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Propeller Safety Warnings


APC Propeller Technical Advisories
All propellers are inherently dangerous. Model airplane propellers are especially dangerous. Model airplane propellers used in high performance racing are extremely dangerous. Model airplane engines designed and modified to achieve maximum operating capabilities create unpredictable and potentially severe loads, leading to various forms of potential propeller failure. Ignoring reasonable safeguards may likely be catastrophic. This concern is the motivation for the following discussion.
Warnings included with propellers are intended to protect consumers. They also protect manufactures against claims resulting from misuse of the product. Most products with potential for causing injury contain ample warnings about misuse. Some advertisements for products now contain warnings, even before the product is sold! There is a strong proliferation of warnings in most products having potential for creating injury or damage. This inundation of warnings may cause consumers to become inured to product warnings.
The warnings about propeller use must be taken seriously, especially for racing applications. It is very risky to assume that a racing propeller blade will not fail, especially when used with state-of-the-art racing engines. Yet, nevertheless, occasionally model aircraft operators are observed standing in the plane of propeller rotation of high performance racing engines running at full power. This is very frightening. The following information reinforces the assertion that dangers of misuse are very real.
Ideally, a product can be designed with credible knowledge of the environment (loads acting on the product) and capabilities of the product to withstand that environment (not fail). There is nothing ideal about designing a model airplane propeller because some major components of propeller loads are very uncertain. The principle load components acting on a propeller are:

  • Centrifugal (from circular motion causing radial load)
  • Thrust/drag (from lift and drag acting on blade sections)
  • Torsional acceleration ( from engine combustion and/or pre-ignition)
  • Vibration (from resonant frequencies or forced excitation)

Another potential source of loading is aero elastic tip flutter. This may be caused by self exciting aerodynamic loads at a resonant frequency.
These loads are discussed next in order.
Centrifugal loads are very predictable, given rotational speed and mass density distribution of a blade. Their contribution to total stress is relatively small.
Thrust/drag loads are somewhat uncertain due to complexities of aerodynamic environments. The relative axial speed at the prop (at any radial station) is aircraft speed plus the amount the air in front of the blade is accelerated by the mechanics creating thrust. The latter may be approximated using first order classical theory. Much empirical lift/drag data (from wind tunnel tests) exists to quantify lift/drag loads, once relative velocity and angle of attack distributions are established.
Torsional acceleration loads are generally not known. Analytical estimating technique used by Landing Products to quantify torsional acceleration loads suggests that they can become dominant when pre-ignition or detonation occurs. These analytical observations are supported by test experience with very high performance engines running at elevated temperatures. The latter causes a high torsional load (about the engine shaft) which creates high bending stresses, adding to those from centrifugal force and lift/drag effects. These torsional acceleration loads depend on unique conditions for specific engines. Engines "hopped up" for racing appear to be especially prone to create high torsional loads when lean mixtures lead to high cylinder temperatures and pre-ignition/detonation.
Vibration causes additional loads from cyclic motions. These motions occur when resonant frequencies are excited or when cyclic load variations exist on the blade. The magnitude of these variations depends on how close the driving frequency is to the resonant frequency and the level of damping in the propeller material. Engine combustion frequency is an obvious excitation. Obstructions in front of or behind the blade can cause cyclic variations in thrust load. Once a blade starts to flutter, those motions alter the flow, causing variations in loading. High performance engines have caused propeller tips to break, presumably due to fatigue failure from vibration.
Aero-elastic flutter is speculated to be a dominant mechanism causing rapid fatigue failure near a tip when insufficient or destabilizing tip stiffness exists. The interaction between variable loading and deflection induces a high frequency vibration with unpredictable magnitude.
Efficient propeller design practice utilizes analytical/computational models to predict propeller performance and stresses. However, the uncertainty in impressed and inertial loading from complex phenomena requires testing to assure safe performance. Unfortunately, it is not possible to assure testing that convincingly replicates worst case conditions. The large combinations of engines, fuels, temperature, humidity, propeller selection, aircraft performance and pilot practices creates an endless variety of conditions. If the origins of severe loads were well understood, quantified, and measurable, structured testing might be feasible that focuses on worst case stack up of adverse conditions. However, since the origins of severe loads are really not well understood, it is essential to provide sufficient margins in material properties and design to assure safe performance. Propellers that are used in fairly routine and widespread applications (sport and pattern) lend themselves reasonably well to test procedures that provide reasonable confidence. In time, a sufficient data base develops that can be used to empirically quantify performance and "anchor" or "tune" assumptions used in analytical models.
However, propellers that are used for increasingly extreme performance applications do not benefit from the large empirical data base sport and pattern propellers enjoy. Assumptions and design practices developed for current generations of engines may not be valid for emerging engines whose technologies continue to push engine performance to greater extremes. Consequently, propellers that are used in applications where performance is already relatively high (and expanding) must be used with great caution.
An adverse cascading effect occurs when propellers are permitted to absorb moisture in high humidity environments. Composite strength, stiffness and fatigue endurance all reduce with increased moisture content. Reduction in stiffness typically causes resonant frequencies to move toward the driving frequency (increasing torsional loads) and, the reduction in strength reduces fatigue endurance. Composite propellers should be kept dry.
In summary, please abide by the safety practices recommended by propeller manufactures. This is especially important for high performance propellers. Assume that propellers can fail at any time, especially during full power adjustments on the ground. Never stand in or expose others to the plane of the propeller arc.
APC Propeller Technical Advisories

