How do these drones control yaw?

[Dan C]

I was watching a SciFi the other night. They had these huge quadcopters; they'd block a four-lane street.

When they yawed, the four blades would tilt to produce, then cancel the yaw.

But the rotors on our quadcopters are fixed. I've watched mine while I yaw it around in a hover. It seems to stay completely level and stable.

So how do these aircraft yaw? Is there a gyroscope in there or something?

 

[Oneball 1971]

Arent the rotors angled slightly on diagonally opposite sides, so I assume when you yaw those motor adjust their speed slightly to create rotation of the aircraft. I may be wrong.

 

[Simmo]

A quadrotor hovers or adjusts its altitude by applying equal thrust to all four rotors. A quadrotor adjusts its yaw by applying more thrust to rotors rotating in one direction. A quadrotor adjusts its pitch or roll by applying more thrust to one rotor and less thrust to its diametrically opposite rotor.

 

[AMann]

Notice how opposite paired props turn in the same direction (front left and right rear, front right and left rear)? When all four props are applying equal thrust, they are canceling their torque out. But if a paired set of props apply more thrust than the other pair, their increased torque on the body produces yaw.

 

[AMann]

A quadrotor hovers or adjusts its altitude by applying equal thrust to all four rotors. A quadrotor adjusts its yaw by applying more thrust to rotors rotating in one direction. A quadrotor adjusts its pitch or roll by applying more thrust to one rotor and less thrust to its diametrically opposite rotor.

Ah, you beat me to the post but I used smaller words!

 

[Dan C]

@Simmo & @AMann -- Got it, cheers.

After reading post #2, however, I remembered that the rotors are indeed angled slightly outwards to the sides, but not front or back. Why are they oriented in this fashion?

 

[BossBob]

The rotors on a Mavic Pro 1 are indeed angled front and back!

 

[Dan C]

The rotors on a Mavic Pro 1 are indeed angled front and back!

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?

 

[Simmo]

A racing quad requires very high camera tilt angle in order to see what’s in front of it when flying fast. Otherwise all you will see is the ground if the camera is too low.

Angle motor mounts reduce the need for high camera angle yet allows you to see better when flying fast.

The second benefit is better Aerodynamics. Your quadcopter pitches forward to fly forward, the faster you fly the more tilted the quad is and the larger area there will be that blocks air.

 

[Meta4]

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?

Early Phantoms had their motors vertically aligned but from the P3 series they were offset to prevent Vortex Ring State (VRS).
But surely you knew that?

 

[old man mavic]

because of what @Meta4 stated,it also help keep the aircraft stable as it descends and allows a quicker descent as well

 

[Dan C]

Early Phantoms had their motors vertically aligned but from the P3 series they were offset to prevent Vortex Ring State (VRS).
But surely you knew that?

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.

because of what @Meta4 stated,it also help keep the aircraft stable as it descends and allows a quicker descent as well

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.

 

[RadioFlyerMan]

because of what @Meta4 stated,it also help keep the aircraft stable as it descends and allows a quicker descent as well

Yes, and I noticed during decent that it’s smoother (less wobbly) if moving slightly forward in order to avoid prop turbulence. It’s interesting to observe much less prop turbulence when descending in modifying sport mode at 22+ mph; very smooth!

 

[Meta4]

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

I suppose that if you can't see it, DJI must have been lying to us.
Funny thing is that VRS was not uncommon with the early Phantoms but seems to have disappeared nowdays.

( Mod Removed )

 

[Rockycat]

A quadrotor hovers or adjusts its altitude by applying equal thrust to all four rotors. A quadrotor adjusts its yaw by applying more thrust to rotors rotating in one direction. A quadrotor adjusts its pitch or roll by applying more thrust to one rotor and less thrust to its diametrically opposite rotor.

Good explanation.

 

[dirkclod]

Early Phantoms had their motors vertically aligned but from the P3 series they were offset to prevent Vortex Ring State (VRS).
But surely you knew that?

Please don’t assume all have had Phantoms Meta.

 

[Keule]

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.

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.

 

[Simmo]

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

 

[kidroc]

It's kinda like a torque cancelling just slowing down or speeding up opposite rotors.

 

[THE CYBORG]

7 minutes into THIS could help understanding motor tilt.

Hopefully the link should start at the correct place.