Multi-Engine Ground School - Aerodynamics and the Critical Engine

0:00Welcome to the OnCenterline Podcast, a show where we discuss the trials and tribulations of learning to fly from both the student and flight instructor perspectives. We feature real aviators in all different chapters of their careers, talking about the things we all deal with, but rarely discuss. So join us as we take on the challenges, hardships, and celebrations that pave the runway to be in a professional aviator as we strive to stay onCenterline. Hello everybody! Welcome back to the OnCenterline Podcast. I'm your host Sam Terrell, the Northwest Air Not On YouTube, and thank you for joining

1:00me. We are getting started today with something I've been meaning to do for a long time. We're going to be starting our series on multi-engine ground school, and hopefully this will be coming in handy for any of you out there who are looking to add on your multi-engine rating to either your private or commercial certificate, or for the few of you out there who may even be doing your multi-engine rating as an initial certificate. This should hopefully all come in handy. So we're going to be breaking it up into two to three episodes, and today we're going to be talking about multi-engine aerodynamics, which is a large portion of what you guys talk about on the checkride. Before we do that, special shout out to all my members here on YouTube. Thank you very much for being members and for your support. And of course, you guys are getting to see this before anybody else, because as a member perk, you get early access to these podcasts and videos on YouTube.

2:05I also want to congratulate Sean on passing his CFI checkride. I love hearing from you guys when you guys pass checkrides, especially if you guys and if we've worked together. I do a lot of mock orals, and I'm happy to do so for you if you are having a checkride coming up. Feel free to reach out to me through my website, northwestaeronaut.com, or you can email me directly at northwestaeronautatgmail.com. We can do a mock checkride, and then I love hearing how the checkride went, especially after you guys pass them. And so congratulations to Sean on passing that CFI checkride recently. So today we're going to be discussing multi-engine aerodynamics, and the majority of that comes down to the way the aircraft behaves when one engine dies and the other one doesn't. All right, because this type of condition creates a lot of asymmetric thrust, which creates

3:07some unique characteristics in the way the aircraft handles, and some unique things that we need to be aware of as multi-engine pilots. The majority of your training for multi-engine aircraft is going to be related to these principles and being able to show that you can control this multi-engine aircraft on a single engine. Now as with many things in aviation, we can use a handy, dandy acronym to help us kind of remember the things we're talking about. And that acronym today is going to be past P-A-S-T. And we use this past acronym to basically describe the critical engine. Now the critical engine is the engine that, when failed, has the most adverse effect on directional control, okay?

4:08That's the definition, the engine that, when failed, has the most adverse effect on directional control. You're going to want to remember that. Now do all multi-engine aircraft have critical engines? Well, actually no. We have two primary types of multi-engine aircraft out there. One we call conventional twins, okay? Conventional, these are twin-engine aircraft specifically. So we have conventional twins and we have counter-rotating twins. What's the difference? Counter-rotating, that might be somewhat self-explanatory. It's basically when each propeller on each engine rotates the opposite direction. So from the pilot's vantage point, each of those props are going to be rotating inward typically, okay, towards the fuselage. Whereas in a conventional twin, both propellers rotate the same direction, which from the pilot's

5:10vantage point is always going to be clockwise, right? If you're looking at it for on the front-on, it'd be counter-clockwise, but they're going to rotate in that direction and both of them are going to do that. So between our conventional twins and our counter-rotating twins, which one has a critical engine and which one doesn't? Well, the conventional twin is going to be the one that has the critical engine and that's what we're going to discuss here is why it has a critical engine. The majority of light twins out there are going to be conventional twins and you might ask yourself, well, why are the majority of twins conventional if counter-rotating takes away this critical engine? And the answer, as is so often the case, comes down to the mullah, the money, right? And to create two propellers that turn two different directions means you have to create

6:13two different manufacturing processes, right? You're basically having to create two different parts and two different toolings and machines to create those parts. So basically you're doubling the cost and that as opposed to being able to simply use one part with one machine tooling and all that and just create the same part twice is a lot cheaper to do it that way. So that's why the majority of twins remain conventional even though we have this pesky thing called a critical engine. So we're going to get into the critical engine here, but between the right and the left engine, which one do you think is critical? Well, in this case, as is the case most of the time, our left engine, our left engine is going to be the critical engine. And that is the engine going back to our definition that when failed has the most adverse effect on directional control, okay? Now fun fact, there are planes throughout history.

