Lift and stuff.

[APenNameAndThatA]

Can I show you where you purported to be a teacher? How about this quote. 

 

"I really do give up. No matter how much I try to clarify and have people's attention hone in on the point I'm making they have to go past the point I want to stop at."

 

If I recall correctly, you complained that by posting here you were casting your pearls before swine and giving what is holy to dogs. Correct me if I got the bible quote wrong. 

 

 

[APenNameAndThatA]

Also, as someone might, and might not have said, if a plane does an aileron roll, the AoA will change unless the roll axis is exactly parallel to the angle of incidence of the wind. I don't think you can guarantee that. For example, in an Extra, before you do an aileron roll, you point the nose up a bit. I expect that the nose is up a bit during the whole aileron roll. That means that when inverted the wings will be providing negative lift (lift against gravity) and when the aircraft is on a knife edge, the AoA could be zero. That would mean that at 45% the AoA would have decreased a little. 

 

Also, a particular pilot in a particular aircraft, might adjust the elevators as they complete the roll. 

 

So, there is every reason to believe that the wing will be producing less lift during the roll. Your answer assumes that the AoA is the same at 45° bank as at 0° bank, which is wrong. 

[APenNameAndThatA]

And this, "Yes. But first I want to get people to really think, and to question what they believe." The idea that you don't think you are here as a teacher is laughable. 

[aro]

27 minutes ago, old man emu said:

I really do give up. No matter how much I try to clarify and have people's attention hone in on the point I'm making they have to go past the point I want to stop at.

Maybe if you post the answers instead we will have more luck trying to work out the question you want to ask.

 

You introduced the question with a video, that suggests that you are intending to post a question that relates to real aircraft behaviour.

 

Incidentally, the video includes real time G readings which actually show us the lift the wing is producing at each point. Not surprisingly they differ from the "perfect" roll I described. There are limits to how perfectly a pilot can fly, and also limits to what an aircraft is capable of.

[APenNameAndThatA]

"Prior to beginning the roll, the aircraft is flying with its wings parallel to the Earth's surface."

 

Wrong. The two planes will be at the angle of attack (assuming that the manoeuvre starts with the airplane straight and level, which is a reasonable assumption. By "planes" I mean the flat plane of the earth's surface and the plane of the wing, defined by the theoretical plane formed by the chord and the span of the wing.

 

Actually, on reflection, even that is untrue, unless there is no dihedral. Airplane wings with dihedral are not parallel to each other so there is no chance that they will be parallel to the surface of the earth. 

Edited March 29, 2021 by APenNameAndThatA

[APenNameAndThatA]

And this from someone who says that they do not think that they come here to teach people. " I just try to expose general misunderstandings."

[Jase T]

I believe it was Bob Tait that said something along the lines of “why ask a complex question that requires clarification when you can ask 5 simple ones”... I think I may have modified the quote but you get the gist.

Edited March 29, 2021 by Jase T

[old man emu]

Since it doesn't seem to be possible to encourage people to consider things from a different perspective, I'll go straight to the point I was raising. This will involve some cut and paste from earlier posts in this thread as I want to be quick. So here goes.

 

Which of you has not seen diagrams or photographs like this:

or this one:  

 

 

In both, we are presented with a view of the side elevation of a wing. These images are implanted in our minds, and affect the way we think about situations. Now, if you are using a mouse, pick it up. If not use a pen or ruler to represent a wing. Put the thing down on the desk and look at the top side. Imagine that is a plan view of a wing. As it sits on the desk it represents a wing that is moving through the air, parallel to the Earth's surface.

 

We know that before the aircraft moves, it is being acted upon by a vertically downwards force caused by the acceleration due to gravity. Call it the "weight force" To get the aircraft into the air, we have to cause the wing to generate a vertically upwards force that, for level flight, has to equal the vertically downwards weight force. For the wing to generate Lift, air has to pass over and under the wing. For best Lift, that flow has to be from leading edge to trailing edge. A flat plate will generate Lift, but we have learned to get more Lift by better design.

