Where’s the RCG thread? I’m curious to see what happened. Probably a bunch of misinformed people.
This is a contentious issue, but there are a lot of misconceptions about helicopters.
One of the biggest ones is efficiency. I’ve kind of gone back and forth on this a few times, and here is my latest position:
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A helicopter is in fact more efficient than a multirotor. This is due to very basic aerodynamics, particularly the momentum theory of lift. That being: it is more efficient to produce lift by accelerating a large amount of air to a low speed, than a smaller volume of air to a high speed. So basically, the load/area of the disk. Helicopters excel at this, just due to the layout.
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However, this isn’t the common perception these days. But that is because people typically compare a purpose-built long-duration multirotor, to an extremely high performance aerobatic helicopter.
First of all, let’s ignore all the 1+ hour duration-special multirotors people have built. These typically are designed to do only one thing: Hover (or fly very slowly) in perfect weather conditions. The frames are barely airworth, and might break-up in difficult conditions. They cannot carry a payload at all, any addition of payload would very quickly destroy their duration number. Even with all this, these machines typically achieve 80-90 minutes of flight time.
Compare that to the world-record holding electric helicopter, which flew for 150 minutes. And this was actually not a very radical build. It was basically a fairly standard Logo 600 helicopter frame, with 800 size blades installed, and the same special batteries the duration special multirotors use. It would easily be capable of lifting a small camera, probably with minimal flight time loss. It’s also easily capable of flying in typical winds of 20-40 km/h.
The longest duration “working multirotor” I have seen were built by Ferdinand K. He has a Quad and an Octo that can fly (at a moderate speed) with a small camera payload for 40 minutes.
However, when one compares that to a typical helicopter that can only fly 4-5 minutes, it looks pretty fantastic. But looking at number for helicopters, you must consider that they are set up for extreme aerobatics. They have extremely high power/weight ratios, which require extremely high head speeds. Much, much higher than needed for a “scale-like” flight. They also typically carry a small battery load, on the order of 1/5th the AUW. This is far shy of the 1/3-1/2 battery weight of a common multi-rotor.
By slowing down the rotor blades, you can easily double the flight time. I have achieved 12 minutes out of my big heli when it was 600 size. And that was simply running 1800 rpm, vs. a typical 2200.
I then stretched this to 700 size, using Spinblade Asymmetric blades, and doubled the battery capacity. I also added a fairly substantial subframe designed for payload management. AUW was 16.5 lbs, including a double-load of batteries. So this was not a lightweight machine at all. Running 1400 rpm, it flew for 24 minutes. And that was with a battery/AUW ratio of only 1/4. I don’t think this was “the sweet spot”. It could take more battery, or even lower rotor speed.
I’m actually shrinking this machine back to 600 size, as I think the 700 size is a bit overkill for the payload and battery load I plan to carry. While it’s true that all-else-being-equal, you can put longer blades on the machine and just slow down the RPM and be more efficient, there are other considerations. Most of it has to do with control. As the rotor speed drops, the control authority also drops. A helicopter has a discrete control frequency which basically has to do with the rate that blades pass a particular spot on the swashplate. For example, pitch control is limited to the rate at which blades pass the pitch servo location. At 1200 rpm, this frequency is 40hz for a 2-blade head. At 600 rpm, this frequency is 20hz which is getting pretty low.
So while a 600 size machine, with 800 size blades might be able to fly at 600 rpm in ideal conditions, it would probably not fly very well. You also have to worry about retreating blade stall in fast forward flight. This also limits how low you can run the head.
So, IMO, there is a “sweet spot” of practical disk loading and blade size that you have to find. I’m still looking for it. I think it’s going to occur at a similar 1/3-1/2 ratio of battery to AUW.
Now all that being said, trying to lift 500-750g of payload, fly for 1 hour, and have it come in at under 10lbs, that is a tall order. Not saying it’s impossible, but it’s that much more difficult.
I am just finishing up an MSH Protos 500 build, and have similar requirements to you. I want to use it for mapping, with a fixed downward pointing P&S camera. But I’m only aiming for 30 minutes flight time. IMO, professional use beyond that time for mapping is kind of pointless, as you need to fly VLOS anyway. 30 minutes at 40 km/h is going to get you 20km of flight path which is quite a lot of mapping in a 500x500m area. That’s 40 passes which means a 12.5m strip width which seems quite good to me? Say you want to map a 1km square area, with the operator in the center, that is still 20 passes, for a 50m wide strip.
Anyway, so the Protos is nice because it is very, very light weight. Typically 1200g without the battery. Add 800g of payload, and you’re at 2kg. That leaves you with a potential for 2kg of battery while staying under 10lbs total. That’s a lot of a battery! I’m going to do my first flights with a 4S 5000 battery, weighing 535g. Targeting 1800 rpm head speed. I won’t guess at the flight time as I have not found a calculator which properly replicates the effect of using Asymmetric rotor blades. If I “build” my 700 setup it says 11 minutes, but I know I flew it for 24. Quite a difference! If I build the Protos as described in the calculator, it says 18 minutes… so who knows. Guess we’ll find out!
It would be downright trivial to get a gas helicopter to fly 2 hours. But it’ll be way over 10lbs.
I wouldn’t recommend a 550, as it’s an inefficient size. It’s basically a 600 with clipped blades. Look at a 600 instead. Though one nice option is a 550 heli, and just put 600 size blades on it. All you need to do to make this work is a 600 tail boom. The 550 frame will be a bit lighter than a 600, but generally all the components are the same strength. My big heli is a 550 frame, but I’ve only ever used 600 and 700mm blades, never 550.
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I think, more than a few people would be interested to follow your build of the mapping helo!
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) but belt drive as to what I have read, belt drive is more forgiving when you graze something with the tail.