Originally Posted by mwhitted
That's why I laughed at the UPS adds talking about washing their planes because carrying around a lot of dirt would cost so much and they run "The Tightest Ship In The Shipping Business"... but all of that PAINT weighs more than any DIRT that could possibly stick to the plane.
It is not just the weight of the dirt but how the dirt interupts the flow of air over the body. A "dirtier" plane will create more drag because the air doesn't flow as smoothly.