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Cowl Pucker

My upper cowl half is attached to the firewall using piano hinge and two removable wire pins. There is a 3 1/2" section of the cowl that is not secured to the firewall to make space to insert the piano hinge pins. In flight, that 3 1/2" space gets puckered up pretty high?maybe by 3/4" or a little more.

Is this normal?

My oil cooler is mounted to the firewall and is fed cooling air via scat tubing off the engine baffle. Oil temperatures seem normal?between 180 and 195 degrees fahrenheit in climb and cruise. Does the cowl puckering have an adverse impact on the pressure differential behind and under the baffling?

It seems like if I had a big enough oil leak it, oil could easily end up coating the plexiglass bubble. I don?t have a need to invent problems where none exist but I am curious to tap into your wisdom and advice.

I have attached the best picture of the area I have available right now but it is not very helpful. If you look just behind the oil fill door you can just make out the two piano hinge pins sticking up:

IMG_0299_zpsjxyfprug.jpg


Better picture added:
IMG_0345_zpsjvrqrgqp.jpg


Thanks in advance for your time and help,
Hank
 
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I am not sure if it is normal since I can't really see the amount that it is puckering, but I can tell you that it happens.

I have seen the scalloping effect from people who have used the skybolt type fasteners across the top of the cowl, vs. the piano hinge method. I think if they got them spaced too far apart, the Scalloping starts to happen.

Also, from the hinge method if you leave a space where the two hinge pins go in you can get the same scalloping effect. The plans call for a SINGLE PIECE of hinge with the eyelets cut out across the top of the cowl. Some people have used two hinge pieces on the cowl itself and leave a 3 1/2 inch gap between them, this will allow for the pucker.
 
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Could you install one or two camloc style fasteners there to eliminate the pucker? and/or

Reinforce the cowl in that area to make it stiffer.

I have seen this on other RV's inflight when using camlocs spaced a little far apart.

Bevan
 
3/4" sounds like too much. I have the same set up and I am seeing about an eight of an inch. (I am using a plenum so that may make a difference?)
 
There is positive air pressure in that area under the cowl at high speed, that does push out on the cowl.

Because of this, the area with the break in the hinge does usually push out somewhat, but usually on the order of 1/8".
If you are actually seeing 3/4 of an inch (big enough to stick an index finger through), I would say you have something seriously wrong.
 
Like Paul K I have sllght "puckering"; maybe 1/8-1/4 at high speed. No plenum. 3/4 sounds like a lot.
 
Does the cowl puckering have an adverse impact on the pressure differential behind and under the baffling?

The additional "exit" would very slightly increase the pressure differential across the cylinders and oil cooler, for a net increase in cooling.

It is adverse in the sense that it probably slows your airplane by at least 1/10 knot ;)
 
The good news, if you have an oil leak, you will know it. Cam locks or sky bolts, or plain ole screws (with tinnermans) would fix it. One should work.

3/4 mm or 3/4 inch?
 
You say in your original post "you can just see the two piano hinge pins sticking up." If I understand correctly what you are saying, this is wrong. You should not be able to see the hinge pins sticking up from the outside. The hinge pins are inserted inside the cowl through the oil door and should not be visible from the outside. Maybe I misunderstood and can't see in the picture you posted. Another picture from closer might help.
 
Brad:

I understand what you and Chad are describing and now I understand the issue better. A good picture would show you the building flaw that has been made. Here is an okay picture:
Piano%20Hinge_zpsbwjsoqus.png

As you can see in the picture above there is a gap of about 4 hinges just above and to the left of the Andair Oil/Air Separator on both the firewall and the cowl. On my plane the two hinge pins are inserted from outside the cowl into the gap you can see in the picture.

It makes a lot of sense that the better way is to have a continuous of piano hinge and access the hinge pin(s) through the oil door.

How would you fix it from here?

