# How To Calculate Arm Of Baggage Compartment, Fuel, and Seats

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#### Marc W

##### Well-Known Member
I used the OP's method to determine the arm to my weight in my little single seater. It is pretty straight forward to weigh it with me in it and without me in it and do the math and get a precise location for my CG. No need to try to measure the location of my belly button and assume that is my CG. Why estimate and assume when you can measure and calculate with precision?

#### Dana

##### Super Moderator
Staff member
I didn’t see that
Nor I. Is everybody missing the point? The OP wants to (since the airplane is (or will be) up on digital scales anyway) add weights, reweigh, and back calculate to determine the arms those weights are at, instead of trying to measure to the unknown center of an irregularly shaped weight. Nothing wrong with that. He was unclear about the math, since the reverse calculations aren't something that's taught to (or generally needed by) pilots.

#### Marc Zeitlin

##### Exalted Grand Poobah
Measure back from the nose to the center of the axle of the nose/left main/right main wheel. They said to confirm the arm location of the pilot seat, rear seat, fuel and baggage compartment using the known weight of a person and bag.
On a kit plane such as a Velocity, I would expect that the variation in fuselage stations for canard leading edge, wing/strake joint at the leading edge, and the two/three axle positions would be much smaller than they are on the plans built canards, but there can still be some variation, since the planes are not built in jigs.

With Long-EZs, COZYs, etc., anytime I do a first W&B on the plane, I drop plumb bobs from all the important locations (axles, wing/strake joint LE, canard LE) and verify them against the POH numbers. They're NEVER exactly the same - usually there's a 0.25" - 0.5" variation in the flying surface locations, and up to a 1" variation in the gear locations.

The math they had in the example did not work out...I believe there was a typo.
Interesting - I never evaluated the calcs in the POH for Velocities, but I've certainly seen two or three Velocity W&B's that were completely unintelligible and useless, and we had to fix it.

My nose job on the airplane is all done now so if I use the nose as the datum it is bench marked in for future reference. My number really closlly match the plans numbers so it is confirmed.
While it's good that your datum and the POH match, I hope that your "nose job" did not involve modifying the position of the nose datum.

I could not find an AERODYNAMIC datum for the Velocity in the POH - don't know if they publish one. For LEs and COZYs, the aerodynamic datum is the strake/wing joint LE, which is supposed to be at FS-113.9". When I do a W&B on a LE/COZY/etc., I DEFINE this as the datum (as in GD&T) so that this is always AT 113.9 - I don't care where the zero location is, as there's no aerodynamic functionality of that point. Then, I measure the locations of the canard LE (so I know how far apart the wing/canard are) and the location of the axles (so that I can reference them to the aerodynamic datum.

With THAT information, the CG actually means something. If the main axles are in the POH location (FS-110") but the main wing was 1" further forward due to MFG variability, then you'd THINK that your CG was far enough away from the NP but in fact, it wouldn't be. So for safety's sake, understanding the ACTUAL location of all the important weighing points and aerodynamic points is critical.

The ONLY way to confirm the moment of the fuel tanks is to weight the airplane...so why bother doing the math for everything else since I had digital scales.
I agree, and it's a very good idea to do exactly what you're doing, whatever the book may say, because of variability in the build as well as differences in body type/shape. Particularly with front seaters in a Velocity, where the front seat can move fore-aft, different people may have substantially different moment arm locations.

So a question I really have for everyone but that too guy...if the airplane is up on scales what would be the advantage of physically measuring and mapping out locations of stuff if you have highly accurate digital scales and a spreadsheet. Put 50 pounds in the baggage compartment and the scales all read higher...add up the three increases in weight and they add up to 50 pounds so the scales seem to be doing their job...right???
It's useful to "map out" locations, if for no other reason than to validate the measurement calculations. If the front seat moment arm is supposed to be at FS-59" (COZY) but you calculate FS-57", well, that's pretty close. But if you calculate FS-23", you probably did something wrong. And with things that the POH may not discuss, mapping them is just another reference.

How would a guy confirm the moment of the fuel in the tanks using a plumb bob???
No can do.

