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John -
> From: John Kleber & Annette Worthington <73761.230@compuserve.com>
> Subject: Turboprop Engine for Legacy
I, too have been researching turboprop engines for my Legacy (kit #62).
> 1. What is the formula to convert "pounds of thrust" into horsepower?
Turboprop and turboshaft engines are rated in horspower, not thrust.
> 2. Does anyone know of a turboprop engine designed (not derated) to
> produce the equivalent of 300-350 horsepower?
The most suitable engine I have found is the Rolls-Royce Allison model
250. It comes in many versions, most of which are for helicopter
applications (6000 rpm output shaft speed). There is a turboprop
version, the 250-B17 model which has an extra gear reduction to bring
the output shaft speed down to 2000 rpm. Horsepower ranges from 350 to
800, depending on engine version. Specific fuel consumption is not as
good as with piston engines. The Allison engines burn 0.6 to 0.7
lbs/hp/hr, compared to 0.4-0.5 for piston engines.
Now the bad news. Used engine prices range from $60-100k for the
turboprop model. I have often wondered what would be involved in
converting a helicopter version of the engine. There are tons of Allison
250 helicopter engines out there, including military surplus (army
designation T-63). Perhaps you could look into it.
Other turboprop engines include the Pratt & Whitney PT6-A and Walter
engines. These are 600hp and up, which IMHO would be too much for a
little two-seater. However, some people are putting them in L-IVp's.
The only other turboshaft engines under 1000hp I have found are
converted APU's from fighter jets and transport planes. Unfortunately,
they all seem to be in the 100-150 horsepower range. Additionally, they
are not particularly fuel efficient.
> 3. What other considerations must be addressed to make a turboprop Legacy
> a reality?
You would have to rethink your weight and balance, since these engines
come in around 200lbs, including accessories. You would need a custom
motor mount. Some people have suggested starting with the stock Lancair
mount (it provides the nose gear attach and pivot points), cut away what
you don't need, and have new tubes welded on to fit the engine. You
would be on your own to develop ducting, baffling, and wiring harnesses.
It is anyone's guess whether the whole assembly will fit under the stock
cowl. Generally turboprop planes have very long narrow snouts. You might
have to throw away your stock cowl and make a new one from scratch.
> 4. Is anyone else interested in installing a turboprop engine on their
> Legacy?
Yes, I still want to do it. The major stumbling block for me is the
price. As an alternative, I am currently researching the Mazda 20B
3-rotor engine. Price for an aircraft conversion is around $13,000
complete. Also, there are lots of race shops and parts distributors
around to support the Mazda car enthusiasts.
>
> Any positive and constructive help will be greatly appreciated!
>
I'm sorry I couldn't give you all good news, but there are still lots of
pluses to turbine engines, mainly low weight, high reliability, and high
TBO. Keep me posted as your research progresses.
-Adam Molny
Legacy Kit #62
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