[rebel-builders] Should you move your firewall back 3"?

[N.Smith]

Just to say thanks to Darryl and Mike for the letter - great explanation and
food for thought.
Nig
745E

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com]
Sent: 29 January 2007 20:04
To: rebel-builders@dcsol.com
Subject: [rebel-builders] Should you move your firewall back 3"? A
letter from Darryl Murphy.


I just received the following letter from Darryl regarding his views on
moving the firewall back 3". He asked that I share it with all the builders
on the list. I have also posted this letter as a PDF file in the file
library.

https://mail.dcsol.com/file/area206/let ... arryll.pdf

Mike

---------------------------------------------------------------------

TO ALL REBEL BUILDERS

Recently there has been a lot of discussion on the need to move the
fire wall back 3" when installing a 320 engine into a Rebel. There are
proponents in each camp as well as a considerable amount of false
information out there.

Lyc 320 powered Rebels have now been flying for 15 - 16 years and there
are many new and older builders who do not know or have forgotten my reasons
for moving the firewall back. Therefore, I would like to re-hash my
reasoning.

First off; I would like to say that although many people believe the
Rebel was originally designed for the 80 hp 912 Rotax, this is not so. It
was really originally designed for the 108 -125 hp 235 Lycoming engines.

The use of the 80 hp Rotax 912 was for marketing. We wanted to show the
public that it would fly well on 80 hp. In the home building industry,
particularly in North America, it is believed by many builders "That if some
is good, more is better", especially when it comes to horse power. If the
first Rebel had started with a 235 or 320, I'm sure we would now be seeing
540 powered Rebels. I know most of you would think that ridiculous but I
have fielded questions about the 470 Continental, 540 Lycs, 220 hp
Franklins, a number of turbines and of course many automotive conversions
from 80 hp to 300 hp for Rebel installations.

The idea behind moving the firewall back 3" for the heavier engine is
very simple. It is done to put the CG range in a more favourable position
for good flight characteristics. I am not talking about the safe CG
envelope of the aircraft but rather the good flying CG range within the safe
CG envelope of the aircraft.

Over the years, I have had the opportunity to fly in many variations of
Rebels and I have never been in one that flys badly. However, those that
were the most delightful to fly, were those built lightly with a more aft CG
(25% - 30%). This was never more apparent as when Robin and I were doing
spin testing. The test aircraft was set up with a heavy lead weight within a
large diameter ABS pipe. The ABS pipe was positioned between the seats and
extended about