Performance

Glider Performance

Ever wonder why some days your wing seems to be a bit off? Sometimes the landing seems faster than other times, the sink rate makes you go home and get on the bathroom scales, unless you are the only one underperforming, then take heart that it could be “Pressure Density” to blame.

The Aircraft Industry supply a performance Manual with every aircraft illustrating the Stall Speed, Maximum Take Off weight, and other parameters which are all based at “Standard” atmospheric conditions, now these standard conditions are rarely met and so some compensation must me made to establish the “true” Stall speed or Max; T/O weight etc;

The standard conditions are typically at sea level:-

Temperature 15C

Barometer 1013 millibar

Now we all know that hot air is thinner than cool air (thermals rise) and indeed the hotter the day, the thinner the air. For every one degree increase in temperature, the air density drops to the equivalent of some 130 ft, With low air density, aircraft need more runway to T/O and land and conversely, require less runway on a cold day.

Standard temperature reduction is 2C for every 1000ft so calculating standard temperature for 3000ft (Blackheath) gives a reduction of 6C, therefore 9C is standard for Blackheath at launch.

A hot day say 18C at 3000ft = a relative take off altitude of 4170ft.

 (((18-9) x 130)+3000)

This is simplistic as the atmospheric pressure has to be considered also, as the pressure drops so does the “air density”, every 30ft of elevation, the pressure drops 1 millibar. Launching at an elevation of 3000ft is 100 millibars, some 10% less than if at sea level.

It can be easily calculated that the “Pressure Density” at seal level on a hot day in the presence of a low pressure system can drastically reduce the performance of an aircraft; this is compounded more when the flight operation is at some altitude.

The opposite is evident on a cool day in the presence of a high pressure system, we sometimes have referred to the air as being “buoyant”

The advantage of flying at high altitudes is that your “true” air speed is actually higher than recorded, this can be verified by comparing a GPS speed with an air driven speed device, (in still air or compensated wind speed). The difference in measurement increases with altitude gained.

Next time you have a good day, go home and check what the conditions had been while you were out flying. It won’t be long before you are checking the conditions before you go out! If the pressure is low and the temperature is going to be elevated, you might have to flare a little more aggressively and run a little harder on take off.

Happy Landings

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