[Bryan Wood]

Here's an update to help anybody with a cold RV cabin. While "None" of these ideas were my own, each helped to some degree. The combination of all of these things however has made the plane more comfortable and should really help on higher flights. Thanks to everybody who has posted their ideas on this subject in the past.

Drafts...

The rear baggage bulkhead allows air to come thru the corregated areas from the tail of the plane. By sealing all of the raised areas cold air hitting the back of my neck is virtually gone.

Aileron boots have eliminated almost all of the cold air that comes up thru the stick boots. There is still an almost inperceivable draft thru the stick boots that is most likely coming from the flap pushrods allowing air in still. For all intensive purposes this cold air coming from between the legs is gone. The boots are available for around $20 for a set and are grueling to add after the plane is flying. I highly recommend purchasing and installing these prior to installing the wings. http://my.execpc.com/~erdmannb/construction.html

Weather stripping under the rear canopy skirt along with the bulkhead mentioned above has cut down on all drafts from the rear of the plane.

Lastly for the drafts was the cheap plastic eyeball vents supplied with the kits. After doing all of the above these dumb vents leak cold air even when closed. Well the last time we went to the airport my wife suggested a bath/tub stopper to stick in the front of the vents to seal them. Having no better ideas I bought two 1 1/2' stoppers that are angled allowing for a good seal when stuffed into the vent. With the vents fully open at cruise speed the plugs seal the vents completely and stay put. Ugly, but highly effective for the coldest flights.

With the drafts cured I took advice that George offered some time back. Reversing the flow on the heater muff so that the end of the muff that feeds the hot air to the firewall valve is the side closest to the exhaust port. I don't know how much this helped because it was done along with all of the above, but the RV is pretty warm now. If the heater doesn't keep up on the most frigid days I'll probably take another piece of advice he offered to put some kind of spun stainless material inside the muff to provide more surface area for a better heat transfer.

So how does all of this work? Last Saturday I went up to 15,500' for around an hour. It was relatively warm that day so the temps up there weren't that low. Still, with an outside air temp or 43F and the Koger Sun Shade slid back allowing direct sunlight into the plane the temps were nice even without turning on the heater. A T shirt was plenty and the sweatshirt laying next to me stayed there the entire flight. Seriously, I never needed it! The heater was tried to see how it worked and it was a little weak, but it did get the plane warm enough that I pushed the cable forward and reduced its output.

Having been cold soaked before in this plane were it took hours after landing to stop feeling cold and achy these little efforts have made the plane better.

Blue Skies,

 

[gmcjetpilot]

What goes in must go out

The heater was tried to see how it worked and it was a little weak, but it did get the plane warm enough that I pushed the cable forward and reduced its output.

Nice work and write up. May I suggest the more you seal the cockpit the worse the heater will be. "Conservation of MASS" as the engineers say. With out a dedicated low pressure exit, the heater is pushing into a fixed volume, thus no flow.

The reason the air comes in from the baggage is because the tail is pressurized. That is why the vents all the way in the access panels under the horz stab don't work. Good idea sealing the baggage bulkhead, but I am going to try to make a dedicated exit out the belly. The vent routes air right from the cabin to the outside low pressure area via a SCAT duct to a reverse belly scoop.

Its a major mod and a hole in the belly, albeit a small one (2"). If you are happy than cool, I mean warm. I bet if you did make the mod you would get HOT air out the heat muff. Of course you where at high altitude and there is less engine power, which means less heat.

If you want to make an exhaust change and super-charge the heat muff, put some "Aircraft Exhaust Inc." studs on it. Send it in and they porcupine the area under the heat muff. They spot weld studs onto the pipe. The increase in surface area will do wonders for the temp, but you still have stagnant flow without the exit.

 

[AlexPeterson]

George, you are right in the absolute sense. But, the two keys to good heat in the winter are lots of heat coming in, and seal as many holes as you can. I have not found any diminishing of the volume of air coming in the heaters as I've sealed more and more cold air leaks. There seems to be enough holes remaining for the exiting of air.

For the lots of heat coming in part, there is an optimum flow rate, below which the air is warmer but not enough of it, and above which the air isn't warmed enough.

As I've commented before, the true tests start at about 15F and lower. I was out this winter with an OAT of -25C (-13F), and remained quite comfortable. I've got two heaters, both running parallel.

 

[RyanM]

Alex, do you have pictures of your install? I'm in Fargo ND, and will need as much heat as possible in my 7.

 

[gmcjetpilot]

YOU MADE MY POINT!

