How to seperate the CG from the MAC?

[Konrad]

Prior to the great crash, I recall this site had some great amateur and professional aerodynamicists. I'm hoping that somebody here can explain what is meant by or debunk this ad copy statement.

In my internet searches I came across this article/ad copy that has me puzzled.
https://medium.com/rc-soaring-digest/designing-for-a-slope-allrounder-c51f49cbbc7a

Halfway down the author says  "It’s best to try to modify the ellipse to separate the MAC and the CG as much as possible."
I thought, not being an aerodynamicist, that the CG and the MAC were inextricably linked. How can you separate them? And if you can how does that help with lift, drag, control and stability?

I think I understand what is going on with Will Schuemann's observation on how the airflow is moving inboard as the tips stall and that we want to remove as much wing area aft of the tip, as the wing stalls. Hence with the Schuemann wing we see the straight TE with all the "ellipse" coming from the LE. (I think this is call the reverse elliptical wing planform. With the wings of the Spitfire and P-47 being thought of as the classic forward elliptical wing planform).

Now I need to say that I've been flying some this designer's latest high aspect ratio wings and love how they are performing. But I have to admit I don't understand his explanation as to why his wing holds on so well at high lift coefficients.

TIA,

Konrad

[Rekitus]

hi Konrad, I can tell you what I think he said.
I can't tell you I have confirmed my thinking in an airframe.

mean aerodynamic (wing) chord
often used to describe swept back wings like an F-86 as a single number
and often said in place of average wing chord or average point of lift
for the entire wing.
I suspect you know that part.

If you take the generic symmetrical ellipse wing
the MAC will be at center of wing.
if you move the wing tip end point back the MAC will
move back...

I tend to think the center of lift will follow in a linear proportion
and the airfoil should be proportioned from the leading edge
and not kept 'centered.'  (a curve , not a line)

thus I believe he was pointing out that he wanted the center of pressure
back from the center of gravity.
victor

[Konrad]

Thank you Victor.

I have to admit I'm having problems following your explanation. But by the last sentance, I think you are describing the static margin of stability. If Hammond (the author) was describing that I think he would have said so. In my trimming I try to get the CP and CG as close as practical within a few percentage points of the MAC. I'm NOT trying to separate the CP and CG as far as possible.  (Note I rarely want the CP ahead of the CG.)

I think what Hammond is describing is the effect of span and aspect ratio if looking a the wing planform. If the CG (center of mass) is assumed to be some point (not a line) along the fuselage Then by changing the wing span or aspect ratio of the wing you can (maybe) be moving the MAC and CG relative to each other. But this isn't some great insight.

[Rekitus]

well, Konrad,
If It makes you feel any better, I find it hard to follow your logic too.

I did not think there was any great insight in the MAC comment by the designer.

the problem is that the terms we use are not always clear for meaning...

center of pressure
center of lift
average chord
mean chord
center of gravity
roll center
pitch center
yaw center

coefficient of drag
induced drag
aerodynamic drag

I have heard all of these get miss-used
most times because the author was attempting to give a simple explanation.

and to let everyone off that hook, I will allow that
what I learned may well have been wrong.

In a world full of great communicators, It is rare that aerodynamists are among them.
For the items I could directly test, the books could have been less obscure in their statements

victor

[Konrad]

Yep, effective communication is very difficult. That is why we have lawyers to make sure nobody knows what we talking about, or have success getting ideas across.  

I was trying to say that you might be correct but that I'm not following. Not that you are wrong, as I don't know what you were saying. In the end I think you are showing what I think of as the static margin of stability. This is to my mind clearly what the author was not talking about.

As to the terms you mention these are pretty well defined in the text books even if we (I) as layman misuse them.

[Rekitus]

yeah, all that.

The books I read were more loose with terms than you are giving credit.
Some terms are very specific and these are often simply not used.
I did a lot of book surveys at maryland's CP engineering library to look for books
I could understand and then hunt to buy... frankly, I believe the most authors
 either did not want anyone else to use the info
or were making claims that had little real testing.

When you know little on the subject, that pair is a biiig roadblock.
I tested and trialed all I could.
These three terms are very often used for the same idea
center of lift and center of pressure and airfoil peak pressure
and in keeping with making the terms obscure, the word pressure means vacuum.

I keep learning about aerodynamics and most of the time
the most effort is sorting out what terms are being used to
describe things I do know. 

now, since you did not think the author meant what I said
and believe me I don't mind. It will not be the last time I am
wrong in my understanding.

what was your guess for his meaning?

