Written by Mark Christy Tuesday, 02 February 2010 17:15
Rotorblades can come in a bewildering range of sections, chords, thickness, weights, sizes, materials, construction and varying C of G's. Some even came with adjustable bolt hole positions.
But what does this all mean and why the variances?
The section of the blade refers to its airfoil. These come as lifting section, semi-symmetrical and symmetrical.. you may even hear the term reflex and washed-out thrown in for good measure. The terms lifting section, semi-symmetrical and symmetrical are fairly self explanatory and refer to the shape around the horizontal centerline. Lifting section will generally have a flat underside and curved top. Semi-symmetrical will have a slightly curved underside and symmetrical will be the same top and bottom. Lifting section blades are generally very rare and most are of a semi-symmetrical design anyway. They are good for scale and large models which spend all their time upright as they are the greatest lift generating type.
Symmetrical are ideal for those machines that spend as much time the wrong way up as they do the right way up. They generate equal lift in both directions but therefore require more pitch to generate the required lift and therefore more drag when compared to lifting section blades. These are the most common type of blades available and are ideal for 3D style of flying. Semi-symmetrical tries to deliver the best of both worlds. Most modern designs are almost symmetrical in appearance. These blades are very popular for F3C flying where the flying is split 70/30 in terms of upright and inverted flight and where a fast forward speed is desired.
Finally we have reflex. This can apply to any section type and refers to the shape of the blade past the center line when considering the section of the blade front to back. A non reflex blade will follow a curve or straight line to the trailing edge. Where as a reflex blade will curve sharply towards the core of the blade before "reflexing" back to create an almost "S" shape to the blade as it approaches the trailing edge. This design increases the efficiency of the blade and in the late 80's early 90's the Sitar semi-symmetrical reflex section blade was very popular for autorotation competitions!
Washout is where the blade literally twists towards the tip to reduce the angle of lift. This is because the tip travels faster than the root and generates more lift. By adjusting the blade angle in this way you can help equalise the lift along the length of the blade. This design is generally only seen on lifting section or semi-symmetrical blades.
The chord refers to the distance from the leading edge of the blade to the trailing edge. The larger the chord of the blade the greater the lift generated per angle of rotation however the level of drag will also increase. Some blade designs feature tapered chord where the blade gets narrower towards the tip. This is for the same reason as washout. By adjusting the chord in this way you can help equalise the lift and drag along the length of the blade.
Refers to the maximum distance between the bottom of the section of the blade and the top. Thickness obviously relates to efficiency but also effects the change in section available to you. Thin blades are restricted in the angles of change in the section where as thick blades can have much greater variance.
This is possibly the simplest to explain. The heavier the blade the greater the potential energy and the greater energy required to swing the blade. Heavy blades will reduce the control response of the model and the ultimate roll rate. They will however autorotate the best.
Why do the same size models have the ability to run different length blades? Obviously the larger the blade the greater the lift, drag and the less the blade loading (weight of model to blade area). A long blade will create a more "floaty" model which will autorotate well. However it will also be more susceptible to gusts of wind and require more power to spin.
Blades are available in wood, glass or carbon fibre forms. Wooden blades are very rare now as composite technologies have taken over. They generally required finishing and balancing and were limited in design so as not to compromise their strength. They must not be spun too quickly owing to a lack in structural strength inherent in wood.
Glass and carbon blades are now almost exclusively all that are available. Glass are generally cheaper owing to their lower material cost. However they are more flexible. This allows them to have a "softer" response feeling compared to carbon blades. The most rigid blades are made from carbon and the most widely available.
This refers to the core material and the lay up of the composite material. Glass and carbon blades feature either a foam or wooden core. Wood will create a more rigid blade. The lay up of the composites can effect the rigidity of the blade along its span and its chord. For hard 3D flying it is desirable to have a blade that is rigid in all directions. For stability its better to have a blade that has some flex along its span to allow the model to have a natural coning angle where the blades flex up from the root to tip in flight. Generally it is not desirable to have a blade that flexes along its chord as this will simple reduce the movement of the blade for a given input and can be replicated by reducing the initial control throw.
C of G
The center of gravity of the blade refers to the balance point of the blade along its span and chord. The nearer the trailing edge the C of G the more aggressive and responsive the blade. The further forward the more stable. Similarly with the span wise C of G. The further towards the tip of the blade the more inertia the blade can store and therefore the more docile in its response and the greater its autorotation ability.
Altering the bolt hole position can have a dramatic effect on the response of the blade. When the blades are spinning the C of G pulls inline with the bolt hole position of the blade. Therefore if the bolt hole is moved towards the trailing edge of the blade then the blade will run in a swept back position. This will make the blade more stable and less responsive. Moving the hole forwards will do the opposite.
As can be seen there is a lot that goes into blade design depending on the type of blade required. Agressive 3D blades feature a symmetrical section, rigid carbon lay up, a light weight and a rearwards C of G. F3C blades feature a semi symmetrical or fully symmetrical ection, a high weight and a forwards C of G. Much like paddles and flybars changing two different attributes can have the same effect but different side effects and a careful combination has to be found to get the exact control response and stability from a blade.
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