Several years ago I spent about 20 minutes talking to Tracy Saylor (sp?) of 230mph 180hp RV-6 fame. He showed me several relatively minor mods to his airframe, & then told me that his big gains were under the cowl. He said that he had a large radius on the bottom of the firewall, & said that the radius on the -8 is much too small. He also said that he has 'guides' from the bottom of the cylinders to the exit at the bottom of the cowl. He also cut his exhaust pipes back inside the cowl to create 'ejector' effect in the cooling exit air. In the old 'Speed With Economy' book by Kent Paser (240 mph 160hp Mustang II), he describes & supplies a drawing of almost exactly the same things, done on his a/c.

A handful of Lyc powered RV guys have built variations on true exit ducts that smoothly re-accelerate & direct the exit air, & some (not all) report significant speed & cooling improvements.

Maybe it's just me, but it seems like a lot of cooling & speed might be left on the table by not using exit ducts.

Charlie

Tracy Crook wrote:

Because it looked easy to modify the bottom cowl outlet on the RV-8 to a cowl flap configuration I decided to try that before installing some louvers in the same area.  Attached is a picture of the cowl after I cut out the area that would form the bottom of cowl flap.
Couldn't resist the urge to test fly the plane with that big gaping hole in the bottom.   I was thinking that it would show if it was the small size of the stock opening that was causing the problem or if aerodynamic factors dominated.

The 'big hole' outlet ran about 10 degrees cooler than stock which was not nearly the improvement I'm looking for so I can't believe that the louvers would work in this location.  Back to working on the cowl flap in the hopes that the low pressure area behind it will extract a lot more air.   I don't have a warm fuzzy about it though.   After this (if it doesn't work) I'll try a bluff body extractor vent as described by some CAFE Foundation reports.

Just as a point of reference, I have about 35 sq in of inlet area and 52 inches of outlet area with the stock cooling outlet.   Now I know that the inlet area is more than large enough (from the cowl off flight test) and the 52 sq In outlet should be enough but I think the pressure is too high under the fuselage where the outlet is.

Tracy

On Thu, Sep 17, 2009 at 6:13 PM, <SHIPCHIEF@aol.com <mailto:SHIPCHIEF@aol.com>> wrote:

    Thanks for that.
    I'm making my radiator chin scoop right now.
    I made a 'buck' out of 2 part pour foam, then cut & sanded to
    shape. After laying up the part, and removing it, I began to
    remove the edges. Good thing I made it too deep. Now you guys have
    revisited the Exit/Inlet relationship issue.
    A little time with a ruler, some quick math, and some trimming is
    leading me right to the reccomended 30% bigger exit area than
    inlet area for the radiator. I was going to use Van's original
    exit, but now I'm adding about 1/2" more depth.
    My oil cooler exit area is really over sized. It is kept separate.
    The right cowl inlet is ducted to the oil cooler, then exhausted
    straight out the right side thru louvers.
    The left cowl inlet has not been made yet. It will feed air to the
    turbocharger and provide surface cooling air for the exhaust
    system and fuel injectors. If that chokes the air exit at the
    bottom, I'll have to make a left side hot air exit as well.
    Tracy, your comment about overheated coils and alternator has me
    thinking about blast tubes?
    Scott


------------------------------------------------------------------------

------------------------------------------------------------------------

--
Homepage:  http://www.flyrotary.com/
Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html

--
Homepage:  http://www.flyrotary.com/
Archive and UnSub: http://mail.lancaironline.net:81/lists/flyrotary/List.html