W&B calculations

[N5246H]

I recently re-weighed my Piper Clipper. I have the weight at each of the three landing gear positions. In order to complete the cg calculation, I need to know the position of the main gear and tailwheel relative to the datum. I was not able to find that from perusing the drawings CD. The TCDS showed the position of items like the seats, baggage area, fuel tanks, etc. but not the landing gear. Where would I find this data?

BTW, the empty weight is about 150 lb heavier than the TCDS says. I expected it to have gained weight, but was surprised by the magnitude of the increase. How much weight do airplanes typically gain in 60 years?

[Gilbert Pierce]

Look at you original pilots handbook, it has the data you are looking for . It has all of the locations and arm lengths. If you don't have that page with the original aircraft weighing diagram you need to get the POH from Piper as it is required to be in the airplane.
The center of the main gear is aligned with the wing leading edge. That is your 60" datum point.. The tailwheel is 165.5" behind the center of the main gear.

[Ed Brown]

The best source I've seen for W&B info comes right from the FAA in their 1999 "Aircraft Weight and Balance Handbook." FAA-H-8083-1. I purchased my copy from Jeppesen Training Products.

Level your plane as per the TC then measure the distance from the datum to the landing gear. That will give you the arms to use in determining the CG. The aircraft needs to be perfectly level to get the correct CG.

Here is the url for the PA-16 TCDS:

http://rgl.faa.gov/Regulatory_and_Guidance_library%5CrgMakeModel.nsf/0/DBB68CFC7C27E4568625720900718FEB?OpenDocument

[N5246H]

Thanks for the info. I do have the POH in the airplane. I was attempting to do the calculation at home.

[Bob Andrews]

It is on the TCDS. Look further down where it says "Data Pertinent to All Models."

[J.J.]

The center of the main gear is aligned with the wing leading edge. That is your 60" datum point..

Gilbert,
To me this is confusing. Why would the Datum be 60"? Shouldn't it be 0"? All measurements and CG envelopes I have seen on the Shortwing TCDS's measure from the wing leading edge. Where did the 60" come from if the main gear axle is in line with the published Datum? Is the front seat at 18 inches like the TCDS says or is it at 78 inches like you say?

It really alarms me when I see outdated or made-up information passed on to people who aren't 100% sure of what they are doing in the first place. The ONLY place you might see a reference to 60" and the main gear is on an original shortwing W&B sheet from the factory which should carry ZERO relevance to anyone making a CURRENT W&B sheet because all CURRENT information indicates the Datum is at the wing leading edge and not some imaginary point in front of the spinner. I submit the possiblity that the original Piper W&B sheets were incorrect on some aircraft because the PA-20 handbook published in 1950 in front of me references the WLE as the Datum which happens to match the legally binding document commonly refered to as the TCDS.

If you use 60" for the mains then 60 must be added to ALL measurements or the W&B will be off enough for potentially tragic results. Personally, I'd stick with information from the TCDS.

Jeff

[Steve Pierce]

Look at the original W&B. Piper used the 60 inches on most all the ragwings I have seen. The original W&B sheet has all the load points. Can't see how this is "outdated or made up information". Been being used on Clippers for 58 years.

[Gilbert Pierce]

Jeff

I am not quoting made up information. Never have and never will. If I don't know I say so.

On my Clipper POH is a w&b drawing of the airplane ( loading diagram), all of the arms for the oil, fuel, front and back seats, baggage compartment, propellor etc are on the drawing. Additionally the drawing shows the aircraft W&B datum 60 inches in front of the wing leading edge. All of the arms are specified in the factory W&B as positive numbers starting at that 60 inch point. It is real simple, you multiply each arm by the weight at that point, total up all of the positive numbers, divide by the gross wt. and subtract 60 from that CG arm. You now have your center of gravity. You did not have to deal with adding positive and negative numbers which stretches some pilots math skills. Piper did this on all of the ragwing Pipers. I suspect they did this so when you loaded your aircraft and did a weight and balance all of the numbers were positive and you did not have to add positive and negative numbers.

