How to seperate the CG from the MAC?

[Rekitus]

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This is generally called superstition. In modern engineering and science we focus on the why as well as the outcome.

and yet you got to the exact conclusion I hoped you would...

A fellow named Davis created an airfoil.  He has his name on it.
There is good reason to think he did NOT understand the aerodynamics involved.

That is why it relates.

slowmatch made a comment I will repeat with enthusiasm.
"...I'm just an interested modeller and not in a position to call someone out either."

Your F3F designer may well know more about aerodynamics than me.
The problem for me is that when I try to state
I know enough to know he is definitely not sharing his knowledge,
you seem to think it is YOUR fault.
(or mine...)

as not many think I know much or have horrible explanations.
 I can say this and you don't need to pay it any mind.

His MAC comment is wrong.  
As stated.  And no matter how we try to 'come to terms' with it, it fails.

I already know you do not trust my comments about the field of aerodynamics
keep in mind I am not trying to sell you an airplane or stop you from buying one.

my only reason for posting is to hand you enough knowledge about this
to possibly help you reason to the answer you seek.

Look at how many ways others are stating
"that mac comment is quite the puzzler"

They are being polite.

victor

[Konrad]

Yep, his position on CFD is also odd. While I don't claim to be an Aerodynamicist I do have some training in fluid dynamics. This the core of how the fluids flow through a turbine engine. And while I do, at times, claim to be a Luditte, I do have a confidence and respect for the output of CFD work/programs.

Thanks again for weighing in on this subject.  

[Konrad]

clip...
...unclip
This is generally called superstition. In modern engineering and science we focus on the why as well as the outcome.

and yet you got to the exact conclusion I hoped you would...

A fellow named Davis created an airfoil.  He has his name on it.
There is good reason to think he did NOT understand the aerodynamics involved.

That is why it relates.

slowmatch made a comment I will repeat with enthusiasm.
"...I'm just an interested modeller and not in a position to call someone out either."

Your F3F designer may well know more about aerodynamics than me.
The problem for me is that when I try to state
I know enough to know he is definitely not sharing his knowledge,
you seem to think it is YOUR fault.
(or mine...)

as not many think I know much or have horrible explanations.
 I can say this and you don't need to pay it any mind.

His MAC comment is wrong.  
As stated.  And no matter how we try to 'come to terms' with it, it fails.

I already know you do not trust my comments about the field of aerodynamics
keep in mind I am not trying to sell you an airplane or stop you from buying one.

my only reason for posting is to hand you enough knowledge about this
to possibly help you reason to the answer you seek.

Look at how many ways others are stating
"that mac comment is quite the puzzler"

They are being polite.

victor

Let's back up a bit. Communication takes two!

I've tried to be clear that it is I that has had a problem with your answers. Not that there is a problem with you or your answers.

I also have a problem with what I see as non sequitur, change in directions. These are my issues.

With that having been said; I thank you for your interest in the subject.

[Rekitus]

Hi Konrad,

gas turbines!!!!
those are fun. the maths in the modeling are killer.

I decided to leave that area alone until I got my aerodynamics straight

you do crushed cubes or compressed flow?

ooops
that is a change of subject.   sorry

I bear no malice arising from this thread. (nor any other)

victor

[OZPAF]

Konrad after reading Hammond's reply to your comments on the Alpenbrise review in this link

 https://forum.alofthobbies.com/index.php?threads/alpenbrise-design-notes.3594/

I may have an idea of what he is saying.

First of all he is definitely talking about the MAC as he spells it out as the Mean Aerodynamic Chord.

From what he says then It seems to me he is considering the spanwise separation between the centre line position of the CG and the semi wing panel span wise position of the MAC at the semi panel's centre of area.

In his view with high AR's of semi elliptical planform  this span wise position of the MAC will be too close to the root -  meaning there is not enough area towards the tip and thus smaller chords!

Yes small wing chords near the tip are or can be a tip stall problem - so why not just mention the problem is smaller tip chords which appears to be hidden in his story of the relationship of MAC to CG.