 

An aircraft is flying with its wings parallel to the Earth's surface (By that I mean that the lateral axis of the wing is parallel to the Earth's surface. With reference to the longitudinal axis, the wing is tilted by an angle set by the designer to maximise Lift generation for the designed performance of the aircraft. This tilt provides the desired Angle of Attack for the designed performance of the aircraft.) For the purposes of this discussion I will refer to the direction of the air (which we call the relative airflow) as if our frame of reference is the longitudinal axis of the aircraft, not the designer-set AoA, is from directly in front. As a result of all the aerodynamic mumbo-jumbo, the amount of Lift produced by the wing is equal to the weight force acting on it. The forces are balanced, so the wing and attached aircraft neither rises nor falls.

 

Pick up you mouse, ruler, pen and hold it horizontally in front of you to represent the plan view of a wing. Now, lower one end of the "wing"  45 degrees. The relative airflow is still coming from the front. Therefore, at that instant, the Lift generated by the wing in relation to the relative airflow is exactly the same as when the wing was horizontal to the Earth's surface. That is at time,T0 .  An incredibly short time later,  (T0 + t)  things go to pieces, and things have to be done if the pilot wants to keep the aircraft on the same heading and the same altitude. We know that because the Lift Force is a vector that is the result of adding (in a two-dimensional representation) the vertical component and the horizontal component, if the Lift vector is offset from the weight vector by 45 degrees, then the vertical component will be less than the weight force.   There is a number of actions a pilot can take to increase the Lift force, but they are things most of us know and mentioning here does not add to this particular discussion.

 

The diagrams shown above make the initial understanding of Lift generation in situations where the aircraft is flying "one wing low". This is the point I was trying to make with this thread. We are so used to thinking of the generation of Lift, and things like Angle of Attack based on the side elevation images that have been planted in our minds from textbooks and whatever. We fail to have in our minds that the air is all around our wings, but our wings don't know up from down. All a wing wants is for the relative movement of air to be from leading edge to trailing edge.

 

We have to be careful about what images we put into our minds as illustrations of concepts since some images can lead to confusion if used in situations they were not meant to illustrate. Another example of misleading illustrations or sayings is the oft repeated description of the Otto 4-cycle combustion sequence - Suck, Squeeze, Bang, Blow. A more accurate description is: Suck, Squeeze, Burn, Blow. It is understanding that the combustion of the fuel creates a flame that takes time to envelope all the volume available, and not an instantaneous combustion that strikes the crown of the piston like a hammer blow that makes it possible to understand mixture control, ignition timing and detonation.

 

11 hours ago, aro said:

You introduced the question with a video,

Yes. I suppose I should have commented on the point I wanted to illustrate, namely the movement of the lateral axis of the wing from "straight and level" to 45 degrees, and that lift is still generated. I was trying to get across the point that at the instantaneous value of Lift was the same in both cases. 

 

I even reject that at if the wing is rotated to the 90 degree position that it produces no Lift. The wing doesn't care what angle it is to the direction of "down" when it is moving in the air. I do agree that the lift generated by a wing parallel to the direction of "down" produces no vector that can negate the down vector of gravitational force. In fact, as aro rightly put it elsewhere, a vector acting laterally to the vector of gravitational force will induce a curved motion.

 

 

 

[facthunter]

A roll is not a turn IF it's with pitch near level. Bank at 90 degrees to the earth's surface and there's no force from the wings whatever opposing gravity. You say you need  lnfinite lift conjuring a concept that there's a gravity opposing effect possible. You ignore the turning effect of the inclined force. You can't IGNORE it as it will affect what happens .  The aeroplane will accelerate  in the direction of that force and the plane will TURN to another heading. Planes turn by tiltinng the lift. BANKING. Rolls  other than barrell rolls have times where negative "G" happens. This it the bottom half of the roll. You will be hanging in the straps. At the 90 degree position you will be forced against the side of the plane. That LIFT (gravity opposing) force is NOT coming from the wing.

