Les Doud asserted that there was a crossover around 300hp where you want to go with 3 blades -- but without offering a really good rationale. I'll make the following observations:
1. Tip Mach is probably only an issue at top speed. A quick hack shows that at 2700 rpm and 250ktas forward speed (basically Legacy max airspeed), the 70-inch 3-blade has a tip Mach of 0.84, vs the 72-inch 2-blade at 0.86 Mach. That difference between .84 and .86 undoubtedly costs wave drag penalties -- at max speed. At 2300rpm and 210ktas, the tip Machs are 0.72 and 0.73 -- negligible diff in wave drag.
2. 3 tip vortices vs 2 --- is somewhat offset by decreased blade angle of attack. The 3blade has more area, and operates at lower angle of attack for a given load and speed. So the vortex shed by each 2-blade tip will be stronger than at each 3-blade tip; assuming similar blade aspect ratios, twist, etc.
3. Better takeoff and climb -- if you're operating at high horsepower and slow speed, the reduced angle of attack of the 3-blade makes operate at a better L/D, so it's a better "low gear". I integrated Les Doud's static thrust for the two props from 0 to 75ktas, at 2100 lb gross weight, and came up with a roll of 400ft for the 2-blade, 342 ft for the 3-blade. The 3-blade is also a better airbrake on approach, if you cut back to idle power.
4. The 3-blade is 20 lb heavier. Assuming that cruise L/D is around 8:1, that 20 lb weight translates into about 2lb of drag, just offseting Les Doud's claimed 2lb cruise thrust increment. (purists note: I've assumed that the CGs are in the same place for 2- and 3-blades, so trim drag is identical.)
5. Noise -- is probably better with 3-blade even if the noise energy is identical -- because it's shifted to a 50% higher frequency that's probably better damped by the airframe.
6. Looks -- no contest.
I think the most significant differences between the two props are purchase price, overhaul cost, takeoff roll, and looks. Also -- you might choose the prop that puts the CG where you want it, if everything else is done and you don't want to move anything. That 20 lb way out front translates to something like a full inch of CG position at middling aircraft weight.
Charley
Date: June 30, 2014 3:10:22 PM CDT
Subject: RE: [LML] Re: Two-blade or three-blade prop for I-550
I understood from conversations with some prop designers a few years ago (can’t recall who) that the fewest number of blades to get the job done is most efficient. One blade would be best except for obvious balance issues. So the remaining factors include how much HP one blade can absorb. I seem to remember that three was the minimum for an engine that goes much over 300 HP at least for the hubs and props we were looking at for the IVP. This info may have come from MT because I was looking at the five blade MT vs four. My understanding at that time was that more blades result in quieter smoother ops but give up small amounts of efficiency. Don’t believe anything I say here. The memories are about 10 years old. John Barrett |