Brett, I was (am) under the impression that the "speed" of an airplane is determined by the pitch and not the diameter.
I'm not trying to stir the pot or start an argument, I just want to know the answer....... 
Isn't the "Pitch" like gears in a car? I'm confused by your statement.
Thanks in advance for clearing that up for me. See you in Woodland.
I can't see how it would start an argument...
Props don't work like a screw screwing it's way through wood. So it's not just the pitch x the RPM, and it's not exactly like the gears in a car. They are little spinning wings. A bigger wing makes more lift at the same speed and angle of attack than a small wing. Or, for the same lift, the angle of attack is lower.
I expect that all else being equal, the in-flight RPM will be about the same, the pitch is the same, and the blade area is much larger with a 10" prop than a 9" prop. So the larger prop will have more lift/thrust at the same airspeed, which means it will accelerate to a higher speed (until the drag again equals the higher thrust). The angle of attack will be lower at this higher speed, roughly proportional to the difference in the blade area.
If you don't like that one, try this. The steady-state speed of the airplane is a function of how much horsepower is applied to it. The horsepower required goes up with the cube of the speed. It speeds up until the power from the prop equals the power required by the airframe. The power the prop puts into the air is a function of the shaft power of the engine, and the efficiency of the prop in turning shaft HP into thrust. A larger prop is more efficient, so the same shaft HP from the engine will translate to more power into the airframe, meaning the airplane will go faster.
Of course raising the pitch would also likely increase the speed, by a different mechanism. The prop with more pitch but the same diameter will end up running at a higher angle of attack, meaning that it too it more efficient, hence faster.
One way to find out, of course. The only debatable point is whether the engine will be able to deliver the same shaft HP, and then, when it is too fast, what happens when you try to needle it down. The 20FP setup works so well with a relatively large venturi (more choke area than David's WC-winning PA75) only because the small and relatively inefficient prop (9-4) permits/requires the RPM to be very much higher to get the same effect, enhancing the fuel suction. When you slow it down, what may happen is that now it will tend to run away to the same RPM you had before, and leave you faster with no way to control it. The solution might be to reduce the venturi choke area so it will run reliably at the slower RPM/reduced HP.
Note this is why the "venturi size vs displacement" chart that Leonard Neumann refers to doesn't work. We have more-or-less the same choke area on successful engine systems from a 20 to a 76, it has *nothing* to do with the displacement. It does have to do with the amount of HP generated, which is not a lot different for stunt no matter what engine you use. Certainly not a matter of 3.6x as much power. In level flight I think David's airplane requires about 20-25% more power than the Skyray, and the difference in shaft power is probably even less.
What happens in the maneuvers is another story, but my comment referred to the basic speed. Based on experience (since this is hardly the first time someone has put a 25LA in a Flight Streak), I would guess the 9-4 still better in the maneuvers, too, but there's one sure way to find out.
Brett
Topic: ARF Flite Streak Again ... (Read 2174 times)
