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How C.G. affects flight characteristics?

Old 12-15-2008, 07:08 PM
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beachbrada
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Default How C.G. affects flight characteristics?

I know that a nose heavy plane will want to pitch down to the ground alot and that a tail heavy plane will fly squirrley. But what Im really looking at finding out is how the C.G. affects different maneuvers. From reading posts on RCU I know that for more sport/pattern flying you want it towards the nose and for 3D you want it aft. But I don't understand why this is the way it is, could someone go into detail to help me figure this out. This whole question stems from the fact that my Hangar 9 Twist .40 is horrible in knife edges, it drops altitude quickly if I dont add enough rudder but if I add too much it wants to roll hard towards the canopy. I came across a post that said moving the C.G. forward will help a plane knife edge better. The C.G. range on the Twist is 4-5", I have mine set at 4.75" and it fly's really nice, just a tad bit of elevator on inverted, but as I said before knife edge's are horrible. I know that certain planes have their qualities and the Twist may not really have the features or aerodynamics to do this maneuver but Im just really looking to dig a little deeper in the center of gravity subject. I've heard once that you could call the Twist a Jack of all trades but a master of none and its a fitting description. Also for those of you who have or flew a Twist what are some of you comments on the Knife Edges and so called 3D capabilites?
Old 12-15-2008, 07:19 PM
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ChuckW
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Default RE: How C.G. affects flight characteristics?

The Twist .40 just doesn't knife edge well. Actually, I will re-phrase that... it isn't as friendly as some other airplanes in knife-edge. It can do it, just takes a littl emore effort. Playing with the CG will make a difference but not completely eliminate all of the coupling issues. I don't know if the problem is with the wing incidence (I'd think that would cause a pitch rather than roll problem), the shape or placement of the wing, the position or shape of the rudder or what. I know fuselage side area will also greatly affect how well a plane does in knife-edge.

I have a basic understanding of aerodynamics but I'm by no means an expert.

In my opinion, the Twist is a fun-flyer rather than a 3D plane. One thing you can do to improve knife-edges is to practice applying aileron opposite of the roll it trys to do. Once you master it, you can get a lot of planes to stay in a knife-edge.

Sorry I haven't answered your CG questions completely. I'm at work but I do have notes on that stuff at home. I'm sure someone else will chime in before I get there and look at them though.

Old 12-15-2008, 07:36 PM
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Default RE: How C.G. affects flight characteristics?

beachbrada:

Here you have something to read about CG and coupling:

http://findarticles.com/p/articles/m...g=artBody;col1

http://www.geistware.com/rcmodeling/...tics/setup.htm

Regards!
Old 12-16-2008, 01:05 PM
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Default RE: How C.G. affects flight characteristics?

Now that you have read all about CG and Coupling I will go back to what Chuck said, the Twist will do most everything but it's a fun fly plane and the way the fuse is designed it isn't going to ever give you that cool down the runway knife edge. Even a lot of the old pattern planes don't KE very well. This season I had to do repairs on my old Kaos and give the poor thing a new covering job. It never did a great KE either so I added a big long fin to it plus added to the rudder giving both more area. This changed the way the plane fly's a lot. Now it does the KE about the way I wanted it to do plus it tracks a lot better. Down side to it is flying in a cross wind, more like landing in a cross wind!! This plane has been with me for a lot of years and I know it like the kid brother I never had so I have no problems with it but you should hear the guys in the pits when it comes in looking at me. I do it for fun but it may bite me some day.
Old 12-16-2008, 02:52 PM
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Default RE: How C.G. affects flight characteristics?

Things are more stable when the CG is ahead of (a contact point or) the wing of plane. As you move the CG aft, you lose stability and gain 'maneuverability' until you get to a point where the system is no longer stable (there's actually a mathematical explanation which is pretty darn ugly and tedious). A forward CG is good for sport flying for more stability, and also so the nose of the plane pitches down in a stall, thus creating a position to get air flowing over the wings again (i.e. trainers).

3D pilots also benefit from an aft CG in hovering and high alpha maneuvers as the plane is less 'top heavy' and thus easier to hold in a vertical or high alpha position.

Pattern guys do both; some like a forward CG for stability and better tracking through a maneuver, while others like it set a bit further back for a more sensitive plane and less 'push' for inverted flight, but definitely not as much as a 3D plane.

Other things like vertical flight and knife edges are affected by CG too, and usually results in a pitch toward the gear or the canopy. There are lots of trim charts to help adjust for this stuff, but it also involves things like wing incidence, thrust angles, etc. Too much to explain here, but basically things that are designed into the plane for level flight become skewed when you change the plane's attitude.
Old 12-16-2008, 05:51 PM
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Default RE: How C.G. affects flight characteristics?

Joe just told you to make a bigger rudder!!!
Gene
Old 12-16-2008, 06:28 PM
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Default RE: How C.G. affects flight characteristics?

Think of your fusalage as a fulcrum with the pivot point at the CG. Look at the picture below and you will see in the exagerated example that the more aft your CG the less you need to manipulate the tail feathers to change the pitch of the aircraft. The example with the more forward CG takes a lot more to get the same pitch change. Hence a plane with a farther aft CG is far more pitch sensitive than the same plane with a more forward CG.
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Old 12-16-2008, 09:58 PM
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Default RE: How C.G. affects flight characteristics?

