AirV wrote:
So I guess the question is when does the alu metal prop start to get hurt. Full rpm 2700 and cruise rpm 2400, there is no big difference in speed.
It’s likely a 25% difference in “collision energy” though.
Cherokee 2 blade prop tip speed:
74" = 1,8796 m diameter propeller
2700rpm
v = 3,14159265359 * 1,8796 * (2700/60) = 265,72m/s = 956,592km/h
2400rpm
v = 3,14159265359 * 1,8796 * (2400/60) = 236,20m/s = 850,32km/h
2200rpm
v = 3,14159265359 * 1,8796 * (2200/60) = 216,51m/s = 779km/h
1800rpm
v = 3,14159265359 * 1,8796 * (1800/60) = 177,15m/s = 638km/h
1200rpm
v = 3,14159265359 * 1,8796 * (1200/60) = 118,10m/s = 425km/h
So I guess the question is when does the alu metal prop start to get hurt. Full rpm 2700 and cruise rpm 2400, there is no big difference in speed.
Euroflyer:
For a 76-inch prop, it would be 76 × 25.4/1000 × π × 2700 / 60 ≈ 273 m/s, or M0.80 at sea level in ISA.
EuroFlyer wrote:
Can some mathematician here tell us who fast the tips of a typical McCauley prop are travelling at 2700rpm ?
“l” being the length of the blades and “s” the time unit second, it is: l*90*π/s. E.g. if the blade is 80cm, then the answer is 226.2m/s:
.8m * 90 * 3.1416/s = 226.2 m/s
Can some mathematician here tell us who fast the tips of a typical McCauley prop are travelling at 2700rpm ?
Must have something to do with Pi, and I don’t have the length of the blades handy right now
I used to live in Vancouver where it rains all the time and various pilots I know would respond to the rain commensurate with their respect for the prop.
A brand new aerobatic airplane with a composite prop… slow down immediately and try to get out of the rain!
A 50 year old Cherokee with an old metal prop… why bother slowing? Presuming you are in cruise and not at full rpm – there is no need to thrash it unnecessarily.
The prop on a floatplane I used to own had a rough life. The spray off the floats can hit the prop when taking off and landing and the tips get eroded and look pretty ugly. Hence the need to be dress the prop occasionally.
They say that propeller can be damaged if it hits a lot of water on the ground
At max rpm the blade tips are not that far short of mach 1, and liquid water is then like concrete. It’s density is only half an order of magnitude less than steel, after all. That’s why the prop blades of e.g. WW2 planes which ditched under power are seriously bent.
Interesting stuff about rain damage though.
huv wrote:
I would really like some evidence for this– granted, just take my word for it, I had damaged composite propeller prototype tips years ago. The manufacturer didn´t think it was necessary to have leading edge protected by “tape” all the way till the tips. After flying through moderate rain for few minutes I have both tips near to delamination….I was lucky I didn´t know that….
Did you know that the opposite causes damage to propellers as well?
I would really like some evidence for this. Some discard the problem as nonsense, while others point out that erosion is quite natural when repeatedly hitting solid water (i.e. drops) at Mach 0,6 to 0,7.
Our motorglider has a composite prop, and our SOP is to avoid rain in the first place, for several reasons, but if flying in rain, then limit RPM (preferably use the coarse cruise pitch propeller setting).
It is a little like discussing shock cooling – of course it could be a problem, but is it really?
