feihu

Jim:

The exactness of the CG setting is really of no consequence so long as it is within the flyable limits of the CG range; that's why the fingertip method works fine.
The pilot of a General Aviation airplane or a Jet Transport (and some RC pilots too) could care less who or how many people get aboard (which causes a change in the exact location of the CG) so long as you're within the flyable CG range.
I use math in the design phase to eliminate the need for unwanted ballast.

feihu

RC Specialties

You are absolutely right. There is always a most forward and most aft cg limit (cg range). Loading can vary to keep the cg within that range. However, in RC airplanes the real only variable is fuel loading. The empty weight cg can be an exact one. It can be adjusted to get the best flight/performance for the full fuel to empty range.

Jim

buzzard bait

Most formulas assume you know what CG you want, and that the problem is compensating for sweepback or taper or whatever. But those things are usually common sense and easy to deal with. The real issue is where the CG should be. To find that I've been using the following formula for about 25 years and I still surprised at how well it works on a wide range of designs:

tail area/wing area x tail arm/wing avg chord = TV (tail volume)

The 'tail arm' can be estimated as the distance from the leading edge of the wing to the leading edge of the stab, taking into account taper, sweepback, etc.

then CG = 16 + 36(TV)

That gives the CG as a percent of the chord. It's all based on "tail volume", which tells you how much power the stabilizer has to keep the center of pressure in the right place. If you think about the formula, you can see that a wide wing chord requires a more forward CG, all else equal, and a long tail arm allows a more rearward CG, for example.

The first time I used it was on a glider that seemed unstable in pitch. I stuck two quarters in the nose and it flew fine. The puzzle was that it had been balanced according to the plans. But the formula showed that the CG on the plans was wrong, and the quarters moved it to the right place by the formula -- and by how it flew. I've used it on every one of my airplanes since.

I've never found another set of plans with the CG too far rearward, but I have found that many have very conservative CGs. For example, the CG on the Balsa USA Stick 40+ was a full inch farther forward than the formula predicted was necessary. By that time I had so much confidence in the formula that I made the very first flight with the CG well aft of the position on the plans -- it flew fine that way.

Many times people complain that a certain design needs nose weight to balance, but in fact, none is necessary if you know that the CG can be farther back. You can adjust later, but it is much easier to know during the building process.

Jim

Mitsu1

My Feedback: (13)

I'll have to look up that April issue of MAN. I probably don't have it, because I don't remember seeing that article and I subscribe to FLY RC magazine instead of MAN. I too like knowing exactly where the actual "0" balance point is, because when I go thru the tedious placement of all components, I want to know that I won't be having to start all over again to get it right.

RC Specialties

Jim,

Very interesting. Do you measure from the nose or the end of the prop shaft for a glow plane? I guess it really wouldn't matter as long as you use a constant. I will have to ponder this a while. Great concept, I will have to try it.

Jim

buzzard bait

Sorry, Jim, it's hard to explain without a diagram. Conceptually, the tail arm is the average distance between the wing and the stab. The simple way is to measure from the leading edge of the wing to the leading edge of the stab.

For a straight, constant chord wing, of course, it is easy. But the stab is almost always swept back some -- just take about the average point, but a little closer to the fuse. The concept is to measure at the point on the leading edge where half the area is on one side and half is on the other (the 'MAC'). You can calculate it, but I always eyeball it. Same with the wing, if it has taper or sweepback.

This formula is in many books. Gordon Whitehead, in his book on scale models, has a different version that is algebraically the same.

Gordon points out a nice use for the formula. Say you want to build a WWI model that is notorious for flipping on its nose when it lands. Using the formula, you can calculate how much to enlarge the stab in order to move the CG back, say, a half inch. It is much less noticeable than angling the gear forward.

If the wing and/or stab are elliptical, what then? I take a piece of cardboard, measure it, and weigh it. Then I trace around the wing or stab outline, cut off the rest and weigh again. The area is proportional to the weight.

