John, I think you may be correct about the overspeed.

That is one of the dangers with using a blow off valve as I mentioned
before.  The compressor wheel is already speeding huffing and puffing faster
at altitude than sea level to produce the same amount of  boost and suddenly
you remove some of the resistance it is working against on the intake side
through a blow off valve.  Already revving at high speed because of the
altitude with plenty of exhaust mass flow spinning the other end - the
turbine the blow off valve suddenly reduces the pressure (and therefore
resistance the compressor wheel sees) and with less load on the compressor
wheel the rotating assembly rapidly increases in speed even more.

A waste gate of course reduces the exhaust mass flow and slows the turbine
down, a blow off valve (at least momentarily) simply reduces the boost by
bleeding off the air the compressor is striving to pressurize to maintain
the boost pressure.  Yes, eventually the lack of boost will cause the
exhaust flow to slow down - but not perhaps before overspeeding the rotating
assembly.

So while blow off valves may be OK for autos at sea level, I would really
hesitate to put one on an aircraft.  That of course just my personal
opinion.

Ed

Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
From: "John Slade" <sladerj@bellsouth.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Thursday, July 08, 2004 10:43 PM
Subject: [FlyRotary] Turbo post mortem


> I took my (ex Rusty's) turbo apart this evening. The bearings seem to be
in
> fairly good shape and the shaft looks ok. It looks like the compressor
wheel
> just "came off the end" of the shaft, much like the other one did. My
> uneducated guess would be that I overspeeded it.
>
> By the way, I was showing 38 MAP at 11,500 ft with the wastegate fully
open.
> However,  there's an open 1/2 inch air bleed on the intercooler (to be
> closed off) and a blow off valve, so the turbo may have been putting out
> much more than the MAP showed.
>
> John Slade
> Rotary Cozy IV
>
>
>
>
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