Learning about lift generation - a waste of time.

[old man emu]

Last month I got into a discussion here of the Newton/Bernoulli theories of lift generation. I decided to research the subject so that I could write an informative paper on the subject and publish it here. Well, I did do that research. I read as much as possible on Newton's Three Laws of Motion, and I studied up on Bernoulli's theorem. I also delved into flat plate aerodynamics which really is the starting point of aerofoil design. 

 

I came up with some really good stuff and figured out some good examples to explain the things I had found out. I had even created a lot of the text in my mind. Then this morning, as I was washing up the breakfast dishes and looking out the into the distance through the kitchen window, I had a Eureka! moment.

 

I realise that for a pilot, knowing the physics of lift generation is not necessary for piloting an aircraft. Does an 8-year-old need to know the aerodynamics of one of these in order to "fly" it?

 

Does any average person know the physics of how their car manages to turn corners? Or the thermodynamics of the combustion process that produces the forces necessary to propel a car? NO. As long as the driver has the skills required to make the car go in the desired direction, and the desired velocity, then the practical operation of the car is achieved.

 

The critical thing a pilot must know is that at a certain angle of upward pitch, the wing fails to produce sufficient lift to maintain the weight of the aircraft at a certain altitude. Call that loss of sufficient lift what you like, a rose by any other name would smell as sweet. Vast amounts of experimentation indicate that this pitch angle, which is the angle between the chord line of the wing and the direction of the airflow relative to that chordline, is close to 15 degrees. So all a pilot needs to know to maintain lift is to keep the angle of the chord line to relative airflow direction below 15 degrees.

 

There is also the other obvious factor in lift creation - the velocity of the airflow relative to the win surface. But that's sort of obvious. You have to keep the airplane moving forward if you want to get from A to B. And we'll ignore the special situations where the velocity of the air approaching the wing is greater than the total Thrust produced by the power train and the aircraft "stands still" or gets carried backwards.

 

 

 

So, how does a pilot maintain that angle? In the majority of aircraft that the average private pilot would fly, there is nothing to use as a visual reference to compare the chord line with the relative wind direction. The obvious tool would be something like a bubble level set on the instrument panel when the aircraft was trestled in its cruise position, and calibrated to show position pitch angle from 0 to 20 degrees with reference to the chord angle.

 

If the DIY method doesn't suit your style, you could always fit an Angle of Attack Indicator.

 

 

So why should the myths, falsehoods and the Newton/Bernoulli debate about lift generation of a body with one curved surface and one relatively flat surface, continued to be pounded into pilots when they do not advance the the skill level of pilots in operating an aircraft? The only "theory" that a pilot needs to know is that if the aircraft is raised in pitch too much, not enough Lift will be produced and the aircraft will begin to fall due to the effects of Gravity, until other aerodynamic forces act on it to alter its motion (Newton's First Law). With that nugget of "theory" the  pilot only has to explore the visual inputs the environment provides as the amount of Lift approaches that break point.

 

[spacesailor]

Exerllent idea !.

BUT if the angle is near vertical, and your Big powerful motor, keeps your speed greater the the stall speed of said " rocket ",

Would you still stall ?.

German " Komet " rocket plane in mind.

spacesailor

 

Edited January 3, 2021 by spacesailor
Missing letter

[Marty_d]

3 minutes ago, spacesailor said:

Exerllent idea !.

BUT if the angle is near vertical, and your Big powerful motor, keeps your speed greater the the stall speed of said " rocket ",

Would you still stall ?.

spacesailor

 

Stalls can happen at any speed - that's the point.  "Stall speed" usually (AFAIK) refers to level flight.   If you're going down vertically in a nosedive and you try to pull out too suddenly, if you don't rip the wings off, the AoA can go past 16 degrees and your plane will stall even though it's doing 500kph.

Plenty of fighter pilots found this out the hard way in WW2.  (Probably in WW1 too, but the speeds would have been lower).  

 

When the air starts separating from the top of the wing instead of flowing around it, the wing is stalled.  Regardless of the speed or direction of the aircraft - it's all about the angle of the chord relative to airflow.

