The Murphy Builders Site

G'day Wayne


Thanks for the info.

I failed to advise that I used the measurement from the centre of the main gear to the centre of the tail wheel, so I will recalculate based on your examples.

It's late now and I will try to come to grips with it tomorrow.


Thanks and regards


Ian Donaldson

Wayne, when you say door post, I assume you mean centre of door post which should be the same as the centre of the main gear axle. Murphy's new manuals specify the datum as the wing leading edge, but the moment arms stayed the same. Wish they would be more accurate with things like that. It may not mean much in the end, but can cause needless confusion. Also, I noticed in their new 0320 manual they still have the pilot and passenger moment arm at 15 inches, whereas it really should be 18 if you moved the firewall back 3 inches. Again, I'm assuming the 15 is correct for the regular firewall. They did fix the fuel one, though, as you mentioned, to match the new tanks.

It is most important for everyone to be sure the plane is exactly in level flight attitude (bottom of Rebel floor or top door sill) when taking weight measurements, as a slight angle can make a significant difference in fore and aft weight numbers.

-----Original Message-----
From: murphy-rebel@dcsol.com [mailto:murphy-rebel@dcsol.com]On Behalf Of Wayne G. O'Shea
Sent: Tuesday, June 11, 2002 8:32 AM
To: murphy-rebel@dcsol.com
Subject: Re: Rebel centre Of Gravity


Ian, only about a 1/2" difference between the center of the gear leg and the leading edge of the wing, in most cases. Murphy offered the centre of the gear leg as a reference because it's the easiest place to measure from and the 1/2" is "rather" insignificant!

P.S. shouldn't you have had all these numbers <before> you flew?? Good you can't really get into trouble C of G wise with the Rebel. The O-320 ones I always throw a 5 gallon jug of water in the back and lash down for the first test flight, just to be safe while sorting out the characteristics of each one!

Cheers,
Wayne

----- Original Message -----
From: Ian Donaldson (allsure@iprimus.com.au)
To: murphy-rebel@dcsol.com (murphy-rebel@dcsol.com)
Sent: Tuesday, June 11, 2002 9:46 AM
Subject: Rebel centre Of Gravity


G'day Wayne


Thanks for the info.

I failed to advise that I used the measurement from the centre of the main gear to the centre of the tail wheel, so I will recalculate based on your examples.

It's late now and I will try to come to grips with it tomorrow.


Thanks and regards


Ian Donaldson

G'day Wayne

Thanks for your reply.

I thought that I had done the Cof G calculations correctly, but I did not know that the factory had more than one way of doing the calculations!

As I said the manual gives one method, and as I later found, the flight manual gives another. It's just another case of the factory not being clear, or precise enough with their instructions or intentions! A little more thought from the factory would have caused many of use unnecessary confusion.

I have done the calculations as you have suggested and provided that the load is kept well forward it appears that I will not have any problems.

Thanks and regards


Ian Donaldson

: Re: Rebel centre Of Gravity


Ian, only about a 1/2" difference between the center of the gear leg and the leading edge of the wing, in most cases. Murphy offered the centre of the gear leg as a reference because it's the easiest place to measure from and the 1/2" is "rather" insignificant!

P.S. shouldn't you have had all these numbers <before> you flew?? Good you can't really get into trouble C of G wise with the Rebel. The O-320 ones I always throw a 5 gallon jug of water in the back and lash down for the first test flight, just to be safe while sorting out the characteristics of each one!

Cheers,
Wayne

Ian, only about a 1/2" difference between the center of the gear leg and the leading edge of the wing, in most cases. Murphy offered the centre of the gear leg as a reference because it's the easiest place to measure from and the 1/2" is "rather" insignificant!

P.S. shouldn't you have had all these numbers <before> you flew?? Good you can't really get into trouble C of G wise with the Rebel. The O-320 ones I always throw a 5 gallon jug of water in the back and lash down for the first test flight, just to be safe while sorting out the characteristics of each one!

Cheers,
Wayne

----- Original Message -----
From: Ian Donaldson (allsure@iprimus.com.au)
To: murphy-rebel@dcsol.com (murphy-rebel@dcsol.com)
Sent: Tuesday, June 11, 2002 9:46 AM
Subject: Rebel centre Of Gravity


G'day Wayne


Thanks for the info.

I failed to advise that I used the measurement from the centre of the main gear to the centre of the tail wheel, so I will recalculate based on your examples.

It's late now and I will try to come to grips with it tomorrow.


Thanks and regards


Ian Donaldson

Hi Ian !

If you have any worries about flying fully loaded, you can
always add a bit of weight up front. We flew for over a year with
10 lb. of lead on top of the 912, just behind the gearbox....

This was later removed when heavy rubber hoses were added
OVER the existing hoses (fuel, oil, and water) to act as heat shields.
Have also seen lead plates added to the front of the prop, partly for
crush protection, but also to make it easier to 'load her up' !!

When packing gear into the baggage area, we were always careful
to put the heaviest stuff right up against the seat backs. We even
stood the big tent up vertically between the seat backs !! :-)
That 912 Rebel sure carried a LOT of camping gear !!!

