Tips: (Tested on Linux Only)
1) Download the application from RealFlight.
2) Optionally but preferably, set up a virtual desktop using Crossover Control Panel "Wine Configuration" for your installation bottle.
3) Install into your bottle of choice (XP works the best so far) as an "unsupported application", "other application" .
4) The install wizard will open and expand the files for installation and display a progress graphic.
5) At completion of file expansion, a long pause of up to 5 - 10 minutes may occur depending on system speed. The application is not frozen and will eventually proceed to the next step so be patient (this delay also occurs in Windows but not as long).
UPDATE: With the release of Crossover 12.x, this delay is reduced to seconds rather than minutes (not much longer than Windows now).
6) The application wizard then attempts to install DirectX9 and displays the MS Agreement so dismiss the box with a "Yes".
UPDATE: Again the newest Crossover releases handle DX9 well so the application no longer needs the MS version. At conclusion of the installation, the installer will leave one with the MS DX9 agreement without trying to install it; just clik "Agree" to clear it.
7) At completion of the install, an information box will appear stating the install is complete with a "launch application" box checked. Un check the box to close the application.
8) When the application is closed, Crossover will finish installation and refresh the desktop and menu with the appropriate links.
9) When the application is opened, the Great Planes logo will appear then an error box "Error: E_FAIL File: .\DXControllerG2Interlink.cpp Line: 119" will pop up. Click ok to dismiss this error (3 times). The application will then open for your enjoyment using the keyboard as the controller..
Edit: Updated 7/14/ 2013 to reflect new Crossover Versions 12.x and newer.