Cowl Pucker

[Henryrifle]

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:



Better picture added:


Thanks in advance for your time and help,
Hank

[704CH]

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.

[Bevan]

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

[Paul K]

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?)

[rvbuilder2002]

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.

[flyinga]

Like Paul K I have sllght "puckering"; maybe 1/8-1/4 at high speed. No plenum. 3/4 sounds like a lot.

[DanH]

Originally Posted by Henryrifle

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

[BillL]

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?

[brad walton]

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.

[Henryrifle]

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:

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