119
edits
Changes
→Total energy in gliding: a more interesting quantity
=== Total energy in gliding: a more interesting quantity ===
What a glider pilot is more interested in is the total mechanical energy of a glider, which has two components: the gravitational potential energy (altitude), and the kinetic energy (speed).
In a mathematical form, this can be expressed as:
\[E = E_P + E_K = mgH + \frac{1}{2} m V^2 \]
Where:
*\(m\) is the mass of the glider, which does not change very often in flight
*\(g\) is the gravitational acceleration, a constant
*\(H\) is your altitude
*\(V\) is your speed
For the simplicity of the following discussions, we shall assume that a glider flies in dead still air, so we can avoid the problem of defining the frame of reference for the speed: the true airspeed and the ground speed are the same. Furthermore, we shall assume that the glider manoeuvres in such a small altitude range that the density of air can be treated as a constant \(\rho\).
Consider a glider which has descended from \(H_1\) to \(H_2\), in which process its velocity has increased from \(V_1\) to \(V_2\). We shall, for the purpose of the discussion, assume that the glider has not lost any mechanical energy during the process, i.e. the glider experiences zero drag (!)
The mechanical energy is conserved, therefore:
\[ E_1 = E_2\]
\[ mgH_1 + \frac{1}{2} m V_1^2 = mgH_2 + \frac{1}{2} m V_2^2\]
==== How to measure total energy ====