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Prepare for Lift Off | Back to Article Index

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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