hello guys,
I am fairly new to RC flying and have aguestion concerning how to identify when the CG is wrong. How does a plane react when the nose is heavy and when the tail is heavy. In other words the CG is off and I don't have plans for my scrach built trainers.

nose heavy = hard to fly
tail heavy = flys once
if the plane is nose heavy it wont react quickly to inputs.
if its tail heavy it'll pitch all over the place.
best advice is to get it right before you fly, and as your new the easiest way is to make it balance on the highest point of your wing ( 25% of root chord )

If the plane is nose heavy it will fly with no noticeable difference. What you will notice is that in turns or inverted the nose will drop. You will also notice that the plane requires more elevator to bring the nose up in straight and level flight or during landings.

If it is tail heavy, you will require no "down elevator" to keep it level or may require up elevator. The second thing you will notice is that on landings or slow speed approaches, the tail will drop.

For many aerobatic maneuvers, a slight tail heavy condition is desirable.

To paraphrase PhillyBaby, A nose heavy airplane doesn't fly well, A tail heavy plane doesn't fly long.

A plane that is nose heavy will tend to fly "Sluggishly" and will have a tendency to not slow down well on landings, while a tailheavy plane will be extremely manuverable.

BUT!

There is a fine balance here. Imagine the CG as an invisible line that goes through the airplane. The MFG might recommend let's say 1/4" on either side of that line as a recommended balance point. If the CG is in front of that range, it will still fly, (although sluggishly). But if the CG is BEHIND that line, you will most likely be bringing your plane home in a paper bag.

Think of an arrow (It has been said that the guy who invented the arrow was the worlds first aeronautical engineer); straight shaft of uniform weight, heavy, pointed weight on one end, Large, lightweight fins on the other. As it travels through the air, the point penetrates the air, while the feathers are held back by the airlflow.

Now imagine taking the point off, and making the feathers out of lead. If you shoot this arrow, as soon as it leaves the bow, the tail will whip around to the front because it's heavier.

This is exactly what a tail heavy airplane wants to do.

If you are not sure of where a plane should balance, Start by balancing it just slightly forward of the thickest part of the wing. Then fly it and see how it responds to controls. If you're capable of doing aerobatics, take it up and see if it will do a spin. If it won't, (or it's difficult to enter a spin) it's nose heavy. Keep moving the CG back until the plane flies the way you want it to. But keep in mind that as you move the CG back, the plane will get more responsive, and that there IS a critical point that you cannot go past.

a tail heavy plane will tend to ground loop. there are two good articles in Model Airplane News for determining the CG. The first is in the April '92 issue on page 54. The second is in the Sept '98 issue on page 66. Lastly find someone that has an Ultra Sport and look in the instruction book toward the back and you should find a list of instructions on flight trimming.
Good luck.

Phillip,

The above answers are good, but bear in mind, this is not an exact science, that is, there is no point that is always the exactly correct CG.

If you fly at full power with a trimmed airplane and cut power, a nose heavy plane will tend to dive. A plane with a correct CG will gently descend. How does a novice tell the difference when it is close? Well, it's a matter of opinion as to what is a dive and what is a gentle glide.

A tail heavy plane will tend to nose up when you cut power. You may have to hold down elevator to get it to descend for landing.

Different types of planes also tend to use different CGs. Scale planes with small horizontal tails usually balance more nose heavy. A larger tail tends to give you more lattitude in CG placement.

Sport planes usually balance more rearward than a scale plane. many fun fly pilots balance their planes in the "slightly unstable" region because they turn and loop tighter, flat spin and do 3D maueuvers better. If you have a fairly large tail, over 23% of the wing area (this is my general rule), the plane is controllable even when tail heavy. i had a really good flying plane that was stable with a full tank, but would oscillate in pitch starting at about a half tank. It would cartwheel really well and fly knife edge all day. So I had to hold a little down elevator to land.

Some RC old timers balance as far back as 70% of the chord. This turns them into essentialls a tandem wing plane with both the wing and the stab lifting upwards and the CG between the aerodynamic centers of the 2 surfaces. They are very sensitive to power when balanced like this.

For a trainer, you want it balanced where you don't run out of up elevator on landing. I say this assuming you can land normally. many novices will get the nose up and the plane in a semi-stalled condition and let it sink, nose high to touchdown. They may be out of up elevator in this condition, but not nose heavy. If you have normal controls at altitude and descend in a gliding final approach, flare at a couple of feet and run out of up, you are nose heavy.

There is a range of usable CG locations. That range is about 5 to 7 percent of the mean aerodynamic chord. The best CG location within that range depends on the purpose of the aircraft, the flying style and skill of the pilot. The location of that range of usable CG locations depends mainly on the tail volume coefficient. The larger the tail volume coefficient, the more aft the range. The horizontal tail volume coefficient is the tail moment arm length (expressed as multiples of the wing's mean aerodynamic chord) times the area of the horizontal tail (expressed as a fraction of the wing area).

There are secondary considerations that affect the location of the usable CG range only a little. A large or unstreamlined fuselage and its appendages slow the flow over the tail and render the tail area a little less effective. The aspect ratio of the tail is another consideration with high aspect ratios being a little more effective for their area.

In full scale aircraft the aerodynamic centers of the wing and tail do not shift appreciably and the neutral point (which is the most aft usable CG location) is stable with changes in angle of attack between the upright and inverted stall angles of attack. However, at model reynolds numbers the aerodynamic centers move around a little with angle of attack. This instability of aerodynamic centers is caused by the waxing, waning and shifting of laminar seperation bubbles as well as variations in thickness of the boundary layer with angle of attack.

A plot of coefficient of lift vs angle of attack is normally a straight line. However, some thin symmetrical airfoils used for tail surfaces have a kink in that straight line near zero angle of attack. The kink is caused by the shifting of the laminar seperation bubble from one side of the airfoil to the other. The symptoms are not unlike deadband in the elevator servo.