We, at Pie Aeronefs, are building Switzerland's first electric race aircraft: the UR-1. But what do you know about the dynamics of flight?
According to many researchers and scientists, it is currently impossible to fully explain how an airplane flies. But don't worry, the knowledges that already exist are more than enough to take you to the other side of the world.
In this article, we will briefly and simply explain the basics of flying.
The first force to which an airplane - like any other body - is constantly subjected is gravity. This force is therefore the one that pulls the airplane toward the center of the earth and thus keeps it on the ground when it is static.
When an airplane accelerates on a runway, the air flows faster and faster around the wing. The shape and the camber of the wing causes the air on its upper side to travel a longer distance to flow than the flow on the lower side.
Since the upper airflow must travel more distance in the same amount of time, it must accelerate. This causes both the pressure and the temperature to drop locally.
The pressure difference between the lower side (slight overpressure) and the upper side (underpressure) thus creates an upward force. As soon as the air flows fast enough around the wing, the lift force is sufficient to counteract gravity: the aircraft flies.
To accelerate and move forward, an aircraft must generate a force called thrust. This is generated by one or more engines. Unlike land vehicles, where the power of the engine is directly transmitted to the wheels on the ground, an aircraft must pull or propel itself through the air.
Whether the aircraft is equipped with a piston engine and a propeller, a turbofan, or a ramjet, the principle is always the same: Air is accelerated backward. Through the phenomenon of action and reaction, the aircraft is then accelerated forward.
While thrust accelerates the aircraft and then maintains its speed, there is a fourth force it faces with: drag. In short, drag is the friction that an airplane is subjected to during flight.
Basically, there are two types of drag.
Parasitic drag: it corresponds to the friction of the air on the aircraft's body and its irregularities. It increases with the square of the speed.
Induced drag: Induced drag is a friction to which a wing is subjected when it produces a high lift coefficient. It means the production of a maximum pressure difference between the lower and the upper surfaces. Indeed, the greater the pressure difference, the more the overpressurized air tends to wirl back on the wing upper side which causes friction. The faster the aircraft, the lower the difference and the lower the induced drag.
Balance between the forces
When an airplane is in straight horizontal flight, the forces balance each other out: Thrust equals drag, lift equals gravity.
When the aircraft turns, changes altitude or speed, there is a change in one or more forces. At some point wihtout more changes, the aircraft becomes stable again.
Simplified example: During a straight horizontal flight, the pilot increases the engine power. The plane will initially accelerate because the thrust force is greater than the drag. If we look at the graph above, we see that the faster the plane, the higher the drag. So at a certain point, the airplane will find a new equilibrium; it will stop accelerating and keep a new stable and faster speed.
We have seen that an airplane can only fly if the wing generates lift. This happens when air is accelerated on the upper surface.
When an airplane flies and increases its angle of attack, it also increases its lift coefficient: the pressure difference between the lower and upper surfaces increases.
At a certain point, the pressure difference becomes so great that the flow under the wing tends to wirl back to the upper surface via the trailing edge. This has for consequences that the upper airflow begins to detach from the wing's surface: the wing no longer create lift, it stalls.
The graph below shows the increase in lift in relation to the angle of attack. At a certain point, the upper air flow begin to separate from the wing surface and the lift decreases until the wing stalls completely: there is no more lift.
There are 3 axes around which an airplane rotates.
The lateral axis, namely the pitch: It allows the aircraft to tilt the nose up or down and therefore to make it climb or descend
The longitudinal axis, namely the roll: It allows the aircraft to bank the wings to the left or to the right and therefore to initiate turns
The vertical axis, namely the yaw: It allows the aircraft to veer the tail to the left or right. This axis is mainly used to coordinate turns or perform crab landings.
In order to initiate a maneuver such as a turn, it is necessary to move control surfaces. These are controlled by the pilot using a yoke or a stick, which influences pitch and roll, and a rudder, which influences the rudder.
Pitch is controlled by the elevator via the stick
Roll is controlled by the ailerons via the stick
Yaw is controlled by the rudder via the rudder pedals
How it works
Now that we understood the basics of a wing, we can lean more about how control surfaces work, with the exemple of a left turn using the ailerons.
With a left stick input, the right aileron will move down. Through the change of the chord's angle, the angle of attack increases. This increase is directly coupled to the lift coefficient, which increases: the right wing goes up.
On the other side, the left wing's aileron moves up, the angle of attack decreases, lift decreases and the left wing goes down.
The airplane banks on its longitudinal axis to the left and can initiate a turn.
This dynamic of chord's variation naturally works for every flight surface. The elevator thus allows the aircraft to lower or raise the tail in order to vary the pitch and the rudder allows it to rotate on its vertical axis: the yaw
But how does the V-Tail of the first Swiss all-electric race aircraft UR-1 work?
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