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Hi Mick, thanks for confirming the more air less
boost. I was a bit concerned when you first mentioned more boost as that is not
what we need.
From what I could tell about the mod from the
photos on Macs web site. This is how their modification works to produce
more air flow but less boost.
First, they replace the stock compressor wheel
with one of larger size. Second, they reduce the effect of the
exhaust mass flow on the turbine wheel by enlarging the exhaust exit around the
turbine blades permitting some exhaust gas to slip by without imparting energy
to turning the turbine wheel and they all also enlarge the waste gate
port. These both reduce the RPM of the turbine/compressor wheel rotating
assembly.
The results is you have a larger compressor
wheel/blades which is turning slower However, the larger/longer
blades of the bigger compressor wheel is "grabbing" more air on each
revolution than did the stock wheel. Now , since the compressor wheel is
spinning slower due to the exhaust mass flow diverted around the turbine
wheel you get more air volume (but less air velocity ) from the compressor blades. Also the outlet
nozzle of the compressor housing is a diffuser (slows down the air and
creates pressure). I suspect that they might also enlarge the exit nozzle
inner diameter as this will also reduce the conversion of air velocity to
pressure but I don't recall reading that so perhaps they leave it
alone..
Mass flow = pVA (density, Velocity, Cross
sectional Area) provides the key to the reason enlarging the compressor
exit nozzle (diffuser) reduces boost. The mass continuity law says that
the product does not change - so if suddenly the mass flow encounters a larger
area (A increases) then Velocity must decrease to maintain a constant
product. Here its simply the
dynamic pressure equation 1/2 pV^2. If velocity is decreased
then the dynamic pressure equation 1/2p V^2 tells us with less V there is less
energy to convert into a pressure increase.
They have
lowered the velocity V of the air flow into the diffuser therefore there is less
dynamic pressure (air velocity) to convert into pressure. In other
words, boost is not conditional on the amount (volume) of air you flow
- but depends on the velocity you accelerate it to before it flows
into the diffuser (among some other factors). So you have lower
velocity giving you less boost but you still have the larger volume
of air produced by the bigger compressor wheel.
Ergo, you have more air flow but less boost for the
same exhaust mass flow.
At least thats the way it appears to
me.
Ed
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
Sent: Thursday, August 12, 2004 10:36
PM
Subject: [FlyRotary] Re: Turbos from the
land of Oz
Hay John
Dropped off my turbo yesterday and
confirmed more air flow less boost! I STAND CORRECTED. I Still
don't under stand. Anyway Max has taken a lot of interest in what we are doing
and is researching more on a way to control boost at alt. He did mention that
the microtec computer that is used down here a lot does compensate boost with
alt but not sure to what alt it goes to ( designed for driving in the hills ),
also has limp home mode. Ill find out more when I see him
next.
Cheers
Mick
----- Original Message -----
Sent: Wednesday, August 11, 2004 12:47
AM
Subject: [FlyRotary] Turbos from the
land of Oz
G'day
Mick
> I must say you did a good job on the turbine or what
was left anyway!!!.
Thank
you, thank you. I try hard to please. :)
Thanks for the update. It's a
little scary dealing with a company so far away, but Max seems like a
straight up guy so I never really had any doubt he'd come through. Just took a little longer than I'd
hoped.
400HP.
Yes. he mentioned that. A bit lower at my max rpm of 6300, but still -
plenty of grunt. :)
> From what he explained to me
was that you will get more boost for a slower turbine rpm,
I thought
it was less boost for the same airflow.
> He did mention
something about throwing in a couple of peacock feathers just incase you
blow it again.
Oh! Nice
touch.
> Now im thinking of changing
prop size. ( currently ivo 70" variable high pitch 3
blade ) can you make any suggestions.
Well
mines a wood 68 three blade with 314 blade area from Performance.
It'll accelerate the plane to take off in 1000 - 1500 feet at 4300 rpm and
climb quickly at 5000. It'll even climb at 2300 rpm. Should be ideal for
about 280 hp in cruise and should get me a better top speed than a variable
pitch. Given that you need 2000ft to land I'm not sure a variable pitch
prop will buy you anything on a Cozy, and it'll probably cost you at the top
end. Are you building a Cozy or an RV? I forget.
Cheers,
John
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