There are many different ideas of how airplane propellers work, and I'm not talking about the basic principle of how thrust is created, but more around practically applicable factors like whether a two or three blade propeller is better for a certain application, or when a large or small diameter prop is better. There are some myths and misconceptions floating around, most of which because correlation is taken for causation. Let's start with the first one. The first myth we are looking at is that three bladed propellers are less noisy than two bladed propellers.
Actually three bladed propellers are usually quieter than two bladed propellers but not because of the amount of blades. Let me explain. Many things contribute to propeller noise like the shape of the blades and especially the blade tips. Then there is propeller vibration which is actually perceived noise. So a well-balanced propeller will seem quieter.
But there is one factor that has a much bigger impact on propeller noise which is blade length. For a given propeller RPM the longer the blades are the faster the blade tips travel. Here's a simple formula to calculate the speed the blade tips are traveling at.
So at any given propeller RPM There will be a blade length that if exceeded the blade tips will break the sound barrier. When that happens propellers lose aerodynamic efficiency and become very loud. One popular and borderline cliched example of this occurrence is the T6 Texan and the distinct noise its propeller tips make as they break the sound barrier.
Of course ground clearance also limits a propeller's diameter, but aerodynamically speaking blade length is a balancing act because longer blades are more efficient, a fact that will get increasingly more obvious as the video progresses. But make the blades too long and the propeller tips exceed the speed of sound, reducing efficiency and increasing noise. Many of you already knew that.
I already knew that. But something not always considered is that the propeller tip as a whole does not need to outright exceed the speed of sound for this loudness and loss of propeller efficiency to occur. Looking at a side profile of a propeller blade we see it resembles an airplane wing.
I'm not getting into how thrust is created again, but But one fact is important and that's air going over the top of the blade is accelerated and the air moves faster than the blade cuts through the air. This causes the faster going air over the top of the blade to reach the speed of sound at blade tip speeds lower than Mach 1.0. Depending on the camber, air going over the fastest flowing section of the blade will be supersonic at blade tip speeds. of as low as Mach 0.8 and thus loss of propeller efficiency and excessive propeller noise is experienced at tip speeds well below the speed of sound.
So at any given rpm as propeller blade length increases propeller tip speed increases. Propeller noise is thus proportional to blade length. Thus a two bladed and three bladed prop spinning the same speed with the same blade length and shape.
and both perfectly balanced would create the same level of noise. But here's the thing, three bladed props does often have shorter blades. For reasons we'll now explore and shorter blades means the tips are traveling slower and thus makes less noise. Next up is a closely related myth that goes three bladed propellers have to be shorter than two bladed propellers. But just because they are usually shorter doesn't mean they have to be.
So why then are three bladed props usually shorter than two bladed props? It has to do with something called power absorption, drag or thrust. At a set rpm the amount of thrust a propeller creates is a result of the combination of its blade length, blade chord, blade thickness or camber, the number of blades and blade pitch.
The longer The wider and thicker a propeller blade is, the more thrust it can create. But also the more aerodynamic drag the engine would need to overcome to spin the propeller. More thrust is more drag and more drag requires more engine power to spin.
And the inverse is also true. The more power an engine creates, the more power the propeller would need to absorb by creating more drag. Let's say we have a fixed pitch two bladed propeller on a plane and the engine can spin it up to 2600 RPM in the takeoff roll at full power. If we then add an identical third blade, keeping the same pitch angle, the engine would not be able to spin it up to 2600 RPM anymore. Makes sense?
Adding a blade increases the drag to spin the propeller and it would require a more powerful engine to spin up to 2600 rpm. Adding a blade also increases weight, but that is not nearly as consequential as the increase in drag. So something has to give in order to let the engine spin the three bladed propeller as fast as the two bladed propeller. And an easy way for a propeller manufacturer to enable this?
is to make the blades shorter. This reduces the propeller's spin drag and makes it suitable for the specific engine. The ideal three bladed prop would have blades just as long as a two bladed prop and thus a more aerodynamically efficient way of achieving optimum power absorption than to make the blades shorter is to make the blade cord thinner. But thinner blades means there is less material to give the blades the structural strength required to withstand all the forces acting on the propeller. This is why it wasn't until the advent of mainstream advanced carbon fibre manufacturing process that propellers with really thin corded propeller blades could be manufactured affordably.
And it's also why we are these days increasingly often seeing 4 and even 5 bladed propellers even on light sport and ultralight aircraft with relatively low powered engines. Some may have noticed I left out one way of reducing propeller spin drag after adding a third blade, which is making the propeller pitch finer. But making pitch finer changes the optimum thrust airspeed. A finer pitch increases thrust at low airspeed and decreases thrust at high airspeed, which is acceptable for a stall plane but not good for a fast plane.
and in this case is not as good a solution as making the blades shorter. To recap, three bladed props does not need to be shorter than two bladed props, but something needs to give if a blade is added, and if it isn't blade cord then it needs to be blade length, which it often is, and explains why three bladed props does usually have shorter blades. The next myth is that fast airplanes should have shorter propeller blades. Or putting it differently, shorter propeller blades will make your airplane faster because it has less drag.