   Your choice to use a hesitation roll as an example complicates it. Not our discussion ensuing. ' Are YOU listening to what others contribute.? Formula's are fine if they are correctly applied to a situation and the correct unit s used but generally to pilot's, not the best way of putting it across especially if it's the MAIN thrust of your presentation.  Regards Nev

[Jase T]

Ahh so you were talking about vertical component of lift? VL. If that’s he case when any rolled AOB is compared to a 0 deg roll total Lift (L) remains the same L1=L2 but VL2 is always less than VL1 (assuming all other factors are equal). If the pilot doesn’t do something else to make up for the smaller VL the aircraft will descend. He can’t increase AOA with elevator or the aircraft will turn (unless he is doing a barrel roll but that’s not the case here). 

Edited March 30, 2021 by Jase T

[Thruster88]

1 hour ago, old man emu said:

Since it doesn't seem to be possible to encourage people to consider things from a different perspective, I'll go straight to the point I was raising. This will involve some cut and paste from earlier posts in this thread as I want to be quick. So here goes.

 

Which of you has not seen diagrams or photographs like this:

or this one:  

 

 

In both, we are presented with a view of the side elevation of a wing. These images are implanted in our minds, and affect the way we think about situations. Now, if you are using a mouse, pick it up. If not use a pen or ruler to represent a wing. Put the thing down on the desk and look at the top side. Imagine that is a plan view of a wing. As it sits on the desk it represents a wing that is moving through the air, parallel to the Earth's surface.

 

We know that before the aircraft moves, it is being acted upon by a vertically downwards force caused by the acceleration due to gravity. Call it the "weight force" To get the aircraft into the air, we have to cause the wing to generate a vertically upwards force that, for level flight, has to equal the vertically downwards weight force. For the wing to generate Lift, air has to pass over and under the wing. For best Lift, that flow has to be from leading edge to trailing edge. A flat plate will generate Lift, but we have learned to get more Lift by better design.

 

An aircraft is flying with its wings parallel to the Earth's surface (By that I mean that the lateral axis of the wing is parallel to the Earth's surface. With reference to the longitudinal axis, the wing is tilted by an angle set by the designer to maximise Lift generation for the designed performance of the aircraft. This tilt provides the desired Angle of Attack for the designed performance of the aircraft.) For the purposes of this discussion I will refer to the direction of the air (which we call the relative airflow) as if our frame of reference is the longitudinal axis of the aircraft, not the designer-set AoA, is from directly in front. As a result of all the aerodynamic mumbo-jumbo, the amount of Lift produced by the wing is equal to the weight force acting on it. The forces are balanced, so the wing and attached aircraft neither rises nor falls.

 

Pick up you mouse, ruler, pen and hold it horizontally in front of you to represent the plan view of a wing. Now, lower one end of the "wing"  45 degrees. The relative airflow is still coming from the front. Therefore, at that instant, the Lift generated by the wing in relation to the relative airflow is exactly the same as when the wing was horizontal to the Earth's surface. That is at time,T0 .  An incredibly short time later,  (T0 + t)  things go to pieces, and things have to be done if the pilot wants to keep the aircraft on the same heading and the same altitude. We know that because the Lift Force is a vector that is the result of adding (in a two-dimensional representation) the vertical component and the horizontal component, if the Lift vector is offset from the weight vector by 45 degrees, then the vertical component will be less than the weight force.   There is a number of actions a pilot can take to increase the Lift force, but they are things most of us know and mentioning here does not add to this particular discussion.

 

The diagrams shown above make the initial understanding of Lift generation in situations where the aircraft is flying "one wing low". This is the point I was trying to make with this thread. We are so used to thinking of the generation of Lift, and things like Angle of Attack based on the side elevation images that have been planted in our minds from textbooks and whatever. We fail to have in our minds that the air is all around our wings, but our wings don't know up from down. All a wing wants is for the relative movement of air to be from leading edge to trailing edge.

