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ACRO E-mail Archive Thread: [IAC-L:1558] Re: Clarke: "Why?"

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ACRO E-mail Archive Thread: [IAC-L:1558] Re: Clarke: "Why?"


Thread: [IAC-L:1558] Re: Clarke: "Why?"

Message: [IAC-L:1558] Re: Clarke: "Why?"

Follow-Up To: ACRO Email list (for List Members only)

From: N10TK at

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


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