Yeah, Barney, maybe it's a cultural change: away from self reliance and to dependency.

Gentlemen.....Lots of good..well made points.....so where do we go from here...I agree whole heartedly that my primary reason for flying is pure enjoyment....probably like most folks...so the door is wide open for whatever amount of knowledge one wants to absorb.....I personally want to learn as much about airplanes and aerodynamics as possible...I still am at awe watching a 747 or 777 T/O as I see it everyday.....just like watching one of our models fly......back to CG....A P-40 (Realtively small tail surfaces) flys just as nice as a P-47(relatively big tail surfaces) if time is taken to balance properly "and" ensure surface travels are correct one must assume both are built straight and have proper wing washout, etc......what I'm trying to say its a matter of personal choice how much knowledge one possesses...and if directions are followed and good help is obtained at the flying field not much knowledge of aerodynamics is required....afterall its only a hobby.....either way works..its just a matter of personal choice....Bill....

Chris,

I do think that you are considering only 2 extremes: Knowing nothing, on one side, be an expert, on another side.

My take is that there is a minimum basis that EVERYBODY should know, then decide by himself if one want to know more, or not.

If you want to be a "full scale" pilot, you will have to learn and understand some aerodynamic basis, yet, you legally will NEVER have to make structural changes to the plane you fly. As a model airplane pilot, chances are that, even an ARF may very well, during his "life" have some "landings" ending up in heavy structural modifications and diverge quite a bit from the "proven design".....

That you are interested or not does not matter much, the model still obey the same aerodynamics rules....

And the basics rules are, at least from my point of view, a lot more simpler than any "college level lecture"....

Bernard

Ollie, I hope that you are not losing sleep over this. With any hobby there is a spectrum of experience and involvement. There was even a point in time when you did not know where the COG was. You were interested and you found out. You also found out how to calculate it and how varying it affected performance.

To have the "full" experience of model aviation one must study a multitude of disciplines. Some people will fly their ARF and be happy. Some will fly their ARF, tinker, learn, etc. Some will crash and go back to RC cars. It all depends on your level of motivation and interest.

Those of you who are willing to share information with us are much appreciated, however mundane the information may seem to you, at times. Ollie, in particular...You've been a tremendous help. Thanks!

I'm finicky in the design phase, and try to get all the math nailed down, so I can build structures with confidence. Sometimes, though, it's good to hear this math verified by someone you trust. (or not...then I learn something.) I want to know ALL the answers, and some of them are things I haven't personally researched. I sometimes feel that to get verification about design issues from particpants in the know here is a sort of "monaker" proving a good design. I then sleep better.

Anyway, thanks to all of you. This has been a great year for me, largely thanks to RCU... sincerely. Sharing information and ideas in a constructive way is the best thing on the planet, to me. Until we all get CFD, virtual wind tunnels, intuitive airfoil design programs, "feedback" feel built into our radios, onboard flight recorders, and better integrated CAD in the design process, (for a few hundred dollars!), I depend on you a lot.

Part to the issue I think is that most plans show suggested locations for the radio and (obviously) the engine, and that little teeny mark for the CG that the builder probably does not fully understand. This will work well if it is an ARF shipped with the a standard radio and engine combination but may not work if heavier servos, lighter engine, etc are used.

Most R/C flyers are not full scale pilots and have never"felt" the difference bewteen a full forward and a full aft CG location in terms of control pressures required and the general stability of the airplane at both low and high speeds.

And as pointed out above, the normal solution for recovering from a bad R/C flight situation is to add full power - a technique that may work in an over powered R/C model, but not in most full scale aircraft nor in many scale R/C models.

Personally, being a full scale pilot I am a believer in proper CG location and, more importantly, in locating the CG intentionally forward or aft in small amounts to achieve a desired performance goal. A slight change in CG aft can make that previously sluggish model very fun to fly, a little farter aft and it can become aerobatic, too far and it can become uncontrollable.

