Questions about proper CG placement are among the most frequently asked on this and other aerodynamics forums. Why is it that beginners and experienced modelers alike need to ask the same questions over and over again? Are the physical principles too hard to grasp? Don't flight instructors teach the subject as part of ground school? Don't flight instructors know the subject? Maybe many experienced flight instructors don't think it is essential knowledge. What percentage of flight instructors teach stability and trim adjustment in ground school? How many flight instructors skip ground school altogether?

Inquiring minds want to know. ?

Easy one -
Models -especially larger models with very low wing loadings- fly pretty well in an EXTREMELY wide and forgiving CG envelope.
The newcomers see-then ask "where is the CG?"
the answers are varied - yet the newcomer sees that those models still seem to fly well .
How could that be so?
Relative to full scale - where CG can make or break a craft with critical wing loadings and very restrained control authority and marginal power, these models can be flown into absurd flight configurations and recovered with extreme power and huge throws.
If - Ollie -you have eve flown a full scale Ercoupe--you se an example of a design which is extremely easy to fly (I love em) yet can kill a thoughtless pilot -who trys to fly it outside the original design parameters. That full scale craft had to kept in the design envelope as the controls were very limited and power was enough to do the design job- that's it.
I had a TOC entrant borrow a model for practice -and the prop broke off at the hub- one blade only - which wrenched the entire forward fuselage from the model.
He actually landed it - using full down elevator and doing rapid aileron recoveries .
We patched it up an back it went to the practice field.
The CG was at the TE of the wing (a straight TE swept LE panels) with the forward section broken from the model.
True story.

Dick,

I think your observations about The Ercoupe hold true for any aircraft, model or full scale, in the sense that an aircraft with the CG much behind the neutral point will be uncontrollable without active stabilization such as a gyro.

I think that your TOC example is readily explainable. The loss of the fuselage forward of the wing moved the neutral point a bit aft. The neutral point even before the accident was just a little forward of the trailing edge of the wing because of the relatively large tail area on a relatively long tail moment arm. After the accident, the CG at the trailing edge of the wing was very near the neutral point. If this were not so, the wounded model would have been completely uncontrollable.

I would disagree with your inference that wing loading has something to do with how wide the range of usable CG's is.

OK- you don't agree- but in actual practice - it has everything to do with usable CG-
You have to try this to se how it really plays out.
My comment on CG is this:
If the plane is too heavy - the CG does not matter
If it is light enough - it still does not matter.
( a little humor)

OLLIE....You bring up an important question about CG.....The answers are probably numerous but certain criteria remain factual.....and needed......from my experience many modelers are not all that familiar with the basic aerodynamics of flight...matter of fact..(I'm an Aircraft Mechanic at an Airline).....I see a number of Mechanics as well as some Pilots that have forgotten or weren't interested in that info....In my earlier years spent time with many Large radial engined planes ...DC-6, Connie ,etc..and helped with W/B computation, fueling etc....and CG better be right....now I work on on some Airbus ..A-320 series and all fly-by-wire and automated beyond comprehension...concieveably could make everyone lazy.....but because of Airbuses Stability Augmentation(they use smaller tail vert/horiz surfaces )CG really better be right.....I fly Large WWII planes and with the high wing loading and less Flt surface travel distances W/B computation is "Crucial"....as compared with the Aerobatic flyers where light wing loading and miles of flt surface and travels give a huge CG window....so my advice is....when doing W/B...Get it right...it just might keep a model from returning back into "kit" form....Bill...

Ollie,

The type of flight instruction you seem to be referring to is entirely outside of my experience, which is not exhaustive. My instructor did check out my plane, and may have asked me about where CG was set, but I don't remember that. There was no "ground school", and no explanation of stability or any other "theory". We flew until I could solo, I soloed, and that was that. This may be atypical, but I was not under that impression.

