I am fairly new to modelling and have a COG question.

I built and am flying a GWS A-10.

The COG on my plane is a bit behind the recommended location, meaning my model is tail heavy. i would have expected this to cause the plane to porpoise upward to a stall, but in flying I have found it has quite the opposite affect of what I was expecting. the plane wants to nose in with the elevator centered. I actually have to apply a good bit of up elevator to achieve level flight. Is this normal? Are my initial thoughts on the affect of being tailheavy 180 degrees off?

Just wondering:
Basically, what are the affects of having a tailheavy plane, and what are the affects of being a little nose heavy.

From my free flight experiments, I would expect a really tail heavy airplane to nose down. A nose heavy airplane can be retrimmed and flown again. A tail heavy airplane may be beyond repair.

I pretty much throw out the reccomended mfg. specs on the proper COG, rarely are they dead on, with at least the kits and arf's I've assembled in the past. I'd say, toss your's a bit forward, see what happens, and you are getting the same results, then go the opposite direction, there could be something happening with your initial build that requires a different COG in the first place. You may have to add some weight to the nose to achieve proper results.

If the CG is far enough back so that the CG is behind the neutral point then it is likely responding to the airfoil camber and nosing down and when that happens it tries to tuck even more. If you add up elevator to recover I suspect you'll find that it tends to run away in nosing up so that you need to correct with some down suddenly.

Otherwise there's so many other trim items that can cause this same response that it's hard to say without studying the actual model.

But basicaly tail heavy doesn't mean that it will nose up. As the CG moves back the elevator response becomes more lively and as it sits on the neutral point you'll find that all sorts of odd non linear responses can occur depending on the model design, thrust angles and other things. As the CG moves behind the Neutral point then the model can be made to diverge from normal flight in either direction and it won't recover on it's own. Airfoil camber can easily cause it to respond by nosing down like your model is doing.

Just how far is it behind the recomeneded spot anyway?

An airplane has an aerodynamic center. It's called the neutral point and it's where all the aero forces equal out. Think of it as the hinge in a playground teeter-totter. An airplane actually is a teeter-totter that has aero considerations on both sides AND weight considerations on both sides of the teeter-totter.

As the CG moves forward from a balanced location, the horizontal tail is called on to carry that force. It is done with elevator trim. But it's a load on the elevator no matter what. And the elevator has only so much aerodynamic strength. So moving the CG away from the NP actually takes some of the elevator's strength and that winds up reducing the elevator's ability to steer the airplane in pitch. And as you move the CG farther forward, the elevator becomes less and less able to pitch the airplane. So your model feels more and more sluggish in response to your elevator input from your TX stick. Most inexperienced flyers interpret this as stability. They think their airplane is becoming more stable as the CG is moved forward. Actually, the elevator is simply losing authority.

Move the CG forward and the weight represented by that CG is down on that side of the teeter-totter. And the tail has to push down on it's side of the teeter-totter to keep the thing level. And in actual fact, with the CG force downward and the tail force directed downward, there is a real load created that the wing is going to have to carry. Most model flyers can't tell it's there, but the wing knows it's there. And the airplane flies with even less "agility".

Move the CG aft from a nicely balanced location, and the elevator gets more effective. Less and less throw is needed to pitch the airplane. Most inexperienced flyers interpret this as instability. The elevator actually needs less deflection to pitch the airplane, and often needs less than the throw it has at 100% of your TX stick movement. If the flyer would simply tune the elevator to move less on the airplane, he would have a more efficient flying airplane that wasn't elevator sensitive.

However, at some point in moving the CG back, the elevator becomes too sensitive to be controlled. Move the CG back even further, and the weight of the airplane has moved across the hinge point of the teeter-totter and it's force is in the same direction the elevator works. And that doesn't work for anyone.

I very much beg to differ on many levels, very much agree on others, so hear me out.

You are wording this upon a very 2 dimensional plane, no pun intended. You do not take into account intertia, power systems, the microscale of RC verses full scale, material densities in construction interacting with enviromental, which really becomes much more pronounced in the typical park flyer.....

Yeah, those new to this should learn, but man, not by someone quoting full scale theory, it has almost no place at all for the average pilot in this section of the forum, go to giant scale, it holds more weight, but not here at all....

edit: ............ removed as simply argumentative........... your opinion of other posters has no place in this or any topic.............

edit: ............ removed as off topic ............. critiquing others' content and style is off topic ............ when it's sucking up, it's off topic, when it's negative, it's probably going to "insult or embarrass"..........

edit: ............. removed as insulting ................

