August 2001

The boom wiring looks fragile, but an American operator, Speck Thornton, flew a whole season with it, got it as dirty as it could be and hit birds with it without breaking it (although the electrostatic system was shorted out until the blood dried). Changing from liquid to solids, however, requires care, as ground handling could damage nozzles and wiring.

Once the spray system is free of leaks, not much special maintenance, other than cleaning and flushing, is required. Check electrode alignment once in a while, change electrode wiring at the beginning of a new season - that's all.

The corona effect

All discharge wicks are removed from the plane and replaced by five special ones at the tips of the wings and tail surfaces. They're wired into a milliampere meter which measures the corona effect in the plane and is used to fine tune the charge being induced into the spray.

This is not indispensable; during tests in Costa Rica, the operator didn't allow them to drill holes in his airplane, so they couldn't install the corona effect meter. They just ran the system at 7,500 volts flat out and got the same good results.

Flying it

Once the plane was ready, the test team took over. Coordinated by Dr. Marcos Vilela Monteiro, who runs Centro Brasileiro de Bioaeronáutica, it was comprised by Prof. Wellington Pereira Alencar de Carvalho (Universidade Federal de Lavras/Engineering Department) and pilot/operator Alan Sejer Poulsen. This system is so revolutionary that regular deposition analysis methods mean little; water-sensitive paper cards attract few charged droplets. They are attracted by live, humid crops. The only effective swath analysis method is to spray ultraviolet dye on crops and use an ultraviolet lamp to see the droplets on the leafs. These were not available for the tests, so a conventional test protocol was used. "We had to have a starting point", said Dr. Vilela.

Alan Poulsen said the nozzles didn't add any significative drag to the Ipanema 2001. I can't confirm this, as I only flew the airplane after the equipment was installed. After takeoff, turn both rheostats to zero, then switch the power on. While spraying, turn one of the rheostats up until the milliampere meter peaks, then turn the other up until it zeroes. This indicates you're using the most effective voltage to induce the desired charge to the spray, which depends on the chemical being applied, air humidity and density altitude. Once set for the present conditions, the system only needs to be turned on and off during loads (keeping it on while on the ground won't damage it, but it won't make you popular with loaders who might get shocked if they touch the boom) and monitored for a zeroed corona. This adds a bit to pilot workload; the Dobbins told me there's an automated voltage adjustment unit being considered to let the pilot concentrate better on flying the airplane.

After the preliminary tests in Botucatu, the Ipanema 2001 was flown to Rondonópolis and Primavera do Leste for field tests on cotton.

BOTUCATU, BRAZIL - Some friends were asking me about going to the Ribeirão Preto Agrishow, Brazil's biggest farm show... Why would I, when I already had an invitation by Neiva's sales manager Fabiano Zaccarelli (manufacturer of the Ipanema ag plane) to observe the installation of a Spectrum Electrostatic Spray System in their new Ipanema 2001?

That's how I met Terry and Blake Dobbins, of Spectrum, and Hal Tom, who's been working with this system for the last eight years as a USDA pilot at Texas A&M University, in College Station, Texas. The three of them are very good-humored people, especially when a new "expert" sees the off-the-shelf components in the Spectrum system and suggests an "improvement" to it. After all, USDA's Dr. James Carlton has been working for 35 years developing this system with a lot of trial-and-error. "That's how it works", Hal says of the way the system is built nowadays.

System description

Although most applications are done in the 5,000 to 7,500-volt range, the power supply is built to withstand 17,000 volts. "More is not better", says Hal, explaining that once you reach the target mass-to-charge ratio, increasing voltage will only add strain to the system, as the charge in the droplets will not increase. The power supply unit is installed behind the Ipanema's cockpit, and takes only two amps from the plane's electrical system; you'd have plenty of juice in the battery to finish a load, in the event of an alternator failure. A 5-amp circuit breaker protects the system. It is controlled by a small box in the cockpit, with one on-off toggle switch and two rheostats for voltage, one for positive and the other for negative. The only other additions to the cockpit are two digital volt-ampere meter combo gauges (you guessed; one for positive and one for negative) and a milliampere meter for the corona effect (no, it's not the beer brand; more about this later).

The forty-four double nozzles mount on the regular Ipanema boom, with a recommended 6-inch spacing. The system uses Spraying Systems' brass check valves, which feed special double nozzles which are mounted inside a round electrode, which induces the charge to the droplets. One of the reasons behind the special nozzles is droplet size; you want them smaller than 120 micron, as bigger droplets won't accept as good of a charge. Usually, one per wing, one boom is positive and the other is negative; this is the most effective setup for deposition. The distance between the innermost nozzles is a whooping 62 inches, with no belly nozzles. This would sound like a perfect setup for streaking, but what happens is that opposite charged droplets attract each other, so the swath closes in the middle perfectly.

"It makes a beautiful, perfect spray deposition", says Hal Tom of the system, explaining that while the opposite charged droplets tend to coalesce in the middle of the swath, the other droplets are attracted to the grounded crops below, falling much faster than an uncharged droplet of the same size. As they reach the crop leaves, they temporarily induce their load to that area; this will keep other droplets of the same charge from adhering to the same spot, as equal charges will repel each other. So, that other droplet will deviate a little and go to another uncharged, i.e., uncovered area of the leaf. This phenomena will cause droplets to distribute themselves in the perfect pattern Hal mentions, "filling the blanks" where the leaf is still uncharged. The electrically charged spray won't just fall to the upper surface of the leaf; the droplets that fall between the leaves and reach the ground will actually deviate and adhere to the underside of the leaves - one of the reasons this kind of application is more effective.