"In the end I think you are showing what I think of as the static margin of stability."
If you give me credit, you got there.
and I did not think you would have believed me if I had just said
"I think he wants the center of pressure behind the center of gravity."

victor

[Rekitus]

Hi Konrad,
I can see some of what you are asking...
as MAC is a geometric description of a wing.
A rectangle is a geometric description of a shape
the Mean Aerodynamic Chord is a a point in the center of the airframe
For a cub, it is at the center of the wing at the center of the fuselage

it is a way to describe a wing geometric shape as one number, one point of the planform.
that is all MAC is.

You know the center of gravity can move to any place forward or back.
The MAC is un-affected. the wing shape did not move.
so MAC is static it will not move until the wing planform changes.

If you put a different airfoil on that previously flying cub, the center of pressure will shift.
That may affect the flight qualities.  like make the tail feel lighter or heavier.

To use a clark-y airfoil because it is a good airfoil and well studied...
if you put the CG and CP in the same place... as the cub goes faster in level flight (more power!)
you will not need to change the trim and the plane will rise from the added lift.

If you move the CG forward and speed up... (more power!)
the extra lift from going faster will also lift the tail.
the plane may chose not to rise. it will run faster.
Depending on the how much the force lifting the tail, These may cause the plane may nose down.
the reverse is true for moving the CG back. plane may nose up and slow.

one more item... the center of pressure is dynamic.  usually moving forward as angle of attack rises.
this last one I have no way I could test that made me confident in it.

I have flown enough RC cub(s) to test the trim/speed claims.

I am intrigued about what you mean when you express curiosity for
"why his wing holds on so well at high lift coefficients.

that caught my attention.  do you know what airfoil is in use?
high angles of attack or strong lift?

victor

[Konrad]

Victor, I honestly don't know what Hammond is try to say/show with the statement; that one must try to keep the MAC and CG as far apart as possible. That is why I'm asking for help in understanding that statement.

I have tried to understand some of his other theories. Such as adding vertical front side area to the nose to control the crabbing (weathervaning) we see with slope ships as we traverse the slope with the wind being 90° to the direction of travel. This theory is what killed his last F3F ship, the Redshift as this model suffered greatly from a lack of directional control. Hammond didn't understand of had forgotten that side area ahead of the center of pressure has a destabilizing effect on directional control.

Now what he was talking about in that part of the article was his idea of optimizing the lift distribution across the span of the wing. Like I said I don't understand his explanation. I will say that his wing planform, where he credits this idea, has shown in a few of his designs the Redshift being one of these, to result in a nice handling high aspect ratio wing.

The best reasoning I've come up with is that he is using the revers elliptical wing as a result of the near perpendicular to the fuselage TE. This inboard flow near the stall was well demonstrated by Wil Schuemann in his full size yarn tuft testing. For reasons of ease of modification he used a faceted LE to mimic the well understood elliptical lift distribution described in Prattle's 1918 paper on line lift theory.  If starting from a blank piece of paper the smooth reverse elliptical LE might show even more efficiency at high coefficients of lift near the stall point. (This is the area I'm most concerned with in trying to make the F3F racer maintain its speed through the turns.) 

While I think Hammond's wing planform (a modified reverse elliptical LE wing) does result in a nice benign stall I don't think what he said is the reason for the nice behaving planform. I say this as I don't see how one can seperate the MAC from the CG.

[Rekitus]

ooo boy.  I went and read the article you posted.

I think Dr. Hammond's book would _not_ have made it onto my "go hunt this one" list.

His airfoil looks good. His reasons for that airfoil are fine and his
graphics to support his statements are not what I learned...

The structural assessment for camber is ... interesting.
I had not considered some relations for strenth of main spar.
The one relation I was taught was wing area will create skin friction no matter what,
so some thickness will cost you no extra drag.
He says that point is 8 percent of chord. plus or minus a half point.
the tail 10 percent of chord has me curious, as a contrast to main wing.

