CG forward vs. backwards - Nose heavy vs. Tail hevy

[Up&Away]

Let's open this can of worms once more!

Please correct me, if I'm wrong. And if I'm right..., well, we'll come to that later maybe.

As I see it:
1. The CG is the point on the plane where the weight in front of it equals the weight behind it.
2. Putting weight in the nose will cause the CG to move forwards.
3. A plane with the CG too far forward will pull out of the dive test fairly fast.
4. A plane with the CG too far to the tail will tuck under in the dive test.
5. If the plane is balanced on the CG (whether the CG is on the recommended/right spot or not) it is neither tail nor nose heavy.

How wrong am I?

[Up&Away]

Scratch 3, 4, and 5

I got myself confused :rolleyes:

3. A plane that is noseheavy will pull quickly out of the dive test
4. A plane that is tailheavy will tuck under in the dive test
5. Both 3 & 4 are measured nose/tail heavy when balanced on the given CG spot
6. There is only one proper CG spot and it cannot be moved. (you'll be either nose heavy or tail heavy)

I think I've expressed myself more clearly now...

[Sparky Paul]

The c.g.'s location is a variable.
The plane's response (to the flier) dictates where a "proper" c.g. should be, for that plane and that flier.
Excessive stability or no stability at all, some people like these qualities on the same airplane type.
Any notation for a c.g. should reference the location where the plane is stable and flyable.
Moving it then is up to the pilot.

[slipstick]

Based on the dive test, the theory is :

Nose heavy means actual CG in front of the optimum CG position.
Tail heavy means actual CG behind the optimum CG position.

That assumes that there is a single optimum CG for all speeds, conditions and flight modes.

Steve

[raptor22]

Based on the dive test, the theory is :

Nose heavy means actual CG in front of the optimum CG position.
Tail heavy means actual CG behind the optimum CG position.

That assumes that there is a single optimum CG for all speeds, conditions and flight modes.

Steve

The dive test refers to the neutral point. If it passes the dive test it is neutrally stable.

--Alex

[vintage1]

...which is not always what you want. Models that are neutrally stable are twitchy and over responsive in pitch, This is good for pattern flying, but a pain for a sport model.

[Salto]

I like this topic, as it contains some of the fundamental concepts that are essential to understanding how to design and trim model aircraft.

It's probably helpfull here to define a few of the terms. I'll make an attempt to do this, plus give my explanation of how it all comes together in a model.

The Centre of Gravity (C of G) on a model plane in the fore/aft direction is where the model will balance in a level attitude when supported say under the wings. It's position is determined by the weights and positions of the various components and structures. It's position is usually given as a percentage of wing chord back from the wing leading edge. It's position relative to the Neutral Point (see below) will determine the Static Stability of the model.

Wings have an Aerodynamic Centre. This is the point on the wing where we consider the lift and drag forces (and the pitching moment) to act. The aerodynamic center is close to the quarter chord point. Other objects like tails and fuselages also have aerodynamic centres. In combination, all the lifting surfaces of a model have a combined aerodynamic centre which we call the Neutral Point. This is the point at which we consider the lift and drag forces of the whole model to act.

A Statically Stable model will try to return to level flight after being disturbed by a gust or other force. A statically unstable model will diverge from level flight if disturbed by a gust. A model with zero static stability will neither return to, nor diverge from its path when hit by a disturbance. This last condition is sometimes called neutrally stable.

A model will be neutrally stable if the Centre of Gravity is arranged to be right at the Neutral Point. If the C of G is in front of the neutral point the model will have some amount of static stability. The greater the distance that the C of G is in front of the neutral point, the greater the static stability will be. This distance in percentage of wing chord is named the Static Margin and is a measure of how strongly the model will try to return to level flight when disturbed by a gust or a control input. If the C of G is behind the neutral point, the model will be statically unstable.

Of great interest to model flyers is the fact that as the static margin is reduced to zero, the pitch response of the model also goes up. 3D type aerobatic models are often flown with close to zero static margin (rearward C of G). They are very responsive in pitch, and continue going where you point them without trying to return to level flight.

Trainers are usually flown with a high static margin (forward C of G) so that they will strongly try to return to level flight after a disturbance or after a control input is released. This high static margin also reduces their pitch response, helping to reduce over-control by inexperienced pilots.

