Notices
RC Pattern Flying Discuss all topics pertaining to RC Pattern Flying in this forum.

CG check

Old 05-28-2014, 06:20 PM
  #1  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default CG check

I've been thinking about this (probably my problem )

I know of the following methods to check CG: inverted 45 climb under power and watch nose, 'near vertical' bank and watch nose, and calculate based on MAC.

Right now my Integral has a little 'up' elevator trimmed in for straight and level. I feel like that would counter what is really going on in the inverted 45 - naturally I will drop the nose since the tail is pushing the nose down when inverted. I don't really understand/believe the 'near vertical' bank since everything aft of the CG during KE flight is basically a wing - why wouldn't the nose drop first?

Anyway, I want to understand this so I can make the right change and make it once. I get a pull to canopy in verticals, 45's, and KE and it seems the amount of correction needed is pretty darn close to the amount of elevator trim I'm carrying. I'm thinking I could use a little more wing attack but want to be confident in my CG location and method before making any changes.

For reference: electric Integral ~4800g, setup (-0.5), (+0.5), (0), CG started at 170 (front of wing tube, now moved back to center of tube). I like a fairly neutral feel during the inverted 45 but moving my batteries back isn't changing it all that much. (Edit: Also I get zero change in pitch when I chop throttle from high power).

Thanks for reading and your suggestions! This plane flies pretty nice already and it will be incredible if I don't need to chase the trim all over the place.

Last edited by Jetdesign; 05-28-2014 at 07:19 PM.
Old 05-28-2014, 06:40 PM
  #2  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,400
Received 114 Likes on 100 Posts
Default

Joe, if it were me I would set the motor to zero and adjust the CG until the up trim is gone then start trimming all over. For the past few years I have been setting up my aerobatic models this way and usually am pretty darned close from the bench. I will also adjust engine thrust to correct up lines only and never to correct horizontal flight issues.
Old 05-28-2014, 09:18 PM
  #3  
OhD
 
Join Date: Oct 2002
Location: west hills, CA
Posts: 1,159
Received 8 Likes on 5 Posts
Default

Originally Posted by speedracerntrixie View Post
Joe, if it were me I would set the motor to zero and adjust the CG until the up trim is gone then start trimming all over. For the past few years I have been setting up my aerobatic models this way and usually am pretty darned close from the bench. I will also adjust engine thrust to correct up lines only and never to correct horizontal flight issues.
I agree you need to get it flying with no trim in horizontal flight. I had been chasing the same problem on my Axiome + and found it was a cg problem. I found a program on the Internet that would let me calculate the MAC of a four panel wing. The Axiome + can be approximated with four panels and the results showed the cg had to be much further back than where I had it. What was really interesting was the fact that the mean aerodynamic chord was not an actual chord on the wing but was shorter than the actual chord at the specified distance from the centerline. You could also calculate the neutral point of the plane which takes into account the horizontal stab but you need to know if the horizontal stab is efficient on your model. I never found a way to determine this, but I'm sure someone knows.

Jim O
Old 05-29-2014, 02:07 PM
  #4  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

Thanks guys. I think the answer to my question is really - there is none. There are guidelines to get you close. I've been reading more of Bryan's triangulation method and the physics make a ton of sense to me. In that case, the location of CG is really a result and not a setting. In other words, you end up with a CG location that works with you and your plane.

I had started with something like Goldsmith's trim process, or one of the similar IMAC trim charts. They start with a CG test, but there is no quantitative answer to where it should be. There are guidelines that get you close.

I think most of us know that changing your CG impacts everything else. So it makes sense that you can't really pick a CG location and stay with it, as you have to balance how the plane flies in all attitudes.

My question then is really where the 25% MAC comes from, but that is another topic! And one I'm sure I can answer with a little research
Old 05-29-2014, 02:43 PM
  #5  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

Here is a good article on the 25% MAC assumption (from the wonderful state of Vermont nonetheless ): http://www.dept.aoe.vt.edu/~lutze/AO...rfoilwings.pdf

Last edited by Jetdesign; 05-29-2014 at 03:47 PM.
Old 05-29-2014, 05:41 PM
  #6  
VerneK
Senior Member
 
Join Date: Mar 2005
Location: Livonia, MI
Posts: 258
Likes: 0
Received 0 Likes on 0 Posts
Default

Joe,
Your answer is in your statement of symptoms and its really quite simple. Everything you have right now results in a pull to the canopy when you're trimmed for straight and level which just happens to be up trim. So you need to do something that will create the need for down trim compared to where you're at right now.

If you increase the wing incidence, it'll climb at the trim settings you currently have and you'll need down trim which should fix all or most of your problems. The other way to get there is to move the CG back which will also make the plane climb compared to where you're currently at and you'll need down trim which should fix all or most of your problems.

