REFLECTOR: Reflector Digest, Vol 98, Issue 33

Dave T Nelson dtnelson at us.ibm.com
Fri May 10 10:52:41 CDT 2013


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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