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Glide Ratio

There are three main components to staying within safe gliding range of the airfield, the still air glide performance of the aircraft, the effect of headwinds and tailwinds, and the presence of rising or sinking air. Compared to most of their powered cousins, gliders have quite good glide performance.

Cessna 150 = 7:1    2-33 = 20:1    K-13 = 25:1
1-34 = 32:1    Discus = 43:1    Nimbus 3 = 60:1

If we take an aircraft with a 30:1 glide ratio, at 45 knots:

  • Feet Lost Per Nautical Mile = 6080/30 = 203 ft.
  • Distance covered per 1000 ft. Height Loss = 30 x 1000/6080 = 4.9 nm

From our average 2000 ft aerotow we see that to join the circuit at approximately 1000 feet we should not be more than 5 nm away under still air conditions. Let’s now consider the affect of the wind.  How high above ground level (AGL) must you be, if in still air if you are 20 nm from the airport and yo want to enter the circuit at the airport at a height of 1000 ft. AGL.

With No Wind

Height = ((Distance / 6080) / Glide Ratio)+Circuit Height
Height = ((20 x 6080) / (30)) + 1000 = 5053 agl.

Headwinds and Tailwinds

GRg = Glide Ratio ((Gliding Speed +/- Tailwind/Headwind)/Glide Speed))

With a 15 kt headwind the glide ratio effectively lowers from 30 down to 20.

GRg = 30((45-15)/45) = 20
Height = ((20 x 6080) / (20)) + 1000 = 7080 agl.

With a 15 mph Tailwind

GRg = 30((45+15)/45) = 40
Height = ((20 x 6080) / (40)) + 1000 = 4080 agl.

Feet lost per nautical mile - Glide Ratio 30/1 at 45 kt

No wind = 6080/30 = 203 ft.
Head wind (15 mph) = 6080/20 = 304 ft.
Tail wind (15 mph) = 6080/40 = 152 ft.

Distance covered per 1000 ft. Height Loss

10/1 = 10 x 1000 ft./6080 = 1.6 nm
20/1 = 20 x 1000 ft./6080 = 3.3 nm
30/1 = 30 x 1000 ft./6080 = 4.9 nm
40/1 = 40 x 1000 ft./6080 = 6.6 nm

We can easily see now how significant the effect of the headwind or tailwind is. The performance penalty for being downwind of the airfield on a local flight is significant, as is the reward for being upwind. So what effect has lift and sink on these matters? A more experienced glider pilot might derate the glide performance of their glider by about 80 per cent to account for the vagaries of sink to maintain a good margin of safety. Rather than get involved in the disappointing performance into headwinds, the same pilot would find it more profitable to stay upwind of the airfield on local flights.  For the just solo pilot the rules should be quite a bit more restrictive. The glide ratio should be derated by 50 per cent. In our example, this would mean assuming a glide ratio of 15 and thus staying within 2.5 nm upwind of the airport on a 2000 foot aerotow regardless of the strength of the wind.