The Murphy Builders Site

G'day


I'm just a little confused with the method of C of G calculations that are detailed in the construction manual, and I would appreciate some assistance.


My Rebel weights are as follows;

Left wheel 348 lb
Right wheel 327 lb
Tailwheel 62 lb
Total 737 lb

Now just what distances should I use to determine the C of G?

One page of the manual says:
Pilot & passenger 15'"
Baggage 43"
Fuel 18"


Whereas over the next page in the manual, the distances are shown as;
Pilot & passenger 15"
Baggage 45"
Fuel 23"

What should I use?
Can anyone do a sample calculation for me please?


Thanks and regards


Ian Donaldson

Wow! That is one very light Rebel>>>What are you using for power??



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Ian, the old 18" was from the days of plastic tanks! The wet wing is around 23 or 24 ". Measure from leading edge to middle of tank to be sure.
Your empty C of G would be Tail weight 62lbs x around 196" (measure from door post to tail wheel axle to get real #) divided by the total weight of 737lb = 16.49" aft of datum.

I am assuming that you put the rear floor in and this is what makes your tail weigh 62 pounds in level flight attitude, thus causing the fairly aft empty C of G thanks to the light engine up front with your 912ULS - 100HP. The 912 powered AULA Rebel I built was only 649 lbs total empty weight with 42 on the tail for an empty C of G of 12.68" (it didn't have the rear floor installed).

Don't know what you pegged on yours for gross weight, but my Rebel worked just fine when it originally had the 100 HP Lycoming in it and still climbed well even when loaded up to, uuuchhumm, 1800+lbs. I know MAM recommended 1450 for the 912 powered Rebels, but you should be able to go the full 1650 on the 100HP x 912S. The problem with the empty C of G being at around 16.5" is that you may go out the rear "limit" before you can get the weight into her.

If you have both 22 gallon US tanks that's 264lbs of fuel. Two 200lb guys and 737 empty would bring you to 1401. Leaves 249 for survival gear ( or 8 cases of Fosters!) Lets see if you can fly that!

737 (empty)x 16.49" =12153.13
400 (2 people)x 15" = 6000.00
249 (baggage)x 43" = 10707.00
264 (fuel) x 23" = 6072.00
1650 34932.13

34932.13/1650 gives a C of G of 21.17 inches so just out the back end of the (old 19.7) recommended limit by 1.47"

If you lower your "case count" to 5 and only had about 150lbs of baggage, the c of g would work out to:
737 x 16.49 = 12153.13
400 x 15 = 6000.00
264 x 23 = 6072.00
150 x 43 = 6450.00
1551 30675.13

That gives 19.77 inches aft of datum and .07 out the aft limit. Move the baggage an inch closer to the seats and your fine!

Cheers,
Wayne G. O'Shea
www.irishfield.on.ca

----- Original Message -----
From: Ian Donaldson (allsure@iprimus.com.au)
To: murphy-rebel@dcsol.com (murphy-rebel@dcsol.com)
Sent: Monday, June 10, 2002 3:48 AM
Subject: Re Rebel Centre of Gravity




G'day


I'm just a little confused with the method of C of G calculations that are detailed in the construction manual, and I would appreciate some assistance.


My Rebel weights are as follows;

Left wheel 348 lb
Right wheel 327 lb
Tailwheel 62 lb
Total 737 lb

Now just what distances should I use to determine the C of G?

One page of the manual says:
Pilot & passenger 15'"
Baggage 43"
Fuel 18"


Whereas over the next page in the manual, the distances are shown as;
Pilot & passenger 15"
Baggage 45"
Fuel 23"

What should I use?
Can anyone do a sample calculation for me please?


Thanks and regards


Ian Donaldson

Ian,

That sounds like a pretty heavy tail compared to my 0320. Did you lift the tail in a level flight attitude when you weighed her? The calculations Wayne gave you are not as complicated as it looks so don't be intimidated if your not a math wiz. the 198 inch moment is the distance the tail wheel is from the datum or leading edge of the wing for the Rebel. drop a plum line to the floor when you have it in level flight attitude and measure it from that mark to the center of the tail wheel. The calculation breaks down like this;

62lb(your tail weight)X198(distance of the tail from the datum)
------------------------------------------------------------------------------------------------------------------
(divided by the total weight) 737lb

That's your empty C of G. Don't confuse this with your loaded C of G which is how it's loaded for flight with you, your fuel, passengers or baggage. This will always change ever time you fly. You want to make sure you are always in the limits. do a most forward possible loaded C of G calculation like the one Wayne showed you for the most aft loaded configuration. Your empty C of G is to far aft to ever be loaded to far forward, but it's good to now. If you know and are familiar with your most aft and forward loaded limits you can be confident your with in limits and not have to calculate it ever time you fly like they told you in flight school.

