Hi All, Just pulled this image in from SUAS News (Thank you Gary) to illustrate a few really good concepts that it incorporates and talk about better Multicopter design a bit.
This copter has a lot of things right and is really a study in design excellence.
1. Most obvious is a sliding ball full enclosure camera Gimbal, very pro and really something we should be striving for.
2. The motors are on the bottom under the arms: More aerodynamically efficient with no prop wash interference and very little intake interference. Noticeably increased flight times and greater lift capability and much quieter.
3. Large slow high efficiency props and pancake motors: These are way more efficient than faster motors with smaller diameter more conventional props.
4. Carbon fiber aerodynamic frame arms: Again more aerodynamically efficient, light and strong.
5. Fully enclosed framework and a simple spring loaded landing gear that provides as little interference with the prop wash as possible.
Basically this quad incorporates the best design features I have seen so far and definitely provides food for thought.
I thought it might be worth soliciting comments and other thoughts about optimal multicopter design techniques.
Best Regards,
Gary
Comments
nearly 2,5 years ago...
http://www.rcgroups.com/forums/showthread.php?t=1483594#post18968812
much quieter? think of pusher drive at flying wings...
What has also become very apparent now that manufacturers are playing around with MR prop.s is that the tips are very critical. having tested a vast number over the years a pattern has emerged that shows that wide cords at the tip are very bad. Any significant tip vortices act at the furthest distance from the hub and as such create drag at the most detrimental place. Fine tips such as the new props. form DJI, Graupner E flight etc. are the way to go.
Oliver
This is another reason why large diameter props are more efficient. The closer the the thrust is to the vertical airspeed the more efficient it is. As this value is zero then the optimum dia. would be infinite. This is based on a formula that is well known but I don't have it to hand right now. It is one of the major design factors for determining the right efflux speed for airliners.
I know from experience that Holgars formula for calculating endurance is cast in stone. Time=54 x bat.rating/current
54 comes from an estimated aprox. 10% loss. The current is derived from the lift curve of your particular motor/prop. lets say that the model weighs 2 kilos then each motor will need to lift 500 grams in the hover. So at 500 grams of lift lets say that our amp. meter says 3 amps. then for a quad 12 amps is your current value.
No matter how many times you do this calculation the less motor you have the longer the endurance.
The efficiency of the prop./motor will reduce as it has to lift a little more weight but the weight of the motor the arm, the esc. and the wiring and mountings will always be more than this deficit.
Denny, notwithstanding the fascinating plethora of secondary factors that affect efficiency, how much do you suppose are we leaving on the table by using props that are designed for airplanes, not hovercraft?
The thing that directs endurance efficiency is not what is optimal for stabilization. However the art of absorbing the available power into the largest blade swept area is what produces the best results in that catagory. If you couple that with the fact that as the RPM gets lower then the motor efficiency also improves then you can see why low high torque flat motors with a low KV are the best which is shown in all of the record attempts. When I get some spare time I'll publish the lift curve of the Multistar motor to show what I mean. You need to be looking at around 12.5 grams of lift per watt. if you want to be in the right ball park at 300 grams of lift. Currently playing with ESC32 to see if I can improve on this.
I watched several of the videos on the Aeryon site and we don't see another one indicator for the arms/props debate.
They claim a 50 minute flight time. Marketing BS?
They actually have two products. One with motors up, the other down.
Hot swap payloads too. Tablet flight interface. Well done and pricey for sure.
We need data, flight time data. The motors could draw 1000A but if the flight time is longer...
Otherwise we are debating angels on pinheads.
-=Doug
Simon, those results run contradictory to expectation, but that doesn't make it less true. I think it's worth investigating why this may occur and if there are specific circumstances that change this, for example pancake vs regular motors and the thickness of the frame. My craft is similar to Thorsten's and also never did an official experiment. It's certainly worth to investigate why this might occur. I hope to have my results ready this afternoon from a small test bench on a single prop, eliminating other issues.
In the video it's clear that the motors even make a different sound, much higher pitched, which is clearly an indication of higher load already, no need even to look at the wattmeter results.
If you can, please dig up that old thread on rcgroups , I'd like to see more evidence and other cases.
One thing that could be related here are changes in stability, as Ricardo mentioned. Putting the motors lower, the CG is pulled down, increasing the stability, making it harder to compensate for attitude changes so the motors have to work harder. In my case, the pancake motors make smaller differences to the CoG position.
Also, if the motor was designed with bearings in a "pull" position, then exerting force contrary could lead to more friction. How can we eliminate friction as a factor?
The other is a question on how accurately you twisted the arms around. In our builds we had some serious issues with unequal motor loading. The issue was related to three of six motors having a one or half degree offset from the vertical. Impossible to see visually, but having large effects on the efficiency of three motors. Or rather, it required three motors to work harder, significantly spiking up the current for those whereas the others didn't do much. This unequal motor load resulted in higher total current as well due to individual motor efficiencies. Please verify.
It would be interesting if you could rerun this test on a bench with one motor instead, which would eliminate other issues like ground effect, CG and how things are mounted on a vehicle.
I was impressed considering a CX4 with gopro and 4S3300 was drawing 20A.
Josh who designed the airframe told me that the ducting gives the aircraft additional lift in forward flight. Only catch was that in my experience during windy conditions the airframe's profile would give the wind a good surface to blow around.
Too many variables Graham. To lower the Kv, you need more turns which means longer thinner wire in your coils - your I²R losses will rise as Kv falls. I think you can combat this to some degree by increasing the diameter and shortening the stator to reduce the length of wire necessary to make the turns. It also increases the torque capacity of the motor and torque is current. I think they have some advantages in theory but how many of them are realized in the RC world of cheap Chinese motors - hard to say...
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