For the garden rotary mowers are a comparatively new design. A steel cutting blade rotates at high speed above the lawn. Basically this rather crude blade beats the grass in two rather than slices through it. Here lies the problem. The remaining grass stalk is damaged and can lead to unhealthy discoloured patches on the lawn.
A second disadvantage of the rotary mower is that it can't cut as close as a cylinder design. The rotating blade covers a larger area of lawn at once than the cylinder design, meaning the blade is more likely to come into contact with bumps on the surface, leaving bare 'scalped' patches. It can also lead to stones and other debris being sent flying out of the sides, leading to lame statements from the operator such as "Oops. Well I used to have a greenhouse".
A sub-design of the rotary class is the familiar "hover" design. This has a fan that creates an air cushion beneath the mower, lifting it clear of the ground so that it can be pushed without being impeded by wheels. Some people like this.
Rotary grass mowers sometimes have a "mulching" feature. This makes a virtue of leaving grass cuttings on the lawn. The mulcher chops the cuttings up very finely before depositing them on the lawn as a simple kind of compost.
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