sportrider_fz6

I've learned a lot by asking questions here, so I'll add another one. I was wondering what the effects are from having the CG to far forward and to far back. do people adjust it for different style of flying or is it something that is always a constant?

HighPlains

My Feedback: (1)

On a conventional aircraft, the CG is normally in front of the center of lift. With this force arrangement the tail is required to create negitive lift in order to balance out the other two forces. With the CG and center of lift close together, it requires a very small force at the tail to offset the imbalance, while moving the CG forward requires much more. As these two main forces are moved further apart, the model has a greater static stability. The down side is that as the airplane slows down for landing, the tail may not be able to created enough down force to bring the wing to a stall (which gives the lowest landing speeds).

On the other hand, as the two main forces are moved closer together, the static stability is reduce, and at some point the control become divergent. In this case, the model is difficult to fly smoothly, and you find it to be very pitch sensitive. When very tail heavy, the model becomes impossible to fly.

On a trainer, a very nose heavy airplane will self recover in pitch when trimmed out. This is because when the airplane is gaining speed in a dive, the extra force on the tail pulling down will cause the airplane to return to a climb. If climbing it slows down which reduces the force and lets the climb go back towards level. A well designed and balanced airplane will go back and forth decreasing amounts until it returns to trimmed flight.

As the CG is moved back, these forces are reduced to the point where the airplane will no longer self recover.

MinnFlyer

My Feedback: (4)

CG is a term that is used for a hypothetical range in which the airplane will fly comfortably.

Let's say a plane gives you a 1/2" CG range. If you balance the plane at the forward end of the range it will be heavier in the nose than it would if it balanced at the rear end of the CG range. At this setting, the plane will be more stable and less maneuverable.

If you balance near the rear end of the range, the plane will be more maneuverable, but less stable.

They both have their good and bad points, but what is worse is when you go too far outside of the range in either direction.

If a plane is too nose heavy it will become very sluggish and stall speed can increase to unacceptable levels (Won't slow down to land).

If the plane is too tail heavy it will rarely make it home in one piece. The reason for this is simple: Think of a dart. Now take the lead weight off the front and make the fins out of lead instead of feathers. If you throw it, the fins will flip around and go first.

A tail heavy airplane will do the same thing - the tail will try to go out in front. It's not pretty.

So where is the best place for the CG? That's totally up to you.

Balance the plane in the center of the CG and fly it. Do you like it? If you like it, leave it. If you want to experiment with other settings, move the battery pack back a bit and fly it again. Is it better? Worse?

You will usually find that the rear-half of the CG range will give you a better flying plane. Then again, you may prefer the way it flies when it's on the nose-heavy side.

So you see, it's totally up to you where the plane balances best.

ChuckW

Here is what happens to a plane with the CG too far back (tail heavy). The thing was very unstable, like trying to balance an elephant on the tip of a needle.

da Rock

Actually, a simple question that has simple answers.

Too far forward... The elevator is maxed out working to carry the load the CG creates being so far forward. Airplane is close to death from the elevator having no spare force available to do controlling as it's busting butt just fighting the bad CG.

Almost Too far aft... But still safely ahead of the Neutral point.... The elevator is extremely effective. Needs almost no deflection to give huge results. Tune the elevator throws to be minimal, and you got a flyable model. Unfortunately, no one does that and they all wind up with an airplane that scares the crap out of them.

Farther aft than that... As in at or behind the NP. No hope. The forces generated by the tail work with the lousy CG instead of against it. Unflyable model.

Charlie P.

I heard it summarized as follows: An airplane that is nose heavy flies poorly. A plane that is tail heavy flies once.

Nose heavy can be difficult to take off and perform poorly in aerobatics.

Models generally are balanced at a neutral or slightly nose heavy position with the tank empty. That way at no time in the flight do you become tail heavy.

One test is to fly at a 45º climb and do a half roll to inverted. If you release the stick and the model falls further onto it's back it is nose heavy. If it tries to increase the climb angle or nose up it is tail heavy. If it continues on course you're gold.

goirish

Hey ChuchW
what's wrong wtih that, I have a bunch just like it.

gboulton

My Feedback: (15)

An explanation I just tossed up in another forum leads me to wander in here, and provide a similar analogy....

Let's use the tried and true teeter-totter (or seesaw if you prefer) analogy.