His comments about the tips are puzzling. As are the stall propagation statements.
Those may be chalked up to wanting a simple explanation.

overall and to answer your MAC question.
he may well state that MAC means some thing other than Mean Aerodynamic Center.
but I read it as center of pressure...
a headsup for the airfoil? the thickest part of the foil is usually the center of pressure.
so he does have it back some from the few I studied. ( don't that sound like I know all of it. )

To share what I know about lift and angles of attack, stalls do not happen until you run out of power
the lift generated on the top of the wing goes up as the angle of attack goes up.
so does the lift generated on the bottom of the wing.
can your elevator push that hard before the elevator stalls?
but if you are still flying it is the bottom of the wing that will push the nose down
as you near stall the bigger that area is, the more nose down force you will get.
for your question of how does it hold on?
I think the airfoil stalls as normal and the long trailing edge saves the flight.

I think his planes look like they will fly well and he has said he has tested them.
so you have little reason to listen to me

[msc]

My conclusion is that Hammond may have some good insights into aerodynamics but he doesn’t fundamentally understand what the Mean Aerodynamic Cord (MAC) is. Some of the efforts to explain have left my head spinning a bit also.

Much of the discussion is treating the MAC as a point rather then the line that it is.

For computational purpose it’s easiest to reduce a complex wing planform to a representative constant cord straight wing. The MAC is the cord length of that simplified wing. The MAC is similar to the average cord but I think the inboard section of the wing has greater influence then the tips so the MAC is often slightly longer then the average cord.

In addition to the length of the MAC it is necessary to know where the MAC is located relative to the actual wing. Typical this is specified as the distance from the leading edge of the MAC to the aircraft datum (often the leading edge of the actual wing at the root).

To understand the influence of CG it’s necessary to know where the CG is located relative to the MAC expressed as a percentage of the MAC. For conventional balance (non lifting tail) the typical range is between 25% and 40% of the MAC. At the front of the range performance is as an ultra stabile nose heavy turd. At the back of the range it’s nimble and responsive. Move to far back and it’s twitchy then unstable.

Changes in wing planform will alter the length and location of the MAC but does not alter the relationship between CG and MAC. This is why MAC is used for CG calculations and determining CG limits. Often the CG limits are expressed as distance from the aircraft datum but engineers calculated the limits based on percentage of the MAC.

Airfoil data is based on a reference point at 25% cord. If Hammond is mistaking MAC as the point at the 25% cord of the MAC then separating the CG from the MAC could be seen as moving the CG aft from the forward  limit. This would be a strange way of looking at it and is hardly an aerodynamic insight. Most would call it trimming the CG for desired pitch stability.

As an interesting side note I know a guy that use to own the prototype glider for the Schuemann wing. It started life as an ASW-12 that Will Schuemann cut up and spliced back together as a 15 meter wing with the now familiar planform.

Mike

[Konrad]

Thank you Mike.

I agree. It is the MAY in your statement that concerns me. "Hammond may have some good insights into aerodynamics but he doesn’t fundamentally understand what the Mean Aerodynamic Cord (MAC) is."

I've been flying his planform for a while and can report that it does have a benign stall and does maintain a lot of energy through the turn. I don't want to discount his findings just because he can't explain what he found. (Not that I really think he found anything, it just looks to me to be the reverse ellipse. But what do I know?)

I also don't want to again, have to deal with his lack of understanding of some basic aerodynamic concepts. Such as the effect on side area ahead of the center of pressure on directional stability.

[Rekitus]

hi Konrad,

you have two different people try to say in a nice way
the Comment about MAC is not being used as they learned it.
Trust yourself, you seemed to have an issue with it or you would not have asked.

I am unhappy that my answers were not able to help you.

My opinion about the resistance of the main wing to stall
is that the elevator stalls first.
the extra camber at 10 percent should cause that.

victor

[Konrad]

Prior to the great crash, I recall this site had some great amateur and professional aerodynamicists. I'm hoping that somebody here can explain what is meant by or debunk this ad copy statement.

In my internet searches I came across this article/ad copy that has me puzzled.
https://medium.com/rc-soaring-digest/designing-for-a-slope-allrounder-c51f49cbbc7a

Halfway down the author says  "It’s best to try to modify the ellipse to separate the MAC and the CG as much as possible."
I thought, not being an aerodynamicist, that the CG and the MAC were inextricably linked. How can you separate them? And if you can how does that help with lift, drag, control and stability?

I think I understand what is going on with Will Schuemann's observation on how the airflow is moving inboard as the tips stall and that we want to remove as much wing area aft of the tip, as the wing stalls. Hence with the Schuemann wing we see the straight TE with all the "ellipse" coming from the LE. (I think this is call the reverse elliptical wing planform. With the wings of the Spitfire and P-47 being thought of as the classic forward elliptical wing planform).