The reason there are so many questions about CG is that beginners are told it is important, but don't really understand it. The question I would have is why so few of them are getting the CG recommendation that the manufacturers give.

banktoturn

BanktoTurn....I think you just answered the question.....very few people understand the "REAL" basic principles of aerodynamics and even fewer understand CG and its relation to flight....so there you have it....just not enough flyers out there to sit down and explain the whys and hows....so confusion remains.....you make a good point....cant go wrong with "FOLLOWING THE INSTRUCTIONS"...have helped a number of new fliers and when I ask them questions related to their kits plans ...many dont know the answers.....BILL..

CG is important - It may sound like I don't understand it's importance - I do.
For good aerobatic setups- the CG placement changes the character of the plane.
BUT the shift permissible is very extreme compared to airliners of Airbus size!
The example I related to Ollie was a cg shift of 6" -- on a 15" chord.
The plane was -for all practical purposes "unflyable "
My point was that with the sudden NEW very light wingloading - (the model lost 35% of it's weight) it could now be coaxed to remain upright.
The neutral point was far from the new CG.(with nose section missing)
On really serious flip flop models -the CG is run back past any decent CG point for solid control -in forward flight -however when flown as a "helicopter" -at these very light loadings - the setup works quite well.
Again extremely low wing loadings and high power loadings allow extreme CG shifts.
These models are not scale P51 types with high loadings -It is a completely different ball game -.
I have flown scale stuff (for scale builders) that have -to me- very high wing loadings - I insist that the CG be kept in proper range -when they are tail heavy - they are a handful!

OK, so only the expert pattern flier, sailplane guider, pylon racer or what ever expert (or aspiring expert) is motivated to tune the CG location for best results. Everybody else is happy with the "appliance user" attitude that the factory recommendations are right because that always allows them to take off, land and do a few simple maneuvers. Their aspirations don't go beyond those simple goals. So, why bother wading through the complication of technical knowledge?

Do I get it yet?

Ollie,

Go down to your flying field, and take a look at the people there. Whatever cynical words you use to describe it, most of the people there have bought a cool, expensive toy, which they now want to have some fun with. I kind of know how you feel, because part of the appeal to me is the aerodynamics & structures involved. That is just not the case for everyone that wants to fly and have fun, and I think they are the majority. Even the people who are serious about pylon racing, soaring, or whatever, usually have a pretty sketchy picture of the underlying physical principles. I guess I don't have a problem with that.

banktoturn

I'll try to keep my cycicism under wraps.

What really gauls me is to see so many of my fellow modelers flit from one product to another looking for better performance when they have never taken the trouble to try to get the most out of what they have by proper adjustment and flying practise. They just don't seem to have the motivation for either. Maybe that takes the fun out of it for most folks but I still hate to see them waste their money that way.

Fun is the basic principle and I believe that everyone should have fun in their own way, as long as they aren't hurting someone else.

Your last statement just about sums it up, Ollie.
Most aren't inclined to go to the technical depths you have, myself included

For me, it's all about fun... as for CG, I want to know a starting point, and I'll play with it
from there, based on how it flies. I'm not the least bit interested in any technicality, though.
I just move it fore/aft of the "recommended" until it flies the way I want it to.

Ollie - --the fact is that learning the interrelationships of CG/ Wing loadingand power loading - -Throuigh applied use - is what modeling is all about-
I once spent a lot of time loking at charts about various foils etc- until it finally dawned on me that these very scientific bits of information are typically for a specific situation - in arobatics - you fly thru a very broad power - and attitude situation.
To put it bluntly - -You ain't going to find a "perfect " setup for aerobatics - till you cut and try - there just isn't any good data that covers our applications.

spel check---read -check -
I dun it again

one day i'm just gonna answer, " on the damn plane "
after all, it must be, where it's ballancing right now, thats the CofG, how you find it?, balance it, no calculation can help you find it.
HOWEVER!!! it might be in the wrong place i sold a kid a 15 fun fly, he fitted a 40 and UC, i laughed and sent him away. the CofG wasn't even on the wing.
i rarely use a calculator, the high point or rear is good enough, plans are good too, for own design sweeps, i just draw a few lines and i've not been wrong yet.

