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3D Flying Discuss everything related to "3D" flying including construction, power setups and flying techniques

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Old 04-05-2011, 06:32 PM   #1
DanWard
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Default Checking CG in flight?

I'm always tempted to post in beginner section because I have so much to learn but I think in this case posting in 3D might have me asking my question to the right sort of folks.

I've been studying and learning about airplane CG. When you build you check the CG on the bench to insure it is within the recommended range and probably start out on the nose heavy side to be safe.

But then once you have it flying and all trimmed for level flight you then need to check CG while in flight?

This is what I've put together from my reading and studying.

1. You start with upright level flight at full speed trimmed to maintain level flight with no hands on the sticks?

2. Then you pull up to a 45 degree line, roll inverted, and take your hands off the sticks?

3. The plane should continue on the 45 degree line. If it falls off toward the ground it is nose heavy? If it climbs towards the sky it is tail heavy?

Do I have this right?

I'm not sure I've got this right because it would seem to me that trim would be comingled in the equation somewhere. It may be that I'm so confused I won't be able to explain in written words.

It would seem to me that a plane slightly nose heavy to begin with, then trimmed for hands off level flight when upright, would require a little down stick pressure when inverted, at least all of mine always have required slight down stick pressure when inverted. Maybe I've never had a plane setup right. To take this thought a step further I would think that when I pull to a 45, roll inverted, and take my hands off the check CG it will fall off towards the ground due to the lack of slight down stick pressure. Make sense?

Somewhere I read tail heavy planes seldom fly twice, they get real twichy, a little up and the jump up, a little down and they jump down.

I always balance on the bench between recommended range but error on the nose heavy side. The more nose heavy the more up trim is required for hands off level flight, right? And the more down stick pressure required for inverted level flight.

So then the trim and slight nose heavy will make it fall off towards the ground when I pull to 45, roll inverted, and take my hands off, right?. At this point I would land, move CG back, which will affect upright level flight trim, right? I would adjust trim for level flight, pull to 45, roll inverted and repeat as necessary?

It would seem that if i keep repeating the cycle I'd end up at one point, and one point only, where CG is spot on? Which seems to conflict with instructions that say balance in a range of from here to here. And it would seem that at this one and only CG point trim would be all but gone and in level flight, whether upright or inverted, it would want to fall off towards the ground.

I'm sure this all sounds like a goober wrote it. Maybe I'm over thinking it. But dange the subject smokes the brain cells

I appreciate in advance any and all who take the time to read and respond!!!
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Old 04-05-2011, 06:49 PM   #2
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Hi Dan
Pleased to meet you
Try building one of these, i use one and they are very accurate and quite easy to use
http://home.mindspring.com/~the-plum...%20Machine.htm
Take care
Yours Hank

"When wild the head-wind beat,Thy sovereign Will commanding, Bring them who dare to fly, To a safe landing."
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Old 04-05-2011, 08:28 PM   #3
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I think you are on the right track, I have seen C/G explained almost like you wrote, I think the author would fly the inverted leg at a level flight, although high enough for safety. I remember him stating he preferred his models to require a slight of down elev, while flying inverted,

This flying test should help you get to the prefect setup for your plane, and the way that you want it to be.

The C/G point given on plans is usually a safe point or a range of points from mild to wild. Only flying a plane over & over and making small adjustments can you find the sweet spot for your plane, then a C/G machine like Hanks will give you a point on the airplane that you can use to check the C/G later, if you have to use a new size of battery or some other weight change, you can put it right back where you want it and go fly

I have read of a pro that puts about 200 flights on a new plane before he has it exactly the way he wants it.
I'm lucky to get 100 flights on a plane before it has bit the dirt, ( I only have one surviving plane that has about 100+ flights )

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Old 04-05-2011, 09:44 PM   #4
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the "correct" cg depends on the type of plane and the style of flying. I like my sport planes to have a cg that will need me to push up on the elevator stick when rolled inverted. For my 3d planes, I set the cg neutral. If I roll it inverted, the plane should continue to fly level hands off. Some 3d pilots prefer a little bit of elevator input to be needed. It's all about personal preference.
You can also test this on a vertical downline. If it pulls towards the gear then it's tail heavy, if it pulls towards the canopy then it's nose heavy.

Again, I wouldn't want to have a sport plane setup to be neutral because that would generally be TOO tail heavy for that style of plane.