One way to experimentally determine the static margin of a particular model is to perform a dive test as mentioned earlier in this thread. The model is put into a dive (say 45 deg.) and the elevator stick is then returned to centre. If the model tries to return to level flight then it's C of G is in front of the neutral point, ie. it has some static margin. If it diverges (dives steeper or "tucks under") then the C of G is behind the neutral point. If it simply continues at the same downwards angle then it is neutrally stable, ie. the C of G is right on the neutral point.

The static margin can be altered (by moving the C of G) to give the desired flying characteristics. The correct position at which to locate the C of G on a particular model is not defined to be at any ideal point. As Sparky said earlier, it is chosen to produce the flying characteristics that are appropriate for that model and for a given pilot.

Graham.

[BMatthews]

....which all goes to say that you got it right Up&away. The others are agreeing with you and adding in additional pearls of wisdom. Well worth reading and considering it all just to help clarify things in your own mind if you aren't sure.

Sorry for butting in guys but no one actually told him he's right yet... :D

[Salto]

BMathews,

Yes, Up&Away was right except for his point 6. There is no "only one proper CG spot...".

In any given model the C of G can be placed over a range of locations. Exactly where it should be placed will depend on what flying characteristics are required, and on the flying style and abilities of the pilot.

5 out of 6 for aero 101 - pretty good!!

Graham.

[BMatthews]

Oops, I missed that one or thought it was correct as far as being close to one spot. Can I use my "get out of Jail Freel" card for this one?

[Up&Away]

OK. How then do you mix the terms "moving the CG forward/backward" with "nose heavy/tail heavy? They are often used in the same breath, but I think there's a world of difference between them.

If the cg can be moved, there is no tail/nose heaviness. Or is there?

[Andy W]

The CG is wherever you put it. It doesn't mean the model will be flyable..
..a

[raptor22]

OK. How then do you mix the terms "moving the CG forward/backward" with "nose heavy/tail heavy? They are often used in the same breath, but I think there's a world of difference between them.

If the cg can be moved, there is no tail/nose heaviness. Or is there?

It means that it is nose/tail heavy for that person's preferance or it means that it is too off for their application. For example, if it is really stable it will go straight to the ground (bad for a plane) but that configuration would be good for a rocket.

--Alex

[globemaster3c17]

Great explanation there Salto.

[BMatthews]

All aircraft have a neutral stability point. This is determined by the wing and tail sizes and moment arms. It has nothing to do with the balance point. It's strictly a charactaristic of the existing planform and is fixed by the shape of the design. This neutral point is typically around the 35 to 50% point on the wing chord of most "conventional" models.... whatever conventional is...

The balance point, or CG, can be moved within the model by moving the weight around inside as you say. The term CG (center of gravity) actually refers to the true center of mass of the whole model and is actually a point located in all 3 dimensions within the model. But we modellers commonly use the term CG. It's incorrect but that's OK as long as you realize that we are REALLY only referring to the longitudinal part of the true Center of Gravity.

The stability of the model comes from the relationship of where the balance point is compared to the neutral point. If you balance the model forward of the neutral point it will be stable based on the need for up elevator trim. If you balance the model behind the neutral point then it'll be UNstable. Right on the neutral point and the model will be either dead neutral or show slight signs of postive or negative stability depending on what you're making the model do at the moment.

If the balance point is far enough forward we say the model is nose heavy. If it's at or behind the neutral point we say it's tail heavy. If it's located within roughly 0 to 10% of the wing chord in front of the neutral point we say the model is balanced.

[radfordc]

OK. How then do you mix the terms "moving the CG forward/backward" with "nose heavy/tail heavy? They are often used in the same breath, but I think there's a world of difference between them.

If the cg can be moved, there is no tail/nose heaviness. Or is there?

When you move the CG forward of the neutral point the plane is nose heavy. Move the CG behind the neutral point and it is tail heavy.

[Up&Away]

When you move the CG forward of the neutral point the plane is nose heavy. Move the CG behind the neutral point and it is tail heavy.
Isn't the cg supposed to be the neutral point?

[Bill Glover]

All aircraft have a neutral stability point. This is determined by the wing and tail sizes and moment arms. It has nothing to do with the balance point. It's strictly a charactaristic of the existing planform and is fixed by the shape of the design.

