What is "lift"?

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RedAndWhiteBaron
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What is "lift"?

Post by RedAndWhiteBaron »

Okay so I am arguing with my flight instructor over the definition of "lift". I respectfully disagree with him, and I may be disagreeing with everyone. I'm just a student pilot - with, true, many hundreds of hours of aerodynamic studies (no degree), but still, I'm a student, and I know nothing. But I disagree with my instructor.

He presented the following conundrum (I will paraphrase so as not to infringe on his copyright, I hope):
You are in a 100 KIAS cruise. You alter your situation so as to attain a climb, and maintain 100 KIAS in the climb. Has your angle of attack increased, decreased, or remained the same?
I will admit that my first answer was wrong.

It's one heckuva rabbit hole, and I'd love to hear other people's opinions.

(Also, I'm hoping this might be a welcome distraction)
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jakeandelwood
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Re: What is "lift"?

Post by jakeandelwood »

at first i thought the angle of attack would increase but on second thought no it wouldn't because you are adding power to keep the same airspeed. if the power setting stayed the same and airspeed dropped then the AOA would increase, am i wrong?

















the same speed is to be kept in the climb the
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Re: What is "lift"?

Post by RedAndWhiteBaron »

Power is not a factor - clearly you have added power. But this is a very academic problem.

There are four forces - and be it in level flight or a climb, they are in equilibrium.
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Last edited by RedAndWhiteBaron on Mon Jul 13, 2020 12:15 am, edited 1 time in total.
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Re: What is "lift"?

Post by TWSC »

jakeandelwood wrote: Sun Jul 12, 2020 11:00 pm at first i thought the angle of attack would increase but on second thought no it wouldn't because you are adding power to keep the same airspeed. if the power setting stayed the same and airspeed dropped then the AOA would increase, am i wrong?

















the same speed is to be kept in the climb the
That’s right. In this case aoa would stay the same. 100kts in cruise results in exactly the same relative airflow and subsequent aoa as 100kts in a climb. Best way to think about it is the air has no up, down, right, or left. Direction doesn’t matter, so constant airspeed will have the same aoa no matter whether it’s in a climb, descent, or level flight. The only thing that changes in this scenario is a higher power setting to maintain the climb
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Re: What is "lift"?

Post by RedAndWhiteBaron »

TWSC wrote: Mon Jul 13, 2020 12:10 am That’s right. In this case aoa would stay the same. 100kts in cruise results in exactly the same relative airflow and subsequent aoa as 100kts in a climb. Best way to think about it is the air has no up, down, right, or left. Direction doesn’t matter, so constant airspeed will have the same aoa no matter whether it’s in a climb, descent, or level flight. The only thing that changes in this scenario is a higher power setting to maintain the climb

This was my first answer.
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Re: What is "lift"?

Post by photofly »

There’s already a 90 post thread on this:
viewtopic.php?f=3&t=93004

To answer the question in the thread title, you could start with page 4 of the Flight Training Guide.
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Re: What is "lift"?

Post by Heliian »

Lift is the opposition to gravity and thrust is the opposition to drag.

To climb, more lift needs to be created to overcome gravity. The lift is created by increasing AOA and then more thrust is added to overcome drag to maintain airspeed.
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Re: What is "lift"?

Post by AuxBatOn »

Heliian wrote: Mon Jul 13, 2020 6:06 am Lift is the opposition to gravity and thrust is the opposition to drag.

To climb, more lift needs to be created to overcome gravity. The lift is created by increasing AOA and then more thrust is added to overcome drag to maintain airspeed.
AOA is lower in a climb as a component of power is countering gravity. Your load factor will also be lower. The extreme would be a 90 deg climb angle where the load factor would be 0g thus the wing generating no lift but the engine(s) acting solely against gravity.
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Re: What is "lift"?

Post by digits_ »

Heliian wrote: Mon Jul 13, 2020 6:06 am Lift is the opposition to gravity and thrust is the opposition to drag.

To climb, more lift needs to be created to overcome gravity. The lift is created by increasing AOA and then more thrust is added to overcome drag to maintain airspeed.
That's only during the initial transient effect. Read the topic linked to by photofly :wink:
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Re: What is "lift"?

Post by PilotDAR »

Okay so I am arguing with my flight instructor......
Which may be a part of the problem. Just take the training as it is presented. Some things are simplified for the purposes of just getting through the curriculum. Sure, if you want to delve into the fine details, go for it, but, find an expert, and be prepared to pay them for their time. You flying instructor may not be an expert at aerodynamics, he/she is there to teach flying, not aerodynamics, beyond the very basics.

