i have many r/c cars (made in china cheapos )

i was thinking i can manage to make an r/c plane without buying a separate controllers set. this would make my model damn cheap... if this can work, what would be the range?

the controllers are all 27 mhz.

also, i am planning to construct a junk 747 or 787   can these work for it?  

Nope. They won't work because they are 2 channel car radios.

Can't we use two of them?

No.

is there any reason why not??? pls justify...

You'll need at least 4 channels to fly a plane. If you think two, two channel car radios are going to work, it will be impossible. You will need an aircraft style transmitter to fly a plane.

 but if i can control two motors with one controllers, then i can control 4 mtors using two controllers

Dear topalle, there can be many reason for this:
1: +1 with iamahuman(regarding channels)
2: these controllers work in 1,0,-1 manner. i.e. either they switch off the motor, or they turn the motor in a particular direction, but with a fixed speed.
for aircraft, you need variable pitch for throttle and controls.
3: the range, you will get with chinese radios is merely 200' at maximum.

 thanks ...then what would be the cost of a basic airplane controller?

The RC Gems will have all your answers. Do some reading and research...

Hi Iamhuman
This is not to contradict anyone but just as a matter of interest for all. Back in the late sixties when RC was out of reach for all of us CL, free flight and glider flyers, one in our group (a catholic priest) had a Pulse RC system. The Tx was a small metal box with one button, a switch and an antenna. One push on the button turned the aircraft left and two quick pulses turned it right. Once launched, you had to fly the aircraft till it ran out of fuel. You maintained altitude by going into steep turns.
So maybe you don't need 4 channels to fly an aircraft always. Maybe an innovative DIY person can cannibalise RC cars to make a flyable 2 channel aircraft. Definitely not a twin engine, and ofcourse the range will be really short. But can it work?

I have my doubts. A toy car transmitter won't have proportional control like Abhay mentioned.
 
First of all, it would be near impossible to use.

Like Abhay said, the range on those radios is abysmal when you need to fly an aircraft.

Even if you were to use a really good 2.4GHz hobby grade car radio, the range would not be enough to fly an aircraft effectively.( Maybe with the exception of the Sanwa RDS8000 which can be used as a crawler radio)