BMAA Microlight Comps

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Laurie Hurman's How to Fly a Constant Groundspeed PDF Print E-mail

 

“How to” article - Flying a Constant Ground Speed

Let’s say the task requires you to fly around a circle on the map at a constant ground speed. The wind is from a constant direction, say, 10 knots from the North. So when you are flying north your ground speed wil l be 10 knots slower than your airspeed and when you're flying south it will be 10 knots faster. Now, you have to manipulate your airspeed so that as you fly round the circle your speed over the ground stays constant.

The first thing to note is that the task will always allow you to decide in advance what speed you want to fly at - in fact it will demand that you declare this before take-off. The point is - you will never be forced to fly at a particular speed! You will always be free to pick your own speed before take-off.

So, suppose you normal cruising speed is 55 knots and the speed range you can comfortably manage is from 50 to 70 knots. The sensible thing to do is say that you are going to fly the task at a constant 60 knots ground speed because then when you fly into wind you will be able to fly at 70 knots airspeed and when you fly downwind you can fly at 50 knots and your ground speed will always be 60 knots.

The problem then comes down to, “How do you know when you're heading into wind and when you are going downwind?” Well, you can't know and you don't try to, although it's helpful to make an educated guess. The trick is to mark the circle on your map with time marks. You calculate how many millimetres you will cover at your nominated speed (in the UK the task map is always 1:250,000) so, in the example I have used, you will cover 7.4 mm on the ¼ mil map for every minute flying at 60 knots. You mark the circle on your map at 7.4 mm, 14.8 mm, 22.2 mm, etc. all the way round the circle. Then, what you have to do to fly the task is to start a stopwatch at the startpoint and adjust your flying speed so that you reach each mark on the circle at the appropriate minute.

Clearly it's going to take you quite a long time to mark the entire circle in a succession of 7.4 mm steps. What most competitors do is to make a bendy ruler specially marked in advance and bring it with them to every comp. To start with you only need one speed marked on the ruler. Just pick the middle of your speed range, calculate how many millimetres per minute for that speed, and mark the ruler accordingly. The ruler has to be bendy because the line on the map is not always straight.

The final point to think about is, “How is the task be ing scored?” Why? Because this dictates how you fly it. Suppose the organiser says that you will be scored on the accuracy of your average speed from start point to finish point. This means that if you arrive at minute 5 and you are a minute late then you have to speed up and regain that time. It is more usual however for your speed to be sampled at two points on the track, the location of wh ich you do not know. What this means is that when you arrive at minute 5 and you are a minute late you have to increase your speed but you are not trying to catch up that minute. What you are aiming to do now is to arrive at each subsequent minute mark exactly a minute late. If you manage this you must be travelling at exactly the right speed – even though you are a minute behind where you expected to be. It all depends on how the task is being scored although it is most common to sample rather than use an overall average speed.

Ground speed is something that is easy to practice on your own. Start by marking your map with a triangle or a square course and mark it with your time ruler. It's surprisingly difficult to judge how much to speed up and slow down so stay with straight line tracks until you start to get the hang of it.

Laurie Hurman

 

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