Your airplane is, essentially, a seesaw. Its current actual CG, or balance point, is the same as the fulcrum, or balance point, of the see saw. Just like the see saw at the park, the CG is the point at which the airplane has equal "moments" on each side. Note, this is ot the same as having equal WEIGHT on each side.

Take the seesaw. Can you balance it with a little kid and big kid? Sure. Just put the little kid farther away from the middle than the big kid. Their "moments" will be their weight times their distance from the center. So, 50 lb kid 5' away has a 250 ftlb moment, 100 lb kid 2.5 ft way also has a 250 ftlb moment, so they balance. Yay.

Now, here's the thing. Your airplane has, at its very end, a device for moving the tail up and down...the elevator. Let's say that our balance point..our CG...is a LONG way away from that elevator. That is, very far forward. You have, essentially, a tiny little 10 lb kid 25 ft away from the center of the seesaw, and, say, a 50 lb kid only 5 ft away on the other side.

What happens?

Well, you can move that little 10 lb kid (remember, he's the "tail" of our airplane) a LONG way up and down...and the bigger kid (the nose of our airplane) will barely notice, since he's close to the balance point. So, your elevator moves the airplane's tail a LARGE amount...and there's VERY little change to the airplane's pitch. in short, a very ineffective elevator.

Now...this airplane is probably flyable...it's not going to do any "jerky" or "sudden" pitch changes to be sure...but you'll need to use a whole bunch of elevator to make just a little pitch change...hence, a "nose heavy airplane often flies poorly".

Now, let's reverse this...and say that the CG is very CLOSE to our tail...which would be a tail-heavy airplane.

Envision that the airplane is moving the other direction now...the 50 lb kid 5 ft from the CG is the tail, the 10 lb kid 25 feet away is the nose...

Move the 50 lb kid up or down just a TINY bit, and the little kid on the other end (the nose) is sent rocketing into space...or, alternately, slammed to the ground.

In this case, the tiniest bit of pitch change results in a HUGE change to the nose, and thus pitch. Just a touch of elevator, and the plane is zooming straight up...which, of course, the pilot immediately "corrects" for by applying down elevator...and getting the same effect, over-control, and slamming the poor 10 lb infant to the ground. Next thing you know, you've got a panicky mother, you've ruined some parenting group's play date, and there's the inevitable lawsuit to deal with.

Hence, tail heavy planes fly only once.

it's absolutely something that's adjusted by different folks for different tastes/preferences.

Within reason, of course, there's not really a "This plane won't FLY because its CG is too ________." issue here. (Ok, sure, put the CG at, say, the tail fin, it ain't gonna fly...but I said within reason *heh*) The manufacturer will, after testing (one would hope), determine a "range" for the CG...they'll say "Hey...balance your airplane between here and there, and the majority of pilots will find it quite controllable for initial flights".

But that recommendation is just that...it's a recommendation for a MAJORITY of pilots, for INITIAL flights.

Once you're beyond that...hey, whatever floats your boat...or, in this case, your airplane.

Some folks (me, for example), like what others would describe as a HYPER sensitive elevator. I want my airplanes to come just shy of a high speed stall if I so much as THINK about pulling back on the stick. So, I fly with the CG "back"...usually either at the rear of the suggested range, or even behind it. That doesn't make me a "better" pilot or anything...it's just what I prefer, and I'm not alone in this.

On the other hand, some guys like a very "squishy" elevator...so "squishy" that the majority of folks would think something was wrong, and some of us would almost wonder if the elevator's even there! They fly with CG's well to the forward end of the suggested range, or even in front of it. Again, these aren't "good" or "bad" pilots..it's just what they prefer.

In either event, it IS safe to say that the manufacturers are usually right...something within their range will prove flyable for a majority of pilots during initial flight testing...and then they can move the CG from there as it suits them. Over time, if you find that you happen to prefer planes "a bit nose heavy" or "a bit tail heavy", then you can "cheat" a little, and balance the plane toward one end or the other of the suggested range initially.

HTH!

daveopam

My Feedback: (9)

No offense to the others but, Highplains that's one of the best descriptions I have seen. Great job. It reminds me of Darrel Waltrip trying to describe loose and tight and a race car. He said"tight is when you see the crash." It made sense to me.

David

HighPlains

My Feedback: (1)

Thanks David. It is a lot easier to explain with a bit of hand waving and a chalk board.

