How do these drones control yaw?

[Meta4]

Please don’t assume all have had Phantoms Meta.

There's no assumption.
It was a very pertinent comment since that's when DJI introduced the offset motors.

 

[Dan C]

Lookup 'Dihedral'
The purpose of dihedral effect is to contribute stability in the pitch and roll axis of a Multirotor.

This <link> explains it in scientific words.

Thank you, that was a challenging but informative read. I gather now that this is about stability.

Unfortunately, the mathematics is over my head, so I relied on the figures and textual explanations. From Fig. 5 on page 3, and supported by what I know of most aircraft, I understand that a dihedral angle is one where the flight surface angles up as it extends away from its root, and the body of the aircraft.

This "up and away" is called "dihedral" and increases stability of the aircraft. In the paper you supplied, this would be a negative βi value.

However, when I look head-on at my Mavic Air, I see the rotors are tilted down and away. This would be an anhedral angle, or a positive βi value, and as such decrease the Mavic Air's (roll) stability, while increasing its maneuverability.

Have I grasped this correctly?

 

[Dan C]

@Simmo and @THE CYBORG Thanks for the links folks. Unfortunately, I'm in China, and my VPN is not cooperating. Can't get onto YouTube to watch.

I'll check them out when I can.

 

[dirkclod]

There's no assumption.
It was a very pertinent comment since that's when DJI introduced the offset motors.

As i said he might not have ever had a phantom so he wouldn't know.
You said

But surely you knew that?

So like I said
Just don't assume evryone knows that.
Nuff Said.

 

[AMann]

I always wanted to use a smoke machine to show how a landing Mavic avoids getting into a VRS, that would be interesting to see.

something like this:

 

[Keule]

However, when I look head-on at my Mavic Air, I see the rotors are tilted down and away. This would be an anhedral angle, or a positive βi value, and as such decrease the Mavic Air's (roll) stability, while increasing its maneuverability.

Have I grasped this correctly?

Yes.

I did a quick check on a 3D model of the Mavic AIR. It has an anhedral (roll) angle of ~4.5°

Front view


Side view:


3D


It's beyond my knowledge why DJI decided this kind of configuration to be used with the AIR.

 

[old man mavic]

the outward facing angle of the motor helps VRS to not develop during descent, it helps prevent the vortices coming off the props from joining up above the craft and so decreases the chances of VRS, of course the best way to get down from height is to move forwards while descending and do large spirals as you descend or equally you could move left or right to achieve a similar scenario

 

[LeRoytheBoy]

Here is a good flight.. real quick, and pilot avoided VRS!

I'm curious to know the brand and model number of the drone in your video.
My MA descends very slowly compared to the one in your video.

 

[Mossiback]

Having the motors at a slight angle against each other might also give a more steady hover; in the way that a chair with angled legs is more stable than one with legs parallel to each other.

 

[Gindra]

I don't own a Phantom.

I can, however, fly a helicopter, and we usually call it Settling with Power. And yes, I know what it is.

What I don't see is how tilting the rotors slightly would alleviate it.

There are three elements to this phenomenon, and all three must be present for it to occur:

1. The rotor is generating sufficient downward thrust
2. The vehicle has a significant rate of descent
3. The rotor disc has insufficient horizontal motion to be in Effective Translational Lift (ETL)

I'm not seeing how the slight angling of the downwash would do anything significant to alter any of these factors.



I've noted that the DJI software limits the rate of descent the pilot can command to a value noticeably less than what you can command for climb. That strikes me as the most effective preventative action against settling with power as it eliminates factor #2.

That said, if you wish to take extra steps to ensure you avoid it, simply maintain a good horizontal component to your speed when coming down.

And if you are descending straight down, and get into the phenomenon, the correct procedure to escape it is to reduce downward thrust (alleviate factor #1), and apply cyclic in any direction (alleviate factor #3). In a helicopter, that usually means forward (most efficient and usually safest direction), but in a quadcopter I don't think it would matter. It is simply that the quickest way to clear the vortex is to move horizontally.

The numbers for rate of descent and ETL depend on the aircraft -- I could tell you what they are for R-22 and R-44 helicopters, but I have no idea what they are for our drones. But ETL occurs essentially by definition from the minimum (or greater) horizontal speed necessary to prevent settling with power.

What I'd like to know is why angling the thrust vector slightly inwards on drones makes a difference. If anyone has information on that, I'd appreciate it.

Thanks for this info! I really did not know about maintaining horizontal motion to smooth out the descent. Now that you mentioned it, I recall all of the combat air missions where we were choppered into an LZ and how much easier it was for the pilot to land when there was room to bring it in at an angle rather than straight down. Some of those chopper pilots were incredible. Almost like parallel parking the chopper into a really tight LZ. Thanks again!

 

[Dan C]

7 minutes into THIS could help understanding motor tilt.

Hopefully the link should start at the correct place.

Thanks for this. It not only added to my understanding of motor tilt, it was an awesome project those guys did as well. It really was impressive looking in flight, and had a motion that seemed to fit perfectly with its scale. I've subscribed to their channel -- they are talented guys.

 

[Lets Fly]

Interesting. I've got an Air, and as far as I can tell, they are straight up and down when viewed side on.

Do you know why they are angled at all? Is it stability?

On my Air there is a full 1/4" difference in hight from one side of the blade to the other when you lay the body on a flat surface.

 

[Dan C]

On my Air there is a full 1/4" difference in hight from one side of the blade to the other when you lay the body on a flat surface.

Right, but they don't tilt fore and aft. I'm guessing this suggests they expect forward flight most of the time?

Had you noticed that their geometry is 20mm wider side-to-side than fore-aft. I wonder what that's about...

 

[Lets Fly]

Right, but they don't tilt fore and aft. I'm guessing this suggests they expect forward flight most of the time?

Had you noticed that their geometry is 20mm wider side-to-side than fore-aft. I wonder what that's about...

The extended legs are what's angled, so straight on they are level. The 20mm difference may just be in the weight distribution??

 

[Dan C]

The extended legs are what's angled,

Yes, but...

so straight on they are level.

Straight-on, you can clearly see the angle of the motor axes, and therefore the rotors. You mentioned this in your first post, so I'm afraid I don't get where you are going with this. Sorry.

 

[old man mavic]

it's all relative to the angle of your dangle

 

[Lets Fly]

Yes, but...


View attachment 90290

Straight-on, you can clearly see the angle of the motor axes, and therefore the rotors. You mentioned this in your first post, so I'm afraid I don't get where you are going with this. Sorry.

Take the same photo but turn the blades 90 degrees looking straight down them. Yes the motors have the tilt, but the blades are now even, or level with each other.

 

[Zbip57]

For comparison, here's the Mavic Mini.

 

[old man mavic]

this thread has been around for a while now,and seems to have lost the OPs original question somewhat, the motor angling is more to do with stability ,and avoiding VRS as has been discussed at length ,YAW on the other hand is simply achieved by slowing down diagonally apposing pairs of either CCW or CW motors which cause the drone to turn on its axis depending on which pair are going slower