Cockpit Venilation Exit Air

Has anyone given much though to cockpit ventilation air exit in an RV-6. I would like to accellerate the cockpit ventilation air back up to speed on its' way out. I have seen someone with a RV-4 who has a relief facing aft around the tailwheel spring with a small fairing riveted over the opening leaving a smooth air exit facing aft. My tailwheel spring reciever is already in the fuse and I would prefer not to cut any skin around that part, so I will not likely copy him. Looks like I will have exit air oppertunity near the aft end of the empennage fairing, but left unmodified, it looks like the air will exit in a haphazard way.

Has anyone engineered a solution for ventilation air exit in an RV-6? Any lessons learned or before and after results from a modification post first flight would be appreciated.

Thanks and Regards,
Dale

RV-6 finishing kit

No but ...

I have not attempted to design an exit air path into the ventillation system of my RV-6A. I have casually tested the ability to slow the airplane down by opening the cockpit vents and the results were positive and repeatable.

I was unable to close the original plastic "eyeball" vent valves and have them stay in the completely closed position. I bought the expensive metal vent valves and this corrected that problem.

There is an opening in the rear canopy skirt around the center canopy guide rail but the flow appears to be inward rather than outward. The pressure differential on the sliding canopy with the aircraft in motion is such that the canopy cannot be opened even after landing until the speed is reduced to at least a fast taxi. I have developed my own shroud and plug fix for the leak problem after reading of such a fix developed by Tracy Saylor. I have not seen his but mine was full of implementation/operation problems. I think mine is finally perfected but relative to your question this naturally occurring opening aft of the inlets does not appear to be a suitable location for the vent you want to add.

In my old 1981 Archer II the exit vent was on the bottom of the fuselage and I believe it was located under the baggage compartment floor. I believe Piper would have done some testing to determine that this was an effective exit vent air location on that particular fuselage design and it may have similar effectiveness on an RV-6. The configuration was a round hole of around 3 or 4 inches in diameter, covered with screen wire with a deflector sticking down into the airstream on the front part of the hole. The deflector that was shaped something like a partially flattened quarter sphere.

I would like to know your results if you proceed with your work and test the results.

Bob Axsom

My rv8 has two holes in skirt area, seems to help.

Air vents

This is a bit premature, but Gamut Services is developing a vent system that will retrofit on already-flying RV's. This is being done because of the "tightness" of the CCS system but should help on all well sealed aircraft. Not giving away any details yet, it will be a 15 minute bolt on job (plus painting time, which can be done before installation) and it will cost under $25.00. NO airframe or canopy modifications! Flight testing should be next week with introduction this month. If flight testing goes as good as we hope, this should solve your problem. Check www.gamutservices.biz in a couple of weeks.

Bob Kelly

whifof100ll said:

Has anyone given much though to cockpit ventilation air exit in an RV-6. I would like to accellerate the cockpit ventilation air back up to speed on its' way out. I have seen someone with a RV-4 who has a relief facing aft around the tailwheel spring with a small fairing riveted over the opening leaving a smooth air exit facing aft. My tailwheel spring reciever is already in the fuse and I would prefer not to cut any skin around that part, so I will not likely copy him. Looks like I will have exit air oppertunity near the aft end of the empennage fairing, but left unmodified, it looks like the air will exit in a haphazard way.

Has anyone engineered a solution for ventilation air exit in an RV-6? Any lessons learned or before and after results from a modification post first flight would be appreciated.

Thanks and Regards,
Dale

RV-6 finishing kit

Click to expand...

Dale:

After flying my RV-6 over 1,932 hours, I have to say that Van did an excellent job with the vent and NO modifications are necessary to the vent system. There is LOTS of air flow. The exit is past the baggage compartment between the "corrigation" of the close out, and out past the horizontal where the elevators are. The air is so DIRTY back there that cleaning it up is not worth the effort.

Gary

What goes in must go out?

RV6_flyer said:

After flying my RV-6 over 1,932 hours, I have to say that Van did an excellent job with the vent and NO modifications are necessary to the vent system. There is LOTS of air flow. The exit is past the baggage compartment between the "corrigation" of the close out, and out past the horizontal where the elevators are. The air is so DIRTY back there that cleaning it up is not worth the effort. Gary

Click to expand...

I agree Van did a good job, but I think modifications are possible to improve the ventilation. I agree with your analysis of where the exit air goes, kind of. However people do have vent problems all across the board, RV-4,6/7/8/9. I think there is room for improvement in my opinion.

Lets call this a fact, if air enters a volumn (the cabin) with no exit, the air will not go in very well. You can keep increasing the inlet size, but if the exit flow is not there, you will not get ventilation air or even heat in. This is the theory called conservation of mass (air flow).

