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On the other hand , the long rotor blades restrict the maximum speed to about 250 miles per hour ( 400 " km/h ) of at least conventional helicopters , as retreating blade stall causes lateral instability .
 retreating blade stall
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 retreating blade stall