Weight & Balance - Pilot location

[psalter]

Brian,

I would double check your numbers that you entered into the W&B spreadsheet, how you weighed it, and the accuracy of the scales.

I did some extensive testing, weighed my plane 32 times with me in it and changing the seat positions each time (I was tired of climbing in and out of the plane). The average numbers in the spreadsheet was based on my testing. I came up with approximately 86-87" for the seat position you described. As stated in a previous post, positions will vary based on Aircraft weight, pilots build etc., which is why weighing it with you in and out is the preferred method.

When I did the testing, the airplane was extremely light, no engine or wings mounted, so I was very heavy in comparison to the plane which should yield a worst case scenario. There were some discrepancies in the data because of this, but it did yield a decent range estimate. I will probably repeat the test now that the plane is nearly done.

As to your question about 5" movement vs 7" movement, the w&B never be a 1 to 1 shift based on seat positions unless you're weight is significantly more than the weight of the aircraft you are sitting in, and/or the pilots moment arm further aft. Very doubtful you will ever have that in a Panther (at least not one that would fly). Having said that however, in the Panther, the pilots weight is usually about 1/3-1/4 of the empty weight of the aircraft, a very significant factor, and the pilot is behind the aircraft CG. So, that is why the variation in seat position produces a considerable shift in CG.

[sowell]

Yes, I weighed it correctly with and without me in it. Twice.

I'm asking for input from others who have weighed their panther like me, with and without a pilot. I'd like to know what other's have determined to be their pilot location.

[JimParker256]

How did you determine the pilot's "actual" CG? I'm assuming you weighed the plane without the pilot to determine the empty CG, then climbed in and re-weighed it with the pilot in position, with hands on the controls, and used the "moment shift" to calculate the CG station for the pilot? Did you also weigh the pilot on the same scales as the plane to eliminate any source of error from different scales? Was the airplane leveled (tail raised to "level" attitude) before you weighed it? If so, I'd say your numbers are probably accurate.

You probably had all that covered, but I asked because it has been somewhat surprising to me to realize that many A&Ps - and even IAs - practically NEVER actually weigh airplanes on a regular basis... And most of them do a lot more tricycle-gear aircraft, so they "forget" that a taildragger needs to be weighed with the tail raised up! I had my Citabria weighed as part of the first annual we did on it, and the preliminary W&B results we got back were just ridiculously out of whack. So much so that I was questioning the entire process. When I asked if they had subtracted the tare weight of whatever they used to raise the tail to "level" before weighing it, I could "hear" the head-slap over the phone... "I'll call you back in an hour," was the next thing I heard... This time, the numbers were right where I would have expected them to be. He admitted they had weighed the plane with the tail on the ground, which shifted everything aft.

One thing that we might not always think about, but which could impact the Pilot's "measured" CG station, is your personal body build. Being somewhat heavier than average, and barrel-chested as well, I have come to realize that my personal "CG" location is likely to be as much as a couple of inches farther forward than the CG of a more slender person, both measured with the seat back in the exact same location. There's just more of "me" occupying space forward of where the other person's more slender build "ends"... So even with the seat adjusted to the exact same location, my CG is likely to be somewhat more "forward" than theirs.

Hope this helps!

[jamesmil]

a good rule of thumb is the location of your belly button from the datum line that's what I use. hope this helps

[sowell]

I weighed my completed panther to do the W&B calculations, and I find that the pilot location (ARM) is significantly further back than the W&B Spreadsheets indicate that the range should be. What have others come up with regarding their pilot location? I need a sanity check on this, please.

The spreadsheet says 84.25 to 89.75 is the range of the pilot, but this is only a 5.5 inch spread. The holes for the seat back adjustment are 7 inches from the most forward to the most aft, so I would expect at least a 7 inch spread, and even more if one considers the difference between sitting upright vs. having the seat recline.

My location came to 91.5 inches aft of datum for the pilot arm. My seat is not in the aft most position. Its in the 4th hole from the rear-most position for the bottom of the seat back, and 3rd hold from the rear-most position for the top of the seat back. I would call it 3/4 of the way back. Rudder pedals are in the forward most position.