Cabin air outlet?

[lelievre12]

I am wondering what folks do regarding letting cabin air back out.  I have arranged my inlets just fine but cant see any info in the build about how that air is getting out again!

Pics/hints appreciated!

[A Bruce Hughes]

See if you can find someone with Debbie's booklet.   I am away from home so cannot give you the way to order one.    It may not be available now as it was written years ago.   I think her name was Iwatate.   The booklet has several neat things to do.

[Marc Zeitlin]

3 hours ago, lelievre12 said:

I am wondering what folks do regarding letting cabin air back out.  I have arranged my inlets just fine but cant see any info in the build about how that air is getting out again!

Nick Ugolini is the only person that claims to have sealed their cabin up so tight that they actually need to provide a dedicated air outlet. Given the leakiness of most of these planes, it's rare that a dedicated outlet would be required. I can tell you that on my plane, I've spend a lot of time sealing my nose gear area, canard perimeter, elevator offsets through the fuselage sides, fuselage top perimeter, and canopy seal. It's pretty tight, with no noticeable air leaks in any of those places. My engine muff heater will keep my feet warm and the cabin at 30F - 40F above ambient. My two large diameter air vents:

https://www.aircraftspruce.com/catalog/appages/largealumavblack.php

will just about blow me into the back seat when fully open, taking in air from the NACAs on the fuselage sides.

But I have no dedicated outlet(s), and I can safely say that I have NFC how the air gets out, but it obviously does. Maybe through the two holes in the top of the landing gear bulkhead area, even though they have loosely fitted covers? Maybe through the electrical conduit channels into the lower cowl? Maybe through the rear curvature of the canopy, above and behind my head, so I wouldn't know if air was flowing in that area? No idea. Almost none of the planes I work on have dedicated outlets, and no-one complains that they can't get air to flow in through their cooling vent.

Now, that doesn't mean that you shouldn't install a dedicated vent, and I've seen a few in the turtleback, next to or aft of the passenger's head area. If you have one and don't need it, it doesn't hurt, but if you don't have one and need it (unlikely, but possible) you'll want one.

My $0.02.

[lelievre12]

The reason I ask is that the cabin doors are big and any pressure  increase inside will lead to a huge burst load. Even 1/4psi over a 36” x 20” door = 180 lb pressing out.  I fly a P210 with a 3.35psi pressure diff and the door locks are super heavy to cope. Given that quite a few canards have had doors fly open in flight I’m going to positively manage cabin pressure to ensure it doesn’t increase at all. Even a slight increase will cause to hiss and this is the door flexing outwards until pressure can escape. Noisy and far from ideal. 

[Voidhawk9]

20 hours ago, lelievre12 said:

Given that quite a few canards have had doors fly open in flight

Due to aerodynamic forces and not being properly latched, AFAIK.

If cabin air is escaping around doors, etc, it may be creating some excess drag (tripping / thickening boundary layer). Is it and of so how much? Who knows, maybe not a measurable amount. But some folks hunt Drag for sport, so...

[lelievre12]

This is the kind of thing I am thinking of.  The rubber 'flapper valves' let air out when pressure builds inside the cabin.

Not as big as this air pressure vent from an F150 but same principle.

 

Edited November 28, 2020 by lelievre12

[macleodm3]

There is a 1” diameter hole at each end of the spar... antenna wires and wingtip light wires exit out these holes.  These holes are where cabin air freely exits.  The holes are protected from weather, so a flapper typically isn’t required.  

[lelievre12]

Sheesh. Of course!  So the cabin air ends up exiting via the spar.  I should have noticed this when I was looking at my airframe. 

 

Thanks for your help Andrew!

[Marc Zeitlin]

3 hours ago, macleodm3 said:

There is a 1” diameter hole at each end of the spar... antenna wires and wingtip light wires exit out these holes.  These holes are where cabin air freely exits.  The holes are protected from weather, so a flapper typically isn’t required.  

This is certainly one place where air might exit (that I forgot to mention), but in my case, I have covers over the main spar openings aft of the seats in the cabin, with only very small openings around some wiring, so while this might be part of where the air is going, it's hard for me to believe that all the air coming in through my vents is going out through a couple of 1/4" annuli around some wiring bundles. If you don't have a cover over the spar, then yes - this could be a major source of exit air.

[A Bruce Hughes]

Andrew and others:   "there is a 1 inch diameter hole" is NOT true on my Longeze.

I had difficulty in passing wiring to the wings.    The entire structure on my project was built by a guy in southern California so he may have deviated from the plans or ................  

I think I am not mistaken,at least on MY aircraft.   I very carefully searched for any opening through which I could pass wires.    Yes, there is a hole BUT there ALSO is a baffle inside the main spar.    Is my airplane unique?    I don't think so.   The only exits that I know of would be where the wires for the rudder control go through the firewall, tiny spaces for many 12 vdc wires, many sensor lines, the fuel line, the primer line, the "hell hole" access door underneath, and the gear spaces.    Altogether that is a LOT.