Datum Line

[pequeajim]

My Rebel has the o-320 in it with the firewall moved back 2" On the Murphy
site, the flight manual for the 320 shows the Datum line at the leading edge
of the wing. I wonder if this is based on moving the firewall back, or the
standard build?



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[Bob Andrews]

You can put the datum (for any aircraft) any where you like ... as long as
all arm measurements (positive and negative) and subsequent calculations and
results are made from and reference that datum.

Bob 612e

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com]On Behalf Of
pequeajim@dcsol.com
Sent: Thursday, November 03, 2005 1:13 PM
To: rebel-builders@dcsol.com
Subject: Datum Line


My Rebel has the o-320 in it with the firewall moved back 2" On the Murphy
site, the flight manual for the 320 shows the Datum line at the leading edge
of the wing. I wonder if this is based on moving the firewall back, or the
standard build?



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[Wayne G. O'Shea]

The datum line for C of G measurements could be 10 feet in front of the
airplane....It's just an imaginary line to take your arm measurements from.
Our Canadian MD-RA prefers that you use an imaginary point in front of your
airplane so all arms are positive...which is nuts since there is no real way
to take quick measurements from later on when you make changes. The leading
edge of the wing is used as it's the %age of wing chord that you are
concerned with when it comes to where your C of G is located...so why not
use the wing itself as the measuring point. The forward door post is the
same as the leading edge position so it makes for a nice easy spot to
measure all your arms from and for reference later on when your installing
floats, moving engines, batteries etc. Just make sure you decide on whether
to use the forward edge, rearward edge or the center of the door post and
stick with it throughout your weighing and calculations.

Cheers,
Wayne

----- Original Message -----
From: <pequeajim@dcsol.com>
To: <rebel-builders@dcsol.com>
Sent: Thursday, November 03, 2005 3:12 PM
Subject: Datum Line

My Rebel has the o-320 in it with the firewall moved back 2" On the

Murphy

site, the flight manual for the 320 shows the Datum line at the leading

edge

of the wing. I wonder if this is based on moving the firewall back, or

the

standard build?



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[pequeajim]

Thanks guys, clear enough. FOr reference, I did speak with Murphy and they
affermed that the manual on their site is for a Rebel with an O-320 mount on
a firewall moved back 3", (not 2 like I mistakenly posted).

So, taking all of this into consideration, I guess I am all set.

I am going to pick up some of the parts this weekend.

Jim!

On 11/3/2005 11:40 AM, oifa@irishfield.on.ca wrote to rebel-builders:

-> The datum line for C of G measurements could be 10 feet in front of the
-> airplane....It's just an imaginary line to take your arm measurements from.
-> Our Canadian MD-RA prefers that you use an imaginary point in front of your
-> airplane so all arms are positive...which is nuts since there is no real
way
-> to take quick measurements from later on when you make changes. The leading
-> edge of the wing is used as it's the %age of wing chord that you are
-> concerned with when it comes to where your C of G is located...so why not
-> use the wing itself as the measuring point. The forward door post is the
-> same as the leading edge position so it makes for a nice easy spot to
-> measure all your arms from and for reference later on when your installing
-> floats, moving engines, batteries etc. Just make sure you decide on whether
-> to use the forward edge, rearward edge or the center of the door post and
-> stick with it throughout your weighing and calculations.
->
-> Cheers,
-> Wayne
->
-> ----- Original Message -----
-> From: <pequeajim@dcsol.com>
-> To: <rebel-builders@dcsol.com>
-> Sent: Thursday, November 03, 2005 3:12 PM
-> Subject: Datum Line
->
->
-> > My Rebel has the o-320 in it with the firewall moved back 2" On the
-> Murphy
-> > site, the flight manual for the 320 shows the Datum line at the leading
-> edge
-> > of the wing. I wonder if this is based on moving the firewall back, or
-> the
-> > standard build?
-> >
-> >
-> >
-> > -----------------------------------------------------------------
-> > List archives located at: https://www.dcsol.com/login
-> > username "rebel" password "builder"
-> > Subscription services located at:
-> > https://www.dcsol.com/public/code/html-subscribe.htm
-> > List administrator: mike.davis@dcsol.com
-> > -----------------------------------------------------------------
-> >
-> >
-> >
-> >
->
->





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[Ken]

I scratched my head over this as my leading edge seemed to be more like
an inch forward of the center of the cage/center of door post/center of
the main gear strut/main gear axle. (With the Mackenzie STOL kit it is
now probably two inches forward). I thought it made sense to define the
datum as the center of the main gear strut. That loses some meaning when
it goes on floats but at least it is a real point that I can find and
measure from and it seems consistent with the info that MAM publishes.

I haven't looked at the aerodynamics and estimated a theoretical c of g
range of the STOL kit modified airfoil as I think the real answer can
only be ascertained by flight testing. However I think it is fair to say
that the STOL kit moves the wing forward and the original 19.7 inches
aft of door post c. of g. limit might be a bit far back for the wing
with the STOL kit. I think I'm comfortable using the newer recommended
18.1" aft limit still but technically I should try and work out a new
number based on % of airfoil. No big deal I'm just pointing out that
there are some minor wrinkles in this for some of us.

It seems to me that adding the STOL kit might be equivalent to moving
the firewall back a wee bit, and that's probably a good thing for the
heavier engines.

Ken





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