REFLECTOR: Reflector Digest, Vol 98, Issue 33

[Grover McNair]

That is a good point Bill. I know from auto racing experience that having
equal length tubes prior to a collector benefits the scavenging effect on
each cylinder. Well made exhaust headers will increase power.

Several key factors to consider are: runner diameter and length and
collector outlet size, and configuration. Generally shorter runner length is
better for lower rpm operation and provides better torque. In six cylinder
engines a tri-y configuration also helps with torque. Six into one with
longer runners will make more horsepower at higher rpm but this is not where
most aircraft engines run at. Another automotive trick is to have a balance
tube on six or eight cylinder exhaust systems. It is located after the
collectors and connects both sides of the exhaust. The benefit is more
efficient scavenging. Be sure, whatever exhaust you have, to ceramic coat
them inside and out for thermal efficiency.

Grover 

 

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Stockman, Bill
Sent: Friday, May 10, 2013 12:09 PM
To: Velocity Aircraft Owners and Builders list
Cc: reflector-bounces at tvbf.org
Subject: Re: REFLECTOR: Reflector Digest, Vol 98, Issue 33

 

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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