A bloke told me this

[facthunter]

We have arrived at a point where you are making an incorrect assumption and there is no way I can just ignore it.  The NETT force  cannot act the way you wish it to and I think this is the  basis of your difficulty, NOT mine. I'm making it as simple as I can as I always do actually. Your reference to wing tip vortices  and skin friction etc is not helpful. I don't think I've ever mentioned such things in this discussion.. OME I've put a lot of thought into what I write here and it's not as if I'm short of things to do. I'm doing it with the best of intentions.. Nev

[old man emu]

Nev, I know your intentions are the best, and I'm enjoying the conversation. 

 

But I am only talking, at this early stage, about the MAGNITUDE Lift force that is generated. I'm not, discussing the vector analysis of that force. I'm trying to get there, but everyone is throwing in distractions.

 

 

I posted this diagram: and people were happy that it was a fair, but simple description of the relationship between air flow and Lift.

 

Now look at this diagram: All I have done is put the aerofoil at a downward facing angle.

 

If you can accept the truth of the second diagram, we can move on the vector analysis of the Lift/Weight vector pair.

2 hours ago, facthunter said:

you are making an incorrect assumption

What is the incorrect assumption that you think exists?

[onetrack]

OME, why is the airflow now at an upward angle in your last diagram? Shouldn't the air flow still be horizontal, as in the previous diagram? 

[old man emu]

54 minutes ago, onetrack said:

Shouldn't the air flow still be horizontal, as in the previous diagram? 

And therein lies what is confusing everyone.

 

Think about the air around you. It's the same in all directions.

Move your arm up over your head, and the relative airflow is from above to below because your arm is moving from below to above. 

 

In this diagram,     if the aerofoil is moving right to left, parallel to the ground, the airflow is from the front of the aerofoil to its rear.

In this diagram, if the aerofoil is moving from right to left, but going towards the ground, the airflow is from the front of the aerofoil to its rear.

 

In both cases,  the Lift is acting at a right angle to the direction of the airflow.

 

Everyone is thinking of AoA and relative airflow, especially as it relates to the disruption of the airflow over the aerofoil. Consideration of that effect is something that comes into the practical aerodynamic  performance of an aerofoil much later in the discussion. At this stage I am trying to concentrate on first principles. 

 

 

 

[facthunter]

 You are OK with your direction of airflow in the descent situation but any discussion of this must treat the existence of 4 forces, thrust- drag (opposing) and lift-  weight in opposition also. You can't just conveniently dismiss the existence of what is also affecting the result.  The wing doesn't JUST produce a force vertical to the planes axis. Any production of lift causes drag at the same time or we have perpetual motion in principle which is not possible A "heavier than air'  CRAFT pays a penalty for staying up there. I'm not complicating things .We KNOW how lift is obtained aerodynamically using specifically shaped surfaces at an angle to the relative airflow to push air around and get a reaction to that. The RESULTANT force is not vertical to the  cord or even the zero lift AoA .You can choose to derive component(s)  PLURAL in any direction you wish but you can't just disregard any still remaining  as if it doesn't matter. Nev

[old man emu]

2 hours ago, facthunter said:

 You are OK with your direction of airflow in the descent situation

Thank the gods for that! That was the only point I wanted to make at the early part of the discussion. If everyone is happy that the aerofoil cant tell up from down, but responds to relative airflow, then we can move on. As long as everybody agrees that the simple, basic definition of Lift is a force that is produced by the dynamic effect of the air acting on the aerofoil, and acts perpendicular to the flight path through the centre of lift (CL) and perpendicular to the lateral axis https://www.aircraftsystemstech.com/2017/05/forces-acting-on-aircraft.html we can move on.

 

2 hours ago, facthunter said:

any discussion of this must treat the existence of 4 forces, thrust- drag (opposing) and lift-  weight in opposition also.

At this stage I am only trying to deal with one vector pair - lift and weight.  Dealing with that pair will introduce the thrust/drag pairing and then I can look at finally putting the four all together. 

 

I'm trying to get agreement, one point at a time, to stop the discussion wandering off into a tangle of related matters.

 

[facthunter]

No! You just took my relative airflow  direction acceptance and tacked your lot on it as well. That's just MANIPULATION of what I've said. Not on .You are too imprecise and conveniently flexible in meaning for your desired outcome to be proven, with you assumptions.  When I try to clarify it, you say too complex. This is supposed to be TWO WAY .Not a manouvering and cornering exercise with Gotcha and Pike and Tuck. Thanks but NO... Nev

[old man emu]

43 minutes ago, facthunter said:

When I try to clarify it, you say too complex

I say that what you say is correct, but I am trying to go step by step. It's like the kid who was given a long division sum to do in an arithmetic test and simply wrote down the answer. The object of setting the sum was to have the kid show that he knew the steps to take to reach the answer. It's an ancient way of teaching to go from step to step, but that's the way the foundations are laid.

 

I have said repeatedly, "I'm trying to get agreement, one point at a time, to stop the discussion wandering off into a tangle of related matters."  I'm not saying FIGJAM. I'm just trying to get to an answer step by step. 

 

There is one thing that I would like to know from a pilot with current experience. If you are on a cross-country and once established on a 500 fpm descent from cruise altitude, do you trim nose up or nose down to maintain the angle of descent?

