I have seen some advantage to using less prop than the biggest the engine can turn. Such as a 10 1/2" instead of a 11" on a .40 muffled engine. The engine ran smoother and the plane flew more to my liking. It is a fact that we use more engine than is totally needed, but it allows so much more adaptation of props and such. Having the engine run in a deep 4 with just a beep sometimes at the top and yet it has the guts to deal with bad air with out having to do a LOT of changes.
Exactly. By the way, since it came up in another thread recently, it's a variation on what Ted told me impressed him about Gene Schaefer's ST46 run - 35-sized airplane, ST46, and a Tornado 10-6. Lots of power available, small easy-to-turn prop, but dead smooth because it was running with little load variation and thus little break. Sort of like putting a Jett 61 in a 40VF airplane, or a PA75 in an ST46 model.
I think David is running a 13-4 3-blade, I think the 75 might be able to handle more than that, like maybe a 14.5 even at that RPM! I know that the PA61 has NO PROBLEM spinning a 14-6 Rev-up with plenty of margin, even when its set up to run at 11,000 RPM. We tried that in a Dyno test. I can easily believe it when Al says his 76 will turn a 16" 2-blade.
BTW, the prop I used to win the 2006 NATs was a 11.875-3.75 3-blade, after buzzing it down at takeoff on the second Open flyoff flight. I also was third at the 2000 NATs using David's old break-in prop -11.5-3.6 3-blade. On the strongest PA61 any of us ever had, and 15% nitro, easily capable of spinning a 13.5-4 Bolly 3-blade.
Point being, when we had engines that ran out of poop at 9000 rpm, the only way to get more power, or to brag about having more power, was to use a bigger prop. Not any more.
BTW, on the topic of pitch distribution, you can have astonishing effect on the turn by just changing the pitch at the tips of the prop. On piped engines it's almost too effective- even a tiny change at the limits of repeatability can be far too much. Particularly with some engine setups. I have flown airplanes that would go halfway through a corner, then engine would kick, and the handle would almost get yanked out of my hand - which of course tightened up the corner almost instantly, too. That could be tamed a bunch of ways. One way would be to take out about .1" of pitch over the last 3-4 stations, which would also reduce the overhead tension. Another way is to use less diameter to reduce the break, and put the engine in a better spot so it doesn't break as hard in the middle of the corner but still can make it to the top of the circle.
Here's a thought experiment. PA61, 14-4 2-blade, lap time 5.3 seconds, ground launch rpm about 10200. Then take 1/2" off each prop tip. What do you do to the lap time? How do you get the lap time back to what you wanted, and then how does that effect the engine run in the maneuvers? Once you get that, what does it likely do to the in-flight shaft HP? And what does it do to the turn?
People were doing that trick with 12-6 VS 11.5-6 VS 11-6 Rev-ups even back in the days of ST46s.
Brett
p.s. this is the topic that just keeps on giving. I would note that a lot of engines are referred to as "luggers". I submit that just means they won't run at higher revs effectively.
p.p.s. and yet more. Might as Doug how he sets up his engines - we discussed on SSW a long time ago.
Topic: Turn - turn and props (Read 2755 times)
