Gross Weight Calculations

Nearing the completion of my Smith Cub build.

Has anyone done or know of calculations to establish a 2200 lb gross weight.

I assume a Back Country would be the same calculations.

My understanding is there is a 5.9 G requirement in the standard category and this can be determined mathematically.

Any help or supporting documents would be greatly appreciated.

Normal category G limit is 3.8, not 5.9. As the builder, I believe you can set the max weight where you choose.

MTV

If you are up for some heavy reading, try looking at Part 23:
http://www.faa.gov/aircraft/air_cert/design_approvals/small_airplanes/small_airplanes_regs/
and CAR 3:
http://www.navioneer.org/riprelay/Yet More Navion Files/car_part3.pdf

For example, it must be able to get off the ground !

mvivion said:

Normal category G limit is 3.8, not 5.9. As the builder, I believe you can set the max weight where you choose.

MTV

Click to expand...

Actually 3.8 is multiplied by the factor of safety of 1.5 which makes it 5.7 for the design load factor.

As the builder of an amateur built experimental aircraft you can set the gross weight of whatever you choose. You do need to provide different weight and balance calculations to support that gross weight and stay within your own specified CG range.

You can increase your gross weight in the usa . In canada , you can do it but need a letter from the designer that says it is ok to do the increase you are adking for. Some of the homebuilt rules differ between the two countries.

My Smith Cub GW is set at 2300#. 10 years and 1800 hrs later all well. Respect the Va numbers.

Lou

Phil Sc said:

Nearing the completion of my Smith Cub build.

Has anyone done or know of calculations to establish a 2200 lb gross weight.

I assume a Back Country would be the same calculations.

My understanding is there is a 5.9 G requirement in the standard category and this can be determined mathematically.

Any help or supporting documents would be greatly appreciated.

Click to expand...

CG is probably far more important than the gross weight. Just something to keep in mind.

I have no doubt that the C of G and a Gross Weight of 2200#'s will be fine.

The issue is to demonstrate the GW mathematically to satisfy the final inspection paper work.

Does anyone know how to do this as is well beyond me.

I understand in the US the builder simply chooses the GW but in Canada it must be published by the designer or supported mathematically.

You folks in Canada have different requirements than those of us in the USA. I suggest that you contact Nick Smith to provide you with the appropriate data. Those of us below your border are more apt to just pick a number out of a hat and say that is it!

Well, if you must mathematically, you would need to recalculate your wing loading based on your new GW, then go find technical data on the spar's you used and calculate that they are able to handle the design limited you are imposing on the aeroplane. Basically look up wing load distribution and calculate the load for the stations between the spar, then integrate that to account for the flight envelope of the airplane based on the load distribution of the US 35B Mod airfoil to validate the weight distribution between the spars. It's tedious, so go get a spreadsheet to work it out and find reference to beam theory to help you get through it. Your are solving for V (vertical sheer) where V=W and Vs (Shear Stress) of the Spar Web...

Don't forget to update your V_speeds with the new Gross Weight!

Hope they don't ask for validation on the wing attach mounts and you should be OK.