Distorted Top Cowl -- Is This Normal??

[Darrel Starr]

I am flying my Super Cub now with revamped cowl seals and baffles. I made the change to McFarland seals that have a Teflon coating because the original seals were eating up my cowling. While I was at it I took special pains to try to seal the top chamber extra tight - see pictures below. Now the top cowl distorts at high cruise speed -- see the picture taken at 98 mph indicated. The distortion disappears below about 70 mph. We flew our recently rebuilt SC last year but teething problems limited the hrs and I can't remember if the top cowl distorted like this before the cowl seals were improved. Is this Normal?
Thanks, Darrel

 

[Longwinglover]

Now the top cowl distorts at high cruise speed -- see the picture taken at 98 mph indicated. Thanks, Darrel

There's the problem! Cubs aren't supposed to go that fast! :lol: Next thing you know, with all that air pressure on the top of the engine you'll be complaining of low cylinder head temps!

John Scott

 

[mike mcs repair]

now Mcfarlane will be contacting you for before and after pics to show how good their baffles stuff works

 

[Darrel Starr]

You guys are funny. OK, I'll just stay below 70 mph.
Darrel

 

[NimpoCub]

Reduce the size of the intake or enlarge the outlet.

 

[Darrel Starr]

Logan, I see the logic but this is a stock PA18-150 cowling so I don't think that chopping it is the answer.
Darrel

 

[bob turner]

Introduce some leaks in the baffles to simulate a real Cub.

 

[NimpoCub]

What Bob said was my thinking.
Your baffle is NOW too tight, so air isn't "leaking" through the system fast enough. "Chopping" the outlet will give you a helluva draft over those rear cylinders!

 

[Steve Pierce]

Break a little more edge on the leading edge of your top cowl. Problem is it will probably crack the paint.

 

[skywagon8a]

Rivet on an angle or channel as far forward as you can get it without any conflict with the nose bowl. Then you will only need to touch up the rivets. This will prevent distortion between the cam-locks.

 

[Darrel Starr]

Steve, that sounds like it would stiffen the edge and lessen the distortion. Might even be able to save the paint by taping both sides. Thanks to you also skywagon.
Darrel

 

[Widebody]

Darrel,

For what it's worth, it looks just fine to me.
But I guess it's your eyes that need to be happy.
More often than not, when you start screwing
with something that isn't really that bad, it becomes worse.

Brad

 

[Paul Heinrich]

What distortion? I don't see any distortion.

 

[Roger Peterson]

Put in a couple of spring loaded flapper type relief valves about 2" in dia in the rear of the baffle. When the pressure gets to high, it will dump excess air. I recently put in new McFarlan baffles in my cub and changed all my temps. had to redo my front baffles to balance my cooling. Those things do seal.

 

[DW]

Darrel go fly it--- everything will vibrate into place and shape soon enough.

DW

 

[Gordon Misch]

A couple more screws into the nosebowl??

DW sure seems to be feelin' good this eve!?! Good on ya, Dennis!

 

[DW]

Just glad the long weekend is over---now I can get some rest.

 

[Darrel Starr]

Thanks for the comments. For now I think I will just go enjoy flying it.
Darrel

 

[Darrel Starr]

I searched the gallery for SCs in cruise and found the following three pictures. They show that a distorted top cowl is common and maybe normal.
Darrel

 

[Paul Heinrich]

Like I said, "What distortion?"