REFLECTOR: Performance

Fri May 10 08:06:11 CDT 2013

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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Subject: Reflector Digest, Vol 98, Issue 27

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Today's Topics:

   1. Re:  Performance (Dave T Nelson)
   2. Re:  Performance (Kevin Baker)
   3. Re:  Performance (Scott Derrick)
   4. Re:  Performance (Rene Dugas)

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Message: 1
Date: Thu, 9 May 2013 21:03:57 -0500
From: Dave T Nelson <dtnelson at us.ibm.com>
To: reflector at tvbf.org
Cc: reflector-bounces at tvbf.org
Subject: Re: REFLECTOR: Performance
Message-ID:

<OF65167550.45874A96-ON86257B67.000B2B13-86257B67.000B58D7 at us.ibm.com>
Content-Type: text/plain; charset="utf-8"

For what it's worth, and my memory could be wrong here, I seem to remember
either Burt or perhaps Mike Melville saying that the exhaust thrust could
contribute perhaps 1-2 knots....

Dave

Dave T. Nelson
T/L 553-4327, Voice 507-253-4327, Fax 507-253-3648
Program Director, ISC ECAT NPI & Test Engineering

      To: Velocity Aircraft Owners and Builders list       
          <reflector at tvbf.org>                             

 Subject: Re: REFLECTOR: Performance                       

Grover,

I think the thrust you'd get from the exhaust would be minimal. ?Think
about this - if you had the exhaust pointed back and took the prop off.
Even with the engine at full power it would not likely push the plane on
the ground a even a little. ?I think it is the disruption of laminar flow
around the cowling that is thought to create drag. ?Intuitively, if I had a
smooth airfoil, but then had a bunch of jets inducing air low normal to its
surface, I would think that would be bad for its aerodynamic efficiency.

Also that the exhaust is throwing turbulent flow into the prop is thought
to be a problem. ?The prop may not be able to perform with the same
efficiency it would if a smooth air flow was feeding it.

Geoff

On Thu, May 9, 2013 at 2:43 PM, Grover McNair <grover at mcnairperformance.com
> wrote:
  I?m not an engineer but I would think the thrust from the exhaust aimed
  to the rear might be part of the benefit. Also when the exhaust exits
  perpendicular to the laminar flow around the fuselage it makes sense that
  it would cause turbulence and drag.

  Grover McNair

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Message: 2
Date: Thu, 09 May 2013 21:12:39 -0500
From: Kevin Baker <flykb at verizon.net>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Cc: "reflector at tvbf.org" <reflector at tvbf.org>,
	"reflector-bounces at tvbf.org" <reflector-bounces at tvbf.org>
Subject: Re: REFLECTOR: Performance
Message-ID: <2FD2DF54-EC23-4B3D-8B24-6669FDA3DDCE at verizon.net>
Content-Type: text/plain; charset="utf-8"

Any/ or also with the removal of down pointing exhaust drag.

Kevin 

.

On May 9, 2013, at 9:03 PM, Dave T Nelson <dtnelson at us.ibm.com> wrote:

> For what it's worth, and my memory could be wrong here, I seem to remember
either Burt or perhaps Mike Melville saying that the exhaust thrust could
contribute perhaps 1-2 knots.... 
>   
> Dave
> 
> Dave T. Nelson
> T/L 553-4327, Voice 507-253-4327, Fax 507-253-3648
> Program Director, ISC ECAT NPI & Test Engineering
> 
> To:
> Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
> Subject:
> Re: REFLECTOR: Performance
> Grover,
> 
> I think the thrust you'd get from the exhaust would be minimal.  Think
about this - if you had the exhaust pointed back and took the prop off.
Even with the engine at full power it would not likely push the plane on the
ground a even a little.  I think it is the disruption of laminar flow around
the cowling that is thought to create drag.  Intuitively, if I had a smooth
airfoil, but then had a bunch of jets inducing air low normal to its
surface, I would think that would be bad for its aerodynamic efficiency.
> 
> Also that the exhaust is throwing turbulent flow into the prop is thought
to be a problem.  The prop may not be able to perform with the same
efficiency it would if a smooth air flow was feeding it.
> 
> Geoff
> 
> On Thu, May 9, 2013 at 2:43 PM, Grover McNair
<grover at mcnairperformance.com> wrote:
> I?m not an engineer but I would think the thrust from the exhaust aimed to
the rear might be part of the benefit. Also when the exhaust exits
perpendicular to the laminar flow around the fuselage it makes sense that it
would cause turbulence and drag.
> Grover McNair
> 
>  
> 
> _______________________________________________
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Message: 3
Date: Thu, 09 May 2013 20:22:54 -0600
From: Scott Derrick <scott at tnstaafl.net>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Subject: Re: REFLECTOR: Performance
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Message: 4
Date: Thu, 9 May 2013 21:45:31 -0500
From: Rene Dugas <renedugas at gmail.com>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Subject: Re: REFLECTOR: Performance
Message-ID: <F8774F3B-9B29-4684-9223-81BB618A81EC at gmail.com>
Content-Type: text/plain; charset="utf-8"

Brian, I did oil studies to show air disturbance from IO-540 down exhaust
exiting about three inches.  The air disturbance for each side was about 14
inches in diameter. I've never gotten around to building tear drop fairing
but I'm certain they would help.  Exit out the back would be even better or
through the prop like my outboard motor.  I know I got carried away.?