You probably already new all this but it sounded from your request for help it was confusing to you. I hope I didn't insult you. Have fun.

I love flying my 912S Kitfox, it's such a fun plane to bounce around in. I can't wait to get the Rebel up. I'm scheduling a final inspection in three weeks.

Bruce G 357R

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The 912 powered Rebels with full floor end up being heavier on the tail than the Lycoming powered ones. Makes sense on the "teeter totter" as the extra 100 pounds+ of the Lycoming installation helps take the weight off the tail!

With a Lycoming powered Rebel EVERYTHING you put into her moves the C of G back, as the empty C of G is usually in front of the position that your butt assumes, which is the most forward item to get loaded into the plane. With the 912S of Ian's may not be the case, but I think with minimum fuel and a light pilot his empty C of G is the most forward. Quick calculation with a 160lb pilot and pushing your luck down to the last 5 gallons of fuel:

737 x 16.49 = 12153.13
160lb pilot x 15 = 2400
30lb fuel x 23" = 690
927 15,243.13

15243/927 = 16.44

Therefore when pushing your luck on fumes and with me on a 35lb weight loss diet it would move the C of G forward, from the empty C of G, by only 0.05", so it is safe to assume in reality that the empty C of G is the most forward condition for the aircraft.

Wayne

----- Original Message -----
From: Legeorgen@cs.com (Legeorgen@cs.com)
To: murphy-rebel@dcsol.com (murphy-rebel@dcsol.com)
Sent: Monday, June 10, 2002 10:47 PM
Subject: Re: Re Rebel Centre of Gravity


Ian,

That sounds like a pretty heavy tail compared to my 0320. Did you lift the tail in a level flight attitude when you weighed her? The calculations Wayne gave you are not as complicated as it looks so don't be intimidated if your not a math wiz. the 198 inch moment is the distance the tail wheel is from the datum or leading edge of the wing for the Rebel. drop a plum line to the floor when you have it in level flight attitude and measure it from that mark to the center of the tail wheel. The calculation breaks down like this;

62lb(your tail weight)X198(distance of the tail from the datum)
------------------------------------------------------------------------------------------------------------------
(divided by the total weight) 737lb

That's your empty C of G. Don't confuse this with your loaded C of G which is how it's loaded for flight with you, your fuel, passengers or baggage. This will always change ever time you fly. You want to make sure you are always in the limits. do a most forward possible loaded C of G calculation like the one Wayne showed you for the most aft loaded configuration. Your empty C of G is to far aft to ever be loaded to far forward, but it's good to now. If you know and are familiar with your most aft and forward loaded limits you can be confident your with in limits and not have to calculate it ever time you fly like they told you in flight school.

You probably already new all this but it sounded from your request for help it was confusing to you. I hope I didn't insult you. Have fun.

I love flying my 912S Kitfox, it's such a fun plane to bounce around in. I can't wait to get the Rebel up. I'm scheduling a final inspection in three weeks.

Bruce G 357R

G'day


I am using the Rotax 921S 100 hp engine, but I surely would have liked to
use a 150 hp engine, but they are just so dammed expensive over here.

At this time the 100 hp Rotax seems OK.

Regards


Ian Donaldson

Wow! That is one very light Rebel>>>What are you using for power??

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G'day Bruce


No, you certainly did not insult me!

I appreciate that you were prepared to assist me.

I will tackle the maths tomorrow and just see what the true picture is.

Thanks and regards


Ian Donaldson

Ian,

That sounds like a pretty heavy tail compared to my 0320. Did you lift the tail in a level flight attitude when you weighed her? The calculations Wayne gave you are not as complicated as it looks so don't be intimidated if your not a math wiz. the 198 inch moment is the distance the tail wheel is from the datum or leading edge of the wing for the Rebel. drop a plum line to the floor when you have it in level flight attitude and measure it from that mark to the center of the tail wheel. The calculation breaks down like this;

62lb(your tail weight)X198(distance of the tail from the datum)
------------------------------------------------------------------------------------------------------------------
(divided by the total weight) 737lb

That's your empty C of G. Don't confuse this with your loaded C of G which is how it's loaded for flight with you, your fuel, passengers or baggage. This will always change ever time you fly. You want to make sure you are always in the limits. do a most forward possible loaded C of G calculation like the one Wayne showed you for the most aft loaded configuration. Your empty C of G is to far aft to ever be loaded to far forward, but it's good to now. If you know and are familiar with your most aft and forward loaded limits you can be confident your with in limits and not have to calculate it ever time you fly like they told you in flight school.

You probably already new all this but it sounded from your request for help it was confusing to you. I hope I didn't insult you. Have fun.

I love flying my 912S Kitfox, it's such a fun plane to bounce around in. I can't wait to get the Rebel up. I'm scheduling a final inspection in three weeks.

Bruce G 357R