The choice for me would depend on how the plane feels when you fly inverted and how it snaps. For now, just stick with the inverted feel. You should need a little down elevator stick to fly inverted. If you don't, you're tail heavy and I'd go with the wing incidence increase.

If you need a lot of stick to fly inverted, you're nose heavy and I'd try moving the CG back a little before changing the wing incidence. You may need a combination of both, but only do one or the other at a time. The key here is to get rid of the up trim you're flying with. That should solve everything.


BTW, if you change the incidence, don't be afraid to crank on the adjusters. For most setups, 1 full turn results in a relatively small change. Just write down everything you do so you can put it back if necessary. I put heavy black Sharpie lines on my ball driver so I can tell when I've turned it exactly one turn.

That plane flies great with 1/2 degree down thrust, at least, the 3 I built did.

Verne Koester
Old 05-29-2014, 06:45 PM
  #7  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

Thanks Verne. That is pretty much what I was thinking. I started this thread thinking there might be a quantitative test to check for CG, but really it's a balance of motor thrust, CG, and wing incidence (hence the triangulation). I read through Bryan's method thoroughly after I started this so I have a pretty good understanding of what I need to do now Looking forward to seeing you and everyone else in a few hours!
Old 05-30-2014, 09:00 AM
  #8  
OhD
 
Join Date: Oct 2002
Location: west hills, CA
Posts: 1,159
Received 8 Likes on 5 Posts
Default

Originally Posted by Jetdesign View Post
Thanks Verne. That is pretty much what I was thinking. I started this thread thinking there might be a quantitative test to check for CG, but really it's a balance of motor thrust, CG, and wing incidence (hence the triangulation). I read through Bryan's method thoroughly after I started this so I have a pretty good understanding of what I need to do now Looking forward to seeing you and everyone else in a few hours!
Think about this. Let's say your plane is quite nose heavy and needs up elevator trim to fly level. You can increase the wing incidence until it flies with no up trim but you still have a nose heavy airplane. Will you be happy with it?

I'm no aero guy, but it seems to me you need to start with the cg in an acceptable range that can be calculated based on the MAC. You don't want too much incidence in the wing or too much downthrust. Then read Verne's recommendation's to fine tune it.

Jim O
Old 05-30-2014, 09:13 AM
  #9  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

Jim, I completely agree. You have to have a place to start. I'm playing with some tools to figure out the MAC but so far they all say that all of us are too tail heavy so I must be doing something wrong.

In your scenario above, nose heavy with added pos. wing angle would result in more pull to canopy when wing is unloaded. If the problem gets worse you know it was the wrong adjustment.

My CG feels pretty good to me esp. after moving it back a little. I can hold altitude inverted better than anything I've flown yet. Responsive but not toucy or mushy. Inverted 45 drops nose a little more than I would like but close to 'pattern standards. But its all relative which is why the CG check is not a definitive test by itself.
Old 05-30-2014, 09:13 AM
  #10  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

Duplicate post.
Old 05-30-2014, 10:17 AM
  #11  
OhD
 
Join Date: Oct 2002
Location: west hills, CA
Posts: 1,159
Received 8 Likes on 5 Posts
Default

Try this:

http://adamone.rchomepage.com/cg4_calc.htm
Old 05-30-2014, 10:39 AM
  #12  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

Thanks! so this is interesting:
Root Chord A) 430
Tip Chord B) 210
Sweep S) 130
1/2 Wing Span Y) 910

Stab Root Chord AA) 245
Stab Tip Chord BB) 165
Stab Sweep SS) 60
1/2 Stab Span YY) 410
Wing LE to Stab LE D) 1160
STATIC MARGIN: 25% (don't fully understand, higher number moves CG forward, and 5% to 15% is recommended)

CG from Wing Root LE: 167.4

Entering 15% moves CG to 140, and pretty much everyone flying this plane is 160 or more.I started at 170 (which a little birdie told me to do ) which is just about 25%. The tool says this should be incredibly stable, and I should be more like 200-230.

Well this would make sense why I'm at +.5 wing and still need up elevator, but it's contrary to what some talented pilots and builders have been using. I'll try slowly shifting back and see what happens.

Thanks again Jim!

Last edited by Jetdesign; 05-30-2014 at 10:45 AM.
Old 05-30-2014, 12:28 PM
  #13  
OhD
 
Join Date: Oct 2002
Location: west hills, CA
Posts: 1,159
Received 8 Likes on 5 Posts
Default

The most important information in Bryan's trimming guide is the starting point.

* Set motor to .5 down, wing .5 positive and stab zero

1. Beginning set C/G 25-30% (of MAC)

I had set the motor and wing but had ignored the cg. The previous owner (a top pilot) told me to put the battery pack all the way back. The problem was, my packs were 6.5 ounces heavier and I had switched to a Contra that is heavier and its cg is further forward than his motor/gearbox was. I adjusted the wing and the thrust line many times with no success. Finally, I found that program and found out where the cg should be, got some lighter packs and added some weight in the tail.