 

We have to be careful about what images we put into our minds as illustrations of concepts since some images can lead to confusion if used in situations they were not meant to illustrate. Another example of misleading illustrations or sayings is the oft repeated description of the Otto 4-cycle combustion sequence - Suck, Squeeze, Bang, Blow. A more accurate description is: Suck, Squeeze, Burn, Blow. It is understanding that the combustion of the fuel creates a flame that takes time to envelope all the volume available, and not an instantaneous combustion that strikes the crown of the piston like a hammer blow that makes it possible to understand mixture control, ignition timing and detonation.

 

Yes. I suppose I should have commented on the point I wanted to illustrate, namely the movement of the lateral axis of the wing from "straight and level" to 45 degrees, and that lift is still generated. I was trying to get across the point that at the instantaneous value of Lift was the same in both cases. 

 

I even reject that at if the wing is rotated to the 90 degree position that it produces no Lift. The wing doesn't care what angle it is to the direction of "down" when it is moving in the air. I do agree that the lift generated by a wing parallel to the direction of "down" produces no vector that can negate the down vector of gravitational force. In fact, as aro rightly put it elsewhere, a vector acting laterally to the vector of gravitational force will induce a curved motion.

 

 

 

I summarized all the above in one sentence for you in post #6

[old man emu]

See John 11:35

 

I don't give a rat's arse about the further effect of the vertical  and the lateral components of the Lift vector. I acknowledge every thing that you are saying about the further effects is correct. Will you acknowledge that 

1. The Lift vector acts at 90 degrees to the direction of the airflow.

2. If the air in the vicinity of the wing, and here let's arbitrarily call "vicinity" anything within a sphere whose radius if which is (1/2 wingspan) + 5 metres, has consistent density then the lift generated by the wing is not dependant on the angle of the wing, or if you like, chord line, relative to the the direction of the gravitational force of the Earth. 

3. If a section of wing, say 1 metre long, was to be placed in a wind tunnel, so that it could be held stationary at various angles to the airflow, with the AoA constant, the Lift force generated by the section would be the same for all angles that it was held at.

4. That when trying to grasp a new concept, we rely on our prior knowledge and experience to apply the old to the new. Therefore, if a person's old knowledge and experience has involved seeing diagrams where the wing is depicted in side elevation, then that can be a hindrance when considering a different situation where the wing is depicted in plan view.

1 hour ago, facthunter said:

the best way of putting it across especially if it's the MAIN thrust of your presentation.

Nev, my big mistake is thinking that people can "think outside the box". In other words look at a situation from a different stance. What I see here is people not stopping to think more deeply. They have given replies (those that are not derogatory) that of themselves are quite correct, but show that they are not thinking deeply about what is proposed. That's why there has been all this stuff about rolling and banking and such. Those replies go further than the proposition raised. That's why I was careful to use the word "instantaneous" and to say that an infinitesimal time after a plane's wing reaches a nominated angle, the pilot has to work to maintain direction and altitude. I agree that as soon as the rotation of the wing starts, the Lift/Gravity pair goes out of balance, but in real life, as shown in the video, getting to 45 degrees is pretty quick, and the pilot wouldn't start acting to ensure that direction and altitude were maintained until the desired angle was reached - remember reaction time of the pilot, even one who knows what is going to happen and what has to be done.

[facthunter]

I don't believe #3 is correct. There is a best angle for all aerofoils  In your following comments the pilot anticipates what control input is required because he has been trained and the coordination is part of the art of doing it right or even the ability to do it at all without losing control. It's pretty easy when flying with somebody attempting these things to know who hasn't a clue and it's deadly at lower levels. Nev

[old man emu]

34 minutes ago, Thruster88 said:

I summarized all the above in one sentence for you in post #6

This is what you said, "Can't wait for your correct answer. In the real world the lift produced will depend on the angle of attack and airspeed,  both pilot controlled. Angle of bank is irrelevant"

 

Can you see now that this is NOT what I was getting at? I'm not denying that what you say is true. All I wanted was an answer that compared the Lift force generated by a wing that had been rotated at an angle to the direction of the Earth's gravitational force with the Lift force generated when the wing was at 90 degrees to that gravitational force.

