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 (...
- 1,912
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 ...
- 4,412
11
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 ...
- 1,912
11
votes
Accepted
Flying a UAV with a banner?
It should be possible so long as the aircraft has enough spare thrust and lift to counteract the drag and weight. Many model aircraft have good power-to-weight compared to their full size equivalent.
...
- 5,996
9
votes
Flying a UAV with a banner?
It is definitely possible to attach a banner to an RC plane. There are many people who have done it and It should be relatively simple. Some things to consider are:
Use a very lightweight material ...
- 5,642
8
votes
Accepted
Why do a quadcopter's opposite rotors spin in the same direction?
This design is to prevent unwanted yaw when moving, or vice-versa.
Consider the following two actions:
When a quadcopter yaws, it does so by creating a speed difference between the opposite-rotating ...
- 5,996
7
votes
Accepted
Calculating the range of an FPV drone
I'm going to give you the answer first:
Yes, it is worth calculating the approximate range if you're building a (very) long-range aircraft. However, theoretical calculations will usually be such a ...
- 2,957
5
votes
Is it better to have a bottom heavy quadcopter or a top heavy quadcopter when it comes to a quadcopter stability?
As far as I know, it doesn’t make too much of a difference in regards to flight stability because of something called the Drone Pendulum Fallacy, which describes how the drone rotates about the centre ...
- 4,412
5
votes
Why do a quadcopter's opposite rotors spin in the same direction?
They don't necessarily have to spin in the same direction, but it gives the best results.
This has to do with the mathematics of drone flight. Pulling from https://drones.stackexchange.com/a/419/46, ...
- 1,912
5
votes
Accepted
Influence of propeller blade shape and tip shape on its behaviour?
The main thing the designers are trying to achieve is reducing tip losses by making the chord of the tip of the prop small relative to the rest of the blade. Tip losses are due to air moving around ...
- 7,003
4
votes
When evaluating the weight of a multirotor, is there a standard way that the components are grouped?
To me the only weight that matters is the total weight of the craft that is airborne. Or commonly referred to as All Up Weight (AUW). That is what performance depends on. That is what regulations ...
- 1,130
4
votes
Why are micro pusher-prop quadcopters becoming more popular?
I have a working theory for why this may be the case; some micro-quad designs support this theory more than others.
Due to Newton's third law, the thrust force created by the propellers also has the ...
- 5,948
4
votes
Contra-rotating coaxial shrouded fans: design guidelines
The starting point is working out the speed of air through your rotor disk(s). This air starts stationary at some point above the disk, and is pushed down by the disk. The force required to accelerate ...
- 7,003
4
votes
Accepted
How to determine the correct specification of servo for a control surface
Most forces in aerodynamics are proportional to area, speed squared, air density and a 'coefficient' that describes how efficient your object is. There's also usually a 1/2 for obscure theoretical ...
- 7,003
4
votes
Calculating the range of an FPV drone
I just want to add to FlashCactus's excellent answer...
Depending exactly what you want, you can get some fairly good estimates on range on various range calculators online. You can get them pretty ...
- 1,033
3
votes
What impact does a shroud have on the torque produced by the prop(s) of a multirotor that incorporates a properly designed shouded propeller?
There are two possibilities, depending on how ducts increase efficiency, but the end result should be the same. It could reduce torque required to spin the motor at a given RPM and thrust, or it could ...
- 316
3
votes
What impact does a shroud have on the torque produced by the prop(s) of a multirotor that incorporates a properly designed shouded propeller?
Ducts and shrouds reduce tip losses; the reason we don't see them more often is that a well designed propeller should not have large tip losses.
Tip losses are greatest when there's a large pressure ...
- 7,003
3
votes
How useful is knowing the torque generated by a motor prop combination?
Not very useful.
If you're designing at the absolute limit of what a frame can handle, such that it's as light as possible, then maybe you'll want to know the maximum torque, but you should be ...
- 1,039
3
votes
Does the tiny helicopter drone black hornet use a swash plate?
No. The Flir Black Hornet Nano UAV does not have a swashplate. The rotor blade pitch is controlled directly by servos at each blade root. The servos are held by a structure that rotates around the ...
- 131
3
votes
When evaluating the weight of a multirotor, is there a standard way that the components are grouped?
Short answer seems to be no standardization.
Personally, I am only concerned with AUW and have no concern for individual groupings. If I'm all that concerned I can either weigh each component ...
- 181
3
votes
Why are micro pusher-prop quadcopters becoming more popular?
Microquads tend to be flown indoors, where the floor is relatively smooth but there are lots of other things to hit. Putting the prop under the frame provides additional protection when the quad flies ...
- 7,003
2
votes
Where can I find a computational fluid dynamics model of a full quadcopter or multirotor?
You can make your own using free software.
Let’s take Fusion 360 as an example, in its simulation suite it has a Computational Fluid Dynamics study option.
If you’re willing to design an accurate-...
- 4,412
2
votes
Is there a benefit to using multiple capacitors on FPV drones?
Capacitors come in various sizes and values, so one big capacitor could equal two smaller ones in value; I do not know for certain if this offers a weight saving, but can reduce PCB footprint size by ...
- 5,996
2
votes
Advice on correct linkage to use to rotate motor mount
I'm no mechanical engineer, but I stumbled across this amazing document which appears to my untrained eye to have basically every mechanism I can imagine:
https://cpb-us-w2.wpmucdn.com/sites.gatech....
- 191
2
votes
Pro and cons of each quadcopter type of rotor arrangement
'H' gives you the shortest arms, minimum drag, and the lightest frame if all parts of the frame were the same strength. However a strong X frame with a lighter frame for the camera and other light ...
- 7,003
1
vote
Does the tiny helicopter drone black hornet use a swash plate?
Based on slide 13 of this 2014 PowerPoint presentation, it looks like the PD-100 Black Hornet does indeed incorporate a swash plate.
Now, a caveat here is that slide appears at the end of the section ...
- 579
1
vote
The Effect of Pitching Moment on Aircraft Stability
I don't think that being 'consistently more negative' will make a lot of difference. What's more important is that the overall aircraft should pitch down at low speeds (i.e. high angle of attack) and ...
- 7,003
1
vote
Why are micro pusher-prop quadcopters becoming more popular?
You can make a small quad with relatively large props without the propellers being in the FOV of the camera.
If you look at the quads you've mentioned, if you flip the motors and props over, you'd ...
- 11
1
vote
How useful is knowing the torque generated by a motor prop combination?
The torque a brushless motor is capable of producing is particularly important when dealing with control loops. The torque is directly proportional to the rate of change of RPMs, which is directly ...
- 1,200
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