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Old 04-05-2011, 09:47 PM   #5
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Originally Posted by kenchiroalpha View Post
Hi Dan
Pleased to meet you
Try building one of these, i use one and they are very accurate and quite easy to use
http://home.mindspring.com/~the-plumber/Vanessa%20CG%20Machine.htm
Take care
Yours Hank
CG machines are good to set the CG the manual recommends, or to get a baseline CG but you'll always need to tweak it in flight to really get a plane dialed in.

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Old 04-05-2011, 11:17 PM   #6
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There is more to setting CG then a single flight maneuver.

You really need to "trim" the entire model because there are a number of settings and adjustments that effect each other. changing one thing generally requires changing something else as well.

For example, poor decolage or an incorrect motor offset can mask or accentuate CG problems.

Trimming is a multi-step process that can take many flights. It involves going through the steps, makeing changes as needed, then going back through the steps again to see how those changes effected other adjustments.

I admit to rarely going through the entire process to completion, however, even once through will make a world of difference in how your plane flys. it will also increase your understanding of how the model works and why it flys the way it does. you will end up a better pilot in addition to haveing a plane that flys better.

Check out these sites for trimming guides and set by step procedures.

http://www.dmac.org.uk/

click on downloads, say yes to the legal stuff, then you can download different trimming and flying guides.


This next one is a simplified version of the one above. It covers all the basics pretty well though and has the advantage of being one page you can print and take with you to the field.

http://www.palosrc.com/index.php?opt...1:ic&Itemid=50


If your really into it, you can also checkout the IMAC web sites and the National Society of Radio Control Aerobatics web sites for additional info on trimming and initial setup of models.

I think I need a signature.
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Old 04-05-2011, 11:21 PM   #7
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Forgot the links.

http://nsrca.us/

and IMAC

http://www.mini-iac.com/DesktopDefault.aspx

I think I need a signature.
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Old 04-05-2011, 11:24 PM   #8
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Originally Posted by Yaniel View Post
CG machines are good to set the CG the manual recommends, or to get a baseline CG but you'll always need to tweak it in flight to really get a plane dialed in.
Hi
Understood
I cut my teeth on F/F aircraft over 30 years ago
Nothing better to learn how to trim an aircraft with
Seems like i could almost do it in my sleep now
Take care
Yours Hank

"When wild the head-wind beat,Thy sovereign Will commanding, Bring them who dare to fly, To a safe landing."
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Old 04-06-2011, 01:29 AM   #9
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I do a static balance first, to be where it is supposed to be.
I've made the hanging tool, and I also use my fingers.

Flying trim usually takes a few days of flying to really get it to where I may like it. I make sure all flight trims are set using the methods described earlier, then perform a few maneuvers to see how the plane reacts. I may adjust the CG by moving weight, or adding weight.

Example: My BME Cap Maniac would balloon up on landing when power was cut, but all indicators were that the balance was fine. It flew straight, passed all the up/down tests and flew great. Adding 2 oz to the nose as reccommended by Herve at BME solved the landing jump on it completely.

To balance an airplane:

1st Static Balance
2nd Flight trim then flight performance
3rd Adjust CG if Necessary within flight envelope if out of balance traits are demonstrable


One of my tell tale items is knife edge flight. Even if rudder mixes are used, I check if up/down is needed to maintain a straight line. Pull to canopy may be tail heavy, pull to wheels may be nose heavy.

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Old 04-06-2011, 07:45 AM   #10
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Originally Posted by Nitro Blast View Post
One of my tell tale items is knife edge flight. Even if rudder mixes are used, I check if up/down is needed to maintain a straight line. Pull to canopy may be tail heavy, pull to wheels may be nose heavy.
I've found the opposite.. in KE a nose heavy plane tends to turn toward the canopy, and a tail heavy one toward the belly.
Or at least that's how I've found it.. and the theory would also indicate that's what you would expect because nose heavy will have up trim and tail havy would have down trim.

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Old 04-06-2011, 08:10 AM   #11
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Steve is correct about KE pitching and CG. However, there are other things that can efect that pitching as well.

If you look through the trimming guiders I linked to above you will see that wing incedence/decolage and aileron reflex and motor up/down thrust can ALL also cause that same pitching either way.

So its possible to have multiple adjustments on a plane contributing to a particular behavior - and they may be conflicting or they may be adding to each other.