The location on the plans is a good starting point but if you are serious about flying (full scale or otherwise) you really should understand CG enough to know what it does for stability and performance.

What many modelers also forget or perhaps never knew is that relocating the CG will also require some adjustment of control throws. You really only want enough control throw to make the plane do what you want and no more to avoid a loss of servo resolution and also to make things less twitchy in the middle of what is otherwise a restricted range of usable servo movement.

R/C models do have a very wide CG envlope which is a good thing as most R/C models also carry all their fuel well forward of the CG. A .40 sized 5 pound sport model can get by carrying 10% of it's total weight in fuel well ahead of the LE of the wing, but I know of no full scale aircraft that would be able to accomplish that. It would either never get off the ground due to insufficent elevator or if if got into the air, would eventually become uncontrollable as the fuel burned off and the CG moved past the aft limit. The fact that R/C aircraft can accomplish this still affronts my aviator instincts as an unatural act that should cause the earth to rise up and smite the plane for it's insolence.

Yep - the power to weight - and weight to wing area of good aerobatic models -is light years away from -say a 182.
But there are some full scale aerobats now that simply kiss off most of the "rules for a man carrying airframe".
I recall -as do many -that you can perfectly trim a --say a Mooney -for level cruise - then extend both arms - lean forward - and the trim is gone -
In the new aerobats full or model - the "trim for cruise" is not even part of the game.
The idea is to have the ability to instantly change attitude - as needed at ANY speed or ANY attitude.
For this control throws are outlandish -(never enough)
Power is gross overkill (never enough)
And the human factor -is the weak link- unless you are stressed +- 15 g's-- or have the reflexes of a mongoose.
The CG? -oh about there - let's try it - really not a big deal--

I STRONGLY disagree here !. How familiar are you with world champ level competitors ? I have seen pilots taking with them weights to have the CAP 232 centered exactly to their liking. And it is a big deal to them.....They can make about 20 flights, and have their mecanic change weight and angle of the ailerons counter-balance, and all flight controls, setup the plane with as much minuties as a formula one pilot will do to his car. That's all behind the scene, you don't see it, but it's there !

And there is no comparison between a "show" pilot, like Patty Wagstaff is now, and a World Champ like Catherine Maunoury is....A meeting crowd will never be as picky as a judge is...

Bernard

I mis stated - what I meant to say was there is not a "fixed" cg -from the factory that must be adhered to - The CG can be shifted to the pilots liking -and that of course - IS very important.
Isee many opinions based on information gleaned from doing work on commercial aircraft -which is a field that demands getting best payback out of a cargo/passenger carrying airframe.
On wild aerobats -the criteria is totally different - be thay full scale -that is -man carrying -or just a big ol toy -

I like, and practice myself, HGU 34's response. I never set my CG according to a calculation. I always set it according to a flight characteristic that I want. For my aerobatic planes, it's set for honest/neutral vertical lines. For sailplanes, it's set with a stop watch. In both cases, I probably end up with a CG aft of whatever some calculation would suggest.

Now, for a beginner, how can he tune his model to a particular flight characteristic that he has never experienced? He can't. That's why you get so many questions. It's simply an attempt to expand ones knowledge. All he really wants is to learn the sticks. The technical questions fill in the blanks until the pilot's skill enable him to exploit the knowledge.

How do you set cg with a stopwatch?

I think lots of people are just simply bored with the technical details needed to give their plane the Wow factor they desire, So they go looking for instant gratification from the experts. Kind of like I hate computer software, but I have sincere technical background in electronics hardware servicing, and I enjoy getting around on the internet. I can wade my way through the program stuff, buts its just not interesting. Its not a direct comparison, but you get my drift. That could all change, as one progress's deeper into the hobby, depending on your levels of natural interest.

Then theres the newbie factor, where you just cant absorb enough information fast enough to satisfy your feeding frenzy.

