Choosing The Right Propeller
All those years ago when I learned to fly rc planes advice concerning choosing the right propeller was brief and unqualified: “For a 40, you’ll need a 10″ x 6″ and if it’s a 60 you’ll want an 11″ x 7”. No reasoning, just accept that this was correct. As my experience grew I discovered that this was a very vague and inaccurate way of looking at prop choice. Experience showed me that models should be propped for the airframe, noise limits, performance and not the engine.
The two numbers that designate propeller sizes, e.g. 10 x 6, 11 x 7 (read as ten by six and eleven by seven) are measurements in inches – the first number indicates the exact diameter of the rotating prop arc or the length of the propeller, whilst the second number is the pitch of the propeller blades. The pitch is the theoretical distance the propeller would travel forward during one complete revolution in a fluid (air) with no drag, resistance or slip. A 10 x 6″ propeller measures 10″ in from tip-to-tip and would theoretically go forward 6″ during one complete revolution.
The theoretical pitch measurement is used as propellers are not 100% efficient, nor is the air it’s travelling through of consistent density and free of movement and air currents. It compresses as the propeller goes through it, and the propeller’s shape, material and finish all affect its performance and efficiency during flight. Some propellers may flex and twist with increased loads, reducing their efficiency. All of this means that the 6″ pitch of our airscrew is just a guide, and in reality doesn’t exactly translate into distance at all.
A propeller is actually nothing more than a wing with an aerodynamic section set at a specific AoA that we call the pitch. ( There’s that abbreviation we came across before, do you remember? If you’ve forgotten go and take another look at the post on Elevators). This generates lift as it is pushed through the air by the rotation of the engine shaft. Each part of this ‘rotating wing’ will have a different airspeed, low near the centre and high near the tip. To ensure that each part of the prop generates the same amount of lift from root to tip, the ‘wing root’ angle of attack is set higher than the ‘wing tip’ (hence the ‘twist’ in the prop).
The Right Choice of Propeller
Choosing the right propeller is just as important as the choice of engine. The propeller size will fundamentally affect the way your model flies. The wrong one may damage your engine and you should always be guided by the manufacturer’s instructions initially. There are normally a number of choices available as manufacturers recognise that a number of factors will affect propeller choice. These include model size, drag, weight, wing loading, engine type, fuel and even the altitude of your flying site. All are important considerations for correct propeller selection.
You also need to consider the material the propeller is made from. This must be suitable for the expected operating rpm of your engine. General sport models and trainers with motors turning props at up to 12,000 rpm are suited to glass reinforced plastic (GRP) types. It is important to understand that propellers for glow engines are totally different to those made for electric motors and the two are not interchangeable.
Having said all this, we will look at making a good selection next so don’t worry that choosing the right propeller is going to be too difficult.
Generally speaking a higher pitch prop will pull the model faster in level flight whilst a lower pitch prop will help it to take-off in a short distance and climb quicker and easier.
You’re not going to go far wrong by using the suggested prop sizes given by the engine or motor manufacturer. For a low-powered .40 – .46 glow engine I’d usually look to fit a 10 x 6″, 10 x 7″, 11 x 5″ or 11 x 6″, using the shallower pitches if extra pull is needed for take-off. If you fly from long grass or the model is heavily built, this might be something to consider. For an equivalent electric outrunner it would normally be an 11 x 5 or 11 x 6, 12 x 4 or 12 x 5. For more powerful motors I’d favour the higher pitches.
A GRP propeller will usually need some preparation before being fitted to your engine or motor. The first thing required is to remove the sharp edges. Don’t try to sand GRP propellers to remove the moulding ‘flash’, use a sharp tool (e.g. craft knife or Stanley knife) with the blade held perpendicular to the edge of the prop to remove it.
Once the edges are smooth, check the hole in the middle. In many cases this will need enlarging to fit your engine’s crankshaft. A special-purpose prop reamer or drill press is required to enlarge the hole, being careful to make it straight and perpendicular to the prop hub.
Under no circumstances should you use an oversize screwdriver or round file to enlarge the hole. Anything that causes your propeller to run out of true will produce damaging vibration and wear problems elsewhere. Click the image above or this “Reamer” link if you wish to order this item on-line.
Balancing Your Propeller
Propellers must always be balanced before fitting to your engine. This can be achieved by using a commercially available prop balancer. Cheaper examples are available whereas others can be quite expensive. Most are totally frictionless magnetic affairs as the examples below. Click on the images to compare and order your selection on-line.
Suspend your propeller on the balancer and carefully remove material from the heavy blade until it is in equilibrium. Don’t remove any more than necessary and be careful not to change the prop’s shape., Never remove material from the blade faces or reduce the length of one side. Alternatively, the light blade can be coated in fuel proofer to get the balance correct. When you’ve finished and it is perfectly balanced, write a ‘B’ on the hub in a permanent marker colour that contrasts with the propeller material to remind you that this one is balanced.
Keep The Noise Down
Depending on where you fly, noise may or may not be a major consideration. In the UK there are strict controls on noise emissions from model aircraft engines. You should check this out with the British Model Flying Association (BMFA) to ensure you don’t exceed the limits. There are also guidelines regarding the distance you are permitted to fly with relation to “noise sensitive buildings”. I believe there are similar regulations in the USA. The AMA will be able to advise you.
These regulations apply to all flying models but those powered by Glow, Nitro or IC engines are the most difficult to keep below the set levels. With many engines of this type, the noise generated by the spinning propeller can often be greater than that emitted from the silencer.
Propeller noise can be reduced to acceptable levels by selecting a propeller that allows the engine to turn at an rpm producing a rotational tip speed of less than 350mph. For the technically minded amongst you, the following formula enables you to calculate this figure.
Propeller tip speed in mph = [(3.142 x diameter in inches) x rpm] ÷ 1056
To achieve this figure isn’t always practical, but it’s a good figure to use as a basis. Noise reduction on many flying sites is now such a concern that some engine manufacturers’ .40-size engines are made to run on 10 x 8″ or 11 x 7″ size props instead of the more traditional 10 x 6″. All of this is aimed at reducing the tip speed of the prop and enabling overall noise production.
So there you have it. With a little experimentation you really can get the best from your model just by choosing the right propeller. Finally a word of caution:- Mind Your Fingers and Never use a propeller that has any sign of cracking or other damage. Even if not for your own safety, at least consider those around you.