Helicopter Presents Attractive Alternative to Multi-rotors and Fixed-wing UAVs

I've been working on a new helicopter platform the last few months. Based on an MSH Protos heli which I chose because it's an extremely light weight platform, weighing in at only ~1200g without battery. It has a full belt drive which I much prefer to gears as it's quieter, lower vibration and more reliable. I've had a few problems with it because the belt drive makes a really awesome Van deGraaf generator... not a good thing on a UAV. But I solved that, and am conducting test flights now.


The flight controller is a modified PX4v1. I replaced the switching regulator with a MIC29300, so that I can run it on 2S direct with the servos. Main motor power is 4S 5000, typically this heli would run on 6S 3300. Using the MSH stretch kit and 465mm Spinblade Asymmetric blades. In otherwise standard form, this heli flew for 17 minutes on an old crusty battery, in -10C temperatures.

I have now added a subframe to hold an extra battery, FPV gear with a camera in the nose, and a vibration damped NADIR camera mount to be used for aerial mapping. The idea is to develop a mapping UAV that is superior to a multirotor, offering a valid alternative to a fixed wing for short to medium range missions. The VTOL capabilities would eliminate all the nastiness of catapults, and controlled-crash landings with onboard cameras in rugged areas.  Even the price is attractive at about $400 for the basic kit with motor and ESC (no servos).

Specifications show the advantage of a heli platform. This machine has an AUW including the batteries and camera of only ~3kg. It is 80m long, and about 15cm wide not including the extended legs, and 30cm high. The blades fold for easy transport, without requiring any lose wires or vibration-prone electrical connectors as a folding multirotor does. It actually looks much bigger on the table than it really is. This seems to be very good compared to multirotors I've seen with the same performance. (payload and duration)

Vibrations are always a problem with helis, but manageable with the right design and construction techniques.

Arducopter really makes helis worthwhile. You could buy two entire heli systems including a Tx for the price of a single DJI Ace One non-waypoint controller.  Or 7 for the cost of a single Ace One waypoint enabled controller.  I strongly prefer the PX4 controller over the APM and Pixhawk, because it offers 32-bit performance in a small package that is easier to mount in a heli frame.

So does it work? I took it up for it's first photo tests yesterday, and it worked beautifully. Better than 80% photos are usable. It flies for 20 minutes in a hover with old, cold batteries (-5C). I'm hoping for closer to 30 minutes while actually moving (helis are more efficient moving than hovering), in warmer weather with new batteries.  It should have an easy cruising speed of 15 m/s with little or no reduction in flight time.  At 20 minutes, this would offer an 18km range, and 27 if it can do 30 minutes.  If you wanted to do FPV and not mapping, you could configure it with a 3rd battery in place of the SX260 and fly for... 30-45 minutes, and a range of up to 36km.  Top airspeed is still TBD, but probably 20-25 m/s.  

Wind penetration and stability is excellent compared to both multirotors and fixed-wing.  You could do a mapping mission in winds up to 40 km/h with little effect on stability or duration.

If the success continues, I'm going to consider building a large gasser heli.  This would allow flight times up to 2 hours, or payloads on the order of 10 lbs for 30 minutes.  So you could map large areas, or even perform light duty spraying operations.  I'm thinking about local application of a herbicide for things like Giant Hogweed elimination, that sort of thing. Such a large heli does pose significant danger and should only be used in industrial, agricultural or remote areas.

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Moderator
Comment by Gary Mortimer on February 3, 2014 at 1:15pm

Alternative to multirotors! Get the fire proof suit on. Looks great and I am oh so tempted.

Comment by Rob_Lefebvre on February 3, 2014 at 1:20pm

Ha ha!  Bring it!

I fly multirotors too.  So I get to see the pros/cons of each.  Main disadvantage of helis is the mechanical complexity.  And I have not yet managed to get usable video from one, but I haven't given up on that yet.  Still a few tricks up my sleeve. ;)


MR60
Comment by Hugues on February 3, 2014 at 1:39pm

Nice and impressive work.

Now my turn to bring a hot potato in the debate multirotor/helis ;)

They look terribly difficult to build and to fly, not talking about their human danger wich is probably the worst of all flying UAVs :"human danger" is by my definition proportional to the physical factors that could endanger a human being, that is : quantity of rotation momentum in the blade(s), size of turning blades, blade exposure to body parts, blade weight (=energy deployed by a rotating mass), dislocation limits of a blade (the easier it breaks on impact the less potential human danger).

I would therefore intuitively score an heli, relatively to a multicopter with plastic props, very negatively:

-Quantity of rotation momentum in blades : Heli : very high / Multicopter : much lower

-Size of turning blades : Heli : very big / Multicopter : much smaller

-Blade exposure : Heli : very exposed / Multicopter : can have "blade" protections

-Blade weight : Heli : heavier / Multicopter : lighter

-Dislocation required strength : Heli : extremely high (because these props are bigger and must resist to tip speeds that can reach the speed of sound) / Multicopter : very low (except FC ones)

What about motor redundancy for an heli ? If your single motor dies (or your driving belt gets used and breaks), it will fall in a horrible way keeping blades turning at high speed and cutting everyone's heads off in its falling path.

