The following is a basic outline of prop theory and conclusions on prop design. A more comprehensive outline would require more mathematics and references than is practical to include here.
For most model aircraft, the propeller is the only means of converting the power of the engine into flight performance. This makes the choice of propeller vital for the performance of a model.
Treat a propeller as a wing, and the theory will be easier to understand. Blade shape, pitch and airfoil selection all become understandable. The only difference is that a propeller is a rotating wing, with the 'Reynolds' number (via different blade width and air velocity) changing along the blade.
One of the biggest problems with propeller design (for all props including ‘full’ size), is that it is impossible to know what a prop is doing in a ‘dynamic’ state. Often static testing does not relate to dynamic performance. It is very easy to develop a prop which gives excellent ‘pull’ on a static test, however a prop designed to these criteria often don’t work that well in the air.
The choice of prop construction material will have a marked effect on prop design and performance.
As the average model propeller operates at a Reynolds number (scale effect) similar to a R/C Glider. A high aspect ratio, elliptical based shape generally works best, whether it be for glider wings or a propeller. If you take the average Bolly Prop and give it a well rounded or swept back tip, this overall shape will be as practically close to an ellipse as possible. This is especially so with the 3 and 4 blade Bolly Props. Practical considerations sometimes prevent the 2 bladed propeller from being the ideal shape, ie, undercarriage lengths and ground clearance.
The choice of blade shape is also determined by the propellers end use. For slow flying high drag models or models which need small diameters, a lower aspect ratio, wider chord prop is more suitable.
1, 3 AND 4 BLADE PROPELLERS
The rules vary when dealing with single blade and racing propellers. A small 2 blade 5.8 x 5.8 with 12mm chord is equivalent to a single blade of 6.2 x 5.7 with 17mm chord. The single blader is more efficient due to the extra diameter and chord (much higher Reynolds number). Large diameter single blade propellers are not practical due to the high weight of the counterbalance required.
For some racing applications a high aspect ratio blade is not possible due to the extreme loads imposed at high RPM, especially if the tip is expected to exceed mach 0.7.
Contrary to popular belief, multiple-blade propellers do not operate in severely disturbed air from the previous blade (when in forward flight). The reason multi blade propellers often appear inefficient is the need to use considerable lower diameter propellers (in comparison to 2 blades), for the same horsepower available. Diameter for diameter a well designed 4 blade prop will in some circumstances perform better than the equivalent 2 blade propeller.
It is rare to find an efficient 3 or 4 blade propeller manufactured from nylon based materials. The reason for this is the most efficient shape (thin, narrow blades) for these propellers is difficult to produce in anything but a carbon or glass composite construction. For this reason Bolly have one of the best and most efficient ranges of 3 and 4 bladed propellers in the world.
As with a glider wing we want the maximum lift over drag performances. Practical strength and aerodynamics means an optimum airfoil thickness of around 15 to 18% near the root, progressively thinning to 10% at the tip. About 12% at the 3/4 span in optimum.
The blade airfoil will vary slightly with use, with the Clark Y shape as a standard. The root which does less of the work and the tip which has ‘tip speed’ problems should have less camber (semi-symmetrical). For some applications the centre portion of the blade may need a lower or higher cambered airfoil.
The Reynolds' number (Re) is a theoretical number used to describe the 'Scale Effect', ie - an exact 1/8 scale replica of a Boeing 747 wing will not behave identically to the full size version. The higher the Re, the greater the efficiency. The equation is - Re = 68500 x Velocity x Length (chord or wing or prop)
Without doubt this is the least understood factor of propeller design. Pitch is the theoretical distance the propeller will advance along the axis of rotation in one complete revolution.
The fact that a propeller of constant pitch will have a twisted blade is also not often comprehended, ie - at 5 inch radius the propeller will travel a circle of 31.4 inches, at 10 inch radius it will travel 62.8 inches (double the distance) it will need half the pitch angle to travel the same distance.
BLADE ANGLE (degrees) AT GIVEN PITCH & DIAMETER DATA CHART
Pitch should always climb from the root to the tip, with the rate of increase being less at the tip - even constant over the last 20% of the diameter. The quoted pitch should always be the 'peak' measurement, although to be more reliable and consistent quoting the pitch at 80% of diameter is the usual practice.
The pitch distribution described above is often described as 'progressive pitch', ie - the pitch progressively increases along the blade from root to tip. Some prop manufacturers quote pitch as say 6 - 10. This refers to a 6" pitch at the root, and 10" of pitch at the tip. This should simply be regarded as a 10" pitch propeller.