It's the ARF generation!
Q: Where's the CG on your model?
A: It didn't come with one. Here's the box!

ARF ARF!

There are 2 types of CG question:

What's a safe CG? - for Newbies, first flights, unusual planform, etc.

AND

WHat's the best CG? -for specific applications, 'batics, soaring, etc.


For the first question - the answer is readily calculable if you have the concepts/formulas. I work out CG for 10% static stability for my first flights, and have never been surprised.

The second is a more subjective and qualitative issue. Unfortunately, people asking AND answering online often confuse which CG issue is in question. That's where alot of the confusion and repetitiveness comes from. Also, the 2nd concept requires more understanding, where the first just requires a tape measure if you can get someone to tell you the value.

As for the example of the TOC plane - not really a valid experiment unless someone has tried to fly the same plane - with the engine still attached AND with the CG at the trailing edge.

That behaviour has everything to do with the size of the horizontal tail w.r.t. the wing and almost nothing to do with wing loading. TOC/ 3d planes have horizontal tail spans about 1/3 the span of the wing and tail area is around 25%(or more) of the wing area. That creates a huge CG range

You can, if you want, point to a WW2 warbird with high wing loading and say "look, high wing loading and the CG location is critical" of course, I would then say "look how small the tail feathers are in relation to the wing". That's where the real answer is.

There is one factor overlooked - and that is dynamic stability - which also works in favor of those planes with big 'ol tails. A large tail surface will create a considerable damping effect, which will make a plane even with it's CG a bit behind the neutral point controlable - as the divergence does not happen quickly and can be handled by human reactions. Also not an effect influenced by wing loading. This effect is what allows a plane to be neutrally balanced (or worse) without being "squirrelly".

Well said John! But, why is this wisdom so rare? Why are so few interested untill they are in a quandry or worse? Why aren't adjusting skills considered nearlyas important as learning to move the sticks?

I would say the vast, vast majority of people don't know that CG range and location is directly related to tail size. When they are newbies, someone says "CG at X location of the chord is good" where x tends to be quoted between 25% and 35% chord.
The discussion ends there. It works for a few planes. No more thought is applied. Talk over the pinic tables at the field rarely strays from a similar theme.
______________________________

FWIW - I got a nice giant scale Privateer old-timer flying boat for a good deal because it wouldn't fly off water - so the seller said.
Anyway, he had it balanced at 25% chord - per the PLANS - and had used about 2 pounds in the nose to do so. After adding the weight, he also had to shim the stabilizer up out of its saddle to get a balancing downforce.

This on a plane resebling an old free-flight glider in dimensions and construction that should have weighed 10 lbs to begin with. The tailplane is almost as big as a wing panel. I believe it's 40% of the total wing area. This is a true lifting tail airplane. The neutral point is at 80% chord. The incidences, natural balance, water step location and of course tail area all indicate this. So, this guy was trying to fly it with 55% static stability and at least 2 lbs overweight. Talk about a cost of ignorance. Why the plans put the CG at that point I have no idea, since the actual designer abviously understood it would be well back from there.

John - the TOC practice plane was not an experiment-I mentioned it as an example of how very lightly loaded stuff is easier to control - no matter how badly it is fu--ed up.
As for tiny tails - gliders have very small tail groups - but long levers of course.
Years back - we went to 25% tail groups on pattern stuff - to gain the stability - but on the current TOC and good IMAC stuff - we have cut back to around 20% volume and pretty well left the scale moments intact-
The wings get clipped - on some -not on others -
Different style flying - -more departure maneuvers.
As for the "neutral balance" stuff- -good for tumble bug - lousy for precision. -----and wing loading - well I never saw a good aerobat -in any scale- with ballast on board - cept for gliders --

The Draftsman did it, with a drafting pen in the DRAWING room.

Changed my mind and deleted all this, but since I had a response, I put it back


Some people aren't interested in learning these things... and it's not needed, for the average Joe.

