Both of your sentences are grammatically correct. Here is a simple, step-by-step explanation a 7‑year‑old can understand:
- Sentence 1: The mighty rotor lifts the helicopter into the air and lets it hover or fly any way it wants.
Why this is correct: "rotor" is one thing (singular), so the verbs are "lifts" and "lets" (singular). The sentence is fine. - Sentence 2: The mighty blades of the rotor lift the helicopter into the air and let it hover or fly any way it wants.
Why this is correct: "blades" is more than one (plural), so the verbs are "lift" and "let" (plural). This sentence is also fine.
One small improvement to make the meaning clearer is to say "in any direction" instead of "any way." A clearer sentence would be:
Improved sentence: The mighty rotor blades lift the helicopter into the air and let it hover or fly in any direction it wants.
Tip: You can use either the first or the second sentence. The second (or the improved version) tells us more clearly that the rotor has blades that do the lifting.