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Thread: PA-22/20-150 weight and balance questions

  1. #1

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    Default PA-22/20-150 weight and balance questions

    I recently purchased a PA-22/20-150 (actually a former PA-22-160) and the weight and the most recent weight and balance paper work completed when it was recovered in 2003 appears to be a mess.

    At that time, the tare weights on the scales were:

    Left Main = 555 pounds
    Right Main = 543 pounds
    Tail = 74 pounds

    So far, pretty normal numbers. However the arms are where things go south.

    Rather than using "0" for the arm for the main gear and +178" for the tailwheel arm (and then adding 60" ahead of the LE to the imaginary datum line in the W&B) who ever did it did this instead:

    Main gear arm = 61.5"
    Tailwheel arm = 255.5"

    I *presume* he leveled the aircraft per the hole in the door frame and center punch point in the seat cross tube, and then dropped a plumb line form the spinner to measure the distance to the center of the main gear axles, more or less getting lost in the weeds and missing the whole point of an arbitrary datum line at the aircraft nose to make the math easier - after the aircraft has been weighed.

    However, I cannot figure out where he got the 255.5" "groundline" he refers to in the W&B diagram. Per the TCDS, 178" is the distance from the center of the mains to the center of a Scott 3200 tailwheel. If I add the 61.5" arm that he used for the main gear, I get a total arm of 239.5" form the point where he apparently dropped a plumb line from the spinner. However that indicates 16" error in the arm used for the 74 pounds of weight at the tailwheel.

    The CG calculations using his numbers:

    1,098 x 61.5 = 67,527
    74 x 255.5 = 18,907
    1,172 86,434

    86,434 / 1,172 = 73.74

    CG = 73.4 - 60 = 13.7 aft of leading edge (Yes, he switched from 61.5" to 60" for the datum line to reflect the normal 60" distance from spinner to wing leading edge, with no adjustment of the total moment to reflect the 1.5" change.)


    He then calculated a "most forward" W&B:

    Empty CG: 1172 x 73.7 = 86376.4 (note the confusion on significant figures here, where he's rounded the 73.74 down to 73.7, but then goes to ".4" in the moment.)
    Fuel 12.5 gallons: 75 x 84 = 6300
    Pilot (front seat): 170 x 81 = 13,770
    Weight =1417 pounds, Total moment = 106,376.4
    CG= 75.1 - 60 = 15.1 aft of leading edge


    He then calculated an "alternate most forward CG" with half fuel and two 170 pound front seat occupants with a CG of 15.9" and a weight of 1620 pounds. (I'll spare you the math.)


    Finally he calculated a "most rearward CG" with a 170 pound pilot, two 170 pound rear seat passengers full fuel and 100 pounds of baggage:
    Empty CG: 1172 x 73.7 = 86,376.4
    Fuel 12.5 gallons: 216 x 84 = 18,144
    Pilot (front seat): 170 x 81 = 13,770Rear passengers: 340 x 109 = 37,060
    Baggage: 100 x 127 = 12,700
    Weight =1998 pounds, Total moment = 103,492
    CG= 84.1 - 60 = 24.1 aft of leading edge

    That's 1.1" aft of the 23" aft CG limit for a PA-22-160, and 0.1" aft of the 24" limit for the PA-20-135. And he was apparently ok with this.

    -----

    If I use the same scale weights, but use a 60" datum line consistently and use 178" as the difference between the mains and tailwheel I get results that appear to be more logical and acceptable:

    Left Main = 555 pounds
    Right Main = 543 pounds
    Tail = 74 pounds

    Main gear = 1,098 x 60" = 65,880
    Tailwheel arm 74 x 238 = 17,612
    Empty Weight = 1,172 Total moment 83,492
    Empty CG = 71.2 aft of datum, and 11.2" aft of wing leading edge.

    The most forward CG is then:

    Empty CG: 1172 x 71.2 = 83,492
    Fuel 12.5 gallons: 75 x 84 = 6300
    Pilot (front seat): 170 x 81 = 13,770
    Weight =1417 pounds, Total moment = 103,562
    CG= 73.1 - 60 = 13.1 aft of leading edge


    The most rearward CG is then:

    Empty CG: 1172 x 71.2 = 83,492
    Fuel 12.5 gallons: 216 x 84 = 18,144
    Pilot (front seat): 170 x 81 = 13,770Rear passengers: 340 x 109 = 37,060
    Baggage: 100 x 127 = 12,700
    Weight =1998 pounds, Total moment = 165,166
    CG= 82.7 - 60 = 22.7 aft of leading edge

    Both of those appear to fall within the allowable CG envelope for both the PA-22-150/160 and the PA-20-135.

    ----

    It gets worse of course as the last documented W&B I can find is the one done after the 2003 recover and it does not include the 2016 addition of Sullivan wingtips, nor does it reflect the removal of a whole bunch of avionics.

    Consequently, I'm going to have it re-weighed anyway, but I want to make sure the basic numbers as I've calculated them are properly done in the interim.

    I also want to confirm that I should still be using the PA-22-150/160 CG envelope.

    Thoughts?

  2. #2
    Pacerfgoe's Avatar
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    Default Re: PA-22/20-150 weight and balance questions

    Sounds like you have a right mess there with that current W&B, and good thing you’ve decided to do a complete re-weight.

    I think the best thing you can do is make a template with all the correct arms for the person that is going to do the re-weight so that there is no scratching of the head and “ohh that looks about right” moments while it’s being done. It’s very easy to screw up if any distractions present themselves while doing this.....I had a small one on mine that I found and corrected during the next annual.

