The primary objective of the lecture was to firmly implant the concept of propulsive forces as arising largely from changes in the net flux of momentum. The two PRS questions (Q1 and Q2) were designed to emphasize this from an intuitive perspective. During the next lecture we will look at this more formally and do several examples. Please read the notes!
It was nice having the CFM56 in lecture because it enabled me to point out the various features. It also seemed to motivate a lot of questions, which is what I had hoped. It made it a little more difficult to do the lecture since I had to write on the overhead. Thank you to those who made recommendations for improving this (I will use darker pens next time).
I loved the engine!!! (and related comments) (6 students)
Viewgraphs worked fine (and related comments) (7 students)
Use a darker pen and leave the viewgraphs up longer (and related comments) (4 students)
Responses to 'Muddiest Part of the Lecture Cards'
(29 mud cards, 52 students attended lecture)
1) Are we to assume because of the speed of air flowing through the engine that we should not take into account the cooling of an engine? (1 student) The air flowing through the engine is in general hot (between atmospheric temperature and about 2000K depending on where in the engine). And it does heat the engine. However, in looking at the behavior of the air (as we did in thermodynamics) we typically make an assumption that the air moves through the engine fast enough that the air temperature is not significantly affected by heat transfer from/to the surfaces of the engine.
2) How is the blade tip speed related to the airplane speed? (1 student) It isn't. The blade tip speed is a function of the throttle setting--low for low thrust, high for high thrust.
3) What were you saying about a normal shock wave on the tips of the compressor blades and how it decreases efficiency? (1 student) I said that the power per unit area of a spinning blade row is proportional to the tip speed squared, so one would like the blades to move very quickly. However, as the tip speeds go much in excess of Mach 1, total pressure losses begin to increase in part due to the formation of shocks on the spinning blades (this is the total pressure change across a shock wave you computed in fluids). Since the goal of the compressor is to increase total pressure, this acts counter to the design objective. Engineers do their best to balance these (and many other effects) in designing an engine. The result is that tip speeds are around M=1 to M=1.5 on many machines.
4) Is this engine primarily driven by a conrol box or is it an older engine driven through analog devices? (1 student) This engine is controlled by a FADEC (full authority digital electronic control). Older engines were controlled by complex fluid-mechanical circuits (diapragms, valves and the like).
5) Are shrouded propellers more efficient than regular props? Why don't you usually see them on aircraft? (1 student) Relative to efficiency, sometimes yes and sometimes no. More importantly there are many more performance parameters besides efficiency to consider to determine the overall benefit -- in particular weight, drag and cost of the shroud.
6) What is an APU? (1 student) Auxiliary Power Unit -- typically a small gas turbine which is used to start the engine (and to provide power for other services when the engine isn't running -- like air conditioning the cabin).
7) It seems as though most of the intake is lost out of the intake and the nose cone blocks most of the inlet to the compressor--why is that? (1 student) Most of the flow in the intake is accelerated by the fan and used to propel the vehicle. A small fraction (10-50% typically) goes through the "core" of the engine -- the compressor, combustor and turbine. The nose cone and the cowling are designed to provide relatively uniform, well-behaved flow into the blade rows at all operating conditions.
8) What are the reasons for the differences between commercial and military aircraft? (1 student) Different mission requirements -- we will talk about this more in lectures P3 and P4.
9) How would this engine behave with a supersonic intake? What do you do to optimize engines for supersonic flight? (1 student) One would typically choose a little bit different engine architecture for a supersonic application. We will talk about this more in later lectures. If you are interested, you may want to take a look at either Kerrebrock, Aircraft Engines and Gas Turbines, or Hill and Peterson, Mechanics and Thermodynamics of Propulsion, both available in the library.
10) How is the outside of the engine insulated? (1 student) I don't think they make any effort to thermally insulate the outside of engine, but effort is expended to make sure all the pieces and parts within the engine cowl operate within their rated temperature capabilities.
11) I didn't understand the use of uexit in the PRS questions (it was a relative velocity). Don't you need to change the velocity to be NOT relative to the boat? (1 student). The problem can be solved using either a relative velocity or an absolute velocity, but it is typically more convenient to use the relative velocity. Only if there is a relative velocity between the boat and the rock is a force realized.
12) How hot would the air be just before the combustor for a fuel ignition system to not be necessary? (1 student) The air would have to be over about 350K (roughly the flash point for kerosine). The temperatures in the combustor are well in excess of this, so the ignitor is only used during starting.
13) Are we going to learn how engine stalls occur? (1 student) No, we will not spend any time on this, but if you ask me sometime after class I can tell you a little about it.
14) No mud (2 students).