Now I need to say that I've been flying some this designer's latest high aspect ratio wings and love how they are performing. But I have to admit I don't understand his explanation as to why his wing holds on so well at high lift coefficients.

TIA,

Konrad

Thank you.

That is why I asked for some input from the aerodynamicist here on HIP. As you will recall I said I thought the CG and MAC were inextricably linked.

It looks like most folks are thinking that Hammond was/is talking about the static margin of stability. He is not. I know Hammond knows about the static margin of stability and he isn't that sloppy in describing it. I might be, but he is not.

I know he is not trying to describe the static margin of stability as we know that we don't want the CP and the CG to be separated too far as a percent function of the MAC. So that statement that; "It’s best to try to modify the ellipse to separate the MAC and the CG as much as possible" is just wrong if we are thinking Hammond is talking about the static margin of stability.

I like Mike's idea of down grading the tips effective cord based on the idea that the tip isn't as effective at generating lift, as the root, as it is subjected to the tip vortices. Now I Iwouldn'twouldn't use the term average especially is dealing with complex tappers or an elliptical wing.

Hammond is talking about his planform and has used this phrase many times separating the MAC from the CG.

It looks like I may have to draw up a drawing showing what I think Hammond is try to say. Even though it doesn't make since to me.

[Rekitus]

too late I think...

I find odd answers in odd places.

I found I was trying to recall who or how I came to know stabilization.

the bf109 was bent in the right place. not sure if it was even...
if that was the one covered in vinyl,  it was an
unerring arrow for the three flights in its storyed career.
I think It was my brother... when asked about the bends on a buzzard bombshell wing
that is to make it more stable
I thought "stable must be valuable for all the work involved"

thus my study of aerodynamics started.

victor

[OZPAF]

Although a little late to the party - after a brief read of J Hammond's article - I find I basically agree with mike - MSC.

It seems to me that he is describing the AC but incorrectly calls it the MAC. I feel he then further complicates the issue by then confusing the AC of the wing and the aircraft Neutral point - the NP is dependent on the tail volume and it's AC - which he doesn't seem to take into account.

He loses me when he talks about extra area near the tip leading to possible tip stalls on high G turns.

His comments on airfoils are also a bit misleading - he talks about various thickness airfoils having different cambers - this does not have to be the case. Thickness and camber are independent variables and can be changed independently.


Also his XFOIL plots in my opinion have been run at too low a NCR number - 6 is almost wind tunnel conditions of turbulence - it is more common to use an NCR of 9 which is closer to normal outdoor free stream levels of turbulence.

This may be a bit hard - but the whole presentation hits me as typical PR blurb and is confusing and incorrect in many places.

Well that's my take anyway.

The Typhoon was and is a very nice flying model. I have a friend flying a current Chinese copy - the Dream. Fast, aerobatic and thermals.

John

[OZPAF]

Another thought occurred to me. Did JH actually write this article or advise someone else who wrote it while not completely understanding it?

John

[Rekitus]

Another thought occurred to me. Did JH actually write this article or advise someone else who wrote it while not completely understanding it?

John

the answer to that question may be in finding who is selling the airplanes.

victor

[Konrad]

Another thought occurred to me. Did JH actually write this article or advise someone else who wrote it while not completely understanding it?

John

John, Thank you for joining the conversation.

Hammond wrote the article and has used the phrase separating the MAC and CG many times before.
But yes that article is written as a sales blurb and not for submittal to a technical journal.

Key to what I think he is trying to say was how he described the delta wing.

I'm making up drawings of what I think Hammond was saying. These will not be trying to rationalize what he is saying.

Here is a link to his toy airplane firm, Aeroic.
https://www.aeroiccomposites.com

[Konrad]

Is there a way to attach a PDF article?

I'm try to attach Wil Schuemann's Article A New Wing Planform With Improved Low Speed Performance. The pictures of the tuft testing are eye opening as to the importance of wing planform

You can find it here in the second post
https://forum.alofthobbies.com/index.php?threads/schuemann-tip.3652/

[Konrad]

This is how I read what Hammond is saying. I define MAC as the area divided by span. Its geometric placement along the span is another subject.

I have no idea what are the units Hammond is using. Is his distance normalized as a function of span? I really have no idea what he is saying. But his high aspect ratio wings do perform better than comparable F3F wings. I'd like to learn why.

[Slowmatch]

I've been trying to make sense of this for the last day or two before posting, and have done some digging.

