This is Why Some Aircraft Use Propellers and Others Fans

Does a propeller pull itself through the air like a screw pulls itself into wood? Or does it push itself forward by blowing air backward? Why do some aircraft use propellers and others use fans? And why are fans placed in a dut but not propellers? These are some of the questions we'll answer in this video by exploring the differences between propellers and fans. A propeller converts rotational motion into thrust, much like a screw converts rotation into linear motion. But while they seem similar, the way they generate thrust is fundamentally different. A propeller doesn't pull itself through the air like a screw. Instead, it accelerates air rearward. If you don't think a propeller blows air back, just look at an aircraft taxiing on a dusty gravel surface. Even at cruise speed flight, the air pushed back by the propeller, called the slipstream or propwash, moves faster than the surrounding free stream air. This clearly shows that propellers blow air backward, like fans do. The equation to calculate thrust proves this, and we'll get there in a minute. The confusion around a propeller being a screw is partly because propellers are often called air screws. But this refers to the helical path the blade tips take as it cuts through the air. And as far as thrust is concerned, a propeller acts much more like a fan than a screw. Yes, it's all based on Newton's third law. Every action has an equal and opposite reaction. Blow air back and the aircraft is pushed forward. But the interesting part is how a fan and a propeller goes about this differently. Thrust is the reaction to the action of blowing air back. Thrust created is equal to the velocity of the propwash minus the velocity of the free stream air times the mass flow rate of air moved. So the faster the propeller blows air, the more air is moved per second and the more thrust is created. However, moving air back as fast as possible is not a very efficient way of creating thrust with a propeller, as described by momentum theory. The engine power required to generate thrust is the square of the velocity of the propwash minus the square of the velocity of the free stream air times half of the mass airflow rate. Thrust thus increases linearly as the difference in velocities increase, but power required to produce that thrust increases with the square of the velocity difference. What that means in layman's terms is, to use the least amount of engine power to produce a certain amount of thrust, we want to move as much air as possible, but we want to accelerate it as little as possible. And we don't do that by spinning a propeller as fast as possible, but by spinning it as slow as possible. A slower spinning propeller is more efficient, but it produces less thrust. To maintain thrust while reducing propeller speed, the diameter of the propeller should be increased. But due to limited ground clearance and the need to keep blade tips below about Mach 0.8 to avoid compressibility problems and keep noise levels low, propeller diameter is limited. In order to create enough thrust without spinning a propeller faster, the number of blades needs to increase. This is why more powerful propeller engines have larger multi-blade props. Fans are typically used inside ducts, especially in turbofan engines. This is for two main reasons. The first is fans generate significantly higher pressure ratios than propellers. This is essential for the engine core and even for air that bypasses the core. Without a duct, this pressure would dissipate into the atmosphere. The second reason is the duct interferes with blade tip vortices and thus helps minimize the blade tip losses that open propellers experience. Fans typically have smaller diameters than propellers, which lowers tip speed and enables them to spin faster. Additionally, the duct reduces shock wave generated by blade tips approaching Mach 1, allowing fans to operate at higher Mach numbers than open propellers. So a fan may look like a propeller just with more blades, but fans are more like compressors, producing a pressure rise behind the blades. The duct helps contain that pressure and uses it to create thrust by feeding it through the bypass nozzle. Air naturally moves from high pressure to low pressure. In a turbofan engine, the fan pressurizes the air. The duct prevents this high pressure air from escaping sideways, and since it cannot move forward through the fan, it exits rearward through the bypass nozzle. As the air expands and accelerates out the back, pressure energy is converted into kinetic energy, which produces thrust. Propellers usually generate a total pressure ratio of around 1.05, only about a 5% increase. Fans, especially in high bypass turbofans, reach pressure ratios between 1.4 and 1.7, so up to 70% higher than ambient. This pressure ratio is relatively low compared to the pressure increase in the compressor stages of the engine, but that's beyond the scope of this video. Fans are designed to compress air, while propellers are designed to move large volumes of air gently to produce efficient thrust. That's why propellers perform poorly when enclosed in a duct. They are not designed to create that high pressure, which is crucial in turbofan engines. Propellers are ideal at lower airspeeds. The large diameters allow them to move more air with less acceleration, maximizing efficiency. However, as aircraft speed increases, propeller efficiency drops. Why is this? Because propellers rely on a pressure differential, slightly lower pressure in front of the blade, about 2%, and slightly higher pressure behind. As the aircraft flies faster, more air is ramped onto the front of the propeller, raising front side pressure and reducing the pressure difference. At high enough airspeed, the propeller can't produce any thrust. Fans don't have this limitation, despite air ramped onto the fan also increasing pressure in front of the blades. The more air is ramped onto the fan and into the duct, the further the pressure behind the fan increases, and the duct helps preserve pressure ratios at high speeds. The duct also allows blade tips to approach or exceed Mach 1 and minimizes aerodynamic losses. Propeller blades usually have more camber and are optimized for moving large volumes of air relatively slowly. This is great for static thrust and low speed efficiency, but is less efficient at high airspeeds, especially close to subsonic speeds at which airliners fly. Fan blades, on the other hand, are flatter and optimized for compressing air and maintaining efficiency at high speeds. They produce less static thrust, but are far more efficient during high-speed cruise flight. In high-bypass turbofans, inner fan blade sections are optimized for compression into the engine core, while outer sections focus on bypass thrust. A ducted fan of a smaller diameter operates as efficiently as an unducted propeller of a larger diameter, and also operates quieter than an open propeller. So why not use them on small aircraft? Well, turns out a duct adds significant weight, adds drag, and decreases maintenance accessibility. Ducts would also drastically reduce forward visibility on single-engine aircraft. As for rear-mounted ducts, they require complex fuselage designs and make engine cooling more difficult. Simply put, it's hard to beat the simplicity, cost and low-speed efficiency of a nose-mounted engine and propeller for light aircraft. Turboprops use propellers, and they are generally quite fast, right? Well, these aircraft cruise at speeds around Mach 0.5, around 250 to 320 knots. At these speeds, the benefits of turbofans don't outweigh its drawbacks, because they only become more efficient exceeding about Mach 0.65 or 450 knots. Around the 300 knots cruise speed mark, using large-diameter props with swept blades and variable pitch mechanisms allow these aircraft to operate on the very limit of propeller efficiency. Turboprops are sometimes also preferred over turbofans for its superior static thrust, which improves short take-off capability, allowing for operation from shorter runways. There's a lot more relevant information on the topic, but this is a basic overview of the differences between propellers and fans without getting bogged down in difficult-to-understand equations. We also didn't cover open-rotor turbines or propfans, which blur the line between propeller and fan. If interested, the Mentor Now YouTube channel did a great video on the CFM Rise propfan engine. If propfans are a subject you're interested in learning more about, let me know in the comments. I have an idea for a video approached from a different angle. Here is a summary table of fan vs propeller pros and cons. Feel free to pause and review. Thanks for watching, see you in the next one.