[spacesailor]

o m e

 "  and simply wrote down the answer. The object of setting the sum was to have the kid show that he knew the steps to take to reach the answer. "

That was my problem at school. I didn't do things the teacher's way, because l had no primary schooling & do multiplication, division by adding and subtracting parts of the subject.

I don't think it works in this metric system. SO l can't add up a shopping list. Just does notesnot work in my head.

half of 45 is 2 !/2 + 20 = 22 1/2.

Then the cane. simples

spacesailor

[old man emu]

Spacey,

Your way of doing "half of 45 is 2 !/2 + 20 = 22 1/2" clearly shows that you understand what a "half" is, and your working out shows it. However, I can accept that you never got your head around trigonometry and as a result you would have trouble doing vector additions. But at least if I try to explain things step by step you'll get the idea. So that's what I'm doing. Starting from 1 + 1 = 2 and moving on to sine of an angle = the length of the opposite side divided by the hypotenuse. 

 

Just bear with me and I'll show my point with diagrams. But I'm not going to write out a lot of algebra formulas straight away and leave you scratching your head because you don't understand them.

[facthunter]

There is no one definite answer to this trim issue. It will vary depending on what you altered and the effect it has on the plane. Slowing up you generally need back trim. Power depends on the thrust line and extent of the power change. 

 Note We don't fly a plane on TRIM (unless there's been some failure). It's flown on attitude(s). Once you hold the attitude you require  you trim out the forces so it's easier to stay there.. Nev

[Yenn]

My usual trim change when descending is to go nose down and to make that work I have to reduce power. I could also go nose down and maintain power, but to achieve 500fpm I would have to travel a lot faster and in my plane probably exceed VNE. Alternatively I could reduce power drastickly and go nose up.

So the answer to your question OME is that it depends upon a lot of other parameters.

I could also maintain the same power and attitude and still achieve 500fpm descent rate, but it would take a pilot to work out how that is achieved.

[onetrack]

I'm sorry, OME, I fail to understand why you're trying to define lift and weight in aviation by reducing them to just two simple factors.

The problem with this approach is that you're narrowing the factors to some highly technical definition that really has no practical application in learning to fly an aircraft.

 

I think the bloke in the link below has a much better approach to explanation of airfoil operation - and it's largely based on "if you do this, that changes that - and if you do that, that changes the response again ..."

 

https://www.av8n.com/how/htm/airfoils.html

[old man emu]

1 hour ago, onetrack said:

I fail to understand why you're trying to define lift and weight in aviation by reducing them to just two simple factors.

The very first sentence of section  3.1  The Airplane and the Air in https://www.av8n.com/how/htm/airfoils.html says,

In ordinary steady flight, the airplane must develop enough upward force to support its weight, i.e. to counteract the downward force of gravity. 

 

The material in the reference you supplied is, as far as you and I can tell, quite correct. It's a great explanation that says, basically, the Lift force comes an interaction between a solid body with the air due to relative movement between them. Who can disagree with that? Not Me. However, the author is explaining how the lift force is generated. I am accepting his explanation, but what I am saying is, "OK we have a Lift force and we have a weight force. How do they interact?" Nowhere in his great explanation does he discuss this interaction, except in that first sentence. He and I are talking about different things whose only connection is that forces are involved.

 

What you are complaining about is similar to complaining about one person describing the Otto cycle in an aircraft engine, and another discussing total drag. They are connected, but by a sinuous thread. 

 

[facthunter]

You called something a resultant when it was only the vertical component of it. . Can't move on this till  that is sorted out.... Nev

[old man emu]

1 hour ago, facthunter said:

You called something a resultant when it was only the vertical component of it. . Can't move on this till  that is sorted out.... Nev

Please indicate where I said that so I can see if the text needs correction.

 

I know that the "RESULTANT" force is the vector sum of all the force vectors. At the moment I have described the two vectors I am working with - Lift and Weight. I've been sitting at my desk for an hour now preparing the vector analysis of those two force. Then I will show the resultant. I keep having to deal with red herrings. I had to read and reply to Onetrack's last post. That was the courteous thing to do, but it took time and was an interruption to reaching my real goal. 

 

2 hours ago, onetrack said:

you're narrowing the factors to some highly technical definition that really has no practical application in learning to fly an aircraft.

 

I'm not applying this to learning to fly an aircraft. If you recall, certain people had great delight in abusing me for "teaching". That's why its on this site in the Science forum. It's a simple discussion of a phenomenon in Physics.

 

5 hours ago, facthunter said:

Once you hold the attitude you require  you trim out the forces so it's easier to stay there.

I agree with that, but which way do you wind the trim wheel in a constant airspeed, constant rate descent - nose up or nose down?

[facthunter]

I answered that about 6 posts ago, quite fully I believe. .Let's all be a bit careful how we use the word "trim" or there will be nothing but confusion. TRIM in OUR planes means relieving control forces that we have had to use to get the right pitch (generally, as we don't usually have in the air adjustable trim on any other axis) and we ARE talking about pitch  here,  IF we aren't doing it by airspeed change. IF we go slower  we can descend (or fly level) at a  more nose up attitude. .Nev

Edited April 20, 2021 by facthunter

[old man emu]

There's no point going on with this. 

[facthunter]

Believe me I'm putting all I can into this . I'm NOT into a contest  either, please note. YOU asked me to get involved but there's no evidence you want my input . Just get an admission out of me.. ... .Nev