None of these tricks are necessary with the Lycoming, of course,
but I think you'll have dazzling performance with the 912-S, and
you'll certainly enjoy the MUCH better fuel burn rates !! :-) ;-)

.....bobp

---------------------------------orig.----------------------------------
At 09:49 PM 6/12/02 +1000, you wrote:

G'day Wayne

Thanks for your reply.

I thought that I had done the Cof G calculations correctly, but I did not

know that the factory had more than one way of doing the calculations!

As I said the manual gives one method, and as I later found, the flight

manual gives another. It's just another case of the factory not being clear,
or precise enough with their instructions or intentions! A little more
thought from the factory would have caused many of use unnecessary confusion.

I have done the calculations as you have suggested and provided that the

load is kept well forward it appears that I will not have any problems.

Thanks and regards


Ian Donaldson

: Re: Rebel centre Of Gravity


Ian, only about a 1/2" difference between the center of the gear leg and

the leading edge of the wing, in most cases. Murphy offered the centre of
the gear leg as a reference because it's the easiest place to measure from
and the 1/2" is "rather" insignificant!

P.S. shouldn't you have had all these numbers <before> you flew?? Good

you can't really get into trouble C of G wise with the Rebel. The O-320 ones
I always throw a 5 gallon jug of water in the back and lash down for the
first test flight, just to be safe while sorting out the characteristics of
each one!

Cheers,
Wayne
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<DIV><FONT size=2>G'day Wayne </FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT size=2>Thanks for your reply.</FONT></DIV>
<DIV><FONT size=2></FONT>&nbsp;</DIV>
<DIV><FONT size=2>I thought that I had done the Cof G calculations
correctly,&nbsp; but I did not know that the factory had more than one way of
doing the calculations!</FONT></DIV>
<DIV><FONT size=2></FONT>&nbsp;</DIV>
<DIV><FONT size=2>As I said the manual gives one method, and as I later found,
the flight manual gives another. It's just another case of the factory not

being

clear, or precise enough with their instructions or intentions! A little more
thought from the factory would have caused many of use unnecessary
confusion.</FONT></DIV>
<DIV><FONT size=2></FONT>&nbsp;</DIV>
<DIV><FONT size=2>I have done the calculations as you have suggested and
provided that the load is kept well forward it appears that I will not have

any

problems.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT size=2>Thanks and regards</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT size=2>Ian Donaldson</FONT>&nbsp;</DIV>
<DIV>&nbsp;</DIV>
<BLOCKQUOTE
style="BORDER-LEFT: #000000 2px solid; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px;

PADDING-LEFT: 5px; PADDING-RIGHT: 0px">

<DIV style="FONT: 10pt arial"><B>:</B> Re: Rebel centre Of Gravity</DIV>
<DIV><BR></DIV>
<DIV><FONT face=Arial size=2>Ian, only about a 1/2" difference between the
center of the gear leg and the leading edge of the wing, in most cases.

Murphy

offered the centre of the gear leg as a reference because it's the easiest
place to measure from and the 1/2" is "rather" insignificant!</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>P.S. shouldn't you have had all these numbers
<before> you flew?? Good you can't really get into trouble C of G wise
with the Rebel. The O-320 ones I always throw a 5 gallon jug of water in the
back and lash down for the first test flight, just to be safe while sorting
out the characteristics of each one!</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=Arial size=2>Cheers,</FONT></DIV>
<DIV><FONT face=Arial size=2>Wayne</FONT></DIV></BLOCKQUOTE></BODY></HTML>

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G'day Bob


Thanks for the hints. I have done the C of G calculations a number of ways
and I don't think that I have much to worry about.

As you suggest I certainly could add some extra weight to the front. I have
already added heatshield over the lines. Maybe if I can afford that
in-flight adjustable propeller, that would add just enough weight!


I haven't had the opportunity to load the Rebel up yet, but when I do, I
will take your advice and keep it all as forward as possible.


It seems that we are getting something around 80 to 85kt (100mph?) cruise,
at about 4800rpm. I have not been able to confirm the speeds with my GPS
because every time I go flying the wind never co-operates and always mucks
up my calculations.
The Rotax is using 17 to 18 litres per hour in cruise. Not all that bad!


Does anyone have a foolproof method of using a GPS by flying a course, that
takes into account of the wind effect.

Regards


Ian Donaldson

Hi Ian !

If you have any worries about flying fully loaded, you can
always add a bit of weight up front. We flew for over a year with
10 lb. of lead on top of the 912, just behind the gearbox....

This was later removed when heavy rubber hoses were added
OVER the existing hoses (fuel, oil, and water) to act as heat shields.
Have also seen lead plates added to the front of the prop, partly for
crush protection, but also to make it easier to 'load her up' !!

When packing gear into the baggage area, we were always careful
to put the heaviest stuff right up against the seat backs. We even
stood the big tent up vertically between the seat backs !! :-)
That 912 Rebel sure carried a LOT of camping gear !!!

None of these tricks are necessary with the Lycoming, of course,
but I think you'll have dazzling performance with the 912-S, and
you'll certainly enjoy the MUCH better fuel burn rates !! :-) ;-)

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