Aerodynamics

[njdem82]

Hello All,
So I understand that we do not have to build airplanes but I am certain that an understanding of aerodynamics is very important and so far as my training progresses I find more and more that aerodynamic theory is a very helpful model for me to place information in. That being said much of the usual theory that I have learned is incomplete and in some cases flat out wrong. I am just starting to read more in depth info on this and have found some contradictions which I am sure that most of you are already aware of, bear with me, I will get to the question part of this. This is what I have learned at flight school on this topic so far:

Lift: relative wind moves more quickly over the top of the airfoil and more slowly over the bottom. This creates high pressure on the bottom and low pressure on the top via Bernoulli's principle. High seeks low and so lift is created. There is also Newton’s third law and as the airfoil pushes down on air the opposite upward force contributes to lift.

The above was basically mantra repeated by almost any cfi which I had asked at an academy which has many instructors. None of it is really seems wrong but it never seemed to explain everything either. I had many questions such as "How do aerobatic airfoils operate in that case" or "how does a plane fly upside down?" the answer I got was that they overcame it with alot of speed- but this didn't really explain it and I am not even sure if it makes sense as speed would probably just increase the downward lift.

Also the deductions that can be made from this; or reasoning’s used to explain this are misleading and can be harmful to the pursuit of a depper understanding. These initial explanations can actually become an obstacle when one tries to really understand aerodynamics. However I get the impression that people are trying to explain these complex ideas in a complete but also simple way which is always going to be very challenging.

Air moves more quickly over the top of the airfoil because it has to get to the same point as the air moving under the airfoil thus since there is more distance to be covered it must move quickly creating lower pressure.
Why would an unconscious molecule have a desire to get to the same place at the same time as its counter-part underneath the airfoil? There are absolutely no scientific laws or even serious theory that suggests that if you take two molecules of air and blast them over an airfoil they would then meet up at the trailing edge at the same time.

As I am beginning to understand it (and I am almost certain that I will get many parts of this wrong or incomplete so be gentle ) . We disturb the air by introducing a moving object into its environment. The air hits the leading edge of the object and reacts according to the shape of the object. Since we are using an airfoil the air hits the leading edge goes over, conforms to the shape because the is a certain amount of viscosity in (all?) fluids (not enough to force the air to conform to the shape at the critical AOA, which is, I think, why there is a critical angle of attack) then deflects most of the air straight down. The amount of the air which it deflects downward and the speed at which that air is moving determines how much lift is produced. Some of the air that goes over the top of the airfoil (for lack of a better word) “circulates” which causes the air underneath the airfoil to move more slowly and is (perhaps) what is referred to as induced drag (or a part of it?). For instance when one gets close to the ground this performance reducing circulation is limited as it does not have the opportunity to disrupt the inbound airflow and so we have “ground effect” a reduction in induced drag causing an increase in performance.

Well once again I want to qualify my statements. I am just starting to understand these complex ideas and I know that I have a lot more work to do. I am also not in any way “bashing” the traditional explanation or any CFI’s who explain it in this way just wondering if we could add to the explanation. Reading over my post as I just did I also see that I did not explain all of my perceived conflict well but I did not want to write an initial post that was two pages long either so I will elaborate/correct as the discussion goes on if necessary.

Basically I have my eye on becoming a CFI at some point in the future (just coming up my instrument check) and I am wondering how I can supplement the traditional explanation to lead to a better understanding of these principles because I do believe that it is not just academic theory but can also be very practical and make someone a better pilot.

As always any input, suggestions, comments or criticisms will be appreciated. Thank You

 

[tgrayson]

just wondering if we could add to the explanation.

Take a look at this:

Circulation Theory

 

[Brian Z]

Wow that is a lot so I will start with this one.

Hello All,

The above was basically mantra repeated by almost any cfi which I had asked at an academy which has many instructors. None of it is really seems wrong but it never seemed to explain everything either. I had many questions such as "How do aerobatic airfoils operate in that case" or "how does a plane fly upside down?" the answer I got was that they overcame it with alot of speed- but this didn't really explain it and I am not even sure if it makes sense as speed would probably just increase the downward lift.

Think angle of attack for inverted flight. You can make a piece of plywood create lift if you give it some AoA. What is any different in an aerobatic airplane. The airfoils are somewhat symmetrical. If you have a symmetrical airfoil and have 0 AoA you will have a net zero lift because both the top and bottom are creating the same amount of lift. Once you introduce an AoA then you can create lift in that direction. For inverted flight you would have a negative AoA relative to normal flight(yoke forward). However, it still would positive when compared to the horizon(longitudinal axis with positive AoA). The wing does not care if the person is up or down.

Brian

 

[PFGiardino]

http://search.barnesandnoble.com/Th...o-Aerodynamics/Hubert-C-Smith/e/9780830639014

This one was good to me.