Jim

RC Specialties

Jim,

That clears it up a little. When establishing the cg, do you compensate for the fuel loading range. This will always move it forward on RC aircraft unless the tank is mounted on the cg. I have been guilty of adding a larger than specified tank and not considering that when full the most forward cg limit will be exceeded.

Jim

buzzard bait

Jim, I have not compensated for fuel, so my planes take off more nose heavy than what I calculated and measured. But a little extra weight up front is safe -- the goal is to make sure the plane is still balanced OK when it has run out the tank. Jim

RC Specialties

I agree. I always set up my warbirds a little nose heavy. Thanks for the info.

Jim

rcairflr

My Feedback: (44)

D x Wa/ Wt= Distance CG will move

D=Distance from CG weight was added
Wa=Weight added
Wt=total weight of airplane

So if I add 8 ounces 10 inches aft of CG on a 25 pound airplane. 10 x .5 pounds/25.5=.196 inch change in CG

Try the above formula, it's what I use and works great. The information came from a copy of 3D flyer magazine. Hope this helps

I only use this to see how much my CG will change when and if I have done a modification to the weight of the airplane. For me I will continually make small changes in the CG of my airplane over numberous flights until it fits my flying style. At that point I don't care where the CG is, just how the airplane flies.

RC Specialties

rcairflr,

You are correct, a change to the cg can be calculated if you have a known cg. In this case let's say 18.0 inches aft of the datum line. Let's say the datum is the end of the prop shaft. If you add .5 lbs at 28.0 inches the following would be the result. I converted to ounces for the example.

Weight x arm = moment

Moment Arm WT
7200 18.0" 400 OZ. (current CG)
+224 28.0 + 8 oz.
__________________________________________________ __

7424 18.19" 408 oz.

Adding 8 oz. at 28.0" aft of the datum moves the cg .19 inches aft. The important thing is to have a fixed datum to do you calculations from.

Jim

timothy thompson

My Feedback: (1)

it is simple physics! and it is important info to know. this is a result of the arf age, 30 yrs ago we all knew these tables now its balance to the manual. ive found many manuals wrong

rcairflr

My Feedback: (44)

Ignore this post. Made a mistake. See next post

rcairflr

My Feedback: (44)

CG is usually measured as a % of MAC ( Mean Aerodynamic Cord), the MAC of a wing can be figured out by squaring the wing as depicted in enclosed drawing. Once you have your MAC it is just a matter of deciding what % is desired for CG. On a aerobatic mono-wing airplane 28-30% is a good place to start.

buzzard bait

This is what I was trying to explain. Every magazine seems to have an article on the MAC calculation every couple of years. But if you understand the principle -- the point on the wing where there will be equal areas on both sides, then and it's easy to estimate. In fact, for most planes it is completely unnecessary because they have constant chord wings.

But if you do the MAC calculation, then you STILL have to decide where the CG should be. So then after all the measuring and calculation...people tell you to guess! I'm not trying to be critical of anyone posting here. It's something I have seen over and over.

Of course, guessing works well enough, especially a conservative guess -- the airplane will fly. But the CG is likely to be well forward of the optimal point. Then you've got to start experimenting with moving the CG back. Why not calculate a reasonable CG in the first place and set up your plane that way? It saves a lot of effort later on.

Jim

rcairflr

My Feedback: (44)

There is no perfect CG for everybody. Even with what I posted above, it usually take me numberous flights in order to get my airplane trimmed and CG'd the way I like it. I continually move the CG aft until the airplane gets very pitch sensitive and then move it slightly forward from there. That is why I agree with the guys above, to not waste any money on a CG machine and just use your fingers under the wing or at the wing-tip to get the CG set within the acceptable limits and then fly/adjust as required for your flying style. Once my CG is adjusted for my flying style I couldn't care less where it is, just how the airplane flies.

