If you have a new aircraft and it is not known to FSXPilot,  FSXP uses a Learjet 45 algorithm for flying. 
But: it is much better to add the aircraft in the aircraft database.
Shareware users will have to do that manually in fspilot.ini, PRO users should always add aircraft through the aircraft options dialog window.
First please write down the model ID that is given in the message box when FSXPilot complains about not knowing the aircraft. 
The rest of the process is described in: How to add your third party aircraft to the aircraft library
Short "tweak" tutorial: 
If the aircraft banks too much -> decrease bank sensitivity. 
If the aircraft banks slow and does not hit runways or waypoints -> please increase bank sensitivity
If the aircraft flies waveforms (pitch) please modify the pitch sensitivity (often you have to reduce that)
If throttle is often overcorrected  -> please reduce speed sensitivity
If aircraft GPS-autolands too short on runway -> please increase flare height or increase approach speed a little bit
If aircraft bumps on runway with too much speed   -> please increase throttle zero altitude or reduce approach speed a little bit.
It is always fun to tweak these values until you have your perfect settings in a new plane. 
AND PLEASE: Don't forget to send the super line you perhaps made for your valuable addon, We could include it in the next update !