Jerry Kaidor:
The other day, I was laying around under my Sundowner cleaning the gunk off the belly, and noticed something curious. Both battery vent tubes had been cut diagonal, where they exit the fuselage, so as to create a venturi effect. The intent was obviously to create airflow through the battery compartment. Unfortunately, venturis had been cut in BOTH tubes, which to my ignorant eye would create a suction at both the top and bottom of the battery, lowering net air movement to zero, or some small value related to the relative sizes of the hoses, the coordination of flight, and the phase of the moon.
So my question to the gurus of MusketeermailLand: How’s it supposed to be? My first inclination would be to create a net air movement upward from the battery drain, past the battery to the outlet hose. That way, battery crap would be less likely to accumulate at the bottom of the box. AC43.13 does not really say: they show a venting scheme with air entering at the top of a battery, flowing along the top of the battery, and exiting at the other side. How did Beech do it? Did the factory do diagonal cuts in both tubes?
Editor:
It is my belief that both battery vent tubes are supposed to be cut on the diagonal. However, the inlet cut should face forward and the outlet cut should face aft. The intent is for the inlet cut to act as a scoop while the outlet cut creates low pressure for the exhaust. If your tubes are two different sizes, the smaller one is the inlet; don’t get them reversed or you might pressurize the battery box. The bottom box connection is the outlet tube, so that any liquid will drain out rather than sit in the box.
As an aside, these tubes are often far longer than they need to be. In an era when most batteries frequently vented liquid acid or acid condensate, the tubes needed to be long to keep the vapor and liquid off the exterior of the aft fuselage. This was particularly true of the aerobatic models. These days, if you keep your battery in good condition (so that the outside remains dry), or you use a sealed battery, and you don’t have an aerobatic airframe, you can trim the hoses down so that the cuts are close to the skin. This very slightly reduces drag, and tends to clean up the look of the older tubes with their frayed and yellowed edges. Make sure that you retain the proper diagonal cuts. The long edge should be at least one inch longer than the short edge.