** Propeller Size Recomendations.
Landing Products does not make specific recommendations for propeller sizes.
Large variations exist in engine/motor performance characteristics and in model aircraft applications. Therefore, Landing Products is unable to reliably offer propeller size recommendations for specific combinations of motor type, size and application.
Please rely on written instructions/recommendations that are provided with most motors. If propeller size recommendations are not provided with the motor, please contact the motor manufacturer for suggestions.
** Engine Shaft Hole Alignment.
The prop hub aft surface and aft hole are precisely defined during molding. However, post-molding shaft hole drilling may induce minor angular mis-alignment of the prop with the engine shaft. This hole mis-alignment is avoided by use of a tapered reamer to slightly enlarge the prop hole forward of the aft surface. This causes the prop to precisely register with the engine shaft at the aft surface and hole.

** Slo Fly and Electric Props off center holes:
The paper insert lists the pilot hole as the precision hole which can be used to precisely drill the non-precision hole. Our preference is to use a tapered reamer from the front side of the prop to avoid contact with the precision hole entirely.

Unfortunately, we are forced to gate the material in through the center of all axis's which necessitates a second operation of drilling out the sprue. For this reason the precision locating rings were adopted for electric propeller production.
It is our experience that most of the time some customers fail to read the instructions for properly mounting the propellers.
** Q40 pylon racing propellers are now available again.
However, a new warning concerning their use is now posted below.
This warning is particularly important when using higher performance Q40 engines now entering the market.

** Pylon Racing Propellers Warning
A previously unobserved failure mode potentially exists when the resonant lateral frequency of the fuselage is excited by the engine rotational frequency. The failures are causing the outer portion of the tip (typically last ¾ inch) to fail during flight.
This failure mode has been observed only with very limited combinations of Q40 engine, propeller, and fuselage design. We suspect that fuselage resonance, in combination with higher performance engines now entering the market, is the most probable cause for the propeller tip failures.
The initial symptoms of the failure mode are audible vibration noise in flight. The postulated mechanism has not been observed on the ground, due to the Q40 engines operating at reduced RPM until airborne.
This failure mode has, to our knowledge, has not yet caused catastrophic failure. However, the potential for more serious consequences is a concern.
Changing propeller characteristics very mildly (pitch change or diameter reduction) effectively mitigates the resonance effects. This rather strong sensitivity to propeller design clearly indicates the presence of a highly tuned resonance condition.
The observance of this new failure mode suggests that this same mechanism could occur with other combinations of fuselage stiffness (natural frequency) and engine characteristics. This might be especially true given the continuous improvements in engine performance. Therefore, the following cautions are particularly emphasizedwhen operating higher performance pylon racing model aircraft.

