Propping Up

When learning to fly R/C aircraft in the late ’70s, advice concerning propeller selection was brief: ‘If it’s a 40, you’ll need a 10 x 6 and if it’s a 60 you’ll be wanting an 11 x 7″. That was it. Black and white, with no shades of grey. As my experience grew I discovered that this was a very blinkered way of looking at the prop choice conundrum, and when I began to fly competitively I realised that these sizes were miles off the mark, learning that models should be propped for the airframe and not the engine. Before delving into this in more detail we first need to know a few things about propellers themselves.

Prop size is designated by two numbers, e.g. 10 x 6, 11 x 7 (read as ten by six’ and ‘eleven by seven’). Measured in inches, the first number indicates the exact diameter of the rotating prop arc, whilst the second number is the pitch. The pitch is the theoretical distance the propeller would travel forward during one complete revolution. A 10 x 6″ propeller measures 10″ in from tip-to-tip and would theoretically go forward 6″ during one complete revolution. The pitch measurement is theoretical because a propeller isn’t 100% efficient, nor is the air it’s travelling through; air compresses as the prop goes through it, and the prop’s shape, material and finish all influence its efficiency during flight. Propellers at the softer end of the scale may flex and twist with increased loads, reducing their efficiency over stiffer examples. All of this means that the 6″ pitch of our airscrew is just a guide, and in reality isn’t easily translatable into distance at all.

You also need to consider that a propeller is actually nothing more than a wing with an aerodynamic section set at a specific angle of attack (the pitch), which is forced to generate lift by being pushed through the air by the rotation of the engine. Each part of this ‘rotating wing’ will have a different airspeed, increasing nearer to the tip. In order to ensure that each part of the prop generates the same amount of lift across the span, the ‘wing root’ at the prop hub is set at a higher angle of attack than the ‘wing tip’ (hence the ‘twist’ in the prop).

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Faced with the ever-increasing availability of sizes, types and makes, how on earth do you decide which to use?

Selecting the correct prop is just as important as the choice of engine or achieving the correct C of G, as the propeller size will fundamentally affect the way that your model flies. Choosing the wrong one may damage your engine, and your first point of reference should always be to the manufacturer’s instructions. There will usually be a number of choices available as manufacturers recognise that a model’s size, drag, weight, wing loading, engine type, fuel and even altitude are all important factors for correct prop selection. Tables 1 and 2 are a good starting guide for general flying, but note that props for high speed flight, 3D flight or specialised applications can’t be generalised so readily.

Another vital consideration concerning prop choice is its material, which must be suitable for the expected operating rpm of your engine. For general sport models with motors turning props at up to 12,000rpm, glass reinforced plastic (GRP) types such as Bolly Clubman, APC or Master Airscrew will suffice. For higher rpm, or on larger motors where the forces are significantly greater, wooden props or even carbon fibre props should be considered. Many large petrol motors will also use wood or carbon props as moulded GRP versions rarely come in the bigger sizes (and the ones that do are prohibitively expensive). Mind you, there are disadvantages. Wooden props are machined from a solid piece of timber and are rarely as thin or efficient as a moulded ditto, despite their increased stiffness. Carbon props give better performance but don’t break as easily as wood in an accident, transferring any shock loads back to the engine and often damaging the crankshaft or even the engine mounting.

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A propeller will usually need some preparation before being bolted in place. Some require more work than others, with the injection moulded GRP jobs requiring more than most. The first thing required with these is to remove the sharp edges. Don’t sand GRP props to remove the moulding ‘flash’, scrape the edges with a sharp tool (e.g. Stanley knife) with the blade held perpendicular to the edge of the prop.

Once smooth, we need to look at the hole in the middle, which in many cases will need enlarging to fit your engine’s crankshaft. Always use a special-purpose prop reamer or drill press to enlarge the hole, being careful to make it straight and perpendicular to the prop hub. Never file or hack your way through with an oversize screwdriver. A rotating propeller makes a considerable flywheel, and anything causing it to run out of true will produce vibration and wear problems elsewhere. With this in mind, all propellers should be balanced before they go anywhere near an engine!

Balancing can be achieved by using a commercially available prop balancer. Cheaper examples are very simple and designed to be held in the hand, whereas others are totally frictionless magnetic affairs that can be quite expensive. Balancing a prop is simply a matter of suspending it on the balancer and removing material from the heavy side until the thing is in equilibrium. Don’t remove any more than necessary, be careful not to change the prop’s shape, and never remove material from the blade faces or reduce the length of one side. Wooden props can be lightly sanded rather than scraped, or the light blade can be coated in fuel proofer to get the balance correct. When you’re done, write an indelible ‘B’ on the hub to serve as a reminder.

