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A. Propeller Size Recommendations
B. General Information about Propellers
C. RPM Operating Limits
D. Gearbox Ratio Recommendations
E. Safety Instructions
F. How to Balance a Propeller
Bore Diameters for Glass-Filled Nylon Propellers
We use the following bore diameters for our glass-filled nylon props. The first number refers to the front of the hub and the second number is the back side. The back side is 1/16" larger than the front for ease in boring out for larger motor shafts.
5.5" dia. through 6" dia. = 1/8" front/ 3/16" back 7" dia. through 13" dia. = 1/4" front/ 5/16" back 14" dia. through 20" dia. = 5/16" front/ 3/8" back
Three-Blade Series Only
8" dia. through 13" dia. = 1/4" 14" dia. through 16" dia. = 5/16"
All Wood Series Propellers have a single bore diameter of 1/4"
A. Propeller Size Recommendations - Back to Top
Modelers who have been in the hobby for a year or two develop a feel for the best size propellers for each model airplane/engine combination. The chart below is intended to give the beginner a starting point so that the best performance can be attained. In general, engines want to be operated at a particular RPM, where they can reach maximum power. Too large a diameter and/or high a pitch and the engine will not rev up to its best power band. On the other hand, too small a diameter and/or pitch and the engine will over-rev and not deliver the best thrust. Often, heavy and slow airplanes should have a large diameter/lower pitch while a light and fast plane should have a smaller diameter/higher pitch.
Use the chart to select a propeller. Check the RPM with a tachometer. Note that RPM will increase from 1500 to 3000 in flight, depending on the weight and speed of your plane. GOOD FLYING!
2-Stroke Engines
.049 - .051 = 5.5x4, 5.5x4.5, 6x3, 6x3.5, 6x4
.09 - .10 = 7x3, 7x4, 7x5, 7x6
.15 = 7x6, 8x3, 8x4, 8x5, 8x6, 8x7
.20 - .25 = 8x6, 8x7, 9x4, 9x5
.29 - .35 = 9x6, 9x7, 9x8, 9.5x6, 10x4, 10x5, 10x6
.40 = 9.5x6, 10x4, 10x5, 10x6, 10x7, 10x8, 10x9
.45 - .50 = 10x7, 10x8, 11x4, 11x5, 11x6, 11x7, 11x7.5
.60 = 11x5, 11x6, 11x7, 11x7.5, 11x8, 11x9, 11x10
.71 - .80 = 12x6, 12x8, 13x6, 13x8, 13x10, 14x8
.90 = 13x6, 13x8, 13x10, 14x6, 14x8
1.08 = 14x8, 15x8, 16x6
1.20 = 14x8, 15x8, 16x6
1.5 = 16x8, 16x10, 18x6, 18x8
1.8 = 18x8, 18x10, 20x6, 20x8
2.1 = 20x8, 20x10
2.7 - 3.5 = 22x8, 22x10, 22x12, 24x8, 24x10, 24x12
4-Stroke Engines
.20 - .25 = 9x4, 9x5, 9x6, 9x7
.40 = 11x6, 12x6
.60 = 11x8, 11x9, 12x6, 13x6
.90 = 12x8, 13x8, 14x6
2.10 = 14x8, 15x8, 15x10, 16x8
B. General Information about Propellers - Back to Top
Propellers are an all-important part of airplanes to provide the necessary thrust for powered flight. Even our jet engines have bypass blades to assist in producing thrust. In the simplest terms, a propeller is an airfoil traveling in a circle with a positive angle of attack relative to the incoming air to produce thrust.
Propeller performance is affected by several factors, among them are diameter relative to RPM, and blade area relative to power absorption and pitch.
Diameter is the measurement (usually in inches) of the prop from tip to tip.
Pitch is defined as the theoretical advancement of a propeller in one revolution (usually measured in inches) and defines the speed and maneuverability characteristics of flying. For example, a 10x6 describes a diameter of 10" and a pitch of 6", or forward movement of 6 inches per revolution. In metric measurement this would be 30x15. Sometimes a particular series letter is used, such as 1260S (12" diameter, 6" pitch, Scimitar Series).
Now, more about pitch, which is the hardest part to visualize. Imagine you turn a 6" pitch prop in a tub of something like butter. It should advance 6" for each turn along its axis. Naturally, the faster you turn the prop, the more rapidly you will advance. Model propellers have a practical limit on how fast they can turn (RPM) based on the power curve of the engine and the diameter of the prop. Another practical limit is due to noise considerations. Prop tip speed limits should be at 600 to 650 feet per second. Tip speed is explained below.
Slow speeds, aerobatics, great take-off's and landings can all be accomplished with low pitch propellers. Higher pitches lead to less maneuverable but faster flying. Because modelers do not have the luxury of variable pitch, most select a pitch based on how they like to fly or compromise speed and maneuverability with a pitch somewhere in the middle - around 65 -70% of the prop diameter. A limiting factor which will decrease propeller efficiency is engine horsepower and aircraft drag, i.e. a high pitch prop can't make an airplane any faster than it's capable of being and too low of pitch can result in lower power/thrust. It should be noted that industry standards are that pitch is measured at 75% of radius.