I used to do Autocross. Setting up the car and learning how to drive took several years, but the skills learned have saved my bacon more than once. I did quite well in my SCCA car class to the point where it became boring. Later I found RC Pylon racing to be much more difficult.

bkdavy

My Feedback: (1)

I've held off long enough. There is one piece of the discussion that is missing, and it is critical to understanding the nose heavy stability vs. the tail heavy instability. That is the relationship between the center of gravity and the center of lift.

The center of gravity is the point at which all the downward forces due to gravity of the plane appear to act. Commonly referred to as the center of balance or the balance point. That is the center of the balanced teeter-totter.

The center of lift is the point at which all the upward forces generated by the aerodynamic forces on the plane appear to act. In the case of the teeter-totter, it is the fulcrum of the lever.

When the center of lift and the center of gravity perfectly coincide, there is no differential force generated between the two (that is rotational force on the airplane). The airplane will move in the direction it is pointed without input.

This condition has no tendency to correct the planes attitude, thus it has no dynamic stability, but it is not unstable. A plane flys with no up elevator trim, and inverted flight requires no elevator input (assuming a symmetrical wing). For an asymmetrical wing (typical air foil), the center of lift changes when inverted, so different discussion.

When the center of gravity is forward of the center of lift, that generates a force that tends to want to drop the nose, that is nose heavy. This condition is normaly corrected by adding some up elevator. Consequently, when a plane with a symetrical airfoil goes inverted, we need to input down elevator to prevent the nose from dropping.

The nose heavy condition has another benefit, that is dynamic stability. As the direction of the wind over the wing changes (turbulence), the wing will generate a rotational force that points the wind into the changed wind direction. Thus the plane will continue to fly with the wing pointed into the wind over the wing. Since the air is normally flowing front to back over the wing, the flight of the plane is stable.

Now the tail heavy condition. When the wind over the wing changes, the rotational force that develops between the center of lift and the center of gravity tends to cause the wing to rotate opposite the desired direction. This happens with any change (such as small elevator movements). The plane is dynamically unstable, because it reacts to worsen the condition. Somone earlier had used the analogy of the dart, but it got edited out, but it was exactly correct. This dynamic instability can be overcome to a small degree by the elevator and pilot, but it doesn't take very long before the plane becomes uncontrollable, and any control input becomes grossly exaggerated. NASA used to have an animation on the web that showed this effect.

I know this is a long explanation, but its not a simple effect.

Brad

sportrider_fz6

thanks for all the replys. that answered every question I had.

plugin

bkdavy, now that you're on the subject of airfoils let me know if I get this right: The dihedral causes the plane to level itself when on straight line. The symmetry of the airfoil will determine if the plane flies straight inverted or if it needs a little elevator. Right?

Osirisf16

Because i'm an optical guy, i found some pictures and want to share them with you.

So, that's how you measure the CG?



And that's a bad balanced plane (tail-heavy). The nose-heavy is exactly the opposite, right? To prevent that unbalance, you move your battery? That's all?

da Rock

Sort of..........

Dihedral helps level the wings when one is higher than the other. The tail helps "the plane to level itself" when the nose is too high or too low.

The airfoil being symmetrical will usually decrease the amount of elevator needed when the airplane is inverted. Airplanes with symmetrical airfoils almost always need different elevator deflection right side up versus upside down.

bkdavy

My Feedback: (1)

The dihedral causes the plane to level the wing side to side. (roll). It does nothing to keep the plane level front to back (pitch)

Lift is generated by the differential pressure between the top of the wing and the bottom of the wing. There are two contributing factors to that differential pressure. The first is the shape of the air foil, and the second is the angle of attack in flight.

For the classic trainer air-foil (flat bottom, curved top) when the wing has a zero angle of attack, the air flow over the top has a higher relative velocity to the wing than the bottom, and the Bernoulli effect results in a lower pressure on top, creating lift. If the CG (just to keep on the topic of the thread) is coincident with the center of lift, the plane will be balanced level, and should not require any downward force generated by the elevator to keep the plane level. Thus as you increase speed, you'll increase lift, and the plane will tend to climb.

For the Symmetrical airoil (typical for aerobatic planes) the airpath is the same over both surfaces (top and bottom) thus the only way to create lift is by altering the angle of attack. The wing must have a slight positive angle of attack to create a differential pressure, and thus create lift. Therefore, a properly balanced aerobatic plane with a symmetrical airfoil will require some downward force on the elevator to keep the angle of attack positive for the wing (up elevator).

Now invert the planes.

