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For example, the CRJ has a flap setting of 0, 8, 20, 30, and 45 degrees.

The Learjet has similar features across their models, as far as I can see which is 0, 8, 20, and 40 degrees

Why do smaller jets have one low flap setting? Why not have 0, 8, 15, 20, 40? Comparing to larger aircraft like the 737, they have a flap setting that captures just about every degree you can imagine.

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    $\begingroup$ The A380 only has five flap positions: UP, 1(+F), 2, 3, FULL It's a design philosophy, keep it simple and easy to use...vs. full flexibility. In a 737 the flap positions 2, 10 and 20 are rarely used at all. Mostly flap 1, 5, 15, 30 or 40... It has pretty much nothing to do with aircraft size $\endgroup$
    – Jan
    Commented Jun 13, 2020 at 16:11
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    $\begingroup$ @Jan And to expand on that, the small Grob 115 which I sometimes fly has an electric flap with near-infinite settings! And, like you say, everybody just uses a couple of pre-marked points. $\endgroup$
    – Dan
    Commented Jun 13, 2020 at 18:04

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The provision of lots of flaps settings is motivated in part by the desire to provide as many profile optimizations as possible. Generally, the less the flap extension for takeoff, the more efficiency you have in a departure plus better engine failure performance, but you need the longest runway, so you would use the least extension allowable for a given runway.

The CRJ was originally certified without an 8 degree flaps setting, and it was added as an option a year or so into production. In general, you will always use Flaps 8, unless conditions require Flaps 20 (shorter runway).

It can create a trap for the CRJ because if you are departing an airport where 20 is required to meet departure performace requirements, and you set Flaps 8 by mistake, there is no Takeoff Configuration Warning mode to warn you you're at the wrong takeoff flap setting (only a warning for 0, 30 and 45... it can't know which 8/20 TO setting you actually want) and you can take off with insufficient flap extension and get into a bit of trouble when you rotate at the Flaps 20 Vr and you don't lift off. When departing a short field that required Flaps 20, you had to be extra careful to make sure Flaps 20 was selected.

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    $\begingroup$ What's the speed difference between VR at flap 8 vs. VR with flap 20? An example would be nice $\endgroup$
    – Jan
    Commented Jun 14, 2020 at 8:16
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    $\begingroup$ It's somewhere around 10-15 kt IIRC but it's been almost 10 years and I'll have to try to dig up an old CRJ200 speed book. $\endgroup$
    – John K
    Commented Jun 15, 2020 at 1:46

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