George, you are right in the absolute sense. But, the two keys to good heat in the winter are lots of heat coming in, and seal as many holes as you can. I have not found any diminishing of the volume of air coming in the heaters as I've sealed more and more cold air leaks. There seems to be enough holes remaining for the exiting of air.

YES!!!!! THAT IS CORRECT AND MY POINT.

I'm glad you are warm. Dual heat muffs is a good thing. Factory planes have one HUGE heat muff that wrap around all 4 pipes, so I agree that is a good thing, but it does not negate my "theory" or predicted benefits of a air vent exit.

No offense you are overcoming the lack of vent efficiency from not having a good exit air vent with extra air, basically pressurizing the cockpit with hot air.

I agree 100% HEAT IN-TO-THE COCKPIT is part of 1/2 of the equation. RV heat muffs are too small, but you can heat a uninsulated metal hanger if the heater is big enough. I am not saying DUAL HEATERS is not a must in extreme winter weather at altitude for an RV. In fact its probably needed, but it does add weight and cost. What if you could just super size ONE heat muff and add the exit vent and get the same result?

If you added an exit air vent with your dual heat muffs, you might have to turn the heat down if my theory or prediction is correct. Take a moment and read my logic or case for the LOW pressure cold air draft sucker outer.


********************************************
How "Flow-Thru" Ventilation improves heating and cooling


Alex look at it this way. You are shoving air in and it has to get out. So it balloons the canopy and side skirt and so on. When it opens it lets air out, but it also leaks air in. It does not take much super cold air squirting thru a small gap to make you uncomfortable.

NOW THE GOOD PART!!!! IF you have a LOW pressure exit, dedicate "sucker" if you will, you will SUCK the canopy down for example. You NOW CONTROL THE FLOW THRU A CONTROLLED EXIT, NOT A BUNCH OF LEAKS.

Also you will gain speed. Little leaks squirting OUT (say around the canopy) into the free air steam makes "plume drag", slow air mixing with fast air.

Look at it this way WE HAVE NO EXIT. It is poor ventilation design at best relying on leaks willy nilly to get air out. If you are getting good air flow thru your heat muff and have no exit you are pressurizing you cockpit and literally blowing the canopy and seems of the plane open.

There is only one thing to do, Try it. Not flying yet but next winter. Every good vent system I have seen car or plane, Acura, Lexus, Beechcraft have a vent scoop exit. Look at a Baron or Bonanza near the aft fuselage. What you see are exit scoops.

I do have two engineering degrees and the basics of HVAC (heating, ventilation and air conditioning) is good air flow. Just on paper the idea of not providing a vent or exit is against basic rules in the industry. What you have is air coming in and swirling around (stagnant air) with leaks all over. Its moving but its not flowing, from high to low, hot to cold, in a controlled fashion, up legs body face and out. Its inefficient and not only lowers the flow of air but you lose ALL CONTROL. Its OK to have COLD leaks if they GET sucked right back out before hitting your neck.

Can't convince you, and I respect your opinion and point of view. I froze my tail off on many winters in my old RV, but I had ways to deal with it, but they where band aids. The exit vent did not occur to me until recently. I think it would be a plus. Again its not only a VENT its a LOW PRESSURE vacumn sucking all the cold air out and making the cockpit tighter because the fuselage is not ballooning and gaping.

Guess I'll be the first? The definitive test would be do all the sealing and than add the vent. You could fly with the exit vent closed or open. Flight test using several temp gauges could measure temps in cockpit under the same condition with and with the exit vent open.

Of course subjective opinion would be the real test. If the leak that hit your neck was gone because it was sucked down and back and out the vent (in a controlled way), before it hit you in the neck, and the canopy sealed better since gaps where not being blown open, it would be conclusive. Also if the heat from the muff was much hotter and/or volume was greater, than that would be conclusive. One Con of higher air flow thru the heat muff is the air lingers less in the heat muff. There is some relation to flow and heat transfer to it heat up. So either a longer heat muff, more internal heat transfer area (studs I mentioned before) or "throttle" of slow the air flow (with the cabin hot air control).

If these "predicted positive results" happened than my theory would be conclusive.

The EXIT vent is also good for COOLING, since more ventilation means more perceived comfort. The cockpit air vents would also be more effective and flow more air. Think about this. If you took a duct and went right from the panel cold air eye-ball vent, right back out side to the free airstream, no cockpit, what kind of flow volume and velocity do you think you would see in the duct doing 200 mph? A lot. The small pathetic air flow we get is the symptom of a non-existent exit vent. It works both for cooling and heating. All HVAC engineers know this.

