• If You Are Having Trouble Logging In with Your Old Username and Password, Please use this Forgot Your Password link to get re-established.
  • Hey! Be sure to login or register!

weight and balance calculation

Good morning,

Put an extended Atlee Dodge baggage in my supercub. In calculating new weight and balance, getting different numbers based on what Atlee Dodge recommends versus what piper's airplane flight manual, report 834, recommends. Anyone else have a similar issue in calculating weight and balance. Thanks in advance for the replies.

John
 
weight & balance question

I don't have the information with me here at my office, it's at the shop. I will post the information when I get it back. Thanks

I believe I have it figured out. Atlee dodge information used firewall as point of reference for one of their measurements versus leading edge of wing and datum for their calculations per piper's manual.
 
supercubdriveroregon,

the Cub's w&b is based on the Wing Leading Edge. If you check FAA's Aircraft Specification No. 1A2 section III, you will find the following numbers for a standard Pa18-150 with Lyc. O320:
Frontseat +11" W.L.E
Rearseat +37" W.L.E.
Baggage +57" W.L.E.
Wing tanks +24" W.L.E.

The distance from the firewall to the W.L.E. is 24"

That means you have to subtract the 24" from Atlee's numbers (W.L.E. to firewall)

Atlee says station 101" and 123" on his installation instruction. So for your calculation it is 77" and 99" from the W.L.E.
The middle of these two numbers is 86". That is what you have to add to your emty weight (5lbs weight of the box) at station 86".

For my w&b calculation I take station 86" with max 20lbs according the STC.

Hope that helped ...
 
Last edited:
weight & balance

supercubdriveroregon,

the Cub's w&b is based on the Wing Leading Edge. If you check FAA's Aircraft Specification No. 1A2 section III, you will find the following numbers for a standard Pa18-150 with Lyc. O320:
Frontseat +11" W.L.E
Rearseat +37" W.L.E.
Baggage +57" W.L.E.
Wing tanks +24" W.L.E.

The distance from the firewall to the W.L.E. is 24"

That means you have to subtract the 24" from Atlee's numbers (W.L.E. to firewall)

Atlee says station 101" and 123" on his installation instruction. So for your calculation it is 77" and 99" from the W.L.E.
The middle of these two numbers is 86". That is what you have to add to your emty weight (5lbs weight of the box) at station 86".

For my w&b calculation I take station 86" with max 20lbs according the STC.

Hope that helped ...



Yes, this helps clarify my question. Thank you.
 
I have found that many Super Cub Weight & Balance calculations have gross errors. One common type of error is to mix up the location of the datum point as was pointed out above. Adding in some numbers that assume a datum 60 inches in front of the leading edge with other data that has the datum at the leading edge then adding moments with this mixture ends in gibberish.
Another very common type of error is to have the datum at the leading edge then reduce the weight of something ahead of the leading edge; for instance substituting a new light weight starter. The moment arm in this situation is a negative number (say -24 inches because the change is ahead of "0", the leading edge). The weight reduction is a negative also (say -8 lbs). When the resulting change in moment is calculated, these two numbers are multiplied (-24)x(- 8 ) = +192 inch-lbs. Most mechanics, even some very good ones, often will enter the moment change as -192 instead of + 192. To many people the idea that multiplying two negative numbers results in a positive number doesn't make sense -- so be especially careful of that case where the calculation involves a negative moment arm (anything ahead of the datum, leading edge in most cases).
 
No wonder SuperCub w/bs are always so screwed up. Three data! 60" forward, Leading Edge, and now firewall.

so the question is, for what reason has Atlee Dodge taken the firewall for his numbers?
maybe the measurement is easier from the firewall to the ext. baggage compartement ?

for the calculation of the ext. baggage compartement you have to know that the distance firewall to W.L.E. is 24" and as Darrel mentioned above, never ever mix up the locations.
 
The reason is no longer important - it has to have been boneheaded stupidity at the Piper plant, and not smart at Atlee either. The cure is what is important. Watch out for incorrect computations after weighing, too - I have seen several errors done by highly respected Cub shops.
 
I have a weight and balance, CG question. A J3C-65 with lots of mods and trying to figure out what CG limits to use or that are accurate for the aircraft as configured? I know it's a J3C-65, but, has Lyc. O-235-C1 engine, 12 Gal header tank, L & R 12 gal. wing tanks, starter, alternator, battery, Com radio, transponder w/encoder, ELT, 8.50x6 tires, STC balanced horizontal elevators. All this installed, the plane is more like a PA-18 configuration wise. I just had the plane on the scales and come up with new current data. My question is, what CG limits or ranges are to be used, empty range and at gross? I do not think that original J3 CG range is what should be used for this configuration for several reasons. Yes my gross is still 1,220 lbs.

J3C-65 as
C.G. Range (+10.6) to (+22.7)
Empty Weight C.G. Range (+8.5) to (+20.3)

PA-11 (A-65-8F)
C.G. Range (+12.9) to (+22.0) or
(+12.9) to (+20.0) when C85 or C90 engine installed.

Empty Weight C.G. Range (+13.8 ) to (+18.3) or
(+13.8 ) to (+15.5) when C85 or C90 engine installed.