It appears the chap in question is quite controversial within the world of F3F (an area of flying I know little about.) He seems to display a good general knowledge of aero subjects, and it seems he claims to be an aerodynamicist but also has designed a number of models that fly well. However his incorrect explanation of the aerodynamic centre (that is what he means by MAC) and CG position does seem to be a very rudimentary understanding of static margin.

In the explanation on this page: https://forum.alofthobbies.com/index.php?threads/the-aerodynamics-channel.3123/page-15 he appears to be assuming the AC is at 50% of the MAC (mean chord) and that the CG is at 25% of the mean chord. This makes no reference to the effect of tail volume and neutral point on the actual static margins influence on pitch stability. He seems to be misunderstanding the aero centre and pitching coefficents, or at the very least ignoring all the many moments about the CG required for stability. Oddly in other posts he gets some of this stuff correct.

He then goes on to talk about the AC and the influence of swept elliptical planform on stall propogation in terms I've never heard before here in post #339: https://forum.alofthobbies.com/index.php?threads/the-aerodynamics-channel.3123/page-15

So I have to admit I'm a bit baffled. I can't work out if he doesn't really know what he's talking about, or if it's just poor communication skills and I've missed something.... If I was being skeptical I could imagine he's rehashing a fair bit from other sources and were it not for the dates I could believe some of it had been written by AI  

However, wishing to think the best, it may well be me that has the wrong end of the stick...

[Konrad]

Slowmatch, Thank you! You were one the guys who's attention I had hoped to get.

I can tell you he is real as I've had some face to face talks with him. I have to admit I often left scratching my head.

I'm tempted to call this BS. But since I don't know much of the aerodynamic arts I'm not willing to say BS to things just because I don't understand them.

So to be clear you aren't aware of a concept were separating the MAC and CG increases the aerodynamic efficiency of the wing planform?

Having tangled with this designer over directional stability and how it manifests itself I'm not willing to hold this designer in high esteem when it comes to the discussion of aerodynamic principles.

I'm building his latest Alpine Slope Ship and I'm appalled at what I'm seeing when it comes to basic mechanics. I hope I'm not allowing these problem to jade my opinion of his overall aerodynamic acumen.

Again thank you for taking the time to look into this.

[Rekitus]

Konrad,
trust you know what you know.
Ever heard of the B-24 wing?
I read of this many years ago.
https://en.wikipedia.org/wiki/Davis_wing  

read to the bottom.
there is no way Davis knew about how useful his airfoil shape was for higher speeds
or laminar flow.
ETA: the thickness he picked didn't help...so he did not know about the high speeds.

There are places where people follow ideas and do not know the "why"

got a lot more I could say. here is a good place to stop.

victor

[Konrad]

Victor, I'm sorry, you again have me at a disadvantage. What does the Davis wing have to do with the separation of the MAC and CG?

And yes I know a bit about the B-24. My father flew in them over the hump during WW2. What a waste of resources!

"There are places where people follow ideas and do not know the "why""
This is generally called superstition. In modern engineering and science we focus on the why as well as the outcome.

[Slowmatch]

It's kind of you to say so, but I'm just an interested modeller and not in a position to call someone out either.

So to be clear you aren't aware of a concept were separating the MAC and CG increases the aerodynamic efficiency of the wing planform?

Nope. The CG is usually represented as a % of the Mean Chord and any planform sweep doesn't change that relationship if the same static margin is maintained for similar stability.

From what I can see Hammond repeats this mantra in several places without really explaining it. The only thing I can think of is that he is imagining the CG as fixed at 25% of the root chord - in which case sweep, by means of a straight TE elliptical planform, would increase static stability and 'separate the CG from the AC'. Martin Simons (Model Aerodynamics p61 6.7) references this as something to be aware of with the straight TE ellipse.

By the way Hammond later clarifies that when he's been using MAC he actually means AC, although he seems to wrongly define this too.

I don't wish to diss the guy - and I must point out I've never looked at the aerodynamic requirements for fast slope models. But it does seem an odd way to talk about planform, especially as he does seem to have broad knowledge over all, and is clearly capable of practical design work. He speaks of wind tunnel testing but is rather scathing of CFD - yet it's not really clear what his method of wing development is.

Straight TE elliptical planforms have been round a long time - Hepcat late of this parish did a paper on wing tips called Tip Topics, that is worth a read. It's in the plans gallery: https://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=130&mode=search