It always starts with an F-450, or a clone of it. Always.
Unless you’re a pro willing to cough up a five digit figure just to get in the air, or you’re going for the RTF1 stuff, you must have come across an F-450 in your multicopter ventures.
I think that DJI should be proud for this achievement alone though. A frame that is versatile, relatively light, sturdy and well-thought of regarding the placement of components, at least for simple setups. And all that for a very suitable pricing (especially for the clones).
Well that’s the way it started for me as well. I got my hands on a new F-450 clone about one and a half years ago. In fact, since the frame was quite cheap I got an F-550 clone as well, to use for spare parts and trying out the silly idea of using a hexacopter as a quad (with two arms removed).
So here it is:
- Motors: 4x iPower ipm2212, 920kv, 150w max
- Props: 1045 Noname, 1045 Gemfan Carbon, 1245 Noname and 1245 Gemfan Carbon Props (4 sets in total, all Slow-fly)
- ESC: Hobbywing QBrain 20A 4 in 1 ESC
- Flight Controller: CC3D nano
- Battery: Gens Ace 3300mah 3S25C LiPo
- Charger, Beeper, wires etc.
There is no radio control in this list because my original plan was to control this beast by WiFi, using an iPhone. In retrospect, I’m not sure how good of an idea this was, but more on that in another post.
Well there’s not much more to it design-wise at this point. The original drone was subject to a few test flights indoors, and after some PID tweaking, it did have it’s maiden flight. These flights were indoors and so there was little to no space for maneuvering, and a lot of noise! And the latter was a thing that I certainly wished to eliminate, so I started poking my nose on what the sources of noise are, and how it could be reduced.
I am not going to explain what are the reasons for the noise multicopters make, since I am not an expert on the topic and there are so many good writings on the topic out there. But there are a number of practical solutions that all people dealing with the topic seem to suggest:
- Use of larger, slower spinning props
- Reduce the angle of attack of props
- Reduce the weight of the craft
The last one doesn’t do any good by itself. It’s the effect of reducing weight on the load of the propellers that matters. Less weight means slower spinning, or less angle of attack at the same radial velocity (or a combination of both).
Now, I was already using the largest diameter prop that the F450 can carry (12 inch). My pitch was rather low at 4.5inch3, and have close to zero experience in prop design, so there’s not much I could do regarding the props themselves at this point. Thus, from the options at hand, lowering the RPM by reducing weight seemed to be the most viable one.
The F450 weighs at around 280grams. For an AUW of about 1050grams and no easy way of shaving some weight from the components themselves, the frame became my main target. Also, the F450 is not the most spacious of airframes, and as such I thought of hitting two birds with one stone, by completely redesigning the frame.
And thus the W500 was born. The frame was designed with the following aims:
- Single-level mounting of components
- Light and stiff (who doesn’t want that!)
- Relatively simple construction with hand tools and screws/zipties
I went for a hybrid between X and H configurations (could it be termed double-Y maybe?), using Fiberglass for the main body4, and pultruded 8mm square tubes for the arms. The motors were fixed directly on the pultruded carbon with two screws, and the carbon arms were fixed to the body with both screws and zipties.
The motor-arm connection, while quite elegant and very lightweight (practically no extra connection weight there), was not suitable for the pultruded carbon arms. The main reason is that pultruded carbon, lacking any lateral fibers, and with minimal epoxy to bind it, was subject to very easy splintering. This is what happened to a couple of tubes and I had to throw them away. I found out later on that if I wrap a ziptie tightly around the tube while processing it, the splintering is greatly reduced.
As for the arm-body connections, and contrary to my expectations, they turned out to be quite strong, and were not very harmful to the pultruded arms.
The frame turned out to be quite lightweight at 150grams5, and reasonably stiff, except in torsion. Torsion was unfortunately too big for any real UAV use. In addition, there were terrible vibrations at some frequencies, which led the FC to go nuts from time to time and almost crashed the craft a couple times. This provided a hint that, even though the frame was indeed stiff, it was not stiff enough to prevent vibrations. Cabling turned out to be quite tidy as well, given the single-level mount of the components.
After a few short test flights I unfortunately had to declare the W500 experiment a failure, due to it’s poor structural behavior, as explained above. With my lessons on structure learned, I went back to the drawing board to continue the quest.
- Ready To Fly; refers to drones that are, well, ready to fly out of the box. No additional components needed, including transmitter.
- Here I should note that where I am located there is not much of a choice regarding available components, especially for drones of the 400 or so class. So, in fact, the actual process was more like a constant back-and-forth between ecalc and suppliers. After “convergence” between design viability and part availability, the final part list for the drone was ordered from 4 different suppliers!
- Could also go to 3.8, but these props were nowhere to be found…
- As I said earlier, sourcing materials, and especially carbon fiber, is quite hard where I’m at. At that time, Fiberglass was the only material I could find in planar form.
- This can be reduced even more, by using carbon fiber plates for the body, instead of fiberglass