db8

db8

Well Known Member
I am building a RV-8 and have read some great threads on heating and cooling of the RVs here. I am at the point of making changes/adjustments for this modification, and based on the previous posts concerning this, there are some smart and knowledgeable people out there, so I am hoping to tap your thoughts.

I have made five 2" hole cuts (with perforated .032 covering the holes) in the rear baggage area of the 8 (one on the lower horizontal shelf area -- viewing holes also, two on the vertical part of the shelf area and two in the removeable bulkhead. These small holes consist of about 60% see throught vs 100% with an unobstructed 2" hole, just to give you an idea. On the 8, for those of you who are not familiar, there are some wedge shaped areas on the sides of the rear seat that air can flow relatively freely to the back storage area. I put these five holes in to allow the air to continue to flow to the rear of the airplane (based on how important that is in the previous threads on this subjust). That work has been done.

Now the big question is how to let the air flow out, to keep the heat or cool outside air "flowing" through the cockpit to stay warmer or cooler. I have decided to make this outflow rate variable, and here are my thoughts. Please post your thoughts, positive and negative (I have thick skin). I just want to give it the best shot with multiple heads to see what might work best.

Here are my thoughts: I made an access panel that is 5 1/2" x 4" in size that is 30" forward of the rear, aft, last small bulkhead. I did this to access the rear area easier (empenage bolds, static syatem, etc). I can put a hinge on the forward part of the panel just inside of the .032 that holds the access panel on (flange). Then I would put a "U" shaped aluminum piece that would drop down into the airstream to "release" air outside to the rear. I will use a MAC motor to be able to change the amount the door is opened or closed. The size of this "U" can be 2 2/4" x 4" (are open to the inside of the flange). It would kind of be like a very very small door compared to the one behind the scoop on a P-51. For the airline guys (or anybody else), it would be a little like the tailskid that comes out when the gear is down (hinge fwd and opens at back). Those are just two generalities of the principle.

It would be great to hear some technical or physics type airflow/volume numbers stuff in basic terms as to whether this might or might not work, or what might be a better idea based on the holes and access plate already done. Is the size of the opening "U" too small? How far in the slip stream shold I have the door open? Will the air just swirl with the door open some, or will it act as a vacuum effect and pull the air out like I want it to? I like using the access plate because if it doesn't work, I can just put the regular acess plate on and nobody will ever know the difference (no like they will anyway since it is smack dab on the bottom rear). Thanks for your replies.
 
db8 said:
I have made five 2" hole cuts (with perforated .032 covering the holes) in the rear baggage area of the 8 (one on the lower horizontal shelf area -- viewing holes also, two on the vertical part of the shelf area and two in the removeable bulkhead. These small holes consist of about 60% see throught vs 100% with an unobstructed 2" hole, just to give you an idea. On the 8, for those of you who are not familiar, there are some wedge shaped areas on the sides of the rear seat that air can flow relatively freely to the back storage area. I put these five holes in to allow the air to continue to flow to the rear of the airplane (based on how important that is in the previous threads on this subjust). That work has been done.
Click to expand...

Unless you've already sealed up some other areas, that air already had a ready path to the aft fuselage, by following the elevator pushrods. With this mod, your airplane will be a little lighter however, and therefore faster.


IMHO, the first job on the -8 is effectively sealing the canopy skirt. I've got some ideas, but it hasnt been a priority for me (there's other stuff I like tinkering with, and tht will have to wait).

Don't let me discourage you--this is definitely an area where there is room for improvement in the design, but if I understand you correctly, you may be going down a blind alley.
 
air flow

I plan on using the aileron pushrod skirts to help seal the air from entering the cockpit. Idealy, I believe the only air I want to enter the cockpit is from the vents where I can control it coming in. I am a little confused about the air getting back there by "following the elevator pushrods." I can see there might be some slight airflow from that to the back area of the fuselage, but in order to effectively heat and cool the "cabin," there needs to be air flow above the floor, and then an area for it to exit?? This is my concern/question. Thanks. Dave
 
db8, just a couple of thoughts

Not the answers that you were looking for, but two points to ponder.

1. While parked on the ground you will want to make sure the insect/bird/rodents can't enter through the vent. Either by screening it or making sure that your checklist always closes it on the ground.

2. If I understand this aerodynamic stuff a little bit (which may not be true) you want the exit air to leave in a negative pressure area. If this is the case why couldn't the exit vent door be spring loaded. Then the more difference in pressure the wider the door would open. On the ground the door would be closed.

Good luck and report back on your mod. I think that there are a number of use that want to do something like this but no one has proved the idea yet.

Kent
 
Air Flow

Thanks for the reply Kent. The first point -- I agree. It will have to be closed when on the ground as part of the "after landing" checklist to avoid little critters. As far as the second point, it seems the air on the bottom of the fuselage is high pressure air in flight. I feel (though I don't know) that this outside air pressure would be far greater than what the air "flow" or movement would be on the inside of the fuselage; therefore, if the hinged door were on a spring, it would never swing down or open because of the greater pressure outside to keep the door flush? Don't know if that's correct or not, but that's what my thought would be? Any other thoughts? Thanks. Dave
 
If the pressure outside the hole is greater...

Then the air on the inside will not exit the hole. Not unless you put a pump on it, but then it would be higher inside and would push the door open into the outside air.

Kent
 
Air Flow

I did think of putting some kind of mini fan or something to draw the air out also, but I am thinking I can vary the flow in the cockpit with the amount the door is -- maybe both would work, but without having any background on the flow of air, I don't want to do a whole bunch of mods for NOT. I wonder with my current design/thought, if the air will get drawn out depending on how much the door is open, or will the high speed air from the outside swirl around the three sided door causing eddies and not allow the air to flow out at any rate (ie worthless)? Questions, questions, questions??? Dave
 
The only way you're going to know if your door is going to work is to test it.

If the exit is aft, there's a pretty good chance that the low pressure created behind the door will be enough to give you positive flow out of the door. It would also help if you put walls on the side of the door so that the only opening is facing aft.
 
Air flow

I agree. I think there will be air flow, but I can't test it right now because I am just building the fuselage (but I know what you mean about doing it later). I refered to it as a three sided door (with sides) just like you mentioned, so that is the current plan. I wonder if that size door will be okay (2 3/4" x 4")? I hope it's not too small, but it's the largest size I can get while using the current access plate. Anything bigger is another hole in the fuselage (or larger access plate where this one is but structural integrity comes into play then) and I don't think it would warrant that? Anybody else have any thoughts on this? Thanks. Dave
 
Vent doors

Just a thought - mount a pair of Vans RV-10 rear vents on the after sides of the fuselage, facing aft, with a light weight push-pull cable running forward to the cockpit. When you open them into the slipstream, the door should create a negative pressure and vacuum out the space. I don't have my plane together to try this with yet, but I was already thinking of building it in if no one else had tried it yet.

Jeff
 
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