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RV-8 Excess Cabin Pressure

flickroll

Well Known Member
When I first flew my RV-8, I experienced a noise that at first scared the h*** out of me because I thought it might be control flutter. It was a high pitched buzz, but after some diagnostics I realized the noise was not coming from the controls, but rather from the rear canopy skirt vibrating against the fuselage. If the pilot (or passenger) air vents were turned off, no buzz, wide open, I get a buzz. My first remedy was to attach fuzzy velcro on the inside of the canopy skirt to stop the noise from the vibration, and it mostly works, although I occasionally still get the buzz, particularly when down low and fast.

But the buzz is really caused by more air entering the cabin than can easily exit the cabin. So the air exits the cabin around the skirt, causing it to buzz as the air squirts outside. So the real remedy is to provide for at least as much exit air volume as the volume entering.

Has anyone addressed this? If so, what sort of exit vents did you provide? I'm thinking about creating exit holes in the rear baggage compartment which will allow air to enter the aft fuselage and then out completely in the empennage area. I think there may be enough 'leakage paths' from the cabin to the rear baggage compartment to channel that air, and then I just need a way for it to exit the baggage compartment. Any thoughts or tips?

Thanks
 
Yes! and they work great!

I drilled two 1" holes in the upper skin, one just on either side of the canopy slider track, just behind the F808 bulkhead. You can not see these holes when the canopy is closed, because they are in the area hidden by the canopy skirt.

These holes allow cabin air, and more importantly, air leaking in from under the skirt along the back, to be scavenged out of the cabin and into the tail cone.

Another builder posted a similar idea in an old thread. His scavenge holes were just in front of F808, but he made a little fiberglass plenum to collect the flow and duct it through the F808 bulkhead into the tail cone area. He then went one further step and provided an exit in the belly of the tail cone, and he ducts the scavenged flow right to that exit.

I don't think the dedicated exit is needed. There are enough leak paths out of the tail cone around the elevator control horn and into the rudder cove, plus the flow out through the rudder cable exits (which I enlarged some).

I just made my first passenger flight yesterday, and asked the back seater (my wife) to evaluate air circulation/drafts with and without the air vents open and with and without the cabin heat open.

We found that we get great vent flow when wanted, and the cabin heat flows from the front seat area all the way to the back seat, and there is minimal draft on the passenger's neck from skirt leakage. (I also have a felt strip on the skirt)

It took all of 30 seconds to drill those two holes with a Unibit, and it has been a great improvement in the cabin flow.
 
These holes allow cabin air, and more importantly, air leaking in from under the skirt along the back, to be scavenged out of the cabin and into the tail cone.

Steve, do think the problem is caused by air coming IN from the skirt? I was thinking it was caused by air exiting the skirt...

Thanks
 
pictures, flow path

I'll see if I can find a picture. Might have to wait til next weekend. (I have to go to LaRC for a couple of days - Paul will know what that means ;-)
But really, its just two holes. 1" diameter, spaced about 2" apart, immediately behind F-808 on either side of the canopy track.

The cabin is being pressurized by the vent intakes, and/or the cabin heater intake. the flow through any/all of these is limited by the available outflow.

The pressure outside, along the back edge of the canopy skirt, is even higher pressure than the cabin. This high pressure recovery is good evidence that the flow is not separating on the canopy, it is attached flow back to the skirt and onto the aft part of the fuselage. Thats why there is usually a significant draft coming forward from the rear skirt and blowing on the back of the neck of the passenger.

The pressure in the tailcone is relatively low. It is being sucked on by the venting around the elevator horns into the elevator coves (the space between the stabilizer and elevators). The pressure at the rudder cable exits is probably about free stream pressure, or a tiny bit above. But these pressures are below the cabin pressure, so they are trying to suck air out of the cabin.

Normally, there is very little flow path from the cabin into the tail cone. The biggest hole is through the F807 bulkhead where the elevator pushrod goes through. But cabin air must first get below the floor, then aft through that hole. Its hard to get air below the floor except where the control sticks are, and those are pretty well sealed with boots. So - its hard to get cabin air into the tail cone. If you don't have any place for air to get out, there won't be much air coming in.

Thats where my two little holes come in. A direct flow path from the rear skirt area of the canopy into the tail cone. By scavenging off the leakage flow under the skirt, directly into the tail cone, you get more flow through the vents (or heater) and the flow is generally moving aft.
 
Thanks for the detailed explanation Steve. That's pretty much what I tought was going on, although I did not consider there would be high pressure on the outside of the fuselage in the skirt area. Question - did your mod completely alleviate the 'skirt buzz' and the cold air reaching the rear passenger? On a cold day I'll feel it hitting the back of my neck as well.

