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Buried in another thread, Battson brought up an interesting discussion about moment arms for the seats in the Bearhawk. Let's collect some data and see if we can find the best way to ensure we are loading the planes in a predictable way.
First, let's talk about flight testing. At some point in the test phase, it's important to fly the airplane at max gross weight and at the aft CG. Obviously this does not mean that we start from a solo, half fuel weight on one flight, and load it right up to 2500 pounds on the next. Rather, we incrementally increase the weight with ballast, and as we do, the CG very gradually moves aft. We evaluate the characteristics of the airplane with each small change, paying special attention to the pitch control. As the CG moves aft, the pilot should notice that the elevator forces feel lighter, there is less pitch trim change required with airspeed change, and the pitch axis will become less stable.
Stable in this context means less likely to correct itself. So that means flying in trim at 80 knots, a bump aft on the stick will raise the nose. How and when the nose drops back down will change as the CG goes aft, up until the point where it doesn't go down at all. Rather, it will continue to increase, until the pilot intervenes. This is generally accepted as bad airplane behavior, and actual flight tests should not reach this point.
We avoid this by honoring the published CG limits. But our ability to honor those limits is only as good as our calculations. If we use a method different from the one that Bob did when he designed those limits, then our calculations may not match.
So let's collect some data. What do you use for arms, particularly on the seats?
Jonathan says "My front seat arm is 690mm (27"), rear seat is 1450mm (57"). I probably measured that with the front seat fully aft, as the most conservative option." We also know that he excels in height and leg length, among his many other favorable traits.
The spreadsheet that I've used for years shows 15 inches for the front seat and 52 for the back seat. This is a big difference!
While I was in the shop yesterday I remeasured and came up with the following, on a Model A kit fuselage. Please measure yours if you have one, and report back.
Front seat, forward-most position:
Front lip of seat: 7.5
Intersection of the top of the seat pan and the front of the seat back: 23.5
Front seat, rearward-most position:
Front lip of seat: 13.5
Intersection of the top of the seat pan and the front of the seat back: 29.5
Rear seat:
46-57.5 (same points as front seat)
Baggage: 65-94.5
So clearly the CG calculations are going to vary depending on which seat position is selected in the track, and where the human CG falls on the seat pan. How do we narrow down these variables?
There are some great minds in the group, and I'd love to hear what you think about arm selection and how that has worked out in flight testing. I found that with my simple numbers of 15 and 52, I didn't like the way the airplane flew before I reached Bob's limit.
First, let's talk about flight testing. At some point in the test phase, it's important to fly the airplane at max gross weight and at the aft CG. Obviously this does not mean that we start from a solo, half fuel weight on one flight, and load it right up to 2500 pounds on the next. Rather, we incrementally increase the weight with ballast, and as we do, the CG very gradually moves aft. We evaluate the characteristics of the airplane with each small change, paying special attention to the pitch control. As the CG moves aft, the pilot should notice that the elevator forces feel lighter, there is less pitch trim change required with airspeed change, and the pitch axis will become less stable.
Stable in this context means less likely to correct itself. So that means flying in trim at 80 knots, a bump aft on the stick will raise the nose. How and when the nose drops back down will change as the CG goes aft, up until the point where it doesn't go down at all. Rather, it will continue to increase, until the pilot intervenes. This is generally accepted as bad airplane behavior, and actual flight tests should not reach this point.
We avoid this by honoring the published CG limits. But our ability to honor those limits is only as good as our calculations. If we use a method different from the one that Bob did when he designed those limits, then our calculations may not match.
So let's collect some data. What do you use for arms, particularly on the seats?
Jonathan says "My front seat arm is 690mm (27"), rear seat is 1450mm (57"). I probably measured that with the front seat fully aft, as the most conservative option." We also know that he excels in height and leg length, among his many other favorable traits.
The spreadsheet that I've used for years shows 15 inches for the front seat and 52 for the back seat. This is a big difference!
While I was in the shop yesterday I remeasured and came up with the following, on a Model A kit fuselage. Please measure yours if you have one, and report back.
Front seat, forward-most position:
Front lip of seat: 7.5
Intersection of the top of the seat pan and the front of the seat back: 23.5
Front seat, rearward-most position:
Front lip of seat: 13.5
Intersection of the top of the seat pan and the front of the seat back: 29.5
Rear seat:
46-57.5 (same points as front seat)
Baggage: 65-94.5
So clearly the CG calculations are going to vary depending on which seat position is selected in the track, and where the human CG falls on the seat pan. How do we narrow down these variables?
There are some great minds in the group, and I'd love to hear what you think about arm selection and how that has worked out in flight testing. I found that with my simple numbers of 15 and 52, I didn't like the way the airplane flew before I reached Bob's limit.
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