If a pilot uses a notch of flaps on takeoff, when do they get retracted? At that time, what changes (i.e. climb rate, speed, pitch)?
NOTE: I have purposefully not specified the type of airplane.
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The short answer is, it depends. The FAA Airplane Flying Handbook merely states that this can be performed once the aircraft has established positive climb, but does not specify exactly when.
Ultimately two situations arise:
With respect to the actual changes in aircraft behavior, you'll likely notice the following things:
Below is sample screen of a Boeing 777's Primary Flight Display. See those flaps marks on the speed tape?
Here's the thing: you don't want to retract the flaps from 5 to 1 when you're slower than the stall speed at flaps 1. So in terms of flaps retraction schedule, you'll:
Acceleration altitude is usually 1,000 feet or 1,250 feet. So that really steep climb right after takeoff roll lasts only about 30 seconds.
If you're flying a small aircraft, chances are you'll only use flap in a short field takeoff. The general procedure would be similar:
According to a Boeing publication flaps are usually fully retracted at an altitude of about 3,000 ft. As you probably know flaps increase lift and drag. So as the plane increases speed (and thereby lift potential), the need for flaps decreases. I imagine by the time a plane reaches 3,000 feet is has attained enough air speed to no longer need flaps.
First and foremost, in light planes flaps should be retracted above VS and below VFE.
In a C172, you have:
VS 47kts Stall Speed in Clean Configuration.
VFE 139kts 10 Degrees Flap Extended, 85kts 20-30 Degrees Flap Extended.
That's a pretty wide range, so it would be hard to get it wrong, and as a matter of safety, you would probably want to use the approach multiplier of 1.3 as the minimum, which is 61.
So next question is, what is the effect of raising the flaps in a climb? Most importantly, raising the flaps decreases the angle of attack, which decreases induced drag, so raising the flaps earlier is better, so somewhere in the range of 61-71 for the Cessna.
[It has been suggested that "positive rate" should be used for flaps, but I disagree. Unlike gear, which is all drag, 10% flaps are mostly lift, with very little added induced drag. Higher flaps settings for short= and soft-field takeoffs have a different set of considerations.]
I fly for a commercial airline, which is different, bit I also fly light planes and as a CFI I flight instructed in them for hundreds of hours. In a nutshell, I taught if you were in flatland areas with no terrain/obstacles to clear, retract the flaps at 300-500 feet AGL in the climbout, as long as you have adequate airspeed and are accelerating. You then have to trim nose up a bit to keep the climb rate up due to the loss of flap lift. No need to over complicate this. In mountainous terrain, or in a canyon, or on very short fields with obstacles nearby or at high altitudes where performance is degraded, pilots often elect to delay flap retraction and gain better slow speed climb performance. Depends on the situation. Hope that helps! Fly safe and have fun!
The lift flaps generate is directly proportional to speed but the drag they introduce is proportional to speed squared. Double the speed, double the lift, quadruple the drag.
As the aircraft accelerates after take-off, the extra lift generated by having the flaps extended will be countered by the extra drag generated by the increased speed so they are retracted in stages to maintain that balance.
at small aircrafts, you usually don't have any intermediate flaps setting between 10 and clean. As we know, when flaps selected, take off roll is reduced, so as the climb performance. Clean configuration increases ground distance but improves climb ability. In ideal take off, you should retract flaps at point where flaps path and clean path meet each other, to obtain best possible climb out. (you always want to be "on the upper line of diagram").
For Cessna 152 it is about 200-300ft
Let's just start by stating that Flaps allow you to fly slower, but they generate a lot of extra drag. You are paying the price of slower speeds by using more power to fly, and also by having a reduced climb performance. This alone means that the pilot usually wants to retract the flaps to a clean configuration as-soon-as-possible. How soon that is, is a bit more complicated to answer.
For starters, you need a specific (safe) speed for each notch of flaps to be retracted. Stall speed will increase every time you retract a notch of flaps so you want to be well above that speed before putting your hand on the flaps lever.
So, in theory, the second the wheels are off the ground, you can just level off, keep flying a few feet above ground and keep accelerating until a safe speed is reached then retract flaps all the way to a clean config.
While some bush pilots might actually do exactly that, in practice there will be obstacles and terrain you might be worried about once you use up all of your available runway. For instance, most runways (unless otherwise stated) will ensure obstacle clearance for a 2.5% climb gradient. (2.5 meters climb for every 100 meters away from runway threshold)
So, in theory, this becomes: the second the wheels are off the ground,you can continue to climb at a necessary 2.5% gradient to avoid the obstacles, speed up, then retract the flaps whenever you reach the speed that you can safely retract them.
It is the most efficient way, and it is what any pilot/aircraft/airline would do if allowed. In practice many regulations, like noise abatement, and manufacturer manual would dictate how your climb profile and flaps retraction schedule would look like:
Eg. NADP 1 will require you to climb in t/o configuration until at least 3000 feet before beginning to accelerate and retract
NADP 2 on the other hand will allow you to start accelerating as soon as 800' AGL (while still continuing to climb at 2.5% mind you), using t/o power on the engines until 3000'