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Help: J3 w&b / wing loading profile

C

CaptnNick

Registered User
Hey forum,

Im newer to the cub world and been flying a lovely j3, however I want to be certain about my W&B and wing loading which I can't seem to nail down in Forelights W&B (fwd/aft cg locations). If anyone is extremely well experienced on W&B and figuring out FWD vs AFT cg on the wing I would greatly appreciate it. Ive included the W&B sheet, sad to say its a bit confusing to me and there's not a lot of helpful detail on the web.

My other question is given the max wt of these planes how does anyone go about flying two adults and decent amount of fuel, especially if you add floats?

j3cub w&b.jpg
 

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When they weigh 835# empty, they become officially one place aircraft. You have a useful load of 385 lbs - enough for two 170 lb adults and 45 lbs fuel - or, if you are like most of us, two 190 lb pilots and 5 lbs fuel.

It seems like almost all J3s wind up that way when you weigh them. I have no idea why. It is legal (although it seems to horrify most folks) to compute your weight and balance using factory original weight. Be very careful - there are two data - use only one of them. Easiest seems to be wing leading edge. The datum 46" ahead of that was because the factory wasn't sure mechanics understood negative numbers.
 
" My other question is given the max wt of these planes how does anyone go about flying two adults and decent amount of fuel, especially if you add floats? "

Cold and windy out so I will start $hit storm early. The answer is simple they fly overweight. Happens a lot in small aircraft especially in small training aircraft.
DENNY
 
bob turner said:
When they weigh 835# empty, they become officially one place aircraft. You have a useful load of 385 lbs - enough for two 170 lb adults and 45 lbs fuel - or, if you are like most of us, two 190 lb pilots and 5 lbs fuel.

It seems like almost all J3s wind up that way when you weigh them. I have no idea why. It is legal (although it seems to horrify most folks) to compute your weight and balance using factory original weight. Be very careful - there are two data - use only one of them. Easiest seems to be wing leading edge. The datum 46" ahead of that was because the factory wasn't sure mechanics understood negative numbers.
Click to expand...

Ya i figured, considering this holds 24gal of fuel its definitely a one place aircraft. I just don't understand how some of the local schools with j3's on o-200s and floats can even teach dual. Is it normal for people to run these overweight and safely? How far out of CG does it get?

Foreflight asks for the FWD most points of CG and AFT most points - there should be two to three for the front and two for the back of the envelope, however piper never supplied wing loading graphs so im unsure how I find them with the included W&B
 
it seems that would be the only way, i guess i want to figure out with the W&B how far out of FWD/AFT cg it gets
 
There's a really nice weight and balance spreadsheet available from the J3 cub forum. I'll put a link to a google drive copy of it.

https://docs.google.com/spreadsheet...ouid=116355310981471044679&rtpof=true&sd=true

There's a few things you can lose to save weight including the starter, battery, and going with a 4 quart oil sump instead of 6 depending on what engine is installed. Regardless, it's going to be on the heavier end of what J3's weigh at but if it flies good that's all that matters.
 
dgapilot said:
Forward limit is 10.6, aft limit is 22.7 per the Type Certificate Data Sheet.
Click to expand...
looking at the W&B sheet I provided, does that then mean 10.6 aft of WLE (which WLE is 48.5") so 59.1?

also what weights would I be looking at for FWD/AFT CG ranges - I assume empty weight and then max gross but there has to be a limit to FWD CG weight since you dont fly from the front

Just trying to get the ranges Foreflight WB youtube pic.jpg
 

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CaptnNick said:
looking at the W&B sheet I provided, does that then mean 10.6 aft of WLE (which WLE is 48.5") so 59.1?

also what weights would I be looking at for FWD/AFT CG ranges - I assume empty weight and then max gross but there has to be a limit to FWD CG weight since you dont fly from the front

Just trying to get the ranges View attachment 58364
Click to expand...

I didn't look at the W&B sheet you had, but it all depends on the datum used. The TCDS uses the wing leading edge as the datum.
 
Doesn't the gross weight go up a bit on floats? And I bet Jack won't let those Cubs get anywhere near a scale.

