Ever feel like you don't know enough?

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Sarah

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Sarah
I'm pretty early in my training and as I'm reading PHAK, I feel... a little like an idiot. The concepts are explained pretty clearly.

But I'm never really sure that I "get" it.

So,

The plane has a left tendency during take off, one reason is because the descending propeller meets the relative wind at a higher angle of attack than the ascending blade. It takes a bigger bite of air and this causes it to yaw to the left. It creates more lift.

Right?

Well, lets say that I read that and understood that.

I would still have a ton of other questions not explained in the text. Is that normal?

Things like:

Why does the tip of the blade move faster than the center of the blade?
(It might be well known and accepted but I just don't understand why and
I can't seem to find the answer anywhere! )

If the cord line of the prop is the measurement of the blade, where do you even measure that?! the whole thing is curved and twisted about it itself.


Maybe I'm over thinking it.
I literally spend every free moment studying, reading and thinking about this stuff.

I stay up until 4 or 5 in the morning at least 3 times a week going over this.

The PHAK is written by the FAA so, I would hope everything I need to know is inside it but, If the Oral exam is a 2 or 3 hour ordeal, the idea that the examiner might say:
" so, why does the outside of the propeller go faster than the hub?" or, " why does the angle of attack change with true airspeed ?" seems pretty certain to me.

I feel like I NEED to know all of this stuff backwards and forward.

Did any of you guys ever feel this way?:confused:
 
Sarah said:
Why does the tip of the blade move faster than the center of the blade?
(It might be well known and accepted but I just don't understand why and
I can't seem to find the answer anywhere! )
Click to expand...
Take a piece of paper. Make a dot in the center, then a small circle around the dot, and then a wide outer circle as big as you can on the paper.

Imagine the dot is the center of a city. The small circle is a city block. The outer circle is a highway around the city.

If you started driving around the block at the same time a race car driver started driving around the highway, he would have to go extremely fast in order to drive once around the city in the same amount of time it takes you to drive once around the block.

And now you know why the tip of a propeller is going faster than the center.

If the cord line of the prop is the measurement of the blade, where do you even measure that?! the whole thing is curved and twisted about it itself.
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It's measured from leading edge to trailing edge. Even though the propeller is a bit twisted, all along the length of it you can still draw a line from the leading edge to the trailing edge. A propeller is nothing more than a wing that spins around a center point. It works like any other airfoil - by moving through the air it creates lift. Since the "lift" side of the airfoil (propeller) is facing forward, the plane moves in that direction.

So cord is measured from leading edge to trailing edge, like this:
Hf99360.jpg

So just imagine a line drawn from a point at the leading edge of the propeller, straight through the center, toward a point directly on the trailing edge. That is the cord.
 
Sarah, first of all.... Welcome.

What you are experiencing is the wonder of flight in a different way. This tells me you are really bitten by the flying bug. ;)

The physics of flying and why things happens in flight is (to me) one of the most interesting and fascinating things about flying.

Keep up the interest by feeding your desire to learn. :D

Again... Welcome!
 
Sarah,

Welcome!

I was told early on that this thing we do will be a lifelong learning experience. And so far, it's held true.

Everytime I go up or spend time around an airplane or with other pilots, I'm learning something. Recently, I've been spending time reading about weather. I hope to start my IFR training down the road and want to be as ready as I can right out of the chute.

Now I'm not able to stay up and study like you're doing, and I'm not sure I'd want to, but I will say that I hope the learning does continue for a lifetime for all of us.

And good luck with your training. Be safe and have fun!
 
Welcome Sarah! :) Your thirst for knowledge is good. We need more women pilots. I know for ME during the oral portion I didn't get very many "Why does x work like that" where my male counterparts got questions like "explain the way spark plugs work" Be sure not to overstudy the book and know how to fly just as well as knowing the book stuff. Just like high school if you are too book smart you may fail at flying the plane.
 
Sarah said:
Why does the tip of the blade move faster than the center of the blade?
Click to expand...

The propeller is spinning at a particular number of revolutions per minute. Let's say 60, to make the math easier. This means the prop is turning once per second.

