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Aircraft Performance

Obstacles, Flaps 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.

Climb-out performance tells us if we can clear that obstacle in our flight path. The correct use of flaps can or cannot help with that. We will also discuss important factors regarding the use of flaps.

Clearing Obstacles

It is advisable to clear any obstacle in your flight path by at least 50 ft. Check your aircraft climb gradient during preflight and gather any pertinent information about the airports you are going to visit on obstacles near the climb out and/or landing path. Also check the latest NOTAMs for any new obstructions not listed on the charts or in the AIP.

Climb gradient

Calculating the height gain per mile (climb gradient) is easy: Rate-of-climb (ft/min) / Ground Speed (NM/min). For example: 500 ft/min climb and a ground speed of 66 Kts (1.1 mile per minute). Dividing 500 ft / 1.1 NM gives a height gain per mile of 454 ft / NM. This will give you a good indication if you will be able to maintain safe terrain and obstacle clearance. Start with a no wind situation for this calculation, any headwind component will increase the climb gradient to your advantage.

Read more on climb performance as it is important and influenced by a number of factors and not all are under pilot control!

Correct Flap settings

Flaps reduce aircraft stalling speed by increasing lift and it enables you to lift-off at a lower airspeed. Resulting in a shorter ground run. It also reduces the rate of climb (and angle) due to a somewhat higher drag. You must always use the recommended flap setting for the given circumstances. Never, ever reduce a flap setting while in a turn or on final. In a turn stall speeds are higher and reducing flaps could mean a low level stall / spin event.

Extending flaps to the first position usually adds quite some lift and a small amount of drag, the second position will increase both lift and drag and the third position adds much more drag and almost no lift at all. The increase in drag helps to stabilize the airspeed and it lowers the nose of the aircraft due to the different position of the tptal lift point on the wing (it moves backwards when flaps are extended). You will need to add power when flaps are fully extended while maintaining altitude.

Reduced AoA

Keep in mind that with flap extension the wing will stall at a lower angle of attack. In turbulent conditions a wing drop is more likely then. Especially so when the flap and ailerons are combined, e.g. flaperons. Pipistrel aircraft are known for this 'feature'.

Slats

Slats are leading edge flaps (where as normal flaps are on the trailing edge of the wing) and have the effect of increasing AoA, camber, lift and wing area thus reducing stall speeds. These slats either move forward or are extended from below the leading edge automatically or under pilot control. Some slats even extend forward, Fowler slats.

Aircraft Performance

Crosswinds

While on approach to a runway with a considerable amount of crosswind, it is wise not to use full flaps on some type of aircraft (old model Cessna's have 40° full flaps and the DynAero MCR-01 even has 45°) without proper training from a qualified instructor. On these aircraft the full flap setting does not lower stall very much (maybe a knot or so) but they increase drag enormously. The glide angle will be very steep (helpful when there are obstacles in the way) and you will need to add power to reduce the flight path angle. Do maintain power until touchdown, especially so in crosswinds.

With flaps extended your groundspeed is even lower and the effects of the crosswind will be that much higher. Possibly resulting in being unable to line up the aircraft with the rudder and banking into the wind at lower speeds to keep the aircraft from drifting. Your only option is then to reduce flaps and approach with a bit higher speed. Make sure enough runway is available, else you need to change to a more favorable runway or divert to another airport.

Written by EAI.


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