Brett has posted that more prop diameter will increase the airspeed, but I don't see that. Less diameter will allow the engine to rev higher, so with the pitch staying constant, I'd expect a bit smaller diameter would increase airspeed. Brett is expecting increased prop efficiency to do the job, while I'm expecting increased horsepower to do it. Brett's probably right, but if we're talking of changing 1/2" plus/minus in diameter, it goes against my experience. 
Steve
Larger props are more efficient (everything else being equal), meaning more HP is transferred from the engine to the airplane, meaning faster. The 20FP uses the opposite effect, you use a smaller prop to reduce the speed and still permit the engine to run the way it wants.
Your theory about more RPM+same pitch = faster is true, also same RPM and more pitch = faster, but only if you leave the prop diameter and other characteristics the same. The prop is not a wood screw wedging itself into a piece of pine, and the airplane definitely *does not* go the speed you might expect from the RPM*pitch, in fact, it's more like 60% for "large" props and lower and lower as the diameter goes down. We have used this effect, whether anyone realized it or not, for ever, even in the ancient ST46 days or earlier, to adjust where the engine ran compared to how fast the airplane went.
By the way, what we measure as pitch is not a direct measurement of the effective pitch of the propellor. It would be if the airfoil was "flat" with the top of the airfoil being the same as the bottom. What actually matters is at what forward speed the thrust goes to zero, divided by the RPM. This is always more pitch than you measure using the back of the blade. That's why when you find the inflight RPM and multiply it by the pitch, the airplane seems to be going "faster" than you would expect, usually, A LOT faster. For example, I know that my engine it going about 10800 rpm in the air in level flight at about 5.35 seconds a lap (~78.7 feet/second), and it has a measured pitch of 3.75". 180 revs/second and .3125 feet of pitch = ~56 feet/second, or MUCH slower than it is actually going.
This is an effect of the prop airfoil elevating the angle where the prop blade lift goes to zero well above the back of the blade angle. You can try to figure out the effects of this analytically, and there are ways to do it by considering the camber of the prop airfoil at a bunch of places. It deviates further and further as you go toward the tips, by a tremendous amount near the tips, typically. Doing that, a 12.5-3.75 Eather UC measures 3.75 at the tip, and through analysis you get something like 8-8.5" of effective pitch. Same thing with a flat-back might be 6", because the camber is lower. I figured the net effective pitch of my particular prop at the time I did the RPM measurement to be about 6.5-7", say it's 7", pitch*rpm = 105 fps, airplane is really going 78.7, only 75% of the pitch*rpm. The efficiency might be around 55-60%.
Effective pitch*RPM = 0 thrust. You actually need about 2.2-2.5lbs of thrust to make the airplane go the speed you need (.32 -.35 HP). If you figure about 60% efficiency, you would expect the shaft HP of the engine to be about (.53-.6 hp) at the in-flight revs.
Being a super-nerd, I tested this. We got out the engine dyno, put my test engine on it, with the flying prop, and set it for the usual launch RPM. Then, we stopped it, and then, without changing any of the settings or the needle, put smaller and smaller props on it, and measured the HP VS the rpm. When we got to the known level flight RPM, the engine was generating about .52-.53 HP at the shaft. The prop was much smaller, of course, but that was just a matter of getting the right load to get the right RPM. Experimental results don't get any better than that. The only effect we didn't model was the slight change in the fuel pressure head (lower in flight).
This was all a long time ago, and a very similar experiment with very similar correlation was the thread where I finally gave up attempting to get people to understand what the word power actually means, when no one "believed" the measurements. The same thread discussed static thrust, where I predicted several weeks ahead of time what the difference between a 12-6 and 12-4 Rev-Up would be, when both are set for launch (difference was a factor of 2.5, which anyone who has launched a ST46 and 40VF airplane could have easily predicted) and also matched the shaft HP required to about 3 significant figures - and no one "believed" that, either.
It's more or less impossible to even have sensible discussion about it because there are so many self-proclaimed "engine experts" who typically descend on these threads to provide their wisdom about how "torque flies airplanes" and other magical thinking bullshit theories that are so ingrained that no one can even listen or "believe" actual measurements, or complain about the "advanced mathematics" like multiplying two numbers together. Howard still likes tilting at these windmills in other areas, I more-or-less gave up about 15 years ago, unless someone asks a specific and well-stated question or situation.
Brett
Topic: How to achieve desired lap times. (Read 2022 times)
