Prop pitch

You can get a good idea by laying the prop flat on a table, cambered side up, and measuring the angle between the bottom of the blade and the table at 70% out from the hub toward the tip. This radius multiplied by 6.28, then multiplied by the sine of that angle, will give the pitch. You'll need trig tables or a scientific calculator. And it will be accurate only if the prop maker left the blade flat at that point; any curvature makes getting an accurate chord line difficult.

Dan

Where does the 6.28 come from?

2 x PI?

That's it. It gives the circumference of the circle the chord travels at that point.

Dan

The easiest way is to lay the prop hub flat then find the point where a

45 degree protractor or piece of cardboard matches the angle at the underside of the blade. Measure the distance from this spot to the centre of the mounting hole and multiply by 6.285 (2pi) to get the pitch. You may want to repeat the exercise on the other blade to find out if the hub is truly flat.

Brian Hampt> Is there a formula to calculate prop pitch from the hub thickness? >

>

Brian wrote in message news:...

There isn't always a linear relationship between pitch and radius on prop blades. There has been much research to find the best pitch distribution for a particular application, and model props will likely be based on some such successful prop. A 45 degree angle will only be present very near the hub, if there's such a steep angle at all. There the underside of the prop is seldom flat and the maker may have increased pitch for better engine cooling at low airspeeds, or decreased it for less drag and more thrust farther outboard where the airplane's nose doesn't get in the way. The 70% point measurement is an industry standard, though FS prop manufacturers publish pitch charts for the entire span. I have some here. My son has an 18" prop he bought for a quarter-scale model, and the pitch at the tips is zero. Never see that in FS airplane props, and someone in the engineering department screwed up. It generated very little thrust. Prop pitch can be roughly calculated, with RPM, to find a cruise speed for an average airplane. Figure on about 90% efficiency; in other words, find out how far the prop would screw itself through the air if there was no "slip," and knock off about 10%. In many models there will be much more slip than that, due to higher drag. Occasionally, in FS aviation, we'll encounter a prop that has no apparent slip, such as my Jodel with its original long prop; it's usually an indication of too long a prop, and the extra drag reduces RPM and therefore peak horsepower. Cruise suffers, but climb is excellent.

Dan

Knowing that many props are not accurately made, I made a LARGE pitch gauge. The same idea as the Prather pitch gauge. I found out that some composite props that claim to be 8 inch pitch are actually 6 at the root and 4 inch pitch at the tips. No wonder they tached 1500 to 2000 high. If you don't have a pitch gauge, tach those big gassers to get an idea what the pitch really is.

If you only used the rear side of the prop to measure, you will be quite a bit off. Many props use airfoils similar to the Clark Y. This airfoil generates lift at negative AOA measured through the mean chord. The bottom side of the airfoil is farther negative from that. The prop generating software that I was in action at APC takes the true airfoil characteristics into account.

Good point, and one I should have mentioned. However, most props have no undercamber at the 70% point and the bottom surface is almost exactly the same as the chord line. Different FS prop manufacturers use either chord or bottom surface, depending on their view of things. The model props I'm familiar with are all flat-faced, using airfoils very much like the real thing, and a leading edge that's rather sharp. Close in to the hub the airfoil will often have an undercamber. A thicker airfoil, like the Clark Y mentioned earlier, might be found only on really slow-turning props. It's way too draggy to be used at typical propeller speeds. The point made that it will generate lift at negative bottom-surface angles is true; our FS Citabria has a NACA 4412 airfoil, similar to the Clark, and the bottom surface is angled downward in cruise. The chord line is at about zero. Some airfoils will generate lift at up to -4° chordline AOA.

Dan

Check out the 5th pic. This is a fast and easy way to get an solid ACTUAL result.