Yes, the shorter a propeller's blades, the less aerodynamic drag it would have. All else being equal in terms of air flowing through the propeller front to back. But since its blades are shorter it will also create less thrust. For fixed pitch propellers thrust equals drag.
So decreasing propeller spin drag also decreases thrust. And airplanes don't generally get faster as thrust decreases. But where small diameter propellers are often advantageous is in fuel efficiency.
Longer blades create more drag and while this does not slow the airplane down at high power settings, spinning a larger diameter propeller requires more power and thus increases fuel consumption. To recap, shorter bladed propellers doesn't make airplanes faster as a blanket rule and a lot of other factors need to be taken into consideration. Not convinced? Let's look at the next myth to dig a little deeper and reveal the full picture. The next myth then again is directly related and is that large diameter propellers are good for stall airplanes but not good for fast airplanes.
One common reason for this myth is that of course longer blades are more efficient but making a blade longer increases propeller spin drag, meaning the engine will not be able to reach optimum rpm and Something needs to give. And since we already have nice, long, thin corded propeller blades, and we're not willing to sacrifice blade length, the only thing left is to reduce blade pitch. Setting pitch finer increases thrust at low airspeed, but decreases thrust at high airspeed. This is perfect for stall airplanes, which needs maximum thrust at low airspeed, but isn't all too concerned with high speed flight.
But this effect mostly just solidifies the fact that large diameter props are bad for fast airplanes. Which isn't true. I think it's possible to bust this myth with a single photograph.
The P-47 Thunderbolt was one of the fastest production propeller airplanes of World War II, yet its blades were freakishly long. In fact, the blades were so long that pilots could only do three-point landings, as a wheel landing couldn't be done with safe propeller ground clearance due to the long blades. Of course, the blades could be so long because of the gear-reduced engine slowing down the propeller's speed, but the choice to use a gear-reduced engine further solidifies the importance of longer propeller blades even on fast aircraft.
This hopefully helps to start dispelling the myth before I even start. But let's cover the theory of how a larger diameter propeller can actually make an airplane faster. The longer a propeller's blades the lower its maximum rpm due to the effective blade tips speed limit.
A larger diameter prop thus spins slower and pushes air back more slowly. But because of the larger diameter it moves a larger mass of air. A faster spinning small diameter prop moves air back faster but in a more concentrated slipstream.
But what does that have to do with anything? Looking at the formula to calculate drag, drag is not only directly proportional to airspeed but exponentially increases with airspeed. In other words, doubling airspeed quadruples drag. A propeller airplane's airframe in flight effectively moves through two different streams of air, which flows at two different speeds. First is the speed the aircraft moves through the air around it, called the free stream.
And secondly there is the speed that parts of the airframe moves through the propeller-induced airflow, called the propeller slipstream. The propeller pushes air backwards faster than the airplane moves through the air. If it didn't, there would be no thrust.
Thus, the area of the airframe that is subjected to the faster propeller slipstream will have more drag than the areas subjected only to the slower freestream. Additionally, the small diameter propeller's faster slipstream is concentrated over the highest drag-inducing area of the airframe, the fuselage. The large diameter propeller's slower slipstream is spread out over a wider area, more of which misses the draggy fuselage and thus further reduces total drag. But how does the slower and wider slipstream create less drag than the faster and narrower slipstream? Looking at the formula to calculate drag, if the drag subjected area doubles, then total drag doubles.
But if air velocity doubles, total drag quadruples. Thus the less accelerated the propeller slipstream, the less the total drag on the aircraft. Now if the airplane in question is a slippery lancer, this actually matters very little.
But if it's a draggy supercub this makes a massive difference. Despite this not being a much talked about phenomenon, this isn't anything new. In fact, by World War II, airplane and propeller designers were well aware of the benefit of large diameter propellers. There's the previous example I gave about the P-47 Thunderbolt, but an even more extreme example from World War II is the FOU Corsair. With the benefits of a longer bladed propeller clear, Vought set out to design the aircraft around the US Navy's requirement to host the largest engine, spinning the largest diameter propeller of any naval fighter at the time.
The biggest benefit of the Corsair's inverted gullwing was to create a low point to mount the landing gear which then creates more propeller clearance while keeping the landing gear short allowing a very large diameter propeller. A shorter but wider corded propeller could easily have absorbed the prototype's 1800 horsepower. But the benefits of a large diameter propeller was well known and a smaller diameter propeller would have compromised performance. So then this myth is false. All else being equal, longer propeller blades usually result in a faster airplane.
But longer blades benefit stalled airplanes much more than fast. and low drag airplanes and it's important to realize that the benefit is both in takeoff thrust and cruise speed. Another reason why slower planes benefit more from longer bladed props is that the lower the airspeed of the plane at high power the bigger the difference between the speed of the propeller slipstream and the freestream.
The bigger the difference the more drag is induced and the slower spinning larger diameter propeller thus reduces drag even more at high thrust and low airspeed. That's it. The video is already pretty long at this point, so I'll stop here. But I'll see you in the next one. Thanks for watching.