lml@lancaironline.net Mailing List Archive

Chris,

I believe you have an engine cooling plenum (N91CZ) and that makes a tremendous difference in the top cowl pressures since the top is then only subject to any whole cowl back pressure caused by the exiting air.

I do use Camlocs and could move my cowl up and down by hand. However, last year I bonded several phenolic blocks on the upper cowl positioned so that the cowl could not move backwards (hence up) over the leading edge of the fuselage joggle. This does not resolve the ballooning of the upper cowl partially caused by the upper plenum pressure that reaches 13" H2O pressure difference from the lower cowl at 200 KIAS.

Scott

In a message dated 12/23/2010 9:11:34 A.M. Central Standard Time, chris_zavatson@yahoo.com writes:

Scott,

Interesting. Some more data points.

The offset in the LOBO photo may not actually represent movement under load, but a slight misalignment. See photo below of the cowl spinner junction of the same aircraft while parked on the ground.

Also, I have many in-flight photos of N91CZ showing no detectable movement (second photo). Do you use camlocs? That might be the biggest contributor to movement.

Continental bed-mount vs Lycoming dynafocal mount may also be a factor based on another post.
The cowl scoop is certainly a weak spot near the rear.

Aircraft in LOBO photo

N91CZ

Chris Zavatson

N91CZ

360std

www.N91CZ.com

=========================================

Rob,

A well built normally aspirated cowl setup with no internal engine cooling plenum is subject to high internal upper cowl pressures (ballooning of the upper cowl), low air pressure over the top of the cowl and some high pressure on the underside of the cowl. This leads to cowl distortion and lifting.

A perfect example is the picture of the aircraft on the LOBO home page ( http://lancairowners.com/ ) - notice the cowling quite a bit higher than the prop spinner. I spent some time on my own aircraft trying to figure out how bug splatter could be on the upper 3/4" face of the cowl right behind the spinner when everything lined up so nicely on the ground. Read on.......

Builders, like myself, that used Camloc fasteners (spring loaded) find that there is a lack of rigidity - that is the cowl can move under the forces encountered in flight. Those builders that made extensive use of hinges or screws would find much more rigidity and less opportunity for the cowl to lift during flight.

If the bottom air exit bulge is not supported as Lancair recommended (stiff arm between the bottom and the firewall), it can certainly deform. One way to eliminate the support and obtain stiffness is to consider the use of some extra foam and carbon along the trailing edge. Rather than a sharp edge that creates turbulence, drag and a virtual reduction in the air exit size, consider a flared exterior ala some Diamond aircraft and/or some interior build up to form a sort of nozzle effect to accelerate the exit air. Either or both will stiffen that part of the cowl.

Good Luck,

Scott Krueger