The flight envelope is the limits of speed and normal acceleration to which an aircraft can be flown.
You'll find definitions of it, with various levels of complexity, in all of the fixed wing light aircraft certification codes. The simplest, and certainly good enough for an HND, will be that in JAR-VLA, which you can download from <a href="http://www.jaa.nl/jar/jar/355000/355325/355325.pdf" target="_blank">http://www.jaa.nl/jar/jar/355000/355325/355325.pdf</a> the important stuff is in part C, structures. The full and unadulterated airliner version is in JAR-25, but unless you are genuinely working on an airliner, I'd not worry about the extra level of complexity there.
Alternatively, there's a fairly superficial version in Megson's "Aircraft Structures for Engineering Students", which you should find in your college library.
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