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.

Repair MLG Hydraulic Cylinder v2

I hate doing things twice…. During landing gear retract testing, I discovered a small hydraulic fluid leak around the shaft end of the cylinder. Since I had replaced all the seals in the cylinder only a few months previous, I was curious to see what the issue was.

I removed the cylinder and took it apart, and on a very close inspection found that the seating surface for the poly-pak shaft seal had a raised area. Microscope inspection revealed that the very thin metal between the NPT tapped area for the fitting was pushed toward the seating surface and that there was a very small hole that was allowing hydraulic fluid to leak. No way to fix this – had to order a new end piece from the factory.

Hydraulic Pump Modifications

It turns out that the version of hydraulic pump supplied to Velocity is configured with the UP side as the high pressure side and the DN side as the low pressure side. The pump itself is symmetrical, producing full pressure independent of rotation direction. Changing the DN side to be the high pressure side requires swapping the internal pressure relief valves at the base of the pump, and recalibrating them for the desired pressure.

As an aside, simply switching the hydraulic lines between the UP and DN side will not work, because the internal valve configuration of the pump requires that hydraulic cylinder retraction (which is our gear up) occurs on the DN side of the pump. The difference lies in the excess hydraulic fluid volume that must be supplied to the cylinders when the UP side of the pump runs (our gear down). Confusing? You betcha.

Scott S. offered to do this PRV swap and calibration at the factory, but I had the required equipment to do it and adjust the PRV settings.

First Landing Gear Swing Test

The nose and main landing gear hydraulic systems were connected to the hydraulic pump and power was supplied to the hydraulic pump bus. The emergency gear operation switch (in the pilot’s emergency panel) was wired up to allow testing of the gear retraction and extension.

As the video shows, the hydraulic pump pressure relief valve opens before the high pressure set point on the solenoid switch is reached. The result is that the hydraulic pump continues to run after the gear is up, and the “screeching” noise is the pump’s DN-side PRV opening.

Further investigation is required.

Nose Landing Gear Door Modifications

A few modifications were made to the nose landing gear doors and their open/close linkage.

During landing gear extend/retract testing it was difficult to find a nose gear door adjustment that satisfactorily held the doors shut, but opened promptly enough to prevent the doors from rubbing the tire on extension. I decided to remove the raised foam area on the doors to help prevent interference with the nose tire. The foam was sculpted and formed with EZ-poxy with flow, and then covered with carbon fiber for extra strength.

The rod ends on the nose gear door closing linkage were replaced with more sturdy and larger -3 versions that will not pop apart. I used left-threaded rod ends on the doors, and tapped an aluminum rod to act as a pushrod whose length can be adjusted by twisting it like a turnbuckle.

MLG Cable Attach

Because space in the keel is at a premium, I designed a smaller gizmo that attaches the MLG cables to the hydraulic cylinder. Fabrication of these parts are beyond the scope of the tools I have in my shop, so I set this out for fabrication – ouch$.

The new device has several features. It has an aluminum cylinder that fits over the actuator shaft, but it is threaded on a 1/4″-28 rod and so the MLG retraction limit can be adjusted by simply screwing the cylinder to the desired location and then locking it in place with a jam nut. In addition, the MLG cable rod ends fit into the device and tension/travel can be independently adjusted with nuts. Once adjusted, the nuts are secured with safety wire.

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.

Main Gear Cylinder Leaks

When trying to swing the gear with compressed nitrogen, it was apparent that the main landing gear hydraulic cylinder was leaking. After pressurization and retraction, if the cylinder was valved off, the gear legs would slowly fall down. The leak seemed to be around the actuator shaft and not internal leakage around the piston.

Since I had to do the same teardown and seal replacement on the nose gear actuator, I decided to pull the MLG cylinder out and replace all of its seals too.