What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Top of cowling air leakage

A

alcladrv

Well Known Member
I attached the top of the cowl on my -7A per plans using the curved hinge.

During flight, I've noticed that it bellies up a bit. I know that some builders have chosen to install 1/4 turn fasteners along the top of the cowl.

If you attached your cowl as I did, have you made any attempt to seal the apparent air leakage at the top aft edge? If so, how? Did you notice any "good" results from doing so, such as cooler CHTs or maybe less drag from more airflow coming out the bottom of the cowl?

Thanks,

Mike
 
I built mine per the plans as well with the piano hinges, and I have the same issue. In flight the cowl bulges up between the hinges. I've been thinking about it for a little while. It would seem on the face of it that the exit area is not enough for the pressure to flow out of the lower cowl. It seems that increased flow woould help things run cooler as well. I've been meaning to call Van's to ask them what they think.
 
Is not the "aft edge of cowl" behind the "high pressure area" you want to stop leaks from? i.e. leaks to be concerned with are forward of where the baffles are against the cowling?
 
Mike, the upward bowing along the top cowl seam is routine with the hinge attachment method. With screw or 1/4-turn attachment the unsupported section of cowl doesn't exist. Even so there can be some bowing between the fasteners. My own cowl (using 1/4 turns) has 6 plies of glass tape added to the edge for additional stiffness, yet because I throttle the cowl exit and increase interior pressure (quite on purpose) I still get a little bit of bowing between fasteners.

Some guys add a short length of 90 angle to the center firewall between the hinge sections, install a few nutplates in it, and use a few screws to close the bowed section.

You could bond a short transverse rib to the inside of the cowl to increase bending stiffness.

From an aero standpoint the gap produces a little bit of drag. The books call it excrescence drag, a jet of air exiting at the gap and disturbing exterior flow. That's about the only downside.
 
Pin Size

All good input here. Check your hinge pin size, should be a close fit, like hard to push in to the hing holes. Any slop here translates into bulges to the rear of your cowling in flight. The piano hinges should give you more support than the cam locks. I added a small door to the rear of my cowling to aid in pin installation and removal. Very tight fit and NO bulges at cruse. :cool: Size does matter.
 
I added baffle material around the entire perimeter of the firewall which significantly reduced the bowing between fasteners but not entirely. The baffle material most likely reduced the leakage but the internal pressure (probably even more now) is still exerting significant force, resulting in residual bowing - to completely solve the issue would require a more rigid structure around the perimeter of the cowl as Dan and I have both done with glass ply. Probably actually need to add ply to the entire aft edge of the cowl prior to fitting the cowl as no mattter how much is added forward of the edge, the edge will be the weaker and is the point where the force is concentrated.

my .02
 
1Oldguy said:
I added a small door to the rear of my cowling to aid in pin installation and removal. Very tight fit and NO bulges at cruse. :cool: Size does matter.
Click to expand...
I don't recommend putting a door in the aft edge of the cowling if you can avoid it. I have one on the -6 I bought flying, and that door is the only spot on the entire back hinge that lifts up. I can see about a 1/8" opening along the entire door in cruise. Figure out how to reach the pins through your oil door, that's really the best way. Or use the Skybolt fasteners along that seam.
 
I used 10/32 SS screws with tinnerman washers in "floating" nut plates on about 2.75" spacing and two plies of re-inforcement. All with no apparent bulging. But I also have a Sam James sealed plenum. I think this helps reduce the pressure applied to the top of the cowl.

Another advantage for this set-up over "standard" baffles seals?? Me thinks so.
 
Pin Door

Hey sorry to hear about your pin door Snowflake. I do not know how it is latched shut but sounds like the door it's self needs to be stiffer. I really have no bulging at all on the back of my cowling. The little door makes pin installation a breeze and most people never notice my little door. :) It can be made to work correctly and worth the effort.
 
