Which way does it go.
rv6ejguy said:
Once the air has entered the cabin through the vents and slowed down in the cabin/ plenum area to almost nil, you won't be able to recover any significant velocity with aft facing vents. The energy is lost over that distance. Most 6s have so many leaks in the aft baggage bay area, canopy area and floorpan/ stick boot areas, the air finds its way out through the path of least resistance.
Well we disagree. You do live in a Canada, a cool/cold climate. A dedicated exit air vent helps heating as well. It is not an energy equation, it's a stagnant air mass thing, i.e., conservation of mass (air flow).
AIR just has NO WHERE TO GO. The whole cabin is surrounded by this high pressure air. The air leaking in does add air, but it really just fights the air you want from the cabin fresh air vents and cabin heat vent. It is not flowing (ventilation) it is just swirling between you legs and elbow and neck, stagnant and not a good design.
If you're going near 200 MPH you should have plenty of air. Getting about as much air as blowing on your hand from your vent, means you have an issue. There is room for improvement.
The leaks you mention are mostly INTO the cabin NOT out, from the canopy, aileron push rod and flap push rod areas. Random drafts IMHO are not useful in cooling your face and body. They also keep fresh air from entering through the panel vents, where you want it, and flowing or ventilating past your body. The little air that can exit out the baggage and tail is a hit-miss random affair. Are you sure more air is not entering from the tail area?
Some air does leak OUT of the canopy, but do you want that? Air squirting out into a fast airstream is "plume drag". Control of exit air will reduce airframe drag by reducing these random leaks, since the air now has a place to go. Do you want air squirting out the side of your cowling?
Taking air into the cabin for crew comfort is going to cost you some drag. It is like driving your car with the window down or AC on. There's no free lunch, but we can control and engineer the cabin inlet and exit vents, while trying to eliminate the unwanted leaks.
If you measure the cabin / cockpit pressure it's lower than ambient, and therefore air leaks in. Because there's a cockpit "vacuum", lower pressure than ambient, one might assume it means air is leaving, creating the lower pressure. That's not so. It's a venturi effect of airflow over the curved shape of the fuselage, canopy and that the cabin is sitting on top of the wing, a low pressure area.
It's just stagnant air at lower pressure.
There's not a lot of flow in/out of the cabin naturally, since there's little air exiting. Yes there's leaks and air swirling around but its not ventilation in the true sense. Ventilation is comfort not random stagnant air swirling around under a bubble, which is really more annoying then comforting. CONTROLLED ventilation by design, not random leaks.
We know ENGINE COOLING is the same. The EXIT air out of the cowling is critical to engine cooling; it's like a valve. So you have all this air coming in, where does the air get out? Well a little gets out some how? No one is sure. The theory is air exits out past the baggage and tail. Where? Not a well engineered exit. It's haphazard, squiring from little gaps and around the canopy, tail wheel, I guess? Going 200 mph you should get more air than a slight breeze.
The intake scoop Van uses, the NACA scoop is another area that could be better. NACA scoop used in the kit is not sophisticated or located in the best place. The stock scoop copies a NACA scoop design, but not exactly. NACA scoops have a radius around the edge and lip, not just flat sheet metal. The fuselage sides, is not a great area, especially due to corkscrew air flow. To be fair it is hard to find a good inlet location as much as an exit location. I like the ones that pop out (round or square), which may be draggy when out, but close flush and seal when you don't need them.
Lack of exit air is especially troublesome with heat muff HOT AIR. The ingress of all the cold leaks you mention, canopy, aileron boots, keeps the hot air getting into the cabin, again due to stagnant flow, but now you have cold air drafts, which are even more noticeable. Leaks are bad for both heating & cooling.
A dedicated exit air vent design should be sealing the canopy, aileron tube and flap push rod areas from random air ingestion, which spoils the flow through ventilation. However the vent may help SUCK some of the (cold) leaks right out in the winter, before they blow on your neck, while promoting more of the warm air to enter the cockpit. Other issues to heating are heat muff capacity and insulation of cockpit and canopy. That's another topic.
YOU WANT AIR BLOWING ON YOU FACE AND TORSO THAT YOU CAN CONTROL, not air between you legs and on your elbow and back of neck.
To get air out the cabin you need to place an exit where the pressure around the fuselage is lower, which is aft of the wing by several feet. That's not going to happen near or forward of the wing, at least easily. Look at high end factory planes. They have reverse scoops (exit scoops) well aft on the cabin. My car has AIR exits in the tail light. Why? That's where the low pressure area is on the car. Also at 60mph my car has more vent air than my RV at 190 mph.
One of the three rules of comfort is ventilation or FLOW (temp and humidity are the other two). Air blowing around randomly is not a comfort like fresh air flowing past is. In the winter to reduce the "breeze" sensation (convective cooling) you can reduce the exit, summer increase for a strong breeze.
SO WHERE DOES THE AIR EXIT?
The concept of air exits through leaks in the baggage area is true to some extent, but for comfort you need flow, not random swirls in the floor-pan or baggage area. The RV is not that full of holes and gaps. There are gaps like the elevator bell crank in the tail, but that is really another area where air can still enter under pressure. The tail fairing can act as an inlet scoop. I would not be surprised if some air comes back into the cabin thru the aft baggage (corrugated) bulkhead panel.
Many RV'ers find they don't get "good" air flow (where they want it) into their cabin. My theory is an exit vent could improve fresh air ventilation and heating. There are other areas to improve besides exit air, like the unwanted leaks, better inlet scoops and insulation. Any mod can add build time, cost and weight. Some want to keep it stock, that's fine with me. IT Works. There is some fun engineering behind the principles of aircraft ventilation worth looking into.
Conservation of mass, what goes in must come out. You increase the EXIT, you increase the inlet flow.
The cabin is in a cocoon of air which traps all the drafts and incoming air, it just swirls around not in and out. That is why in the summer you are hot and winter the cabin heat is not as efficient or effective in providing comfort.