Once upon a time I read a couple not-insignificant statements:
So why don’t more multi-rotor frame designs place the props underneath the arms? Are (1) and (2) not true? Or not worth it because of some disadvantage(s)?
Thanks…
Easier to mount on top
“easier to mount on top”… because…
?
Plus manufacturers could easily just flip motor mounts around - and - they are easy enough for customers to flip around anyway.
Because the props might hit underslung components or retractable gear. They might also more easily show in the FOV of an underslung camera.
That doesn’t mean it’s not a good idea for performance, but it might be a bad idea for many use cases (and a compromise worth making).
I fly a couple commercial drones with props mounted underneath. DJI M300. Acecore Noa. Personally I find the lower mounted props to be a bit of a pain operationally. They are more likely to hit grass or leaves when landing. The blades always seem to be in the way when working with the payload. And for certain suspended payloads the lower props makes it very tense when trying to land with a cable hanging down. I get it that these aren’t issues for everybody, but it’s been enough that I do notice it.
Yeah I hear what you guys are saying - that of course there are use cases where under-mounted props are problematic - just seems like the default mounting would be underneath, and the EXCEPTION would be mounted on top, instead of the other way around.
I’d love to see some hard data on prop “pulse” vibrations, as well as efficiency numbers - and - what shapes of arms produce the least negative numbers when props ARE mounted on top.
Well here’s a start…
There’s actually lots of advantages of keeping the motors up top besides just efficiency, and overall vehicle design considers the entire use case:
(Most importantly imo) Keeping motors out of view of any cameras/payloads
Access to the motors and propellers
Enables belly landings of airframes without landing gears & underslung payloads
Keeps propellers clear of ground for low landing geared vehicles
There’s also something to be said about the efficiency of rotors while the airframe is moving at speed. The prop efficiency might actually be higher on top of the airframe as the rotors arent disturbing the incoming airflow. For most applications, a multirotor might be moving at speed instead of hovering in place.
Best of all, its tried and true, which makes the design decision a really easy and comfortable one to make.
The efficiency while standing still might be true, but overall vehicle design considers a lot of things outside of that.
Just FYI I wasn’t addressing or asking about “the efficiency while standing still” - as of course that wouldn’t be of much use.
I’d think any “disturbed airflow” from the frame while it’s moving thru the air would still effect efficiency less than top-mounted props beating down on the arms - and causing vibration as well in the meantime.
But anyway your points are taken - of course there are many use cases where props on top make more operational sense.
I know this wasnt exactly the original question…
That study mentions the efficiency of coax configurations, but obviously using the same motors and props as for the single configuration. One aspect not addressed (because it wasnt in scope) is the redundancy and a related advantage to the coax format.
Take an octocopter frame of about 1m “wheelbase” and the biggest props you can use is about 14 inches (18 inches for a conventional Hex format).
Now consider a QuadX8 (coax) format you can still have the redundancy but now you can use props of (up to) 25 inches. Probably 20 inches is a more realistic value.
So with the QuadX8 design you can have larger more efficient props, redundancy and huge amounts of thrust.
There are numerous commercial designs that take advantage of the QuadX8 design.
Check this study, that concludes:
A pusher design is more efficient for hovering and slow flight missions, such as
local-area surveillance and inspection; whereas a tractor design is
more efficient for missions requiring appreciable transit, such as
package delivery
Huh.
Ok… Interesting.
I’m not clear though how the pusher generates more “drag” in forward flight. Seems like he may be using the word “drag” in a non-standard way?
All props are more efficient at a certain forward speed as opposed to hover and low speed.
Was he referring to airframe drag? That doesn’t make sense either. I can also see where the size and shape of arms can introduce more variables.
Ok - more research papers are needed. 
All I know is that my quadcopter flies around at a lower average mah/min when the props are mounted underneath as opposed to mounted on top.
But - I’ll try to revisit and make sure I wasn’t deluding myself.
Drag has multiple components as a force. It may be due to parasitic or interference drag that is more amplified for the pusher configuration. Maybe if you start designing a drone with a pusher configuration in mind, you can find ways of minimizing those effects.
In your case, there are other factors to include also. After mounting the props underneath, where is the C.G. in the Z axis? This may affect the average motor output during maneuvers. Have you tried running the whole tuning procedure in both cases (pusher & tractor) and measure again the “efficiency”?
“I’d think any “disturbed airflow” from the frame while it’s moving thru the air would still effect efficiency less than top-mounted props beating down on the arms - and causing vibration as well in the meantime.”
You may be surprised as to the disturbances of the airflow from the frame, feeding into the prop disk area.
I did a non-scientific test of a homemade, foam flying wing, using same motor and prop. By switching from pusher to tractor configuration, noise level dropped considerably and efficiency not as much, but measurable in regards to the amount of amps required for same airspeed.
Perhaps someone could do a test of a multirotor by only changing the motor positions and recording the results of power required, vibration levels, etc.
