- FOUR FORCES OF FLIGHT
- GENERATING LIFT
- AERODYNAMIC DRAG
Understanding the relationship between lift & drag and knowing how to control them with the use of power and flight controls is essential for ALL pilots.
So, without going into college level detail, here are the basics of lift and drag.
FOUR FORCES OF FLIGHT
Lift is the force generated through the wings and acts perpendicular to the relative wind.
- Lift opposes weight.
- In level flight – lift equals weight.
- During a climb – lift is greater than weight.
- During a descent – weight is greater that lift.
Weight is the force generated by gravity.
It acts perpendicular to the earth’s surface and opposes lift.
Thrust is the force generated by the engine(s) and is pilot controlled.
Thrust acts parallel (horizontal) to the airplane’s flight path.
- At a constant airspeed – thrust equals drag.
- When the airplane is accelerating – thrust is greater than drag.
- When the airplane is decelerating – drag is greater than thrust.
Drag is the resistance on an airplane caused by the surrounding air while it is moving.
Drag is a horizontal force acting parallel to the airplane’s flight path and is opposite of thrust.
It can also be partially controlled by the pilot.
Lift is generated two ways by an airfoil:
1. Differential Pressure
2. Ram Air
Differential Pressure is based on the theory by scientist Daniel Bernoulli.
Bernoulli’s principle states that, in the flow of a fluid (liquid or gas), an increase in its velocity will simultaneously cause a decrease in pressure.
To put it simply, when a fluid (air in this case) speeds up, the pressure of the fluid goes down.
Grab a sheet of paper, hold it in front of your mouth, and blow across the top of it. Less pressure on top of the paper causes the higher pressure on the bottom to lift the paper up.
APPLYING DIFFERENTIAL PRESSURE TO AN AIRFOIL
Looking at an airfoil cross-section, we see that the upper camber (top) has more curve than the lower camber (bottom).
Since an airfoil is more curved on the upper surface than on the lower surface, the air moving along the upper surface speeds up (due to the increased traveling distance), creating an area of lower pressure on the upper surface of the airfoil.
Because of this lower pressure, the wing is lifted by the higher pressure generated on the lower surface of the airfoil – just like with the piece of paper in the experiment above.
RAM AIR & SIR ISAAC NEWTON
Differential pressure DOES NOT account for all of the lift on an airfoil.
The other factor responsible for producing lift is ram air and it is exactly what it sounds like.
Based on Sir Isaac Newton’s Third Law of Motion which states that for every action, there is an equal and opposite reaction; air is simply rammed under the airfoil, creating downwash and an opposite upward pressure.
ANGLE OF ATTACK (AOA)
Lift changes with angle of attack—the angle between the relative wind (parallel to the airplane’s flight path) and the chord line (line between leading and trailing edge).
If you continue to increase the angle of attack, you will eventually exceed the critical angle of attack (usually around 15°- 20°), and will stall the wing.
Induced drag is a byproduct of lift.
Induced drag is created by those parts of the airplane which create lift—the wings and the horizontal tail surface.
When any surface produces lift, induced drag will also be present – It’s just the way it is and there’s no getting around it.
So, anytime you increase the angle of attack, you’ll also be creating more lift, which means you’ll also be increasing induced drag.
When airspeed is decreased, Induced drag increases.
Induced drag is also associated with the difference in pressure that exists above and below a wing.
As airspeed decreases, an airfoil must produce an increased pressure above the wing, and an decreased pressure below the wing.
These pressures meet at the wingtip and form a vortex.
The greater the pressure differential, the greater the vortices will be at each wing tip, and the greater the induced drag will be.
This type of drag IS NOT produced as a result of lift, like induced drag, but is caused by the displacement of air around the airplane.
Parasite drag is created by those parts of an airplane that do not contribute to lift.
Think of it as friction caused by the air against the airplane surfaces.
Examples of parasite drag would be the tires, the windshield, rivets, antennas, etc.
Parasite drag increases with increased airspeed.
TYPES OF PARASITE DRAG
There are three (3) types of parasite drag.
- Form Drag
- Interference Drag
- Friction Drag
Form drag is caused by the shape of the airplane and the amount of cross-sectional area that is presented to the air.
Form drag is reduced by streamlining the airplane.
Interference drag is caused by the interference of airflow between parts of an airplane (wings and fuselage or fuselage and empennage).
Friction drag is caused by the air passing over the airplane surfaces.
It is reduced by smoothing the surfaces (flush riveting, smooth paints, etc.).
THE DRAG CURVE
As speed increases, parasite drag increases.
As speed decreases, induced drag increases.
This is often referred to as the drag curve.
The bottom of the drag curve represents the most efficient speed at which the airfoil can generate the most lift and the least amount of drag.
When we express lift and drag as a ratio, this position is referred to as the maximum lift-drag ratio and is also your best glide airspeed.