Mailing List flyrotary@lancaironline.net Message #2805
From: David Leonard <Daveleonard@cox.net>
Subject: Rusty's Power seems about right
Date: Thu, 4 Sep 2003 22:11:17 -0700
To: 'Rotary motors in aircraft' <flyrotary@lancaironline.net>
Message

Rusty,

 

I have been thinking about your apparently less power than you expected for a given manifold pressure (ie. 30 in Hg) and thought of a few points you may not have considered.   You were disappointed that power seemed to be anemic at 30 in. compared to what you would expect an n/a engine to produce at WOT.  However, with the turbo, several things change. 

 

First is back pressure.  That turbo is creating significant back pressure that an NA engine doesn’t see.  This mean that for a given intake pressure you are producing less net torque and therefore a lower h.p., which leads to a lower RPM – and even less h.p.  You should make your power references to n.a engines based on the RPM you are producing and not on the MAP.  You may have to run a higher MAP to get the same RPM as a n/a engine, but that is what the turbo is all about, the availability of a higher MAP.

 

Next is temperature.  Especially without an intercooler, your 30in. air is considerably less dense than 30in n/a air resulting in lower power.  Again, compensate by running a higher MAP until your RPM is where you want it to be.  You will ultimately have more available power, but nothing is for free.

 

David Leonard

The Rotary Roster:

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