Installed Thrust Testing - SR-71 Duct System - RC Groups
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Apr 23, 2011, 07:12 PM
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Installed Thrust Testing - SR-71 Duct System

These tests were made on my nearly completed version of Willmsy’s SR-71. This model is fitted with the DF-70 8 blade fans with Mega 16/15/2 motors. The fan alone had been previously tested and the installed thrust is compared with these earlier tests. These tests reflect the performance of a single fan and nacelle; the model would have twice the thrust since it has two fans and nacelles.

The model was too large to install in my little wind tunnel so the tunnel walls were removed and the model installed on the wind tunnel balance, so only static thrust measurements were possible.

The DF-70 has an FSA of 4.37 sq. in. We had previously done both static and 35 knot speed tests fan alone and also with a short nozzle with exit area of 3.76 sq. in. We also tested and fan alone with and without bell mouth. With 12-volt battery, and a bell mouth, the fan/motor combination by itself would generate nearly 28 oz. of static thrust, pulling 43 amps (450 watts). The short nozzle of exit area 3.76 sq. in. degraded static thrust but helped a little at 35 knots

The basic SR-71 nacelle is 20.5 inches long, with the fan located 7.5 inches from the forward inlet. The large circular inlet has an area of 8.2 sq. in. A smooth .030 styrene duct connects the inlet to the fan and the fan to the exit. The exit area is 4.52 sq. in. (103% FSA). With the fan installed in this basic duct system, installed static thrust was nearly as good as the fan by itself (See Figure 1), 27 oz. pulling 42.9 amps (452 watts). One test was run with the exit area reduced to 3.63 sq. in., which reduced static thrust about 2 oz. An inlet lip on the large intake did not seem to affect static thrust.

A scale like mixing afterburner nozzle into which the efflux is injected and which pulls air in from 11 tertiary inlets was installed, which also did not seem to affect the static thrust results. This was a welcome answer as I was afraid it might degrade performance. The nozzle adds 2 inches to the duct and has an exit area of 5.8 sq. The tertiary inlets have total area of 4.8 sq. in.

The inlet shock cone was installed (see Figure 2 and 3), which reduced the inlet area from 8.2 to 2.47 sq. in. (57% FSA). This resulted in a major reduction in static thrust, while the motor was still pulling 44 amps (459 watts); thrust was only 15 oz., a reduction of static thrust by 44%.

The model had been fitted with scale like auxiliary inlets, similar to the full-scale airplane (see Figure 3). These had not been opened up until now. Opening the first row provided an addition of 7.66 sq. in. The effective area is reduced by screens installed over them. The effect of the screens was not measured. These auxiliary inlets bringing inlet area up to 10.1 sq. in. less the affect of the screens. When combined with the shock cones, these changes brought static thrust up to 23 oz. pulling 42 amps (440 watts), still down 4 oz. (15%) over the baseline performance.

Opening up the second row of 4 auxiliary inlets gave another 3.76 sq. in. of inlet (less screen affect) and full throttle static thrust did not improve, still 23 oz. pulling 42 amps (441 watts). I guess this should have been expected since turning the inlet air 90 degrees and forcing it through screens has to reduce the energy of the inflow air when compared to a smooth straight in flow path.

The results are shown in figure 1. The exit nozzle with external inlets is shown in fig 2. The inlet system is shown in figure 3.

For more on the Willmsy's SR-71 see the thread, ”SR-71 Mk II Scratch Build…Again”.
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Apr 23, 2011, 07:47 PM
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Very interesting William, you have done some great testing performed, it was interesting to note that there was an improvement when you opened up the auxilary inlet ducts but not a dramatic improvement as you advised. From my experience those auxilary inlets are almost like cheater holes so like yourself a little surprised you still have not achieved the baseline thrusts.

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