Take a point one inch from the center of the prop. Every time the prop turns, that point is moving about 6.3 inches (PI * 2). Now take a point one foot from the center. Every time the prop turns, that point is moving 6.3 FEET (PI * 2), compared to the other point's 6.3 INCHES, during the same second.

Take the point at the tip of the prop. If it's a 72" prop, the tip is moving a little more than 18 feet every time the prop turns.

So the one inch mark is moving at about 1/2 of a foot per second, the one foot mark is moving at 6 feet per second, and the tip is moving at 18 feet per second.

Go pick up the book

"Stick and Rudder: An Explanation of the Art of Flying" by Wolfgang Langewiesche

It's considered to be the best explanation by many.

Also, keep coming back here and asking question.

It sounds like you seem unable to move on to the next topic if you have a question on the previous topic. You should learn to get over this problem. When you do, please let me know how. I've been struggling for fifty years.
 
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Welcome.

Please ask questions and keep asking until you "get" the answer.

Sent from my ADR6300 using Tapatalk 2
 
For those "twisty, curvy or slanted" airfoils you mentioned, including the swept wing on many jets, the engineers calculate the "mean aerodynamic chord" as a measuring stick for use by pilots and others. When prop performance is measured, a "station" is sometimes quoted as part of the process. When you see "station" used to describe a dimension on an airplane, it simply means the distance (usually in inches) from a predetermined point.

For example, station 45 might be mentioned when discussing a prop's efficiency, and could be located by measuring the distance from the hub. When examining the aft surface of many props, you'll see some small rectangular reflective stickers adhered to them. They are the sensors for the prop balancing devices now used by many shops, and are attached at a specified station designated by the manufacturer of the balancing device in order to provide the instruments with consistent data to calculate imbalance and vibration.
 
Every day of my entire life, just keep learning. As for tip spend, move your arm. Your shoulder (center of prop) stays still 0 distance, and your hand moves, + distance. Since speed is a measurement of distance travelled over time, the greater the distance travelled in the same time, the higher the speed. The further out on the arm you measure an arc, the greater the distance it will travel to make a revolution.
 
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Welcome Sarah. You do need to learn a lot, and just keep studying. It will come. I'm not sure things are still this way, but back when I was young boys usually spent more time around mechanical things than the girls did, so they learned a lot of things by "osmosis." It will be a little harder for you, but keep trying. You don't have to know everything about everything :). When you are doing the oral, you can look things up, just as long as you don't have to look too many things up.
 
I suppose I'll be the big dissenter here. I think different people, regardless of gender, learn and even think in different ways. Some people visualize something and "see" it. Some are happy when they do the math. Some can hear you say something and others have to read it. Some of your experience here might be that some presenters feel like teaching in a certain way and perhaps you learn better in other ways.

Sometimes learning is a matter of perspective. If you look at the hands of a clock, they move the same number of degrees each second. Yet, the outside tip moves a greater distance than the inside. If you look at it from the perspective of rotation, they move the same. If you look at tip speed, obviously the tip goes a farther distance in the same time the rest of the prop did, so it must have gone faster to do that. If you aren't "getting" something, don't be afraid to look at it from a different point of view and see if it makes sense then.

Some of this "book learning" makes the FAA and certain types of people happy. For most of us, if some one taught us how to ride a bicycle the way we learn to fly an airplane, we'd still be on training wheels. Sometimes "just do it" and figure it out later. In fact, in some aspects, like off and landing, you don't have time to take a leisurely study break. You have to "just do it". As an example, you don't drag your checklist out in the middle of a landing flare. You look at it before or after.

Now people will really get on me. If you can fly the airplane and don't really understand why, study (read "memorize") enough about the "book learning" to pass the test and satisfy your curiosity and then don't worry about it. It's great at any time to really understand why something happens, but don't let it stop you from being a good, happy pilot. Remember riding the bicycle.

Your CFI knows this. He won't say, change the flight configuration of the airplane such that the horizontal lift on the vertical stabilizer establishes the airplane is in coordinated flight. He'll say, "step on the ball". As Nike would say, "just do it".
 