Some RV's are tighter than others. In fact as was mention you can get positive airflow into the cabin from other areas, like the aft canopy. This incoming air fights the incoming air from the fresh air or hot air vents.


What others do
Other planes like the Bonanza have reverse cabin air exit scoops on the top of the fuselage near the tail. One of my cars proudly shows in the owner manual, the rear vents near the tail light to promote the flow through ventilation. So the idea of having a dedicated vent seems sound.

The problem is there is positive pressure all over the plane, even the tail. A classic CFD (computational fluid dynamics) pressure plot of a RV-6 I saw years ago in the EAA Mag shows there is a low pressure area on the belly several feet behind the wing trailing edge. Near or at the wing trailing edge on the belly is very higher pressure. A vent in this area would be counter productive.

The issue a poor exit vent location can either cause air to enter or just be ineffective in improving air flow. It is not a simple problem. People try to do the min and expect great results.


So what kind of vent and where?
I suggest a belly scoop aft of the wing a few feet. Some put vents way aft under the horizontal tail, louvers in the inspection plates. I think that area has pressure problems, making it less than ideal.

The type of scoop is either a plan old protruding reverse scoop (low profile is fine). You could make it a flush scoop with parallel walls with a 10-30 degree ramp. Resist the urge to use a NACA scoop, called a diverging scoop, like the inlet Van provides. It's just not as good for an exit. Both the protruding and the flush scoop will cause a small about of drag, but the flush has more eye appeal. However the protruding scoop will be a conversation piece.

Aerodynamic guys chime in hear, I missed that day of class, but pretty sure about scoop type for exits. A protruding reverse "hood" scoop of course is easy and efficient. That is what Bonanzas do. The flush scoop (parallel wall) will still give a little drag and may not be as efficient as the protruding. It also requires a bigger cut out in the belly than the protruding type which just requires may be a 2 inch hole, verses a larger square or rectangular cutout. You need to install a screen in the exit vent somewhere to keep bugs from getting in. Here is one louver idea and a small vent just for conversation. You can go with two smaller vents verses one large exit vent. You will likely have to make your own. The internal flush ramp exit can be made of sheet metal. vent; vent small; clam shell
The elevator control rod will be in the way of a belly scoop but that could be worked around. The SCAT tube will not be that long, since the baggage bulkhead is fairly far aft of the wing already, so the vent can go near by. Not sure it needs to be on the center line. Two small vents off center line may work as well.


Duct it or loose it
How does the air get from the cabin to the vent. Just hoping the air gets through the corrugation in the baggage compartment to the exit scoop is marginal. Even if you punch some more holes or louvers in bulkhead the air is going to have to fight all the other leaks to get to the exit.

It's a good idea to add a dedicated SCAT hose from the cabin bulkhead to the exit vent. The tail is not necessarily a vacume. In fact sealing the bulkhead (other than the ducted vent to the scoop) may be a plus. Some air comes forward from the tail into the cabin.

The are lots of leaks into the tail and that compete with the cabin air trying to exit. If you just hope the cabin air makes out the exit scoop you may find little improvement. I think the SCAT tube is critical to the design of an exit vent. The are "holes" in the and around the tail which are like positive pressue scoops not exits. These leaks in the tail will cause reduced flow if you don't use the duct.


Questionable locations?
I don't think the louvers in the inspection plates under the horizontal tail, aft part of the fuselage are effective. First you can't really duct them (too long). Second the tail area has high pressure areas. All this diminishes the exit effectiveness. This idea might be fine but some info leads me to think not. It was my first choice but now feel the belly scoop aft of the baggage area is best.

The vent needs to be at a lower (much) pressure than the cabin to work well. The problem is the cabin tends to be at a much lower pressure with all the air going around it. That is why you have air leaks into the cabin from the aft canopy blowing on your neck or your passengers neck. A super efficient exit scoop will suck that cold air right back out.

So in theory a GOOD exit vent with a dedicated duct will improve air flow over nothing. This will also believe it or not improve heating, since the heat now can get in as well, and any cold air leaks get sucked out as I mentioned.


Good Luck
If you want to research further, find that EAA Sport Aviation article with the pressure plot of the RV-6 I mentioned. They sell a CD with ALL the back issues of the magazine for less than $100. The EAA library can copy it and send it to you for a small fee, but only in black and white, which looses the color contours of the pressure plots. The article was within the last 10 years? If you have a copy scan it and send me a copy, I lost the issue with that pressure plot. Keep in mind I have read about this and helped a friend, but not tried this my self. I do plan on trying a small belly scoop. However I would love to hear from others that got good results with other methods.

Exit air vents




Here is what I was thinking. All will cause some drag but over all it may be better.