Would it not be more accurate with digital scales as opposed to measuring tape, framing squares, and bubble levels? Even if I did all of that stuff there is no way I could calculate the moments of the fuel because the delta shape of the wing and the landing gear cut out.
So my position is that your desire to perform the weighing and calcs is exactly appropriate. With planes such as an RV, where everything DOES come out of a jig, all planes are essentially identical, and the seats don't move, the POH locations for moment arms can be used accurately without verification.

For the canard family of aircraft, where those things are not always true (or ever), performing the W&B as you're doing is an important verification of the actual moment arm locations of various loading parameters, and is something that is recommended.

I don't disagree with Toobuilder often, at least with respect to technical issues, but in this case, I think that his interpretation of what happened here is off base.

My \$0.02.

#### HomeBuilt101

##### Well-Known Member
THANK YOU Marc and everybody else...yes even the "too guy"...because on occasion I even appreciate his input (I learn something one way or the other).

When I ask questions on this forum (and I have asked MANY) over these last couple of years I always say "thank you" and express how grateful I am for the help...because I AM INDEED TRULY GRATEFUL.

It is always amazing to me, on this forum there are lots and lots of people scattered all around the world...people who may have never actually met each other... nor will they ever meet...all are people who have real busy lives with family and professional commitments...nobody is getting paid for any of this stuff...but yet you all "meet" here...contribute lots of your time...help each other out...sometimes people REALLY get involved in someone elses project looking up data/ scouring some schematic/ reading manufacturers technical literature...and are really wanting to help a brother succeed in his project...yes there are forums where I contribute as much as I can when I have some kind of experience/knowledge/background to share that could be helpful to some cyber friend who needs help and I have done it a lot...essentially helping a guy do his "homework"...and that same guy is most likely on that forum or another forum helping someone else out on some other area of interest.

Most people here are amazingly helpful...these people are the "salt of the earth" while sadly a very few others are pretty much that stuff the rest of us hose off of the bottom of our boots after we walked across the grass at night...So hopefully the former will be thanked with gratitude and be encouraged and amplified here in this community and the latter...who are clearly seen for what they are...will diminish and drift away to one of the political forums and find a home where their negativity and snark can flourish...

So anyway...THANKS AGAIN to the positive people here...hey if I see you on the flight line I will hold short and let you go first or I will buy you a free pancake if we can ever do a non-Zoom in person fly-in some day!!!

Cheers!!!

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
So this thing has run its course and I think its a clear example of forum miscomunication.

Before I bail out, I want to make it clear that using a moment change to establish the arm of an odd shaped compartment is acceptable in many cases. I do it myself. That said, one has to verify the location of the reaction points. The OP's apparent resistance to do this (baseline the aircraft) in the early posts was my entire objection to his use of this method (the magic spreadsheet). It was only much later in the thread that the OP revealed he understood the importance of this factor. So, word to the wise: If you want ME to help out with a shortcut for something as critical as W&B, you first have to give me the warm fuzzy that feeling you understand the basic concepts.

And to Marc concerning the apparent standardization of jig built RV's... Generally true, except for the gear location. And of course, THAT throws off everything else. The rod style gear is held in place with weldments, which have manufacturing variation. And the amount of weight on the mains causes deflection of the axle CL. Add these together and you may have something quite different from your buddy's, so if you borrow HIS magic spreadsheet your W&B is wrong. One of the main tires on my Rocket is more than a half inch behind the other.

Even with RV's you need to get a tape, level and plumb bob.

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#### Dan Thomas

##### Well-Known Member
W&B spreadsheets are easy to develop and abundant to find. BUT Even the best spreadsheet is of NO HELP if you do not know the raw engineering data such as the Fuselage Station of the components you are attempting to "locate". And if the arm is suspected to be off by .75 inch as the OP suggests, then the location of the reaction points (wheels) is also highly suspect.

Frankly people, a spreadsheet is just about the last thing the OP needs right now. What he needs is a plumb bob, bubble level, and a tape measure.

