@Lancman, barit1, cwaters
Originally Posted by Lancman
If a hang glider is flying at 20 knots in a 20knot headwind it has zero kinetic energy but plenty of potential energy, I hope. If it turns downwind it needs to get some kinetic energy from somewhere, quickly.
An aircraft with it's engines running on the ground with the parking brake set may be doing some work but it's efficiency is zero.
HTH.
Truly amazing in an aviation forum.
But a common misconception that is worth an explanation.
Since 1632 and what is called "Galilean relativity" (no need for Einstein here), it is known that the laws of physics are the same in all inertial frames.
This means that the measurements of interacting forces, accelerations, variations of speed, momentum changes remains unchanged when switching to another inertial frame
although measurements of distance, speed, kinetic energy are changing through frame switching.
Let's look at this glider flying 20 kt in a 20 knot headwind and turning 180° .
A turn always requires an acceleration ; this is the centripetal acceleration.
In the frame of reference of the air, the glider turn changes its speed from +20kt to -20kt.
The integral of the centripetal acceleration has completed a momentum change of -40kt (x the glider mass).
In the ground frame of reference, the glider turn changes its speed from 0kt to -40kt.
The centripetal acceleration has also completed a momentum change of -40kt.
So yes, measurements of accelerations, momentum changes and forces give the same results in the 2 frames.
And that's untrue for kinetic energy.
Kinetic energy alone is irrelevant ; it can vary from zero to infinity just by changing the frame of reference.
The actual meaning of the kinetic energy in a specific frame is that "it measures the work that the mobile object can do
through interacting with a still object of the frame".
It also measures the work necessary to give it its speed through an interaction with a still object of the frame.
So kinetic energy is only relevant
in the frame of reference of the objects that the mobile interacts with.
The common misconception here is generated by these facts:
1. the ground frame has a special meaning to humans since it is the frame of the objects our feet interact with.
2. the concept of inertia is objectively handled in physics through the measurement of mass, but is subjectively felt in all day life as the work required for increasing the speed of a mobile object.
3. the "inertial work" can only be measured in the frame of the interacting object, but humans tend to consider that it is an absolute value measurable in the ground frame of reference.
Back to turning into head wind/down wind.
In summary, one can state:
1. In itself, turning into head wind or down wind makes no difference ; the airplane only interact with the air and so it is the frame of reference of the air that matters.
2. Turning whilst crossing a windshear
does make a difference ; turning into increasing relative wind increases the total energy of the plane (temporary increase of air speed or height gain),
and turning into decreasing relative wind decreases the total energy of the plane (temporary decrease of air speed or height loss).
A turn into decreasing relative wind is a potential hazard at low altitude.
3. Flying perfect circles with reference to the ground makes irregular circles in the air (with non-zero wind). The rate of turn is lower when flying headwind and higher when flying downwind.
Thus flying a ground pattern at low altitude and allowing very high rate of turn when turning downwind is also a hazard.
Luc