Kent Paser (Speed with economy author) found that reducing the leaks in his bubble canopy gained speed. I think you can seal only so much. The canopy will get pushed out from pressurising the cockpit. Per my earlier point when you open the canopy gaps it will leak out but will develop air "LEAK PAIRS" and air will rush in as well.

The mod could be done for less than $50, 5-10 hours and 1 lb, at most.

********************************************

 

[alcladrv]

My flight experience might help with the discussion. Last weekend, westbound at 10,500' over Illinois, the OAT was -30F in my -7A slider with O-360, Vetterman exhaust, and lower right rear baffle output to heater muff. We were reasonably comfortable (60F cabin temps) flying with an unzipped coat on under mostly sunny skies with the standard single heater muff input full open.

I have sealed around the firewall with RTV, lined the firewall, cabin walls and forward fuselage walls with 1/2" super-soundproofing insulation (but not the baggage side walls or aft wall), installed aileron boots made from the arms of a tyvek painter suit (which really stop airflow from coming in around the stick), replaced the black plastic OEM air vents with aluminum air vents from Stein Air (which stop any airflow) and weatherstripped the first 8 or 10" of the canopy side skirts.

There's a cool draft from the baggage are, probably because the aft canopy skirts and doghouse are not weather-stripped. So, I'm part-way down the path to a toasty airplane, but I'm interested in the outflow vent that George describes. How anyone else implemented this idea?

My conclusion is that the standard heater muff installation is adequate with attention given to the areas mentioned above.

Mike

 

[kentb]

George, you could make history here....

There is only one thing to do, Try it. Not flying yet but next winter. Every good vent system I have seen car or plane, Acura, Lexus, Beechcraft have a vent scoop exit. Look at a Baron or Bonanza near the aft fuselage. What you see are exit scoops. .

George, this sounds correct, but I don't want to be the first to cut a hole in my fuse.
I am waiting until after you do it and can report results.

I do have two engineering degrees and the basics of HVAC (heating, ventilation and air conditioning) is good air flow. Just on paper the idea of not providing a vent or exit is against basic rules in the industry. What you have is air coming in and swirling around (stagnant air) with leaks all over. Its moving but its not flowing, from high to low, hot to cold, in a controlled fashion, up legs body face and out. Its inefficient and not only lowers the flow of air but you lose ALL CONTROL. Its OK to have COLD leaks if they GET sucked right back out before hitting you neck.

Guess I'll be the first? The definitive test would be do all the sealing and than add the vent. You could fly with the exit vent closed or open. Flight test using several temp gauges could measure temps in cockpit under the same condition with and with the exit vent open.

You could just measure the vacumn that is pulled on your exit tub. Set the heat and side vents to verious configuration and see what the vacumn is.

Kent - wanting not to be the first.

 

[gmcjetpilot]

Me First, ha ha ha

Kent - wanting not to be the first.

You made me laugh. Good idea, I'll let you know. You are right, I need to pressure survey the fuselage belly and find the absolute min pressure. It will not do any good to put an exit vent in a high pressure area. In fact that would suck, or not suck, which would suck.

However as Alex points out HEAT in is critical. The little tiny single heat muff on ONE pipe with no internal heat transfer fins/studs is part of it. I had a custom heat muff that is longer than the stock ones most buy out of Vans catalog. I also added the studs to my pipe. If you improve the AIR FLOW though the heat muff, you need to improve the capacity or efficiency of the muff it self. SO for those "going for it" consider a MUFF upgrade.

Worst case like Alex, dual heat muffs is an option. I see no down side but a little weight, cost, may be another hole in the firewall. However I do think the exit vent regardless is a win win, not withstanding a small vent hole in the belly, that chicken kentb is afraid of.

 

[AlexPeterson]

George,

Whew, I just wish you would take a stand, you know, have an opinion!

There is a -4 here with exactly what you described, a reverse naca scoop on the belly aft of the cockpit. No perceived difference in heat or comfort. I suspect that there would be a lot of trial and error (and skin patches!) involved with trying to find the right area, if one exists. I believe there is another -4 in SD with one as well, don't know the results, but I believe the local -4 put his in the same place.

Anyway, here are my thoughts. I want the cabin slightly pressurized, as that keeps the cold air from wanting to come in so much. Even with both heaters supplying a lot of air volume, I can still feel some cold air coming in from areas which are obviously even higher pressure. If I put a hypothetical turbo pump in the aft cabin (the idealized vent), it will increase the amount of air coming in via the heaters and all the various leaks. I don't see how a great vent will selectively bring in more warm air but not bring in more cold air. This is the dilemma.

