Newton's Second Law
Force equals mass times acceleration. F = ma
The above is called Newton's Second Law.
An acceleration means a change in speed or direction. Another
way we could think of this law is that anytime there is NOT uniform motion,
that means there is an acceleration, and that means there is a force.
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If the airplane is flying in uniform motion, as shown to the
left then there is no force. We must learn to identify this situation.
We simply ask ourselves:
- Is the heading changing?
- Is the vertical speed changing?
- Is the airspeed changing?
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If we answer yes to any of these questions then the airplane
is accelerating. For example the airplane to the left is "pulling
out of a dive." The vertical speed is changing from negative
to positive, so it is accelerating.
If the airplane is accelerating then there is a NET force acting
on the airplane and that force can be calculated using Newton's
formula:
F=ma |
Whenever we see the airspeed needle move we know we are
accelerating, or if the VSI needle moves, or if the heading
indicator turns. All these things are accelerations, and all will
require a net force.
Conversely: when the airspeed, vertical speed, and heading are constant
then there is no net force.
NOTE: That means there is NO NET FORCE WHEN WE CLIMB, or when we fly
straight and level, or when we descend.
The Four Forces
There are four forces acting on
an airplane all the time in flight:
The above forces are the ONLY forces
acting on the airplane. However, we often run into terms such as G-Force, centripetal force and centrifugal force. So let's take a
moment to discuss each of these before talking about the four forces in
detail.
G-Force
We spend almost our entire lives experiencing two things:
- The force of the earth's gravity
- Zero NET force
As you read this book you are doubtless in a chair that is holding you
more of less stationary a foot or two above floor level. And I assume
you are within the earth's gravitational influence as well. Therefore
the net force on your body is zero, despite the fact that you are within
the earth's gravitational field and are experiencing a downward force
called weight.
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In aviation we frequently talk about a force called G-Force.
In aviation the G-force is by definition the force acting along
the normal axis of the airplane, as shown to the left.
Of course we spend 99% of our time oriented as in the upper part
of the picture, flying straight and level with the bottom of the
airplane oriented toward the center of the earth. In this situation
we have already learned that the NET force is zero. But there is
still a G-Force. It simply follows that there is another force,
which I have not drawn, that acts in the other direction offsetting
the G-Force.
The lower airplane is performing an aerobatic maneuver, a "loop."
In this case the bottom of the airplane is oriented in a completely
different direction. Still that is the direction of the "G-Force."
G-Force is usually expressed in a unitless number call g's (pronounced
gees) which is by definition:
g = G-Force / Weight
In the case of an airplane in straight and level flight we can
see from the picture that G-Force is straight down and common sense
tells us that the only force acting in that direction is weight.
So G-Force = Weight in this case. Therefore g = 1.0 |
When an airplane makes a maneuver such as pulling out of a dive, or a
steep turn, there is a centrifugal force in the direction of the G-Force.
This causes g to be greater than 1.0. We often say we are "pulling
g's." This is important structurally because the airplane is limited
to a certain number of g's before it is over stressed and damaged.
Centripetal and Centrifugal Force
Previously I said that there are three questions you can ask yourself
to decide whether or not there is a net force acting on the airplane:
- Is the heading changing?
- Is the vertical speed changing?
- Is the airspeed changing?
Most people think primarily of #3 when they think of acceleration. I.E.
accelerate most commonly means "speed up" in day-to-day use.
And we can accept that "slow down" is negative acceleration.
In physics we call this linear acceleration.
Both #1 and #2 above involve a curved flight path. If heading is changing
the airplane is flying a circle when viewed from above.
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When vertical speed is changing there is curvature when viewed
from the side, as in the picture to the left.
We can see that in this case the force pulling toward the center
of the circle is lift. However it is possible to think of aircraft
designs where thrust could also act toward the center of the circle
- a Harrier jet for instance.
By definition all the force acting toward the center of a circle
is called the centripetal force. In the example shown centripetal
force = lift.
It is important to realize that centripetal force is not a new
or additional force (i.e. it is not above and beyond the four forces
listed above.) It is just what we call the portion of the four forces
which acts toward the center of a curved flight path. |
To understand the closely related force called centrifugal force we must
now discuss Newton's Third Law.
Next Lesson: The Law of Reactions (Sometimes called
Newton's Third Law.)
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