Smith Cub wide body

[C-FIJK]

Hello I have a 180 hp new build with wirlwind prop and sitting on shock monster gear !

Can anyone tell me the proper way to weigh the aircraft and where to measure from ?

 

[Bill Rusk]

The tail post (trailing edge of the vertical stabilizer) and the firewall should be perpendicular to the longitudinal axis. Make those vertical and it will be ready, (read that level), to weigh. Scales under the mains and one under the tailwheel. You can get your moment arms from the TCDS.

Congrats. Should be an awesome cub.

Best of luck

Bill

 

[C-FIJK]

Bill thanks for the reply what did you use for the datum point the firewall or wing leading edge ?
When you look it up on line Piper used 60 inches or firewall ?

 

[Bill Rusk]

I like to use the prop face. Then all moments are positive numbers. As I recall (please check and don't hold me to this) if you subtract 60 inches from the piper numbers you will get the datums. Yours should be very close to this but please check. Every Cub is a little different.

Hope this helps

Bill


These are the datums I used

Prop Face - 0
Oil sump - 24
Firewall - 35
Inst panel - 51
Wing LE - 60
Gear axle - 62
Front Seat - 71
Fuel - 84
Fuel Pod Fuel - 90
Rear Seat - 97
Fwd wall lower baggage - 105.5
Middle lower baggage - 136
Aft wall lower baggage - 166
Fwd wall upper baggage - 136.25
Middle of upper baggage - 162.25
Aft wall upper baggage - 189.25
Tail ballast - 233.6
Tail Post - 249
Tailwheel axle - 263.6

 

[hotrod180]

The tail post (trailing edge of the vertical stabilizer) and the firewall should be perpendicular to the longitudinal axis. Make those vertical and it will be ready, (read that level), to weigh. Scales under the mains and one under the tailwheel. You can get your moment arms from the TCDS.

IMHO it seems best to use the factory leveling means, and factory datum. From the PA18 TCDS # 1A2 :
"datum: wing leading edge
leveling means: plumb from PK screw in upper door frame channel to center punch mark on enclosure door rear hinge".

If there isn't a PK screw (hole) and /or centerpunch mark,
I believe there's a factory drawing showing how to level the plane,
using the "horizontal reference line" & a water level (or laser).
I know a couple different people who had airplanes on which the firewall & the tail post weren't quite parallel,
they ended up using the reference drawings to level their airplane.

This sheet explains the preferred (factory" means of leveling, as well as an alternate method:

https://fadodge.com/wp-content/uploads/2023/04/PA-18-and-PA-19-Rigging-Instructions.pdf

Also, in my experience, it's best to physically plumb down & measure the arms for various moments,
as what's on the TCDS doesn't always necessarily agree with the actual airplane.

 

[DENNY]

The length of even stock cubs are not always the same and tailwheel/spring combinations can vary also. So make sure you do measure from tire axle to tailwheel axle for a accurate measurement.
DENNY