ORIGINAL: Gray Beard

Joe just told you to make a bigger rudder!!!
Gene
Ha ha
Actually... if you look at planes like the [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXMUX7&P=7]Great Planes Reactor[/link], you'll see that the tail dips down way below the thrust line, and (nearly) half the rudder surface area is above the wings and half below. What this does is balance out the drag forces when the plane is in knife edge flight. On planes where this is NOT the case and you roll into knife edge, the rudder acts as an air brake and creates a ton of (pressure) drag ONLY on the canopy side of the wings, thus creating a roll or pull to the canopy. Many aerobatic planes have the bottom of the rudder just below the thrust line, and have an angular shape that (more) equally balances out the surface area.

(I like fluid mechanics=)
Old 12-16-2008, 11:33 PM
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Default RE: How C.G. affects flight characteristics?

Very well described, gaRCfield!!

I would add that the vertical location of the CG respect to the center of pressure created by the fuselage and the rudder may create a torque around the thrust axis that will add to the coupling effect.
Hence, moving the battery up or down, if possible, could help in a better KE.

Also, the highest the propeller thrust, the easier the KE, because of the vertical component; a propeller of bigger diameter and lower pitch would also help.

Regards!
Old 12-17-2008, 10:27 PM
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Default RE: How C.G. affects flight characteristics?

One of the most important effects of CG is on how much the trim changes with variations in speed. When the CG is forward, you need more up elevator flying slow than flying fast. As you move the CG back, this effect is diminished. Likewise, as someone pointed out, the difference between upright and inverted is minimized.

On most of my planes I like moving my CG back to the point where, if I put it in a dive and then release the elevator the plane stays in a dive - usually called neutrally stable in pitch. A little disconcerting till you are used to it, but it gives you a nice even attitude regardless of speed, and spins and snaps are easier.

My only exception to this rule is occasionally you will have a plane with too little side area in the rear. Your CG does not only affect pitch. The farther back the CG, the less side area in the tail and the more in the nose. That can cause Dutch roll. With one of my planes I had to move the CG forward to get rid of Dutch roll.

Jim
Old 12-18-2008, 08:17 AM
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Default RE: How C.G. affects flight characteristics?

Im going to try and put the C.G. on my Twist at 4" and see how its flys, the range is 4-5" and its at 4.75" right now.
Old 12-18-2008, 08:34 AM
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Default RE: How C.G. affects flight characteristics?

This is from one of my trim charts:
Knife Edge Pitch:
Fly model on normal pass, roll to knife edge, left and right, use rudder to hold model level
A. Model does not change pitch: No adjustment needed
B. Model pitches to canopy: Either move CG aft; or increase wing incidence; or mix down elevator with rudder
C. Model pitches to belly: Reverse of B;

Dihedral
Fly model on normal pass, roll to knife edge, left and right, use rudder to hold model level
A. Model does not roll: Dihedral OK
B. Model rolls indirection of rudder: Reduce dihedral
C. Model rolls opposite to rudder: Increase dihedral
Old 12-18-2008, 09:35 AM
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Default RE: How C.G. affects flight characteristics?

GaRCfield, my Twist rolls towards the canopy in knife edge flight and there is no dihedral in the wing.
Old 12-18-2008, 11:00 AM
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Default RE: How C.G. affects flight characteristics?

Yeah I guess that part only works with two piece wings. I've found that a few planes that don't appear to have dihedral actually have a degree or two, sometimes a half;

maybe you could try moving both ailerons up a degree or two? just a guess. obviously not the same as dihedral, but might push your plane away from the canopy in a knife edge.

here's a link to some trim charts if you're interested. like others have said, the plane just may have it's own unique characteristics.

http://nsrca.us/all/flying/114-flyin...trimchart.html
http://www.rcaerobats.net/trim_chart.htm
http://www.modelaircraft.org/Mag/FTG...31/31main.html
Old 12-18-2008, 12:16 PM
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Default RE: How C.G. affects flight characteristics?

ORIGINAL: beachbrada

Im going to try and put the C.G. on my Twist at 4" and see how its flys, the range is 4-5" and its at 4.75" right now.
beachbrada:

I would suggest moving the CG in the opposite direction, to the tail, even beyond 5" from LE, if your tests still indicate nose heaviness.
That is something to be done in small increments, being alert for any sign of tail heaviness (touchy elevator).

For level flight, you get the balance around a CG that is too much toward the nose, by trimming the elevator up, so the wind pushes the tail down at cruise speed.
When going into KE flight, the elevator trim that tries to pitch the model remains, but the effect of the CG does not match it.
For inverted flight, you get the balance around a CG that is too much toward the nose, by elevator down input, so the wind pushes the tail up at cruise speed.

That is the coupling desirable for a trainer, but not so good for an aerobatic model, which is trimmed (to the very limit of tail heaviness) to go where is pointed, with no auto-corrections.
Most of the available trimming charts are for aerobatics.

Good reading about this here:

http://fatlion.com/sailplanes/divetesting.html

http://fatlion.com/sailplanes/decalage.html

http://www.charlesriverrc.org/articl...GMarkDrela.htm

http://www.dubai*********.org/flight_trimming.htm

Regards!

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