dhal22

Excellent story. Thanks.

doxilia

My Feedback: (3)

Ralph,

I couldn't help but comment on your post. The CG of a model is what it is or what is chosen regardless of how the mass is distributed about it. The ideal CG position doesn't change because we put mass inside the model in different places altering the moment about the CG (at least not in a significant way). The balance changes in flight, in the case of a model which is burning liquid fuel, because the mass, and therefore moment arm, changes over the course of the flight. For this reason we often attempt to put the fuel directly over the CG or close to it so balance changes while flying, and therefore trim changes, are kept to a minimum. In essence, in a glow powered classic, locating the fuel tank is a consideration and generally keeping the bulk of the mass close to the CG is preferable as it makes the plane "lighter" in response to control due to reduced moments about the CG. This is why we try to keep the wings light the closer to the tip we go.

In the case of an electric model where the AUW doesn't change in flight, the CG is what it is and that's that. The ideal location for each pilot will be a little different but there is no need to concern oneself with "tank position in relation to CG" or otherwise.

Maybe this is all obvious but I was a little puzzled by your post.

David

stuntflyr

I'm puzzled by your post, David.

First you say there is no reason to worry about how the components and their weight are distributed within the airframe, and then go on to correct yourself about how one should have the fuel tank over the CG and keep the wings light. Odd.

I think it is obvious to most people flying in competition that keeping concentrated weight to a minumum and closest to the cg. FF, CL and RC competitors all keep the "barbell" effect low by concentrating their weight near the cg and it is a huge trim problem when not doing so. In this case it is very important, "...how the mass is distributed about it".

The way I read the cg comment by Computermonkey was that the cg on the plans for the indicated model were of the rearmost computed cg when the fuel tank was empty. Therefore the cg could be optimal in a more forward position.

Chris...

doxilia

My Feedback: (3)

Chris,

yes, I suppose we're not being as clear as we all hoped. I'll try to better explain myself.

I don't think I said there was no reason to worry about component layout. What I said was that the CG is what the manufacturer decides (presumably based on a number of different factors) regardless of how the model is powered.

There are two issues I was trying to highlight: the first being the location of the CG as indicated on the plans. That is fixed by the manufacturer and is not a "variable position" because the model is glow powered and is therefore burning fuel located off CG. If it was, we wouldn't try to minimize CG changes caused by variable mass located off CG as was the case with many classics with fuel tanks located behind the FW. The fact that the model balance shifts in such a setup because the mass is reducing by about half a kg on one side of the CG during its flight is a different matter. We resort to flying them nose heavy on takeoff and balanced on landing if the fuel is depleted. In the case of a model that is being "converted" to electric power where there is no variable mass (at least we hope not), there is no reason to move this CG purely based on the fact that there is no liquid fuel being burned. In short, the CG is what is specified on the plans or otherwise chosen by the pilot regardless of power plants or variable mass.

Perhaps you are right about it being obvious but I wouldn't bet on it. In any case, we're not all competitors. That aside, a model will fly if balanced and trimmed regardless of whether the mass about the CG is, in a simplistic way, 1 kg on each side of the CG 10 cm away or 50 cm away. The latter will just not fly as well as the former but it shouldn't loose trim because of it. Now, if we change that mass on one side of the CG during flight, that is quite different and trim will be affected.

Yes, this is more or less how I understood it too and is what I was trying to point out is not appropriate in our reading of plans. There is no good reason to move the CG forward based purely on the fact that there is no liquid fuel off CG being burned. If we want to move the CG for aerodynamic reasons that's fine but it has little to do with electric conversions so to speak.

Although not always used appropriately, in the case of the XLT plans, the actual "CG range", as considered appropriate by the manufacturer, is indicated by the CG graphic. This however doesn't call for "moving" the graphic from its position on the plans.

My post above wasn't meant as a pejorative criticism of Ralph's post - we have a friendly rapport. I was just confused by his comments and was attempting to present my view on plans indicated CG as mentioned in my previous paragraph.

