RealFlight expects the controller to appear as a Windows game controller.
Some radios can do that via USB. Another way to connect is through a USB wireless receiver (often called a "dongle"). The wireless receiver behaves like a regular receiver for your transmitter, but converts what it hears to Windows game controller data. Wireless is convenient, but the receivers are often limited to just 8 channels.
If neither one of those is an option, then the InterLink DX controller has a wired trainer input so you can use it as a giant adapter. But if you were to buy an InterLink DX, it's a lot easier to just use the InterLink DX as the controller.