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Speed Polars
All aircrafts, powered as well as unpowered, can be partially described by means of the shown speed polars. Fundamentally they all look alike, but of course the specific values for a B747 will be different from a glider. If you have ever wondered what similarities there might be between birds, gliders and a Boeing 747, please click here!

Although speed polars are important to all kinds of pilots, they become more vital to glider pilots. If a glider pilot wants to develop ultimate flying skills, and maybe even participate in contests, he must have a thorough understanding of those flight dynamics.

Minimum Sink Rate (Vs-min)
Fundamentally the speed polars demonstrate the aircraft's Sink Rate (Vs) as a function of different horizontal velocities (Vv). To put it short, they also show that there is an optimal velocity (V3), where you have a minimum sink rate (Vs-min). If you fly faster or slower than that, the aircraft loses more altitude. So if you are in a glider, and just want to stay in the air as long as possible, just fly around with exactly that speed and enjoy the flight.
Example ASW-24: Vs-min = 0.58 m/s @ V3 = 70 km/h.

Stall Speed (Vstall)
As you can see from the curves, it can be dangerous to fly too slow. Suddenly and often without any warning, your plane will drop almost like a stone. The speed where this happens is called the Stall Speed (Vstall).

If you have plenty of altitude, it is only fun to execute a stall. In fact this manoeuvre is part of the standard training program for all pilots. But if you are too low and experiences a stall, the situation is fatal.

Maximum Glide Ratio (L/D)
If you want to travel a longer horizontal distance in a glider, it is important to know if there should be any optimal speed to travel with. Obviously you would want to go as many miles in horizontal direction as ever possible. And if you think that this is going to happen just as long as you fly with minimum Sink Rate
(Vs-min @ V3), you are badly mistaken.

The diagram shows a higher speed, V1, which claims to be the optimal travelling speed if you want to travel the longest possible distance. It might sound odd to you, because when you increase horizontal speed (V3), the sink rate increases too (according to the polars). So where is the benefit of increasing speed anyway? Well, maybe you could accept that if the relative (%) increment in Vv exceeds the relative (%) increment in vertical speed Vs, then we would have a travelling advantage in a higher horizontal speed? Yes? Ok, that's precisely how it works!
Example ASW-24: Max Glide Ratio = 44 @ V1 = 105 km/h.

Re: Often you see the glide ratio designated as an L/D ratio. This means the Lift/Drag ratio. It can be demonstrated that this ratio equals the glide ratio deduced from the speed polar.

Calculating V1
You can determine the exact and optimal speed V1, by drawing a straight line starting at 0,0 in the coordinate system, and make it a tangent to the curve in question. The smallest (and therefore optimal) angle of the straight line (related to the x-axis) you will ever achieve, is when the line just touches the polar in one single point! OK? You can now read your optimal travelling speed V1 from the x-axis. This speed is inherently locked to the Maximum Glide Ratio (in neutral air).

Also one can see that the speed for Maximum Glide Ratio (V1) is somewhat higher than the speed for Minimum Sink (V3). Therefore, knowing the value V1 of the glider in question is very important, because V1 gives you Maximum Glide Ratio, which in turn brings you about if you want to travel.

The Speed of Weight??
Maybe you have wondered why the dashed curve have been neglected so far? Ok! In all aircrafts the speed polars will move according to payload. If you increase the load like e.g. more passengers, heavier pilots, more luggage etc. its obvious to any school child, that the 'falling' speed of the aircraft will increase too. Consequently the polar curve is descending (so to speak), and at the same time moving to the right. The dashed curve equals an increase of 50% in total weight (Gross Weight).

It can be demonstrated, that the straight line from before, with the very same angle, also will make a tangent to this new 'heavyload' polar curve!

The lesson to learn from this is that with a heavier load on the aircraft, you will still have the same Max Glide Ratio, but now at a somewhat higher speed (V2). You will not travel longer with increased payload, but you will travel faster. That's vital to know, especially at competitions. For this reason it becomes evident that many gliders and sailplanes are supplied with watertanks.

Practical use of Speed Polars
But in the real world, where one can be exposed to headwinds and tailwinds, as well as ascending- or descending streams of air, things looks a little different.

Take a closer look on the above polars. If you experience head- or tailwind, you can no longer just draw a straight line from 0,0. You must compensate by moving your starting point to the right or to the left accordingly. Observe that the Max Glide Ratio Angle now varies a lot.

The best Glide Ratio (smallest angle) is in tailwind situations. Obviously because the wind pushes you in the right direction and brings you far about. At the same time you must fly with an indicated airspeed lower than normal.

The worst case Glide Ratio (largest angle) appears in headwind situations. Imagine that the air speed of your plane exactly equals the headwind. That would'nt bring you anywhere but down! Therefore, to make the longest possible flight, you must fly faster than normal.

Now, if you fly in air that gives you lift or sink, you can easily imagine what will happen. See the speed polar below.

First of all you notice that the Max Glide Ratio Angle varies a lot. It is evident that a nonpowered aircraft exposed to descending streams of air, will not travel very far. The angle is large and you will have to increase the indicated airspeed to find the optimal Glide Ratio.

In the opposite situation, ascending streams of air will compensate for the inherent sink characteristics of the glider. Now, if the speed of ascending air equals or even exceeds the glider's Sink Rate, you can fly 'for ever'.

From the speed polars you can observe an interesting situation; as the speed of ascending air increases, the Max Glide Ratio moves towards infinity, and finally results in a negative Glide Ratio. A nice situation, since that makes us travel upwards instead of downwards. As a consequence indicated airspeed will have to be decreased to obtain optimal flight.

If you want a more visual approach to the Speed Polar issue, here is a link to the internet, which explains some further details: Click here!

/fhp

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Opdateret d. 16.6.2001