Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Thu, 01 Aug 2002 07:55:25 -0400
Message-ID: <redirect-1670465@logan.com>
From: StarAerospace@aol.com
Subject: Turbine power?
MIME-Version: 1.0
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit

<< There are a few other positive considerations:
 1: Weight.  Typically turbines are much lighter than recip engines for the
 same or greater horsepower.
 2: Reliability:  Turbine engines are much more reliable. (Certified anyway)
 than recip engines. >>

One can easily say that these are the two biggest misconceptions about 
turbine engines as well.

1. Weight.  Sea level, static HP or thrust (for turboshaft and turbofan 
engines respectively) is not cruise HP or thrust at M .40 at FL250.  Turbine 
engines are normally aspirated, single cycle engines.  An Otto/Brayton 
combined cycle will always beat an Otto or Brayton cycle on their own.  At 
FL180, your combined cycle TSIO-550N is catching up fast to a comparable 
weight class turbine in terms of thrust to installed weight.  

Therein lies the second caveat on the weight issue:  installed weight vs. 
book weight.  Continental and Lycoming lie like politicians about their book 
weight, but can't hold a candle to most turbine manufacturers.  I don't have 
personal experience with the engine in question, but the difference between 
the book and installed weight of a PT6-67A is almost 40%!  Wait for a side by 
side aircraft comparison before believing that "100 lb." advantage...

As you speed up in a turbine, two things happen:  the SFC rises 
disproportionately vs. the thrust rise from ram effect;  and the total 
equivalent HP drops off due to the loss of residual exhaust thrust with 
speed.  That turbine exhaust is coming out only so fast.  At seal level, it's 
quite a boost.  At speed, it's not so much.

2. Reliability.  Almost no one gets full TBO from a high performance engine, 
right?  Wrong.  The better charter operators have been doing it for years as 
have many flight schools (show me worse operating conditions).  At the same 
time, the owner flown Mirage that makes TBO without a jug change has yet to 
be found.  Most B55 operators make TBO without a jug off, while few B36TC can 
keep their engines out of the shop.  We can only say from this that piston 
engine reliability is mixed bag of make, model, operation and maintenance.

Turbines never fail, right?  Nope, sorry.  IFSR (in flight shut down rate) of 
turbines of the generation being discussed is pretty poor.  Yes, modern 
turbines with either FADEC or rudimentary temperature-based fuel limiting 
controls are quite reliable.  Neither the Walter, nor the ATP has these 
protections.  Shove the throttle up too fast and not watch the TIT like a 
hawk and WOOOOOOFFFFF!  There goes the hot section.  You're now a glider, and 
the rebuild will cost you more than a new piston engine.  This is not 
hypothetical, it's happened on everything from a CF-6 on a DC-10-30 to the 
AI-25 on the L-39.  It'll happen to your turbine Lancair if you throttle up 
too quick.

Power to weight and power to $$$, the turbine is unbeatable at sea level.  At 
altitude, not below 2,000 to 5,000 HP.

Gary Casey summarizes most of the issues with small turbines well.  For me, I 
worry that many are investing in heavy and expensive turbine engines thinking 
they are getting a piece of modern airliner technology when they are not.  
When it flies and it flies side by side with a well tuned piston version of 
the same aircraft, THEN we can make a comparison.  So far, that comparison 
has shown small turbines to be less than we had hoped for.

In the meantime, I will wait for a modern (and cheaper) Jet A piston.  No, 
not the expensive nonsense coming from SMA, nor the auto conversion certified 
by Theilert, nor the government welfare R&D of Zoche.  Yea, the holy grail is 
out there, it just hasn't made it to our market yet...

Eric