I'm not flaming you buddy at all, it's just, you missed some important elements that really need to be included with what you said, that and wording you used is incorrect along the matter with "inexperienced" as stated before, I'm flying prototypes so have a little under my belt in this area.

edit: BTW, not a single word in this post addresses the topic or the question that was asked originally.

Please resist the urge to curse, flame, degrade, insult or embarrass someone in your post. We encourage the free flow of your ideas, but believe that they can be communicated (and received) much more effectively if you keep things civil. If you have to vent, take it offline. We carefully monitor posts and will ban individuals who engage in offensive conduct within the forums. Thanks.

Thanks for all of the feedback. My CG is actually about 3mm behind the aft-most manufacturer recommended location. The plane seems to fly really well, except it requires a bit of up elevator to maintain level flight. I guess it must be something other than the COG making it want to dive. The thing is so much fun to fly compared to my glow model that I really don't mind having to trim in a bit of elevator. Again, thanks for the feedback.

You want to differ, go to it, but address the issues and topic, not the other poster. Your opinion of other's experience and qualifications have no place in an open forum that has a clear directive directly above each Reply box. Where you think he should place his posts is off topic and will almost always insult and embarrass.

If you think important elements were needed, then provide them. You don't like another's wording, then carefully word your own, but keep those opinions to yourself.

Please resist the urge to curse, flame, degrade, insult or embarrass someone in your post. We encourage the free flow of your ideas, but believe that they can be communicated (and received) much more effectively if you keep things civil. If you have to vent, take it offline. We carefully monitor posts and will ban individuals who engage in offensive conduct within the forums. Thanks.

Having read the question about the GWS 10 -
the owner noted :
the cg was slightly aft the recommended point
the elevator trim was up
the plane flew fine.
question: is it tailheavy and how do you tell.
It seems he expected the elevator trim to be related to balance
it is not . on most of these models the setup is about 0-0-0 so some trim is expected
also these do not have light wingloadings the models are light the loadings are relatively high
so they do nose in at low speeds as they simply,
fall out of the air
It helps to have flown some of this stuff now on the market - it is quite different (the small foam electrics) as compared to powered models of years gone by


you can buy/ make an electric made from flat foam with 400 sq inchs area - weighing typically 16 oz ready to rip
or buy a model same size all built up in a very lightwight fashion rivaling the best old stick and tissue models from years gone by - and these weigh - 24 ozs
both fly very well but to expect the latter one to fly like the foam one is just dreamin.
the lighter one can have cg so far aft (50%) that some would say normal flight is impossible
in reality they fly quite well --slowly ,. and there are a lot of flyers now who are used to this kind of setup -and they try to apply it to heavier models -
but this aft cg applied to the 24 oz model won't work at all.
anyway-- cg issues are beat to death and unles you look at the specific model - all this proven theory stuff is meaningless to many flyers.

Dick,
Thank you for the very informative reply.

I am completely new to foam, but am enjoying the experience to far.

I bought and assembled the GWS A-10 pretty much stock with only 7.2 V nicad packs. After reading a few posts and reviews claiming that the model barely flies, I was a bit concerned. However, my experience with the model has been great. It flies very well and seems reasonably easy to control. Doesn't exhibit any bad stall tendencies. My only complaint is that it is extremely fragile, but 5 min epoxy works so well and quickly that repairing even the most severe splits is easy, cheap and quick enough that it is ready to fly before the batts can charge up.

I think I am going to be flying mostly foamie EDF's in the future, so your post is very relevant to me and I appreciate the knowledge shared.

Thank you.

DaRock, if you can't handle being one of the members here, don't post, nobody likes a moderator that's overbearing, you went too far, there was valuable information you just covered up the poster could use, you are now on ignore.

This post has provided me and most likely others, with a wealth of information.
Following up with relationship of,cg of airplane,ctr.of lift and neutral point to each other in determining where to balance the airplane relative to the m.a.chord would be most helpfull.
What say you,DAROCK-B MATTHEWS, et al.
not the sharpest tack in the box,
Fredsedno

Fred, you can't take raw theory and expect it to apply to the real world, we have so many more variables entirely because of scale that can effect it as well. The scale alone brings along it's own set of principals that only apply to this, once you move onto giant scale, it's another, more traditional scene and you can whip out your textbooks and spew off what ever is said. There is still largly much being innovated and explored in the venue, just please, don't be turning your plane into a lawn dart because you are thinking full scale theory alone.

