It seems that multirotors have to be powered by the expensive battery. And the helicopter can use engine and could have much larger load. I’m wondering what’s the advantage of multirotor compared with helicopter in the agricultural UAVs pesticide spraying field? What’s the drawback of helicopter?
I find online that engine vibration is a serious problem to autopilot when the helicopter has a large load and the engine becomes big. But as a newbie to this area, I would think there's a lot of way to reduce the engine vibration so it doesn't look like a problem that can't be solved.
They've been using (gas powered UAVS) in Japan for over 20 years to do small crop spot spraying, so not trying to adapt modern day electric multirotors to this purpose. Here's one good article on the topic: http://deltafarmpress.com/equipment/what-are-key-issues-uav-use-agr...
You catch a couple of windless days that cooperate and end up flying some 10-14 hour stretches just like we do in the real world, and you'd need about 15 batteries to keep in rotation in order to continue through the day.
Not to mention that many of the chemicals, especially the liquid fertilizers, are extremely corrosive and will take it's toll on any electrical connectors, cannon plugs, etc.
In fact, I'm not even sure how you would pressure wash or steam clean a fragile quad after a day of spraying.
Then what's the drawback of unmanned helicopter compared with multirotors? Anyway the multirotors are still on the market.
I don't think there are any drawbacks to helicopters in this application. The only reason anybody is even considering multirotors, is because the multirotor bandwagon is quite strong right now, and everybody is jumping on board. Trying to use electric multirotors for any kind of agricultural spraying application, seems like a bit of bad joke to me.
I worked out the numbers once for a large electric multirotor, and what it would take to just charge the batteries. IIRC, you'd need a 240V 40A circuit, just to charge the battery in 1 hour. But the battery would only last about 15 minutes. So you'd need at least 4 of these chargers to be able to keep an aircraft in rotation.
It's all a bit silly frankly.
I could maybe see electric multirotors used for very specialized cases, maybe for some sort of flower production outfit with really small plots. Or maybe a really small vineyard. But not other than that.
The Yamaha RMAX was in the area of $100,000, the newer Fazer I'm imagining around $120,000 - $150,000 outfitted.
The Fuji RPH2 was quoted at $140,000 - $200,000 almost 20 years ago. Probably $250,000 in today's money.
I don't know of any multi-rotor that's a quarter of a million bucks.
Another advantage of a single rotor helicopter is going to be swath width. One long rotor is going to provide a wider swath than a bunch of small rotors.
Here is some interesting reading on the process from Japan, where they basically have been working on it for 30 years now, and surpassed manned applied acres in 2004.
This photo shows blade tip vortice of an RMAX while spraying, the round curl at the end of the spray pattern.