Thrust refers to the force exerted by the rotating propeller in the direction of travel of the airplane. This is the whole purpose of the propeller - to convert the power of the engine which appears as a rotating force, or torque, into a linear force to propel the airplane. Thrust is usually measured in pounds is a function of air density, rpm, diameter, advance ratio and Reynolds number. It is a long, complicated process to get this number, but what is important to remember is that thrust is different for every shape of propeller and changes with flying conditions.
Power Absorption refers to the power output curve of the engine. Power is the product of torque times rpm. As rpm increases, an engine produces less force (or torque) because the air/fuel mixture is not as efficient at higher rpm's. This is why a power curve becomes flat or decreases at higher rpm's, and means that the most efficient propeller is the one that allows the engine to operate at its optimum power band.
An interesting point in understanding power absorption is that propeller power varies as the cube of the rpm. Consequently, twice the rpm requires 8 times the power.
Tip Speed is measured in feet per second and a formula is provided below to find this measurement.
For model airplane purposes, the best tip speed for efficiency and noise requirements is 600 feet per second. This is due to compressibility losses and the fact that subsonic airfoils do not work well in transonic/sonic speeds with required sound levels.
Feet Per Second (ft/s) = RPM x diameter in inches x .00436
For example, to find the tip speed of a 10x6 on a .40 size engine running at 13,500 RPM, the equation would be 13,500 x 10 x .00436 = 588.6 ft/s.
To find the correct diameter at 600 ft/s, use this formula:
Diameter in inches = 138,000 / RPM
Using a .40 engine running at 13,500 RPM, the equation would read as follows:
138,000/13,500 = 10.22
Rounding down, the correct diameter is 10"
For both of the above formulas, use RPM for the optimum power band of your engine. Consult your owner's manual if you do not know this number.
C. RPM Operating Limits - Back to Top
One of the differences between wood and glass-filled nylon propellers is that glass-filled nylon props have suggested RPM limits for mechanical considerations. This will vary according to diameter. For Master Airscrew props, we suggest the following formula: RPM Operating Limit = 160,000 divided by Diameter in inches. In our .40 size engine example, a 10" diameter g/f nylon prop has an operating limit of 16,000 rpm, well above the requirement of a .40 engine.
D. Gearbox Ratio Recommendations - Back to Top
Wingspan smaller than 2 meters:
- 2.5:1 ratio with 10" - 12" propeller
Wingspan at 2 meters:
- 3.0:1 ratio with 11" - 12" propeller
Wingspan larger than 2 meters:
- 3.5:1 ratio with 12 - 15" propeller
E. Safety Instructions - Back to Top
A rotating propeller can cause serious personal injury. For safety's sake, and the enjoyment of the sport of model airplanes, follow these instructions carefully at the flying site and at the workbench.
1. Select the correct propeller size, following engine manufacturer's recommendations.
2. Before using g/f nylon props, remove any flash along the edges of the propeller by scraping with a sharp knife.
3. Install the propeller using the correct prop nut and washer with the airfoil side forward and securely tighten with a wrench. Recheck after each flight.
4. Only use a "Chicken Stick" or electric starter to start the engine, then make adjustments from behind the engine.
5. Keep spectators at least 20 feet away from and out of the path of a rotating propeller.
6. Wear safety glasses and hand protection when operating model engines. Do not permit any objects to touch a turning propeller. Always remain clear of the propeller arc.
7. To stop the engine, cut off the fuel supply or in accordance with the manufacturer's instructions. In any case, DO NOT stop the propeller with your hand or other object.
8. Inspect the propeller after each flight. Discard any propeller that has nicks, scratches or any other visible defect. DO NOT alter, or in any way modify a propeller.
9. Maximum recommended RPM for Master Airscrew g/f nylon propellers is 160,000 divided by diameter in inches.
10. It is strongly suggested that propeller tips be painted in order to increase visibility while turning.
F. How to Balance a Propeller - Back to Top
Because of slight differences in wood grain and density, and due to variances in the molding process, it may be necessary to balance a propeller before use. Balancing a prop is a simple operation and requires the following materials:
1. Balance Stand (we recommend the Master Airscrew, part no. MA60422)
2. Masking tape - ¾" width
3. Silver solder and or/modeling clay
4. Pocket knife
5. Sanding paper, med. to fine grade
When you place the prop on the balance stand, make sure the cones are placed fairly snug next to the prop hub. The heavy blade will fall. If it fall slowly, the prop is slightly out of balance. If it falls quickly, it is more so. As a test, turn the cones 180 degrees to see if the balance changes. If it does, the cones are out of balance.
To Balance: Take a 1" piece of masking tape and place it on the light blade. Adjust the tape until the prop is in balance. The amount of tape will tell you how much material you will need to add or remove for final balancing.
In most cases, the weight of the tape is so slight it won't show up on a gram scale - say 1" or less of tape. If this is the case, the prop is within spec and can be flown without adding or removing material.
To Add Material: For g/f nylon props, place modeling clay or silver solder in the holes in the back of the hub of the light side until balanced. For wood props, try adding paint or nail polish to the back of the blade.
To Remove Material: For g/f nylon props, use a pocketknife to trim the edges of the heavy blade. For both wood and g/f nylon, use sanding paper and take material from the heavy blade to bring into balance.
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