Don't forget the social aspect of just wanting to be in with the cool guys, that seem to know everything. Lots of us are just hoping up and down with joy, that someone so intelligent will even converse with us

I really hope the repetitive questions are not offending anyone that truly wants to share their technical wisdom, its just the nature of the beast. On the other hand, don't feel obligated to answer the same basic questions over and over again, in hope of saving the entire RC universe. That could become a relentless job. Just respond to the posts you feel comfortable with, unless of course its your job, then you should be working at it

Personally, I mis the "will the wind turn my plane" questions hmm....

Kevin

Kevin - you be right - -kinda like the kid with a new electric guitar - the questions are endless - the experts are everywhere - the technically trained offer info that too often translates as gibberish.
Shee -What he really wants to do is not look too bad when he plays Stairway To The Stars
And perhaps -to share a joint.

For thermal duration, you want the CG at the point where you max more often. This is a compromise between an aft CG for straight-line efficiency, and a forward CG for stability. The better and more aggressive the pilot, the more aft the CG tends to be. It takes hundreds of flights and good record keeping. Virtually every serious model sailplane pilot I ever met does it this way.

I really enjoyed reading this. we have a saying at work."Do not take knowledge for granted" If I ever do have a cog question I will know who to ask...No matter how trivial a question seems it needs to be answered. Ever seen an inexperienced r/c'er try and fly an aft C.G. plane? I have it crashed at the feet of a 10 year old spectator.

i recently rebuilt a glider i haven't flown for about two tears, it still super stable and fast, but it seams to want to roll off a ball when you touch the ailerons, i cut the rates down a little for the next flight and it still wants to do it. now, i'm guessing i have the CofG back a little too far, but you know what... i didn't even bother checking it against the plan before i flew it, it just seamed to be about right, i might check it later to see if it's out, but even us that know what a CofG is and what it does don't always bother to move it to change a planes flight characteristics, we just fly it. am i just lazy? can't be the only one like this

Aft cg affects pitch stability. I don't see how it would cause increased roll rate.
If the plane is not flying the way you like, and you suspect cg, how hard is it to add a few pennies to the nose and see if things improve? That doesn't require a lot of work.
With sailplanes when I notice the plane seems pitch stable I remove 0.1 ozs from the nose each flight until it gets more pitchy than I like. Then I remove 0.1 oz at a time until I like it. I do this with planes I have been flying for more than a year. I have read that once you get the cg right leave it alone. But since the 'right' cg is dependent on the flier and my ability change I adjust now and then.
I have set cg on very calm days, then as turbulence builds I need to add nose weight. I was told to reduce elevator throws but that doesn't quite work for me, at least so far.

everything gets more unstable with aft CofG, at least it seams to.., and easy.. well,, 40mph wind, cold, sooooo cold, and to get in the nose i have to unscrew a wing bolt with my numb fingers, pull the wing back against the aileron control rods ( which is a little hard ) so i can get the location peg out of the canopy and remove it. Thats if it dosn't blow away first. Plus i'm lazy and it was flying anyway.
wheres the CofG on a Phoenix Vagrant? the plans in the shed and i can't be bother to go and get it.
JOKE!!!!

Wow! What an interesting set of messages. It was early when I started reading this thread.

As a newbie, let me add in my CG story:

Last spring I stumbled across an RC flying field. I've always wanted to build and fly my own plane, so I dug out a trainer kit I bought years ago, and started working on it.

Mid-October, on a nice sunny Saturday afternoon (rare in western Oregon this time of year), with my kit still "90%" complete, I finally gave into the club pressure to buy an ARF and get into the air.

After $300 and a few hours of assembly and adjustment, I took my Hangar 9 Alpha to the field. Before starting it up, my instructor gave my plane the once over and asked me about the CG and I admitted that was the one thing I hadn't checked. Being an ARF, he figured it'd be pretty close and if it wasn't he'd bring it down and we'd adjust it. He took it up, added a little right trim, and I was good to go.

