Weight and balance modelling BEFORE you build it...

[JaseM]

Hi guys...

have searched, asked a couple of my RV building/owner buddies - to no avail...

Has anyone ever build a spreadsheet or application model for RV's (even better if it's a 10!) to determine the W&B characteristics of a particular build BEFORE you start?

So for example, what would happen if I build standard QB RV10 kit, with (say) Titan R series (hehe), threw in a dual alternator, few other things in the engine bay, xyz avionics, abc seats, oxy up the back at point xyz... How would that impact the load characteristics once she is built? No good or a bit fine? OK - what would happen if I moved the battery forward?

Get what I am getting at? Last thing I want is to build the girl and the CofG is way aft!!!! I'd rather plan it out first and make decisions on where things will need to be for optimal CofG....

Anyone done that?

Thx

[penguin]

Great idea, but there are a few practical difficulties! Aircraft companies have whole departments whose sole purpose is to do exactly this task, and they often are not that accurate. Its not easy to be accurate.

To be worthwhile it must be within 5lb and (probably) 0.5". So everything must be modelled - that would be quite an undertaking for the basic airframe. Does anyone have a basic number for a bare (no paint, absolutely no extras) airframe?

Paint - internal (if applied) and external - is quite an unknown and also has a significant impact on weight and, more importantly, balance.

Modifications - Anything weighing more than 0.5lb should be modelled, that will be quite an undertaking.

Is a better route to be very careful to add nothing more that you have to have to achieve your goal for the airplane? Saves 3 times over, saves $$$ on buying the extras, saves time in not having to install, and saves weight.

Pete

[RV6_flyer]

Many RV-10s tend to have a front CG and require 50 pounds be placed in baggage compartment for solo operation based on the Initial Airworthiness Inspections that I have done. It would be very easy for you to create a Weight and Balance spreadsheet based on the info that Van publishes in the Preview plans if you have any experience using Microsoft Excel or other spreadsheet program. The hardest part that you will have is coming up with stations for the different items that you may want to add, subtract, or change.

--- insert Copy / Paste from RV-10 Preview Plans ---
SAMPLE SITUATION 4: MOST FORWARD C.G. (STD. PILOT WEIGHT WITH 3 LB BAGGAGE, MINIMUM FUEL)
Weight Arm Moment
Aircraft 1615 106.99 172795
Fuel (5 Gal.) 30 108.9 3267
Pilot 170 114.58 19479
Baggage 3 173.5 521
Total: 1818 196062

CG = 196062/1818= 107.8" aft of the datum. This is equal to the forward limit of 107.8, so is within CG limits. This example illustrates that solo pilots weighing less than the sample situation 3 minimum must carry weight in the
baggage area to remain within the forward CG limit with minimum fuel.
--- End Insert ---

[nauga]

Originally Posted by JaseM

Has anyone ever build a spreadsheet or application model for RV's (even better if it's a 10!) to determine the W&B characteristics of a particular build BEFORE you start?

I did for an RV-4 and still use it from time to time. It helps if you have a good example to use as a basis and know details of what's in your example, then look at increments for the changes you want to make.

It doesn't have to be dead nuts on to the see the effect of changes. I've used it to track the aft cg creep of my -4 as I replace the vacuum instruments and pump with electronics, and to see roughly how much a Landoll balancer and a smaller battery on the firewall would help (a lot and a little, respectively).

Do I know the future cg within 0.1"? No. 0.5"? Probably. Regardless of accuracy, I do know what's going to help and what's going to hurt and what to worry about.

[Snowflake]

For the comparisons you're considering, I think the "book" values for the aircraft would give you a pretty good starting point. Just take the "Vans standard" numbers, and reverse engineer them. Figure out what engine the standard assumes, and remove it from the calculation. Replace with the one you want. etc.

It won't be 100% accurate, but it should be accurate enough to let you make configuration choices like where the battery should go, where the O2 bottle should go, etc.

[az_gila]

Originally Posted by Snowflake

For the comparisons you're considering, I think the "book" values for the aircraft would give you a pretty good starting point. Just take the "Vans standard" numbers, and reverse engineer them. Figure out what engine the standard assumes, and remove it from the calculation. Replace with the one you want. etc.

It won't be 100% accurate, but it should be accurate enough to let you make configuration choices like where the battery should go, where the O2 bottle should go, etc.

I think Vans factory "book" ones are probably much lighter than most customer built versions.

The best thing would be to find a local RV that is close to your planned configuration {added: and talk to the owner} and with a similar paint job and simply use it as a reference for your planned changes.

[BillL]

I had the same thought. The problem is knowing what is on any one build. What I did do was to collect all the model data I could find, with engine and prop as key influencers. I then weighed various components and calculated what the effect would be of location A vs B on the movement of the CG. In my case I moved several things forward knowing what it's collective effect would be on the CG. As to where it really is - that is TBD. Experimental - go figure.

Good luck with your process.

[JaseM]

Well it seems I will have to dig my 30 year old engineering background out and indeed model this...

Will let you know how I go - thank you for the input - some good things to think about...

[David-aviator]

Originally Posted by JaseM

Hi guys...

have searched, asked a couple of my RV building/owner buddies - to no avail...

Has anyone ever build a spreadsheet or application model for RV's (even better if it's a 10!) to determine the W&B characteristics of a particular build BEFORE you start?

So for example, what would happen if I build standard QB RV10 kit, with (say) Titan R series (hehe), threw in a dual alternator, few other things in the engine bay, xyz avionics, abc seats, oxy up the back at point xyz... How would that impact the load characteristics once she is built? No good or a bit fine? OK - what would happen if I moved the battery forward?

Get what I am getting at? Last thing I want is to build the girl and the CofG is way aft!!!! I'd rather plan it out first and make decisions on where things will need to be for optimal CofG....

Anyone done that?

Thx

You are in the planning stage and want to end up a perfect airplane along with doing it your way with stuff being installed.

Nothing wrong with that. But be aware departures from the plan always slow the build and the outcome will be in your court performance wise, cg wise and otherwise. From what you write, the issue may well be a forward cg, not aft.

If you know how to create a spread sheet, what you propose is doable. The reference points are easily obtained from any 10 in the build stage and if the weights of equipment are known, the outcome should be accurate. But factors like paint and interior materials may not be so easy to determine.

Once the build begins changes will occur so the effort may be just an interesting time killer now. If you build it according to the data inputs at this time, it may be worth it but in the long run it is just something to amuse your self for now.

That's one objective here, have fun!!

[Snowflake]

Originally Posted by az_gila

I think Vans factory "book" ones are probably much lighter than most customer built versions.

Exactly... But that doesn't matter.

Armed with a reasonable knowledge of the factory configuration, you should be able to work up a spreadsheet to reflect what you're going to do that's different... You only need the factory data to create a baseline to work from.