ORIGINAL: dirtybird
Once the model begins to move the propeller is relieved of its load slightly so the engine gains in RPM. With higher RPM there is more air moving thru the carb so the mixture must be enriched to compensate.
I hope you learn this quickly or you are going to go thru engines at a rapid rate.
I know how to tune a gas engine and only asked the question because I encountered an issue that was new to me and had not been able to determine the root cause. However, thanks for honoring me with your insight.
Oh by the way there are a number of other issues that will result in a deadstick - bad ignition, bubbles in gas line, clunk falls off inside the tank, defective carb, etc., etc., etc.
This is not my first time at the dance.