Designing For Competiton

With the introduction of the glass-fiber construction, the development of sailplanes has seen enormous progress during the last five years. Since sailplanes of 22-meter span and a gliding ratio (L/D) of about 50 are flying already, the question arises as to what further advances are possible in the future.

The following statements shall be an attempt to show how far the aerodynamic parameters such as span, aspect ratio, airfoil polar, Re-numbers, and wing loading affect the design of new competition sailplanes and which directions the development possibly will take in the next years.

To judge the performance of a sailplane In competition it is necessary to determine its average cruising speed in special weather conditions in comparison with other gliders. Such comparisons make evident that the maximum L/D, no doubt, has an influence on the aerodynamic quality, but must not decide the performance of a sailplane in competition, since a cross country flight consists of climbing at low speeds and gliding at high speeds.

Knowing the polar of a sailplane, the best average cruising speed for the respective average climb can be determined easily.

The derivation can be found in many references on performance soaring flight.

V_cruise =                V_glide  
            ----------------------------------------
                             V_glide                    
             1  +  ---------------------------------
                       e + (V_climb + V_sink)

e = glide ratio 

1. The Influence of Thermal Types on the Climbing Performance.

Assuming uniformity of pilot's qualification, the performance of a glider is not judged from its polar only. It is also of great importance to know the climbing qualities.

A comparison of climbing performances in thermals depends highly upon the form of thermals. Referring to Carmichel we differentiate three forms of thermals which are found most frequently in competitions.

You will normally find strong thermals associated with strong wind after a front has passed. They are very narrow. Wide thermals are found in continental weather conditions. An optimum design of a competition sailplane is possible only for one of these forms of thermals. An adaptation to other forms often can be obtained by carrying water ballast.

The utilization of the best rate of climb depends to a great extent on the wing loading, the angle of bank, and the lowest circling speed. From these parameters the optimum climbing velocity can be found for a certain radius.

The cruising between thermals, of course, must be done with McCready speeds.

2. The Influence of the Wing Airfoil.

The total drag (CD) of a sailplane is composed of

CD = CD(parasite) + CD(airfoil) + CD(induced)

It can be seen in Fig. 3 that the influence of the different drag coefficients varies with the speed.

Whereas the induced drag mainly influences the low speed flight and the parasite drag the high speed flight, the airfoil drag however is effective upon the high as well as on the low speed. It is therefore important to choose an airfoil with a wide laminar bucket considering normal weather conditions during competitions.

Camber flap airfoils of course comply best with the demand for a wide laminar bucket and low drag.

Other than ember flap airfoils, Professor Wortmann and Professor Eppler have also developed normal airfoils without flaps which, with regard to the laminar bucket and drag characteristic, are clearly superior to the former NACA airfoils.

3. The Influence of Wing Loading.

The wing loading has a great influence on the climbing as well as on the gliding performance of a sailplane, both of which determine the average cruising speed. In any case high wing loading means a loss in climbing performance, whereas the high speed qualities are always improved.

The assumptions under which a special wing loading can be regarded as optimum are shown in Fig. 5 and Fig. 6. These curves have been derived from a method developed by Quast and Thomas of the Akaflieg at Braunschweig.

As already mentioned before, a high wing loading is always disadvantageous in respect to the climbing performance, more in strong or weak thermals than in wide ones. In wide thermals where the radius of the circle is not so important, this influence is reduced.

Surprisingly, the wing loading has a rather low effect on the optimum cruising speed which is more influenced by weather conditions, but it turns out that it is possible to fly with a wing loading of 40 kp/cm 2 (8.2 lb./sq.ft.) or even more in wide thermals, as in Texas, For a short task especially, under the best conditions of the day, high wing loading is of great importance for optimum final glide.

When carrying water ballast it is always necessary to check whether the thermals are of a wide or of a narrow nature. In strong narrow thermals it is often useful to drain some of the water.

4. The Influence of Span and Aspect Ratio

The induced drag of a glider depends upon the aspect ratio of the wing. As we have seen from the foregoing figure, the induced drag is of great importance with regard to the low speed qualities and also with regard to the best gliding angle.

Whereas extremely high aspect ratios could not be realized in the past due to structural requirements, there are no problems of this kind in modern glass-fiber construction.

As we see from later figures, the choice of the optimum aspect ratio essentially depends upon the span of a glider. Taking for instance a Standard Class sailplane with a span of 15 meters, an increase in the aspect ratio is necessarily associated with decreasing the wing chord.

Decreasing the wing chord results directly in a lowering of the Re-number.