I am going to do some more experimenting when I get the airplane back from Grady at Glo Cusom, which will be this this week :D :D :D (get the point?). First I'll fly with the upper baggage area panel removed. If this helps then holes in the baggage area only would fix it. If not then I'll fly with the rear seatback removed as well and see what happens. But maybe you've already done all of this??

Thanks again for your detailed response...it gives me a lot to work with.
 
Good description of th problem and solution Steve thanks! Have funny our east....

Jim - looking forward to seeing pictures of grady's work for you! BTW, I have flown with the rear seat back removed before - never thought to check if the airflow had changed, but with the baggage compartment in place, there won't be much in the way of holes for the air to get in to the tail cone anyway. Just guessing it might not make much difference.

Paul
 
Same thought process for a different application - it is common in the sailplane world to have a fuselage vent in the tail just ahead of the rudder - Cockpit inlet air has to go somewhere.

Sailplane guys are trying to keep it from flowing through the control rod holes and coming out at the control surfaces spoiling that smooth long wing! There is flight test data that shows a substantial improvement in performance with vented fuse and sealed control rods.

I added a vent last winter to my sailplane and enjoyed the benefit of better cooling air and a MUCH quieter cocpit environment. Performance improvement remains to be seen and is probably masked by my skills :)

I'd think holes would have to be created to make a path to the tail. Could install a vent similar to the intake vents somewhere too.

I'd be interested in pictures of what has been tried.
 
Thoughts/pictures

I don't know if my mod will work or not, but a while back (after spending a lot of time and thought on the issue), I decided to somehow try to control the flow of air through the cockpit.

I have mounted a fan in the back area (controlled with a rehostat) to "help" the flow of air through the cockpit. I can turn it on high and "pull" the air from the front and "push" it out the area of the tail where the biggest hole is. Everything will be sealed in the back except the hole where the elevator horns come down and meets with the pushrod tube, which is where the fan is direct. I'm hoping the forced air will help with the flow in the cockpit. I control the flow with the rehostat from nil to pretty good flow. Here is a picture of the fan from the bottom rear. I made an access plate to be able to get in there easier.

1262608img.png


I feel there is a large enough gap around the sides of the rear seat back to allow air to flow around the sides to the back. To get flow through the rear baggage area, I have placed five 1 3/4" corregated holes as pictured.

img3318.png


A buddy of mine also has the two holes covered up by the canopy skirt, and I might do that in conjunction with this?

Whether it works or not remains to be seen (errr or felt). At least there is a cool knob up front labeled "flow control." It might be a year or two before it's tested under real world conditions, but it might work? Any thoughts? Dave
 
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And whichever moderator kindly does that, please post how you do that so us less than skilled moderators can do it in the future!:D
 
Size of pics

Sorry about the size of the pictures. It has been a while since I have posted any pics, so I used a free program (Irfan View) to resize them to something more reasonable. Then I deleted the big pictures in the post, and resubmitted the smaller ones via Image Shack. I don't know if that helps you moderators or not? I think it does if you can put the picture on your computer through Image Shack, and then resize it, and then finally resubmit in the smaller size, it would probably work for you. The big question is if you can get the picture file on your computer to do this. I am not much of a computer person. Dave
 
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I'm very interested in this problem. I believe i have a related occurrence.
My canopy skirts are tight to the fuselage and i have foam to seal the
area aft of the rear seat. My passengers tell me that they can feel no
air blowing on the back of their necks. I believe that my the cabin pressure
builds up and then vents out. The result of this is a yaw bump. It also
happens only at lower altitutes and high (+ 140 KIAS) airspeeds. What is
also curious is that it does not occur or is greatly diminished when i have a
passenger. And lately i haven't noticed it at all. I installed an inspection plate
in the aft baggage floor (so as to be able to inspect the bellcrank without
removing the aft baggage shelf). I'm wondering if I remove the inspection
cover (if this works, I'll make it a vent), if it would have the same effect
as the two 1" holes? Both vent air to the aft fuselage, but mine would not
scavange air leaking in the back of the canopy. Comments very much
appreciated.
 
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I installed an inspection plate
in the aft baggage floor (so as to be able to inspect the bellcrank without
removing the aft baggage shelf). I'm wondering if I remove the inspection
cover (if this works, I'll make it a vent), if it would have the same effect
as the two 1" holes? Both vent air to the aft fuselage, but mine would not
scavange air leaking in the back of the canopy. Comments very much
appreciated.

I am going out to fly in a few minutes to check some new runway lighting at our field - I'll open up my battery inspection port before I go, and see if I notice any airflow difference - great idea Tom!