[old man emu]

4 minutes ago, facthunter said:

I don't believe #3 is correct. There is a best angle for all aerofoils  In your following comments the pilot anticipates what control input is required because he has been trained and the coordination is part of the art of doing it right or even the ability to do it at all without losing control. It's pretty easy when flying with somebody attempting these things to know who hasn't a clue and it's deadly at lower levels. Nev

 

For Sod's sake, will you forget about what Pontius Pilate is doing in the aircraft and go back to basics. A section of wing in a constant airflow device with measuring devices to measure the Lift force when the wing is at various angles to the direction of the Earth's gravitational force.

 

Will you discuss the proposition that sometimes the diagrams we use to illustrate one point can confuse the understanding of other propositions?

[Jase T]

3 makes little sense. How can it be at various angles to the airflow and still be at the same AOA?changing the angle to the airflow changes theAOA thats the very definition of AOA? 

 

Are we ignoring the Total reaction on the aerofoil? Wind tunnels have no weight and no thrust so the TR will be different. 

[facthunter]

I'm unable to do it your way OME. I consider my approach to be simple and the minimum complexity required to cover the issue.. nev

[old man emu]

49 minutes ago, Jase T said:

3 makes little sense. How can it be at various angles to the airflow and still be at the same AOA?changing the angle to the airflow changes theAOA thats the very definition of AOA? 

 

Are we ignoring the Total reaction on the aerofoil? Wind tunnels have no weight and no thrust so the TR will be different. 

How can it be at various angles to the airflow and still be at the same AOA?

 

It makes sense if you put a modicum of thought into the set up of the experiment. Here's how you can visualise the set up. Pick up a ruler, say a normal 30 cm one. Hold it in front of your eyes so that the long lateral axis of the ruler is horizontal from right to left, and the short, longitudinal axis is level with your eyes, so that you can only see the thin edge of the ruler. Now hold it there with your finger at the 15 cm mark. 

 

This is what we will call the "normal" angle of the ruler to air blowing at your face from the front. The deflection of the airflow across a flat plate will produce an amount of Lift force. We need to generate some Lift force for this experiment, so we will set the angle between the airflow and the longitudinal axis to an arbitrary 4 degrees. With the ruler set up thusly, we blow a steady airflow at the ruler. This airflow separates at the edge of the ruler and some goes over and some under the ruler generating a Lift force, which our incredibly accurate measuring devices can detect and display.

 

Now, without altering the angle between the airflow and the longitudinal axis which is arbitrary 4 degrees, drop one end of the ruler so that it is 10 degrees below the original horizontal. We haven't change any other factor. Because the ruler is immersed in the air, it will produce a Lift force. That magnitude of Lift force will be the same as when the Lift force was measured initially.

 

Are we ignoring the Total reaction on the aerofoil? Wind tunnels have no weight and no thrust so the TR will be different. Yes, because we are not trying to balance any other force. All we are trying to find out is what the magnitude of the force is when air flows over the ruler, which is representing a wing. In practice, a designer knows that this experiment will give the Total Expected Lift force of a wing, as well as knowing that this Total will be eroded by all the factors that people have mentioned. Having taken all those things into account, the designer can determine the MTOW of the aircraft being designed.

 

You said changing the angle to the airflow changes the AOA that's the very definition of AOA?  What I am trying to get you to think about is "What diagram or picture have you seen that helps you visualise the airflow in this situation? I'm saying that these

           have caused you confusion because the don't show the airflow from the plan view of the wing.

 

What I am saying is that one should be careful about which illustrations they plant in their mind to help understand concepts.