Nearly two months later, I still don't know where the CG is (I will check it before flying tomorrow just in case it has one of those mishaps I've been hearing about), but even in all my ignorance ... I've been having a great time!

I suspect many, if not most, of the new flyers out there are like me. Since we didn't have to build a plane to get into the air, our ground school education has been short changed, but doesn't stop us from enjoying the hobby and mixing in with the experience ones who keep us out of trouble.

Speaking of CG ...

I've got this trainer at home, that's 90% complete. Since I have a good basic trainer I'm leaning to fly on, I've been thinking of making my kit a little sportier, sort of stikish. Maybe making the wing a little thicker, a little longer chord, using a NACA 63014 airfoil instead of the Clark Y, maybe move the whole wing back just a tad.

Any suggestions on where I should place the CG ...

Cheers!

Cub

The range of permissible (empty fuel tank) CG locations is roughly as wide as ten percent of the wing chord depending on the purpose of the plane, the flyi9ng style of the pilot, the skill of the pilot and, how much the CG shifts with fuel consumption. Where the range of permissible CG locations should be located depends on the configuration of the aircraft. A tail first or canard configuration has the CG range somewhere between the fore plane and the aft wing. A tailless flying wing has the CG range between 15 and 25% of the wing chord from the leading edge.

A conventional configuration such as your trainer (probably) has the CG range somewhere between 25% and 40% of the wing chord from the leading edge of the wing. Exactly where does not depend much on the airfoil but, does depend on the tail moment arm length relative to the wing chord and the horizontal tail area relative to the wing area. The bigger the tail and the longer the tail moment arm, the more aft the permissible CG. Your proposal to widen the chord and move the wing back shortens the tail moment arm as a multiple of the wing chord and increases the wing area relative to the tail area. The result will require shifting the permissible CG range forward, probably about 5%of the wing chord or so.

Instead of trying to take all the various considerations into account quantitatively, put the new CG at 25% of the wing chord set the plane up with lots of contro throw and get an experienced pilot to do the first test flight. From that point you can begin a series of dozens of test flights. moving the CG back a little at a time, retrimming and reducing control throws until you have the plane handling the way you want it. Only make one small change at a time and do not be impatient to finish the job. As your flying skills improve and your flying style changes, you may want to go through the testing again and remove a bit more nose weight. Just don't go too far back with the CG or the plane will become uncontrollable. When the plane goes where you point it without trying to recover pitch attitude on a nearly empty tank, you have reached neutral stability which is usually the aft CG limit.

So, why not go through the flight testing and adjustment routine with your present plane to see if you can achieve what you are looking for without changing airfoils, widening the chord and moveing the wing back? In the process you will be teaching yourself about stability, control authority, CG range, etc. This self instruction will fill in for the ground school you missed and you may end up knowing more than your instructor. Get a copy of Model Aircraft Aerodynamics by Martin Simons and read it when you can't fly and don't feel like building. this will better prepare you to be successful with any design changes you have the urge to make and to help you avoid myths about aerodynamics.

Ollie,

You read my mind ... Model Aircraft Aerodynamics by Martin Simons! Thats the answer to the question I was going to ask. Very good, Thank you!

I'm sort of looking at re-winging my kit as my 4th plane, with the kit using the original wing and a ARF "sport" plane as my 2nd and 3rd. With its flat bottom and significant dihedral, I suspect it will fly a lot like my current trainer, which I'm told I'll be getting bored with pretty soon. The thought of putting a symmetrical wing on it is to (a) let me delve into the world model design, (b) end up with a plane I'll want to fly, (c) save me from having to build a new fuselage because my building is REALLY slow, and I'll officially add (d) fussing with my plane to optimize its flight characteristics.

It's been suggested to me that a battery pack and velco are a great combination for adjusting the CG of an aircraft, with the velco acting like a shock absorber just like foam normally does. Any thoughts/objections to that idea?

