Vertical CG ?

LAST EDITED ON Dec-06-04 AT 06:25 PM (CST)
 
There have been comments about the "vertical Cg" and some feel it is different than the fore and aft CG. Not so.

[photo not recovered: 41b4fe2b6716a40f.jpg]

By the way. The CG is a dimensionless point in the real sense and doesn't even need to be within the physical confines of the structure.

Is Nirvana when the CG rest exactly on the Aerodynamic center?

LAST EDITED ON Dec-06-04 AT 06:57 PM (CST)
 
Sorry, Dick - more force semantics. Any plane will need to be balanced about differect axes. Otherwise, we wouldn't worry about tip weight. The technical CG for pitch, yaw, and even roll may be the same, but balance in roll is way different.

That may be where the idea of different "CG's" comes from.

Dick,

Technically you are correct, but for the purposes of trimming a competition stunt ship or any stunt ship, one must trim in roll, pitch and yaw separately. See Chris Lella's superlative articles in Model Aviation from May 1976 through July 1976. He makes the problem simple to understand and correct. Then check out Paul Walker's excellent article on his Impact and how to balance and trim a stunt ship in Model Aviation May 1981. He even supplies a great flow chart on what to do.

Dick,

Nice, clear sketches! I only see a 'static condition' tho, with the model unmoving: No thrust, drag, lift, torque, centri***al force, line weight, curved flight effects, velocity gradient across the span, trimmed orientation...

If we can account for all of those, the sketch will look a bit different.

E.g., the dynamic span center is a bit outboard of the simple ruler measurement midpoint (the effect of the velocity gradient, i.e., the inboard tip flies a smaller diameter lap in the same time as the outboard tip flies a larger diameter lap, thus inboard tip slower & outboard tip faster.)

Also, about half the lineweight is carried by the model. That is pretty much a static effect though. Outboard tip weight should balance it, so that -- in flight -- lift, drag and weight(mass) all act very near where the dynamic spanwise center is.

Many other small and large things, but, then, you were in those longer threads, too. Sure, vertical CG has effects. Let's study it in among all the other stronger forces, in the same force or vector diagram under dynamic conditions. That's the only place it matters, right?

I fly a bit of carrier and having the thrust line below the vertical CG seems to help transition to slow flight. When you have slowed if you "goose" the throttle the plane rotates more easily to a high angle of attack. At least it seems to work that way for me. Some low wing models I have built just could not transition.

Bob Furr

I have built and flown five of the Frank Ehling "Easy". The first three had the bellcrank mounted on the bottom of the fuselage, and leadout guide on the bottom of the wing. All three flew around outside wing down. I warped the wing slightly and got them to fly pretty well. Basically the leadout guide was well below the vertical CG of the airplane. The last two I built with the bellcrank in the wing and the wings built straight in a jig. They fly around pretty much level and I have not installed any warps. So the leadout guide is fairly in line with the vertical CG. Whatever and wherever it may be.

I've finished building my Palmer/Goyet HI-BOY stunter. This is a high wing cabin model with the leadouts in the wing: Therefore, well above the lateral center. Bob, in a recent letter, mentions that the original Goyet design had leadouts at fuselage center, with a wing tip guide. However, Bob put the leadouts in the wing, which is a much cleaner way to go. His original article mentions that low wing stunters are completely successful, with the leadouts in the wing and below the CLA. After all, a wing below the CLA with enclosed leadouts must also fly inverted with the leadouts above CLA! And this doesn't seem to have made too much problem.

>Dick,
>
>Nice, clear sketches! I only see a 'static condition' tho,
>with the model unmoving: No thrust, drag, lift, torque,
>centri***al force, line weight, curved flight effects,
>velocity gradient across the span, trimmed orientation...
>
>If we can account for all of those, the sketch will look a
>bit different.
>
>E.g., the dynamic span center is a bit outboard of the
>simple ruler measurement midpoint (the effect of the
>velocity gradient, i.e., the inboard tip flies a smaller
>diameter lap in the same time as the outboard tip flies a
>larger diameter lap, thus inboard tip slower & outboard tip
>faster.)
>
>Also, about half the lineweight is carried by the model.
>That is pretty much a static effect though. Outboard tip
>weight should balance it, so that -- in flight -- lift, drag
>and weight(mass) all act very near where the dynamic
>spanwise center is.
>
>Many other small and large things, but, then, you were in
>those longer threads, too. Sure, vertical CG has effects.
>Let's study it in among all the other stronger forces, in
>the same force or vector diagram under dynamic conditions.
>That's the only place it matters, right?

Thanks Lou. You are right about this being a static condition. The concept of vertical CG can be visualized without introducing motion. I wasn't trying to "eat the elephant in one bite" so to speak..

There are indeed quite a few forces acting on our little planes.If each can be isolated,explored and understood, then comprehension is much easier. I’m a linear thinker. I like to start at point A and plod along until I get to point B.

I may be wrong but there are probably a number of people on this forum whose eyes glaze over when we start throwing around velocity gradients, moment of inertia, vector dot products, blah...blah...blah. My only purpose is to try to present a simple explanation (and pictures usually work) of what we are talking about. One bite at a time. If I'm preaching to the choir and you guys think it's a waste of time let me know... I'll stop.

So with that said, here's one of the variables Lou is saying is complicating the issue. If we were flying in straight line then things are not quite as complex as flying in a circle. The velocity (speed with a sense of direction )across the wingspan is increasing from inboard to outboard. And lift varies with the square of velocity. So if velocity doubles the lift is 4 times what it was at the old velocity.

[photo not recovered: 41b7779236e2629b.jpg]

This is the velocity gradient. Nice uniform straight line (linear) increase as it moves from inboard to outboard. I’ve calculated some velocities for a typical stunter.

[photo not recovered: 41b778e63d574ca9.jpg]

This shows lift gradient across the wing and it is increasing exponentially. The size is increasing with the square of the velocity. These forces act at various distances across the span produce a torque on the plane. The lift on the inboard side (right side) is trying to rotate the plane counterclockwise about the theoretical aerodynamic center. The lift on the outboard wing (left side) is trying to rotate the plane clockwise. If all of these torques are added up, the result can be represented as one force acting at some point on the wing. In this case , the red arrow is a pretty good approximation of where that sum of forces is acting on the plane. Lou refers to this as the dynamic span center. It appears to be the center where the lift is occurring. So this asymmetrical lift (one side is not the same as the other) produces a torque in a clockwise direction. Do you suppose this lead the Old Timers to make the outboard wing shorter than the inboard wing. An attempt to put the fuselage and its' mass on the center of lift.

Wouldn't it be nice to have an electronic blackboard for this forum!