Complex sentence: In typical designs, as long as the lantern stays upright the paper will not get hot enough to ignite, but if the balloon is tilted (say, by the wind or by hitting some object), it may catch fire while still in the air.
Split into simpler sentences: In typical designs, as long as the lantern stays upright the paper will not get hot enough to ignite. However, if the balloon is tilted (say, by rough winds or by hitting some object), it may catch fire while it is still in the air.