[Thruster88]

I have never been confused when banking an aircraft at 60°

[aro]

7 hours ago, facthunter said:

You say you need  lnfinite lift conjuring a concept that there's a gravity opposing effect possible.

I said lift required was infinite for a turn with 90 degrees of bank.

 

Of course you are right. Lift required trends to infinity as you approach 90 degrees, but at 90 degrees you need to divide by zero: infinity is not the answer, there is no valid answer.

[aro]

This video shows some interesting instruction on rolls:

 

 

The whole video is worth watching, but there is discussion on the slow roll at 17:18 and hesitation rolls at 24:30.

 

The description of the hesitation roll is that it is the same as the slow roll, but with the hesitation added. This really dispels the idea that the pilot rolls the aircraft and then corrects the flight path. It is simultaneous aileron, elevator and rudder inputs.

 

[facthunter]

Yes the hesitation is just a little show off thing added. and just slows the rate and is more difficult. The control movements are quite large, well coordinated but not balanced.  The wings are not doing anything when they are vertical to the ground. I'm not pretending to be good at these as most planes were so underpowered and slow. It was the usual way to get inverted. I'll run that vid when I have a bit more time. Thanks for it. Nev

 

 

 

Edited March 30, 2021 by facthunter

[APenNameAndThatA]

It’s not easy when you’re really clever and no one believes you. 

[old man emu]

This is one of those diagrams that I say is causing confusion:

While this is a totally correct way to explain why the pilot has to take action to increase the resultant lift in order to complete the desired manoeuvre, it does not illustrate how the relative airflow moves around the wing.

 

Here is a scrappy diagram of what I am getting at.

The rectangles represent the leading edge of a flat plate. The red dots represent the airflow meeting the leading edge, and you can assume that the air diverts over and under the wing as we know it does.  In order to get a bit more Lift force we'll arbitrarily assign an AoA of 4 degrees. I don't have the CAD software to draw that in a diagram. Imagine that the plate is connected to some device that measures the magnitude of the Lift force, and that device allows us to tilt the plate around the halfway point of the span. I have drawn the lower rectangle with a 20 degree tilt down with respect to the top one - simply to fit the diagram on a page. 

 

Now if we pass an airstream (relative airflow), having a known velocity and density of the air, at the top rectangular plate, a Lift force will be generated, which we can depict as an arrow pointing upwards. Our measuring device can tell us the magnitude of that force. Next, without changing AoA , airflow velocity or air density, we tilt the plate away from the horizontal and again obtain a value for the magnitude of the Lift force. The two magnitudes will be the same. 

 

In other words, the angle of the plate relative to the horizontal datum will not affect the magnitude of Lift force generated by the plate. This is because no matter how the plate is orientated in relation to the horizontal datum the density and velocity of the relative airflow are the same. The plate is immersed in the fluid the same way that a fish is immersed in the sea.

 

It is incredibly hard to produce a two-dimensional diagram of this, that's why I said to use a three-dimensional object to help envisage what I am saying.

 

If this plate was set up at 90 degrees to the horizontal datum, would the Lift force be the same as in the other two cases? Yes, but when one wants to use that Lift force to balance the weight force of an aircraft, there is no vector component that is equal and opposite to the weight force. aro mentioned at one point that the horizontal vector of the Total Lift force is used to turn the aircraft. When the wing is at 90 degrees vertical, one would guess that it's all turn and no holding altitude. 

 

Phew!

 

 

[Bruce Tuncks]

60 degrees is some bank angle for sure. Once I watched 2 awestruck at 2 big open-class gliders climbing in a narrow thermal core at about 60 degrees of bank. The wings were noticeably curved from the g loading. They were climbing well.

45 degrees seems very steep when you actually do it too. The diagonal instrument screws are parallel with the horizon. The g forces are noticeable too. I reckon few power pilots have ever done much of this.  In a lightly-loaded 15m  glider you need to do 45 degrees at 45 knots accurately and this is an impossible goal, but like a perfect golf shot, you need to just keep trying.