The document summarizes homework 4 for a class on aerospace engineering. It provides statistics on the assignment including that 40 students were enrolled, 39 turned it in on time, and the average score was 72.7% with a standard deviation of 16.35%. It then lists the most common mistakes students made, including incorrectly using static temperatures instead of stagnation temperatures in problem 1, incorrectly calculating work paths in problems 2 and 3, and not properly accounting for air composition in problem 3.

1. AEROSP335 Homework 4 Grading Summary
1 Statistics
There are 40 students enrolled in AEROSP335 and 39 homework copies are submitted on time, one is late.
Title Quantity Average Std dev Max
HW4 40/40 72.7/100 16.35 100/100
Table 1: Homework 4 overview statistics
2 Common Mistakes
1. Problem 1(a) In a large number of homework the work is calculated with the static tempretures, but the correct
way should be using the stagnation temperature
Correct:
wt = wc = cp(T02 − T01)
Incorrect:
wt = wc = cp(T2 − T1)
2. Problem 1(a) It’s also incorrect to directly calculate wt = cp(T03 − T04). There is no enough knowns to directly
calculate T03 and T04. In contrast, T03 is calculated from wt = wc.
Correct:
wt = wc = cp(T02 − T01)
Incorrect:
wt = cp(T03 − T04)
or
wt = cp(T3 − T4)
or
wt = cpT1(
T3
T1
−
T4
T1
)
3. Problem 2(c) The actual turbine work path is defined from 3 to 2 so it’s incorrect to calculate the turbine work
from 3 to 2s
Correct:
wt = wc ⇒ wc = cp(T3 − T2 ) ⇒ T2 = T3 −
wc
cp
Incorrect:
wt = wc ⇒ wc = cp(T3 − T2s ) ⇒ T2s = T3 −
wc
cp
⇒ T2 = T3 −
T3 − T2s
ηt
4. Problem 2(d) This is the most common mistake occured in this homework.The reason why that happens is that
the plot is slightly wrong, however the problem statement is right. Since 2(d) says that the nozzle is not perfectly
isentropic, 4s should be vertically below 2 not 3. The correct one is shown as follow.
1

2. Figure 1: Correction of HW4 Problem 2 Plot
Since that, the isentropic relation to calculate T4s should be operated on the path from 2 to 4s but not from 3 to
4s.
Correct:
T4s = T2 (
p4
p2
)
γ
γ−1
Incorrect:
T4s = T3(
p4
p3
)
γ
γ−1
5. Problem 3(b) A large number of students directly use the ratio of (O2+3.76N2) to C16H34 (which is 24.5) to
compute the corresponding air consumption. It’s necessary to consider the 1:3.76 decomposition of air. So the ratio
of air to hexadecane should be 24.5 × 4.76 = 116.62 not 24.5.
2