Most airplanes do not have constant cord wings. Example: Extra 300, Edge-540, Cap-232, SuKhoi Su-26, Yak-54.

themadmax

My Feedback: (70)

Gee!!! That sounds like something I read over a month ago, 20 posts back...

OOH! I forgot...I wrote it!

KISS Keep it simple ------!

buzzard bait

I put a thin piece of masking tape parallel to the wing at the point I've calculated and then use my finger tips. You can feel the tape, and it makes it easy to tell where you are in relation to the measurement.

In years of doing this I have found many plans that have extremely conservative CGs. Determining this before setting up the airplane keeps things simple, because I'm not trying achieve a balance point that is farther forward than I need.

Then I do flight testing. I rarely find that I need to adjust the CG once I have calculated it. Out of the last dozen or so planes, I moved the CG further back on one; on another I moved forward, not for pitch stability but for yaw control, but that is another story.

Jim

mmiller1

My Feedback: (75)

I just built a Bruce Tharpe Flying King (the smaller one). I have powered it with a TT 91 4S. Battery is about center as are the servos. According to the plans the CG is to be located anywhere from 4" to 5" behind the LE of the wing. I set it up as indicated. The plane took off just fine, but at full up elevator there was no way it would loop. I know other members in our club who have this a/c and they claim it will loop at 1/2 throttle. I can only conclude the CG on my craft must be wrong. Any ideas??
Tim in Tacom

buzzard bait

How much does it weigh?

What prop were you using?

Were you in level flight, full power at the start of the loop attempt?

What happened when you gave it full up?

I don't think the CG had anything to do with this.

rcairflr

My Feedback: (44)

Keep in mind that the manufacturer recommended CG is just that (recommended). If the airplane is nose heavy than slowly move equipment aft until the airplane flies and feels better. Also the recommended elevator throw of 1/2 inch is going to make your pitch axis very limited. You can increase this throw to give you more pitch axis. So my recommendation is move CG aft and increase elevator throw.

tony0707

hi don't know the cg of a given plane ?--use 1/3 the wing cord-as a starting point -finger method----fly the plane--when turning-in the air -input the amount of up elevator-you like to use on a given turn - add or reduce the weight- so the plane-flys level- in the corners at that elevator input setting. (help to give all your planes-the same feel on the up stick --on the turns ) if you move the cg to the rear of the plane the elevator -will become --more sensitive--if cg moved to the front-elevator will become-- less sensitive. do 1/2 oz adjustment at a time.--until plane is flying level on the turns ---do cg of tail draggers- inverted. the plane must always be balanced- front to back--and left to right. balancing left to right-will give the plane greater stability in the air-the plane wants to center itself-or stay were you put it --with the control sticks. find it best to have the plane -just slightly nose heavy-with an empty fuel tank--is a very good starting point (seem to work better than static balancing--the plane completely level-seems to give the plane a better decending rate--on the landing. scratch building- kit building 20 years -flying r/c with great success for 15 years. fly safe-watch those nylon apc props -they take body parts off--love this hobby===tony==

RC Specialties

Tony,

Thanks for the great post. The only comment I would like to offer concerns the amount of up elevator in the turns. It would be hard to have the same input depending on wing design. A high lift wing may take a different elevator input as opposed to a thin less efficient wing. The ratio of lost lift during a turn will vary from wing to wing. I do agree with your comments. A well balanced plane does provide optimum glide and descent rates. It makes it much more easy to establish a stable approach without fighting the elevator to keep the plane level. Great input.

Jim

cyclops2

My C O G calculator works on ANY type of plane that can fly.

Down hill slope with + 2 foot deep weeds ( soft ones ! ).

Gets the perfect C O G everytime.



Rich

xtruckerjim

Tim,
You have probably figured out your little problem by now, but if you haven't, then my only sugestion would be to make sure your "Elevator D/R switch" is off. Some times we over look the simple things.

Back to the subject. If you don't have the experience for trial and error, (myself) then the calculator could prevent a crash.

Thank you every body for sharing your knowledge, I got the info I was looking for.