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  #1  
Old 02-18-2021, 07:08 PM
jeffw@sc47's Avatar
jeffw@sc47 jeffw@sc47 is offline
 
Join Date: Aug 2014
Location: Simpsonville, SC (SC47)
Posts: 364
Default Weight & Balance percent values?

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.
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Jeff Warren
Simpsonville, SC (@SC47 > 10nm NW Triple Tree)
1946 Bellanca Cruisair 14-13-2 (74 YRS OLD 8/1/20)
RV14A (N14ZT), Ser#140195, Start 10/11/14
Contribution made 12/2/20 (USArmy 2/67-2/70)
www.mykitlog.com/jeffw@sc47
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  #2  
Old 02-18-2021, 07:44 PM
JHartline JHartline is offline
 
Join Date: Dec 2012
Location: Carrollton, GA 5GA2
Posts: 274
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Jeff,

Vans uses inches aft of datum versus percent of chord. You donít 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.

Youíre getting close to the end of your doing W&B. Good times!!
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Carrollton, GA 5GA2
RV-14 - First flight 4/7/2019. Phase I complete as of 9/11/2019!!
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  #3  
Old 02-19-2021, 06:13 AM
jeffw@sc47's Avatar
jeffw@sc47 jeffw@sc47 is offline
 
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Location: Simpsonville, SC (SC47)
Posts: 364
Default W&B spreadsheet

James, thanks.

I sent you a PM re W&B spreadsheet.
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Jeff Warren
Simpsonville, SC (@SC47 > 10nm NW Triple Tree)
1946 Bellanca Cruisair 14-13-2 (74 YRS OLD 8/1/20)
RV14A (N14ZT), Ser#140195, Start 10/11/14
Contribution made 12/2/20 (USArmy 2/67-2/70)
www.mykitlog.com/jeffw@sc47
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  #4  
Old 02-19-2021, 06:35 AM
mturnerb mturnerb is offline
 
Join Date: Apr 2015
Location: Ponte Vedra, FL
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I used this reference on Van's website: https://www.vansaircraft.com/wp-cont.../01/RV14wb.pdf

The percentages referenced here are also converted to standard terminology (inches aft of datum). You can measure arm length for each wheel and use that with weight on each wheel to calculate your W/B and use the design CG range in inches - all quite standard calculations.
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  #5  
Old 02-19-2021, 01:14 PM
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Toobuilder Toobuilder is offline
 
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Location: Mojave
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Default

Quote:
Originally Posted by jeffw@sc47 View Post
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
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
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Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #6  
Old 02-21-2021, 05:34 AM
dmattmul dmattmul is offline
 
Join Date: Dec 2015
Location: Spring Hill, FL
Posts: 120
Default Fuel Arm

Did anyone use a "changing" fuel arm to calculate W&B as this actually does change from 0 gals to 50 gals?
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RV-14A QB building, Lycoming 390 Thunderbolt arrived July 2019, Garmin avionics, Vertical Power, EFII-32 Ignition and Fuel, Whirlwind 300-72, Parts became a real airplane 8/15/2020.
Started RV-10 Nov 2020. Empennage arrived and now waiting (And waiting) for QB kits. Garmin avionics, Vertical Power, EFII-32 Ignition and Fuel, Whirlwind 3 blade, real airplane hopefully 2022.
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  #7  
Old 02-21-2021, 06:23 AM
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Bill Boyd Bill Boyd is offline
 
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Location: Landing field "12VA"
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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.
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Hop-Along Aerodrome (12VA)
RV-6A - N30YD - Built '98 / sold '20
RV-10 - N130YD - Phase 1 flying

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  #8  
Old 02-21-2021, 06:28 AM
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Bill Boyd Bill Boyd is offline
 
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Quote:
Originally Posted by dmattmul View Post
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.
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Bill Boyd

Hop-Along Aerodrome (12VA)
RV-6A - N30YD - Built '98 / sold '20
RV-10 - N130YD - Phase 1 flying

donating monthly to the VAF - thanks, Doug
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  #9  
Old 02-21-2021, 07:48 AM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by Bill Boyd View Post
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?
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #10  
Old 02-21-2021, 07:58 AM
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jeffw@sc47 jeffw@sc47 is offline
 
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Location: Simpsonville, SC (SC47)
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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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Jeff Warren
Simpsonville, SC (@SC47 > 10nm NW Triple Tree)
1946 Bellanca Cruisair 14-13-2 (74 YRS OLD 8/1/20)
RV14A (N14ZT), Ser#140195, Start 10/11/14
Contribution made 12/2/20 (USArmy 2/67-2/70)
www.mykitlog.com/jeffw@sc47

Last edited by jeffw@sc47 : 02-21-2021 at 08:05 AM.
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