Decreasing Re-number however increases the airfoil drag coefficient rapidly as shown in Fig. 7. This is true especially for Re-number lower than one million.

Consequently, on Standard Class sailplanes, the airfoil drag coefficient at a certain aspect ratio increases to a greater extent as the induced drag coefficient is decreasing. The total drag increases again at a certain optimum aspect ratio, as Fig. 8 shows.

A further increase in the aspect ratio therefore would be associated with a loss in performance.

Open Class sailplanes, however, offer other interesting possibilities. Here a higher aspect ratio always has a positive effect, because it can be obtained by increasing the span, while the wing chord and therefore the Re-number are kept constant. This means also no variation in the airfoil drag coefficient.

Thus the designer has the alternative of increasing the span and therefore improving the performance, but the sailplane would become heavier and more expensive. Therefore, it is not advisable at present to exceed 22 meters (72 feet) of span, even when employing new material like Carbon or Boron fibers.

An exact determination of the optimum aspect ratio for a certain span however is not so easy because of the differences in the best values at high and at low speed.

As can be seen from Fig. 9, a high aspect ratio is needed in the low speed range, whereas a low aspect ratio is optimum at high speed. This leads to the desirability of changing the span in flight, as the birds do, in order to obtain the best possible flight performance. Here the future lies ahead and offers the designer a wide field of research.

Fowler flaps which lower the wing loading for circling in thermals result in an undesirable variation of the aspect ratio and therefore are effective only in extremely strong and narrow thermals. In normal thermals (even Texas thermals) where circling with a small radius is not required, the increase in airfoil and induced drag causes a remarkable loss in climbing performance (see BJ-4).

It appears advantageous to design modern sailplanes with an aspect ratio for the average speed. This means for a CL value of about 0.7, which approximates the best glide of a sailplane.

As presented in Fig. 10, it is not advisable to design Standard Class sailplanes with an aspect ratio of more than 22, a value which is already commonly used today.

For a span of 22 meters (72 feet) however, an optimum aspect ratio of 31 has been found. This result is exactly realized on the Nimbus.

5. Conclusions.

The foregoing statements show, that the optimum values are already attained for Standard Class as well as for some special Open Class sailplanes. It can be taken as certain, that the sensational improvements in performance of the last years, based on the introduction of new material, cannot be expected in the near future. It does not seem likely that increased performance will come about even when using brand new material like Carbon or Boron fibers unless new airfoils can be developed which associate low Re-numbers with considerably lower drag coefficients. Professor Wortmann says that there is very little chance.

In the Open Class, of course, the possibility exists to develop competition sailplanes with still greater span or variable span. Such gliders. however, are very expensive and become very much more difficult to fly.

A handicap factor has been introduced in Germany for the next Nationals by which sailplanes with a span greater than 20 meters will get a score deduction of 1 percent per meter of exceeding span. If this regulation should become official world-wide (Germany will apply for it on the next FAI meeting in March), it would become unattractive for the designer to engage in great span developments in the future.

The acceptance of the aforementioned span handicap factor would indicate to me that national and international competitions are calming down, and that the modern Open Class sailplanes probably would aim at a span of around 20 meters.

Questions And Answers

Answer: Fowler flaps have remarkably higher lift coefficient, of course, and also much higher profile drag, and in normal thermals the high drag coefficient is worse than the better lift coefficient. Therefore, I feel that it fails to gain performance with the use of Fowler flaps in low speed circling. And you also have much lover aspect ratio since you increase your wing chord and your wing area. The lover wing loading does increase the climb performance but I feel that in normal thermals you lose more than you win. In very narrow thermals or extremely strong thermals, as in South Africa, you may be ahead in this case; but in normal conditions-no.

Question: (Bill Foley) In the British Sigma project they have a mechanism to increase the wing area supposedly without this increase in drag. Would you care to comment on the Signs project?

Answer: It is very difficult to give a complete answer. I know this profile that they are using on the Sigma very well. It looks great if it works on a glider. They will, however, lose performance because they are increasing the wing chord. But they will not lose so much performance with higher drag coefficients. So there may be a chance that this way is better but as I have discussed often, I don't believe in the project for I think the changing the wing when you are entering a thermal is so difficult that you cannot do it in an optimum way. It is already difficult with camber flaps.

Question: (Leo Buckley) Can you say, Klaus, why you feel the CL equal to 0.7 is the optimum number you should use?

Answer: You must compromise. The lift coefficient of about 0.7 or 0.8 is a coefficient where you are in the middle of the usual flying range and applies for very weak weather conditions and on your final glide.

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