To be accurate, that's not the whole story. The planform is a major factor, but there are other things that affect the position of the neutral point e.g.:

* Horizontal stabiliser efficiency (determined by it's position relative to wake turbulence from the wing ... T tail for example)

* The difference between the lift curve slopes for h. stabiliser and wing (determined by their sections)

* The change of downwash angle at the h. stabiliser with change of wing AOA

Also in terms of the planform it's not just the wing and tail that matter ... for example a lot of horizontal fuselage area ahead of the wing will have a destabilising effect.

[Bill Glover]

Isn't the cg supposed to be the neutral point?

No, I think you're getting confused.

If you balance the plane so that the CG is at the NP then the plane will have no stability in pitch. That may or may not be what you want, depending on what type of plane it is and your personal preferences. The 'official' CG position given for kits and plans is always forward of the NP to give some margin of stability (even for aerobatic models). It's then up to an experienced pilot to move the CG back ... if they want less stability and better elevator response. As already mentioned, many 3D models are flown with the CG on the NP. Apart from the 'dive test', if you have a symmetrical wing section you can simply roll the plane inverted and see how much down elevator is required to maintain level flight. Lots means the plane is nose heavy (stable).

[Andy W]

Isn't the cg supposed to be the neutral point?

I already answered this, above.

The CG is wherever you put it. It doesn't mean the model will be flyable..

Bill addresses it in more detail.
..a

[Spinner]

Maybe the best way to undestand this is that the c.g. location is totally determined by the weight distribution, and has nothing to do with aerodynamics. The neutral point is totally determined by aerodynamics, and has nothing to do with weight distribution. The difference in the locations of the neutral point and the c.g. detrmines the stability of the airplane, as described so well by others.

Good Luck,
Joe

[Bill Glover]

Good summary!

[Salto]

Here, here!!

[Al_]

A very important issue with the CG in relation to the center of lift, the center of aerodynmaic drag, and the position of the wing.

If the plane is high winged like a trainer or a piper cub, the CG will have a pendulum effect beneath the wing. With the wing in its high position, it will create drag towards the top of the plane and pull back in a manner similar to pulling back on the elevator. This will not happen to a low-wing plane with a similar cg coordinate relative to the center of lift and center of drag. Instead, a low winged plane will be more likely to stay on its current heading or, if the plane is not set up properly, it will want to go into a steeper dive as the lower wing pulls back harder. (also important here is the thrust angle, but I wont get too technical).

To me, the cg plays another important role during low speed flying:

With the CG in front of the center of lift, the plane will want to drop its nose when the wings lift is reduced and the effectiveness of the stabilizer and elevator lessens. Because of the forward weight, the elevator has to push down slightly to keep the nose up. Like lift, this effect lessens with the dropping airspeed. With the aerodynamic forces weakening, the forward weight wins and pulls the nose down.

If the CG is at/above/below the center of lift, but not in front/behind the center of lift, the plane will still want to drop its nose as lift is reduced. Unlike the nose-heavy situation above, the elevator has no input during level filgt at speed. But once the lift is decreased and the plane's angle of attack increases, the stab/elevator creates its own lift from the air flowing up beneath it. This will cause the nose to drop because the force of the elevator is the only external force acting on the pitch of the aircraft.

Another situation is a tail heavy aircraft (but still balanced. As the lift decreases with the lessening speed, the angle of attack increases. This time, the elevator has been holding this weight during the flight and has been producing lift to compensate. If the effect of airspeed on the lift of both the wing and elevator are equal, the plane will begine to sink without pitching the nose down because all forces, both internal and external, are in equalibruim. This plane could possibly do a flat stall.

Finally, there is a plane which is tail heavy beyond practicality. Because so much weight is on the tail surfaces, the elevator has do do a great deal of compensation for the weight placed on it. When the airspeed is reduced, the wing cans still be producing lift capable of level flight, but the elevator is losing its effect. With the tail effect lessening, the plane will want to pitch its nose up. This situation is not prerable to most because the added pitch could cause the wing to stall. Simpply put, this plane will want to point to the stars when flying slow.

[Al_]

If you have a high wing plane, put the CG slightly ahead of the center of lift