Do you know, perfectly, every system involved in the auto you drive daily?
Can you discuss, with intelligence, stoichiometry with regard to emissions and power? Hydraulic principles,
and how they make power steering possible? Hydraulic brakes? Volumetric efficiency and the effects
of too large or too small induction systems? Flamefront propagation, and anomalies thereof?
Combustion chamber/piston size, shape, defects, and how they affect performance, good/bad?
Symptoms and effects of preignition or detonation? How to prevent or remedy either?
Do you know the difference between oversteer/understeer? Why most autos understeer from the factory?
Do you know where all your airbag system components are, and the significance of each?
Do you have a working knowledge of EGT/O2/TPS/MAP/MAF/ECT/knock sensors and how they
interact with the onboard computer system? How many rows/cells in the default tables on your ECU?
Do you know what the default values in those tables are? Should they be different? Which are dynamic?
Do you seriously ponder all these things on each car you buy?
Well, OK... I see you are a retired engineer, so maybe you do

Most folks "just drive". Doesn't make them stupid... they're just not interested.
Most folks want to get from point A to point B and that's that.

I'd suspect most flyers are the same way. They want the plane to fly, and they know,
if building a proven design, that someone has already done all the dirty work, and they want
the quick answer, not a college level lecture in aerodynamic principles.

My point is that we all have our areas of expertise. Some of us have more than one and,
admittedly, some portion of the population is totally unskilled and/or unmotivated.
It doesn't make a hill of beans of difference when it comes to flying that model airplane.
Either they know how to fly, or they don't (to varying degrees, of course).
They don't have to be aeronautical geniuses, or anywhere close, to accomplish that.
Same with computers, cellphones, microwave ovens, fireworks, television, etc...
There's no real need to understand all the theory and possible adjustments, just to be an end-user.

Not meaning to flame you, Ollie... I rather enjoy reading your spiels sometimes.
You're quite knowledgeable about aerodynamic principles, and it shows.


Just can't help but think that you like the trolling factor sometimes, though

Heck, I bit.

Chris

Oh Chris - sounds like you are a car guy too.
How many car guys like to muse over rod length to stroke ratios?
I love tinkering with engine designs.
Probably cause I never had to do it for a living.
My engine design hero is still Smokey Yunick.
That cat really did some plain and fancy engine stuff.
I also , loved to figure out why aircraft do what they do it.
Probably again, because I did not do it for a living.
It always puzzled me as a kid when I asked why planes flew - and the old Bournelli (sp) theory was rattled off as the "reason why".
Later I found out that the reason it never made sense to me is because it never made sense .
I could see that that the angle of attack of a flat board could provide lift.
So why all the mysterious jargon?
Why not keep the language simple?
Once I was intoduced to techno speak-I found it easy -and it made it convenient for engineers to speak with one another.
it did little tho, to relay info to the unannointed.
Back in the 60's - I had to work with our patent attorney, to make certain that my patents were correctly and plainly worded .
It was a great education.
This guy knew how to ask in a direct simple approach - all questions required to document and explain how a complex pneumatic control device operated.
He knew nothing about pneumatics - but knew how to relate information.
More importantly -how to relate relevant information.
( And this from an attorney!)
I get a bit puzzled - when I hear complex technical info put forth ,when a simple one will do the job.
In working with defense attornies - I learned early on - that if I could not make the information I was to offer, simple enough for the entire jury to grasp , then--
it not worth a damn.
Worse yet it added doubt and confusion.
And now that everyone has drifted off to sleep---- I will steal away---

Ollie man, you are right again. I just went and took a look. About one in fifteen threads were questions about CG. It could be that the "new" guys just don't look around a little to see if their question has been asked.
And just because someone flys pattern or those big 3D's does'nt mean they have that much experience. I see a lot of new guys flock right to that stuff.
I have also seen the "academia inclined" do some real bone head stuff that absolutely shocked the rest of us.
It just keeps going around and around.