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Datums for PA-18 Weight and Balance

Erik

Registered User
This is an interesting one to me:

It appears that there is some inconsistency in Piper's publications regarding what they call the datum for their moment calculations. There is a document that came with my airplane that clearly shows the datum at 60" forward of the wing leading edge. Mechanic "A" added Bushwheels (and removed 6.00's) to my aircraft and used an arm of 62.25". That is consistent with such a datum. However, Mechanic "B" added a climb prop (and removed cruise prop) and used an arm of -54". This did not make sense to me, so when we discussed it, he told me that the "Type Certificate" for my plane shows the wing leading edge as the datum. That means Piper is inconsistent. Two mechanics used two different datums for weight changes, which makes for a meaningless aircraft empty c.g. when you think about it. Has anyone crossed this bridge before?

The c.g. envelope I have in my paper work shows a range of acceptable values "aft of wing leading edge". What that means is that if I use the wing leading edge as the datum point for all weights, then I simply use the chart for the overall arm calculated. BUT, if I use a datum 60" from the wing leading edge, that is fine too, I just need to make sure all weights use the same datum and then that the 60" is subtracted so the chart can be used. Sound confusing? It sort of is. Consistency is the key I think, but look what happened in the above example! There must be a lot of hosed-up w&b's out there.

Example:

10 lbs added at prop, 54" forward of wing leading edge. Mechanic "A" would say the moment added is 6 in*10lbs = 60 in*lbs. (remember, Mechanic "A" used a datum of 60" from W.L.E., so the arm for the new mass would be 60-54=6). Mechanic "B" would say the moment added is -54 in*10lbs = -540 in*lbs.

Say before the change, the aircraft has a weight of 1200 lbs and an arm of 12" aft of wing leading edge OR otherwise stated as 72" from datum 60" forward of wing leading edge:

Scenario #1: Use datum at wing leading edge (consistently) -

aircraft moment = 1200 lbs * 12 in = 14,400 in*lbs
added moment = 10 lbs * -54 in = -540 in*lbs
new moment = 14400-540=13,860 in*lbs
new weight = 1200+10 = 1210 lbs
new arm = 13,860/1210 lbs = 11.45 in aft of W.L.E.

Scenario #2: Use datum at 60" forward of wing leading edge (consistently) -

aircraft moment = 1200 lbs * 72 in = 86,400 in*lbs
added moment = 10 lbs * 6 in = 60 in*lbs
new moment = 86,400+60 = 86,460 in*lbs
new weight = 1200+10=1210 lbs
new arm = 86,460/1210 lbs = 71.45 in aft of datum OR 71.45 in-60 in = 11.45 in aft of W.L.E.

Conclusion: Scenario #1 and #2 are the same and correct.

Scenario #3: All previous weights use datum at W.L.E., but add weight using datum at 60" forward of wing leading edge -

aircraft moment = 1200 lbs * 12 in = 14,400 in*lbs
added moment = 10 lbs * 6 in = 60 in*lbs (this is where the error is induced)
new moment = 14,400+60=14,460 in*lbs (error)
new weight = 1200+10=1210 lbs
new arm = 14,460/1210 lbs = 11.95 in (but from what datum? This is an error caused by mixing datums)

Scenario #4: All previous weights use datum at 60" forward of W.L.E., but add weight using datum at wing leading edge -

aircraft moment = 1200 lbs * 72" = 86,400 in*lbs
added moment = 10 lbs *-54 in = -540 in*lbs (this is where the error is induced)
new moment = 86,400-540=85,860 in*lbs (error)
new weight = 1200+10=1210 lbs
new arm = 85,860/1210= 70.95 in (but from what datum? This is an error caused by mixing datums)

Conclusion: Scenario #3 and #4 both differ from each other and from the correct answers. Imagine the w&b accuracy of a plane with a lot of changes using different datums each time. The end result is a worthless aircraft C.G. #.

Thanks to those taking the journey with me!
 
