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Old 12-15-2008, 09:46 PM   #1  
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Default Weight & Balance shifts

Ok I have been looking into this question for quite some time and still can't figgure out an answer. What I'm looking at is to find a way to compute how much weight must be added at lets say station A, to move the CG so many inches forward or aft. All the weight shift forumuals I have found don't work with my scenario so is there another way to figgure this out?

So in other words, for example the airplane has exceeded the forward CG envelope, so I'm trying to figgure out how much ballast must be added to the baggage compartment to bring my CG so many inches aft. Would be nice to know there is a simple computation I could do without the hassle of making numerous W&B calcuations over and over again.

Last edited by 100LL; 12-15-2008 at 09:52 PM.
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Old 12-15-2008, 11:49 PM   #2  
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Do you have a Jeppesen private or instrument/commercial book?
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Old 12-16-2008, 01:15 AM   #3  
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is this an assighnment? i would say just add some counterweights to the tail.
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Old 12-16-2008, 03:54 AM   #4  
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Quote:
Originally Posted by 100LL View Post
Ok I have been looking into this question for quite some time and still can't figgure out an answer. What I'm looking at is to find a way to compute how much weight must be added at lets say station A, to move the CG so many inches forward or aft. All the weight shift forumulas I have found don't work with my scenario so is there another way to figgure this out?

So in other words, for example the airplane has exceeded the forward CG envelope, so I'm trying to figure out how much ballast must be added to the baggage compartment to bring my CG so many inches aft. Would be nice to know there is a simple computation I could do without the hassle of making numerous W&B calculations over and over again.
There is, these W & B formulas found in aviation textbooks. They can also be derived easily using basic calculus, which is a good way to prove to yourself they work. (Note: to get these to line up correctly I had to insert periods as spacers.)

a)

weight to be added or removed = ...... change in CG
old total weight .................... distance from weight to new CG

b)

weight to be shifted =...... change in CG
old total weight ............. distance weight is shifted

Ex. CG needs to move 1 inch aft to be within limit. Say it's 45 inches from the baggage compartment to the new/desired CG location, and say the airplane weighs 5,000 lbs. Using formula (a)

x ... = ... 1
5000 ... 45

Solve for x: 111 lbs. needs to be added to the baggage compartment.

Use formula (b) when you would rather shift some weight than add or remove it.

Hope this helps.

Last edited by Cubdriver; 12-16-2008 at 04:14 AM. Reason: spacing
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Old 12-16-2008, 07:53 AM   #5  
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I am going to provide a slightly, IMHO, easier formula. At least it is for me, I'll leave you the judge of that.

First it's easier to remember.

We know M = W*A (moment = Weight*Arm)

A = M/W (Arm = Moment/Weight - thats all you need to remember!)

A= (M +- Change in Moment)/(W +- Change in Weight) (Arm is Moment Plus/minus change in moment)/(Weight plus or minus change in weight)

For instance, if you added weight, you would ADD that weight to the bottom and add that items moment to the top (it's weight times it's arm).

If you moved an object backwards, you add to the top the addition in moment (the weight times then arm change, like from 88 to 92 is 4).

For Plus and Minus, just think of it this way. If the moment will increase (moving object aft/away from CG) or just increasing weight, you always use pluses.

With this equation, you can solve ANY problem, including your own. Sorry my online explanation isn't the best - I usually go through some exercises with my students first.
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Old 12-16-2008, 09:38 AM   #6  
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Originally Posted by Cubdriver View Post
There is, these W & B formulas found in aviation textbooks. They can also be derived easily using basic calculus, which is a good way to prove to yourself they work. (Note: to get these to line up correctly I had to insert periods as spacers.)

a)

weight to be added or removed = ...... change in CG
old total weight .................... distance from weight to new CG

b)

weight to be shifted =...... change in CG
old total weight ............. distance weight is shifted

Ex. CG needs to move 1 inch aft to be within limit. Say it's 45 inches from the baggage compartment to the new/desired CG location, and say the airplane weighs 5,000 lbs. Using formula (a)

x ... = ... 1
5000 ... 45

Solve for x: 111 lbs. needs to be added to the baggage compartment.

Use formula (b) when you would rather shift some weight than add or remove it.

Hope this helps.
Yes it does help, thanks, exactly what I was looking for.
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