David

stuntflyr

On a forward fuel tank model, the CG is going to change. The CG is drawn on the plans for a forward tank model with the CG in it's rearmost position because we balance them on the bench with the fuel tank empty. When converting the model to electric a more forward CG is probably better to start with.

I know you do not understand the problem because of what you are typing.

The CG drawn on a traditional plan is for the fuel tank empty on a bench balance of a glow powered model. If you are going to fly the model with a center fuel tank and pump, the CG on the plan is going to aft for flight and should be moved forward. Same with battery powered models.

It just is what it is.

Chris ...

time907356

The information (NOT the debate....) brings up an issue I have thought about while building, but never really dealt with; the distribution of mass. I think most of us (RC Pilots) think about CG in one dimension: the distance fore and aft from features on a 2 dimensional silhouette of the aircraft. However, I suspect that the componant weight distributions have a big part of how the plane actually handles in the air.
Assuming the CG is "correct", and the aircraft is reasonably straight trimmed properly, I would think for smooth, graceful flight, the masses of the componants should be as far away from the CG as possible. It would require more control surface deflection to do the work of rolling, pitching, and yawing the aircraft, and be more averse to external forces imposed by air quality (wind, temperature, humidity, thermals, etc.). The aircraft should respond to such things more as a 'unit', and less like it's combined componants.
If I had weight to spare, I suppose I'd put some lead in the wing, stab., and fin tips to test this out. Or, maybe mount servo's for these surfaces at the extreme of the span of each surface?? It seems to me that a plane like this would be ultra stable in the air. -Thinking on the fly here...quite dangerous!
This would be great for the lower classes that don't have a bunch of snaps and spins. On the other hand, this plane, with high 'inertia', would be pretty bad at accomplishing those maneuvers in the higher classes. For these, we'd want the opposite; the masses all bunched up on top of the CG, to allow the minimum inertial resistance to roll, pitch and yaw.
Does anyone build like that??? I can't ever remember a conversation that went this way. I remember one designer asking me where the CG of a plug in wing panel would be, but he was really talking about the geometric center (of the area), not the mass. I don't think I've ever seen a servo mounted in a wing tip either, where, incidentally, the aileron is having the greatest effect.
Thoughts....while I hijack this thread??

doxilia

My Feedback: (3)

That's my point, it shouldn't. The Center of Gravity (not the balance) of a model, as specified on a plan, should be fixed. Once determined, it should stay there. What happens if you add or deplete mass (fuel) on one side of the CG is the balance of the model changes - that's entirely different.

No it's not. The CG is where it should be regardless of power plant. We don't balance models at say 25% MAC because it is electric powered and 30% MAC because there is a fuel tank somewhere away from the CG - that's nonsense. The CG on plans is (or should be) specified as a range. There is no good aerodynamic reason to move that range forward because you are no longer going to be adding 16 oz of fuel before takeoff. What we do with forward mounted tank glow models is fly them nose heavy or, if we want to avoid trim changes, is move the tank back closer to the CG.

We're all entitled to our view but I disagree with this. The CG should be placed aerodynamically - nothing to do with glow or electric models. If one deems that a more forward CG is aerodynamically preferable, so be it but not because we're burning methanol or running electrons for juice.

Well, I try to write as best I can, not type.

That's your view on CG, mine's different. I'm happy to leave it at that.

David

doxilia

My Feedback: (3)

Tim,

I think this is where both Chris and I agree. The opposite is true. Here's an easy way to figure out why:

Stick your arms out and start turning on your feet as fast as you can. Note how it feels. Now do the same and keep your arms tucked in. You will see that it's much easier to do the spins with the arms tucked in rather than sticking out. Skaters are well aware of this and know there is an increase in both angular momentum and angular velocity if the inertial mass is reduced. Less work is required to spin faster if the mass is concentrated closer to the CG of the moving object.

As they say, the test is in the pudding. Give it a try. I think you'll discover the model will be a rather poor flyer - if it survives the flight. All else being equal, stability is a factor of a well located CG and a model that balances on it. You won't increase the stability of a model by putting a bunch of mass at its extremities.

