The links above talk about it, and it's basically the same principles as in riding a unicycle - if a guy is riding a unicycle, the CG is hardly ever directly over the point of contact with the ground. The forces generated by the rotating wheel are enough to dominate small shifts in mass and the thing stays balanced. I recall from dynamics class that it's the same math as ice skating where there is an infinitesimal point of contact (in theory). You can be forward or back on your skates and still stay up, like one would on a unicycle or like the airplane stays properly balanced.

The point being that none of these machines need a stab to keep them balanced and upright when the CG is within some reasonable range - so there should not be any load on the stab, and CG should not be trying to weight the tail down to increase the attack angle of the wings.

Hi Joe,
The tail stats,, and efficiency that one enters do change the CG result it produces.
However all I was saying is that most people refer to their cg as a % of MAC or an actual distance. That tool requires you to enter a static margin figure to work.
So using your own data you have to use 25% for the static margin (which is overly forward/stable) to get the 167mm result.
167 is 33% of MAC which isn't exactly a forward setting.
I'm not saying the tool is wrong, it just looks from a perspective not normally referred to. It could add to the confusion that all this jargon sometimes causes.

Brian

Hey Brian,
The CG that the tool gives does change, but MAC does not. I don't know where all the numbers come from so I'm not using anything from those tools except MAC right now, and I still have to look at things to make sure I trust what it says and where it's measuring from.

The description pages say the 5-15% static margin should end up around where the MAC is. The static margin is not a location back from the LE, it's a distance forward from another calculated point, behind the MAC.

Note that the MAC is given at the center of the fuse, and the root chord length it is asking for is measured there also. My measurement was at the side of the fuse so I have to redo it.

But I think we are on the same page :)

Ok,
MAC ;

Brian

Edit; Joe we were posting at the same time.

Exactly :)

Joe, I'm not sure you are following me. The load on the stab is not there to increase the AOA but to maintain it. I'm not really getting your unicycle comparison because with the rider leaning forward without forward motion would fall forward. Likewise with an airplane with a forward CG would continue in a nose down arc unless an equal force is applied. This force is your up trim. So say your airplane weighs 11 lb even with a forward CG. At cruise speed the up elevator applies 6 oz of force on the stab so now your wing has to support 11lb 6oz while in level flight. Moving the CG back reduces the needed force to maintain the AOA needed to support the weight of the model. Unless I am missing something here but in the past 36 years of flying R/C a CG change has always required a trim change.

Hi Brian, I believe Joe is correct, the reference I gave calculates the MAC and you can easily figure out where to put the cg based entirely on percentage of the MAC, if you don't want to consider the stab. In fact that is what I did as I didn't have a feel for the stab efficiency. I believe I also used your reference and got the same MAC so I figured they both had it right.

I also found moving the cg had little effect on the trim, until I got in the right ballpark. I was way nose heavy with the battery as far back as I could get it, and yes it flew like it was very heavy. I started thinking it didn't have enough wing area, but it turns out it does.

I suspect we can go more than 30% before we would get in trouble, but I am down to no rudder to elevator mix and no throttle to elevator on up lines so I'm pretty happy. I'd like to get rid of the rudder to aileron mix on knife edge and I still suspect vertical cg is part of the problem. However, Chip says all Axiomes need this mix. Do you guys agree?

Jim O

I understand what you're saying but am after a different approach.

For above, when the unicycle is moving small shifts in balance of the rider have negligible impact on the machine staying up. The rider can move around to a certain extent and stay up, without needing a rear wheel (stab). Another example is a regular bicycle. You don't need really good balance to ride a bike. Once the wheels are spinning, the forces generated are so much bigger than small shifts in body weight that the bike stays up basically on it's own; the rider just needs to keep it straight.

There are stability equations that explain all this. The small changes in the CG are basically treated as zero because the forces of lift and drag are so much greater. I've been moving my CG around for 4-5 days and have not been able to change my stab trim by a single click.