It is important for students to understand that there is so much to get through, that if one distracted from the prime course objectives on various topics, you could make the training take five times as long. The search for more knowledge is great, but understanding the ultimately fine nuances of lift will not make you a better PPL, learning the basics really well will....
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Re: What is "lift"?

Post by Bede »

Heliian wrote: Mon Jul 13, 2020 6:06 am To climb, more lift needs to be created to overcome gravity. The lift is created by increasing AOA and then more thrust is added to overcome drag to maintain airspeed.
Ughh. Shudder.

AuxBatOn is correct, but I believe he flies something with huge thrust to weight. For most GA airplanes, AOA reduction in a climb is negligible but it certainly doesn't increase (except when pulling G).
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Re: What is "lift"?

Post by JasonE »

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Re: What is "lift"?

Post by Bede »

PilotDAR wrote: Mon Jul 13, 2020 7:14 am
Okay so I am arguing with my flight instructor......
Which may be a part of the problem. Just take the training as it is presented. Some things are simplified for the purposes of just getting through the curriculum. Sure, if you want to delve into the fine details, go for it, but, find an expert, and be prepared to pay them for their time. You flying instructor may not be an expert at aerodynamics, he/she is there to teach flying, not aerodynamics, beyond the very basics.

It is important for students to understand that there is so much to get through, that if one distracted from the prime course objectives on various topics, you could make the training take five times as long. The search for more knowledge is great, but understanding the ultimately fine nuances of lift will not make you a better PPL, learning the basics really well will....
All true, but sadly the way aerodynamics is taught to pilots (compared to physicists or engineers) is somewhere between grossly simplistic (lift is 90% Bernoulli and 10% Newton) and demonstrably false (equal time fallacy). If I was an educated student, I'd probably be arguing too. I realize aerodynamics is a massive subject and needs to get taught through somewhat quickly, but that doesn't mean it needs to be taught incorrectly. Just because astrophysics is difficult doesn't mean we should all learn geocentrism.
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Re: What is "lift"?

Post by digits_ »

PilotDAR wrote: Mon Jul 13, 2020 7:14 am
Okay so I am arguing with my flight instructor......
Which may be a part of the problem. Just take the training as it is presented.
As a fellow critical student: absolutely not!
If this happens at the start of a 2 hour ground school session, or even a flying lesson, you've lost me at that point for the next 2 hours. Students that are hung up on a question rarely pay much attention to the remainder of the lesson. If the instructor's only goal is to get through the syllabus (and those people exist), then those students are great, they won't say anything and you'll get through it in no time. If the instructor's goal is to teach them something, then it's a terrible situation.
PilotDAR wrote: Mon Jul 13, 2020 7:14 am Some things are simplified for the purposes of just getting through the curriculum. Sure, if you want to delve into the fine details, go for it, but, find an expert, and be prepared to pay them for their time. You flying instructor may not be an expert at aerodynamics, he/she is there to teach flying, not aerodynamics, beyond the very basics.
I would postulate that understanding when the angle of attack increases or decreases is important. More generally, you could say that a topic is either important, or it is not. If it is important, it should be taught correctly. If it is not, it shouldn't be taught. A flight instructor -or anyone in a teaching role- should answer every question either with a correct answer or an "I don't know". Nobody is perfect of course, but that's why it is important that a student is critical and informs the teacher about inconsistenties in an explanation. If anything, it should improve the knowledge of the teacher.

If a teacher can't handle that, he shouldn't be teaching.
PilotDAR wrote: Mon Jul 13, 2020 7:14 am It is important for students to understand that there is so much to get through, that if one distracted from the prime course objectives on various topics, you could make the training take five times as long. The search for more knowledge is great, but understanding the ultimately fine nuances of lift will not make you a better PPL, learning the basics really well will....
How will it help the student if the is taught that in a climb the AoA is stationary/increasing when in reality it decreases?
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Re: What is "lift"?

Post by Schooner69A »

I originated that little question some time ago as a thought exercise.

It went along the lines of:

You are in level flight at 100 knots. You go into a full power climb maintaining 100 knots.

Question: has the angle of attack increased, decreased, or remained the same?

Hint: what angle of attack would be required to climb vertically?

:D
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Re: What is "lift"?

Post by gustind »

PilotDAR wrote: Mon Jul 13, 2020 7:14 am
Okay so I am arguing with my flight instructor......
Which may be a part of the problem. Just take the training as it is presented.
I've found a lot of your contributions have been absolutely stunning and I am going to voice my opinion on this subject. First off, I condemn the arguing with a flight instructor wholeheartedly. I also condemn your statement which I have quoted above.

