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
There's a thread on RCGroups saying the pancake motors are not more efficient than normal (of the same Kv and weight), any truth in this?
The vertical position of the CoG will give some additional pendulum stability, but this is assuming all rotors are producing roughly equal lift. Most of the stability comes from the balanced forces of the rotor and therefore the flight controller itself.
I think most quads have top-mounted propellers because the landing gear would get in the way. It's possible to design around this, of course, but it's also possible to have upward curving arms to minimise the interference of the arm with the prop thrust. I'm sure it's not an inverse-square law thing going on here, but I'll bet the thrust velocity decays enough with distance for this to be useful.
I was trying to alert about this issue...maneuverability...
Sorry guys, I made a mistake (my English is not so good). Euan Ramsy is right. I used above instead below. Yes, I would like to say that it is more stable when the CoG is on the bottom....or when the rotor is above of the CoG and vice versa. Thanks.
These discussion about efficiency are all very welcome, but we need to keep in mind what is practical.
Re. props up or down:
I suspect the reason most props are "up", are because a) it makes maintenance easier b) the motor does not hit the ground in a tip over (we'll ignore the lateral torque that can damage it from the prop stricking for now). Is that worth a 5-10% drop in efficiency? Yes, probably.
Re: enclosed cases:
When batteries only last 20 mins, you change them. A lot. An enclosed case makes this a tedious activity. DJI are getting there with the Phantom 2, but now the battery itself is propriatory.
Re: props above or below CoG:
Ignoring the balance issue - the lower the props are, the closer to the ground they are. This makes ground effect and dust pollution a consideration. If you believe that having the CoG above the propline is less stable, this is a double whammy. Is it not better to trade some small efficiency gains for smoother landings and takeoffs and less dust contamination of motors etc?
Re: enclosed gimbals:
They offer protection...but from what? They add weight and complexity, and servicing is more complex. The camera is also fixed. Barebone gimbals are lighter, simplier and easier to work on, more flexible and more practical.
@ Ricardo, re: CoG - surely not? Surely a mass hanging below the points of location (ie the props) is more stable?
hold a wide broom by the brush ends, and let the handle hang down. Then hold it with the handle up vertically. Which takes more effort?
Rotor above the frame creates a parasite drag when the downwash hit the arms, thus the efficiency of the air flow will decrease / less lift. In another hand, rotors below will be more efficient, but you will create others problems (lack of motor cooling, damage your propeller etc).
e.g. - Another thing to be considered in this case is about Center of Mass/Center of Gravity. When the propeller is above/up the chopper is more maneuverable or less stable. Below, it is more stable but less maneuverable.
There is no "free lunch", if you do want something, you will need to leave another.
+1
I have tried half ducts, and in fwd flight the results were amazing :-)
Simon
Agreed Andrew - one of the reason I think Curtis' design can *only* be used for performance, because the small props and belt driven system is too complex and too lossy at this scale, and why ducted fans are not optimal for MR use.
The slow progress to bigger props should mean that we start to see less losses though (as you state), even if these are small improvements.
Sadly, I think a simple, actively-controlled and low weight pitch system that can be built directly onto a brushless head will remain a physical impossibility.
Ok... I'll give my idea on efficiency..
The lower mount motor setup should be more efficient as the "High energy" air is not forced over the arms and a lot less turbulent air and no or very little Karman Vortex shedding takes place (huge Impact averted).
So why do some people see a decrease in efficiency?
Well, prop efficiency is determined by the air entering being turbulent "free" if the blades are designed to be laminar. Remember, a blade is only a normal airfoil like a wing.. So just think about a wings efficiency...
So with that in mind, lets look at the "pump/compressor" aspect. I would bet that the decrease in efficiency is due to the pressure dropping in the arm obstructed area and causing the blade to cavitate. Air cavitation happens easily and most people would just ignore it. The effect of cavitation is that the following blade would also suffer separation and stall (think plane wing). And that would be the only reason people would test and find that the efficiency would be lower...
Feel free to reply to this...
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