What are multirotors?
Multirotors are aircraft which has more than one rotor, examples are; tricopters (which have three rotors); quadcopters (which have four) and octocopters (which have eight).
They are very mechanically simple as they are just motors with rotors attached to them. Control is achieved as explained below:
Roll – on one side the rotors spin faster (generating more thrust) and the other side they spin slower (generating less thrust) this means the thrust is uneven and it will roll towards the side with less thrust.
Pitch – pitch is exactly the same as roll but with the front and rear rotors.
Yaw – The motors all rotate in opposite directions to cancel out the torque effect. One motor is rotating one way and creating torque and the other is rotating the opposite way. This way the torque cancels out. Yaw is achieved view increasing the speed of the motors rotating one way and decreasing the speed of the motors rotating the other way. This creates more net torque one way then the other, this causes the multi-rotor to yaw in that direction. These pictures explain (the fatter rings mean more speed on the rotors):
This is in essence how a multi rotor is controlled. However multirotors are extremely unstable – only a computer can control the changes in speed of the motors to make it flyable. All multirotors must have a control unit.
- Multirotors are extremely mechanically simple.
- Multirotors have the ability to hover so can be used in tight spaces and are good for gathering footage.
- The vibrations are a lot higher frequency than standard helicopters, so are better for filming.
- They’re easy to repair if crashed – very robust.
- They require minimal maintenance. It’s easier for the consumer to understand than say helicopters.
- Multirotors are slow.
- They rely on electronic stabilisation.
- They aren’t very good in high wind conditions.
- If the power is cut they drop from the air.
What are fixed winged aircraft?
Fixed winged aircraft are essentially just planes. Where as in helicopter as described below, the wing is the moving rotors. In a plane the wing stays still and the whole vehicle moves – hence fixed wing. As the plane flies through the air lift is generated due to an aerofoil (I will probably explain this in a future post), which keeps the plane in the air, however this means the plane cannot slow down below a certain speed known as the stall speed where the wing cannot function but this also allows high speeds.
Control is done through the use of control surfaces which deflect the incoming air, as shown below:
- Planes are a lot faster, this means more ground can be covered in a shorter amount of time.
- They offer much longer endurances, this is because less power is being used to keep them in the air.
- Because of the above points range is also a lot further, they can travel ground faster and for longer.
- Fixed wing aircraft are very simple and minimal maintenance.
- Planes can glide if there is a power failure to make a safe landing.
- Planes cannot hover, they cannot stop in the air.
- They are much more badly effected by wind.
- Planes require a lot of space, this includes enough space for take-off and landing.
- Due to the added range more specialised equipment is needed to maintain communication with the drone.
- Less payload capacity for size.
- They have to be aerodynamically streamlined.
What are helicopters?
Conventional helicopters consist of one main rotor and a tail rotor. The main rotor is responsible for lift, pitch and roll. The tail rotor is responsible for counter torque and yaw control.
There is a very mechanically complex mechanism known as a swashplate attached the main rotor which is used to control the roll and pitch of a helicopter. It looks like this:
This plate will move up and down to change the angle of the blades,
this will effect the amount of lift produced.
In this position the blades would be pushing the helicopter down.
To control pitch and roll the plate tilts.
In this position one blade is angled in the opposite direction to the other as it rotates around. This creates move lift on one side which rolls or pitches the helicopter.
As the main rotor rotates it creates torque in the opposite direction. Remember ever action has an equal and oppersite reaction. As you can see below.
The tail rotor has a small swashplate to change its thrust. It can yaw the helicopter by changing the amount of compensation.
- Helicopters are inherently more stable than multirotors.
- Lifting capacity is high.
- If a helicopter looses power there is a change of auto-rotation, this is where the helicopter essentially glides to make a safe landing.
- Helicopters can take off and land vertically.
- Helicopters are relatively fast.
- Better handling in wing.
- Helicopters are very mechanically complex as you can see above, this means they require lots of maintenance and there are more aspects to go wrong.
- Helicopters are less efficient due to the tail rotor, in effect that thrust is waisted, whereas it could be used for lift.
- A lot more vibrations produced, which is bad for camera systems.
- Requires a lot of expertise, to IE balance the rotors, maintenance, etc.