Erik said:
It appears that there is some inconsistency in Piper's publications regarding what they call the datum for their moment calculations.

Indeed! Here's what the type certificate (TC number 1A2) says regarding the datum for the PA-18, under the heading of "Specifications Pertinent to All Models":

Datum - Wing leading edge

However, in my own airplane there are several weight and balance computations based on the datum being 60 inches ahead of the wing leading edge. This definitely invites confusion and error.

The main thing to remember, and each mechanic needs to always be vigilant of this, is that the placement of the datum doesn't really matter, so long as ALL arm measurements are taken from the SAME datum. So long as a particular datum is used consistently, there won't be any problem. And since the CG envelope for the PA-18 is called out as inches aft of the wing leading edge, it would seem to me that the wing leading edge would be the appropriate datum (as called out on the TC). However, this means that there will be some negative arm measurements, which some people avoid like the plague (which undoubtedly led to the datum being called out as 60 inches ahead of the leading edge).

Hope this helps!

Joe
 
Hi Erik,

I'm very uneducated in these matters...but have been doing alot of reading on different things.
The following link is the type certificate info for the PA-18 on the FAA site.

http://www1.airweb.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/AD0084716C91470F8525673900566909/$FILE/1a2.pdf

Hope it helps.
Laura
 
Joe, Laura - good responses. Thanks.

The "document that came with my plane" that I referenced in the beginning is titled "Piper Aircraft Corporation Lock Haven, Pennsylvania Model PA-18"150" Model PA-18A"150" Report No. 834". It then says "Airplane Flight Manual - Piper Model PA-18"150" and PA-18A"150" (1750 Pounds Gross Weight)". There are many revisions from 1954 to 1976 listed. When I get to the page with the "Actual Weight and Balance" my plane's serial number and "Certificate No." (N-number) and "Date" are indicated. This is the actual weight and balance new for my plane. It shows a picture of the plane and labels "W.L.E.", shows a distance of 60" forward, and labels a line "Datum".

So, if you are consistent with your datum, it doesn't matter. But why does the original flight manual provided with my plane show datum 60" forward of W.L.E. and the Type Certificate for my plane (held by "The New Piper Aircraft, Inc." and dated September 4, 1996 by the way - thanks for the link Laura) show the datum at W.L.E.? Is one correct and the other incorrect, officially? The September 4, 1996 Type Certificate (that version anyway) did not exist at the time of manufacturer, but the flight manual did exist and was issued with my plane. Has it anything to do with Lockhaven vs. Vero Beach Piper?

Another did-bit of interest! Cub Crafters did an actual weight and balance on my plane in 2001 and they show "C.G. aft of W.L.E." and "C.G. aft of Datum" (+60" difference). Cub Crafters is supposed to know this stuff right? They seem to be siding with my original flight manual on what to call "datum".
 
Erik said:
"C.G. aft of W.L.E." and "C.G. aft of Datum" (+60" difference).

Erik
Welcom to Pipers convoluted PA-18 Weight and Balance system. Would really be interesting to find out what rocket scientist (my apologies to all you real rocket scientists out there) Piper hired to devise this system, but thats just the way it is with the PA-18 W&B. The datum really isn't the datum unless its the datum.
 
It doesn't really matter which Datum you use, the C.G. location on the A/C is the same. Not sure when Piper changed the systems, but the 60" number was used to make the calculations easier to use since all the numbers have a positive value. The Datum on the leading edge causes you to use negative numbers for anything forward of the datum and positive numbers aft of the datum. Use whichever one you are comfortable with, it makes no difference.

John
 
S2D said:
The datum really isn't the datum unless its the datum.
Yes! That's funny.

cubunltd,

Makes no difference IF and ONLY IF you are consistent. You cannot mix datums and come up with correct/accurate C.G. Also, you get negative and positive numbers simpy by virtue of whether you are adding weight (positive) or eliminating weight (negative). I originally thought the same as you stated that the 60" datum was so that all arms were positive. But a positive times a negative (i.e. elimination of weight) produces a negative number.
 
