Motor angle dihedral What’s the deal with this?

[SleepyC]

Ok.. so the spy shots of the new DJI S1000 started to surface and it looks like this:
View attachment 14872

So it looks like DJi is staying with the dihedral concept even with the stiffer carbon tube booms.

Has anyone tried this with motor shims on a normal Cinestar or (insert any other pro frame name)

I can see the benefit being a better centering ability since all the motors thrust are pushing slightly towards center.

So I guess i am asking why has no one else taken this approach? Is there a good argument why this is not good?

 

[kloner]

i think the dihedral makes a bigger prop wash/thrust area for stability. the angle of the motors on the arms pre loads the yaw so they are more even to spin left and right/ for it to correct faster. I've only seen it from dii on the 800 and now this. After playing with brushless gimbals and considering this is evolved from zenmeuse, i bet it has to do with getting the wind off the gimbal to make it perform better.

 

[SleepyC]

I’m with you, so do you think shimming motors like 3 - 5 deg would be beneficial to most rigs?

 

[Benjamin Kenobi]

I also thought it was to help with stability when descending into the downwash as the downwash is cast outwards slightly.

 

[Scott Drysdale]

It all remains a mystery to me..... In addition to DJI not revealing the reasons behind this, MikroKopter does not reveal their reasons for their motor angle adapter kit which is located here:
http://www.mikrokopter.de/ucwiki/en/AngleAdapter

But in the case of MK, they seem to do the angle perpendicular to the DJI method. One explanation for the MK method is that it enables faster Yaw cw and ccw. Some others also claim this improves FC ability to maintain a heading more precisely. Also some other Chinese frame manufacturers are also following the DJI concept with up to 5 deg dihedral.

There has to be a simple laws of physics explanation and some calcs to back it all up. Maybe something to do with gyro effect of spinning motors/props.

 

[kloner]

Just guessing but i think if you gave just the motors dihedral and not the arm, it'd be pushing against the grain on the arm and if the arm did have any engineered dampening affect you'd loose a bit of it..... if you bend the arms at the frame like they do, it'd retain that.

I notice when we get flying in really high wind that the aircraft fights to keep yaw centered, bet doing the motor twist really helps that... wonder how the adverse side is like at high speed or whats the bad part of offsetting or pre loading yaw?

 

[Scott Drysdale]

I definitely had issues with my custom 800mm Hex with flat motor arm layout, specifically with maintaining precise yaw when flying forward in a light crosswind. So I am hoping some motor shimming will help to improve the situation. Also I had a chance to visit with Dr. Alex Ramirez-Serrano in the Mechanical Engineering department at our local university to discuss this and other issues. see here: http://www.4frontrobotics.com/

He had some issues with his original dual ducted fan designs where the gyroscopic effect of the two motors was always fighting against attempts to change vehicle pitch..... so he devised a newer design to eliminate that. The original had two ducted fans inline on a common shaft. By using a differential gear and offsetting the two shafts each by 45 degrees the gyro pushback was eliminated. If you watch the photo slide slow for a minute or so you will see the new one with differential.

 

[Stacky]

A local guy here had problems with his Hexa, a Skyjib that he bought off Droidworx and they supplied it under powered for the specs he supplied ( staggering the number of people that have issues with droidworx these days, its not small...) Anyway an interim way to improve his yaw response was to put some shims under his motors so they all leaned inwards as per the DJI stuff and he got an immediate improvement in yaw response.

 

[jes1111]

One inevitable effect of tilting the motors is to reduce the vertical thrust (in direct proportion to the angle from vertical) - this would translate into less "efficiency", i.e. shorter flight times. You'd certainly need to be definite about the benefits!

 

[kloner]

i'd like to hear a video from an s800 raw, bet there is zero wind on the camera

 

[Stacky]

One inevitable effect of tilting the motors is to reduce the vertical thrust (in direct proportion to the angle from vertical) - this would translate into less "efficiency", i.e. shorter flight times. You'd certainly need to be definite about the benefits!

I seriously doubt that a 5mm shim would have any real world impact on the vertical thrust and efficiency.

 

[jes1111]

I seriously doubt that a 5mm shim would have any real world impact on the vertical thrust and efficiency.

It's straightforward mathematics/physics - if you tilt the motor from vertical the total thrust from the motor/prop doesn't change but there is now a "sideways" thrust vector (the effect you wanted - it's why you tilted the motor) - so there must be correspondingly less vertical lift. If you tilt the motor to 90 degrees, you'll have 100% "sideways" thrust and 0% vertical thrust. At 45 degrees you'll have 50% "sideways" thrust and 50% vertical thrust, i.e. you've reduced the "lift efficiency" by 50%.

If you put a 5mm thick shim under one side of a 40mm diameter motor (for example), you induce a tilt of approximately 7.15 degrees from vertical, equating to about 8% loss in vertical thrust.

[E&OE ]

 

[Stacky]

Go do it, measure the before and after and get back to us on that one.

Theory is one thing but reality is another.

It's straightforward mathematics/physics - if you tilt the motor from vertical the total thrust from the motor/prop doesn't change but there is now a "sideways" thrust vector (the effect you wanted - it's why you tilted the motor) - so there must be correspondingly less vertical lift. If you tilt the motor to 90 degrees, you'll have 100% "sideways" thrust and 0% vertical thrust. At 45 degrees you'll have 50% "sideways" thrust and 50% vertical thrust, i.e. you've reduced the "lift efficiency" by 50%.

If you put a 5mm thick shim under one side of a 40mm diameter motor (for example), you induce a tilt of approximately 7.15 degrees from vertical, equating to about 8% loss in vertical thrust.

[E&OE ]

 

[jes1111]

Go do it, measure the before and after and get back to us on that one.

Theory is one thing but reality is another.

It's not "theory", it's already proven! That's the beauty of science

 

[Carapau]

All this shimming is pointless to me. It might indeed give better stability or agility but my MR with vertical alignment is more than stable and agile enough. More importantly however is how my gimbal reacts to the aircraft. It can handle its movements now without even breaking into a sweat so I would gain absolutely nothing from tilting my motors apart from a loss in flight time (even if it is a relative small amount). With regard to loss of flight efficiency Im fully with jes1111 on this. Science is nicely proven. Just speak to any heli pilot and ask why they have to apply collective as soon as they dip the nose from horizontal.

 

[Stacky]

...

 

[RyanMcCrae]

Would it be possible to to shim a coaxial set up? Seeing as flight controllers need the motors to be perfectly level when in coax configuration to be happy. Is there a way to do it?

 

[DennyR]

This has been covered here many times before. The radial opposite tilting of the motors is to reduce the yaw dead band so that the yaw input is working immediately. This happens because the torque reaction is supplemented with side thrust as well. Use in moderation because you are introducing cross axis errors. The Dihedral works like a fixed wing aircraft that effectively self levels because the side that tilts down now has more lift and the side that comes up has less. Combined with the downwash spread it can help the overall efficiency and protect the gimbal from some of the wind disturbance. In relatively small amounts it works great with the standard stabilization algorithm that DJI use. It works best on larger Hexas and Oktos. Small Quads are better with the thrust lines vertical because the short moment arms are more vulnerable to cross axis error. Yaw stab. being improved with a little more prop' dia. than the standard Phantom uses. 9x5 e-flight works very well or a cut down 10x5 to 9.25x5.

Cross axis error for example, means that if your model receives a roll disturbance it will respond with roll plus yaw. If you do decide to use yaw offset make sure you turn it the right way or it will cancel out the command.

 

[Benjamin Kenobi]

Now that's an answer! Nice one!

 

[DennyR]

Thanks Ben.