That being said, I have to say I admire your courage to build & fly helis!

Just a thought and i bet I probably just triggered a hot debate here.


Moderator
Comment by Gary Mortimer on February 3, 2014 at 2:06pm

Helicopters are just plain cool though.

Comment by Rob_Lefebvre on February 3, 2014 at 2:07pm

Hugues, no hot debate, as what you've stated is not too far out of line.  But there's a couple things:

It's all about risk management.

There's no doubt that helicopters are the probably the most dangerous type of UAV.  However, the risks can, and should be mitigated.  First, the smaller the heli, the lower the risk, This is obvious.  A 450 helicopter presents relatively small risk to a human.  I developed this 500 size to see what could be done with a mid-size machine.  Turns out you can do quite a lot.  Helicopter blades on hobby sized machines do not actually spin anywhere near the speed of sound (around 350 km/h for me), but that does not mean they aren't dangerous.  600+ size helis should not be flown anywhere near people.  And even they can have their risk managed.  I typically fly with greatly reduced headspeeds compared to acrobatic machines, which reduces the risk.

However, I don't think any UAV should be flown near people.  While helicopter blades do pose a great risk, multicopters tend to fall straight down.  And a multicopter capable of carrying a given payload seem to weigh 2-4 times as much as helicopter of similar capability.  Furthermore, helicopters do not tend to fall straight down when they fail.  If helis do fall straight down, it is at a greatly reduced speed (autorotation).  Unless the blades have fallen off or stopped due to catastrophic failure.  Point being, multirotors fall hard and fast.  You could still kill people if you hit them with a falling multirotor.

The redundancy of multirotors is often overstated.  First, multirotors have no redundancy unless they are an Octo.  Lots of people are flying quads and hexas.  Then, even with Octocopters, you aren't guaranteed to be 100% reliable.  First, not everyone is building them such that they can be redundant.  Some are overweight and can't fly with a motor out anyway.  Many still have plenty of single points of failure.

Some other things to consider:  Ground safety.  Multicopters are very dangerous for their operators on the ground.  They can start up cut you in an instant.  Even lift off and fly at you instantly if you make a mistake on the control side.  Happens ALL the time.  Helis can not start as suddenly, giving the operator more time to respond.

I don't think you should ever fly UAV's near people, no matter what type.  I only do test flights in my neighbourhood rarely and when there are no people outside. 

Comment by Assaf Weiss on February 3, 2014 at 2:10pm

Regarding dead motor in a heli - this is actually a major plus, auto rotation of a heli is pretty easy most of the times when the engine dies, in the hands of a moderate pilot a dead motor almost always ends up with a perfect landing.On the other hand - in most multi rotors setups a dead motor means a definite crash most of the times.

Comment by Gary McCray on February 3, 2014 at 2:17pm

Looks great Robert (and very cold).

Interesting that you actually make a compelling case for the economics of a large traditional heli, I wouldn't have imagined that was even possible.

As to whether these are more dangerous than multis, probably a bit, but Trad helis have had a lot more time to have serious accidents than multis so it is no surprise they have a few.

I wouldn't want your copter or a multi falling out of the sky on my head and they all need to be flown appropriately for the circumstance.

Nothing will ever counter stupidity or carelessness.

It is more the unfortunate spread of neophytes with way too much copter and way to little control that is the problem far more than the copters themselves.

In any case really great job.

Best Regards,

Gary

Comment by Rob_Lefebvre on February 3, 2014 at 2:24pm

Yeah, the economics is a really interesting point. A large, high quality Octo is quite a bit more expensive than a large high quality heli.  You can easily spend $4000 for a nice Octo, and it just goes up from there.  But just $2000 could get you a really nice 700 size heli.

It's kind of counter to conventional wisdom, but it you look at it...

Gas power is the "killer app" in my mind though.  It allows capabilities that are impossible with a multirotor.  And VTOL allows usage in areas that you could never use a similar AUW gas powered airplane.  


MR60
Comment by Hugues on February 3, 2014 at 2:29pm

Safety and well prepared flight plans (with escape/emergency route / predefined crash site) makes unanimity (and obviously never fly above people)

About multis flying straight down, the french regulation has (for once) produced a smart requirement : a falling multicopter energy must not exceed 69 Joule when hitting the ground. Which means in practice you have to have a parachute system to slow your multicopter down. The heavier the quad, the heavier the larger the parachute.

Comment by Muhammad Al-Rawi on February 3, 2014 at 2:30pm

Very cool! I've been looking at trying out a traditional heli for some time. I don't know much about keeping one in shape and the mechanical dependency kind of pushed me towards multi-rotors. 

How is maintenance on a heli of this size and type?

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