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# The Physics Behind a Quadcopter

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## Priyanka Majumder

on 22 May 2015

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#### Transcript of The Physics Behind a Quadcopter

What is drag?
What is thrust?
Bernouilli's Principle
for a nonviscuous, nonconducting fluid, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy.
the law of conservation of energy tells us two things:
less kinetic energy = more potential energy = more pressure
more kinetic energy = less potential energy = less pressure
the difference in pressure (high pressure on the bottom and low on the top) results in a
lift
upwards
lift
must overcome the force of gravity in order to accelerate the quadcopter upwards.
the quadcopter hovers when lift and the force of gravity are equal.
differs from helicopters in that:
it does not have a tail rotor to stabilize the craft
lift and thrust is generated from four propellers instead of one
differs from airplanes and gliders in that:
it does not generate lift from the craft's forward airspeed
A quadcopter is a multi-rotor copter with four arms, each of which have a motor and a propeller at their ends
rotor
-several rotor blades that revolve around one mast
a quadcopter is considered a rotorcraft, which is a machine that uses lift generated from wings (rotor blades) that revolve around a mast to fly
other rotercrafts are helicopters, cyclocopters, etc.
Background Information
What is lift?
Newton's Third Law

By: Christina Zhu and Priyanka Majumder
Drag is:
another type of friction, referred to as air resistance
forces acting opposite to the object's direction of motion
parallel to the fluid's flow direction
Lift is:
created by the force exerted by a fluid flowing past the surface of a body
component of said force
perpendicular
to the oncoming flow direction
lift is explained through a variety of principles
for every action, there is an equal and opposite reaction.
as air flows over the surface of a wing and follows the shape of the wing, the air is deflected downwards.
the air exerts an equal (in magnitude) and opposite (in direction) force upwards on the wing, which is called
lift
.

Thrust is:
a reaction force explained by Newton's second and third laws
when the engine does work and accelerates the gas to the rear of the engine. A force equal in magnitude but opposite in direction from the accelerated gas is generated (thrust)
dependent on the amount of gas that is accelerated
used to overcome drag
Hovering
the lift generated must be equal in magnitude to the force of gravity on the aircraft
therefore, there is a net force of zero and a constant velocity
each rotor must be generating the same amount of lift
the torques of two sets of rotors cancel out
hovering is not a fixed setting due to many variable acting on the quadcopter so constants adjustments are required:
a gyroscope independently corrects the thrust of the motors
trim adjustment controls are used manually to correct any drift
Directional axes
the
roll
angle of the multirotor describes how the craft is tilted side to side. Rolling the multirotor causes it to move sideways.
the
pitch
angle of the multirotor describes how the craft is tilted forwards or backwards. Pitching the multirotor causes it to move forwards or backwards.
the
yaw
angle of the multirotor describes its bearing, or, in other words, rotation of the craft as it stays level to the ground.
the root of all the multirotor’s movements is the rotational speed of the motors
rotational speed of motors determines the magnitude of lift each propeller produces
by adjusting the relative speeds of the motors, the flight controller is able to cause the multirotor to rotate around any of the directional axes or change the altitude
Altitude Control
if lift produced by rotors is greater than the force of gravity then the quadcopter will
ascend

in order to do this, increase the speed of all the motors equally (equally, so that there is no net torque causing angular acceleration of the body)
if lift is less than the force of gravity then the quadcopter will
descend

to do this, decrease in speed of all the motors equally
in order to move the quadcopter, the rotorblades opposite to the direction of motion should spin faster
this increases the torque applied on two of the rotors and causes the quadcopter to increase lift on those two rotors
How to control roll
How to control yaw
motors on one side must spin faster than the motors on the other side
motors on one side will have more lift, thus tilting the quadcopter
for example: to make a quadcopter roll right, make the two motors on the left side of the multirotor spin faster than the two motors on the right side

the torque of one pair of rotor blades (which are across from each other) is not equal to that of the other pair of rotor blades.
unequal torques leads to a net torque, resulting in rotation of the entire body due to angular acceleration.
therefore, to control yaw, one must keep the torques of the rotor blades balanced.
How to control pitch
make motors on one side spin faster than motors on the other side to create more lift on one side and tilt the quadcopter
for example: to make a quadcopter pitch down, make the two motors on the back of the craft spin faster than the two motors on the front.
Rotational speed controls movement