Why learn aerodynamics?

T

TDGlider

Filing Flight Plan
Joined
Jan 13, 2023
Messages
15
Display Name
TDGlider
Sometimes, the most difficult student question to answer is "why do I need to learn this"? One of the topics where I struggle to answer this question is aerodynamics. Why do pilots need to understand how a wing creates lift according to Bernoulli or Newton? Many of us learned an explanation of lift based on the equal transit time theory, which is demonstrably false. Yet, we have been able to control our airplanes fine. I was also taught an incorrect explanation of why airplanes with dihedral have lateral stability. Even though I believed this incorrect explanation for years, I had no trouble keeping the wings level. If one can function well as a pilot despite an incorrect understanding of a topic, that calls into question whether understanding the topic is actually important. So, what do instructors on this forum say when asked by students why they need to learn about aerodynamics.

I am not suggesting that we take this material out of the curriculum (except the parts that are false). I would just like to be able to give a better answer to the "why do I have to learn this?" question than "it is on the test".
 
I've heard a local story of a plane getting re rigged incorrectly after maintenance. Caught during preflight.

If pilot didn't know how lift works he would have had a short flight.
 
TDGlider said:
I would just like to be able to give a better answer to the "why do I have to learn this?" question than "it is on the test".
Click to expand...
I would not like to be able to. Any theory that always works and is easy to understand and apply beats an esoteric one. Even science teachers knowingly teach incorrect theory to early physics students when they don't have the prerequisite knowledge to process the full truth. To advance, though, everybody needs to pass the tests.
 
dtuuri said:
Even science teachers knowingly teach incorrect theory to early physics students when they don't have the prerequisite knowledge to process the full truth.
Click to expand...
Are they knowingly teaching incorrect theory or are they teaching theory with additional assumptions that simplify the theory in that specific application? The latter is prevalent, the former is inexcusable.

Nauga,
assuming a spherical cow
 
If I were to guess, I would believe that 90+% of pilots believe in fairy tales like "airfoils are flat on the bottom, curved on top", "cross section of a venturi", "Newton on the bottom, Bernoulli on top", blah blah blah.

So, there is clearly no need at all to actually have any sort of actual understanding other than "If you do this, then that will happen".
 
nauga said:
Are they knowingly teaching incorrect theory or are they teaching theory with additional assumptions that simplify the theory in that specific application? The latter is prevalent, the former is inexcusable.

Nauga,
assuming a spherical cow
Click to expand...
:dunno: A lot of experts don't like the picture of the earth on a stretchy piece of fabric that supposedly explains space-time (I think). I don't understand it either way, so you'll have to tell me. But I've read several explanations about this, that or the other thing where the expert caveats the answer by saying their explanation is not true, but it's the only way to explain it to a layman, or words to that effect. Quora sends me stuff every night and I've seen it a number of times.
 
I had a Mississippi NG student who was a slow leaner. His training record had more red ink than black. One day I asked him out of the blue how a rotor system produces lift. Never forget his confused look and wild hand gestures. “Well it’s…it’s like a ceiling fan. It takes the air and pushes it down…whoosh! whoosh!” Perfect.
 
TDGlider said:
Sometimes, the most difficult student question to answer is "why do I need to learn this"? One of the topics where I struggle to answer this question is aerodynamics.
Click to expand...
This thread is headed by this question. An example is aerodynamics. One example. There are other misunderstood (or not-understood-at-all) subjects that can kill you. One of those is carburetor ice, and many pilots think this is a wintertime-only thing, so it kills their engines in the summertime because they don't expect it or understand it. Another is angle of attack; a lack of understanding there gets them into trouble with accelerated stalls. Or they haven't learned that AoA and airspeed are inextricably linked, and then wonder why their landings are so bad when they approach and flare far above POH-recommended speeds.

The discussion and theories on how lift is generated has been argued for decades. It will go on for decades more. Most of us have a workable idea on it. Mine says that when the air over the top is faster than air over the bottom, lift will be generated, even on a symmetrical airfoil. Just get the symmetrical airfoil's chordline at an angle to the relative wind, and there it is. Bernoulli. A cambered airfoil makes it at much smaller AoAs, even down to -4° on some. Downwash results from it, so there's Newton.

