Pitch up with power

[Keith.Williams.]

The fundamental problem with this string (and almost every other string that deals with JAR examination questions) is that we do not know the exact wording of the original question. If for example it had included something like "what causes an aircraft to pitch up immediately after increasing thrust", or "pitch up as airspeed increases following an increase in thrust", then it would probably have been possible to select the correct answer.

If the pitch up is an immediate effect then it is probably caused by the thrust line being below the C of G, rather than it being below the drag line. To test this theory we might imagine how the effect would change if we raised the thrust line to coincide with the C of G.

If we assume that the aircraft is correctly trimmed prior to the test, the nose up moments will be balanced by the nose down moments. With the thrust line below the C of G, increasing thrust will increase the nose up moments. This will cause the nose to pitch up. The changing tailplane angle of attack may (or may not) eventually rebalance the pitching moments, such that the aircraft stabilises at a new higher pitch attitude.

If we now raise the thrust line so that it passes through the C of G, retrim the aircraft, then repeat the test, increasing the thrust will cause no immediate change in nose up moments. The aircraft pitch will therefore (initially) remain unchanged.

We can take these tests a little bit further by rearranging things so that the drag is above the thrust and the thrust is above the C of G. If we retrim the aircraft then increase thrust, the immediate effect will be a nose down pitch. This is because the nose down pitching moment caused by tailpalne trim force and increased thrust, will be greater than the nose up moment caused by drag.

Finally if we rearange things one more time so that thrust is above drag and drag is above C of G, then retrim and increase thrust, we will again get nose down pitching.

So the realtive heights of thrust and drag will not affect the immedaiate result. But the aircraft will pitch nose up if thrust is lower than C of G.

Getting back to the original scenario, if the increased thrust then causes airspeed to increase, the increasing drag above the C of G will cause more pitch up. The changing airflow over the tailplane and various other factor will of course also contribute to (and may even reverse) the overall effect. But if they are not included in the question, they should not be considerd.

The reall worry here is whether or not the examiners have a good enough grasp of their subjects to understand the possible interpretations of their questions. Much of the feedback from students suggests that they do not. This sounds like a case of an examiner experiencing a sudden flash of inspiration and thinking "That's really clever of me. Let's see if the students can get it".