The point of all this is that in the case of piston engine propeller driven planes, strange as it may sound, at some point, the frontal area of the engine and prop become a
serious liability to faster speed. Unless you can increase the prop pitch or the engine rpm, you are NOT going to go faster. For the first 2 decades I modeled, the model speed record was held by a glider in the Alps. By maximizing L/D for the airframe to include eliminating the prop and engine, they were able to go fast than any driven airplane. To achieve the speed suggested in the video below, you would have to turn 12" of pitch in the 30,000 range. Adding a motor and prop to this slope glider would immediately limit its top speed to rpm x pitch + maybe 10%.
Mike.. to make
your point I've attached a scan of the lift plot for the NACA 4412 previously discussed in this thread. I think the point you are making is that even when the AOA is zero, the coefficient of lift is positive for a non-symmetrical airfoil. The point I am making is that this zero AOA is NOT defined from the flat bottom of the airfoil. Unfortunately, the flat bottom is exactly where APC measures pitch from. If you look at the drawing in the upper left portion of the right hand page you will see where zero AOA is referenced and it is not the bottom of the airfoil. I accounted for these factors by adding 10% to the calculated speed to come up with a ball park figure to answer the question presented in subject line of this thread. That was met with the comment "you cant go off pitch speed for your plane." Again, the calculation is not a substitute for a radar readings, but you CAN go off pitch speed to calculate a good ballpark number for how fast your plane will go.
then explain why my plane does around 182 average with such a low pitch speed?????
I can't answer this... partly because I have serious doubts it is true. Sorry. Your use of an 8x7 prop on this setup raises a flag to me. It is not a good choice if going fast is your goal. I have a Q500 with a Nelson turning a 8.5" pitch prop (as marked by APC) at 19 ish on the ground. It will do 150 ish straight and level and 168 in a long 60 deg dive and a little tail wind. A typical Q40 prop would be something in the 7.5x8 range and that is for close course racing.
To average 180 with 8" pitch prop (that is your 8x7 corrected for the above discussion) you will have to be turning around 24,000. Possible I suppose but you would have to be running a pretty sporty pipe for it to unload that much. Now if you took the prop and engine off, removed the cheek cowel and canopy, added weight and pointed the thing down hill you might see well over 200.
http://www.youtube.com/watch?v=Vi0hrjqU15I
Bill