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Smith Cub Builders - Need Wt&Bal, CG, Weight limits, etc

G

GeneHerzog

MEMBER
Sulphur Springs, Texas
I am within 2 weeks (I hope) of finishing my Smith Cub 180hp. If anyone has their dimensions for the fuel tank moment arm (not sure if I should use the middle of the tank or not) I could use it. Would actually like to see all of your mom. arms. My CG tends to be to the rear of the 10.5" to 20" limits which I hope are the correct specs. I have a Superior XP360 180hp engine and 80/46 Sensenich Wood composite prop.

What did you post as the gross weight? This is a Smith Cub purchased as a kit before he sold to TCOW and does not have the spar doubler reinforcement.

Also would like to have any info about weight limits for the front and rear baggage compartments.

I also would appreciate any and all airspeed limits for marking the A/S indicator before the inspection.

I will be posting a full set of pictures in the next few weeks if it will help anyone.
 
Congratulations Gene and Dorinda!

It must feel so good to be near completion!!!

Hope to see you in Graham next week. I'll be there Saturday through Friday.

Laura
 
Determining these moment arms for YOUR airplane is part of the initial weight and balance task. This IS a bit more involved then what you would do for a production airplane.

For those of you that have never documented and weighed a NEW aircraft (i.e ANY airplane that was NOT built from PRODUCTION tooling) Here's the Drill:

1. Level and support the aircraft for measurement.

2. Using Plumb Bobs, tape on the floor,chalk lines, tape measures, etc. measure the locations of any reference locations that will be used to establish the empty weight and CG limits. i.e. the fuselage station of the jack points or axles, and LE and trailing edge of the wing...(using the reference system that the designer used)

As an aside....The TCDS calls out the datum as being the FS of the LE of the wing (The CG envelope is from 10.5 to 20 inches behind the LE).

I prefer to have FS 0 well forward of any part of the airframe. This makes the math a little more fool proof since you never have any NEGATIVE Fuselage stations. So there's less chance of having a sign error. Thus I would designate the Wing LE as FS 100. This makes the CG envelope come out to be FS 110.5 to FS 120. Easy to remember......

3. Level and support the airplane on the scales (don't forget any tare weights for chocks, leveling blocks etc.).

4. Calculate the Empty Weight, Empty Moment, and Empty CG from the scale readings.

5. Add some fuel to the tanks while the airplane is on the scales.

6. Subtract the empty weight from the fueled weight to get the actual weight of the fuel, Subtract the empty moment from the fueled moment to get the actual moment of the fuel mass. Then divide the fueled moment by the fueled weight to get the moment arm of the fuel.

To be perfectly accurate this should be done for a couple different fuel states to account for the tank shape.

7. Repeat #6 for any seats by putting someone in the seats. If the seat is adjustable be sure to establish the arm for the most forward and most aft locations. As well as the baggage areas by adding weight to the desired locations.

This should really only take one good afternoon with a couple people. It's not Rocket Science.....

DaveG
 
Hey Gene is Dorinda gonna let you come along when she takes it to Lockhaven? :lol: I am glad to hear your on the home stretch.Look forward to seeing both of you.
Bill
 
DaveG said:
As an aside....The TCDS calls out the datum as being the FS of the LE of the wing (The CG envelope is from 10.5 to 20 inches behind the LE).

I prefer to have FS 0 well forward of any part of the airframe. This makes the math a little more fool proof since you never have any NEGATIVE Fuselage stations. So there's less chance of having a sign error. Thus I would designate the Wing LE as FS 100. This makes the CG envelope come out to be FS 110.5 to FS 120. Easy to remember......

DaveG
Click to expand...

A small note. The "units" of arm (fuselage stations) can be anything - MM, CM, Feet or Furlongs. We normally compute using Inches. To make the Wing LE 100 (inches) you would need to make the Datum Point 100 inches ahead of the Wing LE. This is certainly do-able. Just wanted to clarify that you couldn't arbitrarily designate the Wing LE 100.

Carry on.

John Scott
 
DaveG said:
Determining these moment arms for YOUR airplane is part of the initial weight and balance task. This IS a bit more involved then what you would do for a production airplane.