Now, why it goes faster over the top? Mystery. Is the centrifugal force generated by the curving airflow pulling the air outward and causing "suction?" That would suck the air over the top and make it go faster.

Or maybe that whole idea sucks.
 
eman1200 said:
bernoulli. that's all u need to know.
Click to expand...
Newton has entered the chat.

As long as there is a DIFFERENCE in pressure you will get lift (or a force).

The real downside to Bernoulli is that even with today's mega speed computers we cannot accurately model air over a wing.
 
Last edited:
Dan Thomas said:
Now, why it goes faster over the top? Mystery. Is the centrifugal force generated by the curving airflow pulling the air outward and causing "suction?" That would suck the air over the top and make it go faster.
Click to expand...
Turning the flow over the top and reduced pressure over the top go hand in hand (Newton's second law). The lower pressure makes it go faster due to conservation of energy (Bernoulli) - you are trading potential energy (in the form of pressure) for kinetic energy.
 
Dan Thomas said:
Now, why it goes faster over the top? Mystery. Is the centrifugal force generated by the curving airflow pulling the air outward and causing "suction?" That would suck the air over the top and make it go faster.
Click to expand...
Always thought of it as the air flowing over the top has to travel farther in the same amount of time.
 
If you don't have a basic understanding of aerodynamic forces, you may fail to recognize or interpret a situation correctly and die as a result. A classic example is hurrying turns with rudder. Another is AoA. Another is causes of adverse yaw. Anyone who needs more incentive to learn how their machine stays in the air probably should not be a pilot.

The question is how detailed that understanding should be. I would submit that depends on what you are flying and how.

I am a competitive aerobatic pilot. I need to have a solid understanding of gyroscopic precession in order to anticipate corrections in various low speed maneuvers. Does an average private pilot? Probably not.

Do I need to understand "coffin corner"? No. Does an ATP need to know that? Yes, definitely.

Does anyone other than an aeronautical engineer or test pilot really need to understand lift at the molecular or fluid dynamics level? Probably not. But if you are building an EAB with fabric wings, you better understand there is a pressure differential, lest you skip the rib stitching and wind up with an airport in Oshkosh named after you.
 
Last edited:
TDGlider said:
Sometimes, the most difficult student question to answer is "why do I need to learn this"? One of the topics where I struggle to answer this question is aerodynamics. Why do pilots need to understand how a wing creates lift according to Bernoulli or Newton? Many of us learned an explanation of lift based on the equal transit time theory, which is demonstrably false. Yet, we have been able to control our airplanes fine. I was also taught an incorrect explanation of why airplanes with dihedral have lateral stability. Even though I believed this incorrect explanation for years, I had no trouble keeping the wings level. If one can function well as a pilot despite an incorrect understanding of a topic, that calls into question whether understanding the topic is actually important. So, what do instructors on this forum say when asked by students why they need to learn about aerodynamics.

I am not suggesting that we take this material out of the curriculum (except the parts that are false). I would just like to be able to give a better answer to the "why do I have to learn this?" question than "it is on the test".
Click to expand...

Pilots are not expected to know the scientific theories behind lift. They just need to know enough to understand weight and balance, center of lift, center of weight, why an airplane has a tail and rudder, the meaning of drag, stall, Va etc.. etc.. Curiosity is a basic human character. If a student is not asking why and how, I would take that as a red flag. We are not machines programmed to do what's in the POH.
 
MauleSkinner said:
Even an understanding based on simplified assumptions will help retain knowledge that will be useful when adding knowledge and skills or when encountering something you haven’t specifically been taught.
Click to expand...
Simplified assumptions (i.e. inviscid, incompressible): Yes.
Incorrect assumptions ('equal transit time', Bernoulli top and Newton bottom): No.

Nauga,
approximately
 
Ed Haywood said:
Do I need to understand "coffin corner"? No. Does an ATP need to know that? Yes, definitely.
Click to expand...
As an example, at a previous job I was flying with the Chief Pilot one day, and he asked, “Did you hear what happened to [those other guys]?”

Me: “No.”

The follow-up involved phrases like, “watch this,” and “interesting…let me try that,” as we played with low-speed Mach buffet, about which we knew exactly nothing.

