REFLECTOR: Performance

Thu May 9 15:16:02 CDT 2013

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****
>
> ** **
>  ------------------------------
>
> *From:* reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] *On
> Behalf Of *Brian Michalk
> *Sent:* Thursday, May 09, 2013 12:24 PM
>
> *To:* Velocity Aircraft Owners and Builders list
> *Subject:* Re: REFLECTOR: Performance****
>
>  ** **
>
> 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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