I find a lot of problems in flight training result from students blindly accepting the Flight Instructor's word on something without thorough understanding. This problem then repeats itself down the line when that student becomes a flight instructor and blindly teaches something they were taught without really knowing why. It turns into a hamster on a wheel problem with puppy mill students and flight instructors. Blindly saying, just take the training as it is presented is going too far. Sometimes, flight instructors don't even know what they're teaching and the student won't know any better if they "just accept it."

I do agree with the rest of your statement to an extent. In any case, if a student wants to critically think and question my teachings, I'd encourage it. It won't be an argument, it will be a respectful discussion where the student will hopefully learn and take something away from our time together.
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Re: What is "lift"?

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Re: What is "lift"?

Post by Gannet167 »

I used to instruct in an aircraft that had an AoA gauge. Slow flight was an exercise where the student slowed to 15 units AoA and maintained it in level flight. They would climb/descend and conduct turns, all while maintaining 15 units, everything was done solely with power (if done correctly). Trimmed well, it could be done hands off.

The flight path vector in a climb (from a purely academic perspective) would put AoA lower after the initial acceleration from change in path, but considering a 100 ft/min with about 80 kts airspeed, the climb gradient is negligible and for our purposes could be assumed to have negligible effect on AoA. In a fighter, climbing at a 45 degree or 90 degree angle, it would be substantial.

Another way to think of it: flying an approach on AoA requires maintenance of AoA regardless of flight path. A higher AoA puts you close to stall, a lower AoA makes airspeed (and therefore ground speed) higher. If you are high, you maintain the same AoA but merely reduce thrust to increase rate of descent and steepen the flight path (if low, add thrust etc.)

To answer your question, AoA will be an insignificant amount lower, and generally speaking, maintaining the same airspeed will maintain the same AoA. The increased thrust means more energy is going into the vertical as lift, and virtually the same energy is going into the forward airspeed.
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Re: What is "lift"?

Post by PilotDAR »

I find a lot of problems in flight training result from students blindly accepting the Flight Instructor's word on something without thorough understanding.
Believe me, I'm very much in favour of a student having a thorough understanding. My concern is that there is insufficient time in the curriculum for an instructor to convey the thorough understanding. Certainly, the student should seek out as much additional training as they can find, including asking here. However, in the case of using training time to challenge the instructor, in general, the results may not be as desired (displeased instructor, no greater understanding anyway).

Sure, AoA can be familiarized during the ground briefing, and could be a semester study in an aerodynamics course. I opine that a PPL student will get the best benefit from a short briefing, and then time airborne with "let me show you...". What one needs to know about this at the PPL level is pretty easy to demonstrate in flight.

Then, of course, I hope every pilot would delve more into it, as obviously, AoA error accidents keep happening. And, students should have spin awareness training too! In a perfect world, the PPL curriculum would be expanded to assure more intense training of many of these topics, and we'd have even better instructors too.

If your instructor is incapable of providing the detailed instruction on a topic, seek out more expertise. If you think that the instructor is actually deficient in their training ability, that's a different topic. In the mean time, sure, ask for more detail - and be willing to pay for the training time for it too. but arguing with the instructor about the training content, not so much.....
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Re: What is "lift"?

Post by RedAndWhiteBaron »

"Argue" was a poor choice of word. But to be fair, there is very fine line between an argument and a debate.
PilotDAR wrote: Mon Jul 13, 2020 10:09 am If your instructor is incapable of providing the detailed instruction on a topic, seek out more expertise.
He is very capable of providing detailed instruction. His detailed exploration of this question was so thorough that I was able to come up with my own (possibly poor) solution.
Gannet167 wrote: Mon Jul 13, 2020 9:32 am The increased thrust means more energy is going into the vertical as lift.
This is exactly where we disagree, hence the title of this thread. I argue that the downward component of thrust can be considered to be lift, just as the backwards component of lift can be considered to be (induced) drag. Apparently this is anathema to the way lift is generally taught - that a propeller (or any fixed-wing powerplant) can produce lift.

It doesn't change the actual physics of the question, but only the definitions.

Now on the other hand, my thinking here is rather two-dimensional. Lift can also produce a sideways force, and then what would we call that?

It would be negligible, true. With 200lbs of thrust and a 12° angle of attack, the downward force of thrust is 41.6lbs (if my math is correct), which at 2000lbs gross weight, accounts for roughly 2% of the total force opposite to weight - so, not that much.
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