Pitch and Pitch Speed Aeromodeling

Pitch and Pitch Speed

Pitch is the distance (normally expressed in inches) that the propeller "cuts" through the air in a single rotation assuming no slippage. To achieve pitch, the propeller blades are angled to move air to create thrust. The angle of the blade determines its pitch. Propeller blades are aerofoils, just like the flying surfaces on our models. When they have a higher angle of attack they create more lift. In the case of propellers, a higher angle of attack (pitch) at a given rpm will create greater thrust.

Pitch speed is the speed at which the propeller pulls through the air. It is calculated by looking at the pitch of the propeller, and the number of revolutions it performs in a unit of time. Pitch speed does not consider slippage, drag and other forces that may affect the aircraft.

With a high wing loading you need a higher air speed to stay in the air. A higher pitch speed means lower thrust > longer take off > high landing speed. You can get both thrust and high air speed but it will be at a weight penalty as the power needed to get thrust for a short take off will not be in proportion to the power needed to stay airborne.
Warbirds are an often examples of models with high power/high wingloading which are supposed to fly fast, and especially in glow to electric conversions you will need to take the wing loading into account.
Hotliners and F5b models are one of the most extreme examples of high power/high wingloading. The more extreme examples have such a high pitch speed a catapult is needed to get them airborne because of the square (16x16) or over square (16x17) props they use in order to get extreme high speed/climbs. In a perfect world (with zero airframe drag and 100% prop efficiency) you can calculate the speed of your model from RPM x pitch)/1056 = your speed in mph. For example 10000rpm x 7" pitch /1056 =66mph or 105.6 km/h.

Pitch speed isn’t only about wing loading it's also about what you want to do with your model, as I wrote above about hotliners and F5b. With an already light model or of moderate weight you can determine the behaviour from the choice of prop > pitch speed. Without the need of changing anything (keeping the same amps) you can take a GWS Formosa II with a 10x5 from being a sporty low wing aerobatic trainer to a fast aerobatic plane with a 9x6. As a general rule 1" pitch relates to 1" of diameter, if you step up 1" in pitch you need to step down 1" in diameter to keep the same amp draw.
With more normal kind of planes we usually use a prop with the proportion of 1:2 i.e. 10x5, 11x5.5, 12x6 and so on as it is most effective (from what I heard). A High wing trainer could very well use a more square prop like 9x7 instead of 11x5.5, it'll still have a high lift and once airborne you can throttle down, the higher pitch will give it airspeed and you'll get long flying times with low amps, perfect for photography or video.

The following is some extra information about prop selection kindly offered by brucea from RC Groups.

“As a rule of thumb, you want to have a static pitch speed within the 2.5 to 3 times the stall speed. So if your plane stalls at 15 mph in level flight you would like a static pitch speed between 37.5 to 45 mph.

For a particular motor, I know from testing that with a 12x6" propeller the motor is running at 7165 RPM. Each revolution pulls the plane forward 6". So my plane would be making 6" x 7165 RPM or 42,990 inches per minute. Dividing by 12" gives me 3,582.5 feet per minute. Multiplying my 60 minutes gives me 214,950 feet per hour. Dividing by 5280 feet gives me 40.7 miles per hour. The plane I has a calculated stall speed of 14 mph. 40.7 divided by 14 equals 2.9. This ratio falls within the desired 2.5 to 3 ratio of pitch speed to stall speed, which is good!

To select a motor you may have to work back-wards from prop diameter. The plane I have can take a 12" prop. I like to get the largest diameter prop that will fit.”

Lipo packs

Safety: Always use your lipo packs safely.

C rating

Broadly speaking, the "C" rating is a guide to how much current it is safe to draw from your battery. It's expressed in terms of the capacity or C. Beware that constant discharging of your lipo pack at its maximum C rating will almost definitely shorten its life. Depending on the quality of your pack, it is much wiser to keep your current draw to about 10 C with short bursts up to 20 C if you want your pack to last. The easier you are on your pack the longer it will last.

A 2200mAh 10C battery is rated to be discharge at up to 22A (10 x 2200mA/1000) and the same size 12C battery would be good for 26.4A (12 x 2200mA/1000).

The internal resistance in higher C rated packs is lower, meaning that the voltage drop found in higher C packs is not as pronounced giving higher voltage under load and slightly more power.

Here is Brucea’s experience testing two 2100mAh 11.1V lipoly packs which demonstrates higher voltage of a higher C pack under load.

“I tested a 20C versus 15C 2100mAh 11.1V lipoly batteries. Motor volts and thrust are as follows:

15C: 9.3 V motor voltage, 44.6 oz of thrust producing 271 Watts
20C: 10.1 V motor voltage, 48.9 oz of thrust producing 318 Watts

The motor draws 29 Amps from the 15C 30Amp discharge battery, and 31.5 Amps from the 20C 40 Amp battery. For this motor I am using two 2100 mAh 11.1v 15C batteries in parallel. This gives me twice the "C" rating or 60 Amps. This particular motor pulls 31 Amps with a 12x 6" prop and two 11.1V 15C 2100 mAh batteries.”
Also beware that lipo manufacturers often put an overly optimistic C rating on their packs. Unless you see independent test results you trust for lipo packs, use them at about half the stated C rating and you should get many more cycles from them.


mAh
mAh is an acronym for Milliamp Hour, which is how much current a battery will discharge over a period of one hour. Higher numbers here reflect a long battery runtime and or higher storage capacity. For example a 2000 mAh pack will sustain a 2000 milliamp (2 amp) draw for one hour before dropping to a voltage level that is considered discharged. A 1700 will sustain a 1700 mAh (1.7 amp) draw for one hour. 1000 mAh is equal to a 1 Amp Hour (AH) rating.

Like the C rating, the mAh rating also determines the maximum current that can be drawn from a pack as can bee seen in the calculation in the C Rating section above. For example if you have three 11.1 Volt 10C packs, one rated at 1000 mAh, one rated at 1700 mAh and the other at 2000 mAh, we can determine that it is safe to draw the following amperage from these packs. Multiply the C rating by the mAh rating and divide by 1000 to convert milliamps to Amps:

    10 X 1000 mA/1000 = 10 Amps
    10 X 1700 mA/1000 = 17 Amps
    10 X 2000 mA/1000 = 20 Amps