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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Comment by Muhammad Al-Rawi on February 3, 2014 at 3:40pm

@R_Lefebvre,

I can definitely say I've experienced most of those failures between the quads I've built :)

Thanks for the insight. I'll have to try out something small when spring/summer comes around. 


3D Robotics
Comment by Joseph Aletky on February 3, 2014 at 3:44pm

Thanks Rob, I have a Gen 1 Protos so maybe they have done some work to help with the static. I had my Rx brownout from the static in a tailslide that totaled the heli, and I can't rebuild the thing knowing I haven't solved the problem.  I only buy torque tube helis since then :) problem solved!

Can't wait to get at least a trex 450 up in the air with a Pixhawk or APM. Now all I need to do is find the time to set it up..


100KM
Comment by keeyen pang on February 3, 2014 at 4:42pm

Very interesting post. I am also wish to develop a traditional heli platform for my aerial mapping task. Just wonder how is the latest APM traditional heli firmware, is it reliable? I didn't see much development there.


Developer
Comment by Rob_Lefebvre on February 3, 2014 at 4:52pm

Josh, I haven't tached it, but I estimate it's about 1800. Normally I think these are around 2600-2800?  I can't really say if the Asym blades are more or less "floaty" than symmetric.  Probably not really much difference I am running such a heavy load.

That being said, I really do like using Alt Hold quite a lot.  It just makes flight so much easier.  For sure, with the setup like this "settling with power" becomes a big problem you have to watch out for.  Alt Hold completely eliminates it.

Joe: On the larger heli, I'm using a direct drive tail motor.  I like this a lot.  It eliminates ALL the problems with the tail drive.  I might convert the Protos at some point.  One thing I really don't like about the Protos is the belt drag, it's really really high, and I can't imagine it's going to auto well.  Especially at this weight!  My 600 freewheels for ages after powering down, because there's ZERO drag on the main rotor.

keeyen: The Tradheli code is pretty much as reliable as the multirotor code.  90% of it is exactly the same code.

Comment by Gary McCray on February 3, 2014 at 5:26pm

You brought up an interesting point Rob, 30 years of development going into producing trad heli hardware capable of insane aerobatics.

That has unquestionably resulted in the reliability that we see in them now.

Whereas multis are currently mostly just cobbled together with parts that were designed for some other purpose that happened to be lying around.

And on top of that there are a lot of corner cutting toys out there that are being (mis)represented as viable platforms.

Prop design and execution are very primitive.

A lot of the motors are not really well designed for continuous high speed, high load operation (even on good ones bearings are often really marginal). Outrunner motors with only one bearing at one end are already a really compromised design because that one bearing has to take all the continuous hover thrust plus serve to keep the motor itself in alignment. In fixed wings it isn't such a big deal because you really normally aren't spending the whole time hanging the entire airplane from it. Really need some more serious bearing / layouts.

And multicopter frames are all over the place, mostly kind of a joke and lacking in any serious top down design.

Basically the new kid on the block has to play catchup and get some serious hardware designed from the top down by actual engineers.

And then you'll still be able to use gas on a heli and run 10 times as long.

Comment by Denny Rowland on February 3, 2014 at 7:27pm

I have a 700 size Hensliet and it is the best AP platform that I know of. It wins hands down if you want to lift a heavy camera. Bullet proof mechanical and electrical engineering and performance far away better than a multi. 

The problem is that in the unlikely event that it would fail, it could crash into a crowd at 190 kph.  It would not be a case of seeing a fatality, just how many. I get nervous just seeing it fly at high speed on a salt lake. Going smaller would be an improvement but the risk is always lurking from radio interference. 

Comment by Andrew Rabbitt on February 3, 2014 at 7:31pm

The Melbourne group had a meet-up on Sunday - 2 fixed-wing, 2-multi-rotors and 1 heli.  Both multirotors went home for repairs whilst the rest of the fleet was unharmed (despite my best beginner piloting skills!).  As I said at the field, I think tradition had an emphatic victory that day!

Comment by Chris Khosravi on February 3, 2014 at 9:10pm

Sorry for the late reply, but the Pulse Aero Vapor heli posted earlier here is actually a Minicopter Joker (700-800 class heli) with a custom frame. I've seen it in person. Awesome looking heli

Comment by Dan "HotSeat" Neault on February 4, 2014 at 5:32am

Sorry, I forget to tell you about this, I meant too.

When playing with different airframe a year or so ago, I made a mount for my Heli, that support the APM & Battery, with off the shelf vibration stand offs.

 

Extra mass = no lhigh end vibrations, common fix for machine designers :)

 

Worked really well on my heli, didn't need it on other airframes, so I haven't use it since. You may want to try it out.


Developer
Comment by Rob_Lefebvre on February 4, 2014 at 6:39am

Dan, that concept is foundational on most of my vibration damping efforts.  I use it whenever it's practical.  But there are a few issues:

First, no room on this heli to add mass to the PX4 mount.  It really doesn't have much damping at all actually.  On the bigger heli in the back, I've got the APM on a nice vibration damping mount which actually has 3oz of lead on it.

Second, dropping the natural frequency of a damped system isn't always the best choice with a helicopter, since the primary vibration frequency is very low, about 20Hz.  You have to be careful not to drop the natural frequency of the damped mass to that range.  Meanwhile, if you go too low, and drop the frequency to <10Hz to try and damp out the main rotor frequencies, you are now in control-oscillation range.  This would be very bad for the flight controller, or any payload which is significant compared to the mass of the airframe.  It can work for payloads which are light compared to the airframe.

So yes, it's a valid approach, but must be used with caution.

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