Paul
 
I had a similar issue today but the cause seems to be the reverse of what is being discussed here. Cruising at 6,500 at 175 IAS I got the rear skirt canopy buzz, when I opened the cabin heat valve the buzz stopped. I could repeat this cause/effect by opening the heater valve. Appears that without a little cabin pressure to keep the skirt off of the fuse it wants to buzz.

Thoughts?
 
I am going out to fly in a few minutes to check some new runway lighting at our field - I'll open up my battery inspection port before I go, and see if I notice any airflow difference - great idea Tom!

Paul

I'll be very interested to hear what you find out Paul. If having an opening into the rear fuselage helps, then the mod that DB8 did will be just the ticket. And, if you wanted the ability to close off air flow to the aft fuselage, a couple of Stein eyeballs on the upper baggage panel would be just the ticket.
 
Quick and Dirty Results

Well guys, I went out and propped open my back seat several inches to allow airflow into the baggage area, then opened my hatch to my battery - humongous hole to the aft fuselage. I flew normal speeds and felt the same amount of air on my neck as always (I have a pretty well calibrated neck after 1,000 hours in the bird). I tried vents opened and vents closed, cabin heat on and off - felt no real difference.

As to buzz - I have never experienced it. I read about it before I flew my first flight, and that the answer was a good fit and a layer of felt, so I fit the skirt tight and glues on a layer of felt to the edge (about 3/4" wide". I have never had any noise associated with the skirt, so I may not have the same "test conditions" as those experiencing that affect.

Probably not what we wanted to hear - I'd love to solve the wind on the back of the neck problem, especially now that I am spending a little time in the back seat when Louise flies....:rolleyes:

Paul
 
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high speed buzz

I have the same buzz that folks seem to be talking about. Usually happens about 190 mph and happens with the vents closed. What was the result of cutting holes in the baggage bay aft wall?

Jeff Riblet
RV-8
 
I have the same buzz that folks seem to be talking about. Usually happens about 190 mph and happens with the vents closed.

And I get mine down low (denser air), and fast, with the vents open, never with the vents closed.....go figure.

When my -8 carries me back to 'ol Virginny late this week I'm gonna do a lot of experimenting when I get home and see if I can figure something out. Thanks for the data Paul, your test flight information is very valuable.
 
Rear seat vent

And I get mine down low (denser air), and fast, with the vents open, never with the vents closed.....go figure.

When my -8 carries me back to 'ol Virginny late this week I'm gonna do a lot of experimenting when I get home and see if I can figure something out. Thanks for the data Paul, your test flight information is very valuable.

In my '8' it is the rear vent making the "buzz," closed, no noise. It varies with airspeed, so I assume that it has to do with the NACA intake under the wing.

John Clark
RV8 "Sunshine"
KSBA
 
cabin pressure

I HAVE RECENTLY FLOWN WITH BUILDER AND THOUGHT SAME PROBLEM EXISTS FOR HIM I THOUGHT THE SIDE VENT FOR RV TEN COULD BE INSTALLED ON BELLY OPPOSITE AIRFLOW DIRECTION WITH DETENT CABLE MIGHT WORK ANYONES THOUGHTS ON THIS IDEA
 
Reversed NACA vents

Here's a couple of pictures I took at Oshkosh last year of reversed NACA vents in the rear inspection covers on a -8. I have no idea if this does anything to slightly reduce the pressure in the tailcone, but I thought it was interesting.

IMG_0321.JPG


IMG_0322.JPG


-Chris
 
High speed buzz

Jim
After some more flying I found that this buzz I get is related to the vents or the vents postion. Open the vent and it goes away. Could play a tune with it but bet an ipod would be better.

Jeff Riblet
RV-8
KPVG
 
reverse naca vent

A buddy of mine ownes the RV-8 that is in the pictures (a few posts prior to this). He said the reverse naca vents didn't help, or if it did, it's not much (miniscule). Just thought I'd pass that on. Dave
 
No difference

I flew up to Mojave on Saturday for AOPA's airport day with the rear seat
back removed and the 4" X 6" baggage bulkhead inspection cover removed.
I didn't notice any difference in vent or heater airflow. I did think I could
hear more wind noise (probably my imagination!). Still might cut a couple of
slots in the cover anyway.
 
I flew up to Mojave on Saturday for AOPA's airport day with the rear seat
back removed and the 4" X 6" baggage bulkhead inspection cover removed.
I didn't notice any difference in vent or heater airflow. I did think I could
hear more wind noise (probably my imagination!). Still might cut a couple of
slots in the cover anyway.

Thanks for the reply Tom. That's what I was planning to do but never got to it last weekend. Don't know what to try next....not real comfortable drilling holes in the turtledeck....
 
