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You will need to make a folder somewhere in your computer. You it anything but I prefer to use FSX Planes. Now that you have made the FSX Planes folder it is time to find some add-on planes. There are many websites to find large amounts of free planes. These are called freeware planes. Most websites also include scenery, rotorcraft (helicopters), miscellaneous files (different types of experimental or fictional aircraft), cars/vehicles, or AI aircraft. AI Aircraft are the computer controlled airplanes that you do control. Scenery files are files that change how an airport or a certain area of the game looks. These can be very well made and worth a try. You might be thinking that payware airplanes are better than the freeware airplanes. While is MAY be true this is not always the case. I have plenty of extremely well made freeware aircraft. Aircraft that you pay for in Flight Simulator X, or any other Flight Simulator game can be extremely expensive. I have seen a package that includes only one plane for $90. Also payware aircraft are not always the best option because it is quite possible that the aircraft you want is not available.
Once you find an aircraft you like download it and use winrar to extract it. When you extract it put it in the FSX Planes folder you made. After you have done this you need to open up your Flight Simulator folder. This can be found in the Microsoft games folder. Once you get to this go into the SimObjects folder. Depending on if you downloaded an airplane or a helicopter you will place airplane files in the airplane folder and helicopters in the rotorcraft folder.
Open your FSX Planes folder and copy the file you downloaded into the actual Flight Simulator folder (Airplanes or Rotorcraft). Some aircraft come with a built in installer, just follow the directions and you will be fine.
Open up Flight Simulator and look for the plane you just put in. If you do not like the plane you can easily remove it by simply deleting the file in the Airplane or Rotorcraft folder.