into the air would be an advantage.

    I am arguing that those two effects do not overlap. There is a wide size range between the two, where the droplets are too small to break up yet big enough to fall and not drift.
    My argument is based on the relative energy of the two processes. The psi that generates the droplets and the surface tension that holds those droplets together, is hundreds of times greater than the psi due to “wind shear” on release into the air stream. Droplets sized for optimum coverage and minimum drift do not break up on release into the airstream.

    So I think you are wasting your time optimising launch direction and velocity. You should instead be trying to minimise the size range of the droplets generated, so as to optimise crop coverage and minimise drift.
    I believe that is where the rotating generators have an advantage over traditional nozzles.

    Still, if you want to do the experiment we will need to do the numbers. That was the reason for my questions in post 14.
  20. Feb 17, 2017 #19


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    Yes. The range of droplet size is where it is at. A perfect nozzle would produce all uniform droplets. Nothing smaller, nothing larger. Look at the specs of various nozzles and you will find the cheaper ones do not control the droplet size as well.
  21. Feb 19, 2017 #20
    Yes sir, speed is in MPH.
    Orifice is .12 inch flat fan. Also if the flat fan doesn't work with the math I have a .12 inch straight stream to work with. Gallons are 3.785
    Thanks for the help
  22. Feb 19, 2017 #21
    And I don't totally disagree with you about us wasting our time. . A heck of a lot of people with a lot more education have spent fortunes to develop spray equipment.. I just wanted to see what the numbers were and see what happens
  23. Feb 19, 2017 #22


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    You know what you have, but there are too many possibilities for me to guess what shape it is, or how you are measuring it.
    Is that 0.12 inch diameter or 0.12 square inche orifice ?
    Please sketch and dimension the nozzle, or provide a link to the manufacturers detailed data sheet that shows geometry.
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