27
votes
Accepted
Why do multirotors usually have four propellors?
With aircraft, there are 6 degrees of freedom (DoF) we want to control (roll-pitch-yaw, and x-y-z), but for hovering vehicles (i.e. rotorcraft) we can get away with controlling two degrees of freedom (...
- 1,892
21
votes
Why do most drones have upward facing motors?
One practical advantage is increased ground clearance, reducing the likelihood of a prop strike on landing or take-off.
Aerodynamically, a pusher1 design can be less efficient because the propeller ...
- 6,056
20
votes
Why are larger propellers generally more efficient than smaller ones?
Thrust is proportional to the change in momentum of the air passing through the prop - i.e. how much the prop speeds it up.
The power required to do this is proportional to the kinetic energy of the ...
- 7,428
17
votes
Accepted
Why are most multirotors built with an even number of rotors?
Yaw authority and ease of mixing is generally the answer - since yaw response isn't necessarily the most linear, being able to have N number of motors contribute to yaw in one direction with the same ...
- 846
17
votes
Why are larger propellers generally more efficient than smaller ones?
An explanation for why this is can be found in this Aviation.SE answer: Why aren't large, low-speed propellers widely used?, which I'll paraphrase here.
The thrust a propeller generates is a function ...
- 6,018
17
votes
Why do most drones have upward facing motors?
Expanding on @Kralc's answer:
Here's a study (unfortunately behind a paywall) that I'm going to copy key parts of below:
The hexacopter was mounted on a load cell test stand, and data were collected ...
- 752
13
votes
Accepted
How does one hold the motor bell still when tightening prop nuts?
You know those rubber wristbands that are given out at lots of events?
You can wrap one of those around the motor bell to get better purchase on it, and then torque down the prop nut.
There are also ...
- 4,422
13
votes
Accepted
Why do 48KHz ESCs increase flight time?
To clarify, the 48kHz is NOT running on the gyroscope or the FC firmware.
What 48kHz is referring to is the PWM Frequency in the ESC, namely the frequency at which the power MOSFETs on the speed ...
- 846
12
votes
What does Feed Forward do and how does it work?
The way the PID controller works is whenever the actual rate of rotation of the quadcopter does not equal the setpoint, or desired rate of rotation (which you set by moving the sticks), it sends a ...
- 2,997
12
votes
Accepted
What's the advantage of props in VS props out?
As far I know the only difference is on the whoops. They fly much better on props out.
On 2,5-5 inch quads, it doesn't matter. Props in: our camera is dirty, you can stack easier on the tree. Props ...
- 136
12
votes
Why do multirotors usually have four propellors?
This is a very interesting question, and one that is always my favourite to answer.
Firstly, rotary aircraft commonly need an even number of propellers (though not always) or another way to ...
- 4,422
12
votes
Accepted
Why do drone propellers have blades with thin chords?
Overview
The most efficient propeller is single bladed, has infinitely narrow chord, has infinitely thin airfoil, has an infinitely long blade, spins infinitely slowly, and requires infinite torque. ...
- 1,892
11
votes
What does Feed Forward do and how does it work?
30,000 foot view of feedforward as a general concept
Feedforward: I hand you a closed box and you anticipate that it will be heavy, bracing yourself in preparation. Your control of the box is rapid ...
- 1,892
10
votes
Accepted
How do laterally folding propellers affect multirotor flight?
In theory, you can't accelerate and decelerate the props as quickly. If the motor accelerates, the props lower angular velocity keeps it from accelerating too quickly. Same for deceleration: if the ...
- 577
10
votes
Accepted
How does frame stiffness affect a quadcopter?
Background
Stiffness, oscillation, and vibration damping are separate concepts but all somewhat relevant to the question. Each affects flight dynamics, and each is (somewhat) a function of material ...
- 1,892
10
votes
Is it better to have a bottom heavy quadcopter or a top heavy quadcopter when it comes to a quadcopter stability?
It depends on what kind of stability you're after. Nicolas Petit wrote a paper on this several years ago, the graphics below are taken from Pages 4 and 5.
The summary is that a higher CG is better ...
- 1,892
8
votes
What's the advantage of props in VS props out?
The primary reason that I fly props out is to use the props to push the quad away from trees/gates etc when flying forward.
There is a theory that because the leading edges of the rear props are on ...
- 468
8
votes
Accepted
Why was 32kHz gyro sampling removed from Betaflight?
In order to access the 32kHz gyro sampling mode, the gyroscopes in question (e.g. ICM 20689) would be switched to an experimental mode where there is higher gyro data jitter, and that sampling at the ...
- 846
8
votes
How do laterally folding propellers affect multirotor flight?
Surprisingly, that flexible coupling is actually a good thing. If you look at helicopters, this is called a lead-lag hinge, and it allows the propeller to match flight loads when flying around. What ...
- 1,892
7
votes
Accepted
How to clean a drone?
I have found that the simplest and probably one of the safest ways is to use a compressor (alternatively a compressed air can will probably work), to lightly blow away the dirt. This works especially ...
- 2,664
7
votes
Accepted
How to identify and address multirotor motor overheating
Assuming that you've spec'ed a good propeller-motor match and the heat isn't generated by a tiny motor being forced to drive a giant propeller, the heat generation is likely due to a physical issue ...
- 6,018
7
votes
What should I look for when choosing a battery for my multicopter?
When looking for a battery, there are a number of factors to consider: the voltage (or S rating), the capacity, the C rating, the type of connector, and the weight:
Voltage
You will often see ...
- 4,422
7
votes
Accepted
Putting decorations onto a drone?
So long as your additions are very lightweight compared to the drone - like stickers or paint - this should not have a significant effect the balance of your drone. However, you want to make sure that ...
- 6,056
6
votes
Accepted
One motor on quadcopter broken
So this method will vary based on whether they are brushed or brushless motors.
For brushed:
Spin the propeller with your fingers - is anything actively resisting the motion?
If yes, remove the ...
- 4,422
6
votes
Accepted
What's the purpose of airmode and idle up?
Airmode and idle up both seek to increase a control authority and stability during ultra-low throttle moves.
In normal flight, when the pilot commands the multirotor to pitch or roll (cyclic), the ...
- 6,018
6
votes
Why are larger propellers generally more efficient than smaller ones?
Comment number 7 at this link is an excellent source (and fairly trustworthy as it was written by Joshua Bardwell).
The main reason specified for the efficiency is that for a given amount of thrust, ...
- 4,422
6
votes
Accepted
Is there an advantage to having more LiPo cells in a low-power drone?
The advantage of running at higher voltage is that you can use thinner wires and lower current (i.e. cheaper) ESCs, as power lost to resistance is proportional to current (squared) and not voltage.
...
- 7,428
6
votes
Accepted
Does the sharpness of a propeller blade affect efficiency?
Rounded leading edges are better for subsonic aerofoils.
Props and wings have to work at a range of angles of attack. The airflow has to split to pass either side of the aerofoil, and a rounded ...
- 7,428
6
votes
Is there a method to wire a brushless motor to an ESC so it spins in the required direction first time?
Without disassembling the motor, and figuring out how the ESC is designed, there is no other way then simply testing it.
From my experience, the motors spinning clockwise has usually been the ones ...
- 2,664
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