Honest, about all that means is that your model has it's CG behind where the mfg suggests you have it. The mfg's suggested CG is sometimes recommended for amazingly different reasons than those based on sensible design practices. One of my recent ARF manuals used the CG location and surface throws descriptions from an earlier manual..... of a different model..... that wasn't even the same proportions....... no lie

How your model flies is a much better indication of whether or not it's tail heavy.

The way the plane responds is very often a result of where things like the elevator are when you launch the sucker. Or how correctly they're tuned to that model. And the one thing we have to do, is take what the airplane did as being "true" compared to what we thought would be "true". If it doesn't act the way we think it should, then we're either wrong about why it's acting that way, or we're wrong about how things that're that way act.

Having the elevator centered isn't a dependable indicator of anything lots of times. Where it winds up after you've trimmed it so the airplane flies level can be a good indicator, but until you've sorted out your particular model, where stuff points can be the luck of the draw. How the workers glued stuff together and where they put it and how much it weighed versus what it was supposed to have weighed are just a few things that can make your particular model act "funny".


Maybe so, maybe not.

If you're not sure of a CG location and wish to verify it, an excellent way to do that is with the very easy to use geistware.com application. It's easy to use and the computations it does (all you gotta' do is measure some things like spans and chords) have been proven over and over by the airplane industry that designs the planes, and the industry that uses them. http://www.geistware.com/rcmodeling/cg_super_calc.htm

If the CG checks out then look at the airframe to see if the wing is seated completely, the horizontal tail is straight and square and seated completely, the engines pods are aligned, the motors are square within them, the surfaces move the right amount and direction.

Our models don't have really any magic in them that hasn't been found and measured and documented. We do have to find what's wrong when they don't fly right, and sometimes our first guess isn't the reason. But it's a good place to start looking, and looking the right way is the best way to start.

This is a kewl hobby that's fun when things go right, and when they don't, it's kewl that we have a chance to learn. And sometimes things aren't like we thought. This also isn't a soundbyte simple hobby. But there are good tools that help to understand it.

Thanks for the info and the link. I will definately check it out. Not for this model, but I am planning to try my hand at a scratch build from foam or coroplast, so it will definitely come in handy.

I checked the stab and it is properly squared. The nacells are as close to spot on as I can get them with this foam. The only thing I'm not sure on is the wing camber, but since the model is flying well I don't think I'll bother experimenting with shimming it.

So I guess the short answer is:
Since my model flies well once I trim in a little up elevator, then everything is fine and I shouldn't worry about it. Just trim in the required up elevator and get out there and fly and have fun. It's just a park flyer foamie, not a pattern plane, so if it flies well for me, then there is no issue.

The little thing actually flies really well considering how underpowered it is. It will loop out of a dive, but falls out of the loop if you try to go from level flight (obviously an effect of it being underpowered). It rolls well, but does tend to dive excessively when inverted, but I would expect this given the wing design on the GWS A-10. It recovers quickly from induced stalls. It flies so slow and stable while still giving me reasonable control given the limited airflow over the control surfaces with the EDF.

Thanks for all of the feedback and info gentlemen, you have been a great help.

Good idea. But it's not simple. Actually, very little of what we do is soundbyte simple.

There is a good diagram of those things at the geistware.com link I listed above. You can actually play with that application and get a feel for those things. And see how they change relative to each other. When a guy is designing his own model, he very often uses what geistware.com does so easily for you, but he uses it "backwards". He changes the spans or areas or whatever to see how that affects the stability. anyway......

I've got more than one BOOK that explains those relationships. The explanations work for almost everything that flies, from my parkflyer to the C5As that were being built over my head when I worked at Lockheed. Some things like flat foamys have some unique quirks, but for the most part the basics do apply if not perfectly. The basics..........

An airplane with a bigger wing than tail will have a neutral point that's more toward the bigger part. The smaller part is there to stabilize the bigger wing in pitch. For the tail to have a chance of doing that, the CG has to be ahead of the NP. How far ahead it is affects how much work the tail has to do to keep the wing stabilized. Not far enough, and the tail has no hope. Too far ahead and the tail winds up wasting all it's effort carrying the artificially imposed load caused by the CG pushing down on it's side of the NP. Move the CG from it's safe aft location toward the safe forward position and the elevator usually acts more and more sluggish and most people call that more stable. It's usually just an elevator that's being used up carrying a nose heavy CG around and having less deflection to cause pitch changes. etc etc

There are a number of really good model airplane books that tell all about this stuff. The LHSs in my area carry all of them usually. A couple are about trimming for flight. A couple are about designing models. Leaf through 'em and see if they might not be worth the time and money. You won't be learning flying field BS and wasting time.