TJ,

The arm would always be positive. But the moment would be negative if you were removing weight. I.e. if the arm was 100" and the removed item was 10 lbs, then the moment would be negative. 100" x -10lbs = -1,000 in*lbs. You would subtract that moment.

Neither of us is wrong, we are just talking about the same thing in different ways.

Erik
 
Erik,
The arm would only be positive if it were located aft of the Datum. ALL arms forward of the Datum are negative values. Weights are always positive if they are added and negative if they are removed. If you were to remove "a" weight in front of the Datum it would be a minus weight X a minus arm giving you a positive moment.


John
 
Why Why Why didnt Piper use the Spinner as the Datum?

Why didnt Piper use the most forward part of the airplane ( the spinner tip ) as the datum?? My guess is that since the airplane is certified with various spinners (short and long) and some without spinners they couldnt use that reference.

I would have used the firewall but they might hve had a reason to use the wing LE. It doesnt matter, as stated previuosly, as long as all measurements associated with a given calculation are based on the same datum. Now I have to go down and measure those new widgets I installed yesterday.
 
Just cleaning up my paperwork... What W&B arm are you all using for engine oil - assuming datum is the L.E.?
 
I just glanced at this decade-old thread. Forgive me if I repeat something:

Piper used two datums for both the J-3 and the PA-18, often in the same document. The J-3 uses WLE and 46" ahead of WLE, and the Super Cub uses WLE and 60" forward. Using one or the other will not get you in trouble, but mixing them generally creates an aircraft that cannot legally be flown.

I look first for mixed data. Then I look for actual weigh- ins done with the wrong formula. If the weigh- in was done incompetently, I assume the weigher did everything wrong, and go back to computed. That often produces an aircraft that can hold two adults and enough fuel to actually go somewhere.

Be careful - check for landing gear in the wrong spot, not using the Piper formula, and mixed data. If you see an aircraft with CG out of limits, a J-3 around 850# empty, or a Super Cub above 1200# empty, get suspicious.

Each Super Cub should have a ten page or so document on board. Its title is "Report No. 834." Page 2 gives CG info with respect to Wing Leading Edge. Page 5 shows a good formula, and converts Datum to 60" ahead of LE. It says Oil - 8 qts - 15 lbs, 24" aft, 360 moment.

Then, after thoroughly confusing you, it converts back to wing leading edge for CG. On page six it does the same thing, further confusing things. Then in the equipment list it sticks with wing LE. Hopeless.

Very few mechanics get this right. Get someone familiar with math to go over this stuff.

Opinion.
 
One of the reason to use the wing LE is to make it easy to calculate CG in terms of % MAC which become very easy with a straight LE such as the PA18. On the other hand putting it out in front of the airframe eliminates negative math errors as all weights and moments are strictly summed. There are airframes that have imaginary datums several feet in front to acommodate different radomes etc. My view is the very definition of datum is "standard reference point" therefore all distances should reflect distance from a single reference point. It matters not where that point is, only that all distances are measured and stated from that point for ease of understanding.
 
Got it, Bob. Thanks. I have 834. Looking for actual numbers (and datum) most guys are using.
 
I've even seen where guys are using the firewall for the datum (like a lot of Cessnas), instead of the WLE. Also people weighing airplanes but not knowing the means of leveling them-- one guy was weighing a Pacer and used a level on the floorboards. "Well, that's what's level when you're flying the airplane". I told him to check the TCDS.
 
I think I would stick with Piper numbers unless your STC says otherwise. Most Cubs use mixed data - all you need to do is pick one, and make sure you use it consistently. If you do like Piper does and go back and forth, watch it like a hawk. Each time a mechanic makes a change, ensure that he/she used the correct datum. I find it is 50/50. Just saw one last week on a J-3.

Just once, I would like to see a correct weight and balance on a Cub. Just once.
 
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