[APenNameAndThatA]

You put the pic, and idea, of the bubble level as a joke, right?

 

A while ago, you posted a video about the Lindberg Reference. A while ago, the same person who produced the video posted that it was not necessary for a pilot to know about the theory of how an an aircraft works, and that a pilot only had to know that a particular inputs produced particular results.

 

 

[APenNameAndThatA]

16 minutes ago, spacesailor said:

Exerllent idea !.

BUT if the angle is near vertical, and your Big powerful motor, keeps your speed greater the the stall speed of said " rocket ",

Would you still stall ?.

German " Komet " rocket plane in mind.

spacesailor

 

If your airplane is flying straight up and pointing straight up, the angle of attack will be zero, and OME's patented angle of attack meter will show an angle of attack on 90 degrees.

[spacesailor]

I Did say "near vertical " as I don,t think they did vertica takeoff.

But very steep up to target, then power off to turn downwards for another shot at their target, and glide back to base.

spacesailor

[APenNameAndThatA]

1 minute ago, spacesailor said:

I Did say "near vertical " as I don,t think they did vertica takeoff.

But very steep up to target, then power off to turn downwards for another shot at their target, and glide back to base.

spacesailor

Okay. If you are flying near vertical, and the airplane is pointing in the same direction, OME's PAAM will show near 90 degrees. If you were flying a little inverted, say at 100 degrees pitch up, OME's patented angle of attack meter would indicate that you were flying a little backwards.

[facthunter]

PITCH attitude has nothing to do with it. It's ONLY the angle of the relative Airflow to the wing referencing some datum like "zero lift angle" as a suitable choice..  To get zero lift with most airfoils you need a negative angle. The "thing" that determines the AoA is the elevators. Nev

[old man emu]

21 hours ago, APenNameAndThatA said:

You put the pic, and idea, of the bubble level as a joke, right?

I was wondering how long it would take you to start flaming me, yet again. I'm sick of your trolling.

 

Once again you ignore the content of the discussion to incite trouble. You don't contribute in a meaningful way to any discussion of topics I post. And yet you are able to give a sensible response to Spacey.

 

So I used an unusual illustration. Too bad you don't have enough experience to recognise the type of spirit level it was. Just for you, I've replaced the image with something familiar. 

 

Now, if you can't make a sensible input into the discussion, why don't you jump in your widdle plane and see how long you can fly a course of 090T.

[spacesailor]

OME, thanks for your idea,s that spirit level is almost the same as old navigators used for taking star-site fixes..

I was questioning "stall" and angle of attack, surely it differs for greater speed over the wing.

Even F111,s don,t stall at maximum velocity when gaining altitude? .

I could be wrong. But all those rockets don,t seem to have a stall speed UNTILL spent.

spacesailor

[APenNameAndThatA]

2 hours ago, old man emu said:

I was wondering how long it would take you to start flaming me, yet again. I'm sick of your trolling.

 

Once again you ignore the content of the discussion to incite trouble. You don't contribute in a meaningful way to any discussion of topics I post. And yet you are able to give a sensible response to Spacey.

 

So I used an unusual illustration. Too bad you don't have enough experience to recognise the type of spirit level it was. Just for you, I've replaced the image with something familiar. 

 

Now, if you can't make a sensible input into the discussion, why don't you jump in your widdle planet and see how long you can fly a course of 090T.

The spirit level won't show you the angle of attack.

[APenNameAndThatA]

The spirit level won't show you the angle of attack. For example, if you are flying steadily nose up 20 degrees with a two degree angle of attack, the spirit level would show an angle of attack of about  20 degrees. If you were pulling out of a dive, with the aircraft level, the spirit level would probably show about a zero angle of attack, but you might be about to stall.

 

The idea of a spirit level showing angle of attack was so silly, I really thought you might be joking. I didn't comment about you saying you might "write a paper" because I thought that you would just say that you were joking. Looks like you weren't.

 

It seems to me that everyone else is too polite to explicitly correct you. It's not trolling if it stops someone from reading what you write and thinking it might be true, and it's not trolling if you learn that you cannot use a spirit level to determine angle of attack.