Thanks,
Hank
 
The gap in the hinge is per plans. But from this second picture, it looks like installation of the air/ oil seperator interferes with the normal access to insert the pins from inside the cowl and the solution was to insert the pins from outside the cowl. No real harm except the gap from cowl to top skin was widened to permit pin insertion from outside and thus the 3/4 inch gap. Appearance issue and not a danger that I see. Maybe others may have another view on this. You do need some way of keeping the pins from backing out. Maybe someone here can post a picture of the way to make a pin retainer.
 
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Thanks for looking. I don't think there is much likelihood of the pins coming out. They haven't moved in the 26 hours I have put on the plane. Removing and installing them requires a pair of pliers.

Thanks again for all the suggestions,
Hank
 
Here is the best picture I could come up with explaining the issue. The hinge on the cowl should be one piece with the eyelets cut out. This adds strength to that 3.5 inch gap and it will not pucker during flight. Some builders use two hinge pieces but this is not correct and the gap is not reinforced. You can probably reinforce this after the fact by adding some sort of stiffener in the gap and either fiber glassing it in or rivet it. It looks like your cowl is painted, so riveting might not be a preferred option.

Continuous%2520Hinge.png
 
Thanks for the education and your ideas. I really appreciate the time and effort it takes to create and convey a descriptive and prescriptive post. I'll take a look at the cowl and see what options exist.

Thank you again,
Hank
 
Cowl pucker

My pins are inserted from the oil door. The top cowl has the three inch gap, but it has a 4 inch layer of fiber glass stiffener that keeps it from puckering. I would just add a 1/4 inch of glass and it will reduce the pucker. My pins did work out but I fixed that. The short pin came out once and you couldn't tell it in flight.
 
Hank,

I think the problem may be helped by adding enough hinge, so the gap is shortened. My hinge gap between the two sides is only three hinge loops and I don't have any problem with getting both wires in. When flying, the rise in the cowling is barely noticeable.

Jim
 
bend the pin handles so they interlock if they tend to slide

My pins are inserted from the oil door. The top cowl has the three inch gap, but it has a 4 inch layer of fiber glass stiffener that keeps it from puckering. I would just add a 1/4 inch of glass and it will reduce the pucker. My pins did work out but I fixed that. The short pin came out once and you couldn't tell it in flight.

you have probably already done this. I bent the pin handles so they interlock if they tend so slide out. I have no additional pin locking method, works great.
 
Mine does too!

Timely topic. I too noticed that my cowl 'puckers' in the same location. Two piece piano hinges and standard baffles. I usually don't see it since my Stratus blocks my view in that area. Mine is more like a 1/4". My cowl does not have the piano hinge in that section so that appears cause some flex in that area, causing the issue.
 
Pucker

My RV-8 has only 10 hours flight time but no significant pucker noted up to VNE. Hinge is not continuous. Pins are bent in a triangle at the end per Jim Phillips and won't come out. The entire aft edge of upper cowl lifts about 1/8 or less I think.
 
As others have noted, this is pretty normal. There are a couple of things you could try...
Check the diameter of the hinge pins being used. If they are too small in diameter and causing a lot of slop the puckering will be worse. They could be worn, too. Using a little larger diameter would help.
On a couple of planes, I have strengthened the area by laying a couple layers of glass in that area, extending several inches beyounpd the gap area on each side, on the under side of the cowl. It seemed to help.
If you go the fiberglass route, you could create a tab the extends beyond the fuselage edge, kind of like a hinge replacement tab, in the gap. The cowl would not pucker if you did that, but make sure you allow for inserting the hinge pins.
You could rivet a metal tab on to the fuselage to fill the gap, that extends under the cowling a bit. (Or use a piece of baffle material. You could even try glueing the baffle material under the fuselage lip. The positive pressure would push it up against the cowl, but being more flexible, it may not work as well.)
You could add a metal tab on to the fuselage, extending under the canopy, with a nut plate so that you could secure the cowl to the tab with one or two screws. This would have the added benefit of keeping the hinge pins locked in.
Ok, those are all my ideas. Hope you get it resolved.
 
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