Please explain what the second number of a prop is (10x4.7) and how does one add more pitch to a plastic prop and get it even on both side?

Is there a prop pitch gauge and who sells them?

From : Don Stackhouse

Uh-oh, now you've done it. Someone once made the accusation that if you asked me what time it was, I would tell you how to build a clock. I replied that actually, he was wrong. I would first explain how to DESIGN a clock. 'Scuse me a moment while I get out my watch....

Imagine the prop is a screw (such as with the term the Brits use, "airscrew"), screwing itself through the air with no slippage, like a screw going through wood. The distance it would move forward in one complete revolution is the pitch.

For example, your 10x4.7 prop would THEORETICALLY move 4.7" forward through the air with each revolution if there was no slippage.

If you really don't want to know anything more than JUST the definition of the number you find

stamped on the blade, please STOP READING NOW !!!

However, no prop is 100% efficient, and there is always some slippage if the thrust is not zero. For a typical model prop, efficiencies at normal flying speeds are typically around 60% to 80%, with the efficiency usually best at higher speeds and lower power settings. A really well designed prop in a cruising flight condition, and extremely well-matched to the airframe and powerplant at that flight condition, can sometimes do a little better than 90%. However, for models this level of efficiency would be ALMOST unheard of. I do know of at least one (it's on a UAV I'm consulting on), but it's pretty exotic.

The other factor here is how you measure the pitch. There are a number of pitch gauges on the market, and most of them measure the angle of the blade's (usually flat) undersurface relative to the plane of rotation. BTW, in the propeller business, that flat undersurface is called the "face side", and the curved upper surface of the blade is called the "camber side". For best efficiency, make sure you mount the prop with the camber side facing forwards!

The pitch at any given point along the blade will be:

Pitch = 2 * Pi * (radius of that location on the blade from center) * (tangent of the angle, as measured by a tangent to the face side of the blade)

The problem with this measurement method is that:

1. The pitch will not generally be the same all along the blade. There are props out there that advertise having "true helical pitch", meaning that the pitch is exactly what's listed on the prop, and that the pitch in inches (NOT in degrees) is exactly constant along the blade. However, most aerodynamically optimum blade designs do NOT have constant helical pitch. Some parts of the blade work more efficiently than other portions, and so an optimum blade design works those regions a little bit harder.

BTW, the "aerodynamic center" of a propeller blade is about 75% of the way out from the center, so the overall pitch of the prop is generally specified as the pitch that you would measure at that point on the blade.

2. The flat undersurface of the blade is not the angle the air sees. If the airfoil, even a flat-bottomed or undercambered one, has any leading edge radius at all, then the chord line of the airfoil will be slightly higher than the bottom surface. To make matters even more complicated, if the airfoil is cambered (i.e.: the top surface and the bottom surface are not identical), then the zero lift line (the line that really matters) is even higher than the chord line. When you allow for that, it's possible for a reasonably efficient prop to have a specified pitch that is LESS than the distance it actually moves through the air with each revolution!

For example, let's say your 10x4.7 prop was really well designed, and did not have to absorb much power in cruise, so its efficiency in cruise was about 80%. The aerodynamic center of the blade is at .75 * 5, or 3.75" out from the center. The pitch is 4.7, the circumference of that point on the blade is 2 * Pi * 3.75", or 23.56", so the blade angle at that point, measured from the flat undersurface of the blade, is ARCTAN (4.7/23.56), or 11.28 degrees.

Now, let's assume that the chord line of the blade is 3.5 degrees higher than that, and to get to the zero lift line is another 2 degrees, so the blade angle measured from the zero lift line at that point is 16.78 degrees. We're at a low power setting, so let's subtract 1 degree to allow for the blade's angle of attack (the angle between the zero lift line and the local direction of the airflow striking the blade), and we get 15.78 degrees.

The pitch that this angle corresponds to is 6.66" (that's TAN(15.78) * 23.56"), and if we then multiply by 0.8 ( to allow for the efficiency of 80%), we get an actual distance-travelled-per-revolution of 5.33", even though the number stamped on the root of the blade is only 4.7 inches!

However, that actual in-flight pitch depends on the altitude, the flight speed, the weather, the design of the prop, the design of the airplane and motor, the way you're flying it, the interactions between the prop and the airplane/motor, and the weather that day, among other things! Also, you can't measure the pitch from the zero lift line unless you have a template for the airfoil at that point on the blade AND some very good data on the characteristics of that particular airfoil at the Reynolds numbers it will see in your application in flight, which in turn depends on how and where you're going to use that prop. There are few things in an airplane design more complex than the design of a propeller and the matching of that prop to an airframe and motor combination.

By now it should be obvious why most model prop manufacturers (even though they might have used a very detailed aerodynamic analysis when designing the blade) specify the pitch by just measuring the angle from the flat undersurface of the blade.

(former full-scale propeller engineer, but I betcha figured that out already!)

Don Stackhouse
DJ Aerotech