Selecting a Propeller Windsor Propeller Company, Inc. 1
Windsor Propeller Company, Inc.
Master Airscrew
The charts below are intended to give the beginner a starting point for best performance. Modelers who have some experience develop a feel for the best size propeller for different model/engine combinations.
In general, engines want to operate at a particular RPM where they can reach max power. Using too large a diameter and/or too high a pitch may cause the engine to not rev up to the best power band. With too small a diameter and/or pitch, the engine will over-rev and not deliver the best thrust. Often, heavy and slow airplanes use a large diameter and moderate pitch, while a fast plane will have a smaller diameter and a higher pitch. Hovering and lifting applications use an over-sized, low-pitch propeller.
Use the charts below to select a propeller. Check the RPM with a tachometer – RPM will increase
from 1500 to 3000 in flight, depending on the weight and speed of the plane. BE SURE TO FOLLOW ALL SAFETY AND WARNING INSTRUCTIONS. . .GOOD FLYIING!
RPM Operating Limits
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 = 165,000 divided by Diameter in Inches. For example, a 10” diameter prop has an operating limit of 16,500 RPM, well above the requirement of a .40 engine.
Propellers for 2-Stroke Engines
.049 to .051: 5.5x4, 5.5x4.5, 6x3, 6x3.5, 6x4 .09 to .10: 7x3, 7x4, 7x5, 7x6
.15: 7x6, 8x3, 8x4, 8x5, 8x6, 8x7 .20 to .25: 8x6, 8x7, 9x4, 9x5
.29 to .35: 9x6, 9x7, 9x8, 9.5x6, 10x4, .40: 9.5x6, 10x4, 10x5, 10x6, 10x7, 10x5, 10x6 10x8, 10x9
.45 to .50: 10x7, 10x8, 10x9, 11x4, 11x5, .60: 11x4, 11x5, 11x6, 11x7, 11x7.5,11x6, 11x7, 11x7.5 11x8, 11x9, 11x10
.71 to .80: 12x4, 12x5, 12x6, 12x8, 13x5, 1.08: 14x6, 14x8, 14x10, 15x6,13x6, 13x8, 13x10, 14x8 15x8, 16x6
1.20: 14x8, 14x10, 15x8, 15x10, 16x6, 1.5: 16x8, 16x10, 18x6, 18x8, 18x1016x8, 16x10
1.8: 18x8, 18x10, 20x6, 20x8 2.1: 20x6, 20x8, 20x10
2.7 to 3.5: 22x8, 22x10, 22x12, 24x8,24x10, 24x12
Selecting a Propeller Windsor Propeller Company, Inc. 2
Propellers for 4-Stroke Engines
.20 to .25: 9x4, 9x5, 9x7, 9x8 .40: 11x5, 11x6, 11x7, 11x8, 12x4,12x5, 12x6, 12x8
.60: 11x8, 11x9, 11x10, 12x5, 12x6, 12x8 .90: 12x8, 13x8, 14x6, 14x8, 14x10
1.20: 14x8, 14x10, 15x8, 15x10, 16x6, 16x8
Converting from a 2-Blade to a 3-Blade Propeller
To convert from 2 blades to 3 blades you want to decrease the total blade area and increase the angle of attack (or pitch) to overcome the increased drag of the third blade. The general rule is to DECREASE propeller diameter by 1-2”, and INCREASE by 1-2” the propeller pitch. It is all right to keep the same pitch when going from 2 blades to 3.
Propellers for Electric Motors
Motor Size Prop
400 – Direct Drive 6x3, 6.5x4, 7x4
400 – Geared 7x5.5, 8x4, 8x5, 8.5x6
550/600 Direct Drive 8x4, 8x5, 8.5x6, 9x6
550/600 Geared:
2.5:1 Ratio 10x6, 10x7, 11x6, 11x7, 12x6, 12x7
3.0:1 Ratio 11x6, 11x7, 12x6, 12x7, 12x8
3.5:1 Ratio 12x7, 12x8, 13.8.5
Propeller Hole Sizes
Most Master Airscrew Propellers have two hole sizes because they are piloted in the back of the propeller to facilitate reaming should you need to enlarge the hole. The first number is the hole size; second number is the pilot size.
2-Blade Propellers (glass-filled nylon)
5.5” through 6”: 1/8” and 3/16” 7” through 8”: 3/16” and 1/4”
9” through 13”: 1/4” and 5/16” 14” through 20”: 5/16” and 3/8”
3-Blade Propellers
6” through 7”: 1/8” and 3/16” 8”: 3/16” and 1/4”
9” through 13”: 1/4” and 5/16” 14” through 16”: 5/16” and 3/8”
Selecting a Propeller Windsor Propeller Company, Inc. 3
• Wood Series and Wood Scimitar Series propellers all have a uniform 1/4” hole. • All Electric Series propeller have a set of 5 bushing inserts for mounting to various prop adapters.
Balancing a Propeller
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 – either wood or glass-filled nylon – before use. Balancing a prop is a
simple operation and requires the following materials: 1. Balance Stand
2. Masking Tape – 1” or under in width
3. Silver solder, modeling clay, enamel or nail polish
4. Pocket knife
5. Sanding paper, medium to fine grade
Note: If you need to ream the center hole, do it before you balance your prop. If you ream even slightly off center, balance will be changed considerably and you’ll have to balance all over again. When you place the prop on the balance stand, make sure the cones are placed fairly snug next to the prop hub. Hold an end of the prop so it hangs vertically, and let go. The heavy blade will fall and the prop may even rotate once until it finds horizontal balance. As a test, turn the cones 180 degrees and 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 tip of the light blade. Test for balance and add or subtract tape as needed. The amount of masking tape on the blade will tell you how much material you will need to add or remove for final balance. 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 prop hub, on the side of the light blade, until it balances. For wood props, try adding paint or nail polish to the back of the light blade. Industrial enamel or nail polish can also be used on g/f nylon props.

