Why Fit Larger Wheels to your Model ?

Dec 14, 2012   //   by Nigel   //   Tech Info  //  No Comments

Why Fit Larger Wheels to your Model?

Most kits and ARTF models are manufactured down to a price. A price that will offer good value for money and be price competitive against other suppliers. An unfortunate result of these “competitive prices” is the wheels supplied have become much smaller over the years.

If you look at powered free flight and early radio controlled model aircraft in the 1930’s, 1940’s and 1950’s, they all had one thing in common. They all had large wheels mounted on undercarriages close to the front of the model.

Why was this??…. In those far off days, ready made propellers were rarely available and good ones made in wood, were very expensive. Most modellers had to make their own. So large rubber wheels were necessary to prevent the propellers from breaking when landing. And also, many flying competitions stipulated that the models had to “rise off ground”

Not many model flying clubs had access to tarmac runways or finely mowed grass. Most clubs flew from open heathland or farmer’s fields. This is often still the case today where many clubs, including our own, fly from mown grass areas in fields. Which can take many years to convert ordinary grazing pasture to the texture and flatness of a good lawn. So back to the problems of having small diameter wheels, as fitted to so many models.

Many wheels are also made from very hard foam rubber, which although is light in weight, has little “give” to absorb the shock of heavy landings. These landing shocks are then transferred to the airframe structure, which is a point of failure in many current ARTF models. The wooden plywood structures supporting the undercarriage is often so lightly built, that it shatters if any landing is “less than perfect”

Take Off’s

Small wheels have to rotate faster for any given ground speed than larger wheels. This increased rotational speed has the penalty of increased axle friction which will also slow the plane’s rate of acceleration. This increases the length of take off  run before flying speed, and a successful take-off, can be achieved. Small wheels cannot roll over small obstacles, whereas larger wheels will.

ie:        If a 2”(50mm) diameter wheel meets a 1” (25mm) high obstacle it will stop dead !

However, if a 4”(100mm) diameter wheel meets a 1”(25mm) obstacle, it will roll over it.

This is particularly true of large, soft rubber, hollow air filled wheels, which will also absorb most of the shock and prevent much of it being transferred to the airframe. There is also a phenomenon known as “prop-walk” where the tip turbulence of the propeller causes an increase in air friction, at ground level.

This also interacts with the torque required to turn the propeller, and makes a plane extremely difficult to steer straight on take-off. This usually results in the well known, “Ground Loop” as often experienced by pilots of full sized, tail wheel aircraft.

Hence the phrase: ”Real pilots fly tail-draggers….but….Real tail-draggers fly the pilots !!”

Fitting larger wheels also enables larger propellers to be fitted without making any structural modifications. So why fit a larger propeller ? 

Larger Propellers

Engine manufacturers always try to advertise the most powerful engines for their capacity. But the quoted maximum horsepower is often dependent upon achieving very high engine speeds. This is ok until you need to fit the best available “airscrew” to pull, (or push) your model through the air.

On full sized aircraft, the propeller is fitted to suit the horsepower required to give the best cruise performance and/or top speed. Then, a constant speed hub or a dual pitch propeller is fitted. This enables fine pitch to be selected for good acceleration on take-off and a good climb out, then provides a coarser pitch for improved cruise and top speed performance. This is because, at low speeds, a coarse pitch propeller is not efficient, as it will create a lot of turbulence and will absorb a lot of engine power without developing effective thrust until the plane is actually flying at cruising speed.

This can be seen in the very first pre-war squadrons of Spitfires and Hurricanes which were fitted with fixed pitch, wooden two blade propellers. When these were eventually fitted with constant speed, adjustable pitch propellers, take off runs were then much shorter and their rates of climb were significantly improved.

If a small diameter, high pitch propeller is fitted, the high speed airflow along the fuselage creates backwards drag during takeoff, so reducing acceleration and extending the take-off run. The larger the propeller diameter, the greater an area of air can be “pulled” by the “Airscrew”. which gives a more laminar airflow along the fuselage and  the wings and tail surfaces.

As a fundamental rule, the larger the fuselage cross sectional area, the larger is the propeller diameter necessary to achieve the most efficient thrust. Variable pitch propellers for model aircraft are extremely expensive, as well as being easily damaged!! So we have to make the best compromise, between achieving the best takeoff performance and the best flying performance.

Just like full sized aircraft, an engine must develop sufficient power to pull the model through the air fast enough to fly. A larger diameter propeller will pull, (or push) a larger diameter “tube” of air than a smaller propeller. The pitch (blade angle) of the propeller must then be selected to pull, (or push) the air fast enough for the plane to fly, AND for the engine to achieve it’s optimum RPM to do this.

Now this is when an engines maximum torque (Break mean effective pressure/BMEP) can be used to good effect. Rather than achieving maximum power revolutions per minute, on the ground, let the engine achieve this when flying. Thus, using a larger propeller with a finer pitch will give your model better ground acceleration with an improved rate of climb. The engine revs will rise after takeoff and the engine will then develop even more power and the plane will pull through large aerobatic manoevers even better.

As most of us do not race pylon racers, most of us are not concerned with achieving a model’s maximum speed when flying.

So which propeller to Use ?

If you have .20 to .60 sized, slower cabin models, trainers, and/or WWII type bi-planes, a 4” pitch propeller will give a good all round performance, with a larger diameter propeller.

For faster .20 to .60 sized trainers and aerobatic sports models, a 5”to 6” pitch propeller should suit most flying applications with a larger diameter propeller.

With very fast .20 to .60 sized aerobatic and very fast sports models, such as deltas, 6” to 8” pitch propellers are required to achieve the necessary high flying speed.

With larger engines, which normally achieve their best torque and power at much lower RPM, higher (coarser) pitch propellers are required to achieve the best flying speed with even larger diameter props to absorb the power of the engine.

Landings

So now the takeoffs are quite good, and your plane climbs well and pulls through aerobatic manoevers without falling out of the sky…………So what about the landings.?

A large, fine pitch propeller will also act as an airbrake when the engine is slowed for landing. The propeller will then be rotating at less rpm than the airspeed requires, and so a steeper landing approach can be made without a significant increase in airspeed. This is when fitting larger wheels to give your propellers more ground clearance and less rolling resistance will also prevent your models nosing over, due to the wheels “digging–in“ on landing.

So how much do large pairs of rubber, lo-bounce, air wheels cost ?….£4.00 to £6.00 ??

AND, how many propellers will they save from damage, at £4.00 to £8.00 plus, per propeller??

It’s a “No Brainer” as well as preventing  damage to the airframe if any landing is less than perfect.

These guys can’t be wrong ? “ http://www.sportpilot.tv/video/bush-pilots-in-Alaska/  “

 Fitting larger wheels will help your models fly better and save you Money

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