Again, thanks to all of you for taking the time to help us new guys out!

Cheer!

Cub

Cub:

A gentleman who flies at my field turned a high wing trainer type into an aerobat with only one change. He asasembled the wing with ANhedral instead of DIhedral. You can do the same. Do build the wing with a symmetric airfoil, but when you join the two halves put the wing joiner in wrong side up. It's that simple.

What you will have done is decrease the "Pendulum" effect of the fuselage below the wing, and when you go inverted the resultant dihedral (when inverted) will add stability. Again, it's that simple.

Now for the CG argument. I have heard no no one mention the Center of Area. Get out a plan view and calculate the true center of the projected area. That is the truly critical point for most rearward CG, ahead, it can be flown. CG behind CA, a fast computer might fly it, but I doubt it.

Why is this the point of nastiness? Consider an arrow. With the fletching on the nock end the arrow flies with the sharp end leading. Put the feathers next to the point and see what it does. The airplane acts the same way. If there is more surface forward of the CG than behind, the minute it is no longer flying true to the wind, the wind will flip it over. If the CG has more surface behind, it has (at least somewhat) arrow stability, and can be recovered after a departure from controlled flight.

Now, before you say "How the H*** do you figure center of area?" I'll tell you about a very simple computer for finding it, a computer with no electric parts, much less any electronics, and it has only one moving part.

Get a sheet of something stiff that is larger in all horizontal dimensions than your airplane, and draw the exact outline of the plane on it, and cut it out of the large sheet. Now find the balance point of the cutout. The balance point is the center of area. Wasn't that hard?

If your bird is too large for this to be practical you can use a smaller outline, as long as it is scaled correctly the CA will be in the exact scale location of the larger original.

When you think your way through the "Arrow" effect you will see why longer tail moments need smaller area in back, and conversely it will become clear why short fuselages can be made much snappier than the long ones, all else being equal.

This entire thread is interesting and educational.

Now a final note on scratch builders using the "TLAR" design method (I'm one, usually). We think "That Looks About Right" because we are accustomed to seeing airplanes proportioned to fly well. When it looks about right we're generally in the same ball park.

My last two scratches were done with TLAR. One is a pain to fly, a standoff Tigercat, the other, also a twin, is a sweety. It can go either way, but the Tigercat is overbuilt. This made it underpowered, the fault is not the shape or proportions. I expect it to be a sweety after it gets its larger engines.

A twin's the way to go, to stay with the flow.

Bill.

OK Bill,

So you're telling me, a newbie, to show up at the my club's airfield with a wing mounted upside down on a trainer ... hmmm ... sounds like a snipe hunt! It doesn't really fit with the TLAR concept either.

Before I give that a try (at least with anyone else at the field), I'll finish mounting the hardware and see how she flies. It could be that putting a symmetric wing on it (current size, location, and dihedral) will make me happy with it. If not, a new wing is just a sheet or two of balsa away...

Thanks for the suggestion.

Cub:

That's a good thought. Build the wing attachment to allow either side up. One way it's a trainer, and if you can stand the razzing, the other way it's an aerobat!

Or you can build it with zero dihedral, and put an amount of sweep on the leading edge.

Believe it or not, a swept leading edge, with a symmetric airfoil and zero incidence gives aerodynamic dihedral. It works because the nose is always up slightly in level flight, upright or inverted.

Just more to think about.

Bt always have a fin, on your flat single or twin.

Bill.

Hi Bill,

I want to clarify what you mean by adding sweep to the leading edge ... are you talking about a constant chord airfoil with both the leading and trailing edges a little farther forward at the wing root than at the tips ... or a wing with a slightly longer chord at the wing roots but with a staight trailing edge?

A two way wing ... that might be interesting ... I'd still have to find a nice cold damp day when no one else was at the field!

Thanks for the input!

Cub