Remember: Garbage in, garbage out. Baseline the aircraft, THEN start plugging in numbers to the spreadsheet.
And electronic spreadsheets short-circuit the learning and understanding process. One needs to work it out on paper in various configurations to get it down. After that, one can forget all the fancy machinery and do it in a minute on any scrap of paper.

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#### BJC

##### Well-Known Member
HBA Supporter
And spreadsheets short-circuit the learning and understanding process.
Only if one blindly accepts an unverified spread sheet.

The past two or three generations, and many old timers, too, effectively create and use spread sheets for lots of calculations. I see no difference in demonstrating an understanding of the physics and math, and making use thereof, between writing down the problem on paper and calculating the answer long hand verses creating a spread sheet, inputting appropriate parameters, and letting it calculate the results. Both methods require a high level sanity check.

BJC

#### Daleandee

##### Well-Known Member
One needs to work it out on paper in various configurations to get it down. After that, one can forget all the fancy machinery and do it in a minute on any scrap of paper.
True! When my DAR did my current airplane he was quite thorough with the CG & W/B stuff. I had increased the gross by ~8% but that wasn't the point of his focus as he led me to think that many early accidents are caused because builders don't understand this stuff. He looked at the very nice sheets I provided showing most forward, most aft, gross weight, and test flight CG & W/B numbers and them had me do them all again in front of him with pencil and paper while sitting at the table. It was a great exercise.

This was the same guy that made me start the airplane, taxi it , prove the lights & brakes worked, and he checked the instruments to see that all of them were working correctly. I was impressed and felt that I got what I paid for. He told me later that usually when he first walks up to a plane to inspect it the first look at the workmanship will determine the outcome. He was pleased with what I had done.

To compare ... on a previous airplane I had it seems the DAR had the paperwork done and heading to his car before I could get the hangar door open.

#### gtae07

##### Well-Known Member
I've often found that having a spreadsheet enhances my understanding, because the spreadsheet shows me every step of the process (if you look at the equations for the cells). Spreadsheets don't make shortcuts with "it can be shown that" or "it is left as an exercise to the reader"; they don't leave out the middle parts of the work

#### wsimpso1

##### Super Moderator
Staff member
Log Member
I've often found that having a spreadsheet enhances my understanding, because the spreadsheet shows me every step of the process (if you look at the equations for the cells). Spreadsheets don't make shortcuts with "it can be shown that" or "it is left as an exercise to the reader"; they don't leave out the middle parts of the work
My perspective too. If the writer understand the topic, it shows by following the equations through the process. No mysteries, show your work.

#### pictsidhe

##### Well-Known Member
I find spreadsheets hard to read. I have to translate them into a more familiar mathematical form. I use them for complex maths that I need to iterate.

#### Dana

##### Super Moderator
Staff member
I set up a W&B spreadsheet for my plane. It allowed me to quickly evaluate the various loading scenarios and determine that I'm within limits with just about anything I can physically fit in the plane.

#### HomeBuilt101

##### Well-Known Member
Too says>... think its a clear example of forum miscomunication.

See... we finally see eye to eye in something... progress.

Hey... wait a minute.... wasn't it YOU that said...

This is a Velocity, right? The arms for the datum, seats, fuel, and baggage should be... on the web somewhere. No need to figure this out from scratch.

And I was the guy who... on my FIRST REPLY ... said that I really wanted to verify this stuff???

At that point I had already measured the axle measurements aft of datum...

Now I will do all of measurements that Marc listed...

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
...At that point I had already measured the axle measurements aft of datum...
And sharing that small, but critical detail in the first post would have been all the difference.

As I said, "details matter".

TFF

#### HomeBuilt101

##### Well-Known Member
Sorry...you stepped in it...I just couldn't resist!!!

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#### Daleandee

##### Well-Known Member
Reminds me of playground time in the third grade.
You are being quite kind. I was reminded of my granddaughter's "pre-K" class ...

#### PagoBay

##### Well-Known Member
Here, as a sample for a quite common homebuilt, is the two page W&B worksheet (not a spreadsheet) for the Zenith CH750 STOL. Notice the fore and aft limits are very well displayed. Noting also Marc Z's emphasis that a given build may have some variation. And as Toolbuilder said.. "Details Matter".

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