Mauleskinner
Occasionally a ****ing idiot
 
Ed Haywood said:
Does anyone other than an aeronautical engineer or test pilot really need to understand lift at the molecular level? Probably not.
Click to expand...
It's clear that not having a good grasp of the fundamentals of lift and drag does not prevent one from becoming a pilot *and advancing as one* but it certainly makes understanding all of the characteristics you addressed and many others easier.

Nauga,
building blocks
 
Wings fly because of the money we spend on them.

The reason a student needs to know this is because knowledge is power ... and we all want more power!
 
To me, I don’t care about the arguments about how lift is produced. It’s Newton, no it’s Bernoulli, no helicopters just beat the air into submission. I don’t care as long as a pilot understands the outside influences that affect performance.

Do they understand how airfoil contamination can affect lift? Do they understand the effects of increased weight or DA on performance? Do they understand what types of flight profiles (high AoB out of trim) can get them in trouble? Do they have wind or wake turbulence awareness?

Too often I’ve seen pilots get themselves in trouble just because of lack of awareness of their aircraft and surroundings. Basic aero is fine but I like to see knowledge of the big picture.
 
nauga said:
Simplified assumptions (i.e. inviscid, incompressible): Yes.
Click to expand...
Simplifies the crap out of the math. But when you try to explain how a wing generates lift without viscosity, I'm pretty sure you have to invoke circulation which is about as intuitive as quantum mechanics or fall victim to D'Alembert's paradox. :)
(OK. You can fudge a bit and just proclaim that the air flows nicely off the trailing edge, but that pretty much just kicks the can down the road a bit.)

The hard part seems to me to be knowing just how far down a rabbit hole you should go - but no matter what you decide, some people will complain that there is too much math or it's too complicated, others will complain that you glossed over some really essential detail, and some will just call you an ******* for not explaining it the way they learned in middle school.

en.wikipedia.org

D'Alembert's paradox - Wikipedia

en.wikipedia.org en.wikipedia.org
 
sarangan said:
Pilots are not expected to know the scientific theories behind lift. They just need to know enough to understand weight and balance, center of lift, center of weight, why an airplane has a tail and rudder, the meaning of drag, stall, Va etc.. etc..
Click to expand...
The more you know the easier it is to determine that you're being fed bull**** when someone or something is explaining something new to you. People clearly understand what maneuvering speed is, but I frequently talk to people who don't understand why it goes up as weight goes up, and why the POH/placard Va is only valid at one (heavy) weight, and that you don't have the assumed 'protections' if you're at Va but weigh less than that.

Or why some airplanes have Vne as true airspeed
or what control inputs your airplane is designed to be able to withstand at Va (and why you shouldn't use them)
or why there's a leading edge device on your stabilator and why it's important
or...

Nauga,
code brown
 
Capt. Geoffrey Thorpe said:
Simplifies the crap out of the math. But when you try to explain how a wing generates lift without viscosity, I'm pretty sure you have to invoke circulation which is about as intuitive as quantum mechanics or fall victim to D'Alembert's paradox. :)
(OK. You can fudge a bit and just proclaim that the air flows nicely off the trailing edge, but that pretty much just kicks the can down the road a bit.)

The hard part seems to me to be knowing just how far down a rabbit hole you should go - but no matter what you decide, some people will complain that there is too much math or it's too complicated, others will complain that you glossed over some really essential detail, and some will just call you an ******* for not explaining it the way they learned in middle school.

en.wikipedia.org

D'Alembert's paradox - Wikipedia

en.wikipedia.org en.wikipedia.org
Click to expand...
Love that wicked grin on his face as he knows he’s messing with people for the next few hundred years.
 
There is no reason to know. You dispositively do not need to know it at all.

However, SOME form of explanation seems to aid MOST people in the learning process. That’s it.

The most capable welder I know has NO CLUE how ANY of it works. Can’t explain it, can’t teach it, CANT make a bad weld. Hand TIG nitrogen tanks for years. Can pass any welding practical test.

He has simply learned to assimilate the cause and effect. This can be done with flying as well.

The systems we’re taught as pilots IS NOT precisely how they operate, ask any mechanic. But they help us to learn to operate them effectively more quickly.
 
