Alternate Air Door for Filtered Air Box

This photo shows my spring-loaded alternate air door. It bypasses the filter and is screened to keep anything large from getting sucked into the engine. Preliminary testing with a large shop-vac and a water manometer shows that I will loose about 0.5 inch Hg. of manifold pressure with the air intake blocked in addition to the loss of any ram effect. I will do further testing after the first engine start. I would have liked a larger door but it wasn't possible using Van's airbox design. While the engine may not develop full power if a bird or heavy snow gets sucked into the air scoop, at least the engine won't quit. The twin torsion springs used to keep the door closed are captured in a way that prevents any metal from being ingested by the engine even if the springs were to break. The airbox was mounted as far toward the right as possible for three reasons:

1. To align the airbox with the cowl air scoop.

2. To insure that the airbox does not interfere with the mixture lever.

3. To leave as much room as possible for the alternate air door.

Here are some additional pictures of the alternate air door:

Twin torsion springs are used to keep the door closed under normal conditions. The fixed ends of the springs are trapped between the large base plate and the short hinge leaf. A spacer plate between these two pieces has narrow slots for the spring ends. Similar narrow slots are provided in the door to hold the other end of the springs. The hinge is the heavy-duty extruded type. The sealing gasket is a very soft Poron material that compresses easily even under the low force of the torsion springs. Even if a spring were to break due to vibration, the design traps all parts to prevent metal from being ingested into the engine. The hinge eyes will be inspected whenever the filter is serviced. In the unlikely event of all hinge eyes breaking and allowing the door to break free, the door is not large enough to fully block the air flow to the engine, yet it is too large to fully pass through the Bendix servo. Assembling the door with the springs was sort of like solving a puzzle, but some short pieces of hinge pin material helped hold the springs until the long hinge pin was inserted. The springs were left over from a project at work but unfortunately, I no longer have specs or part numbers for them.

The photos below show a simple test I did using a large shop vac and a water manometer to measure the vacuum within the Filtered Air Box under various conditions.

The above photo shows both the inlet and the alternate air door blocked. The vacuum generated by the shop vac was 33.5" H2O (about 2.5" Hg).

With the air door unblocked, the vacuum drops to 5.5" H2O (about 0.4" Hg). The door is about 0.25" open.

With the door propped open 90 degrees, the vacuum drops to 0.88" H2O (about 0.06" Hg).

With the inlet open, the alternate air door is firmly closed by the torsion springs and the vacuum measures a negligible 0.19" H2O.

I will conduct more tests after the first engine start, but my guess is that the engine will only loose about 0.5 inch of manifold pressure (Hg) at full throttle with the inlet blocked (under static conditions). The engine will loose some additional power due the the loss of any ram air effect as well as the power loss resulting from warmer (less dense) induction air from inside the cowl. Despite these three factors, I would expect that there would be plenty of power to maintain a good rate of climb even at relatively high density altitudes.