 

[msmspilot]

Brian already hit on this one, but at some point in your training, either your instructors did not mention angle of attack (or at least did not give it proper priority) or you have forgotten about it. Either way, the rest of the wing shape, while important for particular flight characteristics, is subservient to angle of attack. This one concept answers your question about symmetrical airfoils and inverted flight.

tgrayson covered circulation.

You're correct about the "air goes faster over the wing because it wants to 'keep up' with the molecules under the wing." That theory is total bunk. Experimentation shows that the molecules over the top, even though they have farther to go, actually get there first. This has to do with circulation and anything else I say about it would be simplifying the circulation problem too much.

You're asking good questions. That's the first step in learning

 

[njdem82]

Take a look at this:

Circulation Theory

Thanks for the source it looks very helpful even just at first glance, I'm going to go over that sizeable amount info tonight , I'm sure I'll have a whole new set of questions by tomorrow, or, more likely, at four in the morning

Wow that is a lot so I will start with this one.

Yeah sorry about that I realize I jumped all over the place in the future I will break things up into multiple threads so that they can be more focused.

Think angle of attack for inverted flight. You can make a piece of plywood create lift if you give it some AoA. What is any different in an aerobatic airplane. The airfoils are somewhat symmetrical. If you have a symmetrical airfoil and have 0 AoA you will have a net zero lift because both the top and bottom are creating the same amount of lift. Once you introduce an AoA then you can create lift in that direction. For inverted flight you would have a negative AoA relative to normal flight(yoke forward). However, it still would positive when compared to the horizon(longitudinal axis with positive AoA). The wing does not care if the person is up or down.
Think angle of attack for inverted flight. You can make a piece of plywood create lift if you give it some AoA. What is any different in an aerobatic airplane. The airfoils are somewhat symmetrical. If you have a symmetrical airfoil and have 0 AoA you will have a net zero lift because both the top and bottom are creating the same amount of lift. Once you introduce an AoA then you can create lift in that direction. For inverted flight you would have a negative AoA relative to normal flight(yoke forward). However, it still would positive when compared to the horizon(longitudinal axis with positive AoA). The wing does not care if the person is up or down.

So if I have this right, in inverted flight just like normal flight I would have to pull the yoke back and probably pitch the aircraft so that the nose is pointed more to the ground ( a slight negative AOA) than the artificial horizon in order to maintain straight and level.

So I suppose that Critical AOA is same whether its a positive or negative AOA.
Just out of curiousity . If a plane stalled in inverted straight and level flight would it begin to establish a positive climb before the stall was broken or would the nose down attitude and likely forward cg make it head to the ground?

 

[shdw]

Take a look at this:

Circulation Theory

I haven't read that from you before bud, but that is a wonderful article. Nicely done.

 

[shdw]

So if I have this right, in inverted flight just like normal flight I would have to pull the yoke back and probably pitch the aircraft so that the nose is pointed more to the ground ( a slight negative AOA) than the artificial horizon in order to maintain straight and level.

No, push the nose forward.

So I suppose that Critical AOA is same whether its a positive or negative AOA.

Only on a symmetrical wing.


Side note: Go buy "real flight" flight simulator. You can buy it here. I know it is a little on the steep side, but it does give you a perspective that no classroom or flight ever will.

You learn what each control input will do to the aircraft from an outside-in orientation. You can then apply that to what you might expect to see/do through various maneuvers. Do it all on a computer where crashes cost you nothing. Plus it is fun if you have the money to spend on a toy/training device.

 

[njdem82]

Brian already hit on this one, but at some point in your training, either your instructors did not mention angle of attack (or at least did not give it proper priority) or you have forgotten about it. Either way, the rest of the wing shape, while important for particular flight characteristics, is subservient to angle of attack. This one concept answers your question about symmetrical airfoils and inverted flight.

tgrayson covered circulation.

You're correct about the "air goes faster over the wing because it wants to 'keep up' with the molecules under the wing." That theory is total bunk. Experimentation shows that the molecules over the top, even though they have farther to go, actually get there first. This has to do with circulation and anything else I say about it would be simplifying the circulation problem too much

AOA was mentioned but was presented as far less important to lift than the shape of the airfoil. It came of as more of a control effect (higher angle of attack nose pitches up , lower it pitches down) rather than a primary (if it is fair to say primary) reason for lift itself. This becomes very clear when I ask fellow students at different levels of training.

You're asking good questions. That's the first step in learning

Thanks , I hope I'm not testing anyones patience , I'm just trying to keep up. I see alot of people get by just using the pre-packaged responses but I notice that many of them burn out during CFI training or sometimes even become CFI's who do the absolute minimum .

Im hoping to get as deep of an understanding of aviation as I can so that someday, hopefully I can be a good CFI and help others understand these issues better.