BTW, I have: sealed the baggage bulkhead triangles, flap rod holes, aft canopy (slider) skirt, forward canopy skirts, insulated firewall aft to baggage area, sealed the wing spar boxes, aileron pushrod boots, sealed the crotch strap holes (important one!). About the only noticable leak remaining is the canopy slider center rail, which I haven't bothered with yet, but should. Interestingly, it blows on the pilot but not the px neck! I guess I could solve that by flying from the right seat.

 

[gmcjetpilot]

I agree and you make me laugh

George,

Whew, I just wish you would take a stand, you know, have an opinion!

There is a -4 here with exactly what you described, a reverse naca scoop on the belly aft of the cockpit. No perceived difference in heat or comfort. I suspect that there would be a lot of trial and error (and skin patches!) involved with trying to find the right area, if one exists. I believe there is another -4 in SD with one as well, don't know the results, but I believe the local -4 put his in the same place.

Anyway, here are my thoughts. I want the cabin slightly pressurized, as that keeps the cold air from wanting to come in so much. Even with both heaters supplying a lot of air volume, I can still feel some cold air coming in from areas which are obviously even higher pressure. If I put a hypothetical turbo pump in the aft cabin (the idealized vent), it will increase the amount of air coming in via the heaters and all the various leaks. I don't see how a great vent will selectively bring in more warm air but not bring in more cold air. This is the dilemma.

BTW, I have: sealed the baggage bulkhead triangles, flap rod holes, aft canopy (slider) skirt, forward canopy skirts, insulated firewall aft to baggage area, sealed the wing spar boxes, aileron pushrod boots, sealed the crotch strap holes (important one!). About the only noticable leak remaining is the canopy slider center rail, which I haven't bothered with yet, but should. Interestingly, it blows on the pilot but not the px neck! I guess I could solve that by flying from the right seat.

I hear you, but this proves nothing about the EXIT AIR VENT. Also you could be right, it's yet to be proven, but my Spidy senses are tingling. It takes proper engineering, design and sizing to make it an efficient working vent. Of course the rest of the "system" has to work together for there to be real benifit.

First it would be interesting to see where the RV-4 has the scoop is. Your word illustration of belly patches is a scary thing to think about, but that is where an engineering approach comes in. There are ways to survey the fuselage for the best place for the scoop. If you wanted a LOW impact method you could put the scoop way back in the elevator inspection cover. It would just take $40 of scat tube. If it did not work you could just take the scat out and remake the inspection covers, done.

There are areas of very high pressure on the belly as well as low pressure. Until you move back at least 2 feet or so from the trailing edge. There was an EAA sport aviation article that modeled a RV-6A with CFD (computational fluid dynamics) showing where the high and low pressure was on the fuselage. I need to get that.

Also a NACA scoop (diverging flush scoop) is terrible, awful, terrible, awful, sad pathetic for exit air. The plan old scoop scoop reverse is all you need and will develop more suck. If you want a flush scoop you could use a parallel sided square ramp. There are NACA many technical reports on scoops and its all pretty much old news, researched to death, but the so called "NACA scoop" is not ideal or needed for an exit scoop. In fact its a waste of time; it just looks cool. Most NACA scoops by the way are poor imitations and resemble nothing to what NACA developed in the 40's. Our stock RV side NACA scoops? ha ha ha. Par for the course, not even close. There are not of the radi and lips that where designed and tested way back when. They work but so does a HOLE.

Alex I agree with you 100%, everything you said, dual heat muffs and sealing is job #1. RV's needs larger heat muffs than what VAN sells, no doubt. Again a Cessna has a heat muff the size of a large car muffler and it wraps around all pipes. A Cessna has insulation top, side and bottom. So its no surprise that RV's can be cold with a bare metal interior and bubble canopy. You can just look at that little one pipe 2" heat muff that has to feed the cabin and carb heat in some cases and say, we're going to have a problem.

I can also agree 100% and still think the exit air vent is compatible with your philosophy for heating and ventilation. They can work together. The only diff is you have done it, you know it works to your satisfaction. All I have is words, admittedly. However I think venting could be an improvement. The vent does not have to flow more than what coming in, just a little.

I'll go through the same process of modification and improvements, with all the known must haves, like push rod boots, that you did, before adding the vent. If I where in your shoes and happy with the heat avaiable, I WOULD ABSOLUTELY stop and not add the vent.

Pos pressure is good, but not too much. The KEY is FLOW and CONTROL. Forcing the air to come in at the front of the cockpit, flow up body from feet to neck, head and shoulders than out the back of the plane.

Any way appreciate you sense of humor. I am just frustrated I am not flying yet to try it out.

