REFLECTOR: Reflector Digest, Vol 98, Issue 33

[Clayton Chase]

The increase in performance due to engine output would be noticeable on a
fixed-pitch prop as an increase in what RPM the engine is capable of at any
given airspeed.      You wouldn't be able to check it on a adjustable pitch
prop.

On Fri, May 10, 2013 at 9:09 AM, Stockman, Bill <
bill.stockman at daytonaero.com> wrote:

> You are all forgetting the obvious.****
>
> Accepting the increase in performance as described by Dave, it may also be
> coming from improved engine performance.****
>
> The four pipes out the back are somehow acting like a tuned exhaust which
> will greatly improve engine output.****
>
> I have a four into one exhaust on my Velocity173 that also exists to the
> rear and noticed an improvement over the standard ****
>
> exhaust.    I didn’t get 15 kts improvement, but did gain 4-5 kts.****
>
> bill****
>
> ** **
>
> *From:* reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] *On
> Behalf Of *Dave T Nelson
> *Sent:* Friday, May 10, 2013 11:53 AM
> *To:* reflector at tvbf.org
> *Cc:* reflector-bounces at tvbf.org
> *Subject:* Re: REFLECTOR: Reflector Digest, Vol 98, Issue 33****
>
> ** **
>
> I'll just say these two things - First, I disagree with your assessment
> (esp. #2),  and secondly the speed increase I saw when changing from "out
> the bottom" to "out the back" was very real and quite substantial.
>
> Dave
>
> Dave T. Nelson
> T/L 553-4327, Voice 507-253-4327, Fax 507-253-3648
> Program Director, ISC ECAT NPI & Test Engineering
>
> 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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