There are three trains of thought that I use to generate good ideas for planes or other builds.

(Train 1)

Identify a problem that requires an engineering solution.
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Can it be fixed?
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What design will fix it?
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Mount your solution on a plane or something.

(Train 2)

Hey, it would be cool if I could have something that does (Blank).
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How can I do it? (Notice how you don't ask if you CAN do it)
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Do it.

(Train 3)

I made a pretty nice innovation.
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Good job. Can this cool innovation be applied to anything else? Or can you improve on it?
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Think of something else that your innovation can apply to, or how your innovation can be improved.
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Upload what you make from that existing innovation.

Or, if you don't have much creativity, you can just look at concept art for cool planes or something.