1160 lb. with McKinley floats
1200 lb. with Edo, Wollam or Heath floats
1300 lb. with Edo 60-1320, Edo 92-1400 or Wollam 1200 floats
 
Mine rolled out with metal spar wings at 718 lbs. in June 1946.

We pretty much determined that wood spar wings are lighter - we decided on 8 lbs per set after several weighing exercises. Earlier Cubs often show factory weights in the 690 range. Lighter landing gear, lighter struts, maybe lighter bungees - who knows.

In the olden days, Ray Stits told us his fabric saved weight. I think if you took all the fabric and paint off a J3 it would be between 40 and 80 lbs, no matter what process (except maybe Oratex). So where does that extra 100 lbs come from? I have no idea. Even a C-85 with starter and metal prop and wing tank only adds 50.
 
CaptnNick said:
....Foreflight asks for the FWD most points of CG and AFT most points - there should be two to three for the front and two for the back of the envelope, however piper never supplied wing loading graphs so im unsure how I find them with the included W&B
Click to expand...

My suggestion is to throw out ForeFlight (at least for now) and read the W&B stuff on the J3C TCDS.
It should answer your questions re fwd & aft CG limits, etc.

Microsoft Word - A-691 Rev 34.doc (faa.gov)

If you can do basic arithmetic, it's easy to do W&B calc's the old school way--
weight times arm = moment, moment / weight = CG.
For me anyways, it's easier that way to understand what's going on & why, vs using a whizbang app.
 
There is only one forward point for the J3, I gave you that in a prior post.

Your confusion is all based on the datum being used. The TCDS uses the wing leading edge. Everything aft of the LE has a positive arm, everything forward has a negative arm.

The W&B form you posted in the opening uses a datum 48” forward of the leading edge will all arms positive. It doesn’t matter which method you use, you just need to use the same datum for all calculations.


Sent from my iPhone using Tapatalk
 
Those "point in space" datums are a PITA,
I think the Luscombe does that also.
Dunno if they mix-n-match point-in-space and WLE like Piper does though.
I like the firewall datum used on most Cessnas,
much easier to measure a station, and to deal with in general--
as long as you remember that a positive x a negative = a negative.
 
I think they used datums ahead of the airplane so it was all positive numbers. I never had any issues multiplying positive and negative numbers together, it's like 7th grade math.
 
Both of the above posts are on point.

Avoid like the plague complicated spread sheets. They obscure what is going on.

You may choose any datum you want, but if you know how to manipulate negative numbers the LE is simplest. I am currently using 46" ahead of LE, for no reason other than that is where I started.

My buddy presented me with a complicated eight column spreadsheet on his J3. I am not dumb - although I am incapable of really simple calculations like subtraction, I am a whiz at vector calculus and integral transforms, and I found it incomprehensible.

Do it this way - Weight x Arm = Moment. Start with factory numbers. Use the same datum for each addition or subtraction. At the end, add all the weights, being careful with plus and minus signs. Add all the moments. Then divide the total moment by the total weight. That gives you empty CG. If it falls within the TC specs, you are done.

Then do a couple sample loadings - oil, fuel, one or two pilots, baggage - print them out and stick in an envelope. Present to the feds if you are ever ramp checked.

Make sure you have a good chance to prove you are not overweight on paper. If you really, really need to know how much the aircraft weighs, then weigh it, but avoid putting that number on a 337.

all opinion. Locally there is a 160 Super Cub with a 400# useful load, noted on a 337. A single place Super Cub!
 
bob turner said:
Avoid like the plague complicated spread sheets. They obscure what is going on.
Click to expand...

I created weight and balance spreadsheets for my racing sailplane (variable wing and tail water ballast) and for my FX-3. I have found both to be very useful. Nothing obscure but they do require careful verification. My first draft FX-3 attempt had a data entry error for the fuel arm.

How many people who run close to the CG limits run the calculations for max and min fuel load? I do, and it's plotted graphically. A graphic weight and balance depiction is also given by the Garmin G3X system but, like the spreadsheet, it requires the entered data to be correct.

Data entry errors are as much a risk in a manual calculation as they are in a spreadsheet. Excel is quite capable of error free multiplication and division.
 
