Challenging the Wind |
Introduction
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 Robbert van der Bosch, Netherlands, and his Tsunami in 1996 He is using an engine from engine specialists Metkemeijer, well known for their F2 (control line) engines. |
The influence of headwind can be shown using a simple model: Think of the plane flying in two directions, first against the wind (towards the single pylon) and then back, together with the wind (to the double pylon). At first, one might be tempted to say "well, flying in both directions cancels the influence of the wind", but you probably already know better. If you consider a headwind, blowing with the same speed as your model can reach, your model will zip along the downwind leg, with twice its own speed, relative to the ground. But if you try to fly against the wind, the model will not move an inch against the wind. Such a race would last forever, but you will hopefully never try to fly in a hurricane. The same happens at lower wind speeds, always increasing the lap time. The question is: can we improve this situation by flying somehow different? You always have the freedom to change the altitude, to climb or to dive. Climbing converts energy, which is contained in the speed of the model into altitude, and thus reduces the speed of your model. On the other hand you can transform altitude into speed if you let the model dive.
Calculations have been performed for various climb/dive angles; it has been assumed that your model climbs with the same angle as it dives later back to the initial altitude. The following figure shows lap times for various wind velocities. Along the horizontal axis the dive/climb angle angle is plotted, a positive value means climbing against the wind and diving with tailwind.
Lap times versus climb angle for various wind speeds.
Looking at the curve for zero wind, the minimum lap time of approximately 6.2 seconds can be found for an climb angle of zero degrees, which means horizontal flight. If you compare with the result for a (rather high) wind speed of 15 m/s, you learn, that the lap time increases to 7.0 seconds, if you still fly in a horizontal plane. You can achieve a faster lap by choosing a negative climb angle of 15°, which brings the lap time back to 6.7 seconds. For 10 laps this will make 3 seconds - which is not bad. The negative climb angle means: dive against the wind and climb on the downwind leg of the course. The diagram also shows, that for each wind speed, a different optimum climb angle exists; you could also say that you have to tilt the flight plane according to the wind speed.
| Boundary
Layer of the Earth Besides the global wind effect described above, there is also another effect to consider. The wind blowing over the surface of the earth develops a boundary layer like the air flowing over the wings of our models. The long distances covered by the wind lead to very large Reynolds numbers and thus thick boundary layers. While the thickness of the boundary layer on a model aircraft wing is in the order of a few millimeters, the boundary layer on the surface of the earth is several tens of meters in height. The graph to the right shows a typical boundary layer profile for a smooth grassy surface as we would find it on the contest scene. It can be seen, that the wind speed increases with height. Close to the ground the wind velocity reduces to zero. If we measure a speed of 5 m/s in a height of 2 meters, we measure only one point in the velocity profile. We must be aware, the the velocity at a height of 6 meters above ground has already grown to 6 m/s and at 20 meters we have about 7.5 meters per second. The scale on the upper border of the graph gives the ratio of the velocity at any height to the velocity measured at a height of 2 meters above ground. This scale can be used to find the velocity at any height for any measured wind speed. The boundary layer profile adds to the same tactics as described. It is advisable to fly or dive low against the wind, and to fly high or climb with the wind. Thus we can make use of the stronger tailwinds at altitude and avoid stronger headwind by flying low. |
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When the wind speed stays below 10 m/s, the time lost due to the wind is marginal, but if it gets stronger, a climb and dive flight style should give better results. The general rule is to dive against the wind and climb on the downwind part, but I am sure that most of the F3D pilots already have enough workload. Nevertheless, for some national, slow ´racing´ classes, there might be enough time to think about applying such a technique.
Fixed or retractable
landing gear ?
Challenging the
Wind Flaps in pylon
racing models ?Last modification of this page: 21.05.18
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