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Right then, before looking at specific choices, given what I said earlier about the prop fundamentally affecting the way in which your aircraft flies, I reckon an analogy is in order.

Think of your prop as a gear on a car that you can’t change all the way through your journey. Let’s consider the base elements and break that journey down. You’re going to need to set off (or with your model, take-off) from a standing start but would probably want some decent speed at the top end, too. Fifth gear (or in prop terms, a very high pitch) would be out of the question. Sure, you’d be going fast eventually if you could get off the deck in the first place. First gear (a shallow pitch) would get you off the mark quickly but you won’t be flying anywhere fast and would soon reach maximum speed and rpm. You could still get away in second gear and would go a little faster at the top end, but third would be tricky (though still manageable) with a long distance required to get up to cruising (take-off) speed.

Generally speaking a higher pitch prop will pull the model faster in level flight whilst a lower pitch prop will cause it to take-off in a short distance and climb quicker and easier. Some full-size aircraft have adjustable pitch props so pilots can use the most efficient pitch for each situation, determining the pitch for take-off then climbing to altitude before adjusting to a higher pitch. Lets look at a few model related cases in point:

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Basic trainer
You’re not going to go far wrong by using the suggested prop sizes in the charts. For a low-powered .40 – .46 I’d usually look to fit a 10 x 6″, 10 x 7″, 11 x 5″ or 11 x 6″, using the shallower pitch if I needed extra pull for take-off. If you fly from long grass or the model is heavily built, this might be something to consider. For more powerful motors I’d favour the higher pitches, adding an inch or so if the motor could take it.

Fun fly
The BMFA Fun Fly class requires four distinctly different flights to be made with the same model, and good prop selection here can make all the difference. Let’s take each task in turn to illustrate a point, with the example model having a .46 up front.

To be competitive you have to loop the limbo gate. Your model must accelerate and decelerate quickly, have good engine pick-up and a reasonable climb. I’d use a wooden 11 x 4″ or 11 x 5″ prop: the large diameter gives good airbraking at idle, exploiting the lightness of the model to decelerate rapidly, whilst the shallow pitch ensures the quick acceleration and climb needed to loop the limbo gate. The prop’s lightweight wooden construction allows the engine to speed it up and slow it down quicker than if it were heavier and moulded. The main disadvantage is that the wooden prop will surely break the first time you hit anything, but as the object of limbo is to miss the floor, you hopefully won’t be interfacing with it!

Touch and go
Much of the same applies here except that the intention is to hit the floor every time. Consequently I would choose a moulded GRP 11 x 3″ or 11 x 4″; the heavier prop would mean slower pick up, but the shallower pitch should compensate against this a little. I’d use the same prop for the Triple Thrash task, which also includes a touch and go manoeuvre.

Climb and glide
Here, take-off leads straight into a prolonged climb and you don’t need to throttle the motor until you shut it off at the top, so throttle pick-up and good idle are less important. What’s needed is a hard, aggressive, endless climb, and a GRP moulded 12 x 5″ prop is best in this situation. These pull very hard on a .46-size motor. The extra diameter takes a bigger bite of the air whilst the shallow pitch ensures a good climb, but the motor won’t throttle well and in level flight at full throttle the tips may go supersonic, making a very loud ‘barking’ noise.

High speed model
If you want to fly fast, you need a powerful motor and high revs, typically in the high teens of thousands. This demands a smaller diameter prop, typically 9, 8 or even 7″ on a .46. High speed also means high pitch of around 8 to 10″, which does present a problem. These models are usually hand launched, but setting off in such a high gear makes this very tricky and a compromise is required. A drop in pitch to say 7″ will get the model away much easier, but top-end speed will be slow. You can also drop the diameter towards the lower end to get the revs up, but you won’t be grabbing as much air. You really need to look at your engine. Most manufacturers will quote peak horsepower at over 15,000rpm, so don’t be afraid of getting some revs on with a smaller prop, however higher-end engines designed to turn faster will always be best for this type of flying.

Glider tug
Much of what’s said here also applies to 3D models as the flying characteristics provided by the prop are very similar. Good acceleration and a hard, endless pull are equally useful for dragging gliders off the deck as they are for prop hanging. Watch Steve Holland at a show, aerotowing six gliders at a time with his Titan, often combining the two very different flying styles. A large diameter and shallow pitch are what’s needed, typically an 11 x 4″ for a .46, a 13 x 6″ for a .60 right up to, perhaps, a 16 x 4″ for a 1.20 four-stroke (large petrol engines might run pitch as low as 6″). These props provide rpm near the power band of the motor, assisting the throttling and giving plenty of prop wash over the tail feathers for control at low or zero air speed.

