REFLECTOR: Plume Drag

[Brian Michalk via Reflector]

How far away from the prop was your exhaust exit?

On 07/04/2014 05:15 PM, Geoff Gerhardt via Reflector wrote:
> 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 <mailto: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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