I have been asked by somebody to share my thoughts on rotor
blade design. As the email was turning into a lengthy page spiel I
thought I might as well post it here too for those of you battling
with insomnia!
I don't pretend to be an aerodynamicist or physicist, and this is all
from my personal experience. Factual errors and c*ck ups are
apologised for in advance and will be corrected in due course
Rotorblades
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?
Section
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.
Chord
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.
Thickness
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.
Weights
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.
Sizes
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.
Materials
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.
Construction
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.
Bolt Hole
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.
Conclusion
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
section, 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.

I have been asked by somebody to share my thoughts on rotor

blade design. As the email was turning into a lengthy page spiel I

thought I might as well post it here too for those of you battling

with insomnia!

I don't pretend to be an aerodynamicist or physicist, and this is all

from my personal experience. Factual errors and c*ck ups are

apologised for in advance and will be corrected in due course

Rotorblades

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?

Section

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.

Chord

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.

Thickness

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.

Weights

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.

Sizes

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.

Materials

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.

Construction

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.

Bolt Hole

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.

Conclusion

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

section, 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.