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P51 Mustang Net Thrust - Net Downforce

Jay Chen
Aug 26, 2008
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Sccp_0808_04_z+p51_mustang_net_thrust+p51_mustang Photo 1/3   |   P51 Mustang Net Thrust - Net Downforce

Long ago, aerodynamicists stumbled upon the concept of net thrust when they were busy designing warbirds like the P51 Mustang. While the definition of net thrust can apply to a lot of things, what I'm talking about is getting thrust for free.

Before turbo jets, turbo fans, ram jets, scram jets and pulse jets, planes were powered by piston engines-similar to car engines, just a lot bigger. Since aerodynamics came first, most planes were either limited to air-cooled radial engines (like on most navy fighters) for serviceability on board a carrier. Or they had water-cooled in-line or narrow V-bank engines that fit inside the long, slender nose of something like the Mustang. Water-cooled engines obviously needed radiators, but because of the lines of the fuselage and turbulence from the prop wash, placing a sufficiently large radiator in front, automobile-fashion, doesn't work.

So designers took advantage of the fast airflow over the plane and placed heat exchangers under the body or in the wings. Even though fast-moving air through a heat exchanger is efficient, big flat surfaces like the face of a radiator is bad for drag, whether in a plane or in a car. In order to maximize cooling without adding massive drag, designers were forced to use a radiator with a smaller frontal area and a thick core. The problem there is that it takes pressure, not speed, to drive the airflow. Without adequate pressure, the front face of the radiator would become a stagnation zone.

To change flow velocity into pressure in a confined space, you have to slow down the flow. Bernoulli's equation states a proportional relationship between pressure, flow area and speed. Since flow rate doesn't change much, the inverse relationship between velocity and pressure means that if flow becomes slower within an enclosed area, the pressure would rise. The easiest way to reduce velocity is with a diverging nozzle. If the airstream enters through a small opening (small cross-sectional area) and exits a larger opening, the air velocity going out would be slower than what's going in, and the pressure would be higher at the outlet than the inlet.

A diverging nozzle or duct at the inlet of the radiator cuts the speed and increases pressure in front of the radiator. The pressure ensures that air has enough potential to push all the way through the resistance in the radiators' fins and tubing. Without that pressure, the flow would stop and all subsequent air would go around the radiator duct, making it useless.

On the outlet side, the opposite effect is desired. If there is little pressure on the back of the radiator, the flow in front will be sucked through. Dropping the pressure means using a converging nozzle and trading pressure for speed, so the outgoing flow is now accelerated. But that outgoing flow will never be as fast as the incoming flow, because of drag, and frictional losses induced by the radiator and turbulence generated off the converging and diverging nozzles. The net result is still drag.

Here's where net thrust comes in. Since the four-stroke internal combustion engine has a real-world thermal efficiency of under 50 percent, over half the fuel's energy is wasted as heat through the exhaust or sucked up by the cooling system. In a few instances, we scavenge some of this to pre-compress the intake charge, such as a turbocharger. But the majority is still dumped into the atmosphere.

Sccp_0808_03_z+p51_mustang+p51_mustang_net_thrust_diagram Photo 2/3   |   P51 Mustang Net Thrust - Net Downforce

However, energy is energy whether it's heat or velocity and it can be converted (with some loss) from one form to another. Transferring heat from the radiator to the air flowing through it energizes the airflow by heating it up and changing its density. The air in front of the radiator has less energy than the air behind it. With more overall energy, that heat can be converted into exit velocity, making the outgoing flow faster than the incoming flow and voila, you have thrust. Do it right and the total thrust gained from the exit nozzle is more than the total drag incurred from the entire radiator, ducting and cowling assembly.

That's exactly what North American engineers stumbled on in the belly radiator ducting of the Mustang. Pilots reported that, at the right altitude and airspeed, the Mustang was flying faster than the engine should be able to pull it. That's because the radiator ducting had now turned into a thrust nozzle.

So how does this have anything to do with a car? While thrust is good for straight-line speed, downforce is better for the track. Just like free thrust, if the exit flow is turned up slightly, the thrust pushing forward is now also pushing down. With the right design, this becomes net downforce, where the downforce gained is worth more in terms of lap times than the drag incurred by the radiator.

Sccp_0808_02_z+p51_mustang_net_thrust+go_kart Photo 3/3   |   P51 Mustang Net Thrust - Net Downforce

Designing one that works is a whole other story and is typically better with sleek formula cars with side-pod-mounted radiators that vent up and back. But some road cars can take advantage of this, especially mid-engine cars where the hood doesn't have to clear an engine, and air is forced through diverging ducting in front of the radiator into a converging duct out and back. This is why GT cars all have that large, gaping hole in the hood, even if the engine isn't in front.

While I doubt any of us have the resources to develop net thrust, it's a great concept to play with while testing at the track. The angle of convergence and divergence is critical to prevent flow separation, which results in massive drag and reduced cooling. But some basic calculations and ambitious CFD tests by the industrious closet engineer will get you into the ballpark before testing begins. Did anyone say Project NSX?

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By Jay Chen
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