Sent from Rene's iPad. A pleasure.  Go Apple.

On May 9, 2013, at 11:23 AM, Brian Michalk <michalk at awpi.com> wrote:

> I still don't understand the drag mechanism for normal angle of exhaust
being a drag.
> Does anyone know where I can find NACA or other research on this?
> 
> On 5/9/2013 11:07 AM, Richard J. Gentil wrote:
>> Geoff,
>> 
>> If you make the exhaust change I would be very interested in your results
since mine currently exit straight down.
>> 
>> Richard
>> 
>> Sent from my iPhone 5
>> 
>> On May 8, 2013, at 10:59 PM, Geoff Gerhardt <geoff.gerhardt at gmail.com>
wrote:
>> 
>>> Scott,
>>> 
>>> Thanks - that was the insight I was looking for and what I suspected.  I
would actually prefer to have the normal, shorter wing - I'd love the 15kts
in speed and would gladly live with a little higher stall speed.
>>> 
>>> Now, given that, I wonder if there's anything I could do that could
improve my performance.  The two things that I'm considering are changing my
exhaust config from exiting straight down to exiting inside the cowling at
the prop.  I've heard some folks state they have seen significant
improvements with that mod.  The other was an intake ram scoop - would that
give me more MAP at altitude and a little more power?
>>> 
>>> Thanks for all the performance comments/comparisons...keep them coming.
>>> 
>>> Geoff
>>> 
>>> On Wed, May 8, 2013 at 8:31 PM, Scott Baker <scottb33333 at gmail.com>
wrote:
>>>> I notice that several responders are giving numbers for a Velocity
Standard RG, while Geoff's aircraft is a Velocity 173 (aka LW 'Long Wing').
>>>> Velocity Standard models, with their smaller wings and reduced drag,
are faster than their 173 brethren ... by 15 knots on average (with
comparable engines and propellers).
>>>> The factory V173FG trainer is hard pressed to see 145KTAS in cruise.  I
think the RG version cruises between 155-160KTAS (about 15knots faster than
the fixed gear model).
>>>> These numbers are pretty close to what Geoff is reporting for his
aircraft.
>>>> Scott B.
>>>> 
>>>> 
>>>> 
>>>> 
>>>> On 5/8/2013 10:38 AM, Geoff Gerhardt wrote:
>>>>> Folks,
>>>>> 
>>>>> I'm getting pretty consistent performance out of my 173RG over the
last 8-9 months since                           initial flight testing.  My
question to you guys is, are the performance numbers I'm seeing typical, or
should I be seeing better performance?  Normally, on a cross country, I'll
fly b/w ~8-12,000', ~60deg LOP which typically has me going 150kias at
8-8.5gph.  Full power, 160kias is about my max speed.  
>>>>> 
>>>>> I have a fixed-pitch Catto prop (66x74, design target 200mph @
2800RPM).  When I'm doing 160kts, my prop is spinning at ~2800RPM.  That
tells me that I have the power to spin the prop, but may have air frame drag
that's keeping me from seeing the 200mph predicted by Catto at 2800RPM.
>>>>> 
>>>>> I have a standard 200hp, Lyc IO-360, Lightspeed EI on one side, Mag on
the other.  Timing as per spec for both EI and Mag.  About 90hrs on the
engine (rebuilt).  Compression/oil consumption is normal.  Very stable
performance from the engine.
>>>>> 
>>>>> My plane is fully finished and pretty tight.  Main/nose gear doors are
not hanging out and are reasonably well tucked in when the gear is raised.
Main wing angles of incidence are spot on, no rudder trimming was necessary
to center the ball - she flies nice and straight.
>>>>> 
>>>>> The factory predicts I should be able to cruise at 175kias - that's
just not possible with my current setup.  Can others with Std Velocities
relate the performance you are seeing?  Is this as good as it gets, or does
anyone have suggestions for things to check to see if I can eek out a bit
more speed?
>>>>> 
>>>>> Thanks.
>>>>> 
>>>>> Geoff
>>>>> 
>>>>> 
>>>>> 
>>>>> _______________________________________________
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>>>> 
>>>> 
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>> 
>> 
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> 
> _______________________________________________
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> 
> Visit the gallery!  www.tvbf.org/gallery
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