You can find the type designation to use for your aircraft when filing a flight plan using the following resources:
If you have an Experimental aircraft and it has not yet been assigned a unique aircraft type designator, you can enter the ICAO designator "ZZZZ" with a description of the type in the Remarks section of a flight plan.
If needed, use these designators for the following aircraft:
a. BALL – Balloon.
b. GLID – Glider/sailplane.
c. GYRO – Micro-/ultra-light gyrocopter.
d. SHIP – Airship.
e. UAUH – Micro-/ultra-light autonomous unmanned helicopter.
f. UAUL – Micro-/ultra-light autonomous unmanned aircraft.
g. UHEL – Micro-/ ultra-light helicopter.
h. ULAC – Micro-/ultra-light aircraft.
i. UOPH – Micro-/ultra-light optionally piloted helicopter.
j. UOPL – Micro-/ultra-light optionally piloted aircraft.
k. URPH – Micro-/ultra-light remotely piloted helicopter.
l. URPL – Micro-/ultra-light remotely piloted aircraft.
Source: US DOT FAA Order JO 7360.1