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.
Comments
OK, I can only respond to the number of blades issue based on full size helos so it may not be exactly true for scale - More blades are better. The Apache uses 4 blades. The Hind uses 5 the CH-53E uses 7. You just run out of lift with fewer blades given the disc diameter. Most early generation helicopters used the NACA 0013 series symmetrical airfoil because its pitch and lift characteristics were well known. The lower number of blade helicopters had to go to cambered airfoils to get more lift and keep the drag (read efficiency) somewhere near acceptable. I don't buy the argument that more blades are less efficient. The Bell 206 and the Hughes 500 for the same power get substantially different performance numbers - the 500 is faster by a wide margin and has a higher out of ground effect hover ceiling. Granted, the airframes are a factor. The CH53 E with 7 blades is a much better lifter than the D model. Here again, it has an additional engine as well, but to deliver the power, the 6 blade rotor could not handle the blade loading.
The mass of the blades is not as important as the moment (weight times arm). outright blade weights are no indication the blades will achieve a balance in the dynamic state.
N-per-rev(number of blades times rotor RPM) is aggravated by disc loading, and by non-uniformity of disc loading as previously stated. At hover, the disc loading is generally uniform and hence there is little N-per-rev.
Keep in mind, the helicopter is essentially a positive G machine. Unload a teetering system and watch your rotor stability go out the window. Only rigid rotor systems (where the blades are constrained to only pitch) can tolerate low and negative G. Now for the compromise...the rigid system, for all it's control ability also transmits the N-per-rev directly into the airframe.
Four bladed heads will only exhibit 2nd order harmonics if the disc load is not uniform on opposite pairs. One can create 2nd order harmonics in 4,5,6 et al by having the lift distribution unequal across the disc but uniform about the rotating axis. 3 bladed systems are generally immune from this.
Teetering rotors will mast bump when disk loading goes below a certain value and cyclic is applied. Isn't it then odd that the Cobra Gunship has one?
About the 3-4-5 blade heads and balancing, it seems that for a mass difference of 0.05 g paint or stickers can be used. The only down side that I can see for the 4 blade head is that it will excite the 2 blade harmonics as well. Whereas 2-3-5 blade heads excite prime number harmonics.
A trade off then presents itself, Time balancing vs. solidity ratio
Using COTS parts, the power optimal solidity ratio is defined by blade count and blade chord. Harmonic vibrations can be used to select between multiple power optimal solutions, but two stage balancing by hand may be required.
The genesis of scale multiblade rotorheads was not defined by engineering concerns; it was done for looks not reynolds number. An opportunity then presents itself; we need an engineered rotorhead and blade set that solves for the power optimal and vibration conditions. Flapping hinges may be used, but FEA and modern composites are more autopilot compatible.
Have you seen this webpage? www.unicopter.com He does great conceptual helicopter designs. The humble Canadian is a treasure trove of intel.
David, thanks for the tip. Looks like they have some interesting heads there. Too bad there is so little info. This head here appears to be a free-teetering 2-blade head, which could work. But it's not entirely clear if it's truly free-teeter or must simulated.
http://www.scaleflying.com/Bell-206-Jet-Ranger-Long-Ranger-Scale-Ro...
One thing to know about free-teetering heads, is that they cannot exert any cyclic control on the helicopter frame unless they are generating lift. As such, there's a risk that if you are not generating lift A) you have no control and B) this lack of control could allow you to chop the tail off.
Furthermore, I believe rigid multiblade heads will not produce an N/rev vibration due to cyclic, but they will still produce an N/rev vibration any time the flow through the rotor disk is not uniform... which is to say "always". ;)
A company called Rcaerodyne has some interesting rotorheads. They actually have a "plug n play" system that would slide into a Trex 550 - 600 with everything included at what seems to be a reasonable price. The sell semi scale large Helicopters up to around 20lbs. They also have a variety of other Heli stuff. Still looking around at maybe trying a four blade system.
Regards
David Boulanger
Hmmm... that's a good point about that head. There are a bunch of other complicated things about head design, such as Delta-3, which is where the pitch link is purposefully offset from the pivot. But this head appears to have negative Delta-3.... I think... which could be really bad. Or I'm not looking at it right. Will have to consider this more carefully.
I do balance my blades dynamically, but it would be tough to take a 2-blade set and make an odd number of blades. This is because to achieve dynamic balance, you have to start with the blades at *exactly* the same mass (I have a 0.01g accurate scale). Good rotor blades come matched by the manufacturer (Spinblades are within 0.05g) on a pair. But two pair could be significantly different.
But I think if you used a 4-blade head design, you could use 2 pair, only matched to eachother, as long as you keep the pairs on opposite sides.
@R_Lefebvre
The three blade head with flapping hinges worries me. The blade grip's feathering axis ball is not collocated with the axis of the flapping hinge. Thus flapping angle change creates feathering angle change. If you look at the rotor head that the artist was trying to copy, you will note that there is not flap induced feather. In forward flight this defect should induce a port roll moment.
http://www.cnchelicopter.com/servlet/the-2668/AS341-high-simulation...
Has any one manually balanced a blade set statically and dynamically? Theoretically, we could put wide-chord blade pairs on a five blade head. This should increase max ineducable power and drop hover power required.
It will be a golden day when rc helicopter aeromechanics becomes it's own scientific discipline.
We really appreciate your creation of this thread. Lets raise the bar. Do you really want to make a tandem? I plan to. If you would like to chat about advanced aero_design pm me.
Here is my bird(Trex450) with a Tamarisk640
I know what you mean :) It's been a long time since I've bought what I would call "important" servos. I have 20 Futaba/Jr, a dozen little micro Hitecs, and I haven't needed to buy anything else for a few years. Thank you so much for the help! Unfortunately, the blades you recommended seem to be right hand only, bu at least I know how far I can deviate from 725mm. :) I hope to be joining you soon in the autonomous heli club!
Helicopters with failed servos won't runaway very far. ;) Luckily it's not that common if you use good servos. The trick is figuring out what good servos are. Futaba is kinda the benchmark. JR historically every good. Both are very expensive. So that's why I'm looking for alternatives myself.
If I could find MI BL HV servos from one of the big name brands, I might bite the bullet. But I haven't seen them. And the Itair servos seem quite well made and I've not had a failure that was their fault. ( I have killed a couple through my own fault though).
@R_Lefebvre: Thank you so much! That should cut a huge chunk out of my expenses, as well as increase efficiency. As for the servos, those MIBL servos put up some impressive stats. This will be a working man's helicopter, doing videography and residing on lease for a few customers who have interest in using it as a platform for their experiments. I live in the heart of defense contractors and military, right next to a NASA base.... I could really use durability and reliability ;P Runaway heli's on an airforce runway is not my idea of a good day. Thank you so much for your help, you've been nothing but kind!
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