Firewall Relocation

[Monocoupe]

Hello All,
Since there has been recent discussion about the merrits of moving a Rebel firewall back three inches for a Lycoming O-320 installation...... Here is a copy of the letter from the archives that has been referenced. Now you don't have to search the archives for it!

Cheers,
Nigel

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 ¾ of the way down the tail cone. The weight was
attached to a hand crank through a series of cables, bicycle chain and pulleys. With the
movable weight we were able to stall and introduce many spins at various CG positions
all in one flight. When things started going wrong (after 34% - 35%), we would madly
crank the weight forward to a CG position that allowed normal recovery. Having the
advantage of flying the aircraft at various CG positions at the same weight within
minutes, really showed the difference between forward and aft CG.
If a builder could reduce the weight of his Rebel by 60 – 80 lbs. with only 6 – 8 hours
of extra work, I’m sure every builder would do so. Consider a Rebel on floats flying at
the design allowable gross of 1730 lbs. with the CG at 10.8” or 18% of chord. The
aircraft will require a downward load on the tail of:
1730 x -4.2”
162” = -45 lbs. (downward)
Where: 1730 = Gross weight in pounds on floats
4.2” = Distance FWD of aerodynamic center (C of L)
162” = Distance from C of L of wing to C of L of tail.
C of L is at 15” or 25%
The wing of course has to create enough lift to counter the downward load on the tail
so the wing now has to produce: 1730 + 45 = 1775 lbs. of lift.
Now let’s consider another aircraft which has its CG at 18” or 30% of chord which is
still well within its safe CG range. We now need an upward load on the tail to balance the
aircraft. .
1730 x (+3)
162 = 32 lbs. (upward)
The wing is now being helped by the tail by 32 lbs. so it must now produce lift of only
1730 – 32 = 1698 lbs.
Since the second aircraft is required to produce less lift, it will stall at a lower speed.
In this case, about 1 mph less which can be significant on a float plane. The second
aircraft would also fly slightly faster as its angle of attack relative to its direction would
be less, thereby reducing profile drag. Climb, of course will also be better. And since
induced drag is a product of lift, the induced drag will also be lower.
Another issue to consider is the `moment of inertia` of the aircraft. The lower the
moment of inertia the more maneuverable the aircraft will be. The moment of inertia is
increased by not moving the firewall back and increased even more when weight is put in
the tail to balance the aircraft. This increased moment of inertia reduces the effectiveness
of the rudder in yaw and the elevator in pitch. Any builder who has installed a 320 and
has not moved the firewall back and has added lead to the tail should go slowly when
spin testing.
Over the years I have had a number of builders state that they do not have enough
elevator authority on a three point landing. This can almost always be attributed to a too
far forward CG. Such aircraft normally have 320s installed with the firewall not moved
back.
Lets now look at the structural benefits. The ability of an object to resist buckling is
dependent on the square of its length. The shorter it is, the greater the resistance to
buckling. Fus-070 (lower corner wrap) on a Rebel without the firewall moved back is
approximately 25” long. With the firewall moved back, Fus-070 is approximately 22”
long. The increase in buckling strength at the shorter length is:
(25 x 25)/(22 x 22) x 100% =129%
Or put another way the buckling strength at the longer length is only: 100/129 = 0.77
of the shorter length. By not moving the firewall back you are asking the airframe to
carry as much as 10% (possible difference in weight of the 235 and 320) more load with
only 77% of the strength. An added benefit of moving the firewall back is a weight
reduction of approximately 1 ½ lbs.
So, by moving the firewall back with a 320 we have a reduction in stall speed, an
increase in cruise, better climb, more maneuverablility, greater elevator authority in flare,
greater strength and reduced weight. What is the downside? At the most, 6 to 8 hrs of
building time.
It has been said by more than one person that if you move the firewall back there is no
room for radios. This is not correct. The manual very clearly states that when moving the
firewall back you also move the dash back 3”, so the room for radios and other
instruments is exactly the same.
It has also been said that 3” of leg room is lost. Again this is incorrect as the builder
positions the seat rails to suit himself. I should mention that with the seats and rails
moved back egress is even easier. I have yet to meet a pilot whose legs are long enough
to operate the pedals with the seats in the aft position.
The belief that the windshield must be positioned at a greater angle is also incorrect.
The use of a windshield shelf allows the windshield to be mounted exactly the same as in
the standard Rebel.
It is true that with the seats back 3” there is a 3” loss of cargo space, but with the size
of the Rebel cargo area, it’s hard to imagine running out of space before running out of
useful load.
Years ago, we were asked many times about putting the 360 into the Rebel. We took a
good look at it and decided they were not a good match. The Rebel airframe was not
designed for that amount of horsepower. We instead felt it was better to develop a new
airframe for the 360. The result was the Elite. With the extra 150 lbs. gross weight and
the simpler to build stronger airframe, it is a much better match for the 360.
Because these aircraft are experimental we have no control over how they are built or
what power plants are used. I would ask that our builders remember that as aircraft get
older, new maintenance issues arise. I would also like to point out that as aircraft are
passed on or sold, there is no guarantee the new owners will fly and treat the aircraft as
you might.
Another subject that has been mentioned to me recently is the angle of incidence of the
wing on the Rebel and the use of leading edge cuffs. It seems a few builders are
considering increasing the angle of incidence. The use of cuffs will certainly increase the
lift of the wing. A cuff that adds 3” of chord will for no other reason give more lift
because it has 5% more wing area. A properly designed cuff will add even more lift.
One possible downfall of a cuff depending how it is designed is the reduction in the angle
of incidence. This reduction could be ½ degree or greater. Aircraft with such cuffs
would probably benefit by an increase in the angle of incidence. The Rebel like all
aircraft is a compromise of designs. If its sole use was as a float plane we would
probably have increased the angle of incidence during the initial design. But since it is
expected to perform a variety of roles with various horsepower the best overall angle was
chosen. The Elite, which we expected to normally have greater horsepower installed than
the Rebel and therefore higher cruise speeds has a lower angle of incidence.
What makes aircraft hombuilding such a great recreational pursuit is our freedom to
change, create and express ourselves through our machines. With this freedom comes the
responsibility to ensure that the aircraft are safe not only for our selves but for our
passengers as well.
Regards
Darryl