Static stealth aircraft
If the aircraft is static (parked on the ground) the bats echolocation would enable them to "see" the aircraft and avoid it. Bats' echolocation uses sound waves at 15-100 KHz. Stealth aircraft are not designed to reduce their reflection of sound waves, they are designed to reduce reflection of electromagnetic waves (primarily radar).
Flying stealth aircraft
If bats are in the path of any aircraft flying at typical subsonic aircraft speeds (say 50-200 m/s), the bats would probably not have time to evade the aircraft. Like us, bats evolved on a planet where nothing much flew above 10-20 m/s (some hawks can dive at 50 ms or faster, but not at night when bats are active)
- Bats fly at less than 10 m/s
- F117s cruise at over 300 m/s
Therefore it isn't accurate to think of a collision in terms of bats flying into an airframe, the bat is relatively stationary.
Bat echolocation range
The bats reacted to insect prey at dis-
tances of about 70 to 120 cm. Given the flight
speed, the detection distance was estimated to
about 110 to 160 cm.
lets say a bat echolocates a cruising F117 closing at 160 cm distance, the sound echo travelling from F117 to bat at 340 m/s reaches the bat after 4 ms. The F117 arrives 1 ms later. I doubt the bat's brain has even time to register this, but if it could react in less than 0.01 ms, it can only fly a few mm before the F117 strikes it.