 

Experience is not the issue. Things like force and velocity vectors, and mass, force and acceleration are dealt with in high school maths and physics.

[APenNameAndThatA]

2 hours ago, spacesailor said:

OME, thanks for your idea,s that spirit level is almost the same as old navigators used for taking star-site fixes..

I was questioning "stall" and angle of attack, surely it differs for greater speed over the wing.

Even F111,s don,t stall at maximum velocity when gaining altitude? .

I could be wrong. But all those rockets don,t seem to have a stall speed UNTILL spent.

spacesailor

The stall happens at the critical angle of attack (about 16 degrees) regardless of the velocity.

[facthunter]

It's imperative we don't have confusion on these matters, so it's worth persisting with. Nev

[Kyle Communications]

Another name for a AOA is a lift reserve indicator. jets flying at say 900km at 35,000 ft will stall about 40 or 80 knots lower than 900kph..well thats what  commercial jet pilot told me...and it makes sense as the air is much thinner. AOA's work and are regularly used but they measure airflow..really nothing else. I would think they work the same at 35,000 ft the same as at 1000 ft...its just the density that changes

 

[Jerry_Atrick]

I think where OME may have originally wanted this thread to go is, is the theory syllabus for gaining the requisite flying qualification required to the extent it currently is:

17 hours ago, old man emu said:

I realise that for a pilot, knowing the physics of lift generation is not necessary for piloting an aircraft.

 

17 hours ago, old man emu said:

Does any average person know the physics of how their car manages to turn corners? Or the thermodynamics of the combustion process that produces the forces necessary to propel a car? NO. As long as the driver has the skills required to make the car go in the desired direction, and the desired velocity, then the practical operation of the car is achieved.

etc.

 

The presentation of using a spirit level and a bank angle indicator, although worthy of discussion, narrowed the discussion somewhat  - although @old man emu - please correct if I am wrong.

 

I don't know how detailed the RPL theory exam is, but looking here, it looks very similar to the PPL theory subjects: https://learntofly.edu.au/wp-content/uploads/2020/08/Recreational-Pilot-Licence-RPL-Theory-Course-Learn-To-Fly-Melbourne-Course-Guide.pdf , though I am assuming it is probably a lighter weight version.

 

Anyone who used to read Jim Davies in Australian Flying would know one of his mantras was to know the systems of your aircraft because when the proverbial hits the fan, knowing those systems can be the difference between continuing to enjoy life or burning a smoking hole in a paddock. I personally think this is a very good idea.. However, I do qualify it with these points:

• If it is a system that I cannot control in flight, then it is nice to know, but hardly going to save me.. How it interacts with systems that I can control in flight, I should know.
• As a PPL, I can't do any but the most basic work myself (legally), so having to know know things like how the ignition system works, or the suck, squeeze, bang, blow cycle of a 4cyl engine are not really adding any value to the safe conduct of my flight. However, the theory should labour the point of how I can manage things associated with these systems to ensure safe flight. For example, in pre-flight I should know to look for chaffing of HT leads because that will affect the performance of the engine due to resistance/electrical loss. I shouldd also be taught to identify a rough running engine.. and what to do to determine if it is a bit of water in the fuel or an ignition issue. But knowing the clloapsing a magnetic field to induce a spark, or the 4 stroke cycle is not really going to help me when I get rough running in flight.

 

Don't get me wrong - I am glad I learned what I did, but I can't help thinking if the theory spent more time on what I need to do amd monitor to ensure a safe flight and less of the periphery to this, would mean less clutter in the brain. So, yes, I agree with @old man emu - we don't need to know the theory of lift.. We need to know, for example, to keep our speeds correct and listen for the stall warning and feel for buffet, and that for certain flight manouvres (sp?), expect a higher airspeed to not stall. There is more we need to know, but themathematical equation for lift (which apparently, is wrong), is not one we need to know.

 

On the AoA indicator - remember - as pointed out, it is the critical angle of attack that, when exceeded causes an aerodynamic stall.. this can be done with, say, A 4G pullout of a dive at a high IAS.. However, you have exceeded the critical angle, and I would also argue your airspeed (as opposed to velocity) has rapidly deteriorated to next to nothing as your pitot tube will be suddenly not be having air rammed into it.. unf. your airspeed indocator lags a bit and won't show it.

