I'd like to try a quadcopter with motors facing down. This setup could possibly provide better stability in the wind (hypothetical), I'm only guessing here. Since nothing will obstruct the prop wash, this could help. Now I'm not sure if I can use normal quad APM firmware for this (with motors facing up).. What do you think ?
No, No & No performance improvement that you can detect !
Why so strict ? Have you tried it ? Others apparently did but not running Pixhawk obviously as far as I know.. E.g. http://copter.ardupilot.com/wiki/advanced-multicopter-design/#a_com...
you can do it but to no benefit that can be observed! to implement properly you would only mount 2 motors upside down the other motors do the reverse . The motors that force the air down on to the booms.
Sorry but I don't understand what you mean.. Mount 2 motors down and 2 motors up ? My idea is simple,all 4 motors facing down. No restrictions in the prop wash, clear of obstacles..
I can see that Friend! No No benefit to doing that at all! if you do move all 4 motors upside down you will still have the booms in the way of the prop wash still! 2 normal props 2 pusher props both move air in different directions not just downward.
I see, thanks.
On of my next builds will probably be upside downers.
I did see a thread where it showed a measurable if small benefit (can't find it now but it was on DIYD). The prop wash under a prop isn't the same as above. Ever noticed you can't rally feel the the air moving behind a desk fan, but obviously a lot of air moving on the other side?
The side that is sucking is drawing from all around, the blowing side the air is in more of a uniform coherent cylinder.
The air velocity on the sucking side is lower, so the boom will have less drag force. I believe this will cause less beating due to the boom shielding air flow as that blade passes over.
The benefits were small though, just a few %.
The Arm directly under a prop represents a serious restriction in thrust.
It directly reduces thrust by its equivalent flat plate area.
A 1/2" square rod has 1/2" flat plate area, because it is more aerodynamic a round 1/2" rod has only 1/4" flat plate area (half as much) and an aerodynamic (wing shaped rod can have considerably less.
This is the actual amount of force that is actually wasted by "pushing against the arm".
With the prop under the arm, there is still some loss (even at hover), but it is much less because the air is not organized and directed as it is below the prop and is generally not even worth considering.
With the motors above and depending on Arm size and shape, the loss can generally be from about 4% to 7% or even more from the combined effect of flat plate area and turbulence.
With the motors below for all practical purpose at a hover there is no significant loss.
I may be off a bit on this, but not much.
You are also right that there is a vibration inducing interaction with the periodicity of the blade thrust when it is coincident with the arm.
Especially on quad copters with really big motors and props this is a noticeable and undesirable effect because of the relatively low frequency but high amplitude.
The main problems with putting the motors / props below the arms relate to landing gear placement and camera line of sight interference, but from an aerodynamic and flight endurance standpoint it is definitely superior.
I have gotten this information from Dr. Paul Pounds PHD paper on multirotor vehicles
and from Brad Huey a DIYD member who is a true expert on propeller design and implementation for multirotors.
Brad contributed a lot of info in the past but hasn't been around so much lately.
The older posts that you reference include this information.
Thanks, sounds encouraging ! I'm considering to build a quad based on GD1400VZX frame with octogonal arm booms and I'm considering down facing motor/prop option. I would try this and compare with upward standard version to see the difference. The setup idea consist of 110KV motors and 26-28" props running on 12S1P. Calculated flight time at hover with standard quad upward configuration is 81 minutes. So if down facing option would be better in any aspect, I would use it for sure. However I feel this is not the right setup for tests or experiments because of higher costs of components involved. So probably I will try it with something smaller, cheaper first. The main question is the Pixhawk and APM code.. Is it ready for this ? Can I use standard quad APM firmware for this ? I'm not sure..
APM code absolutely does not care whether motors are up top or underneath.
Depending on vertical CG relative to prop height, some tweaking of Rate PIDs may be necessary.
BTW it is optimal in any case to have vertical CG roughly in line with the props for maximum maneuverability.
But a bit of offset doesn't really cause any significant problems.
With the truly large copter you are considering, you will definitely see benefits in endurance and reduced vibration and wobble if you put the props/motors below the support arms.
On big multis with big slow props, there is also a wobble effect caused by real time relative prop blade position, most noticeable when accelerating upward or slowing the rate of descent while descending.
Can be very pronounced for big quadcopters.
Thanks Gary ! Worth a pint of beer advice !
B Hurly same guy how told my my 30mm square tube copter design would not work! works fine one most power full copter I have made! this stuff makes sense on paper but useless in practice. if your quad was the size of a 747 then yes it would make a difference.