Regarding the curvature of the Earth I have a doubt in Earth's curvature. This question may be dumb.

How much we need to travel on the Earth to feel the curvature of Earth? Hypothetical case: for example, we are building a vehicle. How much is the length of that vehicle to feel the curvature of the Earth?

A more question, what is about the speed of the Earth. Again a hypothetical case. If we are travelling in aeroplane from one country to other, whether we can take Earth's rotational speed (rotating around it's own axis), and aeroplane speeds as comparables and find the resultant speed?
About curvature, you can see it. Just look at a ship some hundred meters away: you'll notice if you are careful that you can not see where the ship touches water.

You can see at http://mathcentral.uregina.ca/qq/database/QQ.09.97/dyck2.html that the curvature is 8 inches per mile.

On the other hand, about rotation, planes do actually take advantage of earth rotation, but not directly. This was answered in https://physics.stackexchange.com/questions/16390/does-the-rotation-of-the-earth-dramatically-affect-airplane-flight-time