cicda

You know what’s better than flying? Hovering, just levitating mystically through the air and keeping it still. And it’s one of the reasons why the multi-copters enrapture me to certain a degree more than the other flying aircrafts.

Few days back I started to work on one such multi-copter with goals to make it capable  of way-point navigation and carrying a gimbal with head tracking functionality. Though I chose a hex frame initially-for payload requirements, but later chucked the idea out realizing that I can get almost the same thrust with a Y6 configuration without sacrificing the control sensitivity and the extra hours of work. For multi-controller I chose APM 2.5, since it is really simple to use and comes with pre-written firmwares which you can just flash onto your controller EPROM and you are good to go. Besides it’s easy to interface it with the optional hardware like gimbal unit, payload latching mechanism etc. But primary reason which motivated me to use APM was that it was the only controller I had available with me at the time.

Although Y6 configuration has better control sensitivity as compared to the hexa, here is the rub. With a Y6 configuration you lose on efficiency since the rotors are aligned co-axially. The aerodynamic view point would be that the bottom rotors pull in the the wake released for the top rotors, messing with the efficiency. Also since the incoming velocity increases, the effective angle of attack of the bottom rotors becomes low and it needs to function at even higher speeds to get the same lift; one more reason for inefficiency. All in all, the net efficiency decreases by almost  15-20% as compared to the hex configuration, for the coax props of same size and pitch. But hey, who cares when you are still getting the required flight time, better control sensitivity and a wider field of view for your camera. And anyway that problem can be solved to a certain extent if you are a bit of aerodynamics nerd. How? Well you can use blade element+momentum theory (google it, I am not going to spoon-feed) to optimize the pitch and diameter of bottom prop for the highest possible efficiency.  Generally the pitch and dia of the bottom prop will be higher than that of top one. Higher pitch will compensate for the reduced AOA and bigger dia will ensure that your tip doesn’t stall due to wake from top prop. I will share the Matlab code for optimization soon.

The Y6 configuration, itself has two versions A and B. Except the tail rotors, all the top rotors of Y6a has clockwise motion, while bottom rotors move in opposite direction; for the tail rotors top one is clockwise and bottom one counterclockwise. However, for Y6b, all the top rotors move in clockwise and the bottom ones move in opposite direction. I prefer Y6b over Y6a since it offers better yaw stability. Below is the pic of the configuration.

After having flown with this config several times, I can attest that the controls and stability are just fine and it flies like a dream.

Frame construction

I wanted the frame to be as lightweight  as possible- obvious, isn’t it? I had a 3D printer at my disposal; thanks to the awesome Aerodynamics club at the campus which I am part of, you can like our facebook page https://www.facebook.com/AeroD.BITSGoa?fref=ts . Now, I had few options available for the the arm rod material-aluminum, wood and unidirectional carbon fiber rods. I chose CF rods since the specific strength is much better than that of aluminium. Only problem with unidirectional CF rods, unlike the woven ones, is that the strength is really low in radial direction and you can split the rod just by pressing it between your hands. And the problem with the 3D printed parts is that they are really weak in axial direction(perpendicular to the layers) and strong in radial direction. But if you cap the ends of the unidirectional  CF rods with 3D printed parts, voila, it will be adamantium strong; okay, may be a bit less. So the frame is mostly made up of 3D printed parts and  unidirectional Carbon fiber which is cheap. The total weight of the frame came out to be 230 grams- pretty lightweight, right?

Here are the specs:

Motors- 6, 1200KV                                                                          Power distribution board-1

Batteries-3, 3cell, 2200mah (3s, 3p)                                        Net weight -1670 (with gimbal)

ESCs- 6, 30 Amps                                                                              Payload Capacity- Upto 1 Kg

Multi-controller- APM 2.5                                                              Throttle at take off- 30%

cicada 2

Problems faced with Gimbal

3D printable gimbals are a bit problematic because if you mess it up, they tend to vibrate a lot. The trick is to place the COM of all the moving parts of gimbal as close as possible to the primary servo- the one which is stationary w.r.t to the body, while designing it. If you don’t, it will create significant moment around  the gimbal-servo joint and result in huge amount of vibrations.  Also you have to get the thickness of the gimbal parts right, if you don’t they will tend to bend a lot.  Instead of designing or printing gimbals, I will advice you to make it using balsa, it will give you high stiffness and low weight. You don’t need to worry about the strength, you can stick two pieces of balsa such that the fibers are perpendicular to each other- it gives you really good strength.

Magneto-meter problems

Ahh! A lot of magnetic interference due to the the wires going to power distribution board. I am getting 50% interference at 55% throttle. That’s the reason why there is a provision for an external Magnetometer. But since I am trying to get the cost of the whole copter down, I will figure out first how to do without an external one. One idea is to use a ferromagnetic sheet between the APM and Power distribution board and create a magnetic shielding which will reduce the intensity of magnetic field lines from the wires, near the APM. It will be extra weight of course but not sure how effective it will be. Hmm..  will need some calculations. Other idea is to simply mount the power distribution board farther away from the APM, won’t do much change in COM position but will have to redesign it- not a big deal I guess. I will probably go with the later one; will post updates soon.

Views: 1246

Comment by Vladislav on October 16, 2015 at 8:13am

All in all, the net efficiency decreases by almost  15-20% as compared to the hex configuration.

it is not completely true: http://www.ecalc.ch/calcinclude/help/xcoptercalchelp.htm#hints

Comment by Abhinav Pandey on October 16, 2015 at 9:04am
Yep! I haven't mention that explicitly- that this efficiency decrease holds true for coax rotors of same size and pitch. That why I have mentioned that I am working on the optimizing the pitch and size of the bottom prop.

Anyway, I will make it explicit.
Comment by Vladislav on October 16, 2015 at 9:30am

Even if you uses same props you lose no more than 5%. If you pick the right propeller to the bottom of the engine you get even more traction. And coaxial scheme gives a bonus to the stability in strong winds...

Comment by Sergios Zafeiropoulos on October 16, 2015 at 9:56am

Plus one must take into account the reduced weight of the y6 compared to a traditional hexa due to the parts missing : arms, motor mounts etc

Comment by Vladislav on October 16, 2015 at 10:28am

and reducing vibrations from the engine as they are in antiphase :D

Comment by Abhinav Pandey on October 16, 2015 at 3:04pm

There are two causes of the inefficiency-

a) The downwash from the top prop, lessens the effective angle of attack of the bottom prop, reducing the thrust generated (at the same speed). To gain the same thrust bottom prop will rotate faster and at faster speeds efficiency is less as lift by drag ratio reduces. The swirl velocity( responsible for traction) is not much when compared to down-wash.

b) The wake from the top prop, especially at tips.

The efficiency decreases for sure but whether it's 5%, I am not sure. Will have to run a CFD simulation for that. I said around 20% because I saw it from the sources-ecalc etc. Can you site the source of that value please?

Yeah, I also got tempted to mention that the if the weight will be reduced efficiency won't decrease that much after all. But in my case only parts which are missing are 3 unidirectional CF tubes and some fasteners (I have used 6 motor mounts)  which weigh about 35 grams in total- insignificant.

I converted this Y6B from a hexa. And I can't say whether the vibrations have reduced significantly. They are not quite antiphase. The vibrations interfere constructively and destructively, alternatively after a displacement phase angle of pi/2 each. So yeah they have decreased but not really enough to have a large effect on efficiency. Also the torsional vibration is increased.

Can please tell why it will be more stable in strong winds?

Thanks

Comment by Vladislav on October 16, 2015 at 3:15pm
Comment by Vladislav on October 16, 2015 at 3:20pm
Comment by Vladislav on October 16, 2015 at 3:22pm

and yes, these people understand in the helicopter better than anyone

Comment by Andrew Zaborowski on October 16, 2015 at 6:25pm

It's an interesting issue that I haven't thought about before.  How the coaxial configuration will affect efficiency depends a lot on the distance between the two props I guess.  Too far and they start to perform like in still air (that would be very far though), too close and the effect will also be reduced..  It resembles the consecutive compressor fans in a jet engine with their different AoAs...

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