I would not condemn the idea of an EXIT AIR VENT based on your RV-4 friend. YOU STILL HAVE TO GET AN EFFICIENT HEAT SOURCE AND SEAL THE PLANE. It does not do any good to have exit air venting when you have lots of leaks and no heat coming in.

Here is a RV-8'er that did something like I am thinking about:
http://bidasst.bizland.com/heatvent.htm
(His conclusion is it helped drafts for the rear passenger. I think it would promote more hot air coming from the front to flow to the rear. The vent idea probable has more benefit than for side by side. Look a camp fire can keep you warm and reasonably comfortable even though your back side is hanging out over a log, so a vent may not be a MUST, but a possible option. As I said about the RV-4 above, his choice of a NACA scoop may not be the best one.)

 

[kentb]

If control is important...

Then the exit tub/vent should have a valve so that the amount of air from the heater or vents should be balanced to the amount going out the exit tub.
Each time you adjust the input controls, you should change the output control too.

Kent

 

[kentb]

Ok, I did a small mod...

I place close-cell foam in the baggage bulkhead triangles and took to the sky's. Sure enough the cold draft that I had felt along the outside of each seat was cut down allot.
But the air coming up through the stick-boot increased about 3 fold.

Duh.. This air must be coming under the floor next to the elevator push tube. Now I need to make a seal for that tube (I already have aileron push tube seals.

Will it ever end.

I am thinking that I must have negative pressure along the outside area of the canopy and that until this is sealed better I will continue to draw air up from the tail.

I was not running the heat or vents as the temp were just right in the cockpit, just a little drafty.

Kent

 

[dan]

I am thinking that I must have negative pressure along the outside area of the canopy and that until this is sealed better I will continue to draw air up from the tail.

Fix the canopy leaks and things get a lot more comfortable. http://www.mcmaster.com P/N 1120A712

 

[TravisHamblen]

To block or not to block...

I have got my cabin fairly draft free, the biggest help was sealing up the aft skirt on the slider canopy. The MAJOR leak I have is the plastic eyeball air vents. I can't tell if they are leaking around the back where the hose connects or just through the closed eyeball itself. Anyway, my dilemna is that here in frigid Minnesota, you don't want any leaks or it will get VERY cold. I fundamentally hesitate to totally block off the air vent in case I ever needed fresh air in an emergency. If it is internally sealed, then if needed in an emergency I would not be able to get fresh air. I guess if things got really bad I could open the slider about 1 inch and see what happens. I just don't know if it is dangerous to block all your air off. I've though about making a cap to go over the aoulet, but I don't know how effective this will be if it is leaking from behind due to pressure.

Any advice is appreciated. I have also thought about putting an insert into the naca duct at the point where it becomes a circle and you attach the hose. The other alternative is to put in the blocking insert and to put a small hole in it, so only a small amount of air can get through. Maybe start at a pin hole size and increase until I can get a little air through the vent, but so that it doesn't leak due to the high flow.

What do you guys think??

Travis

 

[AlexPeterson]

Travis, someone very recently just posted something on these forums that is exactly what you need - they were some sort of cap that fit nicely on the Van's vents. I didn't find it with a quick search, but it couldn't have been more than a week or two ago. Maybe it was for something else - others will have better memories than I.

I sealed mine with little "wiper" sheets of rubber with small aluminum backing plates right on the vent "throttle". I don't have pictures, but could get some.

 

[az_gila]

Great idea

.......
installed aileron boots made from the arms of a tyvek painter suit (which really stop airflow from coming in around the stick),
......
Mike

That's a great idea, nice flexible airtight material and relatively cheap...

I made this a separate post so this idea didn't get lost in the text....

gil A

 

[TravisHamblen]

Travis, someone very recently just posted something on these forums that is exactly what you need - they were some sort of cap that fit nicely on the Van's vents. I didn't find it with a quick search, but it couldn't have been more than a week or two ago. Maybe it was for something else - others will have better memories than I.

I sealed mine with little "wiper" sheets of rubber with small aluminum backing plates right on the vent "throttle". I don't have pictures, but could get some.

I guess the easy solution to testing if they can be sealed from the front without forcing air out the back is to just put some duct tape over the front of the two vents and see if it seals up. If that works then I could just make some type of cover for them that looks a little nicer than duct tape.

 

[Mark Burns]

Here's how I sealed the vents.

That's a great idea, nice flexible airtight material and relatively cheap...

I made this a separate post so this idea didn't get lost in the text....

gil A

Here's what I did.

I haven't flown with these yet but you can't blow through them!
I used 1/8" sticky back closed cell foam. I had it left over from when I used to repair camera's as a hobby

This is a take on how butterfly valves are sealed in the industrial world.

Mark