Pietenpol-List: Center of Gravity
Posted: Mon Feb 16, 2004 12:23 pm
Original Posted By: "DJ Vegh"
Folks:I'll make a couple more comments on CG, and then shut up. I'll stick by my originalrecommendation to express and compare center of gravity as a percentageof MAC (or simply "chord," if you prefer, since we assume a rectangular Hershey-barwing), and here's why: using "inches aft of wing leading edge" is fine ifyou assume everyone sticks to the plans and builds an identical "FC-10" airfoilwith a 60" chord. I'm not sure that is necessarily a valid assumption. Notethat recently there has been discussion on the list about using differentairfoils. As soon as someone "improves" on BHP's design and goes to a 59" chord,or a 63" chord, or whatever, your "inches aft of the leading edge" comparisonsbecome meaningless. Deriving your safe operating CG range based on percentage of chord is consistentwith well-established aeronautical engineering practice. You might want tosearch the archives for a post on this subject by Doc Mosher back on 4 Jul 2000,in which he references the old CAM 18 standards for monoplanes as allowingan operating CG range of 22% to 34% of chord. Of course, BHP's limits of 25%to 33 1/3% are right in there. For loading graph purposes, you just convert toinches from the datum, like on a factory-built aircraft. The math is prettysimple, really.Actually, I'm not sure why the comparison of airplane A to airplane B does anythingfor you. What you want to compare is your airplane against the establishedobjective engineering standard (i.e., operating CG within 25% to 33 1/3% chord.)Just because someone might manage to get a Piet with a 60" FC-10 to slitheraround the sky with the CG at, say, 25" aft of the leading edge doesn't meanit's "O.K."Of more use than comparing airplanes is computing the extreme fore and aft loadingconditions for your particular aircraft, and making sure the aircraft can'tbe loaded outside the allowable CG range, or else developing a set of loadingrestrictions to keep yourself in the safe range. AC 43.13-1B, chapter 10 tellsall about it. If you want to maximize your safety, you're going to have todo some math.Ted Tuckerman________________________________________________________________________________
Folks:I'll make a couple more comments on CG, and then shut up. I'll stick by my originalrecommendation to express and compare center of gravity as a percentageof MAC (or simply "chord," if you prefer, since we assume a rectangular Hershey-barwing), and here's why: using "inches aft of wing leading edge" is fine ifyou assume everyone sticks to the plans and builds an identical "FC-10" airfoilwith a 60" chord. I'm not sure that is necessarily a valid assumption. Notethat recently there has been discussion on the list about using differentairfoils. As soon as someone "improves" on BHP's design and goes to a 59" chord,or a 63" chord, or whatever, your "inches aft of the leading edge" comparisonsbecome meaningless. Deriving your safe operating CG range based on percentage of chord is consistentwith well-established aeronautical engineering practice. You might want tosearch the archives for a post on this subject by Doc Mosher back on 4 Jul 2000,in which he references the old CAM 18 standards for monoplanes as allowingan operating CG range of 22% to 34% of chord. Of course, BHP's limits of 25%to 33 1/3% are right in there. For loading graph purposes, you just convert toinches from the datum, like on a factory-built aircraft. The math is prettysimple, really.Actually, I'm not sure why the comparison of airplane A to airplane B does anythingfor you. What you want to compare is your airplane against the establishedobjective engineering standard (i.e., operating CG within 25% to 33 1/3% chord.)Just because someone might manage to get a Piet with a 60" FC-10 to slitheraround the sky with the CG at, say, 25" aft of the leading edge doesn't meanit's "O.K."Of more use than comparing airplanes is computing the extreme fore and aft loadingconditions for your particular aircraft, and making sure the aircraft can'tbe loaded outside the allowable CG range, or else developing a set of loadingrestrictions to keep yourself in the safe range. AC 43.13-1B, chapter 10 tellsall about it. If you want to maximize your safety, you're going to have todo some math.Ted Tuckerman________________________________________________________________________________