PA-11 Item 311. D. (6) Lycoming O-235-C engine
C.G. Range: (+12.9) to (+17.3)
(No Empty weight range given)



PA-18 w/O-235-C1 engine
C.G. Range Normal: (+14.0) to (+21.0) 1,500 lb.
(+11.5) to (+21.0) 1,200 lb. or less

C.G. Range Utility: (+14.0) to (+19.0) 1,500 lb.
(+11.5) to (+19.0) 1,200 lb. or less
 
Last edited:
Does the paperwork/STC for the O-235 or the balanced elevators list any applicable CG limitations? If yes, then use those numbers. If no, then it is still a J-3C65 and you will use those numbers. In other words, unless it has been legally modified to another set of numbers, use the original. You need to use the most recently approved numbers.
 
I have a variety of paperwork, and not yet finished with the research, but the engine was field approval 337 in the 1950's no CG change located, the balanced elevators was a STC install in later years, but no CG change located. The latest I find is a sheet from 1993 with a CG range for +12.9 to +17.3 same as the PA-11 with O-235 which seems a bit narrow or conservative. I probably won't really know until flight tests. Currently the CG empty calculates to be 13.01" no oil. It will probable require some ballast weight in the tail, which the TCDS mentions up to 11.5 lbs if needed, for the PA-11 O-235 install.
 
... but the engine was field approval 337 in the 1950's no CG change located,....

Those 337s from prior to October 1956 (date?) were similar to present day STCs. Is there any reference to a PA-11 on the 337? Perhaps if you read the 337 closely it will become more clear.
 
Yes, was a July 1954 337, but a little sparce on information, its a two page sheet. and one paragraph on engine, and engine mount referenced to a J5. I have copies of other 337's from other J3's with this engine, and they are much more comprehensive with diagrams, photos, and W&B info.
 
I have a variety of paperwork, and not yet finished with the research, but the engine was field approval 337 in the 1950's no CG change located, the balanced elevators was a STC install in later years, but no CG change located. The latest I find is a sheet from 1993 with a CG range for +12.9 to +17.3 same as the PA-11 with O-235 which seems a bit narrow or conservative. I probably won't really know until flight tests. Currently the CG empty calculates to be 13.01" no oil. It will probable require some ballast weight in the tail, which the TCDS mentions up to 11.5 lbs if needed, for the PA-11 O-235 install.
I think I would go with the paperwork from 1993. Sounds like what you have done is converted a J-3 into a PA-11, plus the ballanced tail from the mods you listed.
 
Yes, does seem to have most of the PA-11 conversion items. What constitutes a J3 to PA-11 conversion, is there a new data plate issued, or what? I guess a person would have to conform to the TCDS for the PA-11, and reference several Piper Drawings on the conversion of such. Removing the 12 gallon front header tank, which mine has still installed. Otherwise I think I meet all other critera for a PA-11. Or will it always just be considered a J3? A heavier than normal J3. So many talk about keeping them light, and the heavier thay are how miserable they fly, but take the lightest J3 known, and compare againt the one I have and both are losded at 1,220 gross, wouldn't they fly the same? Other than mine has enclosed cowling, and more horsepower, and balanced tail, I will probably out perfrom?
'
 
What constitutes a J3 to PA-11 conversion, is there a new data plate issued, or what? ...

It sounds as though yours is still a J-3 in the eyes of the FAA, particularly since it still has the nose tank.

What is that sheet from 1993? Is it an approved, signed official document or just a piece of paper with some writing on it? If it is the latter, it is just a piece of paper and means nothing.
 
The only way to change the CG range is with either a field approval or an STC. I think that a CG change is by definition a major alteration. Look for one of those things - the field approval is a 337 with an FAA stamp in block 3, and as stated above, prior to the mid- 1950s the 337 need not be specifically for your airplane. Still has to have that FAA stamp.

There are only two exceptions to that (that I am aware of): One - a DER or VDER can create approved data like a field approval, and two - there is a paragraph in the 8900-1 that says an IA owns the approval on a Field Approval he receives from the FAA.

as always, Opinion.
 
See if you can figure out what is wrong on this one. Second weight and balance since new. Every subsequent W&B after this was jacked up.
 

Attachments

  • W&B screw up..pdf
    279.2 KB · Views: 332
I'll bite - arithmetic is close but not exact. "Existing data per??" Looks mighty heavy if this is the second w&b, i.e. first out of the factory. Other than that?
 
The tailwheel arm is wrong. The 200 number is about right if the datum were at the leading edge. So the arm should be 260.
 
Happens all the time, and it is because of that AFM. I found a 1976 airplane - first change ever was an Airwolf filter, and they switched datums right then and there. It went downhill after that. Mechanics are not supposed to be mathematicians, and the average pilot does a weight and balance for the checkride. Period. Find a good math teacher, and have your W/B gone over with a fine tooth comb.

Opinion.
 
Sent this to the builder to tell me what was wrong. They didn't have time to look at it. Had two pages of equipment to adjust. Will weigh it next time and see how close calculations are.

Sent from my SM-N900V using SuperCub.Org mobile app
 
Back
Top