REFLECTOR: Plume Drag
Geoff Gerhardt via Reflector
reflector at tvbf.org
Fri Jul 4 21:42:38 CDT 2014
I was using the standard exhaust that I've seen in many Velocities, but I
agree, that is the worst way to do it.
Brian - I had my exhaust that exited inside the cowl clipped as far back as
possible. I don't know the exact measurement, but I'm not sure I could
have had them back any further.
Geoff
On Fri, Jul 4, 2014 at 10:23 PM, Jeff Barnes via Reflector <
reflector at tvbf.org> wrote:
> 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
> <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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>
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