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I'm not gonna beat up on you, Jim. The FAA specifies a certain minimum standard of knowledge, which we all possess (however briefly) at the time of our testing.

What people need to know to satisfy their internal hunger for understanding varies widely.

Sarah, now that I'm on a laptop instead of a phone I can give a little longer answer. Follow your desire to understand things, and don't be frustrated if your CFI cannot explain them beyond a certain depth. He's not supposed to be a physicist or aero engineer.

However, on this forum we DO have physicists, mathematicians, engineers, doctors, musicians, and Henning and others who defy easy description. A question asked here will get answered. Many times. Some of those answers will even be correct, keeping in mind that "correct" is not absolute, as things can be often described in multiple "correct" ways.

Anyway, again, welcome to the community, and best wishes on your flying!
 
You're getting deeply into the PHAK. Really good.

Do you "have" to understand the details at the level you're going after? Not really. You'll get through a checkride without really understanding how chord lines are drawn or why P-factor gives you a yaw to the left. But it helps to do so. Massively. For some people, and I think that includes most or all of the posters in this thread so far, deep understanding here helps motivation. Don't underestimate that! An aircraft really is an interesting physical, geometrical, and dynamical system.

The PHAK is the official source, but there are some significant errors in it. Make sure you read the errata. And the four-force diagrams showing climbs, descents, and turns are not correct (they confuse frames of reference) in the 2008 version at least.
 
Flying is science and emotion. Skill is understanding enough of the science to matter and controlling the emotion. Experience is knowing when to ignore some of the science and pay attention to the emotion, and knowledge is understanding the science at an emotional level.
 
Thanks everyone!

I feel so much better after hearing your thoughts.

You guys are spot on. I have a really hard time moving on when I feel like I don't completely understand how the information would translate in different situations.

For example, I was re- reading information about the elevators and stabilators.
I read how the elevator can change camber on the trailing edge and
The stabilator changes at the leading edge.

Then , I found myself thinking " What would that look like? The stabilator moving? how exactly does the wind pass around it.. if It has a negative angle of attack, how exactly does the process happen?"

When I read the responses I realized you're all right.

by the time I get closer to the exam, I will be able to put the theory that I am reading about into practical use. When I feel the effects of the elevator, I will have a much better understanding of why and how things work..

I was worried that if the examiner asked me how a propeller created lift, he might not accept the PHAK's explanation he might then say.." well, how exactly does the outer blade move faster?" and a analogy of the process might not be the answer he was looking for.

I think, my real problem is that I have never wanted something so badly in my entire life, and I feel that If I have any doubt at all about this, I may jeopardize the only thing that has every truly made me feel... alive.
 
If you get tired of reading airplane books, squatting on an electric fence will also help you feel alive.

Sarah said:
Thanks everyone!

I feel so much better after hearing your thoughts.

You guys are spot on. I have a really hard time moving on when I feel like I don't completely understand how the information would translate in different situations.

For example, I was re- reading information about the elevators and stabilators.
I read how the elevator can change camber on the trailing edge and
The stabilator changes at the leading edge.

Then , I found myself thinking " What would that look like? The stabilator moving? how exactly does the wind pass around it.. if It has a negative angle of attack, how exactly does the process happen?"

When I read the responses I realized you're all right.

by the time I get closer to the exam, I will be able to put the theory that I am reading about into practical use. When I feel the effects of the elevator, I will have a much better understanding of why and how things work..

I was worried that if the examiner asked me how a propeller created lift, he might not accept the PHAK's explanation he might then say.." well, how exactly does the outer blade move faster?" and a analogy of the process might not be the answer he was looking for.

I think, my real problem is that I have never wanted something so badly in my entire life, and I feel that If I have any doubt at all about this, I may jeopardize the only thing that has every truly made me feel... alive.
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Sarah said:
I think, my real problem is that I have never wanted something so badly in my entire life, and I feel that If I have any doubt at all about this, I may jeopardize the only thing that has every truly made me feel... alive.
Click to expand...