Since two of you have moved the rear seatback forward (our out) and it hasn't made a difference, I think it concludes there is enough of a gap on both sides (with the rear seat like it normally is) to not restrict any air flow? I would have thought with Paul's hatch open on the shelf, that might have "helped" the flow of air to the rear. When I get mine done (with the fan to "pull" the air from the front), I don't think I will seal off everything like first mentioned (except the big hole where the fan is pointed). That whole part of the project might have been a waste of time?

I guess the big question is if air is being drawn out through the leak holes in the empennage during flight? Or, because of air and pressure differentials, is air getting sucked in possibly (a little)? It would be interesting to put some yarn back there all over the place and a video camera that doesn't interfere with the elevator pushrod. Or, if any of you or your back seaters are cigar smokers, light up and see where the smoke goes?? Just some thoughts, but leave the cards on the ground ;-) Dave
 
I've been a little skeptical watching this thread.
From my experience with my "open canopy" mod, I know that the rear of an RV8 canopy does not lift, it in fact presses down on the turtle deck.
I have a golf club mod, that allows the aft baggage bulkhead to lay down, and I flew that way last summer. I didn't think about seeing if I got more heat because, well, it was summer. I was disappointed when Paul verified that he got no increase in flow with the door open.
Since I built the golf club mod, I can easily open the rear compartment and check things in the aft fuselage area.
I found something directly attributable to another "mod", my smoke system!
The entire inside area of the fuselage "belly skin" was coated with smoke oil residue.
It had been entering from an unplugged "future Nav antenna" hole, just fwd. of the tail spring bolt hole and migrating forward.

I'm thinking the best fix for more heat in the rear of the RV8, is moving two things.
The first will be, to move my condition inspection gradually to January.
The second will be to move my hangar to Florida.

Good luck,
 
Boots?

Would be interesting to know if those posting have aileron control boots and see if that has any effect on flow?
 
FWIW

In the for what its worth column, I do have airleron push tube boots.
Finally got around to venting the baggage compartment inspection cover.

rv8baggagevent003.jpg


Nine 5/8" holes (easier than cutting slots with a cutoff wheel). Can't tell
any difference.......
 
That looks good!
How does it apply to an RV-8?
No I'm not being sarcastic.
Would it go under the horizontal stab where the fiberglass fairing covers the elevator horn? which side?
:confused:
 
http://www.aerologic.com/example/m2.html

And that's the million dollar question. The link above is a CFD workup on the one off Melmoth2. It's certainly not an RV, but you get the idea. The color spectrum runs from blue(high pressure: good for air intakes) to pink(low pressure: good in thoery for air exhaust), with green in the neutral pressure. Most would agree that it would be best to avoid exhausting cabin air out on the wings messing up the airflow. One place that really seems like a possibility is on the low pressure area up on the vertical stab(about a foot from the top and say at 40% chord.) It would require a length of ductwork from the cabin and up the verticle stab to the exhaust vent. If this were feasible, the disturbed exit air would not be running down the length of the fuse, just a brief spurt past a small section of the rudder blade then gone. I believe that some gliders have cabin air exits in this area for this reason.

I'd be curious to know if anyone has done a cfd workup on an RV?

George
 
Same question here. Any updates? Is there a proven modification to keep the air off the back-seater's neck? Pressurize the aft fuselage like Cessna 180's with little forward scoops?
 
In Formula 1 cars, the cars have small ducts that route the high pressure air to the lower pressure region to excavate the stagnant region. If there is ducting that excavate the high pressure at the aft skirt to the lower pressure region at the floor, then that can solve it. It can probably solve another problem of not able to open the RV8 at speed if the pilot wants to eject because the high pressure pushes the canopy forward to the front windshield roll hoop.
 
Same question here. Any updates? Is there a proven modification to keep the air off the back-seater's neck? Pressurize the aft fuselage like Cessna 180's with little forward scoops?

Seems most have had good success with various types of "D" shaped rubber seal strips.

I have some of this rubber D strip material but I have not yet taken the time to install it.

I have a lot of air movement in the cabin coming up from the back, but I wear a lot of warm layers and pull up the hood on my hoodie if my neck gets too cold.

The real problem is if there is any moisture, like rain - it comes in and gets everywhere.
 
It’s an old thread but I have a new question. Has anyone done a smoke test to see where the air is really exiting?
 
It’s an old thread but I have a new question. Has anyone done a smoke test to see where the air is really exiting?
Encountered the canopy skirt buzz around 16O KIAS. Applied 1/2" wide sticky back foam insulation to underside aft portion of the canopy skirt. Problem solved. Glad I didn't drill holes in my fuselage.
 
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