Nearly all the current F5J RC gliders have AR's of 18-20 with a span of around 4m. They control the span distribution with careful plan form mods and by using airfoils chosen to meet the flight requirements at the RE no's through the flight. Modifying the plan form becomes harder as the AR rises but the designers of these models have managed. The Arthobby Avatar I was flying had an AR of 21 at a span of 3.4m - but it was not as refined in performance(older design) as the current flock of West European models.

I'm afraid it all still reads to me as a lot of sales blurb - which may sound harsh on my account but that is my impression.

John

[Rekitus]

what I was taught is a MAC
the red line across a delta wing.

victor

the edit was from not knowing if the picture loaded

[Slowmatch]

First of all he is definitely talking about the MAC as he spells it out as the Mean Aerodynamic Chord.

John, in the link I gave he corrects himself as having been using MAC when he means AC. In your link (more recent) he's gone back to MAC. He defines AC incorrectly as the Centre of Area (not 1/4 mean chord) and he also mentions projection the AC onto the centreline.

Having Googled the language he uses it seems that a lot of people just use 25% MAC as a CG position regardless of neutral point/static margin/tail volume. So maybe he's assuming the CG to be fixed at 25%.

The reason I thought he meant fore-aft separation of the points is because sweeping the ellipse doesn't change the MAC position laterally. And if he means lateral separation, surely it's just a function of aspect ratio, not planform?

It's all rather baffling...

He keeps saying the MAC (AC?) will interfere with the CG if too close - this is really odd - and I can't think of any aerodynamic concept that fits the description of the 'problem'. It doesn't have any bearing on the 'deltas don't stall' point he makes, which is for other reasons.

The fact he states he's using a single airfoil and manipulating lift distribution with area (no twist or changes in section mentioned) suggests that his wing design is rather basic too.

You might well be right that he's talking about small wing tips though...

[Konrad]

Thanks guy!

Ok I'm going to call a spade a spade!

To my way of thinking this was just ad copy. I think Hammond was trying to add some mystique to try to separate his product from the rest. I wise man (my father) told me not to trust what a sales man is saying. After all he is trying to separate you from your money.

I'm confident in saying that aerodynamicists have the behavior of the wing well understood. And there is no need to generate new "concepts".

If he is trying to say that narrow wing tips stall too soon. That is known and described with the use of Reynolds numbers. If he is trying to say that high aspect ratio wing are more efficient than short aspect ratio wings that is also well known.

So if he is trying to describe (or define a new variable) the spanwise separation of the MAC and CG it seems to be just a misunderstanding of aerodynamics or at best an over simplification of the known forces involved.

I'm coming to the conclusion that his idea of separating the MAC from the CG is just hog wash!

I'm still trying to keep an open mind, but the door is rapidly coming to a close.

[JohnOSullivan]

A wise man once told me " If the CG is in the wrong place, just move your fingers"

[Konrad]

A wise man once told me " If the CG is in the wrong place, just move your fingers"

LOVE IT!!!

[OZPAF]

To add my final comment to this discussion - I did a quick drawing highlighting the MAC for equal area wings of 2  different AR's and different taper ratios.

The MAC does move a little with increased taper ratios - but not all that much.

The MAC in every case depends on the area and span and is located with it's mid point on the centre of area.

I have shown a Delta plan form - but in this case it may not be strictly correct as Delta's have a different Vortex generated lift pattern to normal wings.

A wise man once told me " If the CG is in the wrong place, just move your fingers"

Hope the model was just as wise

John

[Konrad]

John, Thank you.

Your drawing aligns well with my cartoon in reply #19.

I think we all understand MAC. What is still a mystery is how does one separate it for the center of gravity.  MAC and the CG are understood to be used in setting the static margin of stability. But that is separating the Center of Pressure and Center of Gravity by a percentage of the Mean Aerodynamic Cord. This to my mind is not the same as seperating the MAC from the CG, whatever that is!

[Rekitus]

hi Konrad,

I've been trying to sort out a bunch of related thinking in regards to MAC.
for the most part I have ended up at some combo of
who does one trust when the info one has is not clear and
what one is sure one does know.
Probably best to start with--> what I thought was MAC is not what you are sure is a MAC

I am off by 90 degrees :--D.  more on this later.

in the early 80's I tried to learn what was being taught as aerodynamics.
I did a lot of reading and related math problems/calculations.
said to show an earnest effort.  I don't think I learned the aerodynamics maths
enough to reach any novel understandings.
and I did learn that buzzword bingo got into some books.