Thats also why trimming isnt a simple single step process. If you only do a single test and only adjust a single thing on the model, you will almost certainly not have all the information you need to come up with the correct solution.

I think I need a signature.
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Old 04-06-2011, 02:33 PM   #12
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I've read through some of the procedures and I end up with more questions.

For example adjusting wing incidence based of results from a step in the procedure. On most of my planes wing incidence is set by the designer. Carbon fiber wing joiner tube and locating pin on wing fixes wing position, the wing sort of plugs into a predetermined location set by the designer. Or in the case of my Multiplex Acromaster the wing plugs into the fuselage. How do you change the wing incidence?

In another step you adjust wing dihedral based of performance results of a step in the procedure. How would you adjust dihedral? In my case all of my planes are staright wing with no dihedral.

I appreciate all the feedback and I'm sure I'll be better for having gone through all of this but some of it...at this point...I'm struggling with.


Originally Posted by Larry3215 View Post
There is more to setting CG then a single flight maneuver.

You really need to "trim" the entire model because there are a number of settings and adjustments that effect each other. changing one thing generally requires changing something else as well.

For example, poor decolage or an incorrect motor offset can mask or accentuate CG problems.

Trimming is a multi-step process that can take many flights. It involves going through the steps, makeing changes as needed, then going back through the steps again to see how those changes effected other adjustments.

I admit to rarely going through the entire process to completion, however, even once through will make a world of difference in how your plane flys. it will also increase your understanding of how the model works and why it flys the way it does. you will end up a better pilot in addition to haveing a plane that flys better.

Check out these sites for trimming guides and set by step procedures.

http://www.dmac.org.uk/

click on downloads, say yes to the legal stuff, then you can download different trimming and flying guides.


This next one is a simplified version of the one above. It covers all the basics pretty well though and has the advantage of being one page you can print and take with you to the field.

http://www.palosrc.com/index.php?opt...1:ic&Itemid=50


If your really into it, you can also checkout the IMAC web sites and the National Society of Radio Control Aerobatics web sites for additional info on trimming and initial setup of models.
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Old 04-06-2011, 03:52 PM   #13
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You are quite right; there is no easy way to adjust incidence or dihedral on most planes. I assume those instructions are written for serious contest aerobatic flyers who are prepared to do serious changes to their models or develop new improved models based on their findings.
For most of us 'near enough' is good enough.

To be honest I would question some of the recommendations. For instance adjusting incidence would have to be done in conjunction with adjusting CG, the two are highly interdependent and canít really be treated separately.

Steve
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Old 04-06-2011, 04:11 PM   #14
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Originally Posted by JetPlaneFlyer View Post
You are quite right; there is no easy way to adjust incidence or dihedral on most planes. I assume those instructions are written for serious contest aerobatic flyers who are prepared to do serious changes to their models or develop new improved models based on their findings.
For most of us 'near enough' is good enough.

To be honest I would question some of the recommendations. For instance adjusting incidence would have to be done in conjunction with adjusting CG, the two are highly interdependent and can’t really be treated separately.

Steve
Exactly Steve. That statement I highlighted is why the instructions say to go through the steps multiple times Each adjustment you make effects other things. For example, CG changes also effect the camber settings (aileron droop or reflex) as well as down thrust.

The steps listed at the DMAC web site I linked to first above is a lot more thorough than the short list at the PalosRC web site. It has more steps covering more areas with better instructions and more details as to why you do each step. It also goes into the initial setup of the model in more detail. The IMAC and NSRCA web sites also have more setup details and tips specifically for pattern flying.

As far as changing incidence and dihedral - these things were written back before the days of ARF's. You were expected to know how to work on your plane

You can often shim under the leading edge or trailing edge on top or bottom mounted wings. On plug in wings, its an easy mod to plug the hole where the locating pin is and drill a new one - unless the wing is recessed into the fuse.

Dihedral I admit is somewhere between a lot more trouble and buying a different model

I think I need a signature.
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Old 04-15-2011, 12:41 PM   #15
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For adjusting CG its a feel thing for me but when I want my 3d planes to be perfectly neutral I trim for upright level flight at about ĺ throttle. Fly the plane nice and level and roll inverted. If it stays on track with no inputs inverted I'm done. I then take my plane inside and put it on a CG machine to find out where the CG is located on the plane. That way I can use other batteries in the plane that weight different amounts and still have the plane fly the same. (see only have to find my CG once in flight)

3dhs junky
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