7:16I don't know how many of them are currently flying or being manufactured, but there have been planes throughout history that in fact have two critical engines because the props do counter rotate, but they counter rotate the opposite direction towards the outside of the fuselage. That would make both engines critical technically. I mean, realistically, you could also make the argument that neither engine is critical in the case of a counter rotating prop or that both are critical. You could look at it either way, but there are also aircraft particularly not made in the US that might have their propellers turning the opposite direction, which would make your right propeller, your right engine critical, all right? So left engine is going to be the standard critical one throughout most aircraft in conventional twins, but that's not always the case.

8:16Okay, let's get into our past acronym and talk a little bit more about these critical engine here. Okay, our past acronym describes the forces involved when we lose an engine that cause our aircraft to want to yaw and roll a certain direction. Okay, many of these you probably are going to be familiar with from your single engine flying. One in particular you probably won't be familiar with. Let's start with the first one. P stands for P factor. Yes, that P factor, the same one that you've dealt with since day one in private pilot training, the thing that causes your right wing to drop when you rotate on take off and your nose to yaw left that P factor. And just because we have two engines, doesn't mean we don't have P factor. So the way P factor works here is, of course, we are getting a the center line of thrust

9:19kind of comes off of the, if we're again, from the pilot's vantage point comes off of the right side of the propeller disc, okay? And this is because the descending blade of the propeller has a higher angle of attack, takes a bigger bite out of the air, a bigger chunk, bigger bite out of the air there. And therefore, with that higher angle of attack on the oncoming wind, it creates more thrust, whereas the ascending blade doesn't have as high of an angle of attack, it's actually somewhat retreating, it's retreating in the oncoming air stream. So it's not going to be producing as much thrust. So if you're thinking about the propeller disc, okay, the circumference of the propeller, the majority of the thrust is going to be coming off of the right side of that disc from the pilot's perspective, assuming, you know, particularly when we are pitched upward, right?

10:23So because of that, we, when we lose an engine, the thrust that's being created from the right side of that propeller disc is going to be yawing the aircraft to the left, okay? So why does that make the left engine critical? Well, basically, guys, everything we're about to talk about from p-factor on here comes down to one fundamental fact, okay? And that fact is that the arm, okay, just like when we're talking about weight and balance, the arm between our center of gravity, roughly in the middle of the aircraft, outboard to that center line of thrust on the, on the engine is longer going from the center of the airplane to the right side of the right propeller. Then it is going from the center of the airplane to the right side of the left propeller. If you think about it, the right side of the left propeller is right up cozy next to

11:28the pilot seat there, right? Right next to the fuselage, but the right side of the right propeller goes way further out, outboard on the wing. And so you have a longer arm between that center of gravity and that right side of the propeller with the majority of the thrust is being produced. So was this mean? Well, we know that leverage, you get more leverage from a longer arm, right? When we apply a force, so when we have the left engine dead, the yine motion created by that right engine pushing forward from the right side of the propeller is going to be a lot stronger, a lot, you're going to feel it a lot more pulling you to the left. Then you will from a short left arm, okay, a short left arm trying to pull you to the right, okay? This comes down to the same idea, you know, when we're thinking about doors, right? If you try to open a door with the handle, which if you think about the hinges on a door

12:35being the center of gravity in this case of the airplane, you've got a long distance between the handle and the hinges. And so opening or closing that door is relatively easy, right? It doesn't take a lot of work on your part. And that's why we put handles there on doors. And if we were to move that door handle towards the hinges, or again, back to the airplane if we're moving that propeller that thrust towards the fuselage, it's not going to be as easy for the same amount of thrust, or it's not going to be as easy for you to open or close that door when you're so close to the pivot point, okay? And our pivot point is the center of gravity on the aircraft. All right, so that's kind of the analogy I like to use thinking about a door. We've got the handle where it should be on the door when we're talking about the right engine, which makes it very easy for that right engine to yaw that airplane to the left.