And that lands us on the trust part of learning. After a while,
I knew some books were little beyond "I need to write a book" for the author.
The info in these just was not available. Whether my inability to assimilate or the author's
inability to disseminate makes little difference. these are not
a trust problem and have to be noted because of the next group.
Where I could match what I know to a book's info or lessons, then
I read and tried to confirm. I have built small aircraft in an attempt to confirm.
With that limited gauge, I would trust the author.
Last group were the ones that made logic leaps that made no sense to me.
these I did not trust.

I think I was reading a description of how to calculate where you should put
the center of gravity on a delta wing type. My mind's eye says a Dassault Mirage of some sort was the example.
The author may have mentioned MAC.  I am sure all I got was
"the CG is calculated by guesswork based on some geometry we thought might pass a sniff test"
in place of doing a couple of averages calculations to get a CG.

After a lot of learning to trust and not trust,
I got to where I trusted my bogus meter. 
That MAC thing... became a buzzword to me.
even If I learned the wrong thing.

If a MAC, as you know it, is good for anything else, I will assimilate it.

victor

[Konrad]

M.A.C. Mean Aerodynamic Cord is just a mathematical representation of all the cords. Mean is not the same as average. Cord is the fore and aft measurment of the wing.

I'm sorry I don't see a trust issue. There isn't any smoke and mirrors here.

I hope the CG isn't a guessing game. And again I fail to understand the 90° comment.

[Rekitus]

Hi Konrad,

take what you know to be a MAC and turn it 90 degrees.
across the wing. 

I know the 'mean' is halfway.  Not an average.

someone said a Mean gives you three useful data items.

I am not sure a mean will give you much of a difference over an average on an aircraft wing.

the trust item is my trusting a book author to speak truth.

you I trust.
(mostly :--D )

victor

[Konrad]

Turning the cord 90° would be defining span.

Are we confusing the definition of MAC with how one uses it to define the control and stability of the airframe?

[Rekitus]

Hi Konrad,
Go with,  confusing a MAC with the line to get over to the CG.

Since I think MAC is a poor construction to aid for getting a proposed CG,
I tend to give it little weight in my thinking or subsequent calculations.

the center of pressure for the airfoil at the halfway point of span
strikes me as a better path to CG goodness.

Please keep in mind I am not trying to build a brand new delta wing or sell it.

I can see how I confused you.  I'd prefer to remove the confusion I created.

victor

[Konrad]

Victor, we are definitely coming at this from a very different perspective.

I think of the MAC as an accurate way to define the cord needed in defining many aspect of the wing's aerodynamics. It is just a mathematical variable used in the mathematical modeling of the wing.

Again I think you, when adding CG or CP,  might be confusing it with the static margin of stability that is called out as a percentage of the MAC.

The question was how does one separate the MAC from the CG? Not how does one find the MAC and use the CG in conjunction with the MAC?

[Rekitus]

Hi Konrad,
From the post where you shared your thinking on possible ways to trim a fw 190
I knew we had different perspectives.
I also have had many people tell me I think very different from them.

I am not going to use my definition of a MAC any more.
That does not imply I have changed my mind of its vagueness.

To try to be as careful as I can get. I see MAC as a term that one might some how use
to convey a mathematical idea or to help describe a point on a wing...
or even something as useful as where to go to find an example airfoil.
All that does not save it from being a biiiiig red flag marking the bulldroppings.

To make a point about moving a MAC from a CG,  all you need is a wing with a normal root chord
 and have the wing tip chord a very long distance away.
the fact that you can't built it does not stop the math from driving the mean away from CG
as the wing gets longer. 
I just do not see a use for the ideation past teaching how to deal with infinity.

An aside, I have been reading and enjoying your posts about F3F aircraft
I think your posts are fascinating and I have absolutely nothing to add there.
modern RC aircraft are just amazing.

victor