Thoern

When I have balanced my aircrafts I have settled with that the plane 'seems' to be level when balancing it on the CG.
But now I wonder if there is a more precise way to find the CG.
What is it realy that should be level, is it the stabilizer ? I have tried to water-level the stabilizer when the plane is hanging on the CG recommended for the model.

Geistware

My Feedback: (16)

If the stabilizer is installed with zero degree incidence and it is a flat airfoil then use you can set your level there. The problem is that your level will add weight, and change the reading. What I would recommend is that you measure at the CG point and 2 or 3 degrees tilt will not make much difference if you can balance it at the CG point. Most planes have a CG range where the CG point is someplace in between.

Ollie

Tie a plumb line near a tip of the prop. Suspend the plane so that it hangs by a string yoke tied to the tips of the prop. The yoke must be suspended at its center. Rotate the prop until the plumb line hangs near the side of the fuselage. Tape the side of the fuselage with masking tape whose edge follows the plumb line. Then rotate the prop as far as you can to a new position so the plumb line lies alongside the fuselage again. Apply a second piece of masking tape to the fuselage side so that the edge of the second tap is now aligned with the plumb line. Where the tape edges, that were aligned with the plumb line, cross is the location of the CG. This proceedure gives you the location of the CG in the side view and is independent of the plane's orientation.

To minimize error, use a weight for the plumbob which is as small as possible relative to the weight of the plane. A thread makes a good plumb line.

RSands

I'd not get too concerned about having the CG exactly "per plans". What you're doing is fine. There's always some final tweaking to do with it once you get it flying. The plans are just a starting point to get you safely/controllably in the air.

majortom-RCU

My Feedback: (40)

RSands has got it right. If you want to get precise about your cg, the way to do it is fly it, see how it stalls, how it glides with engine at idle, how it lands, how it responds to elevator control. If flying tests show it close but not quite where you like it, then you can start nudging it forward or back by relocating battery and other components. By this time you don't need any measuring device, other than your eyes on the plane and thumbs on the sticks. For first flight and general trimming out, balance according to the plan recommendations, and if it balances within a couple degrees of horizontal--measured by eyeball--then go ahead and fly it.

Consider that if your fuel tank is in the usual spot behind the firewall, you're going to shift your cg considerably as tank goes from full to empty. There's little point in trying to nail it down to an eighth or a sixteenth of an inch without first going to a pumped/regulated system and putting your fuel tank at cg. Even then, if it measures right but flies wrong, it's wrong. When you get it so it flies right, there's no point to measuring it.

Geistware

My Feedback: (16)

I have to agree with majortom. This is how I adjusted my Giles!

BernieG

I entirely agree with that, BUT ! The big problem with most design is that you can't adjust the incidence, so you can never get it "right " !

What I mean is, balancing an airplane is just that, balancing. When you balance something, you have 2 sides, Most of the time, when we balance an airplane, we adjust one side, never touch the other....

The difference of Angle of Attack between wing and tailplane, or incidence, is the other side. Each time we move the CG, we should adjust this incidence. Without doing that, the setting is always a compromise ( cheated with trim on the elevator), but never "right".

Bernard

majortom-RCU

My Feedback: (40)

BernieG raises a hairy issue with the matter of wing incidence, tail incidence, decalage, and all that. I have spent some time in the past fooling around with wing and tail incidence and their relation to each other. All I can say is my experiments with decalage have brought me nothing but grief. If I encounter a model with specs for anything other than zero degrees incidence in any flying surface, I am inclined to move on to something else. On occasion (high-wing trainer types) I have been able to just set main wing and tailwing at zero incidence and do whatever I have to do with CG to get things right, and that has generally worked OK.

I am able to fool around with engine thrust--down and sometimes up--and get good results. Maybe Ollie or someone can tell us if my intuition is correct that there is little practical difference between let's say 2 degrees downthrust on the engine with zero incidence on main & tailwing as compared to neutral vertical engine thrust coupled with main & tailwing both at 2 degrees positive incidence.

No question that thrust offsets, incidence offsets and cg are all crucial to pitch behavior; my way of dealing with it is to isolate the variables down to engine vertical thrust offset and cg, and simplify the decalage issues by keeping flying surfaces neutral.

Ollie

Incidence is the angle between some arbitrary reference line, like the centerline of the fuselage, and the chord line of a flying surface like a wing or tail. The purpose of incidence is to make it easier to establish the angular relationship between the wing and tail in order to rig the plane for initial flight. After that the arbitrary reference line is never used directly again because its location can only be found in reference to chords and thrust lines. It serves the same intermediary purpose for establishing the angular relationship betweeen the thrust line and the wing chord line. The aerodynamic significance of the centerline of the fuselage (or any other arbitrary reference line) is not nearly as great as the decalage (angle between the wing and tail) or the thrust line to the wing and tail chords.

