10

Most freestyle quadcopters I have seen are specifically designed to fly with the motors facing upward and have the arms of the drone underneath the propellers like this:

A "normal" quadcopter

As you can see in the picture ^ , the arms of the drone are fairly wide (as is common in lots of quadcopters of this type) and it seems to me that the arms would block a portion of the air coming from the props. I imagine that having the arms like this would reduce efficiency or top speed and maybe cause some amount of prop wash.

There are significantly less quadcopters which have the motors mounted under the arms like this:

(and almost all of them are home-made projects like this one which modified an existing frame to work upside down) A "less normal" quadcopter

What I am wondering is, why are so many manufacturers and hobbyists making so many drones with upward-facing motors as it seems that downward-facing motors would be better in many ways? What are the advantages and disadvantages of upward-facing motors vs downward-facing motors?

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    I think the exact same efficency arguments you cite for the upside-down configuration apply identically to the normal configuration. The air still has to flow around the arms no matter whether it's being pulled or pushed around them. – ifconfig May 15 at 6:41
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    @ifconfig Not necessarily, because a lot of the air entering the top of the propeller comes from around the sides of the top as well. And the air exiting the propeller is almost all pushing directly downward. – Jacob B May 15 at 7:12
  • I don't think the intake plume is that tall or wide. All I'm trying to say is that your claim isn't necessarily true and shouldn't be asserted as fact. – ifconfig May 15 at 7:16
  • @ifconfig oh. I didn't mean to make it sound like I was stating a fact. I just meant to put it as what I thought. I have adjusted it to hopefully make it a bit more clear. – Jacob B May 15 at 7:22
  • An answer below compares apples to oranges. We need the owner of your second picture to bench test it in both configurations, as little of the aerodynamics would change because the armatures are straight and flat, and it'd be easy to do so, it not being molded plastic with a curvature. Comparing pusher types doesn't control enough variables; need the exact same one, either pushing or pulling. – Mazura May 15 at 21:35
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One practical advantage is increased ground clearance, reducing the likelihood of a prop strike on landing or take-off.

Aerodynamically, a pusher1 design can be less efficient because the propeller is spinning through the wake of the fuselage - this results in 'lumpy' air, causing vibrations. I don't have exact figures, but I've heard anecdotally that this can be a 10 to 15% drop for fixed-wing designs and the exact fuselage configuration will make a difference!

1: Pushers are often defined as '[motor/engine] shaft under compression'; which seems to apply to the 'upside down' motor configuration.

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    I'm pretty sure your first para is the real reason for motor under prop being the prevalent orientation. I'd really like to see reference for the second paragraph's statements. Still, +1 for your answer (please get some references for the 2nd para!) – Pᴀᴜʟsᴛᴇʀ2 May 15 at 14:40
12

Expanding on @Kralc's answer:

Here's a study (unfortunately behind a paywall) that I'm going to copy key parts of below:

The hexacopter was mounted on a load cell test stand, and data were collected in the University of Michigan’s 5 × 7 ft wind tunnel over different freestream flow speeds, motor thrust percentages, and hexacopter angles of attack. To validate wind-tunnel performance findings, outdoor autonomous flight tests were conducted. Results show the pusher configuration generates approximately 15% more thrust (lift) than do tractor propellers; however, they exhibit a relatively poor lift-to-drag ratio. These results suggests that a pusher configuration hexacopter will have higher efficiency for local-area surveillance applications requiring hover and slow flight, whereas a tractor configuration is more efficient for payload transport applications that require forward flight at appreciable velocities.

Pusher vs Tractor type quadcopter comparison

RPM vs efficiency and thrust

Thrust to weight ratio comparison

To answer your question, Why do most drones have upward facing motors?, the answer lies mostly with the last graph which shows (from a fairly small sample size) that pusher type drones typically have a better thrust to weight ratio.

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    I don't think that last paragraph works at all. If pushers are better, why do most drones not use them? – Robin Bennett May 15 at 13:36
  • To be completely honest, after doing the research I'm not entirely sure myself. @Kralc's answer's probably more succinct and better suited. – Krish May 15 at 19:52
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    @RobinBennett The last paragraph sounds clear to me. It doesn't say pushers are better, they have more thrust but more drag. Tractors have less thrust but less drag. So if all you want to do is lift a lot of weight a pusher is better but if you want to race a tractor is better. This is similar to the trade-off you make with the number of prop blades: the more blades the less efficient but more thrust. – slebetman May 16 at 22:55
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    The brief explanation is that when hovering, the propeller acts like a fan: all the lift comes from the motor spinning it. When moving, the propeller acts like a hybrid fan/wing: for the forward part of the blade's rotation, it produces extra lift, while during the retreating part, it produces less drag, for a net increase in efficiency. A tractor propeller is moving through less-turbulent air, enhancing the "wing" part of the behavior, while a pusher has fewer obstructions to the exhaust flow, enhancing the "fan" part of the behavior. – Mark May 18 at 20:13
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    For those interested in reading the research paper mentioned by @Krish I was able to access it through Researchgate.net as a guest for no cost. researchgate.net/publication/… ~~ I found the entire paper quite informative. – HansenJC001 May 18 at 21:32
2

The DJI Matrice RTK has rotors on the bottom. I believe this is to avoid impact when flying upward for inspections. (Under side of a bridge for example).

I'm not sure about the aerodynamic effects, but I will say the majority of my near misses (or crashes) are due to upward/tilting into movements, and not downward tilting away.

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-1

They are more stable since the drone is suspended from a higher point. If the drone tilts, the side that gets lowered has its propellers move outwards, giving them more leverage while the side that gets raised has its propellers move inwards, giving them less leverage. That makes the drone right itself again. The lower the propellers are, the less effective this self-stabilising system becomes.

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    In either configuration these systems are inherently unstable and only becomes so as a result of the actions of the rate limiting control loop, possibly driven by a second level-seeking one. – Chris Stratton May 16 at 3:36
  • This is a fallacy. A drone is "suspended" from it's center of gravity regardless of where the props are – slebetman May 16 at 22:56
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    This answer appears to be a version of "the pendulum fallacy". – Russell McMahon May 17 at 4:22
-1

It is because practically an object can achieve equilibrium easily while it hangs from the rope rather than balancing(making it stand) in the ground.

Some similar thing also happens in the case of drones A drone need much more perfection in the positioning of the motors and the weight to balance the COG if you want to face the rotors downwards. Upward facing rotors of a drone give it more stability.

The old fighter planes had their wings in the front so the body behind the propeller will be pulled and balanced, if the wings will be tried to installed in the tail of the plane then the plane might not balance itslef.

PRACTICALLY a pull is more controlle than a PUSH that's why..

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