Flush scoops will have some "air plume" that will mix with the free air stream and cause drag. The one with the small scoop will cause some small drag but it may be a wash since it directs the air more in-line with the free airstream.

In fact overall drag may go down. If you have a slider you know air can leak all around the canopy. Any air flow in or out will upset the airflow outside. This kind of drag, hidden air flow current interaction, goes under different names. The bottom line by controlling how the air gets out the cabin, where you intentionally are sucking it in, may be an overall benifit.


I personally have no doubt that you will improve (increase) the airflow in the cabin, both cool and hot air with better or dedicated exit air outlets. This is just my idea based on what I have read. I have not tried it to date. One 2" duct may be plenty. Not sure of the optimal distance from wing trailing edge to located scoop on belly, but see the eaa article I Ref. above.

How about

Should the exit hole on the outside of the bottom skin be a smaller diameter to excellerate the airflow?
I am thinking that a fiberglass S shaped tube with the inside part being 2" to connect the scat tube to and the portion that stick through the bottom skin tapering into a 1" tube pointed aft. This should excellerate the air flow.
This could almost be a jet boost when you wanted to go faster you would open up your air vents and turn on the cabin heat.

Just kidding about the jet part, but how are we going to measure if anything is being improved by the verious solutions?

Kent

RV 10 Backseat Vents

I am going to install the RV 10 vents into my -7A. I will put (2) in the front where Dan and others have incorporated their access panels and put (2) more reverse in the back to let the air move through the cabin. Building a tip up in Texas and trying to avoid some ground heat. I am a ways from getting to this part of the project but some of my friends will be incorporating this idea into their -7's.

Bruce Pauley
VAF #582
Empannage
Kitlog
177 WD (Res)

There is an RV-8 flying around this area with vertical louvers pressed into the aft inspection panels. I have not asked the owner/builder the reason but I suspect it is to let air out.

Thanks

George,

I really appreciate the posts. You have given me really some good insight and ideas. I will look for the EAA article you reference. I'm really interested since I want to spend extra effort to try to make my 6 fast. If anyone knows month and year, please let me know.

I suppose one way to approach this would be to install the exit vents after she is flying. I was thinking about testing for places that don't see alot of air over the skin as a good exit point. Perhaps putting small drops of dirty oil on the belly (the planes, not mine ) and looking for where the oil does not run. That way I would be introducing exit air at a point where it would not interfere with fast moving air over the skin and cause drag. Am I thinking about this correctly? I know low pressure is where you want it for max differential pressure and max ventilation, but I am primarily interested in low drag and adequate ventilation.


Regards,
Dale

Give it a try later

whifof100ll said:

George,
I really appreciate the posts. You have given me really some good insight and ideas. I will look for the EAA article you reference. I'm really interested since I want to spend extra effort to try to make my 6 fast. If anyone knows month and year, please let me know. Regards, Dale

Click to expand...

Sure, give the EAA librarian a call and have her search RV-6, RV-6A, CFD, computational fluid dynamics. They will send you a copy for a few bucks.

The good news about making a RV fast is that it's not hard to do. If you are worried about drag, I doubt exit air will be a significant drag, if even measurable.

Flow and rate of air out the cabin air exit vents is small comparied to the free airstream, drag will be small. Like I said it's better to have controlled exit air by design than squirting out in all directions from under the canopy, around the wing root and around any holes or gaps in the tail area.

Also for "Racing" you can always speed tape over the vents for the event or race. I think the stock NACA vents on the sides of the fuselage near the firewall are a bigger drag, even when not in use. Again I speed tape the heck out of my plane all over when I raced it.

Clearly its a mod you can do later. Also some find the vent with no exit is OK for them. I do think it's adiquate to marginal depending on part of country you live. You can try the louvers in the elevator control inspection plates and see if that does anything. I think it's a wash, but you can always try it. If it does not work, just make some new inspection plates. Cutting a 1" or 2" hole in the belly makes some sweat. There's no doubt in my mind a properly designed cabin air exit vent will enhance the cabin airflow.

Ideas about "how to" design a scoop (exit air) can be found in aerodynamic books and old NACA reports. The trick is to size it and locate it properly. "Tricks" like lips and ramps can increase the flow and even reduce drag slightly. I like the KISS method. Less parts and work to go best results. Like I said the drag will be small and you do want the air to exit from the belly / fuselage as shallow as possible. A flush design is probably best overall for weight, drag, performance and looks. You can make some external scoop that will suck the dirt off the floor.

whifof100ll said:

Has anyone given much though to cockpit ventilation air exit in an RV-6. I would like to accellerate the cockpit ventilation air back up to speed on its' way out.

Click to expand...

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.

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.

Click to expand...

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.