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Radio
Control Model World - Apr '95
by
Stan Yeo
INTRODUCTION
Get
any seasoned group of modellers together discussing model aerodynamics,
particularly a gaggle of glider pilots and invariably a discussion
will evolve around to wing sections. Numerous articles are written
and even more hot air is expended discussing model airfoils than
probably any other related model topic. Unfortunately very little
of these discussions are understood by the majority of club flyers
yet it is important that all modellers, particularly glider flyers,
understand the fundamentals of lift production and how it affects
a model's performance. Understanding the rudiments of lift and to
a lesser extent drag will improve your flying and reduce the number
of 'arrivals' / crashes you have. This will have an impact on both
your wallet and your enjoyment of the hobby!
So
before you 'switch off' and turn the page let me reassure you that
this article is not a theoretical diatribe on the latest in model
section design but a discussion on the concepts of lift production
and the main factors affecting the characteristics of a section.
Typical accidents that are often the result of a lack of understanding
of lift and sections is the failure of a model to recover from a
high speed dive or a model falling out of the sky in light lift
conditions. Power flyers will readily recognise the dive recovery
accident whilst most slope pilots will have experienced a model
falling out of the sky in marginal lift conditions at some point
in their flying career. Both these types of accident are avoidable
if the limitations of a particular wing section are appreciated.
WHAT
IS LIFT?
Lift,
in simple terms is the force created by the pressure difference
of the air between the top and bottom of a flying surface i.e. a
wing. This pressure difference is caused by the air molecules on
the top surface of the wing having further to travel to reach the
trailing edge than those travelling along the under surface due
to the deflection of the air by the upper surface. For the technically
minded Bernoulli's theorem states that the sum of the energies at
any point in a fluid remain constant. Put simply this means that
if the air is travelling faster, as is the case of the air travelling
over the upper surface of the wing, then it has more Kinetic Energy
(KE = 1/2 x Mass x Velocity2) and less Pressure (Potential) Energy
than air upstream or downstream of the wing. It is the resultant
pressure differential that we call lift. Not all lift is produced
by the top surface of the wing, some is produced by the lower surface
as a result of an increase in pressure under the wing. The ratio
of lift produced by the top and bottom surfaces will vary depending
upon the wing section and the angle of attack of the wing (see diagram).
WHAT
AFFECTS THE AMOUNT OF LIFT PRODUCED?
There
are three main factors that affect the amount of lift produced.
1.
Wing surface area
2 Flying
speed
3.
Coefficient of Lift
There
is a forth variable, that of air density but unless you are going
flying at high altitude sites there is no need to worry about it.
Air density is only mentioned as it is part of the general lift
formula of which is:
LIFT
= 1/2 x Coefficient of Lift x Air Density x Velocity squared x Wing
Surface Area
Surface
Area & Velocity (Flying Speed)
Wing
surface area is self explanatory. The bigger the wing the more lift
it can produce. The same applies to flying speed. The faster the
air travels over the wing the more lift it produces except that
with speed the lift increases with the square of the velocity (KE=1/2MV2).
This means if we double the flying speed the lift increases four
fold. It is worth remembering this when adding ballast. Doubling
the weight will only increase the speed by approximately 40%. We
would need to quadruple the weight to double the flying speed.
Coefficient
of Lift
The
Coefficient of Lift (CL) is similar to the Coefficient of Drag (CD)
or Drag Coefficient which is often quoted for the aerodynamic cleanliness
of new car designs. It is a constant used to balance the lift equation
for the different amounts of lift that sections can produce. When
the lift coefficient is divided by the corresponding drag coefficient
(CL/CD) then a guide to the section's efficiency can be obtained.
Sections can have a maximum CL ranging from 0.5 to 2. The factors
affecting the CL of a section are:
1.
The camber of the section
2.
Section Thickness
3.
The Angle of Attack the section is operating at.
1.
The Camber of the Section
The
Camber of a section is the curvature of a section. It is important
because it determines how much the air travelling over the section
is deflected. The more the air is deflected the further it has to
travel to reach the trailing edge and hence the greater the reduction
in pressure as discussed previously. A flat plate section, with
a rounded leading edge, will have negligible camber and only deflect
the air a nominal amount and consequently only produce low lift
coefficients before stalling whereas a curved flat plate will deflect
the air considerably more producing higher lift coefficients (see
diagrams).
A bi-product
of lift production is drag. This is mentioned because there has
been alot of research in recent years on model glider sections in
an effort to optimise the camber of a section to improve the lift
and drag coefficients in order to produce a more efficient section.
The success of these endeavours is seen in the improved penetration
of today's thermal soarers. However, to realise these improvements
in section efficiency it is necessary to observe the section profile
to within a few thousands of an inch or fractions of a millimetre
if you prefer. This has led to the creation new wing building and
flying techniques in order to realise the full potential of these
sections.
2.
Section Thickness
Allied
to the camber of a section is its thickness. This is normally expressed
as a percentage of wing chord (width). A wing with a 10 inch chord
that is 1.5 inch thick has a thickness chord ratio of 15%. Generally
the thicker a section is the more camber it has and the more lift
it can generate. Increasing the thickness of a section not only
increases the amount of lift it can produce but also increases the
stalling angle i.e. the angle at which the airflow over the wing
becomes turbulent resulting in a dramatic reduction in lift. This
characteristic is often used on aerobatic models to prevent tip
stalling. Tip stalling is where the outer sections of the wing stall
before the inner sections of the wing. It is desirable that the
outboard wing stalls last so that lateral control (ailerons control)
is retained up to the point of stall.