For those of you that have never documented and weighed a NEW aircraft (i.e ANY airplane that was NOT built from PRODUCTION tooling) Here's the Drill:

1. Level and support the aircraft for measurement.

2. Using Plumb Bobs, tape on the floor,chalk lines, tape measures, etc. measure the locations of any reference locations that will be used to establish the empty weight and CG limits. i.e. the fuselage station of the jack points or axles, and LE and trailing edge of the wing...(using the reference system that the designer used)

As an aside....The TCDS calls out the datum as being the FS of the LE of the wing (The CG envelope is from 10.5 to 20 inches behind the LE).

I prefer to have FS 0 well forward of any part of the airframe. This makes the math a little more fool proof since you never have any NEGATIVE Fuselage stations. So there's less chance of having a sign error. Thus I would designate the Wing LE as FS 100. This makes the CG envelope come out to be FS 110.5 to FS 120. Easy to remember......

3. Level and support the airplane on the scales (don't forget any tare weights for chocks, leveling blocks etc.).

4. Calculate the Empty Weight, Empty Moment, and Empty CG from the scale readings.

5. Add some fuel to the tanks while the airplane is on the scales.

6. Subtract the empty weight from the fueled weight to get the actual weight of the fuel, Subtract the empty moment from the fueled moment to get the actual moment of the fuel mass. Then divide the fueled moment by the fueled weight to get the moment arm of the fuel.

To be perfectly accurate this should be done for a couple different fuel states to account for the tank shape.

7. Repeat #6 for any seats by putting someone in the seats. If the seat is adjustable be sure to establish the arm for the most forward and most aft locations. As well as the baggage areas by adding weight to the desired locations.

This should really only take one good afternoon with a couple people. It's not Rocket Science.....

DaveG
Click to expand...
8. Find a guy that is good with excel!!!!
 
I guess I need to differentiate the "Datum" from the "Coordinate System" used for the CG calculations.

The "Datum" is usually some readily measurable point on the airframe, like the LE of the wing. It is the reference point from which all other points are measured.

The coordinate system is REFERENCED to the datum but the coordinate system zero doesn't need to be (and RARELY is) at the datum. It's just way more convenient to have all of the airplane in positive space by having the zero well out in front of the airplane. i.e. the axle is at FS 96 instead of FS -4

I like making the LE FS 100 to keep all the arms positive. Then.... you can convert to the TCDS LE reference by just subtracting 100 from all your results.

As a designer I always look at the CG calculations from the design standpoint......You end up with the same CG numbers in the end. I just like to keep the math as simple and intuitive as I can.

DaveG
 
Thanks to all for the posts. Do you have any more ideas or data about the specifics I need regarding Smith Super Cub 180Hp??

I have a good program for calc'ing, but am trying to pin down weight limits, Airspeeds and changes in the envelope based upon gross wt (which I have been told is the case).

Laura,
I will be at the staying here in Sulphur Springs and helping with the Clyde Smith seminar for Legend.

Everyone should understand they have a place to stay if they are passing through the Area
 
Tim,
I forgot to mention that FAA publication FAA-H-8083-1 "Aircraft Weight and Balance" is a very complete source.
 
Since the Smith Cub is aerodynamically essentially the same as a PA-18-150, you could just use the data from the TCDS.

http://www.supercub.org/woe/pa18type.pdf

Scroll to page 4 for speeds and CG envelope. Gross weight may be different. but the forward and aft CG limits should be valid.

That's where I'd start at least....

DaveG
 
Gene,

Is your Smith Cub significantly different structurally from a Piper? Fuselage, wing spar, wing leading edge, windshield thickness?

If not, the maximum Piper speeds (top of white, green, yellow) should work just fine. Even if things are more robust, Pipers numbers would give some additional "cushion".

Performance speeds like Vx and Vy could be calculated from some trials.

Of course you are the builder and test pilot so you can set whatever you want! :eek:

John Scott
 
John,
All of the mentioned items are stronger, bigger, etc. than the Piper SC and I will definitely try your suggestion if I don't get better data.

I think I have some pretty good numbers on a 180 hp STC for a standard Super Cub. I was hoping to get numbers from a current owner/builder of a 180 hp Smith Cub specifically.

Thanks for the answer and input.
 
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