 

[shdw]

AOA was mentioned but was presented as far less important to lift than the shape of the airfoil.

A level of importance can be quickly derived from the formula:

Consider that Cl = coefficient of lift which is determined by 2pi * AOA in radians. Now 1 degree = 0.017 radians. So a 1 degree change in AOA:

1 degree change = 2(3.14) * 0.017 = 0.11 or 11%

So a change of 1 degree AOA changes the total lift by 11 percent, direction isn't relevant. (Did I do this right, I am kinda wasted from easter woot!)

 

[tgrayson]

http://search.barnesandnoble.com/Th...o-Aerodynamics/Hubert-C-Smith/e/9780830639014

This one was good to me.

That's the book that I always recommend that people buy. He doesn't get into circulation theory, but he otherwise does a pretty complete job of providing the basic concepts behind every major area of aircraft flight.

 

[msmspilot]

AOA was mentioned but was presented as far less important to lift than the shape of the airfoil. It came of as more of a control effect (higher angle of attack nose pitches up , lower it pitches down) rather than a primary (if it is fair to say primary) reason for lift itself. This becomes very clear when I ask fellow students at different levels of training.

Yeah... Shape of the airfoil is important... but AOA is basically the whole story. (btw, the increase AOA and nose pitches up, and vice versa is only sorta right)

The shape of the airfoil has more to do with the stall characteristics than producing lift. A properly designed airfoil may produce lift more efficiently and stall with less bad stuff going on, but as has been mentioned in this thread already, a flat piece of plywood can produce lift at the right AOA.

 

[shdw]

AOA was mentioned but was presented as far less important to lift than the shape of the airfoil. It came of as more of a control effect (higher angle of attack nose pitches up , lower it pitches down) rather than a primary (if it is fair to say primary) reason for lift itself. This becomes very clear when I ask fellow students at different levels of training.

Well I showed you relative importance in my previous post with the lift formula. Now that I am sober I can tackle the rest of this paragraph.

Be careful about saying things like "higher angle of attack nose pitches up, lower it pitches down." Remember in my earlier post I said that direction is irrelevant. A high negative angle of attack is either (a) inverted climb/level or (b) a sharp dive from upright flight.

Also, AOA and pitch are not effected equally by degree changes. What I mean is if you change pitch 10 degrees, AOA will not change 10 degrees. However, pitch and AOA vary directly in all instances I am familiar with. That is, if you pitch up 10 degrees, AOA won't go up 10 degrees, but it will go up.

As for airfoil shape, that is a whole different bird. Sure it has something to do with lift, but not a whole heck of a lot. Let me see if I remember these all properly, if not someone can straighten them out:

Basic Information:

• General: As per the lift formula, the only part of the airfoils "shape" that matters when creating lift is the wings area.
• Stalls: Wing shapes often have wide effects on stall propagation.
• Drag: Drag varies mainly from wing thickness as far as the shape of the design goes.

The 4 main shapes:

1. Square Wing: Referred to as the hershey bar here a few times and is nothing fancy. A typical wing on a cessna/piper trainer. Stall will commonly start at the inboard trailing edge and progress toward the leading edge and outward.
2. Elliptical/Tapered Wing: Seen in many higher performance singles, warbirds, and aerobats. This wing will stall from the trailing edge to the leading edge.
3. Sweepback Wing: Seen on some single, but mostly for jet and military use. This wing will stall from the trailing edge wingtip and progress inward and towards the leading edge. Hence one of the reason for stick shakers/pushers in transport category jets.
4. Delta: Not much different than the sweep back in stall characteristics. This wing generates more lift because it has more area.

Random Terms/Purposes:

• Camber: Has to do with the difference in shape between the lower and upper curves of the wing. Here is a list of what each different one gives the designer: Wing Camber List. Camber can increase/decrease lift and drag based on the design used.
• Thickness: Commonly falls in with camber as the shape determines the thickness. A thicker wing produces more lift and more drag, vice versa for thinner. Generally speaking of course.

If you are interested in looking more into the structural design and its effects on dynamics you should aim answering this question: What effect does each of the above have on an aircrafts stability? Which axis are effected? Are the effects positive or negative, static or dynamic, stability?


PS No time to edit, I have a meeting.

 

[njdem82]

Well I showed you relative importance in my previous post with the lift formula. Now that I am sober I can tackle the rest of this paragraph.

Be careful about saying things like "higher angle of attack nose pitches up, lower it pitches down." Remember in my earlier post I said that direction is irrelevant. A high negative angle of attack is either (a) inverted climb/level or (b) a sharp dive from upright flight.

Also, AOA and pitch are not effected equally by degree changes. What I mean is if you change pitch 10 degrees, AOA will not change 10 degrees. However, pitch and AOA vary directly in all instances I am familiar with. That is, if you pitch up 10 degrees, AOA won't go up 10 degrees, but it will go up.