3.
The Angle of Attack
As
the angle of attack of a section increases so does the lift coefficient
until the section stalls. This increase in lift is also accompanied
by an increase in drag. The drag generated by a section will, initially,
increase slowly relative to the increase in lift but as the section
approaches the stall so the drag will increase disproportionately
reducing the section's efficiency. A notably improvement in glider
sections in recent years has been the ability of some sections to
maintain their efficiency over a wide angle of attack range. The
angle of attack at which the stall occurs will depend upon the thickness
and camber of the section as well as the shape of the leading edge.
A section with a sharp leading edge will produce lower lift coefficients
and stall at a lower angle of attack than a more generously radiused
one with the same co-ordinates. The stall will also occur more rapidly
with little or no warning!
TYPES
OF SECTION
Wing
sections used on model aeroplanes can be divided into four distinct
types:
1.
Under-cambered
2.
Flat bottomed
3.
Semi-metrical or Asymmetrical
4.
Fully Symmetrical
These
sections can be further sub-divided into laminar and non-laminar
flow sections. This is a crude description as all sections are laminar
flow but the distinction I am trying make is the difference between
a Clark Y generation airfoil of generous camber with the thickest
part of the section well forward i.e. 30% of the chord back from
the leading edge and the more modern sections with less camber and
the maximum thickness point at 40 - 50% of wing chord. The difference
between the sections is that the early generation airfoils are capable
of producing very high lift coefficients at high angles of attack.
They are also more stable but have high drag coefficients, particularly
at these higher angles of attack. More modern airfoils in contrast
produce lower lift coefficients but are much more efficient due
to the markedly lower drag coefficients. They also need to be flown
at higher speeds / lower angles of attack to achieve the optimum
performance.
This
does not mean that the early airfoils have outlived their usefulness,
on the contrary they are ideal for trainers where their aerodynamic
abusability is a desirable asset. A good training model should give
the trainee pilot plenty of thinking time and not build up excessive
speed when out of control. All attributes of the early sections.
Another use for this type of section is on aerobatic power models
where excessive speed build up in a dive is undesirable.
Under-cambered
Sections
It
was not so long ago that slow flying under-cambered sections were
all the rage in thermal soaring circles (I once came forth in F3B
at the Nationals flying a Graupner Amigo!). Since then however things
have changed and this type of section is rarely used except on light
wind thermal soarers where the ability to make use of light lift
and not penetration is the prerequisite. The latest glider sections
do incorporate some under camber but they are also designed to be
flapped to achieve the best of both worlds i.e. a high rate of climb
in thermals and travel at high speed with minimum sink between thermals.
Flat
Bottom Sections
Flat
bottomed sections were once the workhorse of model airplane design.
They were used for both the wings and the tailplane but now this
section is only in widespread use on trainer slope soarers. It has
been replaced on power models by semi-symmetrical sections of generous
thickness. Whilst for tailplanes it is normal to use a flat plate
section.
Semi-symmetrical
Sections
These
are now the most used sections. They are surprisingly efficient
and have predictable handling characteristics. The venerable Eppler
374 is a favourite of mine and I have yet to find another section
in this class which has the same all round performance.
Fully
Symmetrical
This
type of section is used on fully aerobatic models and tail surfaces.
It is low drag but also low lift. If using the flat plate variant
on an all flying tailplane it is worth remembering that the section
stalls at angle of 8 to 10 degrees so there is no point in setting
the tailplane up with any more angular movement than this.
HOW
DOES THIS AFFECT MY FLYING?
Poor
judgement in selecting a section / model and a lack of understanding
of a section's characteristics can have an impact in a number of
areas. If you are a beginner it could affect the longevity of the
model and the speed at which you learn to fly. If you are an accomplished
flyer it could be the difference between a run of the mill model
that does nothing in particular to one that dreams are made of.
Tyro pilots get themselves into trouble at regular intervals and
if they are not I would suggest that they are not making progress.
This often results in panic control movements that are very demanding
of the wing section. If the section is not capable of producing
the required response without complaining then there is the risk
of a disaster. Thin, low cambered, sharp leading edge sections are
not capable of producing the required results and should therefore
be avoided.
Sometimes
it is necessary to use a high performance section to achieve the
desired performance, particularly on gliders. If this is the case
and these sections are used the pilot must be aware of the section's
characteristics and limitations. This way not only can the optimum
performance of the model can be realised but the risk of a flying
accident greatly reduced. When flying the model control inputs should
be planned in advance and applied progressively and not pulsed.
The model should be allowed to fly at its optimum flying speed i.e.
with a slightly nose down attitude in the case of a slope soarers,
and not with its nose in the air on the back-end of the drag curve.
That is a sure way to end up down the bottom of the slope with a
pile of bits.
As
modellers progress onto higher performance aeroplanes there is sometimes
a pre-occupation with reducing the drag generated by a model. This
is often done by designing models with thin, low drag wing sections
that have sharp leading edges. These sections invariably have low
maximum lift coefficients and whilst they may be very good at flying
fast in straight lines they are not so successful when it comes
to performing aerobatics. Here there has to be a compromise between
section efficiency at a fixed angle of attack and the need to be
able to produce high lift coefficients during aerobatic manoeuvres.
SUMMARY
Once
again I have tried to squeeze a quart into a pint pot and deal with
a complex topic in an easy to understand way without becoming too
technical. I hope I have succeeded and the article has added to
your understanding of the basic concepts of lift and sections. To
my critics please forgive my simplistic approach, we all have to
start somewhere!
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