ORIGINAL: elmerfud
feihu-RCU
this may sound not so good but...(im not a full scale pilot, but it intregue`s me):
you are saying flying (a cessna 150) with 10º flaps it takes fwd trim....to maintain 75mph, but.. when adding 30º or 40º of flaps...it takes back(up trim ) to maintain the same 75mph.... why would you want to maintain the same speed>?(with 10º or 40º of flaps) i thought the idea of flaps was so you could approach at a slower speed>?..and land. does it become more stable with the flaps at max deflection>?
you may of just used this as an example for arguments sake....
so ..when coming in for a landing you start with 10º of flaps ...it pitch`s up slightly .you compensate by pushing fwd.....then once it begins to "settle in".. and you add more flaps.(and i would guess throttle)then you begin to pull back. to maintain same speed,, during aproach>?... then it sinks in at what ever speed the aircraft was designed to land at>?
you have to understand that the effects of flap is not a linear think........
flaps from zero to lets say 10 degrees are making the wing produce more lift for only a small penalty in drag. if you extend the flaps more the increase in drag is much more important for only a very small gain in lift.
in a cessna the approach speed flaps 20 is aproximatly the same than the aproach speed flaps 40. But when aproaching flaps 40 you will need much more power on the glide.
the main reason to approach flaps 40 is that the plane will decelerate much more faster when you will cutof the power and make your landing short.
in a cessna 172 aproaching full charge flaps 40 the go around can be a problem because of the enormous drag generate by the flaps. So it's A GOP to select landing flaps when you are sure you are going to make the runway with that kind of airplane.
extreme positions like "flaps 50" on some airliners has a other main reason.
such a flaps seting will keep your engine runing at very hig RPM during aproach (to compensate the enormous drag). keeping the engine at hig rpm makes the spoolup time in case of go around much more short , that's great in case of go around at very low altitude (go around in 747 can be performed up to 20 Ft without touching the runway)