Riff,riff_raff wrote:vonk,
A liquid-to-air heat exchanger core requires higher air pressure ahead of the core than behind it, otherwise the airflow would stagnate. So one might consider that a "forced" condition.
As for the size and orientation of the core, it is a compromise between conflicting requirements of core surface area, thickness, pressure drop, heat rejection, rules, etc. The core would produce the least drag if the face (and airflow channels between the fins) were oriented normal to the airflow. But given the heat exchange surface area required and the side pod space available, this would result in a core with a small frontal area and an extreme thickness. Such a thick core would likely produce a high pressure drop and unacceptable flow loss.
Apparently, the better compromise is to use a thin core with large frontal area, but mounted at an angle to the local airflow direction so that it fits within the side pod space. There are still flow losses with this arrangement, since the airflow loses velocity when it has to change direction to pass through the core. But still maybe less than those of a very thick core.
Of course, one notable exception to this was the radiator on the P-51 Mustang. It had a small frontal area and an extreme core thickness, but was purported to have no drag penalty.
So, a well designed heat exchanger and duct installation should produce little, if any, drag penalty. In fact, it may even produce a small net thrust. You need to consider that there is lots of thermal energy being imparted into that cooling air mass that is being accelerated out the duct exit.
riff_raff
I agree with you, although I understand that the Germans denied at the time that the Mustang oil cooler really produced any thrust.
In theory, a well ducted radiator system can produce net thrust depending on the heat added by the radiator.
Conversely, the more turbulent the cooling flow becomes inside a moving vehicle, the more drag derives from it. Seen from inside the vehicle, the incoming air losses its directed momentum to turbulence and must be pushed out by increased ram pressure at the inlet. In F1 cars, ducting is a big problem, particularly as concerns the outlet.
As for the zigzag fins in the core, they serve to increase the convection contact area for the passing air, but they come at the expense of skin friction drag. It’s hard to tell from the RBR picture, but if they use fins, they should be staggered to minimize flow redirection. This would also provide a longer contact path for the air inside the core. Since fins increase heat transfer, they might even allow a smaller core size.
