Yep, the table reflects the pressure that a pitot tube would see and could
be used for calibration (maybe) of such a tube. Think of it as
the maximum pressure that could be pitotically (invented word) obtained if
there is no bleed off of the pressure by some sort of leak - like consuming the
fuel in the tank. 1/2 the pressure sounds good for a vertical tube with a
45 degree forward facing cut with the aforementioned leak. Maybe it is
less for a NACA duct. It is preferable to have a slight positive pressure
in a fuel tank provided by the vent.
Uh, after all that, does this force water into the tank? If the flow
thru a NACA is backed up because it is impeded by back pressure, does water
enter the duct at all? Is there yarn small enough, yet big enough to see,
that could tell us the flow of water droplets about a teeny NACA? I have
further questions, but my wife wants to go to dinner. Tata for now..
Grayhawk
In a message dated 5/11/2007 7:16:36 P.M. Central Daylight Time,
kyrilian_av@yahoo.com writes:
This table looks like simple dynamic pressure.
The third report referenced in Chris Z's post (
NACA RM
A50E02) provides pressure recovery ratios for various
mass flow ratios (discussion on p 5, plots on pages 14-20). While it
appears nothing was tested to ratios around zero (1/4 dia wing tank vent
lines at 15gph and 200 kts are in the 0.005 range--essentially zero),
extrapolating a bunch makes me think we should expect mass flow ratios of 0.5
to 0.8. This is based on -2 to +2deg angle of attack at 0.3 Mach.
So divide in half the pressures in the table below for a (maybe) conservative
estimate.
- Kyrilian
Sky2high@aol.com wrote:
Mike,
|
|
|
Pressure in Units |
|
IAS
mph |
Knots |
"H2O |
"Hg |
PSI |
|
|
|
|
|
|
|
40 |
34.8 |
0.79 |
0.06 |
0.028 |
|
50 |
43.4 |
1.23 |
0.09 |
0.044 |
|
60 |
52.1 |
1.77 |
0.13 |
0.064 |
|
70 |
60.8 |
2.41 |
0.18 |
0.087 |
|
80 |
69.5 |
3.15 |
0.23 |
0.114 |
|
90 |
78.2 |
4.00 |
0.29 |
0.144 |
|
100 |
86.9 |
4.94 |
0.36 |
0.178 |
|
110 |
95.6 |
5.98 |
0.44 |
0.216 |
|
120 |
104.3 |
7.12 |
0.52 |
0.257 |
|
130 |
113.0 |
8.37 |
0.62 |
0.302 |
|
140 |
121.7 |
9.72 |
0.71 |
0.351 |
|
150 |
130.3 |
11.17 |
0.82 |
0.403 |
|
160 |
139.0 |
12.72 |
0.94 |
0.460 |
|
170 |
147.7 |
14.39 |
1.06 |
0.520 |
|
180 |
156.4 |
16.15 |
1.19 |
0.583 |
|
190 |
165.1 |
18.03 |
1.33 |
0.651 |
|
200 |
173.8 |
20.01 |
1.47 |
0.722 |
|
210 |
182.5 |
22.10 |
1.62 |
0.798 |
|
220 |
191.2 |
24.29 |
1.79 |
0.877 |
|
230 |
199.9 |
26.60 |
1.96 |
0.961 |
|
240 |
208.6 |
29.03 |
2.13 |
1.048 |
|
250 |
217.2 |
31.56 |
2.32 |
1.140 |
|
260 |
225.9 |
34.21 |
2.52 |
1.236 |
|
270 |
234.6 |
36.98 |
2.72 |
1.335 |
|
280 |
243.3 |
39.86 |
2.93 |
1.440 |
|
290 |
252.0 |
42.87 |
3.15 |
1.548 |
|
300 |
260.7 |
45.99 |
3.38 |
1.661 |
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