I know you won't believe me, but you are gonna do just fine. I have not seen such passion about flying and wanting to "get it" in a long time. Be patient with yourself, and ask a lot of questions.

Don't be a stranger. Your honesty and passion is like a breath of fresh air here.

Good luck.
 
wabower said:
If you get tired of reading airplane books, squatting on an electric fence will also help you feel alive.
Click to expand...

One of the funnier things I have ever seen was a rooster lighting on a charged wire carrying juice to a charged fence on a farm. The wire gradually sagged under his weight until his feet touched the ground!:hairraise:
 
Geico266 said:
I know you won't believe me, but you are gonna do just fine. I have not seen such passion about flying and wanting to "get it" in a long time. Be patient with yourself, and ask a lot of questions.

Don't be a stranger. Your honesty and passion is like a breath of fresh air here.

Good luck.
Click to expand...

Thank You so much.
I cant even begin to tell you how much that means to me.
 
Geico266 said:
I know you won't believe me, but you are gonna do just fine. I have not seen such passion about flying and wanting to "get it" in a long time. Be patient with yourself, and ask a lot of questions.

Don't be a stranger. Your honesty and passion is like a breath of fresh air here.

Good luck.
Click to expand...

+1

Well said, Larry.
 
Sarah,

Propellers turn so many revolutions per minute. That means every point on the propeller traces out a full circle so many times a minute.

If you want to measure the linear distance each of those points travels, you can apply geometry to calculate the circumference of a circle.

That formula is:

Linear distance around a circle =2 x Pi x r, where r= radius and Pi is a constant 3.1416.

Notice that as r approaches 0, the linear distance around the circle approaches 0 for every revolution.

As r gets large the linear distance around a circle gets large for every revolution.

Technically, this is how Pi is defined, it's the ratio of the radius of a circle to it's circumference.

Hope this helps.
 
Sarah said:
I was worried that if the examiner asked me how a propeller created lift, he might not accept the PHAK's explanation he might then say.." well, how exactly does the outer blade move faster?" and a analogy of the process might not be the answer he was looking for.
Click to expand...

Just so you know, I could never see an examiner asking you this question. You don't really even need to know that it does. The only reason that is explained to you in the book, is so you know why the prop is twisted.

I guess you might get asked why the prop is twisted, I don't know.


This has been explained many times in this thread, and you might already have it down, but in case you don't, I will explain it in a slightly different way.

First let's talk about straight and level flight, and make sure we understand this principle:

Let's say your flying along at 100 knots, and not gaining or losing altitude. If you add more thrust so your going 120 knots, but change nothing else, what happens? You start to climb, because air is running around the wing at the same angle of attack faster. this produces more lift. The way you counter gaining altitude is to point the nose down a little, so the angle of attack is reduced, so the wind traveling at 120 knots produced the same amount of lift as it did when it was traveling 100 knots.

So make sure that makes sense before you read the next part.

Lets say we are now going to design a propeller, and we want the entire length of the propeller to create the same amount of trust. The prop works just like the wing, however it's working in the forward direction, and not up. It's a tiny wing stuck to the front of your aircraft.

Let's backup for just a second, and we will come back to this. Let's imagine we are looking at an aircraft head on. Kind of like this one:

b-17-press-kit-b17tight34angle.jpg


On a piece of paper, we were going to trace the outline of one of those props with string, and then one about half way down the prop, and then one right around the point were it connects to the airplane.

So we have three circles made from strings. If you then take those strings and straighten them out. the outside one will be a lot longer then the inside one. However for one rotation of the prop, that's how far it has to travel. As you can see, the outside one has a lot farther it needs to go in the same amount of it. because of this, it's traveling faster.

So we have the same problem we have above if we make the prop just like a wing, and have it the same angle of attack all the way down it. The "wing" on the outside of the prop is going a lot faster at the same angle of attack, so it will produce more lift. We need to reduce it's angle because we can't reduce it's speed.