To Remove Material: For g/f nylon and wood props, use sand paper to remove material
from the heavy blade and bring into balance. For g/f nylon props, use a pocket knife to
trim the edges of the heavy blade.

SAFETY TIPS: Choose an appropriate size spinner for the airplane, engine and propeller combination that you are using.
Balance all propellers before use
Inspect the spinner before each engine run to be sure that the spinner is still firmly fastened to model correctly & that the prop nut is tight.
Inspect the spinner frequently to make sure their are no cracks in the spinner or the propeller before starting any engine. Do not try to repair a damaged spinner replace it immediately
A... Ensure the proper fit of the spinner backplate on the engine shaft, If the hole in the backplate is too small, carefully enlarge it using a drill ( preferably a drill press ) making sure that the hole is drilled square.
B... Slide backplate back onto the engine shaft and check for clearance in between the cowl & the spinner , the clearance should be at least 1.5mm minimum.
C... Install the prop, washer & nut to the engine and test fir t he spinner cone the spinner cone MUST NOT TOUCH the propeller at all. Use a file to enlarge the openings so that the prop is not touching the front cone of the spinner at all, DO NOT leave any sharp edges, corners, gouges or knife marks that may cause cracks to form and cause a failure in the future.
D... Tighten the prop nut firmly using the appropriate tools to the engine manufactures specification’s.
E... Install the front spinner cone to the model making sure the back plate is aligned correctly and secure it using the supplied screw or screws.
Please note these are sport Nylon or aluminum spinners and is not designed for use with high performance engines
Operation above 14,000rpm is not recommended