Flow Modeling of NACA Cooling Ducts

My engine choice is currently a TSIO-550. I want to make sure that I have plenty of cooling air available to the engine and oil cooler. Recall that I am not electing to install an oil cooler in the nose of the aircraft.

I am planning to modify the factory-provided NACA ducts to enlarge the entrance height to 3.5″ and also allow for a 1/4″ thick airfoil lip at the duct entrance plane. I don’t really want to extend the length of the duct because of the headspace encroachment in the cabin. The concern is that the ramp angle for this configuration is about 9.6 degrees. This is higher than the recommended range from the old NACA literature.

To give myself some confidence, I ran some flow simulations in Solidworks and validated the performance of my modified NACA duct for takeoff conditions (0 MSL, 15C, 100 kts TAS) and medium cruise (10,000 MSL, -4C, 160 kts TAS). The unmodified (factory stock) duct provides 78% of free-stream mass flux and the enlarged entrance duct provides 77% of free-stream mass flux – only a tiny decrease in efficiency, but with a 1.75 times larger mass flow due to the enlarged area.

Fresh Air Door and Ducting

Instead of a nose-mounted oil cooler, I am using that location for a fresh-air inlet with a variable-opening door. I fabricated a door for the inlet using a foam-core sandwich panel, fit it to the opening in the fuselage, and attached it with a hinge just forward of the opening.

A fresh air distribution box was fabricated with flanges to attach to the fuselage and holes for standard SCAT ducts to route the fresh air.

SCAT Hose Flanges

I refuse to buy a bunch of the expensive metal flanges for the attachment of SCAT/SCEET hose for heating and cooling.  So I bought one each of the two most probable sizes if hose I will be using, and used them as a template to make a mold so that I could replicate them in fiberglass.  Mold release wax was used liberally on the metal, and then a liberal layer of cabo on the surface (poor mans gel-coat) followed by 4x bid was laid up on the outside of the metal template to make the mold.  I added graphite to the epoxy to make the mold black.  After smoothing and filling and waxing, I could punch out replicas of the SCAT hose flanges.  I will use these on the various air boxes and ducts for the cabin heating and cooling system.

Sidebar – Pneumatic Measurements on Fan

I purchased a squirrel-cage blower to use to recirculate air in the cabin, and as a side experiment, I wanted to measure the airflow characteristics of the fan. I tend to geek out on the physics aspects of things, so I set out to measure the zero-flow pressure and the no-back-pressure flow rate, and several intermediate points in-between. In analogy, its like measuring the source characteristics of a power source, where one would measure the open circuit (zero current) voltage, and the no voltage (short circuit) current, and points in between to get the source impedance.

So I rigged up a flow tube with several fixed-sized circular orifices, with dynamic pressure measured on both sides of the orifice to obtain (via the equations for compressible orifice flow) the air flow rate, and then an independent pressure measurement (re: atmosphere) at the fan output.

Measurements indicate (see graph below), that at a fan voltage of 13.5V, the measured airflow vs. back pressure (dots) agrees in shape, and is consistently above the manufacturer’s data (solid curve), which was taken with a fan voltage of 12.0V.

I think if I keep the resistance to flow low enough, this fan will deliver an adequate air flow. Further experiments will include measurement of flow resistance of SCAT and SCEET tubing.

Nose Submerged Inlet Door

I have decided to depart from the plans regarding oil cooler location and cabin heat.  I plan to put the oil cooler in the rear of the aircraft, and use the nose NACA inlet for cabin fresh air only.  Details, calculations, and full plans to follow later. To control the amount of fresh air entering the aircraft, I am building an electrically actuated door that will fit flush in the NACA inlet when closed, sealing it completely against incoming air, and also retaining the fuselage contour. When open, outside air will be routed to cabin ducts for cooling in the hot Texas summer.  In the winter, and at altitude, the nose NACA duct will be closed, and hot air from exhaust muffs will be mixed with outside air and routed from the engine compartment into the same ventilation ducts.