My money would be on top-mounted configuration.
This is an interesting question, that I have also been thinking about for a long time. But in my thinking, a lot of importance goes into making the center of thrust agree with the center of mass, a point that many copter builders seem to totally forget!
Please correct me if I’m wrong, but I believe that the “pendular stability” achieved by having the center of mass lower than the center of thrust does not help at all in actively/automatically controlled aircraft, because all the required stability is provided actively, by the combination of various sensors and fast data processing, that uses several configurable parameters to optimize stability. We just don’t need to add passive stability. Whether we place our propellers above the center of mass, or at the same height, or even below it, changing pendular stability from positive to neutral and even negative, makes no change in actual stability, after the parameters in the software have been properly set. Right?
But there is an important other point: Power distribution between motors. If the center of mass and center of thrust are coincident (in all three dimensions), then during stable flight conditions (hoovering, or flying at speed, with or without wind) all motors will be required to produce equal thrust, and thus run at the same power. Power changes occur only briefly, while changing the craft’s attitude, or accelerating it.
If instead the center of thrust is higher or lower than the center of mass, then whenever the copter is in an inclined position, half of the motors need to continuously produce more thrust than the other half. This condition limits the thrust reserve for manoeuvering, the cargo capability, and it also reduces efficiency, and thus flight time. Particularly flight time is affected, because a higher final battery voltage is required to maintain adequate manoeuvering reserve when the craft is imbalanced in this (or any) way.
When flying fast enough, aerodynamic effects come into play too, but I believe that due to the mentioned thrust distribution between motors, it should always be best to place the propeller plane so that it intersects the center of mass.
From a practical, operational point of view, I dislike having the propellers face down, with nothing below them, because it’s a fact of life that some landings happen with the copter not totally leveled, or with some residual horizontal speed, and I just don’t enjoy landing on my prop tips…
So I think that an optimal configuration when designing a copter is placing the center of mass as high as possible (battery on top), then place the props as close as possible to the height of the resulting center of mass, and then place the arms as low down as possible, under the props, using the arms as landing gear. That way the arms are as far away as possible from the props, minimizing interaction, the motors can all work at equal thrust even when the copter is inclined, and the structure of the frame is simple (needs no separate landing gear).
Of course the arms should be as narrow, smooth, and aerodynamically shaped as possible, consistent with stiffness, to further reduce aerodynamic interaction with the props. But the importance of this might not be very high.
About vibration: In my own, admittedly very limited experience, the largest contributor to vibration is mass imbalance, rather than aerodynamic interference with the arms. I see that very few people dynamically balance their motors and their props. At best they do a static balancing of their props, which is definitely not sufficient to achieve low vibrations! I invested a lot of effort into doing a sort-of dynamic balancing of each of my motor+prop combos, fully assembled. All it takes is loosening the prop and retightening it, and the balance is gone, the vibrations are back! So it’s of little use to balance a propeller by itself, then mount it on a motor. When I get the balance just right, after much trial and error and many little pieces of tape, small spots of hot glue, layers of varnish at various places, the vibrations are very low. If I then wrap my copter arms in something that drastically changes their aerodynamic behaviour, not much changes with the vibrations. But all it takes is a prop catching a blade of grass, and the balance is gone, the vibrations shoot up.
Surely this is not the same in all copters. Depending on prop size, type, speed, arm shape, size, surface, the interference effects on vibration might be stronger in some copters, and vibration might be more or less of a problem, but in mine (a good(?) old F-450), mass imbalance dominates by far. Which makes me think that the vibration advantages of placing the props under the arms instead of above them might be irrelevant in most cases.
A lot of vibration can also come from aerodynamically imbalanced props. For example, think about a 2-blade prop that has a slightly crooked seating plane, so that one blade ends up with a higher angle of incidence than the other. The result much more thrust from one blade than from the other, and fierce vibration even if static and dynamic mass balance is perfect.
I went through dozens of props of different brands and types, selecting the least crooked ones, rectifying their seating planes, enlarging their center holes to make them straight and well centered, then inserting precise bushings with a ring-shaped contact to the shaft, to remove play without crooking them again… And it all helps a lot, but all it takes is a slight hit against something, and the propeller will again be out of balance and alignment.
So what I suggest is: Don’t see it as “props over arms” or “props under arms”, but rather see it as “center of thrust coincident with center of mass”, and then place the arms wherever it’s more practical for your intended use. In most cases that’s arms below propellers. Keep the arms reasonably thin and smooth. And do watch the static and dynamic balance of the complete, assembled motor+prop combos, and the aerodynamic balance of the props!
Pendular stability is a myth. There are some aerodynamic effects on translating rotors that cause pitching moment opposite to the direction of flight in helicopters but it isn’t considered as a source of stability at all. If anything placing rotors above CoM would benefit from this effect causing aeromechanical damping.
Only thing that matters is that CoM lies on the thrust line.