  • Like
Reactions: cdb
TDGlider said:
Sometimes, the most difficult student question to answer is "why do I need to learn this"? One of the topics where I struggle to answer this question is aerodynamics. Why do pilots need to understand how a wing creates lift according to Bernoulli or Newton? Many of us learned an explanation of lift based on the equal transit time theory, which is demonstrably false. Yet, we have been able to control our airplanes fine. I was also taught an incorrect explanation of why airplanes with dihedral have lateral stability. Even though I believed this incorrect explanation for years, I had no trouble keeping the wings level. If one can function well as a pilot despite an incorrect understanding of a topic, that calls into question whether understanding the topic is actually important. So, what do instructors on this forum say when asked by students why they need to learn about aerodynamics.

I am not suggesting that we take this material out of the curriculum (except the parts that are false). I would just like to be able to give a better answer to the "why do I have to learn this?" question than "it is on the test".
Click to expand...
If you can't think of how misunderstanding aerodynamics could cause an accident then you aren't using your imagination. I had a CFI student tell me that adverse yaw was caused by the outside wing traveling faster than the inside wing and causing more drag on the outside wing. Now imagine if this incorrect knowledge caused you to use extra inside rudder during a steep turn. Imagine if the steep turn is at a high g-load and low airspeed.

Here's another example. If the topic arises, some people will point out that the stall speed vs. bank angle graph we've all seen only applies in level flight, but that's only half true. Load factor and stall speed increase essentially just as much in a constant rate descent, whether 200fpm or 2,000fpm. It would be unfortunate if someone were to perform steep turns at low airspeed while descending with the mistaken idea that the descent ameliorated the typical stall speed increase associated with bank angle.
 
Last edited:
eman1200 said:
bernoulli. that's all u need to know.
Click to expand...
Plenty of people know what Bernoulli's principle says, but dont actually know what pressure is. Most people learn some classical physics at some point(ie Newtons laws), but dont get to the point where they learn that "P = F/A."

Knowing what Bernoulli's principles says does not actually teach you what it means. It's just more practical to teach the basic aerodynamic model of lift via Newton's third law, as it's covered earlier in the physics curriculum, so more people may have covered it in school, and may actually understand lift.
 
SOME people think E = IR, but we all know it’s REALLY the magic smoke. Yet we can all operate our cell phones. Just sayin…
 
Do you explain to your 6 year old the physics of how a bicycle stays upright when they learn to ride? (as an aside, not sure I know to this day in spite of 10's of thousands of miles on one) To some degree, flying is the same thing. Do I really need to know how an engine works at the level of detail in PP training? But I do need to know that it requires air, fuel and spark to run and how I might diagnose/fix a failing engine in flight.
 
Racerx said:
Always thought of it as the air flowing over the top has to travel farther in the same amount of time.
Click to expand...

The misconception here is thinking that two air molecules separated at the leading edge are somehow still “linked” so that the upper one “knows” it must speed up to rejoin the lower one. They’re not.

My working visualization is that there is a venturi formed by the wing and the undisturbed air that exists somewhere above it. That’s why it speeds up and the pressure drops, thanks to Bernoulli.
 
dmspilot said:
If you can't think of how misunderstanding aerodynamics could cause an accident then you aren't using your imagination. I had a CFI student tell me that adverse yaw was caused by the outside wing traveling faster than the inside wing and causing more drag on the outside wing. Now imagine if this incorrect knowledge caused you to use extra inside rudder during a steep turn. Imagine if the steep turn is at a high g-load and low airspeed.
Click to expand...
Or imagine thinking that picking up the wing with in a stall or during an engine-out in a multi instead of applying rudder wouldn’t have adverse effect.
 
Last edited:
dmspilot said:
I had a CFI student tell me that adverse yaw was caused by the outside wing traveling faster than the inside wing and causing more drag on the outside wing.
Click to expand...
If I were sitting at a desk I'd be banging my head on it right now.
I hear the same argument for dihedral effect* (why an airplane rolls with rudder) but adverse yaw?! Daaaaaaang.
*How dihedral does what it does is a whole 'nuther hot button.

Nauga,
and his fivehead
 
FastEddieB said:
My working visualization is that there is a venturi formed by the wing and the undisturbed air that exists somewhere above it.
Click to expand...
I visualize Marisa Tomei as a molecule of air stopping to take a sip of a molecule of water when BAM! an effing wing shoots its brains out:
 
You must log in or register to reply here.
Back
Top