# Energy changes of an airplane

#### MaxBrownsmith

We have this exercise about energy. We had to learn all different types of energy (kinetic, potential gravitational, potential elastic, electric, heat, light, sound, chemical). Then there was this exercise where we had to write down the energy changes for some examples such as: "a light bulb changes electric energy into light energy and heat".

Now there was some discussion about this example: "landing an aircraft". The book says it is potential graviatational energy turned into kinetic energy (which makes sense to me, because in the air it has potential energy and when it is coming down it is moving), but my teacher says the book is wrong and it should be potential energy into heat, because the airplane comes down with constant speed. What does he mean by that and who is right?

#### skeeter

While performing an approach for landing, the pilot maintains a constant speed during descent by increasing the plane’s drag moving in the air mass. This is accomplished by physically changing wing aerodynamics (angle of attack) using approach and subsequently land flaps. Bottom line is KE remains constant during approach (and actually decreases some just prior to touch down) while GPE decreases ... clearly, GPE is not converted into KE. Your teacher is correct.

Once the plane establishes itself on the runway, KE is decreased by means of reverse engine thrust and/or air brakes. Conventional wheel brakes on the landing gear are not used until the plane has reached a certain slow speed so as not to burn them up.

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#### MaxBrownsmith

Thank you for the explanation! It is a lot clearer than my teacher's!