Lecture P4: Connecting Aircraft Performance and Engine Requirements

I thought the class went well (except for the blackboard problem). People seemed to be in better spirits (and more awake) than last week. I covered all of Chapter IV of the notes. The most important concepts are 1) given aircraft aerodynamic and weight information you can calculate thrust required for steady level flight, 2) flight at max range requires flying at a condition that minimizes drag (for a given weight--i.e. that maximizes L/D), flight at max endurance requires flying at a condition that minimizes power required (drag times velocity), and 3) Power available minus power required = time rate of change of potential plus kinetic energy. Requirements for T/W are set by maneuverability requirements. These as well can be found knowing aircraft weight and aerodynamic information.

We did two PRS questions (PRS #1, PRS #2). A question was asked in class about lift not exactly balancing weight during climbing flight. If you look at the solution to the second PRS question as well as the notes you will see there is only one place where I used a small angle assumption (saying L=W to replace D with W/(L/D) instead of L=Wcosq).

Next lecture we will discuss rocket performance. Please read Chapter V and VI of the notes.

Responses to 'Muddiest Part of the Lecture Cards'

(32 respondents)

1) How does thrust in 2D work in terms of the engine? Do you have vanes thrusting at an angle? And if you want to minimize thrust, which of the below (equations written on card) is more precise? (1 student) I think I understand the first part of the question, but not the second. Some aircraft have thrust vectoring nozzles (designed to change the direction of thrust relative to the axis of the aircraft--this is done to provide enhanced maneuverability). Most aircraft, however, are designed with a fixed orientation between the thrust vector and the airframe (except when thrust reversers are deployed). I don't understand the part of the question regarding minimizing thrust, please ask me again in recitation.

2) I didn't have the problem, but asking for "thrust required" meaning "thrust available required" confused as to go to solve the wrong term. (1 student) I don't understand exactly what you are asking, but I think I see where some confusion could arise. Let me clarify the terminology and see if that helps. Power required is typically used to refer to drag*velocity for steady level flight. For maneuvering flight, more power is needed to accelerate the vehicle. So power available must be greater than the power required for steady level flight. It gets confusing when I ask "How much power (or thrust) is "required" for a particular maneuver?" I will try to be a little clearer and note whether I am talking about power required for steady level flight or that for a maneuver.

3) No mud (30 students). Good!