The Murphy Builders Site

Is there some effect on horizontal loads or rotational dynamics as well? I
need to brush the dust off old text books too, but I seem to recall
something about having a large weight at opposing ends of a beam behaves
differently dynamically than having the same weight over the centroid. For
example. a dumbbell with 100 lbs at each end may weigh the same and have the
same CG as a single 200 lb weight in the center, but once you get it
rotating, it tends to want to keep rotating and be harder to stop. Seat of
pants engineering suggests that this is not the kind of help I would want
when trying to stop a groundloop. The term "rotational inertia" jumps into
my head, but there is too much fog between my ears to resurrect dynamics
stuff from classes long ago slept thru. Compensating for a heavy engine on
one end by adding more weight on the other may solve the CG problem, but it
seems it would both increase stress because of a longer moment arm about the
CG as well as increasing the rotational inertia (if there is such a term) if
the plane ever tried to swap ends on the ground. Elite taildragger comes
standard with the firewall moved back 3", so I guess the choice has been
made for me anyhow. Great discussion topic and good responses by all. I've
learned a lot while lurking.
- Chuck Skorupa -
Elite SN 500, Taildragger.
----- Original Message -----
From: Brian Cross <rebelair@idirect.com>
To: Murphy Rebel Builders List <murphy-rebel@dcsol.com>
Sent: Sunday, December 05, 1999 11:19 AM
Subject: RE: Rebel "652" Wing tanks

Hi There Folks

The thought that Bob expressed about moving the firewall back is a very
valid one when dealing with the heavy 0320 engines. A 3" movement does

not

sound like very much, but, if one were to do the stress analysis on it, it
means a great deal. I would have to double check my text books, but I'm
sure that the induced bending stresses in an overhung load such as this

goes

up the square of the distance away from the structure, i.e. double the
distance and you have 4 times the bending stress. In other words, a 3"
movement on a moment arm of about 27" will equate to a significant

reduction

in stress of the nose area. Will it be enough to prevent problems? So

far

so good in my case, but time will tell or should I say one of my bad
landings will tell!

That was a strong enough argument for me to move the firewall back. With
careful placement of the stick i.e. mounted centrally over the 2" cross
tubes as per the original design but with bent sticks to prevent it from
touching the dash, there is no difficulty at all with stick interference

or

forward visibility especially with the speed cowl which really improves

the

view over the nose. With this setup I am very happy with results. After
flying Cessnas, I feel I can see the whole world out front and the only
difficulty I have with the aircraft is to make it stop climbing!
(Seriously, until you get used to all the view over the nose).

Best Regards


Brian #328R

-----Original Message-----
From: Bob Patterson [mailto:bob.patterson@canrem.com]
Sent: Friday, December 03, 1999 10:24 PM
To: Murphy Rebel Builders List
Subject: Re: Rebel "652" Wing tanks



Ken, to confuse things further ...

The Rebel that Wayne is talking about, that he just finished
rebuilding, without moving the firewall, was originally powered by a
modified Formula Power Subaru Legacy. As far as I know, it flew very

nicely

in that configuration, on wheels and straight floats ....
(until the floats - NOT Murphy floats - soaked up a little too much water

!)

:-(

.....bobp

<I> would worry about the forward fuse side walls, and the firewall,
and the top panel on the instrument panel, if that heavy engine were not
moved back, though ....

(and everybody wonders WHY I really like that 158 lb Rotax 912-S !! ;-) )
====== (Great Big Grin !)

-------------------------------orig.--------------------------------------
At 09:13 AM 12/2/99 -0500, you wrote:

Bob
Just to add more confusion.... At the 1998 OSK builder's dinner, Daryl

Murphy

said that he thought all Rebels, regardless of engine, should have the

firewall

moved back. I am not delighted with the moved back seat position, but I

did

it

anyway, partly as I expect the 1800 amphib floats to slightly pull the C

of

G

forward. I think my Subaru weight and c of g is going to be very close to

an

0-320.
The mod also pulls the instrument panel back to maintain instrument space

behind

it.
Ken

Bob Johnson wrote:

One more question and that

is in regarding firewall placement with the "O320". I will gladly build

it the

suggested (3" back) as the instructions indicate, but I know there are

Rebels

out there at the original position supporting this power plant-What are

your

thoughts-(I'm probably about 4/6 weeks from starting the

fuselage)-Thanks

again-Bob

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The Murphy Rebel Builders List is for the discussion
between builders and owners of Murphy Rebel aircraft.
Archives located at:
http://www.dcsol.com/murphy-rebel/lists/default.htm
*----------------------------------------------------*

*----------------------------------------------------*
The Murphy Rebel Builders List is for the discussion
between builders and owners of Murphy Rebel aircraft.
Archives located at:
http://www.dcsol.com/murphy-rebel/lists/default.htm
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