Tying the model down and letting it run for extended periods on the ground will ruin any engine.

If you were able to do so, you would see a very fine cloud of dust being pulled from the ground and into the engine's carburetor.

Go to a pattern contest and see how much effort most pattern pilots go to to minimize their engine's running time on the ground. Those in the know will have a drill worked out to keep this time as a minimum. Dust/dirt is abrasive and is similar to running lapping compound through your engine.

No engine will tolerate this kind of abuse for very long without showing a drop in power production.

Years ago, in RCM, IIRC, there was a picture that was taken of a model airplane with running engine at dusk. Car headlights were illuminating the dust cloud as it entered the engine. It changed my behavior around engines permanently.

Ed Cregger