So you took three measurements. If we were going to build that prop, we would make three separate angle of attack wings, and glue them together (on each side). We would then have a better prop then if we just has the one wing all the way down. At the three points we measured, we would have the same lift. However everywhere else on the prop, we would be off a little, if we went back and did it again, but with 6 measurements, we would be accurate in 6 places, and that's even better.

So in that case, we are solving the problem down the prop for 6 points, creating 6 little wings, and gluing them together. If you stepped back, and looked at your propeller at this point, it would looked twisted. Crude, but twisted.

This is where that math people say is of no use in the real world comes in. Calculas. What calculas allows you do, is take a problem like this one, and instead of solving it for 6 points, you can solve it for all points. So someone has done that math, and knows the angle of attack at any point down the length of the prop. They then take those thousands of little wings, and glue them together. In the real world, they just machine it out of one piece of material, but if you went through the work of gluing 10,000 little wings together, you would end up with the same result.

I hope that was not to much rambling, and it made sense :)
 
Actually, that's differential equations and analytic geometry, not calculus.

But it's solved in real life by drawing a whole bunch of line segments to approximate the surfaces and getting a big computer to solve a huge number of linear equations.

The Wrights did it by trial and error with a wind tunnel, and were very obviously aware of the angle-of-attack argument, and the view of a propeller as airfoil.
 
You can see in this picture the tips of the blades are blurred but get clearer the farther you go in. The tips are traveling a greater distance than at the hub therefore they're traveling faster. This speed is above what the shutter can "stop" therfore it's blurry.

Also what you brought up (P-factor) is one factor in asymetrical thrust on take-off. You also have torque and slip stream effect as well. You could have an argument from pilots as to what has the greatest effect, but I won't go there. :D
 

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somorris said:
Welcome Sarah. You do need to learn a lot, and just keep studying. It will come. I'm not sure things are still this way, but back when I was young boys usually spent more time around mechanical things than the girls did, so they learned a lot of things by "osmosis." It will be a little harder for you, but keep trying. You don't have to know everything about everything :). When you are doing the oral, you can look things up, just as long as you don't have to look too many things up.
Click to expand...
Best advice you could give. Couldn't have said it better myself.
 
Velocity173 said:
You could have an argument from pilots as to what has the greatest effect, but I won't go there. :D
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And they all miss the point. P-factor is a function of angle of attack; you'll feel it most strongly during climbout, and to a lesser degree during descent (and in the opposite direction). Gyro effects are a function of the rate of change of any attitude angle, and is zero at any constant attitude including high angle of attack (you'll feel it during rotation, but not after). Rotating slipstream is independent of attitude and weakly dependent on rate of change; you'll feel it as yaw during RPM changes. Torque effects are only felt as roll when the RPM changes.

All of these factors appear at different times, and are controlled separately. Left-turning tendencies on takeoff occur mostly due to torque when the throttle opens, gyro effects on rotation, and P-factor during climbout. Rotating slipstream is present throughout the whole thing.
 
If you want to get away from math completely, just picture an old phonograph record spinning on a turntable (unless you are too young to have seen one). Put a dot on the outside edge of the record and another one close in toward the spindle. The outside dot will clearly be moving faster than the inside dot.
 
MAKG1 said:
And they all miss the point. P-factor is a function of angle of attack; you'll feel it most strongly during climbout, and to a lesser degree during descent (and in the opposite direction). Gyro effects are a function of the rate of change of any attitude angle, and is zero at any constant attitude including high angle of attack (you'll feel it during rotation, but not after). Rotating slipstream is independent of attitude and weakly dependent on rate of change; you'll feel it as yaw during RPM changes. Torque effects are only felt as roll when the RPM changes.

All of these factors appear at different times, and are controlled separately. Left-turning tendencies on takeoff occur mostly due to torque when the throttle opens, gyro effects on rotation, and P-factor during climbout. Rotating slipstream is present throughout the whole thing.
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Good points. Totally agree. :)
 
Sarah,

I think that even a 50,000 hour pilot is still learning. With your piqued interest, coupled with your curiosity about flying, I expect you will do well.
 
Thanks guys!

I really appreciate your encouragement and the detailed explainations.

