ACRO E-mail Archive Thread: [IAC-L:1558] Re: Clarke: "Why?"
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Thread: [IAC-L:1558] Re: Clarke: "Why?"
Message: [IAC-L:1558] Re: Clarke: "Why?"
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From: N10TK at aol.com
Date: Sun, 26 Oct 1997 23:41:50 UTC
Airfoils are only a small part of setting CG limits. Forward CG is usually set by the limit at which the aircraft can flare in ground effect (further forward would yield insufficient elevator control power). In "conventional" airplanes, it takes more control power to bring the nose up in ground effect than in free flight. Biggest factor is size of the horizontal tail and its distance aft of the wing, then a lot of smaller factors add in such as sstabilizer to elevator chord ratios, deflections, wing airfoil pitching moment, and pitching moment contributions of the fuselage and landing gear. Forward limit can also be set by landing gear nose-over limits or, in the case of some large WWI antiques, by excessive control forces required to maneuver. Aft limit is usually set to provide acceptable static stability margins at full power and stick free. In a tractor airplane, the prop is destabilizing so the key is to get acceptable stability margin at full power. In some of our hot-rods with oversized engines, the original recommended aft CG can be a bit exciting.... Aft limit may also be set to insure spin recovery characteristics. Spins with CG aft are generally harder to recover from. "MAC" is Mean Aerodynamic Chord, or the average chord length of the wing. Divide wing area (including the area which passes through the fuselage) by wing span to determine length of 100% MAC. With your straight wing, the leading edge is at 0% MAC, Trailing edge is at 100% MAC. If the leading edge is at fuselage station 100, and if you have a 48" MAC, the trailing edge is at station 148 etc.... The wing center of effort is at roughly 25% chord, the eliptic wing tips of the Monocoupe taper proportionatly forward and aft of 25%, so for most purposes your wing can be treated as a straight wing for purposes of determining longitudinal position of center of lift (ignore the tips). Forward limit for flaring in ground effect and aft limit for acceptable stick free (hands off) stability can be fairly easily calculated and can be easily determined by flight testing at progressively further CG positions until pilot in command says no more! Aft limit for spin recovery is set by flight test, computer models cannot adequatly calculate what happens in the spin. Tom Kerns Pitts KS-2B