ASP - Fixed Pitch Props

Hello and welcome back, today we're going to talk a little bit about propellers and we'll start with

fixed

pitch

and I'll break down constant speed and governors in a separate video, a little bit of basic propeller theory. Propellers are basically rotating wings. rotary wing assemblies, the relative airflow is derived from a combination of the rotation of the blades and the movement of the propeller through the air, that is, the forward air speed of the aircraft in terms of vectors, looks like To this, we have the rotation speed represented by this arrow and we have our forward speed here and if you draw a diagonal between them we get the relative wind.

If you take that relative wind and look at the difference between that and the chord line, we get the angle of attack, the rotation speed. varies along the blade because the tip moves much faster than the root of the blade and therefore to maintain the same angle of attack they have to rotate the blade, that's why it's crooked, it's just to compensate that geometry of the fastest component of rotational speed there is at the tip, so if you find a written test question that asks about blade twist, the answer will be to maintain a constant angle of attack throughout. of the sheet there.

There are a series of forces in a propeller and this also applies to the rotor blades, by the way, there is a centrifugal force which is an imaginary force. In reality, it is a centripetal force. We're pulling the blade in to keep it attached to the hub, but it's significant that it's on the order, even for a moderate-sized general aviation propeller, it can be on the order of six thousand pounds. There is a bending torque where the power strokes of the engine pull the blade and therefore it vibrates back and forth. There is a push flexion. forces that push the blades forward and also drag forces, if you have the engine idle to the point where the relative airflow spins the propeller instead of the engine then the blades would be pushed back, there are aerodynamic torque forces , so you know any lifting airfoil will tend to twist downwards and that happens and then there are also centrifugal torsional forces on it, so the propeller is under a lot of different forces, very dynamic forces and it's actually flexing quite a bit. .

From the pilot's point of view, it is important to note that he became a leader before the flight. It is very important that you inspect the blade carefully for any Nicks, because these forces will cause even a small Nick on the propeller to concentrate the tensile forces at that point and a crack will occur and you really don't want a tip failure, a piece of your propeller that breaks, that is quite dramatic when we talk about the

pitch

of the blade, we are talking about the distance forward with each revolution that the propeller would make. move if it was perfect if it had perfect efficiency if you increase the pitch you decrease the RPM the motor will spin because it is trying to go further with each revolution if you are thinking about the pitch there are a couple of metaphors you can use or little thought experiments if you imagine submerging a propeller and a large vat of gelatin so that when turned it would perfectly cut the gelatin and determine how far forward it would move.

In that kind of substance, another way to look at it is if you imagine a nut on a bolt, how much does the nut move down the bolt with each revolution? And that's what the step tries to describe if we reduce the step it makes. So we're not trying to go as far with each revolution that requires less energy and that results in less resistance to the propeller and therefore the motor accelerates, so if you have a high pitch, you have less rpm, low pitches and high. rpm and remember that rpm is a really important piece of the power formula, so that's a big impact that we have on the powers by changing those rpm, there are several types of

fixed

pitch propellers that we'll talk about here in just a moment.

A couple of minutes there's not much to say about it, but we'll talk about it anyway. The next step would be ground adjustable. There are light sport aircraft that still have adjustable propellers on the ground, but you probably won't come across one in your career, they are just as they sound, you can actually adjust the pitch of the propeller on the ground by loosening a few nuts, turning the propeller and then tightening the nuts again, there are controllable pitch or variable pitch propellers and these you really have direct control. about what the propeller pitch is, you're unlikely to find one I've flown, but you're unlikely to find a constant speed, it's very common and allows the pilot to set the rpm and then the governor adjusts the pitch accordingly. necessary to maintain those rpm and then there are reverse pitch propellers that allow you to change the pitch of the prop to the point where it is pushing air back and forth and then there are feathering propellers that allow you to increase the step until it lines up with the wake so the engine stops spinning, we'll talk about that in the next video and then there are the tractor and pusher propellers that have to be configured differently so that the airplanes that we are flying or all the accessories for tractors , there are also thrust attachments, so the two most common and the ones that you will surely have to deal with during your training and during your tenure as a flight instructor are the fixed pitch and constant speed, you will continue to deal with constant speeds if you use turbo

props

, but for now we will only focus on the fixed pitch propeller.

Fixed pitch propellers have the great advantage of being simple and because they are simple, they make it really fantastic for a student pilot to learn to fly. They are relatively light, relatively inexpensive, and quite durable. They are a big piece of metal. They aren't even. Typically hollow ones, of course, wooden fixed pitch propellers exist and are still being manufactured and have some advantages of their own. Wood is actually a very strong material for its weight, but the ones we deal with are metal and are quite strong, usually described in terms of diameter and pitch, so for example the 152 has a McCawley, a 103 TCM 69-58 and it's those last four digits that you're interested in because what they give you is the diameter of the propeller from tip to tip and the pitch, which is 58 inches.

One of the things that is important and not discussed enough during primary training is the effect of air speed on a fixed pitch propeller. As the airspeed indicators decrease, the blade angle of attack increases in rpm and this has a dramatic effect, especially during a grade, as the airspeed decreases, the blade angle of attack increases in rpm. rpm decreases, so the bottom line here is that you are speeding the air forward. of the airplane has a direct effect on the RPM of the engine with a fixed pitch propeller, so this is what the vectors look like.

We have a vector here once again that represents the rotation speed of the propeller. It's long because it moves pretty fast. We have this forward speed of the plane and in this case the plane is not moving very fast, so this is our angle of attack right here, as the Earth says, we apply full power now and start taxiing down the runway as that our forward speed increases. The angle of attack decreases and that means that with less angle of attack there is less resistance to the propeller and the engine starts to spin faster. Now some of that angle of attack is automatically recovered because as the propeller spins faster, the rotational speed increases and we gain some. of that angle of attack back and it somehow automatically stabilizes for us, but the next time you go out to fly your fixed-pitch airplane, take note of the fact that you have a little more rpm right before takeoff than you did in the moment of takeoff. start of the takeoff roll and that has an effect on power, you actually gain a little bit of power as you accelerate, there are different types of fixed pitch propellers and many general aviation aircraft had options for other propellers, so normally The 152 and 172 have some sort of compromised propeller, but it's possible that with a climb strut or a cruise strut a climb strut has a smaller pitch, which is fine, it's taking a smaller bite, which means there is less resistance and that allows the motor to run at a higher rpm remember our braking power formula 2 pi times torque times rpm divided by 33,000 so that we get more power by letting the propeller spin faster, on the other hand, a cruising propeller has a higher pitch, which means that in a cruise we can leave the manifold pressure higher at the same rpm, which allows us to produce more torque at the same rpm, which means more horsepower or horsepower equals to a faster cruise.

Unfortunately, it also results in higher fuel consumption. This is a big mistake. Pilots think that if you have a cruising propeller you save fuel, not per hour, you don't, you'll actually burn more fuel per hour with a cruising propeller than with a climb problem and the other thing about cruising propellers is that you lose performance in climb because you turn slower during that climb and that means you've lost some power and since both climb support and cruise support have some disadvantages and advantages, most of these airplanes were delivered with a Sort of a medium engagement propeller that does an adequate job both climbing and cruising, that's all we'll say about fixed pitch propellers right now, we'll talk more about constant speed propellers next time.