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center of gravity

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Lew Gallagher

Joined: 04 Jan 2008
Posts: 402
Location: Greenville , SC

Posted: Mon Aug 18, 2008 6:22 am Post subject: center of gravity

Hey Guys,



I've been wondering about this for awhile and Doug's "I have the Airflow system & my CG is 108.4" on the AC thread prompted me to just

 ask:



Where is this 108.4" measured from?  Probably obvious, but I haven't found it yet, and other general literature on COG talks about a reference point somewhere in front of the plane.  



It seems to be a very critical measurement.  So far, I'm still head-down-building, but getting closer to needing to know stuff like this.



Later, - Lew

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_________________
non-pilot
crazy about building
NOW OFICIALLY BUILDER #40549
Fly off completed !

bcondrey

Joined: 03 Apr 2006
Posts: 580

Posted: Mon Aug 18, 2008 7:44 am Post subject: center of gravity

Lew,



Here's the info I've got at hand:



Datum: 99.44" forward of the wing leading edge

Allowable CG range: 107.84" - 116.24" (15-30% of wing chord)

Front seats: 114.58"

Rear seats: 151.26"

Fuel: 108.9"

Baggage: 173.5"



Main wheel (right): 124.44"

Main wheel (left): 124.31"

Nose wheel: 50.44"



Bob

N442PM (flying)



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AV8ORJWC

Joined: 13 Jul 2006
Posts: 1149
Location: Aurora, Oregon "Home of VANS"

Posted: Mon Aug 18, 2008 8:51 am Post subject: center of gravity

Another way is that your CG (empty) is 8.96" aft of the leading edge of

your wings (in theory).  The usable CG is a dynamic which moves forward

and aft from that imaginary point based on your current Load

considerations.  Tim did a great job talking about aft of Empty CG and

Forward of Empty CG changes to the flight characteristics in your hand.



Flying forward of 15% or Aft of 30% of the MAC is "Not a good thing".

Sean Tucker safely crashed aircraft #3 when he lost the prop and hub

which radically changed the Aft CG more than 8" beyond the maximum.  Oh

by the way, he was on the ground 53 seconds later and he had onboard

video of it.  Do not try this at home.



Many builders relocate or locate components with little regard for their

impact on the empty CG.  A few builders have announced that they have a

left of center/right of center change as well.  One builder took

exception to why anyone would want to relocate avionics off the panel

and remotely mount them.  It is all about manipulation of the final

empty CG.  VAN has designed us a conservative and flexible platform.

Those flying can comment more meaningfully on this important issue to

insurance rates and flying pleasure.



YMMV



John Cox

#40600



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jcumins(at)jcis.net
Guest

Posted: Mon Aug 18, 2008 9:56 am Post subject: center of gravity

John



Since you brought up MAC you might need to explain it in better detail and

in a e-mail that might be tuff.  OK pulling out the old Flight engineer hat

for this one.  MAC stands for Mean Aero dynamic cord and when referenced, cg

is usually a % of the mac.  



In flying the c-5 for almost 15 yrs our cg ran from 41% of mac for aft cg to

29% of mac for fwd cg.  The airplane empty an 397,000 lbs was usually at 39%

of mac. And remember this does not include any fuel, crew, baggage, or

cargo. 



It really is just a math calculation to figure it out.  WG is still based on

weight x arm for the moment then added together and referenced to the

appropriate table to insure your in the cg envelope for your flight

conditions.



I have nod sat down and plotted the cg envelope for the -10 but I will and I

will make a spread sheet and reference diagram that I will share with

everyone. 



This brings up another issue I have seen on the list about when people weigh

there plane.  I think people do not understand what is required to get the

basic airplane weight.



The basic weight of the plane, Is the airplane with zero fuel, Engine with

full oil as called out in the engines service manual.  It the manufacture

states 12 qts then there needs to be 12 qts in the oil system. No

accessories in the plane, no head sets, tow bars, maps, or any pilots

accessories, just the basic airplane.



The airplane needs to be perfectly level on both axes vertical and

horizontal, this is very critical to get accurate weights.  The manufactures

maintenance manual will give you the level points.



Once these conditions are met then you can get a good weight and figure out

the arm and moments then your basic empty weight cg.



 	  Quote: 	
		  From what I have seen with the large variance of numbers that have been

posted here, It looks to me there needs to be some re-weighting that needs
	


to be done very carefully.

Ok that's my 2 cents worth.  



John G. Cumins

#40864 Virtal Stab done starting rudder

 



Your Total Technology Solution Provider

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AV8ORJWC

Joined: 13 Jul 2006
Posts: 1149
Location: Aurora, Oregon "Home of VANS"

Posted: Mon Aug 18, 2008 11:34 am Post subject: center of gravity

The Excel spreadsheet has been done and is in the Matronics archives

(Tim and I have them as well).  The weighing process is as you describe

but many builders do not achieve balanced nirvana on the vertical and

horizontal. Many are casual with the scale accuracy as well.  Many weigh

the aircraft Sans wheel skirts and fairings to get a beginning weight

for the DAR.  Those items come later in the Phase One completion (and

may never be updated).  VAN has designed the airfoil of the wing and

established the minimum and maximum MAC as you describe.  Rob Hickman's

AFS Efis even allows a graphic presentation for up to the minute/up to

the flight Weight and Balance changes.



The large variance of numbers is due to/but not limited to avionics

choices, trim servos, body putty on the composite pieces, size of

battery and quality of fire suppression and noise suppression materials.

All that up to each builder.



What you left off is the aircraft being weighed needs to be out of the

wind and elements to truly get a correct reading.  If a DAR blows off

the importance of an accurate list or proper weighing it is in the hands

of the individual builder.  If we are building... we should sure know

what M.A.C. is.



Note:  When buying an aircraft.  Have your Pre-purchase inspection

include a Weight and Balance as part of the agreement.  Takeoff 5% of

the price for every 1% of weight variance and watch the seller squirm.

If the Gross Weight is over the VAN approved RV-10 weight, take another

5% for every 1% of weight that is over limit without full FAA spin

testing.  Watch how quick the ink erasers come out.  Though Dan

Checkoway's weight table has fewer submittals than mine, his is easy to

read and understand.



John



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