kaysee:
kwn rasa topik ni pernah dijawab oleh bro umran, tp da try cari xjumpa. time nak mencari ni plak search box xjumpa (da xder ker thread search function?).
cerita nyer begini. kwn nyer simplestick ntah mcmana smp sekarang asyik nose heavy ja (bila cut throttle). daripada CG kat 1/3 wing chord, smp kwn bwk CG 1/2 wing chord. still nose heavy. even kwn tambah weight kat tail pun.
aper yg kwn suspek skg, ader yg x align ngan wing incidence n stabilizer kwn (maklum la tukang tgn pengkor
).kemusykilan kwn, adakah wing n stabilizer incidence akan menyebabkan cog kapal kita berubah maksud kwn, tidak berada di tempat yg kita agak (1/3 wing chord) ?.
apa plak perkaitan dia?
umran:
c of g beza daripada c of p. apa benda yang kita cakap ni?
C of G = Center of gravity - is the center of mass of the aircraft - roughly not an angle dependent.
C of P = center of pressure - is the center of resulting pressure force acting on any lifting body at a given angle. Very much angle dependant.
Apa kena mengena? ngan soalan? at any given airspeed, an airfoil produced lift, the sumation of the rpoduce lift will do work by lifting the body. depending on where the c of g is in comparison with c of p, the aircraft may rise vertically level or rise while tilting forward, or rise by tilting backwards.
Now what controls the c of p? the effective angle of attack which depends on the incidence angle of a wing and current tilt aircraft angle.
Hope this highlite the basic...
eragon:
Contoh macam kertas 4 segi, C of P dia di tengah lah?
Macam layang layang. Betul?
umran:
dekat dah tu Ali, cuma lebih sikit la complex dia... kedudukkan dia bergantung kepada tilt angle vs airflow angle.. nak kata exact kat tengah pun tak berapa kena gak..
kaysee:
relate pada keterangan bro tu, body dlm citer tu ble saya consider sebagai wing saya la kan?
yg saya paham dari keterangan tu, CoG ni kira tempat di mana wing tanggung berat kapal n pivot up n down (pitch axis).
n CoP plak wujud bila wing sdg berkerja. so by changing angle of attack of kita nyer wing ie. elevator input. kita sebenarnya mengubah CoP ni tuk climb or dive kapal kita tul?
kalu betul pemahaman kwn ni. meaning kapal kwn skg naturally having -ve incidence which move the CoP behind my CoG. thus making CoG kapal kwn too far aft.
betul ker pendapatan saya ni bro? saya ikut paham2 sendiri ja ni.
mungkin ujung minggu ni saya nak try up sket wing incidence, n see how my stick behave.
tq bro.
umran:
Okay sedikit detail lagi...
For any typical aerofoil shape crossection wing, C of P will first occur at the frontal area of the top wing (+ve incident or semi symetrical airfoil), as the effective angle of attack increases, the C of P stated to move backward untill stall occur. This is basic.
Now when we say fwd c of g, we means that the c of g is foward of c of p as such that when ever we take out the thrust, the airplane may glide down properly.
But when we say aft c of g, when ever we take out thrust, the airplane will quickly lose air speed and tilt nose-up and stall... this occur because c of p is aft of c of g.
Now what is the behavior of the aircraft bila kita remove thrust?
kaysee:
skng nih problem, bila test glide, meaning throttle down, plane akan dive as if plane tu nose heavy. da try bring the cog aft, smp 1/2 wing chord then saya stop, just put in uptrim. sbb saya start suspect bukan salah cog.
so pendek kata my understanding to bro nyer explaination betul la kan?
then my wing will get its +ve incidence.
satu lg. mungkin x stabilizer saya yg skg nih saya nampak cam ader sedikit -ve incidence yg nyebabkan nose down attitude ni?
xble tido bro memikirkan mender nih. cari dlm internet xjumpa2 exact answer yg saya nak. last2 kat sini gak jumpa. xpuas hati bila pemahaman saya xbetul. sbb bg saya, kalu pemahaman xbtul, camner nak troubleshoot kapal.
tq skali lg bro. nnt bro jwb saya tq lagi
umran:
increase angle of attack will some how move the c of p kebelakang sikit, input daripada up elevator will alter the lift amount of the wing as well as the actual position of the c of p, but for a normal airfoil, this c of p position will varies between 1/4 to 1/3 of a crossection. If somehow the c of p moved beyond 1/3 from leading edge(which may occur) this leads to a stall condition.
So the point is, if the airplane c of g is within the normal boundries, 1/4 - 1/3 is a save place for the C of G.
hope you guys learn something as well ...
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