Lecture P3: Integral Momentum Equation and Engine Efficiency

I found it extremely hard to concentrate/pay attention/think today. I guess it is the lack of sleep all week. (1 student)

Another poor turnout for the lecture (43 students). I will repeat what I said last time: This will cause some people to struggle with the application of the integral momentum theorem. This is one of the most important concepts in the propulsion material. If you are one of these people: 1) read the notes, 2) start your homework early so you can ask me questions, and 3) come to the recitation on Tuesday--I will do some examples. The information on efficiency is more straightforward and you should be able to pick this up reading Chapter III of the notes.

We did two PRS questions (PRS #1, PRS#2). We used the cell phones. There were pluses and minuses with this. Although the technology worked smoothly, we need to fix the problem of having people see the responses as they come in--it is better to arrive at an answer independent of knowing what others are doing. From my perspective, it gave a little too much information for me to handle and still concentrate on the lecture (people with questions and % confused -- it is easier if you have a question if you raise your hand, and I usually get some input on % confused based on your facial expressions). But we will try this a few more times so we can understand what works and what doesn't work. Below are your comments (keep them coming please):

It takes me longer to figure out how to ring-in than it does to solve the problems. (1 student)

I don't like seeing the real time graphs on the screen cuz I don't like to be influenced---> but we need to see the time remaining for the question, yeah? (1 student)

So I can check stocks and netmail while in lecture ... interesting. (1 student)

Next lecture we will discuss aircraft performance. Please read Chapter IV of the notes.

Responses to 'Muddiest Part of the Lecture Cards'

(20 respondents)

1) General confusion about the momentum equation, probably due to missing Wednesday's lecture. Just not comfortable doing the integral. Will be at office hours to get questions. (1 student) See you there. Read over the notes.

2) What if the airplane is super massive an otherwise efficient engine poduces little velocity, Tuo is small. Does engine efficiency depend on the design of other parts of the plane? (1 student) I think I understand your question, but I am not sure. Engine efficiency does depend on the design of other parts of the plane, but not significantly (this was the propulsion airframe interaction, PAI, I briefly mentioned). Overall efficiency is how much thrust power is produced per unit of fuel power. And since thrust only relates to the flow through the engine (not the drag on the outside of the engine or pilon) it is influence mostly by the engine design. what changes with flight velocity is the drag and the power required for flight--so these set what the thrust must be.

3) How would one calculate the thermal, propulsive and overall efficiencies for when in Charlie and the Chocolate Factory, Charlie and Grandpa drink sode and float to the top of the room? (1 student) Not a clue. What does momentum transfer look like when an astronaut farts?When on a space walk and when just chilling on the shuttle? (1 student) It is all F=d/dt(mv).

4) No mud (13 students). Good!