I was involved in topic about propellers on here that got very hostile when I started asking simple questions, so I decided to make myself a safe space.
I am confused by the “stats” of propellers.
If a prop is a “82/41” for example I know that has to do with the pitch but how would I look at that and know if its a climb or a cruise prop for example?
If a prop is “760lbs” I assume that’s not not what the thing weighs if you put it on the scale, but it doesn’t seem like it would be the thrust either, as that seems to low to me.
I know from swapping out boat propellers throughout the years that “static” as in “it statics about 2050” probably means the wide open RPM with the prop.
Different pitch propellers will affect the top end RPM. Typically as a kid I was trying to get the highest RPM out of my inflatable boats to make the cruise/top speed the fastest. But you would want something that caused a lower RPM if you were towing a skier, wakeboarder, another boat etc…
If someone could explain this to me as if I was retarded I would appreciated it.
Alright,
As I was falling asleep last night realizing that I needed to invite someone onto Pilot Ground that understands propellers at an instructor level, (not just a frustrated level of rote knowledge), I realized that I was mistaken in my recollection about what the different propellers were doing on my inflatable boat.
The issue was that the propeller that came on the outboard motor would hit the rev limiter when I had the boat super light trying to get as much speed as possible. (Think light inflatable boat combined with a 70lb kid) The manufacturer correctly assumed that most installations of that outboard would have the engine pushing more weight through the water.
This lower pitch rev limiter inducing prop was great if I had a few buddies with me and/or I was towing a knee boarder, tuber another buddy’s broken down boat etc.
In order to get more speed out of the boat (for Kelly’s Island runs and such) I kept swapping the prop for higher and higher pitch’s. Finally getting 30+ knots out of the boat when I was by myself.
That higher pitched propeller was terrible if I had a bunch of people on the boat and/or was towing though.
82/41 means the prop is 82" long and 41 relates to the pitch. Lower the number, shallower the pitch. Shallower the pitch, the smaller the “bite” of air. The smaller the bite of air, the easier to spin. The easier to spin, the more RPM. The more RPM, the better climb performance. I read somewhere that it’s supposed to mean the prop would travel 41" forward with one full rotation but not sure that’s true.
This guy explains it better than I ever could: Collin McDonald - The Flying Mechanic | Climb vs. Cruise Props The difference between climb and cruise propellers comes down to their blade pitch and the resulting performance… | Instagram
I think Lee misspoke when he said 760 lbs bew relates to the prop. It doesn’t. His airplane is 760 pounds Basic Empty Weight.
You are correct as you your understanding of “static.” The RPM produce if you were to tie the tail to the ground and run it wide open.
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Thanks @IanArendt!
I know you were involved in the convo, but the passive aggressive comments were for that other guy… hope you understand! haha
I was over complicating the numbers. We had a cigarette boat when I was younger and we spent an entire summer swapping performance props and they had a third number for the “cup” of the prop in addition to the width and pitch. I think we were messing with gear ratios as well which is confusing.
Yeah, the BEW in lowercase through me off as well!
I have to go re-read my post. I thought referencing my bew was pertinent for the performance conversation. It’s not a 1100lbs+ Super Cub or Super Cruiser. So the C90 isn’t all that anemic feeling that I can’t bias more towards cruise and retain decent climb. In reference to this post @RobertBerger the 41 in 82/41 is the pitch of the prop at the 3/4 of diameter station. This references the distance the prop would travel in inches, in one rotation as if it were in a solid like a screw into a piece of wood. There is a bit of efficiency loss because we’re spinning the prop in a fluid. Most prominent at low speeds. The faster you go, say cruise flight, the less slip there is and the math ends up starting to work out. So for example for everyone an 82/41 prop will turn to a higher RPM with a given Horsepower than a 82/44 would. Lending itself to better takeoff and climb performance. The 82/44 would not turn up as high an RPM for a given Horsepower, so a longer takeoff and slower climb, but when you level in cruise it will go faster due to each revolution of the prop taking it farther forward than the 82/41. @IanArendt Accurate?
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What Is the diameter station?
I mean there is also, the factor of the longer the prop is, the more prop there is between the tip and hub that functions at a pitch greater than the stamped pitch. This makes it difficult to compare a prop of one pitch and diameter to another. A general rule of thumb is an inch of pitch and an inch of diameter can be interchanged to create a similar static RPM. But what does that translate to, climb or cruise? a 74/40 prop can be shortened to a 70/44. Will go faster in cruise but climb worse. a 74/40 can be lengthened to 78/36 and climb great but not go anywhere. Longer props ARE more efficient due to the principle of aspect ratio as well. Spanwise airflow and such…
@RobertBerger the 3/4 of diameter, location.
@LeeGriffing Sounds right to me! I know just enough to get myself in trouble.
I’d say the same for me @IanArendt