REFLECTOR: Plume Drag

[Jeff Barnes via Reflector]

Hi Geoff,


      If your pipes were just sticking out perpendicular to the plane, you have nearly one of worst drag configurations: CYLINDERS. A cylinder is .95 the drag of a flat plate, in other words, damn near as bad.


      Look at Jean Prudhommes exhaust in the upload section of TVBF.ORG,


Regards,

Jeff Barnes


________________________________
 From: Brian Michalk via Reflector <reflector at tvbf.org>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org> 
Sent: Friday, July 4, 2014 8:41 PM
Subject: Re: REFLECTOR: Plume Drag
 


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).  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
>>_______________________________________________
>>To change your email address, visit http://www.tvbf.org/mailman/listinfo/reflector
>>
>>Visit the gallery!  www.tvbf.org/gallery
>>user:pw = tvbf:jamaicangoose
>>Check new archives: www.tvbf.org/pipermail
>>Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
>
>
>
>_______________________________________________
To change your email address, visit http://www.tvbf.org/mailman/listinfo/reflector Visit the gallery! www.tvbf.org/gallery user:pw = tvbf:jamaicangoose
Check new archives: www.tvbf.org/pipermail Check old archives: http://www.tvbf.org/archives/velocity/maillist.html


_______________________________________________
To change your email address, visit http://www.tvbf.org/mailman/listinfo/reflector

Visit the gallery!  www.tvbf.org/gallery
user:pw = tvbf:jamaicangoose
Check new archives: www.tvbf.org/pipermail
Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.tvbf.org/mailman/private/reflector/attachments/20140704/4445de7c/attachment-0002.html>