A well built normally aspirated cowl setup with no internal engine co= oling=20 plenum is subject to high internal upper cowl pressures (ballooning of the= upper=20 cowl), low air pressure over the top of the cowl and some high pressure on= the=20 underside of the cowl. This leads to cowl distortion and lifting.

A perfect example is the picture of the aircraft on the LOBO hom= e page=20 ( http://lancairowners.com/ = ;) -=20 notice the cowling quite a bit higher than the prop spinner.&nbs= p; I=20 spent some time on my own aircraft trying to figure out how bug splatter= could=20 be on the upper 3/4" face of the cowl right behind the spinner when= =20 everything lined up so nicely on the ground. Read on....... &nb= sp;=20

Builders, like myself, that used Camloc fasteners (spring loaded= ) find=20 that there is a lack of rigidity - that is the cowl can move under the for= ces=20 encountered in flight. Those builders that made extensive use= of=20 hinges or screws would find much more rigidity and less opportunity for th= e cowl=20 to lift during flight.

If the bottom air exit bulge is not supported as Lancair recomme= nded=20 (stiff arm between the bottom and the firewall), it can certainly deform.&= nbsp;=20 One way to eliminate the support and obtain stiffness is to consider the= use of=20 some extra foam and carbon along the trailing edge. Rather than a=20 sharp edge that creates turbulence, drag and a virtual reduction in= the=20 air exit size, consider a flared exterior ala some Diamond aircr= aft=20 and/or some interior build up to form a sort of nozzle effect to accelerat= e the=20 exit air. Either or both will stiffen that part of the cowl.

Good Luck,

Scott Krueger

In a message dated 12/21/2010 12:25:03 P.M. Central Standard Time,=20 rwolf99@aol.com writes:

<<When in flight, the bottom= of my=20 cowling is compressed upwards by about 1.5" to 2". Make sure you do not= have=20 anything that this compression could effect.>>

Wow! Lorn, you're talking about th= e back=20 section of the scoop deforming upwards, I hope? I mean, you're not= =20 suggesting that the front of the cowling elevates 1.5 inches relative to= the=20 spinner?

I can see how the scoop might deform nea= r the=20 back -- I just pushed on mine with my hand and it deformed almost an=20 inch. But I can't seem to make it move significantly near the=20 front.

Please let us know if that's what you=20 meant.

- Rob

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