View Full Version : Measuring prop pitch ??
02-07-2003, 09:44 PM
I have often wondered how the pitch of the prop is actually measured.I have seen a pic of the Prather gauge, looks like it measures in the middle.
Also I guess that the pitch does not truly match the increase in diametre?
Anyone know the facts on pitch? :wstupid:
smoke and mirrors :) IMHO...the pitch on most props decreased from hub to tip, so any number they give is a nominal one ........
02-08-2003, 06:40 AM
For the purposes of this discussion lets ignore the prop airfoil and prop inefficiences.
Compare a prop to a screw that moves forward when turned. A 6" inch pitch prop will move forward 6" in one revolution. This is determined by the angle of the blade.
Now pay attention! the tip of a 10" x 6"P prop travels 31.42" in one revolution, but moves forward 6". The 5" dia station of this prop travels 15.71" in one revolution, but still moves forward 6". It is kind of like a grade on a hill. You go along 31.42" and up 6" or along 15.71" and up 6". It follows therefore that the blade angle will be steeper at the 5" station than at the 10" station.
The Prather pitch gauge measures exactly that. The blade angle at various stations along the prop. The prop is screwed to its block which locates in the slotted base. As the prop is moved from slot to slot the swinging arm is aligned to the back of the blade. It should always then give the same reading of pitch on the scale.
Sorry. it is not smoke and mirrors.
02-08-2003, 07:44 AM
Thanks for the excellent explanation. In all the prop articles in the mags over the last few years never heard it explained why the pitch is progressive.
So, that would also explain why a 7-6 prop appears to have a steeper blade than say a, 11-6 prop - the circumference the smaller prop has to go in one rev is much shorter, therefore the smaller prop has to have a steeper pitch to go the same "6 inches forward" - is that correct?
Thanks again for making it clear!
02-08-2003, 08:30 AM
Ed has hit the money here. :D The angle of the blade is all relative to the pitch, and the diameter of the prop. As the length of the blade gets longer, the angle lessens to account for increased circumference. This is so that as the blade travels forward, the angle of attach remains constant along the entire length of the blade. In reality though, this also depends on airspeed achieved once in the air. In the static position, the hub has a much greater angle of attach than the tip does.
Thanks Ed, Your explanation makes a lot of sense.....
02-08-2003, 06:00 PM
most props are measured on the flat backside of the prop except APC which measures it on the chordline of the prop which is why they show higher RPM readings for the same pitch. Actually it's the proper way to measure a prop, as in wing incidence.
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