Wizo,
Typically the kinematic viscosity of air is ~10^-5, which means that a commercial aircraft tends to fly in a regime where the Reynolds number is ~10^7.
You can see as a result, that you'd need to change the velocity of a flow by a hell of a lot to get a significant change in Re.
As John said, Re is a dimensionless number used to denote similarity of flow regimes. If you are testing a model of aforementioned commercial aircraft in a wind tunnel, you need to match the Re during testing.
Depending on what you're trying to measure you may need to match the Mach Number as well. Mach is just another dimensionless number like Re.
Re denotes the ratio of inertial to viscous forces in a flow.
In the regime we are typically interested in, inertial forces are large compared to viscous ones.
You won't see much dependence of lift coefficient with Re but, conversely, it will be important to match Re if you want a good drag estimate.
I remember when doing my PhD some colleagues trying to obtain laminar flow over wings at high Re with the intention of applying this technology to commercial aircraft.
They laser drilled thousands of minute holes in the upper surface of their wing and attempted to suck away the boundary layer, thereby delaying transition.
Of course, in any technical undertaking like this one has to deal with bugs.
In this case, the bugs kept blocking the laser drilled holes and the exercise was less than satisfactory.
NASA claim to have made progress on this front though:
http://www.nasa.gov/centers/langley/...HSR-Wings.html
with their F-16XL:
