REFLECTOR: Performance

Fri May 10 14:06:26 CDT 2013

Hi Geoff, 

   I disagree with one itsy-bitsy change at a time. Unless you're really good at doing about an hour of testing for every little change. I tested props, with an experienced co-pilot doing recordings onto a Team Rocket data sheet, each test flight regimen took about an hour, for 3-4 different altitudes, triangular courses, etc.

   You want a faster airplane. Go straight for that. 

   Also note that Scott put a bit of fairing in front of his pipes. A cylinder is the second-third worst thing you want to expose to an airstream - a flat plate being a 1.0 rating.  A forward facing cupped half cylinder is actually worse (+ over 1.0), but a cylinder is .90 of flat plate drag.   See http://sacrvators.com/Building%20Tips.htm Dave Anders part.

   Turn the pipe while it is still inside the airplane, and you're not exposing parts of a cylinder to airstream.  You might look at this...
http://www.tvbf.org/gallery/index.php?dir=engines%2Fexhaust_jean_prudhomme

Regards,

Jeff Barnes

--- On Fri, 5/10/13, Kevin Baker <flykb at verizon.net> wrote:

From: Kevin Baker <flykb at verizon.net>
Subject: Re: REFLECTOR: Performance
To: "geoff.gerhardt at gmail.com" <geoff.gerhardt at gmail.com>, "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Cc: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Date: Friday, May 10, 2013, 1:12 PM

I think that would be a perfect start.
One small change at a time is the way to go

Thank you Geoff for doing this.
Kevin Baker

.

On May 10, 2013, at 12:31 PM, Geoff Gerhardt <geoff.gerhardt at gmail.com> wrote:

I could likely do something similar, just put an elbow on my exhaust exit so it points to the rear instead of down as a first pass and see if any improvements in speed are observed.

Heres what my exhaust currently looks like.

Geoff

On Fri, May 10, 2013 at 12:44 PM, Scott Derrick <scott at tnstaafl.net> wrote:

I did change from straight down to rearward facing exhaust. I saw a 3-5 knot increase.

I attached before and after images.

Scott

-------- Original Message --------
Subject: Re: REFLECTOR: Performance

From: Richard J. Gentil <richard at naples-air-center.com>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>

Date: 05/10/2013 09:47 AM

I would say we need someone (Geoff, Geoff) who only changes the exhaust and nothing else, runs a test and reports back to the Reflector with the findings

Richard

Sent from my iPhone 5

On May 10, 2013, at 11:38 AM, Kevin Baker <flykb at verizon.net> wrote:

I would agree.
This is the type of info I've read about as well on the cozy /long forums with similar results

Thanks
Kevin Baker

.

On May 10, 2013, at 10:12 AM, David Rene Dugas <renedugas at gmail.com> wrote:

David,
I would suggest that #2 is significant because the large plume of hot exhaust is forcing a dramatic change in direction on a large shout of air traveling at 200 mph. This would act like a speed break.   The energy to move this air would be parasitic drag and would be substantial.   I realize there is no mechanical connection but there is a gaseous connection the same as one has with the earth when in flight so very real. I'm not an engineer so I do not has the formulas to calculate it but my 2 cents worth is that that plume and pipes are a big source of drag. Just my maniacal machinations. 😳

Rene' Dugas
Sent from an Apple.

On May 10, 2013, at 8:06 AM, "David Ullman" <ullman at robustdecisions.com> wrote:

IF I added up all the speed increases I could get from the topics discussed
here, my plane would be supersonic.

I too would like to see proof that rear facing exhaust helps.  I have now
heard four theories: 1) the exhaust gas acts like a jet, 2) the
perpendicular flow adds drag, 3) The exhaust disturbs the boundary layer on
the cowl, 4) the exhaust gas disturbs the flow seen by the prop.

The first one I calculated, and the thrust due to increased velocity of the
exhaust gas is minimal.  For #2, the plum has no way to transfer its drag
back to the plane.  Picture the plume as a cylinder projecting into the
airstream.  It is not connected to the plane in any way to slow it down, so
#2 is out.  The boundary layer at the back of the plane is already well
chewed, so #3 is not much of a possibility.  I hadn't thought of disturbing
the prop flow before.  The wings are already doing a good job of this, could
the exhaust add to this disturbance in a significant manner?   I can see
where moving the exhaust to the hub area (or outside the prop radius) may
have an effect, the first three are just noise.

David Ullman
N444DX
President EAA 292
541-760-2338
david at davidullman.com

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Sent: Thursday, May 09, 2013 7:47 PM
To: reflector at tvbf.org
Subject: Reflector Digest, Vol 98, Issue 27

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