There is a range of acceptable CG's on any plane. The best location of the CG within that range depends on the purpose of the plane, the skill level of the pilot and the flying style of the pilot. Trimming the plane for the best CG also requires trimming the decalage. This usually is done with a click ot two of elevator trim which changes the camber and incidence of the horizontal tail. Most people would rather live with the minor offset in the elevator rather than remount the wing or tail so that the elevator would be perfectly aligned to the stabilizer geometrically.

Make no mistake. Trimming a plane involves adjusting the decalage one way or another. Exactly the same thing could be said of thrust line adjustments.
Incidence and thrust alignment to an arbitrary reference line during building and initial rigging makes the plane ready for safe flight testing to prepare it to change those angles a bit as the plane is tuned to its purpose and pilot.

BernieG

Don't fly any bipe, sailplane or Scale model, then....Don't add the thrust in the picture, as the thrust has something to do only when the engine run, and when there is an engine....Aerodynamic is more generic than that....

Bernard

BernieG

Ollie

OK, Sorry, my use of the word "incidence" is because of a french/english thing. We use incidence for both the "builder" incidence, which the way I see you define it, and the "aerodynamic" incidence, which you name decalage.

About trimming to compensate for decalage, problem is that this correction will vary with speed, that's why most pattern design have a way to change the "incidence" of the tailplane, as it is the only way to have a really "neutral" setup for decalage.

Bernard

Ollie

The way to set a plane up for neutral stability is to gradually move the CG aft until the CG coincides with the neutral point and the plane goes where it is pointed without any terndency to correct. With this set up there is no particular trimmed flight speed. The trouble is that most power planes have the fuel tank forward of the CG and the neutral stability is only practical with a nearly full tank. With the tank full, the plane is nose heavy relative to neurtal stability. One answer to this problem is to put the tank at the CG and use a fuel pump and regulator. The tank has to be equipped with baffels to prevent the fuel from sloshing around and changing the critically located CG. Another problem is that the CG has to be in the same location as the center of drag and the thrust line must go through both. This will prevent any change in pitching moment with speed or throttle. Also, the wing tips have to be at the same level as the CG and center of drag so that the center of drag does not move around with changes in induced drag. Those are a lot of requirements to meet and it is probably why so many designs do have changes in stability and trim over the course of a flight. A plane that is designed and adjusted to meet all the above requirements would reduce the pilot work load and be a sweet handling aerobatic ship.

BernieG

is not that true ONLY with fully symmetrical profiles ?

Bernard

Ollie

My understanding is that the airfoil type has little to do with it.

However, there is something I forgot to mention. Most airfoils in the range of model reynolds number have laminar seperation bubbles that change size and location on the airfoil with changes in angle of attack. The moment coefficient versus angle of attack curve is "S" shaped with a negative slope on either side of a positive slope. This can be interpreted as the aerodymanic center's location changing position. This in turn causes the neutral point to shift a bit. With a fixed CG set for neutral stability at the most unfavorable angle of attack, the stability goes from neutral to slightly positive over a small range of angles of attack within the useful range of angles of attack. This can frustrate a perfectly neutral setup a little.

BTW, these shifting laminar seperation bubbles can put a kink in the coefficient of lift versus angle of attack curve for many symmetrical airfoils.

majortom-RCU

My Feedback: (40)

Don't fly any bipe, sailplane or Scale model, then

Right you are, Bernard. I can have a good time flying a bipe that someone else has trimmed, but when I try to experiment with trimming a bipe myself (more than tx trim settings), I get into trouble. Sailplanes I can handle because they eliminate the power variable. Some scale models (mostly of the personal aircraft type) I have no problem with, but scale warbirds or other sophisticated designs are beyond my present ambitions. It would be great to develop the understanding necessary to manage these models, but I still have a good time within my current limitations.

Don't add the thrust in the picture, as the thrust has something to do only when the engine run

Power is what I fly, and I like my model to be reasonably neutral to throttle changes. It may not be theoretically orthodox, but it works for me.

I don't want everyone to fly as dumb as I do. I admire those who can tame the wild model. When someone asks me about a model behaving badly, I am happy to explain what I understand, as I understand it. When I don't understand, I suggest they consult their personal physician, or post to Ollie.