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Whistling Wemacs


Getting fresh air into your airplane is important. There’s so much high pressure air outside the aircraft, and the cabin or cockpit being low pressure you’d think this task sounds pretty simple. Somehow though some builders don’t pay the right amount of attention to this. A LongEZ I’ve been flying had some pretty fancy air ventilation valves, often referred to as “Wemac” or “eyeball” vents. They were great quality vents but unfortunately not the right design for the application. The source for these cockpit vents was a slot in the tip of the strake just outboard of where it meets the fuselage, a very common location. That’s some pretty high pressure air. Aft of those slots there wasn’t any space to allow that high pressure air to decelerate and slow down. The net effect was the Wemac valve acted like a jet engine nozzle, converging its opening and accelerating that already fast moving air.

It was so pronounced that when talking to ATC with the vents open it was almost impossible to transmit because all that came over the radio was a screeching air noise. In the pattern taking to the Tower I’d have to close the valves. In the Texas summertime heat that makes for a toasty cockpit. I did a little shopping and found a version of the Wemac valve that had about double the diameter opening than the ones the builder installed. It also had a constant diameter opening so there’d be no convergent nozzle action. The next flight was a completely changed environment. I can now fly with fresh air flowing into the cockpit and not interference with the radios or intercom.


When air accelerates it makes turbulence which makes noise. The roar of a jet engine is the high velocity air exiting the nozzle. Make sure your fresh air vents aren’t acting like jet engine nozzles and you’ll enjoy a much quieter and more comfortable cockpit.


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