REFLECTOR: Plume Drag

[Brian Michalk via Reflector]

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