Kewl.................
You know, your thread has caused me to look up that A-10 on Tower. Best info on anything there, but not quite enough this time. So I HAD TO DRIVE down to the best hobby shop in the area JUST to check it out. (Jeez, it's awful to HAVE TO DO this stuff.) After seeing it, I WANT ONE.

After seeing it, I have no doubt that it would be tricky if it was the least bit underpowered. And I'd bet it would have to be dialed in on trim or it'd be a handful. I've got a couple of the little guys and they were snakes until they're sorted.

Thanks for pointing out that airplane. I really need another one.

Been fun talking with you.

BTW, I forgot to look closely, how hard would it be to upgrade the power?

(And any way to put a cannon under the nose....... hehehe)

It should be pretty easy. The EDF 55 version holds to the motor shaft with set screws, so as long as you chose a can that will fit in the EDF unit and has the proper shaft dia, it should be a breeze.

If your thinking about building one I would recommend:

Buy the newer EDF 55 version, not the older versions, as they are extremely underpowered (from reading, not personal experience).

If you want to upgrade the power, do it before building. the nacelles are not going to be disassembly friendly.

Build without the landing gear if you don't mind belly scratch landings.

Reinforce the nacelle mount points with a tube through the fuselage that protrudes enough to also seat into the nacelles.

Reinforce the rear horizontal stab with a rod of some sort. Without reinforcement it is too fragile.

Mount the elevator linkage wire in some type of tubing glued to the fuse. GWS just has it running through a gap in the fuselage and over time is widens and gets really sloppy.


If you plan to upgrade the power, reinforce the wing with something stronger than the provided bamboo rod and consider extending the reinforcement further out the wing.

just as an example of what you might try -- here is a foam model, under 7 ozs per sq ft (430 sq with huge power a 480Eflite) and using simple paper tape and CA to assemble and to attach parts
lightweight servo extensions allow the servo pushrods to be 4" long -all of em.
the model has a flight speed envelope of zero to rocket speeds and is extremely easy to control even for a beginner
th secret ?
light strong easy to repair - with something like this you can easily sort out the questions of cg and how it affects flight --without having a model which is a drag to fly. (It is now painted and so is a lot better looking ) as a bonus -you will discover that the flat plate wing is the best shape for low speed flight at low wing loading
I whacked this out of two sheets of 6mm Depron -in a couple of hours .
One other secret -
all of the years of hard work trying to design best airfoils -- for powered craft -- was simply because power plants weighed too much for the power produced
doubt it?
look at modern fighters ---

Actually for the basics like this you can use the raw theory. While I agree that our selected size affects some things the nuetral point calculation and resultant CG selection is pretty cut and dried and scales faithfully. The only difference being that for full size stuff they insist on a far larger %of stability than we do so full sized stuff often will call for much more forward CG's than us modelers use. But that is a choice and has nothing to do with the physics of scale.

There's some MAC and Neutral point calculators linked in the sticky thread titled Aerodynamic Tools, Calculators and test links. Check 'em out.

I agree, raw theory is a good starting point, it's better then nothing at all. I'm always fiddling with different configurations, both in scale and design and find some pretty neat results at times. With new pilots, I tend to tell them to place it forward of the reccomended COG, this helps them with stablity, as you stated, but it's also fun to place it slightly aft, you get to really push the envelope as to what acrobatics you are able to pull off that would be more difficult if it was dead on. Then you get into 3d land where it's rarely set for stability and unpowered, tend not to glide so well.

Thanks all,
The information you have put forth is what I was looking for. I now have documentation I need to ask the boss [wife ] for pocket change to order Andy Lennon's book,unless you more knowledgeable guys have a different favorite.

Still not the sharpest tack in the box
Fredsedno

Lennon's book is one of the best.

I've collected everything I've ever run into.
There are a number about model design, and some are sort of loose on the design side, some that aren't up to date, and some that are just a mess. Lennon covers all the theory that is useful to us. In enough depth to be helpful. And illustrates things using his own designs. And he's got a bunch of very different designs.

If I could keep only one, it'd be his.