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  #1  
Old 10-29-2011, 10:39 PM
alcladrv alcladrv is offline
 
Join Date: Oct 2006
Location: Southeast
Posts: 569
Default Top of cowling air leakage

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
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  #2  
Old 10-30-2011, 03:52 AM
Chappyd Chappyd is offline
 
Join Date: Feb 2006
Location: Connecticut
Posts: 321
Default

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.
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  #3  
Old 10-30-2011, 04:06 AM
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Andy Hill Andy Hill is offline
 
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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?
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  #4  
Old 10-30-2011, 05:16 AM
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DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
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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.
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  #5  
Old 10-30-2011, 07:18 AM
1Oldguy 1Oldguy is offline
 
Join Date: Aug 2010
Location: Guthrie Oklahoma
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Thumbs up 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. Size does matter.
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  #6  
Old 10-30-2011, 07:47 AM
RV8R999 RV8R999 is offline
 
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Location: na
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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
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  #7  
Old 10-30-2011, 10:03 AM
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Snowflake Snowflake is offline
 
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Location: Victoria, BC, Canada
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Quote:
Originally Posted by 1Oldguy View Post
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. Size does matter.
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.
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  #8  
Old 10-30-2011, 10:09 AM
gereed75 gereed75 is offline
 
Join Date: Sep 2007
Location: pittsburgh pa
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Default

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.
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  #9  
Old 10-30-2011, 02:44 PM
1Oldguy 1Oldguy is offline
 
Join Date: Aug 2010
Location: Guthrie Oklahoma
Posts: 65
Thumbs up 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.
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  #10  
Old 10-30-2011, 03:56 PM
Bob Axsom Bob Axsom is offline
 
Join Date: Mar 2005
Posts: 5,685
Default 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
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