I know there are airplanes out there that require load on the tail to fly straight. However I don't intend to set my airplane up that way, and I don't believe that is an ideal way to setup an airplane. There is some talk about that in the Pilot's Encyclopedia of Aeronautics too. It explains it really nicely. If your CG is forward that it pushes the nose down and needs a downward force on the tail, it is really nose heavy. It feels stable in some aspects because now the forces on the nose and tail overcome other stuff. control surfaces feel less touchy because they have to work harder to be effective.

Anyway I think I'm on the right track and have the resources I need to get where I want to be. The links above were really helpful, so thanks :)

Hi Jim,
Yes , was just trying to stay away from stuff that clouds the issue.
On re-reading some of the posts I think that maybe MAC is being confused with AC !!??

As a matter of interest what % of MAC did you end up with and then using that tool what static margin needs to be specified so as to derive that ?

Brian

Brian is correct

working out the MAC on just the wing, is all thats needed to get a accurate placement However there are other factors involved in where the plane can use a specific location like Engine thust, and wing inc. they all three work together to make the flight envelope Happy.

The biggest misconception is you can force the wing to lift more than it can, either by adding inc. or adding tail weight. You have to work with the effeciency of the wing, Every wing has a limit, finding that limit gets you on the right track. This limit sets the CG bounderies and the reason you can move the cg from 25-33% with little change in trim. The less inc you use in the wing the more CG affects the elevator trim , because thats when cg affects the flying OAO of the wing, causing a need for elevator trim to keep the wing flying level, to lift the 1G load. Wing inc. is a lifting force, no inc in the wing, no lift. This is also why some don`t understand that the wing is always lifting in up or downlines it never stops working. Down thrust is positive inc, rear cg, is positive inc. and Positive inc. is positive inc. :) how to use the correct positive inc. to fly the model precisely is where my trim guide helps. The correct positive inc. adjustment must be used.

The reason we see excessive right thrust, ( more than .5) or excessive down ( more than 1.0 deg) is because we exceed the limit of the wings effeciency.#1 ( cg too for back for the wing inc. requires right thrust) Or #2poor wing effeciency ( not enought inc, or the need for down thrust) simple as that. This is where the rudder to throttle mixes and throttle to elevator mixes are required. This is not a bad thing , just an inclomplete trim or a design on the fringes. The cg range is very critical in setting the flight envelope of the airplane,Finding that range is the magic.

Catagorizing the adjustments in the proper order, and compartmentalizing the laws of the adjustments or ,( finding what each adjustment contributes) to the proper flght requirments is where my trim guide takes out the confusion.
However , there is entry,or Basic trimming methods and advanced trimming methods, Such as what it takes to trim an airplane to fly Sportsman, or even just a good knife edge ,and what it takes to fly unknowns. To that point, Experience and Skill seperate the plane deficiency, from the pilot or trim deficiency. This, makes it hard to help everyone.

Some think a pattern plane should fly 45 up lines by them selves with no input , or roll to inverted with no rudder needed. or fly hand off inverted. These are missconceptions perpetuated by so called experts of days gone by, Input and Skill is still required to fly a precision Pattern plane. Years ago we got away with running the cg at 40-43% MAC not any more the patterns are too complicated and demanding, and the speed of flight has been cut in half.

The next problem to work out is how to accurately check the CG , to be able to duplicate the standard and make meaningful adjustments. without a solid standard your spinning you wheels. First find the zero line This is easy, use the stab set to zero trim,
Or if your flying my design use the conopy base. Make a stand to put under the rear of the plane to rest the plane on at Zero.
Put the airplane together on the bench, find a spot to lift the airplane , and mark it in increments of 25-32 % in 2% scale lines.
Here I use the flange on the canopy base and reinforce this area for lifting. I use a carbon tube spread across the flanges,pick the plane up till it perfectly balances and you can set it down with the wheels and tail touching at the same time. That, is your actual CG ,,Now you can procede with confedence and make accurate adjustments. This ,is the most important step in the trimming Process.