 

Edited January 3, 2021 by Jerry_Atrick

[facthunter]

You run into Vmo and M crit, both limiting converging line s getting closer together, as you fly higher.  Often called "coffin Corner"  where you get close to both. Not a place to be when in turbulence or manouvering. 

Thin air? Your IAS allows for that and the INDICATED stall speed won't vary much, for us.  Your stall warning horn is activated by an AofA sensing vane. Most planes experience a pre  stall buffet. and your joystick HAS to be way back, unless something else is wrong (like CofG) tail heavy. YOU have to stall it.  IT won't do it by itself.   Nev

Edited January 3, 2021 by facthunter
clarity

[APenNameAndThatA]

4 hours ago, Jerry_Atrick said:

I think where OME may have originally wanted this thread to go is, is the theory syllabus for gaining the requisite flying qualification required to the extent it currently is:

 

etc.

 

The presentation of using a spirit level and a bank angle indicator, although worthy of discussion, narrowed the discussion somewhat  - although @old man emu - please correct if I am wrong.

 

In what way is discussing using a spirit level or a slip (not bank) indicator worthy of discussion about angle of attack? Particularly if said discussion says that they have anything to do with each other. As far as I can tell, OME *still* thinks that you can tell angle of attack with a slip indicator. Also, a slip indicator will not tell you your angle of bank, unless you are not turning. This is just bizarre.

[old man emu]

5 hours ago, APenNameAndThatA said:

OME *still* thinks that you can tell angle of attack with a slip indicator. Also, a slip indicator will not tell you your angle of bank, unless you are not turning. This is just bizarre.

Not bizarre. The problem is with the inability of a certain person to think outside the box, or to have any experiences outside their restricted world.

 

This is what is commonly called a builder's spirit level This spirit level can be used to lay out lines in the vertical and horizontal planes as well as a line which is at 45 degrees to both the other two planes. Silly me assumed when I was talking about using a spirit (bubble)level that the reader would have had experience of using one. Obviously, in one case, I was wrong. Unfortunately, I initially chose the wrong item to illustrate what I meant by a bubble level. Consequently, I removed that image and replaced it with the image of something pilots would recognise. Unfortunately, I failed to remember that some people can only read the words and see the pictures, but cannot comprehend their deeper meaning.

 

So let's go into an explanation of how one would buy a slip indicator and apply it to be used as an AoA guide.

 

An aircraft has three planes of symmetry - vertical (yawing plane), longitudinal (roll) and lateral (pitch). A spirit level can be used to align the aircraft with reference to any one of these planes. For example, when taking measurements for the weight and balance of an aircraft, the longitudinal axis of the aircraft is aligned to the horizontal  and lateral planes. When an aircraft is aligned to those two planes, it should be in its straight and level configuration. At that time, a "slip indicator" can be fitted to the instrument panel aligned to the lateral plane. This is the usual way one of these devices is fitted and used.

 

However, a slip indicator can also be aligned to the longitudinal axis of the aircraft. In this configuration it becomes a "pitch" indicator. To turn it into an AoA indicator, the aircraft would be lifted by the tail until the chord line of the wing was aligned to the longitudinal plane (usually about 4 degrees nose down-tail up). Once that alignment was made, the slip indicator can be fitted to the aircraft so that the bubble is centralised. The aircraft is then returned so that the longitudinal axis is aligned with the horizontal plane. At this point the bubble should be off-centre by the same angle as the chord line is to the longitudinal axis of the aircraft. The position of the bubble can be marked on window of the indicator. This would indicate the position of the bubble when the aircraft was flying at the best AoA for Lift generation. The aircraft can then have the nose raised until the chord line of the wing is at 15-16 degrees above longitudinal plane. The location of the bubble is marked. That mark shows the position of the longitudinal axis of the aircraft when the wing will begin to stall.

 

So, does that clarify the use of a spirit level as a tool to monitor an aircraft's AoA ?

