RE: Importance of engine break-in ?
CW,
Although safety is of utmost concern in this hobby, it is not the reason some recommend a flying break-in.
The concern of break-in technique, is its safety to the engine's innards, not to the operator's fingers, or to anyone's physique...
It is needless to say that if the break-in is done on a test-stand, it must be firmly tightened to a solid object, so it cannot 'fly-off'...
The same goes for the 'poor' model that its owner decided to punish, by doing the break-in on it.
After you got the engine started, any adjustments must be done from behind the prop - not over/around its spinning blades.
Any proper test-stand will allow this, at a much lower risk to anyone's fingers, or self...
A model may not be that user-friendly, if the engine is installed and access is somewhat limited.
Once a prop has created a vortex around the engine; for all practical purposes, it is flying... Its running temperature is set using the needles and this temperature is virtually the same; whether it is running in 110ºF of sweltering heat, or at -25°F.
Despite the great difference in air temperature, your car's coolant temperature is the same 195°F; in the dead of winter and in mid summer.
It is just controlled differently...
And, of-course the setting of the needles will not be the same in both hot and cold conditions.
The engine will be running cooler, neither on the stand, nor in the air; but on the stand the user can make adjustments, which would be impossible when in flight, so for the 'flying break-in' the user will be advised to set it a bit richer than ideal, to avert a lean condition that may develop during the flight.
This may, in turn, prolong the break-in process...
On the ground, the user can set the needle 'on the spot' and change the settings on the run; if they prove to be a bit too rich, or too lean.
In the air, the setting you take-off with is the same as the one you land with... You cannot make changes during the flight, to compensate for the advancing break-in, that you can easily make on the ground.