SAWE Report - Cal Poly San Luis Obispo

Diameter (in)
70
60
50
40
30
20
10
300
250
200
150
100
50
Length (in)
0
0
12,000 17,000 22,000 27,000 32,000
Thrust (lbF)
Figure 6.4 - Engine Sizing Plot
6.2 Inlets
Sizing the inlet for supercruise flight at 1.6 Mach posed an interesting problem. A pitot inlet is
good up until about 1.6 Mach and it is by far the cheapest inlet possible. However the
performance of the inlet above Mach 1.6 is very poor. The pressure recovery of a two shock inlet
(one oblique and one normal shock) and a three shock inlet were analyzed. The optimum
deflection angle for Mach 1.6 flow was found for a two shock inlet by finding the stagnation
pressure loss across the oblique and normal shock for different deflection angles. The results
were graphed in Figure 6.5 and the resulting deflection angle for the greatest pressure recovery
was found to be 10.75 degrees yielding a pressure recovery of 97.65%. Finding the optimum
deflection angle for a three shock inlet is more involved therefore a rough estimate of a six
degree deflection angle followed by another 6 degree deflection angle was used to compare
against the two shock inlet. The difference in on design pressure recovery is about 1% however
the larger the deflection angles become the better the pressure recovery will become. The
pressure recovery comparison can be seen in Figure 6.6. The military specification for inlets is
given below and is represented in the graph.
Mil Spec MIL-E-5008B
η
rSpec
⎧ 1 M ≤ 1
⎩1
0.075( 1) 1 5
0
= ⎨ −
1.35
M0 − < M0
<
41