Aerodynamics Index

Definitions

Aircraft Axis
CG definition
Define Up and Down
Define Pitch, Bank, Hdg
Define: Lift, drag, etc.
Define Power

Physics Review

Newton's First Law
Newton's Second Law
Newton's Third Law
Reaction = Lift
Reaction = Drag
Conservation of Energy
What is a Vacuum
Action at a Distance

The 4 Forces

Spotting Forces & Moments

Performance

Drag Overview
Induced Drag
Induced Drag Equation
Total Drag

Jet Climb Performance
Prop Climb Performance
Range Jet
Range Prop

Forces in a Turn

Misc

Pitch Controls
Roll Controls

Configurations

Previously we looked at a box on a table and we said that the table reacted to having the box on it such that when the box pushed down on the table, the table pushed back up on the box.

The same would of course be true of a model airplane sitting on a table, as shown to the left.

The reaction on the part of the table is automatic. The table cannot choose to not push back against the weight of an object resting on it ( whether it be a box or a model airplane.)

Newton's Third law tells us that there is always a reaction.

For example when the air generates the upward force we call lift we can be certain that the airplane also generates a force pushing down on the air.

In short there cannot be a lift force holding the airplane up without an equal force pushing down on the air. In the case of the model on the table that force pushes down on the table. in the case of an airplane, it is called an air plane because it pushes on the air. (an airplane cannot fly in a vacuum.)

Based of Newton's third law we should realize that if air was visible, then we would see the air below and immediately behind an airplane being pushed downward. This is just as true as the more obvious observations that the air behind a propeller must be pushed backward, or that the chemicals in a rocket must accelerate downward if the rocket is to go upward.

We will consider just how a wing accelerates air downward later. For now we are just trying to grasp that this is necessary.

Relationship Between G-Force and Lift

Previously we looked at the picture to the left. We also previously defined G-Force as the force acting down along the normal axis of the airplane. It should therefore be clear that in the picture to the left G-Force and centrifugal force are the same.

We can also see in the picture that IN THIS CASE lift equals centripetal force. Therefore, the previous formula for g's:

g = G-Force / Weight

can be reworked to:

g = Lift / Weight

Strictly speaking this reworked formula is wrong, because Lift has the opposite sign to G-force. We solve that by arbitrarily reversing the sign of weight. In other words an airplane doesn't really weigh 10,000 pounds it weighs -10,000 pounds. But we will agree to count weight as a positive number. We must do this because the Lift vector is opposite to the G-Force vector, in other words has positive sign, whereas G-Force (centrifugal force) has a negative sign. (Dividing two negative numbers gives a positive number. Positive g is by convention "down" along the normal axis.)

Now consider the picture to the left, which shows an airplane making a balanced turn in level flight.

Clearly centripetal force does not equal lift in this case. In this case centripetal force equals Lift plus Weight.

Nevertheless we can see that the G-Force is still equal but opposite to Lift. Therefore, the formula above:

g = Lift / Weight

still works, if we count weight as positive.

Throughout this text we will use this as our definition of g's. We will be assuming balanced flight at all times, which means that we assume the lift vector acts in the plane of symmetry of the airplane. That of course is not always the case, as for example in a sideslip. However, if we limit ourselves to considering only balanced flight then lift is equal but opposite to G-Force, and the equation for g is valid.