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Aircraft Performance

Weight and Ground Effect

Most aircraft accidents occur during the take-off and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. In this section of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.

The effects of increased weight of the aircraft on stall speed or take-off run might be more than some pilots bargain for. More so when the runway is not perfectly flat or level or the grass is long and wet.

Aircraft Weight

The weight of an aircraft directly influences its stall speed. A rule of thumb says that 10% increase in weight equals 5% increase in stall speed. And this has its effect on the performance of the aircraft. This is a formula: VS new = VS old weight x √(new weight / old weight) should you wish to calculate the difference.

Take-off

Normally, lift-off speed is about 15% above stall speed. Thus if weight increases -> higher stall speed, your lift-off speed increases too. As a result more time is needed (higher weight also means slower acceleration) to get to that lift-off speed and you will need more runway at the same time. The general rule is that a 10% increase in weight means 20% more runway needed for take-off distance.

Landing

During landing the same effects apply. A heavier aircraft has a higher approach speed (1.3 VS, and stall speed is higher) and therefore needs more runway length to stop. Rule of thumb: 10% more weight means 10% more runway needed when landing.

Into Ground effect

Flying within one wingspan of the ground has the effect of reducing the induced drag from the wing. The result is that the wing is more efficient (wingtip vortices are reduced) and generates less drag. During take-off the aircraft 'wants' to fly sooner and during landing it refuses to land (floating) and you will need more runway before touchdown, and every knot above the recommended 1.3 VS will increase this effect even more.

Ground Effect

Take-off

During take-off roll the pilot can be under the impression that the aircraft is ready to fly. Again caused by ground effect which reduces induced drag by reduced wing tip vortices. But when the aircraft leaves this ground effect it can settle back onto the runway due to a sudden increase of induced drag. Best remedy is to lift-off and remain in ground effect, accelerate to VX or VY then continue climb out. I call this a step climb, it is also used when climbing out over obstacles from short runways.

TODR / ASDA

Usually take-off distance required (TODR) must not exceed 85% of the take-off run available (commercial operators). For general aviation: make sure that 2/3 of your lift-off speed must be reached before 50% of the runway is used. These decision points make it easy to abort your take-off before anything nasty happens. Also check the accelerated stop distance available (ASDA) and the clearway before departing that particular airport.

Landing

On landing, ground effect can result in floating. Even more so when higher (or too high) final approach speeds are used. And as far I can see it, most private pilots just come in several knots too fast. The result can be an overrun of the runway resulting in a go-around. Use at minimum 1.3 VS with the applied gust factor (should there be any) as a final approach speed.

Turbulence

Turbulent air constantly changes the angle of attack of the wings and during landing or take-off and within ground effect this can result into a stall. Make sure to add half the gust factor to your approach or take-off speed and be mentally prepared for this. Just make sure that you can clear obstacles and that enough runway is available.

Written by EAI.


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