    What does the conversion paperwork say to use for the W&B envelope? There should be some guidance there.
    Forrest Miller

  3. #3

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    Default PA-22/20-150 weight and balance questions

    Best thing is to forget what you have, and forget the 60” forward of LE. Do all the calculations based on the wing LE per the TCDS. I would also use the tail post, not the tailwheel. I would redesign the airplane and start over. Remember, under CAR 4 oil is not part of empty weight, it is payload. I would also verify once level the main wheel center line is indeed station 0.

    Oh, by the way, you still have a PA-22-160, that won’t change unless you get a new data plate. If you read earlier versions of the TCDS you will find the 150 hp engines are optional on PA-22-160, but not the other way around.


    Sent from my iPad using Tapatalk
    Last edited by dgapilot; 11-18-2020 at 07:49 PM.

  4. #4

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    Default Re: PA-22/20-150 weight and balance questions

    I like setting the tailwheel on the scale so I use the arm for the tailwheel instead of the tail post. I have used scales that would have been extremely impractical to use to get a weight at the tail post.
    "Good judgement comes from experience, and experience comes from bad judgement." - Fred Brooks

  5. #5

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    Default Re: PA-22/20-150 weight and balance questions

    The problem with using the tailwheel is when it pivots, or the spring compresses, the station changes. The tail post is a fixed dimension that won’t change.


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  6. #6

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    Default Re: PA-22/20-150 weight and balance questions

    Yeah, I can see that but your only talking, what?, eighth inch difference in the initial GG location between a properly arched spring and a flat one? Maybe a little more. It isn’t likely to change at all unless the tail spring takes a lot of abuse and I doubt it would affect future update computations by anything measurable with a tape measure. Anybody ever measure the tail post arm? How many are exactly where Piper said they are? From reading other forums it sounds like Cubs can be pretty variable.

    It occurred to me putting the scale under a nose wheel is variable too. Nose gear struts are raked forward so being under or over serviced would change the arm.
    Last edited by Jeff J; 11-18-2020 at 11:13 PM.
    "Good judgement comes from experience, and experience comes from bad judgement." - Fred Brooks

  7. #7
    Administrator Steve Pierce's Avatar
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    Default Re: PA-22/20-150 weight and balance questions

    I believe you have the dimensions Piper used to the tail post and the previous w and b was done using the tail wheel. Way easier for me to put a scale on a 55 falling drum and shim accordingly and weigh. Then plumb bob off the center of the tailwheel axle and off the mains.

  8. #8

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    Default Re: PA-22/20-150 weight and balance questions

    Just to clarify... the last few airplanes I weighed were done with an older version similar to this platform scale: https://www.globalindustrial.com/p/p...iABEgIUSfD_BwE

    We put the scale on a palette and used a forklift to raise the tail already sitting on the scale.
    "Good judgement comes from experience, and experience comes from bad judgement." - Fred Brooks

  9. #9

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    Default Re: PA-22/20-150 weight and balance questions

    Quote Originally Posted by dgapilot View Post
    Best thing is to forget what you have, and forget the 60” forward of LE. Do all the calculations based on the wing LE per the TCDS. I would also use the tail post, not the tailwheel. I would redesign the airplane and start over. Remember, under CAR 4 oil is not part of empty weight, it is payload. I would also verify once level the main wheel center line is indeed station 0.

    Oh, by the way, you still have a PA-22-160, that won’t change unless you get a new data plate. If you read earlier versions of the TCDS you will find the 150 hp engines are optional on PA-22-160, but not the other way around.


    Sent from my iPad using Tapatalk
    It has three separate data plates, two on the cabin floor in front of the passenger seat and one on the tail.

    One of them is for a PA-22-160, the other is for a PA-22/20, and the third is for a PA-20. All have the same serial number.

    It's in the FAA registry now as a PA-20. That might not be how it is done now, but it's apparently how it was done on this one in 1994 when the conversion was done.

    (For what it's worth, conversions of 7 series Champs and Citabrias (7AC or 7FC to 7EC, etc, or 7ECA to 7GCAA, etc) are also very inconsistent in the registry in terms of how the model number and even serial number prefixes are recorded. I don't know if that reflects policy, procedure and practice changes over time for the FAA registry branch as a whole, or just differing practices in different district offices, different staff practices at different times or in different offices, or just defaulting to however the owner or AP/IA chose to report it.)

    However, the flight manual supplement for Univair's STC SA45RM includes various CG limits and max weights for:
    PA-22;
    PA-22-135;
    PA-22-150 and PA-22-160; and
    PA-22-108.

    That will take precedence over whatever data plate was last installed.

  10. #10

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    Default Re: PA-22/20-150 weight and balance questions

    Quote Originally Posted by Steve Pierce View Post
    I believe you have the dimensions Piper used to the tail post and the previous w and b was done using the tail wheel. Way easier for me to put a scale on a 55 falling drum and shim accordingly and weigh. Then plumb bob off the center of the tailwheel axle and off the mains.
    The original information from the conversion in 1994 indicates 0" aft of datum for the main wheels and 177" to the "tail point" when the aircraft is leveled so that a plumb line falls from the hole in the upper channel of the front door to the center punch mark on front seat cross tube.

    ----

    I'm not sure how you plumb bob off the center of the tailwheel axle when it's on a scale on a 55 gallon drum.

    What matters is that the tape used to measure the distance to the tailwheel is perpendicular to the plumb line dropped from the hole in the door frame. Similarly, if the plumb line doesn't pass through the same plane vertical plane as the axles when the plane is leveled, that distance also must be measured perpendicular to the plumb line. That said of course, eyeball calibrating "perpendicular" will be close enough for practical W&B purposes and measurement accuracy limitations.
    Last edited by LarryV; 11-19-2020 at 09:10 AM.

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