 

Can we now get back to the discussion of whether or not knowing the mathematical description of lift generation is of greater value to the average pilot than learning by experience the simple fact that as the nose of an aircraft goes up beyond a certain angle, the wing is unable to produce sufficient to "Lift" cause the aircraft to be supported at a constant altitude?

 

 

[Garfly]

Hey there, OME,

I would say that your basic question does sort of come back to your spirit level.

While I agree that high maths and physics ain't needed to fly a plane, the (practical) facts about pitch angle (deck angle?) are not all that simple.

For instance, the show-off pilot who does a max speed pass over a mate's house and then does a rapid pull-up may find himself stalled even though his pitch angle is well under the 'stall' sight picture he's used to. (And his airspeed well over Vs).  I imagine it as a kind of tail down longitudinal skid through the sky so that the air is not hitting the leading edge head-on but rather, in large part, from underneath which means that the angle of attack of the wing is much higher than the deck angle would suggest.  So the (simple) way I see it is that the bubble is about gravity and AoA indicators are about airflow, relative to the chord.

 

But still, I'd like to hear more about your research into Bernoulli versus Newton.  It has always amazed me that most of aviation history progressed - it seems - on a misunderstanding of the physics.  We always learned that the wing was sucked up by the negative pressure. It was only in recent decades we were told "Oh, we were only kidding, that's just a small part of it.  The wing deflects air downwards and flight is the (to be expected) reaction."  Wow, so many great airplanes invented - and flown - on such wrong assumptions.  Go figure!  😉

Edited January 4, 2021 by Garfly

[APenNameAndThatA]

1 hour ago, old man emu said:

Not bizarre. The problem is with the inability of a certain person to think outside the box, or to have any experiences outside their restricted world.

 

This is what is commonly called a builder's spirit level This spirit level can be used to lay out lines in the vertical and horizontal planes as well as a line which is at 45 degrees to both the other two planes. Silly me assumed when I was talking about using a spirit (bubble)level that the reader would have had experience of using one. Obviously, in one case, I was wrong. Unfortunately, I initially chose the wrong item to illustrate what I meant by a bubble level. Consequently, I removed that image and replaced it with the image of something pilots would recognise. Unfortunately, I failed to remember that some people can only read the words and see the pictures, but cannot comprehend their deeper meaning.

 

So let's go into an explanation of how one would buy a slip indicator and apply it to be used as an AoA guide.

 

An aircraft has three planes of symmetry - vertical (yawing plane), longitudinal (roll) and lateral (pitch). A spirit level can be used to align the aircraft with reference to any one of these planes. For example, when taking measurements for the weight and balance of an aircraft, the longitudinal axis of the aircraft is aligned to the horizontal  and lateral planes. When an aircraft is aligned to those two planes, it should be in its straight and level configuration. At that time, a "slip indicator" can be fitted to the instrument panel aligned to the lateral plane. This is the usual way one of these devices is fitted and used.

 

However, a slip indicator can also be aligned to the longitudinal axis of the aircraft. In this configuration it becomes a "pitch" indicator. To turn it into an AoA indicator, the aircraft would be lifted by the tail until the chord line of the wing was aligned to the longitudinal plane (usually about 4 degrees nose down-tail up). Once that alignment was made, the slip indicator can be fitted to the aircraft so that the bubble is centralised. The aircraft is then returned so that the longitudinal axis is aligned with the horizontal plane. At this point the bubble should be off-centre by the same angle as the chord line is to the longitudinal axis of the aircraft. The position of the bubble can be marked on window of the indicator. This would indicate the position of the bubble when the aircraft was flying at the best AoA for Lift generation. The aircraft can then have the nose raised until the chord line of the wing is at 15-16 degrees above longitudinal plane. The location of the bubble is marked. That mark shows the position of the longitudinal axis of the aircraft when the wing will begin to stall.

 

So, does that clarify the use of a spirit level as a tool to monitor an aircraft's AoA ?

 

Can we now get back to the discussion of whether or not knowing the mathematical description of lift generation is of greater value to the average pilot than learning by experience the simple fact that as the nose of an aircraft goes up beyond a certain angle, the wing is unable to produce sufficient to "Lift" cause the aircraft to be supported at a constant altitude?

