Luke Maximo Bell has published an informative video documenting how he created and flew a quadcopter capable of over 4hrs of flight time. He used a CubePilot CubeOrange+ running ArduPilot and I hear that Flying Robot also pitched in.
It’s best to watch the video to get the details but for me I found it interesting that the vehicle was most efficient at a relatively slow ~5m/s and while circling. I’ve always heard that hovering was less efficient than flying forward but circling being more efficient than flying in a straight line surprised me.
I’m really not sure. I’d also thought of weather. Perhaps it’s because when flying in a circle the back motors are less likely to fly through the prop wash of the front motors?
Glad to see I’m not the only one who was caught off guard by the turns.
So just to throw another idea in the mix: I wonder if there was some sort of imbalance (twist in the arm creating torque split, or C of G, etc) that was some what relieved in the turn? Would a turn in the other direction have the same improvement?
That was my guess as well. Just a small imbalance in yaw such that spinning slowly improves efficiency, but also getting the translational lift efficiency.
This is the efficiency plot from the video. To me it looks like textbook rotorcraft power required curve albeit with lower speeds. I would guess that multicopters have pretty terrible aerodynamics compared to helicopters (and there is little you can do unless optimizing for one speed*) so drag contribution kicks in much sooner.
I wonder how much improvement you would get from flapping blades like on Alta X.
* Technically you can have moving aerodynamic fairings but that’s quite a lot of complexity.
@geofrancis yeah that’s what I was thinking. Considering the size of the circles and therefore a very slow yaw speed, it could be a tiny imbalance. When he’s pushing up against these limits each little change looks even bigger too.
It’s all speculation of course, but that was just my guess.
Completely agreed that this is standard rotorcraft behavior in the plot. Later in the video for the final flight he gets better efficiency when doing continuous circles instead of a racetrack which is what I think some of us were discussing.
That graph is really useful. 13% improvement comparing hover vs best speed.
@Elmtree it would be interesting to see:
how much of a difference X vs + frames make
propeller direction, would having the front propellers rotation sweep out cut down frame drag?
if there is efficiency gains to be had from a slightly twisted motor geometry could it be optimised for a circle nascar style, tweak it to only turn left more efficiently by sacrificing right-hand turns and some straight line efficiency.
After thinking about it, basically all you would need to do is twist the 2 motors that are having to increase to yaw the aircraft so they have some lateral thrust that reduces the rpm and thrust load on the blade so it matches the other 2, keeping efficiency higher.
I would guess that any rapid maneuvering is bad for efficiency. Though it is more likely that he had different conditions, for all we know he could have flown in some thermal and with such low rotor loading it could make a noticeable difference.
IMHO if you want meaningful still air efficency numbers you should do the test at night as during the day there is too much energy moving through the atmosphere.
@geofrancis I think X vs + will change which arms get swirl to decrease lateral inflow component for the arms. But I would not expect meaningful difference between them.
That kind of tests would need to be done at night to avoid thermals and wind.
There’s definitely a lot of variables, but it’s fun to speculate
In terms of + vs x, I did read a paper awhile ago showing what percent benefit you got from giving the rear rotors clean air by lifting them up a bit, but I can’t quite remember.
SiFly’s solution to the control problem with big rotors is super interesting. It seems like it’d be more efficient than traditional quadcopters at attitude stabilization, but I couldn’t say for certain. @LupusTheCanine they also have airfoil shaped arms for forward flight, but I’m unsure if they’re freely aligned, or fixed.
