Syd Cohen mentioned that the turbulent airflow caused by ridges in the wings caused the inboard section of the wing to stall before the outboard section, so that the outboard section of the wing continues to fly even at low airspeeds. Another factor that causes the inboard section of the wing to stall first is the high amount of wing twist in an Ercoupe wing. I forget the exact number, but wing twist is only about 3 degrees in standard aircraft. This twist causes the inboard section of the wing to be at a higher angle of attack than the outboard section of the wing, so that the inboard section stalls first. In standard aircraft, the twist keeps the ailerons effective during a stall because there is still airflow going over them. This twist in a 'Coupe wing is pretty apparent if you look down the wing from the wing tip.
Jeff Lewis N3888H > -----Original Message----- > From: Syd Cohen [mailto:[EMAIL PROTECTED] > Sent: Monday, June 21, 1999 8:20 PM > To: Doug Davis > Cc: Coupers > Subject: Re: Fw: Elevator throw for "D" > > > Doug, the spin-proof characteristic of the Ercoupe has > nothing to do with the > elliptical shape of the wing. A spin is produced by first > stalling the > airplane and then cross controlling it, causing one wing to > remain in the stall > configuration and the other one to be flying again. A > properly rigged Ercoupe > cannot be stalled due to the fact that the elevator is > limited in it's up > travel, and the ridges in the leading edge of the wing to > fuselage fairing > which cause turbulent air to be produced when the wing is at > a high angle of > attack. This turbulent air follows the fuselage back to the > elevator, reducing > the effectiveness of same, thus allowing the tail to come > back up and reducing > the angle of attack of the wing. These ridges also caused > the stalling of the > wing to start at the inboard portion, so the outboard part of > the wing is > flying satisfactorily even at very low airspeeds. Also, > with the two-control > Ercoupe, cross controlling is impossible, therefore spinning > is impossible, > although the 3-control version was also spin-proof. The ERCO > factory offered a > $500 reward (a lot of money in 1946) to anyone who could > prove they could spin > an Ercoupe, and many tried but no one could do it. Fred > designed his baby this > way after working at the NACA and observed accident reports > where pilots had > stalled and spun in at low altitudes, usually turning base to > final. He also > designed it as a low-wing to keep the thrust line above the > wings (as opposed > to Cubs, Luscombs, Champs, Taylorcrafts, etc.) > In an article entitled "Enviable Ercoupe" in "Plane and > Pilot," November > 1982, Fred stated "I remember once seeing a fellow coming > into a small field in > a J-3 Cub, and he was overshooting. He got slower and > slower, just hauling > back on the stick, and then he decided to go around. He > opened the throttle > wide and the thing just stalled into the ground and he killed > himself. If he > held the stick back in the same position, he'd have 10 > degrees or so higher > angle of attack just above the stall, and he'd get way into a stall. > "To get away from that I wanted to have, as closely as > feasible, the same > speed power-on and power-off. That was one reason for having > a low-wing > monoplane with a high thrust line. If you have a tractor > that's the way to do > it because that means the propeller has a thrust that's above > the center of > drag. > "(When power is applied) that tends to nose it over. I > went even further > in that direction. In order to get a bigger nosing-over > moment I tilted the > engine down five degrees." > > Doug Davis wrote: > > > Hi to you too Steve,and I wish I knew the specific answer > to your question. > > Although an engineer, I'm not the aeronautical type. What > I DO know is, > > that aircraft with the design characteristics mentioned, > don't particularly > > like to spin, and must be held into same. My point was that > since the coupe > > has limited rudder travel, it might not be enough to create > a condition of > > spin. To answer specifically your question of "how", one > might have to go > > back to the history of the P-47, and Seversky Aircraft of > the late 30's, > > where the elliptical shape and dihedral combination was > born to produce an > > aircraft with reduced spin qualities. Many designers of the > time, picked up > > on this including good ole Fred, and consequently some high > performance > > military aircraft of that time, share these same characteristics. > > Doug >
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