Glide ratio (aircraft)

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The glide ratio E is the ratio of aerodynamic lift and drag . Aircraft with a high glide ratio have a long range with low energy consumption.

Definition and meaning

The glide ratio is the ratio of lift and drag. In the case of non-propulsion gliding , the glide ratio also corresponds to the ratio of the distance covered and the loss of altitude, from which its name is derived. In motorized level flight, the glide ratio corresponds to the ratio of weight and engine thrust; with a glide ratio of 20, the engine only needs to apply one twentieth of the weight. Since lift and drag differ only in the coefficient, the glide ratio can also be understood as the ratio of lift and drag coefficient. Consequently, there are five equivalent interpretations of the glide ratio:

The glide ratio is dependent on the angle of attack , indirectly also on the flight speed, the aircraft mass and the load factor . If the glide ratio is given as a simple numerical value, it is usually the maximum achievable glide ratio. It is an important parameter for the aerodynamic quality of an aircraft and is approx. 15–20 for powered aircraft and 40–50 for gliders. The glide ratio for different angles of attack and speeds can be read from the polar diagram or the speed polar .

Different definitions

The five definitions mentioned above are only identical if the aircraft is in a non-accelerated state and all speeds and distances are measured relative to the surrounding air mass (travel, airspeed). With speed and distance over the ground (ground speed) slightly different values ​​result depending on the wind direction and strength. In common parlance, both are referred to as glide ratio. A distinction is often not necessary or results from the context. In aeronautical engineering and aerodynamic specialist literature, all speeds generally refer to the surrounding air mass, so there is no inaccuracy here. Nevertheless, there is an inaccuracy in the specialist literature because the glide ratio is used both as a property of the wing profile and for the entire aircraft. The glide ratio of the entire aircraft is always slightly lower because the fuselage creates additional drag and the limited wing extension leads to tip vortices and further losses. As the fuselage shrinks and the aspect ratio increases, the glide ratio of the entire aircraft approaches that of the profile.

Maximum glide ratio

The glide ratio of an aircraft depends on the angle of attack, i. H. indirectly on the speed and gust of the ambient air as well as on the pilot's control skills. Unclean flying ( sliding angle , unnecessary rudder deflections) reduces the glide ratio. The speed at which the aircraft flies with the angle of attack of the best glide depends on the weight, the load factor (influence of centrifugal forces) and the cable pull in the winch start . A determination of this angle of attack is therefore only possible indirectly with an airspeed indicator . A direct determination can be made with a side thread that is glued to the cockpit canopy and shows the angle of flow. Since the best rate of climb occurs at the angle of attack of the best sink (optimal rate of climb and descent are at the same angle of attack), it is advantageous to fly at this angle of attack when starting the winch and when making an intercept. This is often associated with higher speeds and with high g-loads.

The glide ratio is not influenced by the mass of the aircraft. In the state of best gliding, however, the flight speed increases with increasing wing loading. A heavy aircraft climbs more slowly in the updraft because the inherent sinking is greater and, due to the higher speed required, the turning radius when circling also increases. This is a disadvantage as the updrafts usually get stronger towards the center. High-performance sailplanes are therefore built as light as possible, but equipped with water tanks. In good thermals , the pilot can make his aircraft heavier and faster with water ballast. If the thermal gets worse on the way, he can restore the original light state by draining the water. When landing, the water is usually drained off in order to be able to approach more slowly. With a high wing loading, the influence of downdrafts and headwinds is smaller, since the aircraft flies faster with the same glide ratio, and generally higher average speeds are achieved with strong thermals.

In general, when gliding, the energy to overcome the air resistance comes from the loss of altitude due to the lack of a motor - the propulsion is a component of the weight force. Furthermore, the speed can only be regulated via the inclination of the flight path. As can be seen from the polar of an airfoil profile , the ratio of lift to drag changes; H. the glide ratio, at different angles of attack.

The maximum glide ratio is not reached at the speed of the lowest or best descent, since that is where the lift is greatest, but the resistance is also great. So more altitude has to be surrendered at a given distance in order to maintain speed. The approach with the lowest resistance is also of little advantage, since the lift is also low there. The maximum glide ratio is in between and can be read from a diagram of the power polar. This speed of the best glide is a compromise between the highest possible lift with the lowest possible drag.