The CG trumps All then the wing inc. Then Thrust , Get these out of order and you will chase your tail forever!

Bryan

put a

When setting up my designs, I use one of the popular calculation tools on-line to get the ball park cg figure. It's a reasonable starting point. For example, for a typical (for me) wing area, stab area, moment arm (ac to ac) and 15% static margin, the tool often returns the "IDEAL" CG AT 32% mac. This cg location will typically push to belly in knife edge flight. I have to trim quite a bit more to minimize the push. I get close to zero mix but find my planes can use a couple % mix so I add it and go play. I don't sweat the small stuff...2-6% is very small actual movement, so it's no biggie in my book.

Brian, I'm going to have to go back and do some work to answer your question. I printed out the data for 5% and 15% static margin for the Axiome, and it would suggest I could go further back yet with my cg. (This is with std. stab efficiency). The problem is I'm at 33% of MAC now and I'm not between 5 and 15 on the static margin. Doesn't sound right to me but maybe that's what happens with these swept wings. Anyone know where CPLR's Axiome cg is/was?

Jim O

Hi Jim,
It may be the assumption in the tool re the tail.
Changing from std to low changes the outcome significantly, though I have to say I've not looked at the math it's doing.
The math in the Palos club tool is fine - and straight forward.

Brian

' Brian is correct '

Hi Bryan,
You just brought one of my favourite movie quotes back to mind.
It's from The Legend of Bagger Vance, and is by one golfer commenting on another's good shot.
Anyway it goes like this;
' Even a blind squirrel finds an occasional nut. '
:)

Brian

I am never a fan of using tools without knowing where each calculation comes from. The MAC is pretty straight forward, but when you get into the safety factors and stab stability, it's not worth it to me to risk my investment on someone else's assumptions.

People give me a hard time at work for this all the time. Then I find a flaw in their tool and show them. It is better to figure it out and learn for yourself the first time, so you know what you are doing and can make your own changes. Then you can figure out if the tool is good and use it the next time.

' Even a blind squirrel finds an occasional nut. '

Brian ,That includes all of us ,, you know what they say about border line insanity, and expecting different results from the same experiments. :) Well ,I resemble those remarks lol

What we must all remember no matter the design the Wing is the Heart of any adjustment and when we try to compensate or "force"the wing to do a task it`s not capable of, that`s when we see the mixes. The harder the task to more the mix. It`s why why I`m asked to help trim a plane I ask what are the CG Location, wing Inc. and Thrust and then all mixes in the radio. If I get a inconguent reply,(or a mix that does not make since with the airplane setup) I find out if he`s hiding a mix from me, Hiding the fact that he actually measured it, or Hiding the fact that he knows how to measure it :). Only then am I able to give the advise,and find out where to plug some knowledge in, or just give the answer to him ,and hope he can use it correctly.

All Pattern airplanes respond the same, the feed back gives me insight to the weakness of the design, or the weakness in the skill to set it up correctly.

So I`ve had plenty experience in stumbling on the truth :)

Bryan

Bryan,
Perhaps not quite at the criminally insane stage but if ever they start locking people up for OCD , we're fcuk'd. :)

Brian

CG moved forward, tuck in KE and with rudder input is 95% gone. Very slight pull in verticals but almost perfect. I've moved my CG a total of about an inch or more and still no change in elevator trim and my resolution is pretty high (short servo arms, long elevator horns). Sensitivity of control surfaces is going back down and I can feel the difference in the way the plane flies - personally I liked it better a little AFT but I'll take this over the pulls and tucks any day :)

I had no idea how sensitive CG was. Small changes make a difference. Before this I never really had a full set of matching batteries and am now realizing how important that is. I feel like I've flown 3 different airplanes and all I did was move batteries around the tray, maybe about 2 inches total shift.

