"Jose" wrote in message
. net...
[...]
Does the pilot have to wait until the airplane's speed down the runway is
twice Vr before he can take off?

As long as the fan is blowing the airplane forward, it will be impossible
for the airplane to take off. The only way for the fan to move the airplane
forward would be for the airplane to be moving more slowly than the air,
which means the airplane has a continuous tailwind.

If, on the other hand, you make the assumption that you can remove the fan
at any time, you can arbitrarily decide to remove the fan at the normal Vr
and the airplane can take off normally.

There are a variety of other assumptions one could make, due to the
incredibly ambiguous nature of your question, and the exact outcome depends
on how those assumptions are made. I simply provided the two "most
reasonable" assumptions that came to mind, for the sake of discussion.

Pete