There’s one more very important aspect of prop selection to consider, one that might throw everything else I’ve said out of the window. Noise. The Department of the Environment has a code of practice for the noise emission from model aircraft (it’s reproduced in full in your BMFA members handbook).The figures recommended are many and varied, but the underlying aim is to restrict models to a noise emission of 82 decibels measured at a distance of 7 metres. I’ve done many tests in this area both at club level and nationally for the BMFA and one thing’s for sure; propellers are noisy! In most cases on engines over a standard .40, the noise made by the propeller is louder than the noise made by the exhaust. You can reduce this noise, but you have to accept that this may be to the detriment of the desired flying characteristics of your model.

To keep the propeller noise down you need to select an airscrew that allows the engine to turn at an rpm that gives a rotational tip speed of less than 350mph. Achieving this figure isn’t always practical, but it’s a good figure to use as a basis. Noise reduction on flying sites is now such a concern, it’s interesting to note some engine manufacturers acknowledging that today’s .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″. Irvine recommend running their Q40 on a 10 x 9″ APC, which is a very high pitch for a .40. All of this is aimed at reducing the tip speed of the prop and overall noise production.

As we’ve established, gone are the days of screaming .61s on 11 x 7″ props. You’re now more likely to find them turning at least 12 x 9″. In the mid-1990s, sixty-size FAI aerobatics engines were quite capable of turning props of 12 x 12″ and above with ease, and this development has continued with the larger engines currently common in the class, turning props of around 16 x 14″. Again, development is aimed at lowering the tip speed and hence the emitted noise. There’s a useful prop tip speed chart for an easy reference in Table 3, but if this doesn’t suit your circumstances you can use this formula:

Propeller tip speed in mph = [(3.142 x diameter in inches) x rpm] ÷ 1056

A brief note on petrol engines. Even small petrol engines (and bigger glow motors) can be fitted with propellers of around 22″ diameter, and if under-pitched these can reach as much as 10,000rpm as the engine unloads in the air. A quick calculation shows that such a 22″ propeller turning at 10,000rpm will have a tip speed of 655 mph! Compare this with a 10″ diameter prop at the same rpm, which has a tip speed of just 300 mph.

The material of the prop’s construction also raises its head in the noise debate. The general rule is that the prop should be as stiff as possible. Accordingly, the best props are the carbon / epoxy types, which are the stiffest you can buy but hellishly expensive, too. They’re also not generally available in smaller sizes. Wood and well designed, stiff GRP props come next, providing a wide range of propellers from which to choose. For general sport use the modern GRP, scimitar-styled props such as APC, the Bolly Clubman series or the new Graupner G-sonic range have proven to be amongst the quietest props available. Some modellers avoid using these because they’re prone to breaking about an inch from the tip in a bad landing, opting instead to use the very popular softer construction props such as the Graupner Grey or Master Airscrew. Although quite reasonable at low to medium revs, some of these propellers can produce high noise levels as the engine begins to unwind. This can be down to the shape of the blade tip, where square-tipped props are without doubt noisier than one with a scimitar-shaped blade.

Before I finish, a quick word about multi-bladed props. The more blades there are, the less efficient itll be. The reason for adding a blade is, in most cases, ground clearance. Look at the full-size Spitfire, for example. The early prototypes had two-bladed props but as the war progressed and more power and airspeed was demanded, blades were added to soak up the extra power whilst the airframe largely stayed the same. By the end of the Spitfire’s reign versions were flying with 5-bladed props! Aircraft such as the Chance Vought Corsair got around this problem by jacking up the fuselage centre line, giving the plane its famous Gull wing; all to increase ground clearance and save an extra propeller blade whilst keeping high efficiency and range.

Whilst multi-blade props aren’t generally available in smaller sizes they’re sometimes popular with large scale aerobatic modellers wishing to properly replicate the full-size look. However, many a modeller has spent a small fortune in this area only to be disappointed with the flight performance.

So there you have it. With a little experimentation you really can get the best from your model with nothing more than a simple prop change. A final word concerns safety. Never use a propeller that has any sign of cracking or other damage. If you don’t care too much for your own safety, at least consider those around you. Oh, and mind your fingers!

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