REFLECTOR: Performance
Mark Magee
edjonesbrady at gmail.com
Fri May 10 09:06:05 CDT 2013
The biggest advantage of rear facing exhaust exiting with the engine
cooling air as in a Long EZ is that the pipes can be shortened to be an
inch or so inboard of the trailing edge of the top/bottom engine cowl. This
will provide scavenge cooling via venturi principle while in ground
operations. OTW the rest of the suggestions posted on this thread should
really be in the category of marginally minimizing drag, marginally
increasing speed.
We pay Velocity all this money, but they can't figure out that downfacing
exhaust costs us 15 kts?
If that were the case we all got fairly well suckered in buying our Velos...
Really guys??
Mark Magee
N34XL
On Fri, 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
>
>
> -----Original Message-----
> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
> Behalf Of reflector-request at tvbf.org
> Sent: Thursday, May 09, 2013 7:47 PM
> To: reflector at tvbf.org
> 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)
>
>
> ----------------------------------------------------------------------
>
> 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
> Message-ID: <518C59FE.40508 at tnstaafl.net>
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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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--
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