Ever wonder why the rotary lawn mower can cause the lawn to be damaged. Here on this lawn, this spot is obviously caused by the mower itself. Harder to see in the photo, but the grass blades are bent in a circular direction. Why is it that the centre is green? Then an area with the most damage and then areas that seem okay.Rotary mower engines are made to spin at 3,600 revolutions per minute when not under load. During cutting, the rpm drops to between 2800 and 3300.
The centre of the blade is spinning at 3600 rpm, but the velocity of blade itself will vary depending on the length of the blade. An 18 inch blade tip will travel through the air at one speed, a 21 inch blade at a faster speed and so on. This also means that the centre of the blade is travelling slower than the tip.
Does the speed of the mower affect how well the grass is cut? Often if the grass is thick, I find there is a strip of grass that does not get cut very well. If the riding mower is set at a lower speed, this strip in the centre get cut better than if the speed of the mower is faster. In this case, the grass in the middle does get the mower blade passing over it, but at some times, the centre of the blade is also over this area and not cutting is done. At the side, the cutting edge which is at the end of the blade is over the lawn longer -therefore that area gets cut better.
So now in the case of the circle in the picture, did the brown area get cut more by the centre of the blade, that part that is not sharpened. So that there was more tearing of the blades, rather than cutting with the sharpened part? Tearing of the blades would cause more stress and more damage to the lawn than being cut by the sharper part of the blades at its ends.
The cente didn't get cut at all when the mower was sitting over this spot. It got cut when the mower moved on. Then it was cut by the sharper part of the blade -hence very little damage.
I don't know if I am 100% right, but that's my theory.