As for airfoil shape, that is a whole different bird. Sure it has something to do with lift, but not a whole heck of a lot. Let me see if I remember these all properly, if not someone can straighten them out:

Basic Information:

• General: As per the lift formula, the only part of the airfoils "shape" that matters when creating lift is the wings area.
• Stalls: Wing shapes often have wide effects on stall propagation.
• Drag: Drag varies mainly from wing thickness as far as the shape of the design goes..

Thanks all, I have been looking into the concepts that have been brought up here, took a couple of breaks to bash my head against the wall, but it is starting to take some sort of shape. One of the things that I had trouble with is “stall propagation” I found some info in Caltech and MIT papers, apparently the term is also used in networking but for the most part it went over my head.
But from the context here it seems to have something to do with where and in what condition a stall begins and how it proceeds from that point. Also that a part of wing design is based on “how” a wing stalls. I am sure that this is an oversimplification, if it is even in any way true, so I’m not going to hold onto that idea.

If you are interested in looking more into the structural design and its effects on dynamics you should aim answering this question: What effect does each of the above have on an aircrafts stability? Which axis are effected? Are the effects positive or negative, static or dynamic, stability

So far it does seem like looking into relationships is a better way to gain some level of understanding. All of this goes very deep and it seems like I’m going to have to develop some better math skills since it will make it easier to understand the relationships described in the lift formula.

 

[DaveC]

Lift: relative wind moves more quickly over the top of the airfoil and more slowly over the bottom. This creates high pressure on the bottom and low pressure on the top via Bernoulli's principle. High seeks low and so lift is created. There is also Newton’s third law and as the airfoil pushes down on air the opposite upward force contributes to lift.


Air moves more quickly over the top of the airfoil because it has to get to the same point as the air moving under the airfoil thus since there is more distance to be covered it must move quickly creating lower pressure.

Two very common yet very wrong misconceptions here.


First, the airfoil doesn't create high pressure on the bottom. The static pressure on both the top and bottom will be lower than ambient, but the relative pressure will be higher below the wing.


The next paragraph is totally off. The acceleration in flow has nothing to do with it being a longer path. It's due to the law of conservation of mass applied to choked flow. The two air paths don't need to meet up at the end. In fact, the air flowing over the top of the airfoil will reach the trailing edge before the air flowing under the bottom.

 

[tgrayson]

First, the airfoil doesn't create high pressure on the bottom. The static pressure on both the top and bottom will be lower than ambient, but the relative pressure will be higher below the wing.

That depends on the AoA. At low angles, you're quite right, but the pressure on the underside rises and quickly exceeds ambient pressure as the AoA increases.

 

[N18040]

I am surprised nobody has mentioned the Coanda effect yet. I was taught that it caused the circulation as opposed to the vortexes as mentioned in tgrayson's link.

 

[tgrayson]

I am surprised nobody has mentioned the Coanda effect yet.

:banghead: Do a search on previous threads on the topic.

 

[Mike H]

Now that I am sober I can tackle the rest of this paragraph..

Wait a minute- let's not play musical chairs here...

errr. let me get back to this htread later

 

[Gunnar971]

Two very common yet very wrong misconceptions here.


First, the airfoil doesn't create high pressure on the bottom. The static pressure on both the top and bottom will be lower than ambient, but the relative pressure will be higher below the wing.


The next paragraph is totally off. The acceleration in flow has nothing to do with it being a longer path. It's due to the law of conservation of mass applied to choked flow. The two air paths don't need to meet up at the end. In fact, the air flowing over the top of the airfoil will reach the trailing edge before the air flowing under the bottom.

So just to clear up what you are saying:

1. Both the upper and lower surfaces of a wing have a lower pressure than the undisturbed air. However, because the upper surface essentially disturbs the air more (due to its camber) the pressure on the upper surface of the wing is only at a lower pressure relative to the lower side/ Or to look at it from another POV- the lower surface is a higher pressure because it has less camber than the upper surface, meaning it will displace less air COMPARED to the upper surface.

This gets into point 2. The reason the upper surface displaces more air is because of its camber. Air, just like water or any other liquid or gas, is a fluid. This is where Bernoulli's principal comes in. As speed increases, temperature and pressure decrease. (this is where it gets hard to explain over the Internet without picts!). The air at a certain distance in front of your airplane is undisturbed. As your plane (wings) begin to move through the undisturbed air, it has to go somewhere. This air moves around your wings and (for the lack of a better visual picture) is sandwiched against the undisturbed air that is above and below your wing (the air that your plane did not move through). This sandwich effect is what creates bernoulli's principle on your wing and your REALTIVE higher and lower pressures.

Hope this helps