The aerodynamics you cite are fascinating. Please forgive me for continuing to try to clear up the 415-CD model confusion.
ERCO wanted to increase the gross weight. During testing, they found that the full power, full 13 degrees up elevator flight characteristics were bad enough that the FAA wouldn't certify the plane as spin-proof without the 9 degree up limit. ERCO came out with the 415-D for serial numbers 4424 through 4500. There was some dissatisfaction with the landing speed which ran about 5+mph faster. Therefore, the 415-D was put on hold and the next models were sold with all the 415-D improvements but the elevator limitation went back to 13 degrees and the gross weight went back to 1260 lb. -- this was the 415-CD and were serial numbers 4501 -- 4868 (368 planes). 415-CD models are eligible as 415-D models if the items listed in the type certificate are complied with. See A-787. For that matter, every coupe after about serial number 813 is eligible as a 415-D if the same changes are made to them. The 415-CD just came from the factory with those extra changes already made. My own plane was born as a 415-CD, serial number 4503. I've since had the changes made to fly as a 415-D with the 1400 lb. gross weight and 9 degrees of up travel. 415-C and 415-CD models are only allowed 1260 lb. gross weight. 415-D, E, G and Forney F-1 models were allowed 1400 lb. gross weight. I have seen no evidence that any 415-D or the later 415-CD models were sold with the C-85 engine. My sources (and my plane) say the 415-D and 415-CD were all sold with the C-75 engine. Of course many were later modified to be C-85s through the minor changes specified by Continental. [EMAIL PROTECTED] wrote: > > Ed, > I should have been referring to the C-D model; that is a Coupe with 85 hp, > 1260 lb and 13 degree up elevator. We lack a name for the model which is a > C-D operated at 1400 lb or a D model operating with 13 degree up elevator > limits. > > The 43013 wing used on the coupe came about as a result of NACA > investigations into wings which had low pitching moments and useful > coefficients of lift. It was thought at the time that elevator trim drag was > a problem and that the planes would be faster and require less trimming if > the wings did not cause the nose to pitch down. Their studies found two > airfoils that were thought to be useful: the 23000 series commonly used on > Cessna and Beech twins and the 43000 series found on the Ercoupe and the > ATR-42/72 which is loved and praised by none of its pilots. > > The wings are formed by taking a symmetrical foil shape which has good drag > characteristics in turbulent flow and bending the nose down to delay the > stall at high angles of attack. That is why you will see a hollow on the > lower surface of the forward 25% of the wing. Cessna on the C-172 and others > have applied this leading edge treatment to many wings to improve the low > speed behavior of the wing. > > The camber line of the 43000 family is a straight line from the leading edge > to 15% then a straight line to the trailing edge. Intuitively we might > expect some gentle curve in the camber line with the camber distributed > evenly but by bringing all the camber very far forward it was possible to > create a wing which behaves as if it were symmetrical in cruise with little > trim drag. The 23000 family is the same except the point of camber bend is > at 9%. > > The problem with the wing, which was properly documented although generally > ignored, was that while the wing might not stall until the angle of attack > was 15 degrees, once stalled the wing did not begin flying until the angle of > attack was lowered to 10 degrees. When you learn to fly an airplane that > stalls gently you learn that the plane will begin flying as soon as you > release back pressure. The 43000 series will not begin flying until you gain > speed to reattach the flow. > > Thinking has changed since the 1930s when the 5 fidget wings were developed. > Trim drag has been found to be less of a problem than was thought. My > sailplane has a modified 65-618 airfoil which means maximum camber of 6% at > 50% cord with a thickness of 18%. > It is flown with the cg at 35 to 45% and performs best at 45%. There is no > elevator trim and none is needed from 45 to 150 mph. > > If the coupe were made today it would be a Cherokee 140 which is where Freds > thinking went by the early 1960s. The 23000 and 43000 series of wings were > successful but they are not suited to flight near the stall. > > ref. Harry Riblet, Modern Airfoils > > Bob Condon -- Ed Burkhead Peoria, Ill. N3802H
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