Caveat: This is the procedure as I best recall and it worked for me, and the approach is done in a way that has the least chance of giving you any problems. I have FSX Deluxe with Acceleration, but I don't know if this works with the basic version of FSX. You need to be very comfortable with editing your FSX configuration files and you should make a backup copy of any file that you modify. Don't blame me if you try this and your FSX gets messed up. I highly recommend you read at least the first of the above FS Insider articles and make sure you understand it before you try the instructions below.
Another Note: RealAir has a C172 for FSX coming soon: http://www.realairsimulations.com/list_box.php?page=coming. I'm sure it will be a lot nicer than this freeware one, so you can wait for this one if the instructions below seem like too much trouble. :-)
OK, here's what to do:
First, download the RealAir C172 here: http://www.realairsimulations.com/list_box.php?page=downloads&sim=all&product=allfiltered&fileType=freeware&submit=Go. You will get a zip file. Unpack it in a temporary location.
Make sure FSX is not running while you fiddle with files in its installation folder and with fsx.cfg.
Now go to your FSX installation folder (where fsx.exe is located) and add a new folder called "Aircraft". This folder is where you'll add the new airplane. Now go into the SimObjects folder and under that the Airplanes folder and Copy the C172 folder. Then go back to your new Aircraft folder and paste the C172 folder there. At this point you have made a copy of the default C172. Rename this new folder "RealAir C172". Now copy all the files from the temporary folder with the contents of the downloaded zip file into this folder and let Windows merge the folder structures. Make sure you are copying these files into the new RealAir C172 folder and not the original C172 or you'll mess up your default airplane! Tell Windows to Copy and Replace any files so that the file from the downloaded zip will overwrite the file in the directory.
Go into the RealAir C172 folder and delete the texture.1, texture.2, texture.3, texture.4 folders and also the folders that say .G1000 on them. Also delete the Cessna172SP.air file, as the RealAirSimulationsC172SP.air file will be used instead.
Now you need to open the panel.cfg files for both the new RealAir airplane and also the default C172. To do this, go into the panel folder and open the panel.cfg in both airplane directories. In the new RealAirC172 panel.cfg, delete the sections of the file marked as [VCockpit01] and [VCockpit02], that is delete that text in brackets and all the lines underneath up to but not including the next section which should be called [Default View]. Now go to the default C172 panel.cfg and copy its [VCockpit01] and [VCockpit02] sections and paste them into the RealAir panel.cfg in place of the text you deleted. Save the modified panel.cfg.
Now you need to edit the RealAir aircraft.cfg, by just adding the following lines under ui_variation in the [fltsim.0] section:
ui_typerole="Single Engine Prop"
One last thing, you need to tell FSX how to find this new airplane. Find your fsx.cfg file and add the following line in the [Main] section:
If you already have a SimObjectPaths.6 defined, use the next number in the sequence. The point is to tell FSX to look into your new Aircraft folder for any airplanes you may have added. In the future you can use this Aircraft folder to add additional FS9 airplanes.
That's it! You now have the RealAir C172 with both 2D and 3D cockpits. Start FSX and try it out. Notice the added suspension sounds and wind sounds. NOTE: You need to have your Realism slider full right to get the benefits of the new flight model, this is typical for addon aircraft and especially for any RealAir product.
For repaints, it appears that FSX repaints for the default C172 work OK.
If you want to improve on the sounds, you can go to to simmarket.com and download a C172 sound pack, they have several products there for the C172.
By the way, I did try doing spins in both the default C172 and the RealAir. In the default plane, you just get yaw. In the RealAir you properly go into a spinning dive, you can look at YouTube and see examples of doing a spin in the RealAir.