Nitro Quad-copter How does it yaw?
 

Saw an ad and video for a nitro quad-copter. I can figure out how it will roll and pitch, but how to make it yaw? It looks like all props spin at the same speed.

Same as all of them, differential thrust

No Andy, I don't think it's just like all of them. I'm no expert but regular quads don't use differential thrust to create yaw. They use differential rotation, right? The 2 props rotating one direction will speed up and the 2 going the other direction will slow down, thus rotating the body of the aircraft.

At first I was thinking some sort of differential to allow opposing props to run at different speeds but I wonder if it really might all be done with pitch. Increasing the pitch on clockwise props and decreasing the pitch on counterclockwise props will keep total lift the same but might change the angular forces and cause yaw.

Again, I'm no expert and have no idea if this is how it's being controlled, just my best guess. I look forward to seeing a reply from someone who knows how this aircraft is set up.

Dave

Maybe I should have posted this question in a quad-copter forum. No, more fun to try to figure it out!

I started to say you might get an answer sooner on another forum but I'm really wanting to see the answer too, so I didn't mention it. Can't wait for the real answer.

Dave

Looks like I need to post in a multi-rotor forum. I'll come back with an answer, I hope.

Hey RC_Fanitic,

I haven't quit thinking about this one. I didn't think about it at all when I watched the video. But after you asked, I realized that it might be something different. I'm still thinking.

Consider this. Two rotors going one direction, two going the other. All of them variable pitch instead of variable speed.

If you increase the pitch on the two clockwise rotors and decrease the pitch on the counterclockwise rotors so that the total lift stays constant, what happens?

When I go through these kind of mental puzzles I tend to think to extremes. So increase the clockwise rotors to an extreme pitch and the counterclockwise rotors to zero pitch. Those counterclockwise rotors aren't doing much of anything. No lift, not much drag, just hanging out. The two high pitched rotors are doing all the work. But there's another force they are creating. At high pitch and at opposite corners of the aircraft, the clockwise rotors are each creating forcers other than the downward push of their rotors. Since outer tips of both blades are moving in the clockwise direction with relation to the entire aircraft and they are farther out than the inner tips, they have a longer moment arm. They'll act like paddles and push the airframe in the counterclockwise direction.

Again, I'm no expert. I'm only a physics fan (physics phan?) and an engineer who enjoys thinking out how to build things.

There it is, that's my guess. Increase the pitch on two rotors, decrease on the other two rotors and the outer end rotor drag overcomes the inner end rotor drag, yawing the entire airframe.

Just a guess. But nobody else was bold enough to posit an opinion on how it worked.

Dave

Sounds plausible. Also, the clockwise-spinning blades will be imparting rotational momentum to the air and thus the opposite to the aircraft. Recall that the P-51 with that big Merlin engine could flip over if you went to full throttle at low speed, or so goes the story. My dad said that when he flew one, he would lift one main off the ground.

I did post on the multi-rotor forum but no response yet.

All of the quad-copters that I have seen have fixed pitch propellers. Two of the rotors turn clockwise and two turn counter-clockwise. In order to produce counter-clockwise yaw, the clockwise propellers will have to increase speed and the counter-clockwise propellers will have to decrease speed. Clockwise yaw will require the opposite. I suspect that some of the yaw is produced by the acceleration/deceleration forces on the rotor and some yaw is generated by the difference in torque required for the differential rotor speeds, but the end result is yaw in either case.

All multi-rotor craft share the same characteristics.

I agree with you on the electric quads. However, when you look at the Nitro-Stingray it appears to have variable pitch props. I think that with a single motor powering the props, it would be quite difficult to make them variable speed. It probably has to do with increasing the pitch on say the clockwise props, and decreasing it on the ccw ones, or vise versa.

There's information here on a single motor quadcopter that uses constant speed rotors with variable pitch controlling yaw. https://www.youtube.com/watch?v=fkSx3fSz0tE

Also, diagrams of the MantaRay here http://www.dronetrest.com/t/mantaray...adcopter/175/2. Definitely all rotors are the same speed. It's the independent variable pitch.

Dave

Same as a Chinook, like I said in the beginning, differential thrust :)

I think that the Chinook, along with most two rotor designs, tilt the front rotor one way and the rear rotor the other way to accomplish yaw. If differential thrust were used, it would also add in the complication of unwanted pitch.