The plane is flying much better now.

Getting the cg right is very important.

Jim O
Old 05-31-2014, 04:14 AM
  #14  
nonstoprc
My Feedback: (90)
 
nonstoprc's Avatar
 
Join Date: Sep 2003
Location: Central, TX
Posts: 2,466
Likes: 0
Received 1 Like on 1 Post
Default

I would also try the following two.

1. Reduce the down thrust angle
2. Reduce the angle of the stab a little bit, say half of the up elevators input needed during level.

Depending on what sequence you fly, it is better to optimize for the entire sequence with the objective to reduce the workload and to achieve repeatable results.

For my Visa, a little bit of elevator inputs are needed for level flight at both position.

On uplines, no elevator input. On down lines, none.
Old 05-31-2014, 08:09 AM
  #15  
rcflyer4fun
 
rcflyer4fun's Avatar
 
Join Date: May 2007
Location: Plainfield, IL
Posts: 111
Likes: 0
Received 0 Likes on 0 Posts
Default

You are going to get 20 different opinions on this one. Here's mine. From my experience making plane adjustments changing the CG will not change your trim setting at all. I believe this might be because there is so much lift in the tail that changing CG will not have an effect on trim. Changing CG will affect how the plane knife edges though. Basically what a more advanced flyer had me do was get the CG very close to recommended setting and to set all my surfaces to neutral trim and fly the plane out of trim and see what the plane was doing. I basically landed the plane out of trim and started making wing incidence adjustments till the plane flew straight and level with no trim added. Then I focused on the CG. If the plane pulled to canopy I move CG back and if you pull to the belly move CG up. Dave Snow says that the stab will follow the wing pretty much no matter what. So what he means is most of your adjusting should be to your wings assuming all other stab and thrust settings are on their mark.

Hope this helps you out.

Evan

Last edited by rcflyer4fun; 05-31-2014 at 08:10 AM. Reason: wrong spelling
Old 05-31-2014, 08:49 AM
  #16  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,400
Received 114 Likes on 100 Posts
Default

Evan, it has always been my understanding that the reason for the pull or tuck in knife is because the influence of elevator trim. Now elevator trim could be needed for a few reasons including CG being off. A good example is the MK Aurora I recently started flying. Initially it was very nose heavy and pulled to the canopy. As I started shifting equipment and adding weight to the tail the up trim has been reduced as has the pull to the canopy in knife edge.
Old 05-31-2014, 09:31 AM
  #17  
rcflyer4fun
 
rcflyer4fun's Avatar
 
Join Date: May 2007
Location: Plainfield, IL
Posts: 111
Likes: 0
Received 0 Likes on 0 Posts
Default

Originally Posted by speedracerntrixie View Post
Evan, it has always been my understanding that the reason for the pull or tuck in knife is because the influence of elevator trim. Now elevator trim could be needed for a few reasons including CG being off. A good example is the MK Aurora I recently started flying. Initially it was very nose heavy and pulled to the canopy. As I started shifting equipment and adding weight to the tail the up trim has been reduced as has the pull to the canopy in knife edge.
I think your right that if you have some up trim it will influence your knife edge. This is very true. Been my experience that changing CG has little to no effect on elevator trim setting and certainly I have never been able to eliminate all my up trim with just a CG adjustment. All it did was make inverted flight more hands off but knife edge still had a pull due to the up trim in the plane. It's my belief that you must get your CG as close to the design CG that you can pre-flight . Test fly your plane with trims neutralized and make wing panel adjustments to establish straight and level flight. Then you can revisit your CG position for optimal performance. This is a very subjective subject. If there is 30 responses there will be 30 different opinions most of which will hold true for one reason or another.
The Integral is a pretty solid proven platform to start out with. I'm pretty sure he is only going to need a positive wing incidence adjustment to fix his problems then a CG adjustment to fix knife edge flight. Like I said make a flight with neutral trims and note how bad it wants to dive. Land the plane and and adjust the wing. Make another test flight and I bet you will be amazed at the difference. Keep adjusting it till it flies straight and level. After you dial it in check your thrust line with vertical power on climb tests and straight and level flight tests. Start out at mid throttle and then ease throttle to full and note any dive or climb and adjust thrust accordingly. It will take many flights and lots of very calm air to get it right.
Old 05-31-2014, 03:24 PM
  #18  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,400
Received 114 Likes on 100 Posts
Default

We definitely have different trimming methods. I bet we end up at the same point though. What has always worked for me is to only adjust engine thrust line to correct vertical track. I would think that adjusting the wings to more then 1/2 degree would take away from inverted flight too much. Then again everything is a compromise. The best we can do is get the airplane to do everything reasonably well. If we get lucky it does some things exceptionally well.
Old 05-31-2014, 05:56 PM
  #19  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

I'm sure there are many different approaches. To me it makes sense to consider cg, wing angles and thrust angle. I am going to try Hebert's method as it makes sense to me and has worked well for many others.

