REFLECTOR: Plume Drag

[Geoff Gerhardt via Reflector]

I changed my exhaust from the typical perpendicular to the fuselage to
exiting out the back inside the cowl and did see a significant improvement
(6-8ktas).  I think its a combination of plume drag as well as disturbing
the air going into the prop.

I am currently re-jiggering my exhaust again, this time exiting out the
bottom of the cowling rather than inside the cowling (similar to what Brett
described
<http://www.velocityxl.com/content.php?162-Section-Exhaust-Augmentation>).
 I have the factory exhaust fairings (VEFL-01 and VEFR-01) and I'm just in
the process of modifying my cowl to add them.  I found the scorching of the
prop excessive with it venting in the cowl.  With a three bladed prop and
the exhaust pipes located 180deg apart, there's no way to clock the prop so
one blade doesn't get a hot exhaust blast.  By moving the exhaust to exit
below the cowl I'm hoping that I get less prop scorching.  I don't mind
cleaning off the soot, but I was getting enough heat effect on my Catto
prop so that I could see it was affecting the fiberglass.

Geoff



On Thu, Jul 3, 2014 at 12:02 PM, Brian Michalk via Reflector <
reflector at tvbf.org> wrote:

> I just finished an email conversation with Marc Zeitlin on plume drag.
>
> My question to him was if he knew of any references to research in this
> area.  He does not, but believes the reason for why people think plume drag
> exists is due to three factors.
>
> 1) Perpendicular pipes at the fuselage will have some portion impacting
> free air, and will therefore have some parasitic drag.  If the tubes are
> arranged such that they do not impinge free air, then there is no
> associated parasitic drag.  This would be the directly aft configuration
> where tubes never really leave the cowl envelope.
> 2) There is some thrust to be gained from pointing the exhaust aft. I'm
> not claiming its much, but I acknowledge this.  Not pointing it aft is
> wasting that extra little bit of thrust.
> 3) Perpendicular ejection of exhaust will cause some separated flow from
> the fuselage due to the exhaust plume, increasing parasitic drag.
>
> He agrees that the plume itself cannot cause the drag, because it's not
> attached to the airplane, but there could be other factors increasing drag,
> and two of them have nothing at all to do with the plume.
>
> He also pointed me to this conversation:
> http://copilotco.com/mail-archives/matronics.2003/msg53894.html
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