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