When I flew a King Air, I even did the loading with gear retracted and gear extended. Pretty easy to get out of the envelope when you have a long fuselage!


Sent from my iPhone using Tapatalk
 
As a BAC 111 first officer, I was responsible for a horrible run-around chart for wt/bal before each departure. So I wrote a computer program (in Basic) - it would ask for fuel, baggage fore and aft, and passengers, and spit out gross weight, I think CG, and V-speeds.
Did the same thing for the 737.
Then the airlines got smart and put a program in the big computer for the qualified gate agent.

Even with such relatively complex computations, eight columns of data were not required. All you need is weight and arm.
 
Then they did this,

On Board Aircraft Weighing System

Some large transport airplanes have an on board aircraft weighing system (OBAWS) that, when the aircraft is on the ground, gives the flightcrew a continuous indication of the aircraft total weight and the location of the CG in % MAC.

The system consists of strain-sensing transducers in each main wheel and nose wheel axle, a weight and balance computer, and indicators that show the gross weight, the CG location in percent of MAC, and an indicator of the ground attitude of the aircraft.

The strain sensors measure the amount each axle deflects and sends this data into the computer, where signals from all of the transducers and the ground attitude sensor are integrated. The results are displayed on the indicators for the flightcrew.

Gary
 
BC12D-4-85 said:
Then they did this,

On Board Aircraft Weighing System

Some large transport airplanes have an on board aircraft weighing system (OBAWS) that, when the aircraft is on the ground, gives the flightcrew a continuous indication of the aircraft total weight and the location of the CG in % MAC.

The system consists of strain-sensing transducers in each main wheel and nose wheel axle, a weight and balance computer, and indicators that show the gross weight, the CG location in percent of MAC, and an indicator of the ground attitude of the aircraft.

The strain sensors measure the amount each axle deflects and sends this data into the computer, where signals from all of the transducers and the ground attitude sensor are integrated. The results are displayed on the indicators for the flightcrew.

Gary
Click to expand...

I thought they had something like that. That's why the 737 tipping on its tail at Lewiston ID seemed so strange.

https://www.news18.com/news/auto/un...g-company-blames-shift-in-weight-4223750.html
 
Makes sense. In the olden days it was an estimate, based on random loading and 170 lbs average.

But note - the most sphisticated system does not give you an eight column spread sheet. Just weight and CG. All you need.
 
Right on all accounts regarding the long spreadsheet. However FF allows me to keep multiple aircraft W&B on file within fingertips on my iPad or in this case, on my phone. Any rampcheck I have it on my phone as current as it’s going to be. Forgot my phone you ask? Rarely, but I do have a paper one stashed behind my aircraft flight log folder.
 
bob turner said:
But note - the most sphisticated system does not give you an eight column spread sheet. Just weight and CG. All you need.
Click to expand...

I like to see how you make a working weight and balance spreadsheet with only two columns.

You only have mass and CG. Where does the arm go? Where does the description of the line item go (empty, fuel, front seat, back seat, baggage, etc,)
 
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You don't. But you do not need eight columns, and the answer is only two numbers. If you have to print out things like flight number and number of passengers, destination, fuel load, etc, you may need some format other than gross weight and CG.
What I saw was an eight column spread sheet for a J3. Serious overkill. Next time I open the big computer I will print my simplified version here.

Simplified version (I cannot get tabs or spaces to reproduce here - but there are three distinct columns in the original):

Empty weight 743.8 lb Empty C/G 61.5” Moment 45750.41

Note that the new datum is 46” ahead of wing LE. This aircraft left the factory at 711 lbs @ 15.94” No significant changes have been made to the airframe since then, with the exception of the above equipment list.

Sample Loading – Full Fuel

Aircraft EW – 743.8 lbs 61.5” 45750.41
Oil – 4 qt 8 17 136
Fuel – 12 gal 72 (main tank) 28 2016
Fuel – 6 gal 36 (wing tank) 70 2520
Front seat 195 55 10725
Rear seat 0 82 0
Baggage 0 95

Total 1054.6 lb 58 ” 61147

Approved: 1220 lb 56.6” to 68.7”

If empty C/G is aft of 59.8”, any load will fall within allowable range, including front seat solo. No placard is required on this aircraft.
 
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