Sometimes, Ill read something over and over and it just doesn't make sense.
Then all of a sudden, I'll just get it!

I study a lot and sometimes when my brain is feeling s little fried, I'll just go to a local air museum and hang out. Just chat with folks and enjoy the environment. Thats usually when everthing clicks.

I got pretty frustrated trying to understand manifold pressure and how its regulated by a constand speed prop.

When to the museum, (had the honor of sitting in the cockpit of a DC3!) and on the way home I realized, the MP fluctuates when the engine is failing. The governor keeps the rpms constant. When Rmps fluctuate it means somethings not right in the powerplant.
Didnt even think about engines the entire time I was there. Read about PSA, The flying tigers and listened to ATC on the radio.

Guess a little time to digest was all I needed.
 
MAKG1 said:
And they all miss the point. P-factor is a function of angle of attack; you'll feel it most strongly during climbout, and to a lesser degree during descent (and in the opposite direction). Gyro effects are a function of the rate of change of any attitude angle, and is zero at any constant attitude including high angle of attack (you'll feel it during rotation, but not after). Rotating slipstream is independent of attitude and weakly dependent on rate of change; you'll feel it as yaw during RPM changes. Torque effects are only felt as roll when the RPM changes.

All of these factors appear at different times, and are controlled separately. Left-turning tendencies on takeoff occur mostly due to torque when the throttle opens, gyro effects on rotation, and P-factor during climbout. Rotating slipstream is present throughout the whole thing.
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P-factor... I know high-power induces greater p-factor...

But for me, in learning to fly, the most challenging handling p-factor induces is during flare. All about the coordination with aileron into wind with opposite rudder...at my strip the prevailing cross-wind is from teh right. So it took me a bit to learn how to transition on the rudder pedals to maintain best coordination comin' in.
 
Sarah, we are all learning, all the time. Your enthusiasm is infectious!
Rather than answer the questions that have already been answered, I'll tell you that no, you're not the only one asking "why".

I had learned at 15 that differential pressure under the wing is caused by the air on the top of the wing moving faster than the air on the bottom. But I didn't understand WHY the air on the top and the air at the bottom HAD to meet at the back of the wing?
And I couldn't ask, because everyone else seemed to get it, and I didn't.

I could quote it, got through my checkride 20 years ago, and had been a pilot for two years before it clicked in my head that the wing was moving through the air, not the air moving over the wing.
That's why the difference in relative speed, because the air is stationary and the wing moving through it.

So, don't feel bad. We all learn differently. And different things don't make sense to different people.
 
Sarah said:
and on the way home I realized, the MP fluctuates when the engine is failing. The governor keeps the rpms constant. When Rmps fluctuate it means somethings not right in the powerplant.
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The manifold pressure fluctuates when the pressure in the engine's intake system fluctuates. The pilot operating the throttle will cause it to change and it will also decrease as you climb (in a normally aspirated bird). There are also various other reasons manifold pressure changes.

What you should understand though -- is that the pilot typically uses the manifold pressure gauge, tachometer, and mixture to change the amount of power the engine is generating.

Moving the throttle will change the manifold pressure but it will not change the prop RPM unless the prop's pitch has hit the stops.

RPM's fluctuating is more likely an issue with the governor/prop but could possibly be an issue with the power-plant just depends on the implementation and failure mode.

The key to understanding a constant speed prop is to get into an airplane with a good instructor and have them show you how to operate everything. Once you see it you'll realize it's quite simple.

If you want to really understand how it works you must first understand how a governor works.

A constant speed prop actually makes flying easier and you soon start to realize how big of a pain in the ass fixed pitch props are for the pilot :)
 
PilotAlan said:
Sarah, we are all learning, all the time. Your enthusiasm is infectious!
Rather than answer the questions that have already been answered, I'll tell you that no, you're not the only one asking "why".

I had learned at 15 that differential pressure under the wing is caused by the air on the top of the wing moving faster than the air on the bottom. But I didn't understand WHY the air on the top and the air at the bottom HAD to meet at the back of the wing?
And I couldn't ask, because everyone else seemed to get it, and I didn't.

