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
David, I run the red dampers. Make sure you have that plastic sleeve in the head. Align only added it in the past 1-2 years. But it makes a difference as it turns the head into more of a "teetering hinge" design, allowing the soft dampers to do their job without getting compressed just from lifting forces.
Gaui has a nice looking head called the "Formula" which has a true teetering hinge. Could be interesting to try:
Huh, looks like this confirms my theory:
http://www.rotaryforum.com/forum/showthread.php?t=31099
Rob. Very well written. Thanks for the time. I have noticed a big difference using different dampers. The stock Align setup came with the black ( 85D) dampers. I switched to the gray ones ( 70D ) and there was a big difference. I have the red ones ( 60D ) and will be giving those a try soon. My digital pitch gauge and turnbuckles came today so I can try to fine tune my blade tracking. It looks good to the eye but thats just another source of vibration that is easy to overlook.
Regards
David Boulanger
Like I said, we need a "special" camera. The new Powershot does almost all of it, and it's not "huge".
Austin, I fly an HDX-500, with flybar. That has been replaced by the HC-500. Both are very well made, but, going back, I would have just gone with Align. They are kind of a standard. But there are a lot of cheesy clones out there, so beware. Parts can be had almost anywhere, and they are very high quality. A 500 will still scare the crap out of you, and it should, but they are not too enormous, if it happened to go bye-bye.
@ Josh
I think it has more to do with it being a fixed head. You know how two blades jump about until they get straightened out by some rpm.
One of the tricks of the trade with full size helis is to change your rotor speed if you start to see some jitter.
The full frame sensor is actually not as good as a smaller one when it comes to video. A full frame 35 mm sensor is great for differential focussing effects for filming on the ground but in the air you want the largest depth of field. And you certainly don't want any camera with a mirror system. Sorry you DSLR lovers but facts are facts. Nikon and Canon have never yet been able to create the same rez.. that the 50mm zeiss lens has on a well known compact body. The reason is because of the design compromise to fit the mirror between the rear element and the sensor.
Re: Multi Blade Heads.
I have not tested this yet, but I have a theory that goes something like this:
First, it must be understood that commonly available 2-rotor RC heli heads are not free-flapping. They all employ dampers in the head of varying degrees of stiffness. Some are quite stiff, especially if you use a DFC head. Whenever you enter an aileron control, even if it's just to hold position in a hover, the blade disk tilts relative to the main shaft. I'm going to look at ONLY aileron control, since it makes it easier to visualize, and we tend to have more problems along the Y-axis because the helicopter has much higher moment of inertia along the X-axis.
Since you have a two-blade head, you get torsional vibration pulses, with peak amplitude that occurs when a blade reaches the highest point in the arc. When the blades are pointing directly forward and rearward, there is no force, because the dampers are not deflected. The force/vibration happens, because when the blades are not perpendicular to the shaft/head, the dampers are transmitting a force into the head. And this force comes and goes as the blades go between fore-aft, and perpenpendicular, resulting in a torsional vibration. The softer the dampers, the lower the force. A free flapping head would have much lower vibration due to flapping.
Now, if you have a rotor weight imbalance, you will get a vibration frequency equal to the rotor frequency. 1800 rpm = 30 Hz. However, the torsional vibration due to the blade flapping is actually at twice that rate, since you have two blades which each reach that highest point of deflection every revolution. So, 1800 rpm = 60 Hz.
This is important to understand because, it's possible to do a very good balancing job on the blades so that you won't have any imbalance, but you'll never get rid of the torsional vibration because the blades always flap in flight.
Now, if you used a 5 blade head, two things happen. First of all, the vibration will now be at 150hz intead of 60. This is because 1 of the 5 blades passes the maximum deflection point 5 times per revolution. Now the vibration is quite a bit higher frequency, which actually makes it easier to dampen. A damper system tuned for say... 50Hz will attenuate a 150hz vibe more than it will a 60hz vibe.
But, I also believe that the vibration *amplitude* will be much lower. This is because there are always blades rising up to the maximum deflection point, at every point in time. So that torsional force does not come and go as it does on a 2-blade head. Instead, there are always blades rising and falling.
There are a number of 5 blade heads available, mostly for scale applications. But it's not clear to me if they have any flapping hinges at all, or if they're rigid. They sort of look rigid. I'm not sure if a rigid 5-blade head would be better than a normal damped 2-blade head.
This 3 blade head does appear to have flapping hinges:
Multiblade heads will typically be less efficient than a 2-blade head. The only reason to use one would be to reduce vibration, or because you need to lift more weight while retaining given disk diameter.
@Thomas, I'm not a fan of the the microscopic, super-resolution camera concept. At least with technology available today. Much is made of the 41MP camera sensor, but I've never seen any demonstration that shows the net-result quality is anywhere near a Nex5. Let alone a real full-frame sensor. It's not all about megapixels. Squeezing more and more MP onto a smaller chip generall results in much more noise, particularly at lower light levels. You also have to consider the quality of the lenses that are available. I'm not a proponent of carrying a full frame DSLR on a UAV either. But somewhere between these two extremes...
@Jim: yes, I have a bag full of the rubber bulb dampers of varying stiffness that I use to tune my vibration mounting systems. That's the nice thing about them, is you can get them in different stiffness, and they are easy to change for heuristic tuning.
This has turned out to be a great discussion. Rob has mentioned a five blade head and Denny has talked about 3 or 5 being better than 2 blades. Whats wrong with 4 blades? The frequency a blade is moving over a certain point on the swash doubles. I just did another video test 1 hour ago and the video is outstanding at a rotor speed of 1900. The mount has about zero give to it. 1600 or 1700 gives me jello. I realize a head with more blades may need different gearing or a different KV rated motor. I was thinking about trying a 4 blade head but I think I will wait until I research more and see where this discussion goes. GREAT INPUT EVERYONE!!
Regards
David Boulanger
@Thomas - Thanks for the advice! I've noticed that too while watching people fly 450's. What would you recommend I get in the 500 size class for learning to fly larger heli's? I've flown coaxial and some CP before, but I'm planning on building up to that Vario. Thanks again!
Austin, retire that 450 and go to a 500. They're not squirrelly like 450s. I skipped learning to fly on a 450 and went straight to a 500, after learning the basics on cheap coaxials. BTW, I get jealous just even hearing about Vario. I have their catalogue. Do they have a Dolphin fuselage yet?
Denny, the more blades, the less efficiency you will have. I put 11" tri-blades on my quad, flew good, sounded real cool, but over-heated everything. Went to 12" two blade and everything cooled off and flight time went way up. Nasa even built a one-bladed heli, (big teardrop counter-weight on the other side), but it proved only practical in very fast forward flight. Very strange, it just looked "wrong". Cobras-2, Apaches-2, Hughes 500-5, because it makes them quieter. Dude, stick to two, any more is really for scale looks, IMO.
I'm excited to report that I found all of the replacement parts we need for my custom heli on Vario. Turns out, it's based off of Vario mechanics, and I can still get parts right from them, so I will be joining in the Arduheli community soon! Question though, after almost 2 years, my 450 heli is physically untunable due to the mechanics on it. What would you recommend I get to replace it, with the idea of putting ardupilot on it? Thanks!
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