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#1
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Looking For W&B Using Arm Instead of Moment
I'm looking for an example of a W&B worksheet using CG arm values instead of
moment values. Basically, I'm curious to see how these are done and what the worksheets look like. Either online or emailed examples are acceptable. Thanks! -- John T tknoflyer[remove] at hotmail[remove].com http://tknowlogy.com/tknoFlyer _______________ |
#2
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"Ron Natalie" wrote in message
m You can't do it with just an arm. You need the arm plus the weight. And then your calculation is just going to multiply them together (yielding the moment). Yes, but recent readings suggested that my understanding my have been...incomplete. For instance, every W&B graph I've seen (Skyhawks, Skylanes, C150s, Warriors, Katanas, Tomahawks - all under 35 years old) have shown this graph using only weight/moment. All the sample loading problems I've seen have all used moment instead of arm, too. Recent readings revealed folks referencing balance by arm instead of moment - something I'm not used to doing or reading. It raised the question in my mind whether there are charts that use weight/arm instead of weight/moment. So that's the real question: Do such charts exist? If so, where can I take a peek at one? Thanks. -- John T http://tknowlogy.com/tknoFlyer __________ |
#3
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"John T" wrote
For instance, every W&B graph I've seen (Skyhawks, Skylanes, C150s, Warriors, Katanas, Tomahawks - all under 35 years old) have shown this graph using only weight/moment. All the sample loading problems I've seen have all used moment instead of arm, too. In those airplanes, the "arm" is generally fixed by the location of the seats and bagage compartment. On the graphs, the upward sloping lines each represent a different arm. Regardless of how it is presented, a w/b calculation always involves weights and arms. Moment is simply weight times the arm. Bob Moore |
#4
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"John T" wrote in message ws.com... For instance, every W&B graph I've seen (Skyhawks, Skylanes, C150s, Warriors, Katanas, Tomahawks - all under 35 years old) have shown this graph using only weight/moment. All the sample loading problems I've seen have all used moment instead of arm, too. Which graph are you talking about? The CG envelope itself is almost always the CG position (arm) versus weight. The other graphs (where you follow the weight out various lines for the different positions) are just computing the moment (multiplying the arm for you). The cessna books give you the arms you can use in lieu of these charts if you really want. Recent readings revealed folks referencing balance by arm instead of moment - something I'm not used to doing or reading. I'm sure you're confused. The CG point is always an arm. A moment tells you NOTHING about the CG. A moment of 1000 could be 1000 pounds at the datum or 1 pound 1000 units away from the datum. If you've computed the moment, you divide that by the weight to get the arm. It raised the question in my mind whether there are charts that use weight/arm instead of weight/moment. They all do. Googling for cg graph yeilds this page, which is quite representative of just about every plane I've ever come accross... http://www.eaa838.org/wtbal.htm |
#5
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In article om,
"John T" wrote: I'm looking for an example of a W&B worksheet using CG arm values instead of moment values. Basically, I'm curious to see how these are done and what the worksheets look like. Either online or emailed examples are acceptable. Thanks! If you can use Excel, you can easily construct your own W&B worksheet. Put the weight of each component in column "A", its arm in column "B", its resultant moment in column "C". All you have to do is multiply column "A" by column "B" in coulmn "C", add the sum of columns "A" and "C", divide the sum of column "C" by the sum of column "A" and you have the CG in column "B". You can get better by adding fuel in gallons, converting it to lbs in the weight computation, oil in qts, converting it, etc. You can build pax/fuel loading scenarios and get a real handle on what's going on. It is really quite elementary. A year ago I helped a retired AA captain build a W&B program for his plane -- it really opened his eyes! |
#6
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Recent readings revealed folks referencing balance by arm instead of moment - something I'm not used to doing or reading. It raised the question in my mind whether there are charts that use weight/arm instead of weight/moment. Yes, such charts exist. I take it you are referring to the "envelope" into which you n eed to fit. I've seen it with weight plotted against CG (arm), which I prefer as it makes the envelope fit better on the page, and I've seen it with weight plotted against moment, (which makes a skinny sloping envelope) There are other ways to represent the envelope too, but in all cases you need some term involving weight plotted against some term involving arm. The "arm" term could be arm (CG) itself, or moment, which is just armxweight. Whichever is used, I'll call it "balance". To get the final point, you still need to figure out the contribution to total weight and total balance due to each factor (pilot, passenger, fuel, etc). This is a simple multiplication, but a multplication can be represnted on another graph by a sloping line. This is just the old algebra y=mx+b rearing its friendly head again. The charts are designed to be compatible, so there's a minimum of arithmetic involved. It is actually possible to plot the contributions from each station as gridlines on the same graph as the envelope, but the lines are curved if you use arm for balance, and they are hard to distinguish from each other if you use moment for balance. I did actually set a few up that way; it allows one to figure out the weight and balance with no math whatsoever. But before I finished the program to do that, I found CoPilot for the Palm, and that was that. Jose -- (for Email, make the obvious changes in my address) |
#7
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Thanks, folks.
While I'll grant you that my math skills aren't nearly as polished as my piloting skills, I did know that "weight * arm = moment". The item I was looking for (included in Ron's helpful link) was a CG envelope (thanks to Jose for the correct term) using arm instead of moment. As I said, all the envelopes in the POHs I've used for the planes I've flown have all used moment - not arm - in the graphical representation. After seeing the graphic from Ron's link, I realize that I've seen similar charts - just not in any of the POHs I've used. Thanks again! -- John T http://tknowlogy.com/tknoFlyer __________ |
#8
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And not to belabor the point, what John wanted (and finally found) was the good-old-days CG "box" instead of the dumbed-down parallelogram that Cessna foisted on us starting around 1960. Seems that some marketing genius thought that the flying public was too stupid to do a "real" weight and balance by doing the multipication of weight*arm, adding the moments, and dividing by the total weight to get the point of balance. I started flying in '64, and it wasn't until I went for my A&P in '70 that I ever saw (or used, for that matter) the good old box style CG method of calculation. Jim "John T" shared these priceless pearls of wisdom: -Thanks, folks. Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#9
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I wrote a weight & balance program some time ago that will display the
envelope either way. If you're interested in looking at it, you can find it at http://users3.ev1.net/~medcalf/avinfo.html As noted, I accept no liability for use of the program, as you are still responsible to properly calculate weight & balance for your specific plane. Enjoy. On Thu, 30 Oct 2003 12:41:30 GMT, "John T" wrote: I'm looking for an example of a W&B worksheet using CG arm values instead of moment values. Basically, I'm curious to see how these are done and what the worksheets look like. Either online or emailed examples are acceptable. Thanks! -- John T tknoflyer[remove] at hotmail[remove].com http://tknowlogy.com/tknoFlyer _______________ Karl Medcalf - WK5M PP-ASEL-IA (N43CS) |
#10
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"John T" wrote in message ews.com...
I'm looking for an example of a W&B worksheet using CG arm values instead of moment values. Basically, I'm curious to see how these are done and what the worksheets look like. Either online or emailed examples are acceptable. Thanks! Moment is the vector product of force (weight) and displacement (distance, or "arm"), in other words, F X S |
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