 

No. The best angle of attack will be whatever it is when the aircraft is at max weight and is flying at Vx or Vy. Most people would choose Vy as best, when total drag is at its least. This has nothing to do with the alignment of the airplane, although if the aircraft was designed well, they would coincide.

 

A spirit level cannot do the same thing as an angle of attack indicator because the the spirit level cannot take into account acceleration of the aircraft. If a spirit level was useful for determining angle of attack, the people would use them instead of a commercial angle of attack meter because they are far cheaper. 
 

Just to reiterate, the best angle of attack is at Vy, where drag is at its lowest. That’s how come Vy is published and airplanes don’t come fitted with slip indicators aligned with the aircraft.
 

The best angle of attack for lift generation is however much lift you need, up to a maximum, at about 14° angle of attack.   
 

The position on the spirit level will not show when the aircraft is about to stall as when an aircraft is about to stall, it will likely be accelerating and the spirit level will no longer be accurately reflecting the angle of attack. For example, if someone is recovering from a spiral dive, the aircraft could be 10° pitch up and have an angle of attack of 18° and be stalled. 
 

Likewise, if someone is climbing or descending, the slip indicator would not reflect angle of attack. Likewise, if the aircraft was speeding up or slowing down the slip indicator would not indicate the angle of attack. It would only reflect the angle of attack in straight and level flight where the aircraft was not speeding up or slowing down - not the time when aircraft stall. 
 

If you want to know when the aircraft is about to stall, you can mark a particular amount of control movement of the rudder (for a particular flap position).

[Yenn]

OMEs bubble and his A of A meter would not agree with each other.

I agree that you don't need to know why an aeroplane flies to be able to fly it, but it is nice to know. It is also nice to understand what happens and why, so that you can work your way out of a difficult position.

I wrote this yesterday and it never got posted, so I will just add to it.

The spirit level or OMEs builders version of it relies on the contents of the bubble being lighter than the surrounding fluid. The lightest portion rises to the top usually. The same can be achieved by hanging a plumb bob from the cockpit roof. It will give exactly the same information.

Go ahead and fit a plumb bob to the cockpit roof and fly. Do you really have to fly to see what happens or can you visualise it.

I would suggest that if you need to fly it to see the results, that you think long and clearly about where you hang that weight and how heavy it is.

Angle of attack is well defined and the definition cannot even remotely be likened to a lift reserve indicator. That can only be likened to an A of A meter. There is adifference between what you are measuring and the instrument used to do it.

[Kyle Communications]

6 minutes ago, Yenn said:

 

Angle of attack is well defined and the definition cannot even remotely be likened to a lift reserve indicator. That can only be likened to an A of A meter. There is adifference between what you are measuring and the instrument used to do it.

I agree I should have qualified my statement...A  "AOA meter" as most use in aircraft on a separate gauge or EIFIS as they show the lift reserve you have. A "AOA indicator" is a different animal..its just a vane with a pot on it to show angle only. The ones available for our normal use are run with software

 

[old man emu]

I have now struck through the obviously offensive words in my original post in the hope that the discussion can now be centred on the theme of the thread, a discussion of whether or not knowing the mathematical description of lift generation is of greater value to the average pilot than learning by experience the simple fact that as the nose of an aircraft goes up beyond a certain angle, the wing is unable to produce sufficient to "Lift" cause the aircraft to be supported at a constant altitude?

 

And can we keep the discussion within the usual envelope of the majority of pilots who don't have teh need to cross the boundaries of the envelope of enjoying a flight?

 

[Garfly]

10 minutes ago, Kyle Communications said:

I agree I should have qualified my statement...A  "AOA meter" as most use in aircraft on a separate gauge or EIFIS as they show the lift reserve you have. A "AOA indicator" is a different animal..its just a vane with a pot on it to show angle only. The ones available for our normal use are run with software

 

But if the vane's position is simply relayed to some coloured lights on the dash, couldn't you call it a lift reserve instrument?

'I've got two bars of orange and one of red left!" ?

Edited January 4, 2021 by Garfly