The Trainer is generating a lot of lift, but its in the wrong direction. The angle of attack of the wing must be adjusted to compensate for that. Thus you must push the nose up (by using down elevator) until the effect by the angle of attack overcomes the airfoil.

The aerobatic plane requires only a small amount of down elevator to reverse the condition that was trimmed into the plane for straight and level flight, since it doesn't have the big airfoil effect (the wing looks the same from an aerodynamic standpoint)

Does that sufficiently muddy the water?

Brad

da Rock

Yes, moving your fingers as shown in the picture is how you find out where the airplane balances level. And when it's balancing level, the CG will be directly below the balance point. When manufacturers tell you where the CG should be on the wing, they're not saying it exactly right. They are saying where they want your fingers to go to get the airplane to balance level.

If you've got your fingers placed where you want the airplane to balance level, and the nose is lower than the tail, the nose is heavier than the tail. And the plane is nose heavy. And you move whatever you can easily move in order to bring the balance point where the mfg wanted it (or you wanted it). If you can't move enough things in the airplane for it to balance level, then add weights to the light end. That's why LHSs sell lead, stick-on weights.

Osirisf16

In the picture, that guy's fingers are not exactly in the middle of the wing. How can i tell when i find the balance point? I must first do that the guy in the picture do without adding receivers, batteries, servos etc?

MinnFlyer

My Feedback: (4)

The CG should not be in the center of the wing.

It is usually back 27% - 35% of the wing's chord from the Leading Edge.

Check your manual for where your plane should balance.

pkevinb

But I like my planes balanced with the tails hanging low. Does this mean I need to re-balance them all?

Just Kidding!

Charlie P.

What MinnFlyer said. The plans or instructions will give the optimum C.G. point, or an acceptable range. Most trainers are 23% to 25% from the leading edge, but that's not chiseled in stone. Swept wings, tapered wings, high wing, low wing (balance the model upside-down), two wings. All different. It should be on the plans or in the manual.

I prefer two pencils with erasers to my fingers as the pivot point is then stiffer and smaller. Finger pads are too wide and forgiving. But it's just a "ball parky" starting point anyway. How he model flies and reacts is how to balance her out.

Put in everything you will be flying with except the fuel. I even pre-balance before covering fore-and-aft and side-to-side (strings around prop shaft and rudder hinges with model hung from ceiling) and add lead is necessary so it can be the smallest amount needed under the covering at the wingtips or vertical fin root/fuselage tail. Lighter always flies better.

With ARFs I have come to rely on cents or flattened lead pieces taped to servos in the wings or fuselage for balancing if battery placement alone won't do it. Easy access and well secured.

da Rock

Remember that the CG location you're trying to find is the STARTING location for your first flight. The suggested CG is really of no real importance after you've gotten the model safely into the air.

With your first flight, you decide your elevator throws. You figure out how the airplane reacts to the suggested elevator throw and you adjust that throw to fly the plane. You adjust it to make your stick less sensitive or more sensitive.

Then you decide if you want the airplane to be more agile or less twitchy. You move the CG to get that like you want it.

Step 1 is to set the CG for a safe maiden. No more no less.
Step 2 is the maiden and you might need to change the recommended throw.
Step 3 is from then on. You decide the best CG location by flying the sucker.

da Rock

There is nothing sacred about a CG location. And it's not as critical that we get it EXACTLY perfect, as it is to just do it.

You really need to check it before your maiden. You really need to get the CG near the suggested location. THAT's important. But near is good. Exactly on the spot is not needed at all.

And you need to have your elevator throw near the recommended. THAT's also important. But exactly on the measurement? Nope.

I'm about to maiden a new Corsair. I did the math to see what range I had for the maiden's CG range. (Actually, I had www.geistware.com do the math. I just did a couple minutes worth of work with a yardstick.) The wing on the sucker has a MAC just over 11". The CG range is an inch. I knew if I got the CG within that inch, the model would respond decently on the maiden if the throws were close. And it'd be safe if the CG was close. But, the airplane is a taildragger with a history of being easy to trip up. So I wanted to be sure the CG wasn't toward the front of where it could be. So for this time, I was slightly more interested in a relatively accurate CG location. I did want the CG to be in the back half of that 1" range.

It'll fly safely and the response to the stick will be safe too. Because the CG is near where the mfg suggested, and the throws are as suggested. But after the maiden, it's whatever the airplane tells me where the throws and CG goes.