On the other hand, when we need to balance a model laterally, we do add mass on (or in) one of the two wing tips. Additional tip weight is not great for rolling performance but it allows the minimal amount of ballast to be added as the moment arm about the model's CG is the greatest resulting in a laterally balanced model with the minimal amount of weight gain. In any case, a model that is not laterally balanced rolls and tracks poorly so even if we need to add some tip weight for balance, it is preferable than not doing so.

It would be bad at maneuvers in any class - it would just fly poorly.

Now you're on to something. Yes, many build like that keeping wing servos (aileron and flap) as close to the center as possible. Likewise with the radio gear in the fuse - we keep most of it close to the CG above the wing. However, in a model where there is roughly a pound of fuel (16 oz) that is being burned in flight, it is preferable to keep that close to the CG and move the relatively light radio to either side of it as needed in order to minimize not only the shifting balance of the model but also to reduce the moment of inertia the model has when it is being pitched or yawed. Generally, we distribute the fuel symmetrically about the thrust line in the fuse with the tank centered laterally so rolling performance is unaffected by a tank that is moved fore/aft. Of course, all controls interact with each other and the model is a 3-dimensional flying object so a model that is out of balance longitudinally will affect its lateral maneuver (rolling) performance as well.

Yes, the CG of a wing panel alone in the context of it being an aerodynamic flying object is nonsensical. As far as it being any object, all objects have a CG which in the case of a typical wing is somewhere inside the panel. If the wing panel has a typical mass distribution, it's CG often coincides with a location along the MAC. If you pick up the panel with a finger along the MAC, you may well find out it balances span wise.

Chris,

I think Tim's post above and my comments here are good evidence that it is not obvious to all (including experienced modelers like Tim) that it is best to concentrate mass close to the CG in a flying model..., even if we just finished writing a few posts on the very subject.

David

time907356

David,
Why did I know that you would bite on this subject?! I appreciate your comments...all except the last one, which I find a little condescending.
As in all pattern maneuvers, smooth scores better. If your wings are 'twitching' on a Straight Flight Out maneuver, you'll score lower than if they're locked in. Same in a Loop, Top Hat, or Vertical Square 8. Not many maneuvers require spinning like a balerina, or figure skater, in fact, none. That's for the 3D guys.
But on to the point, When you're NOT trying to spin or snap (99% of the time) and fly smooth, in breezy conditions, distributing component masses should make the airplane be less affected by these conditions.
As an illustration, stick your hand out of the window of a moving vehicle, and twist your palm into the wind direction quickly once or twice, noting the amount of movement the wind induces on your hand. Now, strap on a 1lb. wrist/ankle weight and repeat. Your hand will move a lot less, and a lot slower when weighted.
The same effect is realized by the aircraft as it's flying along. A breeze manifests as an upward force on one of the wing panels (usually), and knocks the aircraft into a roll until the breeze relents. If the wing were heavier at it's tip (more inertia) it will be less affected (less induced roll) than a lighter tip. Simple dynamics.
Will the plane snap/spin as 'quickly', ballerina style? I doubt it, as the same attribute that combats choppy wind conditions fights these maneuvers. I seriously doubt however that I've conceded many points in a round by snapping too slowly. I have conversely been tossed around like a sheet off of the laundry line by 20mph winds (especially with my EU1A experience), enough to know that there is a significant advantage if I can beat the breeze.
And by the way, I'm a Mechanical Engineer..I get it. I guess I'll go for some pudding!

flywilly

My Feedback: (121)

Don Lowe discussed this issue many years ago in his RC Modeler column. The further from the center of mass on the airplane the less mass you want (keep the flying surfaces as light as possible). His argument against using weight to dampen wind induced oscillations was that less correction would be required for lighter surfaces which would also respond more quickly to corrections due to lower inertial forces. He also suggested that maneuvers would be easier to fly as the inertial forces of a lighter airplane would make starting and stopping directional changes require less energy and thus less control movement making precision flight 'easier'. I will try to find the column as he explanation was a bit more elegant.

doxilia

My Feedback: (3)

Tim,

apologies if my tone was inappropriate - I didn't intend it to be.