How do you do a weight and balance when you load your Pacer? I find the loading diagram much easier to use then to decipher the TCDS which does not give the information in a straightforward manner. Do you carry a TCDS in your airplane so you can do a weight and balance. I doubt most Short Wing owners have ever seen the TCDS for their airplane much less keep in the airplane to do a weight and balance. One reason the POH is required in the airplane so you will have this information plus the empty weight and empty wt. cg. from your Weighing form to work you weight and balance.

The arms on the loading diagram are all given as a positive number referenced to the 60 inch datum. Most military aircraft I have been involved with are done the same way with an imaginary datum in front of the airplane. It is done on military transport aircraft so all of the numbers are positive when you add up the loads at the numerous stations.

If a person can not deal with the 60 inch datum point when weighing their airplane they probably are not qualified to be working on it themselves anyway.

[Wayne]

Having the datum 60" forward of the leading edge of the wing makes all calks. positive. This makes the calks. much easier to do and less chance of errors. Most aircraft have their datum points somewhere in the area of the tip of the spinner for this reason.

[J.J.]

The W&B information for my Pacer is based on the TCDS and my owners handbook specifying wing leading edge as the datum with with charts and info using that datum. Looking at the TCDS for all other Shortwings led me to believe they were all the same (I also checked Cubs) or I wouldn't have ever brought it up. To me it makes a lot more sense and is a lot easier to make all computations from the same point than to switch back and forth, especially for someone not used to making the computaions. Assuming an airplane has been recovered and weighed, the factory W&B sheet no longer applies anymore than a 337 from 2 covering jobs past. Some of this reminds me of the time I was told you can't compute W&B if you use zero inches as an arm somewhere.

Wayne, I wouldn't say most. My experience is with Pipers and Cessnas and there are several out there that use the wing leading edge or the firewall according to the TCDS and the manuals. I don't know which is more prone to errors, hitting an occasional negative number or performing all the computations nose then converting to a different point to use the loading graph. I did the computations from the tailwheel once just to prove it could be done just as accurately and as fast as having a nose datum because some "guru" said it couldn't be done.

Jeff

[taildraggerpilot]

It doesn't matter where the datum is, what matters is the sign in front of the number for the moment arm (i.e. (+) aft of the datum or (-) forward of the datum) Piper probably picked the tip of the spinner as the datum reference so all the moment arms would be positive. After you arrive at your CG location aft of the datum, it is easier to use the wing leading edge for seating and cargo positioning and gives the pilot a better reference.

Boeing and most other airframe manufacturers use datums at distances very forward of any physical part on the airplane. They do this because when they stretch the airplane, they don't have to monkey with the datum reference and all the moment arms stay positive.

[Wayne]

Yes Jeff, I guess it may be 6 of one, a half dozen of the other. There is the advantage that the moment numbers are significantly smaller with the shorter arm lengths when measured from the leading edge. Smaller numbers= less chance of errors.

My plane gained weight as well. 1168 lbs. empty. The upholstery and a lot more radios account for a lot of it. Aerothane paint on Stits fabric might account for some as well. The tail wheel is almost a wash.

[Ed Brown]

Negative numbers are just part of life when doing W&B calculations. After doing them a few times you get the hang of it.

Our short wing Pipers were certified in an age when engine oil was not part of the empty weight. (In aircraft certified under Part 23, which came after our short wings, engine oil IS considered part of empty weight) So, when our aircraft are weighed the mechanic has to either drain the oil, or subtract it mathematically.

For example: If a mechanic weighs a short wing without draining the oil, (using the leading edge of the wing as the datum,as per the TC) he will have to use a negative number to subtract the oil. Since the oil is forward of the datum, the moment will be a positive value. Then when the pilot does a W&B calc before a flight, he/she must add the oil to the empty weight, which will be a positive number, but the moment will be negative.

[13010]

I really have problems when the tip of the spinner is used as the datum. It is really hard to calculate when the spinner is sitting in the corner of the hangar.

[Gilbert Pierce]

JD
Just start at the tail when nose is in the cornor of the hangar and make all of the numbers negative (you are going backwards) and you will be alright.