I have a small plate

I installed a small aluminum plate at the aft end of the cowl for accessing the two loop end hinge pins that I safety wire together after installation. The plate is s small - maybe 2" side to side and 1" fwd and aft. I have two small tabs underneath with #8 platenuts that hold it in place using flathead #8 screws and dimple washers. The plate mounting holes are 3/8" diameter. I can't go to the hangar and measure anything at this time but the opening is small and there is no bulge of any kind. I have custom lower cowl baffling that isolates the the area between the engine and firewall from the engine cooling air path. That is all covered in this forum.

Bob Axsom
 
Carl J. Niedermeyer said:
Here is a pic showing the cowl on the ground, and an in-flight pic showing the bowing out of the aft part of the cowl (attached with hinges). My RV-8 has over 1000 hrs on it.

cowl_inflight_small.jpg
Click to expand...

That's what mine looks like inflight on my -7A with 700 hrs. on it. I just thought if there's any benefit from reworking it that was worthwhile, I'd work toward that, but it sounds like a bunch of work for next to no gain.

Thanks for all your input,

Mike
 
The simple fix. The hinge pin is undersized to allow it to feed through the arc of the cowl down the sides. You can use the correct size pin for the hinge (sorry don't have plans and can't remember what size that is). Lay the hinge pin on the cowling and mark the area where the curve of the cowling starts down. Chuck up the pin in a drill and taper the pin with a belt or disk sander while spinning it in the drill. This makes a tapered hinge pin that will feed through the corner but still hold the hinge in alignment on the top. No more top lifting up.
My buddy did this on a couple of eights and it works well.
 
JonJay said:
The simple fix. The hinge pin is undersized to allow it to feed through the arc of the cowl down the sides. You can use the correct size pin for the hinge (sorry don't have plans and can't remember what size that is). Lay the hinge pin on the cowling and mark the area where the curve of the cowling starts down. Chuck up the pin in a drill and taper the pin with a belt or disk sander while spinning it in the drill. This makes a tapered hinge pin that will feed through the corner but still hold the hinge in alignment on the top. No more top lifting up.
My buddy did this on a couple of eights and it works well.
Click to expand...

This simple fix sounds like the optimum fix. I will give it a try.

Thanks,

Mike
 
1Oldguy said:
Hey sorry to hear about your pin door Snowflake. I do not know how it is latched shut but sounds like the door it's self needs to be stiffer. I really have no bulging at all on the back of my cowling. The little door makes pin installation a breeze and most people never notice my little door. :) It can be made to work correctly and worth the effort.
Click to expand...
My door is "latched" with a single countersunk screw through roughly the center of the door, that goes into a nutplate on the back of a 1/8" thick by 1/2" wide aluminium bar that crosses the opening. The bar makes it hard to get into the opening to grab the piano wires, and the location of the screw doesn't adequately seal the door. It looks like it was done as an afterthought, and not to the quality standard that the rest of the plane received.

It's not changing though, so i've resolved to let it be until such time as I overhaul the engine. I expect i'll be making enough changes at that time that I may want a new cowling anyway...
 
Rather than tapering the correct size pin for the hinge, use the thin pin but slide some thin walled SS tubing over the horizontal part. Small Parts has lots of sizes, the .094 ID by .120 OD 304 SS hypodermic tubing would probably work.

Stewart Willoughby
6, finishing.
 
Friend of mine used heat shrink on the horizontal part of the pin. worked a treat and has not been replaced in 200 hrs.;)
 
Stoo said:
Rather than tapering the correct size pin for the hinge, use the thin pin but slide some thin walled SS tubing over the horizontal part. Small Parts has lots of sizes, the .094 ID by .120 OD 304 SS hypodermic tubing would probably work.

Stewart Willoughby
6, finishing.
Click to expand...

Sounds like it might work. You can try it on your 6 as it will have the same issues if your going the hinge pin route. The taper pin is really simple and cheap to execute and I know it works but I am sure there is more than one way....
While your at it, beef up the area between the hinge pins where the oil door is with a couple additional layers of glass. This area pillows up on the 6.
Some day I am going to fix mine, but for now, I just fly it....
 
You must log in or register to reply here.
Back
Top