Hi Joe,
That is one advantage with E power - having that big battery to move around.
Rule of thumb; ( depending on batt,, size ) The batt,, is from 20% to 25% of the total mass. So moving the pack say 10mm = a CG move of 2 to 2.5mm. (maybe not as much as one would think)

Moving your pack forward 1 inch = 3/16 CG forward.
When I'm testing (searching) like you are right now I make big changes first so as to get a distinct change - say move the CG 1/2 inch at at time till It's too far.
Then you get a zone to zoom in on to make fine adjustments.

Good little thread.

Brian

I ran the calculator with different static margins until the cg matched my actual cg and it came out about 17%. This is with the stab efficiency set to low.

I believe the cg calculator looks good for finding the MAC and the AC but is suspect when it comes to the neutral point. It can calculate the tail volume coefficient with the data supplied, but needs to make a bunch of assumptions regarding lift curve slopes and downwash angles. I ran through the calculation making my best guess assumptions including a stab efficiency of .5, and came up with a neutral point an inch forward of what the tool calculated. It is still way aft of the AC suggesting I still have a lot of static margin. The tail volume coefficient kind of dominates the results and maybe we shouldn't be surprised as we do have long tail moments and now relatively small wings on our models.

I'd have to make a major change in the battery mount to move the cg back any further and I don't want to add any more tail weight so this may be the end of the discussion. Then again I'd sure like to see how it would fly with a lower static margin like you'd expect and want on an aerobatic airplane.

Jim O

Edit: I just noticed that my scratch calculation of the NP is nearly right on the money if I compare it to the tool's calculation with low stab efficiency. In other words the difference between using standard and low changed the neutral point by one inch which would change the static margin by 7.5%. This kind of makes me believe it works.

I base my CG off actual balance points, not by what I move. The numbers I gave were approximations. I'm more certain about the CG shift than the amount I moved the battery - actually, it is well over 2" because they were overhanging from the battery tray, so it is 2" plus the overhang, so maybe 4".

But the CG changes I marked on the wing. Each flight I shifted the batteries back a little until I got to where I felt the flight characteristics were deteriorating. In this case I was getting a bad tuck in KE and the plane was diving a little during rudder corrections. Both are characteristic of a rear CG. So that set my rear limit, then I worked forward until I was happy with what I had and went one move farther to prove I was where I wanted to be.

It is a nice advantage of electric. It's very easy to make small adjustments. One downfall is that you need batteries of equal weight and need to be sure they are in the same place each time. I have not implemented anything into my plane to make sure that happens yet, but I should.

I just lost one of my 5s packs so now I'm wondering what will happen with an eventual change to larger packs, and how the extra weight will impact flight characteristics.

Brian, I think you may be right and I think definitions vary depending on the authors. It looks like the AC (wing AC) in this cg calculator is defined as the point on the center line corresponding to 25% of the MAC. I always believed the AC (aircraft AC) and NP were the same thing, that is, the point on the aircraft where a change in angle of attack produced no pitching moment. In the missile business we used center of pressure instead of neutral point. I find all disciplines have there own confusing lingo to guarantee we commoners need them to solve our problems.

Jim O

For those few of you that really want to understand how to balance an airplane, I thought the following article might be of interest.

http://soartech-aero.com/Design methods Update.htm

Jim O

Jim,

While that article is interesting it refers to pitch stability. I think we all know that it's possible to fly a typical pattern model with the CG well behind the optimum for our purposes but it's the directional stability or how "locked" it feels that suffers.

The reason for this I suspect is the recent trend to bulkier fuselages without a corresponding increase in vertical tail area. What is interesting is that the very latest designs such as Chip's new model seem to be addressing this. Perhaps with this particular design aspect correctly tackled it would be possible to consider a running less pitch stability.

Malcolm

High pitch stability is for trainers...LOL!

And yaw stability can be "simply" and "easily" achieved by lightening the tail and bringing the wing fore......