Examples of the maximum glide ratio

  • Gliders of the 15-meter class have glide ratios of around 42. Those of the 18-meter class have around 50. And those of the open class around 60, each at a speed of around 110 km / h. Gliders used in the training have glide ratios from 25 at approx. 85 km / h (wooden-steel tube) to approx. 38 at 100 km / h (plastic). The glider eta comes to about 70.
  • A commercial aircraft (e.g. Airbus A340 ) has a glide ratio of around 16 at a speed of around 390 km / h.
  • For the US space shuttles , the glide ratio on the final approach was around 4.5.
  • A paraglider has a glide ratio of around 9.
  • A kite for hang-gliding has a higher glide ratio of 10-15. Fixed-wing pilots reach 16-19.
  • A wingsuit reaches 2.5 to 3.5 at a horizontal flight speed of 130–150 km / h - the rate of descent is around 14 m / s (approx. 50 km / h).
  • A tracksuit , a textile suit in which wide sleeves and trouser legs fill with air and which has no areas stretching between the limbs, reaches a speed of 1.8 at around 180 km / h.

Importance of the slip angle

The minimum glide angle that can be achieved determines whether a distant and lower point can be reached as a landing site from a specific starting point or whether it can be overflown as an obstacle. It should be noted that when starting with a paraglider or even more when jumping from BASE with a wingsuit, at the beginning of the flight phase until a steady airspeed is reached, a significantly larger glide angle is flown, or, viewed differently, a certain additional loss of height until one is reached shallow slip angle occurs.

In particular, to  check the flyability of a planned flight route - a line - for a wingsuit BASE jump, the use of an inclinometer, possibly with laser sighting, is widespread.

The typical minimum glide angle is achieved when flying in still air. The effective flight path results from the temporal concatenation of path contributions via the vector addition of flight speed compared to air plus local wind speed over the ground. Air currents especially vertically across the intended flight path are relevant. Local updrafts increase the range, while downdrafts increase the sink compared to the ground and reduce the range of the flight path.

Glide ratios in gliding

In gliding , long distances are often flown, whereby for reasons of air traffic control and because there are no thermal updrafts at night, it can only be flown during the day and the distance is therefore also limited by the average speed. Therefore not only the speed of the slightest sink rate is important, which ensures a quick gain in altitude in updrafts , but also the speed of the best glide.

A higher maximum glide ratio allows you to cover a longer gliding distance to the next updraft zone. This leads to a higher average speed.

Many performance gliders can also be loaded with water ballast - for days with particularly good thermals. This allows the best gliding of a glider to be shifted to higher speeds, which means that you can cover a distance faster with the same gliding power . However, due to the additional weight, the climbing performance when circling in thermals deteriorates. Gliders with cambered flaps are advantageous here, in which a “slow flight profile” with a high lift coefficient can be selected for circling in thermals - through positive flap position . When the thermals weaken, the water ballast is released in flight. The water ballast does not change the best glide ratio of a glider.

In order to allow a steep landing approach , the glide ratio should be as low as possible. This is achieved during the approach by extendable landing aids, which increase the aerodynamic resistance of the aircraft and at the same time destroy part of the lift. This achieves a significantly higher rate of descent than normal driving. Air brakes (Schempp-Hirth flaps, see illustration), which are extended vertically into the air flow in the middle of the wing profile , and can reduce the glide ratio of 40 and more to 5 to 10 on approach , are widely used . The maneuvers - is additionally - especially in older wood aircraft sideslip (slip) effective, for which the aircraft by counter-rotating pressing transverse and rudder banked and the aircraft fuselage in an angle diagonally is rotated to the flight direction.

See also

literature

Individual evidence

  1. ^ Space Shuttle Technical Conference p. 258
  2. ↑ Cédric Dumont: This is how a wingsuit works. At: redbull.com. August 9, 2013, accessed October 3, 2016. - Dumont speaks of a glide ratio of up to 4.5.
  3. ↑ FAQ. At: base-jump.at. Retrieved October 3, 2016.