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Weight & Balance percent values?

jeffw@sc47

Well Known Member
I am sure the answer to this is 'stupid-simple' but I cannot figure it out.

For both the 14 and the 14A on the Van's W&B examples they include percentage (%) numbers that appear to be related to the ARM numbers.

I cannot figure out how those percentages are calculated, derived from what values? I've tried relating the numbers in each table every which way and can't find the relationship that derives the percentages.

Give me a hint or explain please.
 
Jeff,

Vans uses inches aft of datum versus percent of chord. You don’t need the percentages unless you want to use chord values which are not shown on their W&B form. I can send you my W&B I did for mine if you want. (Excel spreadsheet that does all the calculations) I had to do a bunch of sample calculations for my DAR to show heavy/light, forward/aft CG examples including aerobatic numbers. All CG and arm values were expressed in inches aft of datum. Math is much easier that way.

You’re getting close to the end of your doing W&B. Good times!!
 
I am sure the answer to this is 'stupid-simple' but I cannot figure it out.

For both the 14 and the 14A on the Van's W&B examples they include percentage (%) numbers that appear to be related to the ARM numbers.

I cannot figure out how those percentages are calculated, derived from what values? I've tried relating the numbers in each table every which way and can't find the relationship that derives the percentages.

Give me a hint or explain please.


The Percentage number is referencing Mean Aerodynamic Chord. With a hershey bar wing this is dead simple - its simply the measurement of the distance from LE to TE: 56 inches. The LE is 0% chord; the TE is 100%

The calculation is also simple: %MAC = CG - LE MAC/MAC*100

Using FS 86.3 as a example CG:

LE MAC = 72
MAC = 56
CG = 86.3

86.3-72/56*100 = 25.54% MAC
 
Fuel Arm

Did anyone use a "changing" fuel arm to calculate W&B as this actually does change from 0 gals to 50 gals?
 
From my Sunday morning armchair with inadequate coffee, it appears that the formula offered is missing some very important parentheses.

Standard "My Dear Aunt Sally" calculations will get you way into the weeds on this one.
 
Did anyone use a "changing" fuel arm to calculate W&B as this actually does change from 0 gals to 50 gals?

I don't think the fuel arm changes as significantly as the fuel weight does, although with a D-cross-section LE tank, you are correct.

The occupant arm changes with seat position, and the baggage arm assumes simplistically that center-mass of the loaded baggage is the middle of the compartment. Somehow these things seem not to matter too much - until they do.
 
From my Sunday morning armchair with inadequate coffee, it appears that the formula offered is missing some very important parentheses.

Standard "My Dear Aunt Sally" calculations will get you way into the weeds on this one.

Could you be more specific?
 
My guess is >
Order of operation: PEMDAS.

First (Parenthesis), Then (Exponentials);

Then >My Dear Aunt Sally

(Multiply), (Divide), (Add), Last (Subtract).

I think that the order is universally applied in math operations.
 
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Fair enough. But if you plug the values into a calculator as read (left to right) you get the number - which is what the OP asked.

Just trying to show the sequence within the limits of typing on a phone.
 
Fuel Arm final answer

=82.58-((B15-5)*(1.22/45))

I'm not going to defend, elaborate or debate but you can judge for yourself. B15 is the input in gals to quantify the Fuel Arm in inches. Yes other assumptions as to where the baggage is placed also adds to the error. I just like to define as close as possible the inputs if possible. The weight (In lbs.) of the fuel still needs to be used to calculate the moment.
 
I made a spreadsheet that plots the W&B. This is not a real aircraft empty weight, I just used the Van's supplied demonstrator weights until ours is finished. Helps to see when you're in the aerobatic category or not. Also calculates Va at specific W&B. Here's a screenshot....
 

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What is the formula for calculating the Va?

Here's a link to the Van's website wherein they discuss the speed limitations of the RV-14 series.

https://www.vansaircraft.com/wp-content/uploads/2019/01/RV-14_V_speeds.pdf

Short answer: Va=(stall speed)*squareroot(load factor) For example Va=62kt*sqrt(4.4g) or 130kt

Long answer: First you need to know if you're in the aerobatic load category (6g) or the utility category (4.4g). Then you need to know your stall. My spreadsheet extrapolates the stall speed based on the weight, then the load factor based on both weight and CG. Then Va as described above.

An interesting note about the load factor: depending on weight and fuel burn during the flight, you could start in the utility category, pass through aerobatic, then end up back in utility at landing. Thus my personal minimum is to calculate Va at my zero fuel weight and utility load factor. That's not a recommendation, just my personal preference.
 
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