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In a message dated 1/13/2003 8:30:13 AM Central Standard Time, harrelson@erols.com writes:
Scott, can't agree with you on this one. The wings DO carry more than the weight of the airplane.
Bill,
Mea culpa. I was somewhat wrong because a series of force vectors collided in my mind thus creating mild confusion and a simultaineous loss of near and long term memory. However, unlike my dog, I accept corrections easily without too much choking from jerking the collar.
The aerodynamic lift of the wings magically carry more than the weight of the aircraft having to do with pressure centers (Bernoulli, et al), air flow (Newton), bug distribution on the leading edge, AOA, speed and the amount of spit in the wind. So, let's try to understand this by flight configuration and wing, specifically the 320/360 small tail, short engine mount, normal empty weight CG.
At cruise (184 Kts, Vno)- The longeron should be level by design. Note that, as cruise speed is approached, the nose is continually trimmed down, thus reducing AOA and eventually resulting in the elevator horn faired in with the horizontal stabilizer (mounted at negative 3/4 of a degree). The wingtip AOA reduced even further than the inboard section because of washout. OK.
With the plane loaded at max gross weight (1800), no baggage, no wing fuel, full header, and 2 265 pound people in the front seats results in the furthest forward CG at max GWt.
The forward CG is at 27.7", the center of lift (CL) is 25% MAC (mean aerodynamic chord) which is about 30" and the tail MAC at about 165. Assume the airplane is balanced on the center of lift, the forward down force is 2.3x1880 = 4680 / 135 = 35 pounds of down force on the tail. So the wing is carrying 1835, only slighty more than the limit. Note also that any movement of the CG to the rear would reduce the force on the tail (more nose down trim), thus less drag due to negative lift and less overall drag due to less overall aerodynamic lift.
Hold on- how about less weight, more forward CG? OK, replace the front seat guys with a 240 pound pilot only, plus the header. This moves the CG to the forward limit of 24.5 at 1510 pounds. 5.5 x 1510 = 8305 / 135 = 61 pounds of tail down force for a total of 1571, nowhere near the wing breaking limit. Side note, 160 pound pilot moves the CG 1 inch past the most forward point, but burning off all the header fuel puts it back pretty close to the forward limit. You did placard your airplane for the minimum weight of a solo pilot, didn't you?
If you slow down, the nose must be pitched up to hold altitude. Greater AOA, more tail down force, greater wing load, but vertical gusts are less likely to break the wing because of speed. Putting some flap in (taking out of reflex) pitches the nose down, increasing even more the tail down force (I need to check this), increasing wing lift and increasing drag.
The above chart from Cessna Professional manual of Flight.
A- Wings level stall
A to B - Max lift before accelerated stall.
B - Load limit of aircraft at Va
C- Vno
D- Vne
Tomorrow, maybe we can talk about approaches and climbs and maybe even turns.
Scott Krueger
N92EX
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