RV-8 Excess Cabin Pressure

[flickroll]

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

[scsmith]

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.

[Ironflight]

We need pictures of those two holes Steve!

[flickroll]

Originally Posted by scsmith

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

[scsmith]

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.

[flickroll]

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 (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.

[Ironflight]

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

[selhardt]

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.

[db8]

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.



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.



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

[islandmonkey]

Is there any danger of some kind hearted moderator resizing the pictures in this thread?