Principles of Aerodynamics – Aerodynamic Lift

Consider this!

What if I want to squirt a friend with the garden hose, but the pressure is not quite enough for the water to reach them.

Is it possible for me to do this?

How does covering the end of the garden hose make the water go further?

When you put your finger over the tip of the hose you partially block the end, or in other words – decrease the amount of space the water has to flow through!

…but what if we just PINCH the hose.

When you squeeze the hose, you decrease the amount of space the water has to flow through, and in the pinched area of the hose the water flows faster!

Since the same amount of water has to flow out of the hose as flows in to the hose, the water must shoot out faster – to keep the amount of water flowing out a constant.

The hose can’t expand to accommodate more water, so the water has to shoot through the ‘reduced’ opening faster. Pressure has to do with how an object will feel as a result of a force exerted on it. Because pressure causes the water to shoot out of the hose faster, it will feel harder, and it will travel farther.

FLOW IN = FLOW OUT.   In fluid dynamics, the continuity equation states that, in any steady state process, the rate at which mass enters a system is equal to the rate at which mass leaves the system.

Area is inversely proportional to Velocity

Reduced cross sectional area therefore Increased Velocity

Basically:   Halve the Area, Double the Velocity

• Velocity of airflow increases through the restriction
• Velocity after restriction is equal to velocity before the restriction

Airflow “Velocity” through Venturi can also be referred to as the “Dynamic Pressure” of the airflow through the venturi.

In fluid dynamics, a fluid’s velocity must increase as it passes through a constriction in accord with the principle of mass continuity, while its static pressure must decrease in accord with the principle of conservation of mechanical energy.

• Static Pressure decreases more through the restriction
• Static Pressure after restriction is equal to Static Pressure before the restriction

Bernoulli’s principle: At points along a horizontal streamline, higher pressure regions have lower fluid speed and lower pressure regions have higher fluid speed.

Relationship between Velocity & Pressure

Total Pressure (H) = Dynamic Pressure (q) + Static Pressure (p)
(H =q +p)

It might be conceptually simplest to think of Bernoulli’s principle as the fact that a fluid flowing from a high pressure region to a low pressure region will accelerate due to the net force along the direction of motion.

This still causes a reduced area in the hose for the water to flow through, and in the reduced area of the hose, the water will still flow faster!

This is exactly what happens with air as it moves over an aerofoil. A fixed wing is the shape of an aerofoil.

…its all about SHAPE

Consider the flow of air:

The fluid on top of the wing is accelerated and the fluid on the bottom of the wind is slowed down compared to velocity of the aircraft itself because the wing geometry and angle narrows the flow area above the wing and widens the flow area below the wing. (Venturi Effect!)

This is the Lift formula:

• Increase in air density increases lift and vice versa
• Increase in surface area increases lift and vice versa. A larger wing creates more lift
• Speed v^2 affects lift. A faster airspeed creates more lift.
• Wing shape affects lift. Extending flaps (& / or slats) will create more camber and create more lift Increase in Angle of attack increases lift and vice versa. (Relative to the airflow) an increase in angle of attack

For Level flight (Lift is Constant)
Angle of Attack is traded for Speed
Or Vice Versa