I'll just add that Chris and I were differing in opinion on a related subject (CG location and moments of inertia) and your follow up post seemed to address some of the issues being discussed. You and I have a similar background with mine in physics and applied fluid dynamics. Because of that, it is hard for us to not have an opinion on matters like these and since you raised questions in your post, I offered an opinion.

I believe Chris is an experienced full scale pilot and happens to share my opinion on the questions you raise - airplanes fly better with mass concentrated near the CG. It appears that Don Lowe also shares that opinion (thanks for commenting Will). I believe that even in commercial airlines, when a flight is half full, the seats assigned are concentrated above the wing area of the cabin. Granted, commercial jets don't do aerobatics but the same mechanical and aerodynamic principles apply.

My last comment to Chris was merely to point out that not all aspects of dynamics are immediately obvious to all, including those with relevant backgrounds like yours and mine.

David

time907356

Hey David, absolutely no problem. I realize we get carried away with our technical topics sometimes, and don't always weigh the social implications....forgive me: I spent 3 years in management! ������....don't hold it against me though: i also spent 12 years simulating visco-elastic, non-Newtonian fluid flows (molten plastic in Coex. extrusion dies).
Your technical perspective far outweigh any social fopa. I'm challenged on this subject and will look for a suitable test venue (pudding!). I don't disagree with the assessments of your analogous scenarios, just whether theyre what we're looking for in pattern.
ive noticed flying an eflite Splendor that the wing panels are exceptionally light, and the power/weight is very high. I will look for a chance this fall to add some appreciable weight in both concentrated and distributed methods and see how it does. It will be subjective of course, but welcome to pattern!
For now though, my first contest in 22 years is next weekend in Charlotte NC. It'll take a little bit of focus this week to be ready.

flywilly

My Feedback: (121)

Good luck, Tim! What airplane & power set-up will you be using? As for the 22 year layoff... well, it's just like riding a bicycle; except the 'bicycles' have changed a lot in the past 22 years
Get a calm, experienced caller if you can.

MTK

Let me add that in 2 meter Pattern, many planes are designed with plug in wing panels. The panels are quite large in some cases, nearing 500 squares each and more ( I really like large wings in my designs), yet typically weigh less than 1 pound RTF, each, and in some cases as little as 12 ounces (for smaller 400 square panels). This is precisely at the heart of Dave's comments.

I've learned over the years that in addition to light wings, area distribution is also a significant contributor to smoother flight particularly in windy lumpy air. I prefer root to tip chord ratios at 3 or greater. This wing design brings the MAC closer to the fuse CL, concentrates mass nearer the fuse CL and reduces mass toward the tip side of the MAC. Disturbances at the tips are just not as reactive as they are for more "standard" planforms. Consider the typical classic wing which had ratios of at most 1 1/2.

Another benefit of this planform is that it enables a smaller LE radius outward (tip-side) which enhances ability to start and most importantly stop some key aerobatic maneuvers in current schedules. But it probably means less in classic pattern since such maneuvers are not done.

time907356

Hey Matt,
Maybe you read in here that my worst experience with choppy air was with the eu1a. It was a really bumpy ride in anything over 5mph even breeze. It cost me lots of points and places competing. That bird has something like 3.5:1 chord ratios, with its massive 22in root.
I realize the new 2m plans look somewhat hefty and bulky and generally large like the eu1, which is one of the aesthic appeals to me, so we'll see. I hope to be getting a BJ Invitation before next season, if money, and momma will allow?????

MTK

I believe it,,,,That model had a penchant for heavy wings as I recall. Heavy wings will not perform as well as light wings.

Small tip chords reduce the tip area and weight but still, I'm talking 530 square inch panels that weigh 14 ounces RTF. I have not flown anything better in bumpy (or smooth) air