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________________________________________________________________________________
Pietenpol-List: Center of Gravity
Re: Pietenpol-List: Center of Gravity
Original Posted By: Ted Tuckerman
Ted, I agree with everything you have said in that post. Particularly the point aboutdoing what it takes to make "YOUR" plane balance at the proper CG. these planesget changed so much... pulling here, stretching there, etc. Who really cares what others balance at in inches aft of firewall, leading edge,front cabane, whatever. IMO it's 100% useless info that can eventually causesomeone to make a serious mistake.The real deal about CG is make YOUR loaded plane balance at 25-33% of MAC (FC10).PERIOD. and that works whether your wing is 6" aft of plans or 50 feet forwardof the plans. a wing is a wing is a wing..... it's center of pressurehas no clue how much fuse is in front of it or behind it.DJ----- Original Message -----
Ted, I agree with everything you have said in that post. Particularly the point aboutdoing what it takes to make "YOUR" plane balance at the proper CG. these planesget changed so much... pulling here, stretching there, etc. Who really cares what others balance at in inches aft of firewall, leading edge,front cabane, whatever. IMO it's 100% useless info that can eventually causesomeone to make a serious mistake.The real deal about CG is make YOUR loaded plane balance at 25-33% of MAC (FC10).PERIOD. and that works whether your wing is 6" aft of plans or 50 feet forwardof the plans. a wing is a wing is a wing..... it's center of pressurehas no clue how much fuse is in front of it or behind it.DJ----- Original Message -----