13:38But the handle is essentially right next to the hinges when we're talking about the left engine. So the amount of yaw that it creates to the right is not that great, okay? And we're talking about here in this past acronym comes down to that fundamental fact, the fact that we have a longer arm between the thrust and the center of gravity on the right side than we do on the left, okay? So that's p-factor, and that creates a yaw in moment to create a yaw to the left when we lose the left engine. Next we have a and a in our past acronym stands for accelerated slipstream. Now you guys might be familiar with another type of slipstream, which we may or may not be talking about here shortly, but accelerate slipstream you probably are not familiar with. And what we're referring to when we talk about accelerated slipstream is the slipstream

14:41of the air off of the propellers off of the engines. But in this case, you know, if we think about the fundamental or one of the fundamental differences between a single engine and a multi-engine aircraft, well, in a single engine, the engine is generally in front of the fuselage or in line with the fuselage at least. And that slipstream goes over the fuselage, whereas with a multi-engine aircraft, the slipstream of the engine is in front of the wings, which means that accelerated air from the propellers are going over your wings. And of course, we know that when we accelerate air over wings, what happens? We create lift. So this accelerated slipstream is actually constantly assisting the wing with lift. And that's why whenever you're landing a multi-engine aircraft, you will find that if you pull that power to idle, or as soon as you pull that power to idle, you are coming right down out of the sky.

15:42You are going to lose a pretty significant amount of lift, a noticeable amount of lift. That's why we usually keep power on until the last couple of seconds in the roundout when we're landing a multi-engine aircraft. But what does it have to do with the critical engine? Well, if we lose our left engine, our critical engine, again, that accelerated slipstream is still producing lift on the right wing, and is producing lift further outboard on the right wing, right? And now we have no lift being produced on the left wing from that slipstream. So what does this create? It creates a rolling moment, okay? We got extra lift on the right wing. That wants to go up. Left wing wants to drop, and you get a rolling moment to the left. It's like you would get a rolling moment to the right from the left engine, except again, the point at which that center of lift is occurring, based on the slipstream, is

16:44further inboard closer to the fuselage with the left engine. So it's not going to create nearly as much of a rolling moment. So that's our accelerated slipstream. It's the accelerated air coming off of the propellers going over the wings, creating extra lift, that when we lose our critical engine, either engine, we have an unbalanced amount of lift from one wing to the other, and it's going to create a rolling moment. All right? So we started with P-factor. P-factor is going to create a yine moment, accelerated slipstream is going to create a rolling moment. Moving on, we have S in our past acronym, and S stands for spiraling slipstream, and you guys should be well familiar with spiraling slipstream. It's something we deal with with single-engine aircraft, just as much as we do with multi. And it works the same way with the exception that, again, our engines are off to the side

17:50of the fuselage. So when we are looking at our spiraling slipstream, coming off both engines, the spiraling slipstream when the air or when the propellers are rotating clockwise is going to come back. And again, if we're looking top down from the pilot's vantage point, so we're looking forward at the plane, but we're looking top down at the plane, the slipstream is going to come straight back off the propellers over the wings, but it's going to then start moving to the right of the aircraft. Why does it move to the right of the aircraft? Well, we talked about how that, the majority of that thrust, that kind of centerline of the thrust is coming off of the right side of the propellers with the descending blades. Again, this is primarily prominent when we are pitched up, okay? So we're pitched up, we got more thrust coming off of the right side of the propeller disc. That air is moving faster, right?

18:53And we know that as air, thanks to Bernoulli, we know that when a fluid is accelerated, what happens to the pressure of that fluid, it decreases. So the air is moving faster off the right side of the propeller disc, which means there's a lower pressure on the right side of that slipstream than the left side, which means the slipstream itself moves off to the right, okay? And when we're talking about the slipstream off of the left engine, that slipstream coming back and moving to the right is going to hit your vertical stabilizer. So if we lose our right engine, you know, the aircraft's going to want to yaw a little bit to the right, but you also have that slipstream coming back, hitting the vertical stabilizer, which is making the tail want to go right, and therefore the nose want to go left. So the spiraling slipstream, if we were to lose the right engine, the spiraling slipstream

19:55from the left engine is actually helping your aircraft fight any right turning tendencies, right yawing, right rolling tendencies that it's experiencing, which are not strong to begin with. But now let's look at the right engine. If we lose our left engine, which is critical, well, now we have all those yawing and rolling tendencies from the P factor, the accelerated slipstream that are wanting to yaw and roll us to the left, the slipstream off of that right engine is going back and off to the right. Well, what's off to the right? Nothing. There's nothing off to the right. It's just going off into the free air stream, saying, see ya. Good luck, buddy, okay? There's nothing there. There's no vertical stabilizer for it to hit. So there's nothing in peeding that tail from swinging to the right and that nose from

