Lecture P4: Connecting Aircraft Performance and Engine Requirements


General comments

I began this lecture with a discussion of expressions for efficiency for aircraft engines. It is convenient to express the overall efficiency in terms of the propulsive efficiency and the thermal efficiency. Considering these independently allows a better understanding of engine architecture choices (e.g. high bypass ratio commercial engines versus low bypass ratio engines for high speed military aircraft) and engine development trends (e.g. increased compressor pressure ratio for increased thermal efficiency). We did a PRS question to allow you to exercise these expressions in the context of a design trade-off.

Then I covered much of Chapter IV of the notes. We did a second PRS question (PRS#2) designed to test your understanding of the conditions under which endurance is maximized (the goal for your dragonflies). 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 can be found knowing aircraft weight and aerodynamic information. Note that there are typically three kinds of calculations requested for manueverability: a) steady climbing flight (so no acceleration term), b) constant altitude acceleration (no term for change in potential energy), and c) steady, constant altitude turning flight (no terms for acceleration or change in potential energy, but drag increases in response to increased lift required to overcome centrifugal acceleration).


Responses to 'Muddiest Part of the Lecture Cards'

(12 respondents, 60 in class)

1) The system performance stuff was a good review for our .05 exam coming up next. Thanks. (1 student) My pleasure.

2) The B-52 Stratofortress has 8 "small" engines which are much more inefficient than 4 "big engines". The reassons for the smaller engines are less drag, more maeuverability and lower observability. Wouldn't the larege size of the B-52 negate all improvements made in these three categories? (i.e. we might as well put 4 big engines on?) (1 student) I recently served on a Defense Science Board panel to evaluate re-engining the B-52 (a 4 for 8 swap). The B-52 doesn't effectively take advantage of the benefits of a smaller engine. And it loses the efficiency benefits of a large engine. But the reason for the engine choice is historical -- in the 1950's we didn't know how to build high bypass ratio engines (the technical challenges had not been overcome). Today we could do the replacement and gain about 35% in range and/or save 35% in fuel burn -- but the price tag probably isn't worth it right now (but it might have been 5 years ago). But there are a lot of interesting details with this case. Remind me in class and I will tell you a few stories about it.

3) What engines do commercial/military people choose and why? (1 student) It depends entirely on the mission. For commercial applications, range is a principal mission objective so the engines tend to be optimized for fuel efficiency and weight. For some (but not all) military applications requirements for maneuverability and supersonic flight lead people to choose engines that are smaller, lighter and lower drag but have worse fuel efficiency.

4) No mud (9 students). Good.