How do you reckon they "tilt" the rotor? We are talking a variable pitch rotor head.

Step on the right pedal and front rotor has more pitch on the left than the right, and the rear has more pitch on the right than the left.

Don't forget about gyroscopic precession. I suspect that when the pilot steps on the right pedal (assuming counter-clockwise rotation of the front rotor and clockwise on the rear), both rotors have more pitch at the front than at the rear. This causes the front rotor to tilt to the right and the rear rotor to tilt to the left. To be perfectly honest, there is entirely too much monkey motion in helicopter operation and thinking about it makes my brain hurt.

A good reason to think about this in terms of a quad copter rather than a helicopter as the effects are quite different.

Considering I have a few hours in them....
Precession is taken into account at the swashplate, the input occurs 90 degrees before the action is needed.

Is precession a significant effect in quad copters with rigidly mounted propellers? I can't see that with fixed pitch props or with the variable pitch props we are considering on the subject in question.

The generalization that it's "The same as all of them, differential thrust", is still wrong.

First of all, most quadcopters don't use differential thrust to create yaw. Increasing the rotational speed of two rotors and slowing the others causes yaw due to conservation or rotary motion. In the process of changing rotor speeds on those fixed pitch props, they do change thrust, but that's not what causes the yaw of the vehicle.

Secondly, in the example of the variable pitch, glow quadcopter, again in the process changing pitch of 2 rotors and reducing pitch of 2 rotors does create differential thrust, but it's not the differential thrust that creates the yaw. In the case of a rotor aircraft, thrust is in a downward direction. Differential thrust makes you go up and down. Or increase some, lower others, differential thrust can hold the same altitude.

Let's take an extreme example. This won't create any lift but consider what would happen if the two clockwise rotors were set at 90 degrees pitch (straight up and down) and the two counterclockwise rotors were set at zero pitch. Now power that crazy thing up. What will happen? Seriously, think about it for a minute before reading on.

First consider that all rotors are rotating the same speed. This means that there is no vehicle rotation due to different rotor speeds. The feathered counterclockwise rotors aren't creating much in the way of other forces. The clockwise rotors are beating the heck out of the air, pushing air and creating lots of drag and the create the same force all the way around their path. But the forces applied to the airframe are multiplied by their distance from the center of the airframe. So the outer two tips of the clockwise rotors create a force of let's say X which is multiplied by their distance from the center Y. Note that the direction of the force of both outer rotor tips is in the same rotational direction with respect to the center of the airframe. Now consider the inner ends of those rotors. They also create a force of X and it is in the opposite direction of the outer rotor tips. But it has a much shorter moment arm, for our example let's say it's 1/2Y. So the outer tips each create a force of XY in one rotational direction while the inner tips each create a force of 1/2XY in the other rotational direction.

What will that non-flying quadcopter do? It'll rotate in a counterclockwise direction because of those blades slapping all that air.

Now, tone down the exaggeration. Take a balanced, flying, variable pitch quad. Increase the pitch of the two clockwise rotors, decrease the pitch of the two counterclockwise rotors. Yes, there is differential thrust, but it doesn't cause anything because it's balanced. However, the changes in drag will cause the same thing that happens in the above ridiculous example, just to a lesser extent. But it will still cause the vehicle to rotate.

Science is a wonderful thing.

Dave

Dave, you are not taking into account the rotational motion of thrust. Thrust is not just straight down. And since the two rotors turning one direction are creating more thrust than the two rotors turning the other direction there indeed seems to be differential rotational thrust.

BTW, helicopter rotors do tilt, they don't just stay in one plane. The rotor tilt is what causes the body to tilt, not the other way around.

Thank you for your service.

Sorry, I misunderstood. I thought that you were referencing the rotor blade angle, but apparently you were referencing the rotor disc angle. I agree that precession is dealt with at the swashplate by installing blade control arms that are approximately 90º leading offset to the blade mount. Thanks for the correction.

Agreed. I cannot think of any meaningful way that gyroscopic precession has to be dealt with on a quad copter with rigid rotors. ------ Since we are only theorizing here (and since Andy brought it up by throwing in the Chinook), what if the quadcopter was equipped with tilt rotors?

I think that Dave did take into account the rotational motion of thrust. He dealt with it separately by discussing the differential pitch (or speed) of the counter rotating blades and the resultant differential torque on the airframe.