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 -----

From: Michael Haynes

To: Rotary motors in aircraft

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 -----

From: John Slade

To: Rotary motors in aircraft

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