Open-Fan Engines: Why a Bigger Fan Burns Less Fuel

CFM's RISE demonstrator has no cowling around its fan. The blades turn in the open air, which looks like a step backwards towards the propellers everyone abandoned in the 1950s, and is being done in pursuit of more than 20% lower fuel burn than the engines flying today (CFM International). The reason the shroud is coming off is a piece of physics that has been true the whole time.

Thrust is easy. Efficient thrust is not

An engine makes thrust by taking air in and throwing it backwards faster than it arrived. The thrust you get is the mass of air per second multiplied by the speed you added to it. So there are two ways to make a given amount of thrust: hurl a small amount of air very fast, or move a large amount of air gently.

They are not equivalent, and the reason is that thrust scales with the velocity change but the kinetic energy left behind in the jet scales with its square. Every metre per second of exhaust speed you do not need is energy the engine paid for and threw away. Air departing the back of the aeroplane faster than the aeroplane is flying is, from the engine's point of view, waste heat with a nozzle around it.

This is propulsive efficiency, and its form is unforgiving: it improves as jet velocity approaches flight velocity. Perfect efficiency would mean pushing an infinite amount of air by an infinitely small amount, which produces no thrust at all. Real engines live on the slope between those extremes, and every generation has crept along it in the same direction — bigger fan, slower jet.

What the cowling was for

If bigger fans are better, the obvious question is why fans are not already enormous. The limit is the duct around them. A turbofan's nacelle has to enclose the fan, which means its frontal area and its wetted surface grow with the fan — and both of those are drag. Past a certain diameter, the drag of the housing eats the efficiency the larger fan was supposed to deliver. The engine also has to fit under a wing without the aircraft needing longer landing gear, which is weight, in an entirely different department's budget.

Delete the nacelle and that limit goes away. The fan can be far larger, bypass ratio rises sharply, jet velocity falls, and propulsive efficiency climbs the slope. That is where the fuel saving comes from — not from a better core, but from a better bargain between mass flow and jet speed.

The cowling was doing real work, though, and all of it has to be replaced. It shaped the flow into the fan, so the blades saw clean, aligned air; an open fan meets whatever the airframe hands it, at whatever angle. It contained a failed blade, which is a certification requirement, not an aspiration. And it was an acoustic liner, absorbing the fan's tonal noise before it reached the ground.

Why there are two rows of blades

A single open rotor leaves its wake spinning. That swirl is angular momentum the engine paid to create and gets nothing back for — the same waste as excess jet velocity, in a rotational direction.

A ducted fan solves this with stator vanes behind the rotor, straightening the flow and recovering the swirl as useful pressure. Without a duct there is nothing to mount stators on, so the open-fan answer is a second row that also turns, the opposite way. It straightens the flow, and because it is doing work rather than merely redirecting it, it recovers the swirl energy as thrust.

The second row helps acoustically too, for a reason worth stating plainly: two rotors passing blades at different frequencies produce pressure waves that can be tuned so their peaks partly land on each other's troughs. It is not a trick — it is the same interference any two sound sources exhibit — but it only works if the blade counts and spacings are chosen for it.

The trade the whole design turns on is easier to feel than to read. The turbofan demo lets you move bypass ratio and watch specific fuel consumption respond, and the turbofan anatomy shows where the ducted version puts its stators.