So, don't feel bad. We all learn differently. And different things don't make sense to different people.
Click to expand...

That actually does make me feel a lot better.
Whenever I feel like a dunce, Ill be sure to re read all these posts. :D
 
Ok, so I just read the article and I think I get it but, this is what's hanging me up:
If a fluctuation in MP is an early sign of engine failure, isnt it because the governor sets the RPM ti a constant speed? The RPM shouldn't change.. the prop pitch should.

Or, am Interpreting the info incorrectly?
 
You've got it, I think.

The engine is quickly slowing down and speeding up but the governor on the prop is working hard changing pitch angle to keep the prop turning at the same speed. The inertia of the prop is also helping smooth out how fast it's turning.

Thus, "constant speed prop". The governor will fight to keep the prop RPM constant. Even when the engine isn't.

Let's say the failure inside the engine is a fairly common one. An exhaust valve has a big leak in it.

When the valve goes to close, it can't. That piston will move up for the compression stroke and there will be almost no resistance on it.

What happens to the other piston that is on its air/fuel intake stroke? It goes faster and "sucks" harder than the others because of the one that had no resistance. All interconnected via the always turning (we hope!) crankshaft.

That little extra suck will register on the MP gauge. Briefly.

Additionally, the piston with the busted exhaust valve will have an all goofed up fuel/air/pressure ratio and won't be firing correctly, which will change the force it can apply to the crankshaft for moving other cylinders through their strokes.

So MP fluctuates rapidly up and down depending on which cylinder is firing, doing nothing, working normally, etc. The whole system is out of balance.

That is only one sample of how an engine might be malfunctioning. Similar problems with ignition, fuel injection, induction air leaks, whatever...

Frankly in most cases, theres also a boatload of vibration once you get all the way to a full cylinder failure, so MP fluctuating is a sign something is wrong but not bad enough to notice tons of vibration yet.

The reason the general rule of thumb is that engine problems show up as a rapidly oscillating MP is because there's usually some cylinders firing properly and some not in most failure modes.

All kinda depends on the engine and what exactly went wrong or is starting to go wrong inside. Fluctuations may be the first indication of a failure that will progress to being much more violent as a cylinder completely dies. Perhaps because it ingested chunks of metal from as lightly leaking valve, etc.
 
Sarah -- It's great you're really digging in to the information. But, to the extent that it is frustrating you, keep this in mind - the PHAK is not written to be the body of knowledge you need to know for your private pilot certificate. It serves other uses (ratings, certificates) as well. For instance, I didn't spend any time thinking about MP and prop governors until I was working on my complex endorsement. So, to the extent you like to go beyond the scope of your training (which you'll find a lot of folks on here do), then go for it. But if you feel like the water pressure of the fire hose you're drinking from might be a bit too high, your CFI should help you focus your study efforts a bit.

Have fun,
Sam
 
PilotAlan said:
I had learned at 15 that differential pressure under the wing is caused by the air on the top of the wing moving faster than the air on the bottom. But I didn't understand WHY the air on the top and the air at the bottom HAD to meet at the back of the wing?
And I couldn't ask, because everyone else seemed to get it, and I didn't.

I could quote it, got through my checkride 20 years ago, and had been a pilot for two years before it clicked in my head that the wing was moving through the air, not the air moving over the wing.

That's why the difference in relative speed, because the air is stationary and the wing moving through it.
Click to expand...

Hi Alan et al. - sorry for the thread creep, but I really latched onto the thought about the wing moving through the air, rather than the air over the wing as explaining WHY the air flowing over the top of the wing and the air flowing under the wing HAD to meet at the back of the wing. That made intuitive sense. I could picture it. I was forever grateful for the word picture.

Then, two hours later, I remembered the phrase "relative wind" and realized that it's my understanding (albeit potentially incorrect), that I could use a fan to cause the air to move over the wing, and it would still be the same - lift from a pressure differential due to the two air flows meeting at the same time at the back end of the wing, and therefore, the top flow moving faster than the bottom flow.

And, it's gone.

Any thoughts? Appreciate it!
 
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