20:58swinging left. So in the case of spiraling slipstream, it's not so much that the characteristics of the right engine or left engine are enhancing the left turning yaw or bank of the airplane. But it's that when we lose the right engine and that slipstream's coming off the left, that spiraling slipstream actually helps prevent yaw and roll where we don't have anything to help prevent it when we lose the critical engine, okay? All right, hopefully that makes sense, guys, moving on to our T, T in our past acronym stands for torque. And again, this is something you guys should be familiar with. Torque is kind of thinking about our Newton's third law motion, right? For every action, there's an equal and opposite reaction. So when our propellers are turning clockwise, our aircraft is going to want to rotate counter

22:00clockwise. Now this is kind of the only one that doesn't specifically affect or get affected by the difference in the length of arm between the right side and the left side. The arm, because what this does, the torque happens around the crankshaft, right, or the center rotation of each engine. And the center rotation of each engine is the same distance from our center gravity on either side, okay? So this one doesn't really affect that one, however, if you think about, I got my little airplane here for those of you watching on YouTube, but if you think, hey, I'm going to hold this airplane, I would hold it as broken. I'm going to hold the airplane by the wings just underneath the wings with one finger. If I lose my right engine, the left engine is rotating counterclockwise, it's going

23:03to want to rotate the aircraft, sorry, the propeller's rotating counter, the propeller is rotating clockwise, it's going to want to rotate the aircraft counterclockwise. So if I'm lifting from the left wing, if I'm pushing up, I've got to kind of get leverage. If you push up from the left wing right next to the fuselage and push down on the top of the left wing further outboard, that's essentially what's trying to happen is you're trying to twist the plane this way and it holds itself up, right? You have to basically lift the full aircraft, the left engine is trying to lift the full aircraft with the torque, which is a losing battle. You're not going to feel hardly anything, okay? Now let's think about the right engine. The right engine is turning clockwise, the airplane is trying to turn counterclockwise.

24:08Well, if I try to turn the aircraft counterclockwise from the right wing, is that going to be easier or harder? I'm going to be pushing down on the in-board part of the top wing, of the top of the wing, and I'm going to be pushing up from the outboard side of the bottom of the wing. The aircraft's weight is going to go with that rotation. It's going to be very easy. In fact, you know, if you're doing it with a model, the model's just going to fall right out of your fingers or your hands because the weight is trying to go counterclockwise as well. So the torque from that right engine is going to have a lot easier time trying to rotate the plane, therefore causing a banking or a rolling moment, okay? Then the left engine would, if the right engine was dead. I know that's hard to kind of maybe visualize if you're listening to this out there in podcast land, but what you got to understand is that the aircraft wants to move in the

25:09same direction that the torque is trying to make it move when the left engine is dead. The aircraft does not want to move in the same direction that torque is trying to make it move when the right engine is dead, okay? So that's torque, and you know, for most of our light twin aircraft, the amount of horsepower in our kind of power to weight ratio is not so great to where you're not going to feel torque that much, but certainly in higher power, higher performance aircraft, then torque will play a much bigger role. All right, guys, so P factor, accelerated slipstream, spiraling slipstream, torque. These are the four factors, the four things that we kind of think about when it comes to defining what and why the critical engine is, okay? What we have, why we have a critical engine and what engine that is.

26:11These are all the forces that are acting on the aircraft when we are flying on a single engine in a multi-engine aircraft. For P factor, going to create a yine moment, accelerated slipstream is going to create a rolling moment, spiraling slipstream is going to create a yine moment, and torque will create a rolling moment. So we've got four forces creating a mixture of yine and rolling moments, and all of those combine are what make our left engine, the critical engine in conventional twins. Hey guys, I hope you found that helpful. I hope I did a decent job of kind of explaining this over the air waves of the podcast. I know it's a lot easier to have some visuals, and I'm going to be putting some visuals up on the YouTube video. So if you don't, if you have, if you're not watching on YouTube right now, check out the video on YouTube, some of these visuals will help.

27:13Hey, share this video with anyone else you think would find it helpful. Don't forget to hit that like button here on YouTube, and that subscribe button. Hit that follow button if you're out there in podcast land. I really appreciate you spending time with me today. And we will see you next time on centerline.