20th March 2012

[87] Aspects of Aircraft Design and Control, project 7

lecture 7

lecture 7 (Aircraft Trim) v1.5

The majority of the lecture sees only minor updates here; however, we made important mistakes in the last part ("A pilot’s perspective"). While our conclusions were correct, the explanations leading to them were not.

In this case, the center of pressure on a airfoil does not in fact move when speed is increased at constant attitude, regardless of whether it is cambered or not. An increase in speed at constant attitude (such as during a gust) would neither change the elevator neutral position, nor cause a net moment around the aircraft center of gravity.

It turns out that the reason why aircraft pitch towards their "trim tab setting speed" is much simpler. Any change in speed needs be compensated by a change in attitude; that in turns changes the lift force on the tail. This change must be compensated by the elevator, either with pilot control input or with the help of a new trim tab setting. Failure to compensate the net change on the tail force (by letting go of the controls) will result in the aircraft pitching. A quick sketch will reveal that the aircraft will pitch down if slowed down, up if accelerated. I hope the slides explain this clearly now.

project 7

project 7 (Aircraft Trim) v1.1

No change at all since last year. I am still unsatisfied with the data and know some of it to be erroneous (in particular the mean aerodynamic chord). But I lost the source for the corrected information, and now lack the time to experiment with this ("if it ain’t broken").

project 6

project 6 (Propulsion) v1.1.1

Corrected a typo (the turbine power is in kJ/kg not J/kg).

Tags: aspects_aircraft_design (lnk)