[FONT=Verdana]Knowledge Saves You Money
We answer a lot of questions in these forums about finding the correct CG. Here, you will find a collection of useful of information on that subject.The CG is also called the CoG, or Center of Gravity
The best in-flight ideal CoG test is:
With your radio, adjust your trims on the plane flat and leave it there. Now, Instead of adjusting the trims with the radio, you will adjust them by repositioning the battery. Fly your aircraft at a normal cruising speed. If you feel the need to trim the nose up, it's nose heavy (move the battery back). If you feel the need to trim the nose down, it's tail heavy (move the battery forward).
- If it flies flat, it’s at the ideal CoG; when you land, mark that spot.
- If it doesn't feel right, land and repositioning the battery, then keep on flying, landing, and adjusting untill it does fly flat. When you finish, don't forget to mark that spot.
The CoG mark on any set of plans is a good starting point, but the above method should be used to fine tune it. Never use the radio trims to compensate for a poorly adjusted CoG. If you ever have to adjust the trims very far from flat, redo the above procedure.
The location of the ideal center of gravity varies with wing loading (weight), velocity (speed), and any aerodynamic changes to the aircraft. It usually doesn't change much, how much it changes depends on how drastic the changes were. But that is why you need to fly the plane the way you normally would while doing the CG finding flight test. That way, you will find the CG that best suits your style of piloting.
Let's look at some in-flight center of balance adjustment effects:
There are some advantages to using a center of balance point that is different from the CoG.
Nose heavy is safer, has sluggish maneuverability, and will increase stability in high winds. Favor nose heavy for maiden flights
. If the center of balance is way nose heavy, you will have to fight to keep the nose up. You know it's nose heavy when you are having to spend a fair amount of time on the stick fighting to keep the nose up, or your trim is adjusted high.
is good for high-alphas and radical maneuverability, but is inherently unstable and should only be tried by seasoned pilots. The nose will pop up very easy, this is great for cobra maneuvers. If the center of balance is way tail heavy, you're all over the sky, hoping you can land it without crashing. You know it's tail heavy when you are having to spend a fair amount of time on the stick fighting to keep the nose down, or your trim is adjusted low.
The higher the wing loading, the more sensitive and critical the ideal center of gravity adjustment becomes. Light, floaty jets are much more forgiving.
The ideal CoG moves progressively forward a small amount as the aircrafts speed increases. The reference point in the end should be a compromise that favors your normal cruising speed. Fast aircraft should feel a little nose heavy at slow speeds.
If you intend to fly in high winds, adjust the CoG a little nose heavy to increase stability.
To find the ideal center of gravity:
Assemble the aircraft without any electronics. Gently glide test the aircraft and adjust its balance by taping coins to it. Do this several times until the plane is balanced enough to glide straight and flat. Pick the aircraft up by the fuselage loosely using your thump and a finger. Adjust your hold position until you find the spot that makes the aircraft hang level. Make a mark on the aircraft fuselage one inch forward of that spot (see NOTE 2)
, this mark is the reference point for the first powered flight test. Remove the coins and Install everything except for the battery. Hold the aircraft in the same way at the reference point, and use the battery size (mAh) and mount position to balance the aircraft. The aerodynamics of the airframe has the biggest influence on the ideal center of gravity, but the thrust location, (component and surface) friction, airfoils, and velocity, also have an effect. So after the powered flight test, move the battery fore or aft a little, as needed. Readjust and retest until you are happy with the flight performance.
Now that you have found the ideal center of gravity, mark that spot as a reference point so you can balance your plane before every flight.
Any changes that you make (different size battery, different size motor, added servos and airfoils) can necessitate a reevaluation of the ideal CG. Now for the good news. The old and the new ideal CoG will be pretty close to one another.
NOTE 2: We have found that after the electronics are added, the ideal center of gravity moves forward one inch
This next part is from another thread "The controls we use
Adjustment and positioning:
You can adjust the throw amounts with some radios, but you can also adjust them by your control horn hole choice. There is sometimes more than one hole in the servo and control surface horns. Having a choice of which hole in the horn the control rod is secured in, allows you to choose between more of a throw in the control surfaces movement, a more powerful throw, or anywhere in-between the two.
If you choose a powerful throw:
Lessening the throws by hole choice has the added advantage of increasing the authority the servo exerts on the movement of the control surfaces. This control movement will be strong and sure. Its speed is a little slower, but not by much. This is good for speed planes where a lot of torque and less movement is needed. Gyros work better with strong movements.
If you choose more of a throw:
- At the servo, the hole closest to the servo moves the control surface the least.
- At the control surface’s control horn, the hole farthest from the control surface moves it the least.
This control movement will be fast and weak, but it will move a longer distance; this will make the control surfaces move farther. This is good for 3D planes where fast control surfaces and more control is needed. High travel movements cause a gyro's gain adjustment to be more sensitive. Weak is a relative term; The weaker setting on a strong servo is still strong.
Servo mounting, and Servo and Transmitter Adjustment:
The servo's placement must be functional. First, let’s choose where you want the servo to go:
- At the servo, the hole farthest from the servo moves the control surface the most.
- At the control surface’s control horn, the hole closest to the control surface moves it the most.
After you chose where you want the servo to go, do not glue it down yet. Turn the radio system on so it is operational, and set the transmitter trims to zero. After you're finished installing a control system, the control surfaces should be flat with the surfaces they are mounted to. With the control surface and wing flat, the control rod should be close to 90 degrees to the servo arm. Also the control horn holes on the control surfaces should be directly above the hinge. This will give the movement even throws; that is, the control surface will move equally in both directions from the centered position. Now that everything is set up just like it will be used in flight, Z bend both ends of the control rod to the correct length and install the control rod. Now, you can prepare the servos mounting position by cutting the hole where the servo will be glued, then glue it in place. By doing it this way, the control rod will be light, strong, and at the perfect length.
If you have installed everything correctly, with the transmitter trims set to zero, the control surfaces should be flat with the surfaces they are mounted to. If you have to adjust your trims very much on your first flight, the aircrafts center of gravity is misadjusted. Land, set the trims back to zero, reposition the battery and try again. The Center of gravity adjustment is a lot more sensitive on heavy planes.
- Place the servo to where it has the least effect on the aerodynamics of the aircraft. An airflow blockage can create big problems in some places.
- The linkages of the control system have to be smooth, free flowing, tight, and at a good mechanical angle.
- When possible, install the servo to where it can’t be easily seen.