I tried moving my CG back, it had no effect on elevator trim and only resulted in the planea little more touchy. I will find the 25% mac and see how close i am, I bet I'm pretty close. So then some wing adjustment or thrust adjustments to clean up the stab, recheck performance, etc. My hunch is a 10th of a degree more positive in the wing will get me there but we will see.

I just had a good flight with some positive comments from the judgesbut I. I think the airplane is really close.
Old 05-31-2014, 07:10 PM
  #20  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,400
Received 114 Likes on 100 Posts
Default

Joe, just curious, how far back did you move it? You are correct, if you do nothing more but move the CG back a tad you will feel the elevator get more effective. As you stated before CG will effect everything. One thing that I try to do is make a single adjustment at a time and then put 10 flights on the adjustment until I decide to keep it. In this case when moving the CG back I would have reduced elevator throw at the same time. I would have considered this one adjustment and then flew it for a couple outings. When the adjustments get this small I feel I need to fly them a while to rule out a habit we have developed around the airplane.
Old 06-01-2014, 10:03 AM
  #21  
serious power
 
Join Date: Nov 2007
Location: wexford, IRELAND
Posts: 1,112
Likes: 0
Received 0 Likes on 0 Posts
Default

Hi Joe / Jim,
I prefer to calculate CG just as a % of Wing MAC.
That tool is considering tail area and efficiency !??? - really an assumption.
Normally when people refer to say 25% or 30% etc , they are referring to a straight % calculation of wing MAC.
Try;
http://www.palosrc.com/index.php?opt...1:ic&Itemid=50
http://www.palosrc.com/index.php?opt...1:ic&Itemid=50

Brian

Last edited by serious power; 06-01-2014 at 10:59 AM.
Old 06-01-2014, 11:51 AM
  #22  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

It doesn't matter what you put in for the stab, the MAC is for the wing and doesn't change. The one linked by OhD looks pretty good. I clicked the 'back' button on the bottom of the page and found the reasoning behind the tool. It makes sense and is inline with the article I linked above:

Basically the 25% MAC is about where the "Aerodynamic Center" of the wing is - a position on the wing in which the CG will generate no pitch forces (moments) regardless of the attitude of the wing.

This goes with the thought that changes in CG should not have an impact on elevator trim. If you change your CG and it affects your elevator trim, you are outside of the 'zone' - either too far forward or too far back. This also goes with the idea that the stab should not be generating lift for the airplane. The wing flies the plane, the stab guides the plane.
Old 06-01-2014, 12:04 PM
  #23  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

This is turning out to be a lot of fun and a good learning experience. It is cool to see what happens when you move CG around - what changes, what doesn't, etc. As I've moved my CG back I noticed I went from a pull to a tuck during KE, again with no change in elevator trim. It must have to do with the way the wing is loaded, and possibly to do with the wing taper.
Old 06-01-2014, 01:09 PM
  #24  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 8,400
Received 114 Likes on 100 Posts
Default

Joe, if you went from a pull to a tuck then your CG shift was fairly large. Honestly I'm still not getting why your trim is not changing with a CG shift as it does on every airplane I have ever set up. I think it has to do with how the stab is loaded. If you think about it if you are nose heavy then the up trim is going to place a certain amount of load on the stab and has the effect of making the airplane fly heavy. As you move the CG aft, the load decreases as you don't need as much pressure to hold the nose up. That decrease in pressure should directly translate into less elevator trim. If we continue to move the CG aft we get to a point of having almost no load on the stab and that's when the airplane starts to hunt and will not hold a line. I not only apply these techniques to my aerobatic models but my pylon airplanes as well. Typically on all my airplanes I move the CG back until I notice the lack of stability and then move back to where it will hold a constant line without a large amout of correction inputs. Ironically this is the point where I have almost no elevator trim if any at all. With the aerobatic airplanes this will almost eliminate any tuck or pull while in knife edge, a small amout of mix takes care of the rest. On the racers it leads to minimal elevator travel needed to get through the turns. Less travel= less drag= more speed.
Old 06-01-2014, 01:28 PM
  #25  
Jetdesign
Thread Starter
My Feedback: (8)
 
Jetdesign's Avatar
 
Join Date: May 2008
Location: Honolulu, HI
Posts: 7,056
Received 0 Likes on 0 Posts
Default

The stab shouldn't be loaded and CG shouldn't change elevator trim if CG is near MAC.

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.