The often quoted alternative to progressive pitch is 'constant or helical' pitch, ie - the identical pitch measurement from root to tip. This type of prop tends to be extremely efficient .... but at only one rpm range.
The mathematical explanations to why progressive is best are very involved. Basically it involves the angle of attack that the airfoil is operating at, coupled with the need to slightly washout the root and tip to reduce drag.
The diagram below shows how most Bolly Props are pitched...... The diagram has 4 lines.
The lines are drawn separated for added clarity. They should all be identical at about 70% of radius.
Is a word often used to describe the propellers lack of efficiency. To say a propeller has 15% slip is in fact saying a propeller is only 85% efficient at converting the pitch to forward motion.
It is also often described as the difference between the angle of the blade and the angle of the relative airflow (which is less than the angle of the blade).
Most Bolly Props are in the 90 to 95% efficiency at converting pitch to airspeed, ie - very efficient. Some props we have tested struggled to have 60% efficiency at converting pitch to airspeed.
NOTE... this assumes a correctly matched prop to airframe.
NOTE... the above is different to overall efficiency, which is the measurement of converting the engine energy into kinetic energy via the propeller. This measurement is considerable less than above.
Over the last few years noise has become an important issue to many model clubs. The exhaust noises are fairly easy to reduce, leaving the propeller as the main source of noise. The fact remains, to convert engine power into propulsion must by its very nature be noisy, FACT = the more powerful the engine, the more noise it will produce.
There is a big difference between true and accurate noise levels as measured and the perceived noise of the average person. An outstanding example of this is the noise level produced by a model with a high rpm small engine, (.049) compared to a large scale model (60cc). To the ear both may sound equally noisy, but a noise meter will tell a different story .... the large model will be far more noisy, and if the noise is measured from a long distance away, the small model will hardly register at all.
The overall design and even the materials from which the propeller is constructed, also have a significant effect on noise. The softer materials such as nylon produce a ‘softer’ noise, but as they are more flexible, the flexing will also create extra noise. To make such a prop rigid will require a thicker blade (heavier and more expensive to produce), which in turn operate at lower rpm (due to absorbing more power), and reduce the propellers overall efficiency. As can be seen, designing for low noise as opposed to high efficiency is a compromise.
The most obvious factor in prop noise is tip speed and shape. Reducing tip speed and using a good tip shape is the most productive method of reducing noise. This will generally mean using a lower diameter higher pitch prop than before.
Propeller noise is predominantly a product of tip noise. The propeller tip is travelling much faster than any other part of the blade, speed = noise.. A high drag tip shape will always create more noise than a low drag shape, the tip shape will have an effect on the props overall efficiency
A well rounded or raked tip should always be used, never leave the tip square. The very end of the propeller should be rounded on all sides, not only the leading and trailing corners, as would a model wing tip - never leave it cut off square.
Although not necessarily the best shape a well rounded tip is easily reproducible and less prone to damage than some more exotic shapes.
By all means, use other shapes. A raked tip of between 20 and 35 degrees is often used successfully.
Important .... not only should a tip be rounded etc in plan view (as shown), it should also be rounded in side view, as if it were a well rounded wing tip i.e. the tip should be a fairly sharp edge as per the leading or trailing edge.
TIP SPEED Vs RPM & DIAMETER DATA CHART
The speed of sound is approximately 760mph.
A propeller should NEVER be used over 550mph without paying attention to tip shape, due to compressibility and shock waves as mach 1 is approached.
An optimum maximum tip speed for achieving a low noise is 400mph. There appears to be a marked increase in noise above this speed. The best example of this is R/C Aerobatics where low noise is an advantage. These models have developed a basic set of rpm vs. diameter equations, which are reasonably accurate to the below chart ... As can be seen, F3A models avoid going above 375mph tip speed.
15" diameter = 8,500 rpm
14" diameter = 9,000 rpm
13" diameter = 9,500 rpm
12" diameter = 10,000 rpm
RPM x 1000
As can be seen from the diagrams, a large diameter spinner or fuselage doesn't have as large an effect on propeller efficiency as expected. Using an exposed propeller nut will reduce propeller performance, instead a short blunt spinner flaring into the fuselage should be used.
Many large scale models have a problem with large radial cowls. It is important to pay attention to the shape. Where possible, allowing air to flow through the cowl will help.
When designing a model pay close attention to the shape of the ‘front end’. The shape and drag of this area is very important for performance.