update, I thought I would check the engine thrust angle first. I first made sure the plane was sitting as level as I could get it. Then I used a protractor with the string attached with a nut hanging down. I then put it against a level cabinet to make sure it read zero. Then I put it against the hub & to my suprise it read 4 degrees down thrust, I had to lift the BOTTOM of the protractor to make it level. That seems quite opposite of what the plane was doing in air, any throttle at all it went straight up & I mean any throttle. I guess my next project will be what capinjohn showed in those pictures & try to figure out that method.

man was I in trouble, knees knockin against each other, sweatin like a pig.....lol I was happy that with no power, it seemed to float in & land ok. But to answer the question, I dont know how the cg as figured. I just went with what redwing rc posted in the build thread on youtube

I am not good at explaning another persons way. This was a online thing I found. The way I see it you are just finding the CG the airplane has. The airplane needs to be level ( I do not see a measurement needed for that) and when this is done as shown...that is the point of the CG you have. As I look at the photo...I see the more weight you add forward it will make the plumb bob move forward, showing you how much weight you need to move the CG a given distance. I know you will want too fly the plane and check more...like rolling into inverted flight and see if it climbs or dives.

One thing I liked is once you have the device you could easy use it on all your airplanes and maybe some of your friends too. The cost is not that much....and you could use it to store one airplane up and out of the way! Best Regards Capt,n
Note...if you add say 3 ounces to the engine end...the aireplane,s nosewill drop...then as you rotate the dowel with the string around it and get aircraft level again...you will see the point of plumb bob move forward.

Are you sure you do not have a mix set up in on that throttle? Like does you elevator go up a tad when you throttle up? All the airplanes that I flew that was tail heavy was very sensitive to elevator...even worse as the airplane slowed down. I had one very nose heavy....but landed good. As far as thrust...that should be built into the engine box...on a lot of airplanes.

One more time...read this important part on photo. Remember the dowl is a tight fit...but you can rotate it to level the airplane after adding some weight. New CG will show up when you bring airplane back to level.

I got it now, I thought the dowel didnt move....... makes way more sense now......thank you for your patience

You can make a slot in the part that the dowel goes through...and use a small bolt & wing nut to adjust the tension...that is how hard the dowl turns. Got to go. Its time to have some fun!!!!!!!!!!!!!!! Capt,n

Excessive incidence !

Hey, That is the device my late friend Jim Archer called Vanessa! after his ex.

Thanks Dan,
I somehow missed the changing left bar links on aircraft proving ground. The aircraft supercalculator I was after. I used to have a copy on my old computer, and now am glad to have it again.

@ all:

If the CG is too much aft, the nose of the (trimmed) plane will come up when speed decreases. Power application with properly aligned engine will initially just accelerate the plane. Incidence will enter the picture only after the plane speed changes.
If the OP's plane reacts violently by pitching up by power application, the engine thrust line is too much up! He wrote that the plane behaves well the moment throttle is reduced, so COG and incidence must be very close to OK.

I have had planes with too much up thrust on engine(and these were arfs directly from factory), with power nose high loops, w/o power good flying. What I did in the field to determine what was happening was took a yard stick to the bottom of the tail horz stab and pulled the cowl to see if it was Parallel with engine mounts. Mine was easily visible and off about an 1/8". I shimmed it and it was flown with no issues. Its not the best method, but it worked for what I needed to know right then.

dave de, I am sure it is a nice device to see where a airplanes CG is...but after reading all this thread....it is clear the airplane has too much up thrust. Shim the engine to give it more down thrust. Then go test fly it. I bet it will cure the problem. You did say you checked the thrust line ...but just going by the hub would seem hard to measure the thrust angle real accurate. Good luck ! P.S. pe rivers, how did you friend like the CG device?

I'm inclined to go along with Captainjohn,Check to see if you have any elevator mixed in with your throttle.
Are you sure you have 4% of down thrust?.You can just eyeball the profile of your plane and see if you have up or down thrust.
I'm also inclined to think you don't have any.As far as CG goe's if it was that tail heavy,you most likley couldn't fly it anyways.Finger ballance on the spar line should be a good place to start i think.Just my two cents worth.

Dick

4 degrees of down thrust would not even begin to generate the pitching up with power issues this plane has. 4 degrees of up thrust would make for a lot of upwards pitching but not enough to go vertical without something else influencing the plane. Too much back thumb on the elevator, expo the wrong direction with too much thumb, too much up trim, horizontal stab incidence, or a throttle-elevator mix left in the radio from another plane.

Pitching either up or down with power application is always an engine thrust issue but it can be combined with other factors to make it better or worse. It's the first thing I check during the first flight, with the next being the center of gravity. If I land a plane with good thrust response and there is any elevator trim present to maintain level flight at a medium flight speed after the first flight the trim direction tells me what I have to do with CG. Up trim means it's nose heavy while down trim means tail heavy. After those are dialed in I can go to work on other more subtle trims and offsets.

A seriously aft CG makes the plane difficult to control because each control input often have to be greater than the last one to reverse the condition. Planes can get out of control quickly, or be highly reactive to small control inputs. Landings can be quite difficult because the plane would have to be horsed onto the runway. Once past neutral stability things get dangerous. A seriously nose heavy plane has to be landed fast or it would stall before the wheels were close enough to do any good. Too much up elevator is required to hold the nose up, generating a lot of drag and slowing the plane down, generally at a height a lot higher than you wanted it to slow down. The word "snap" comes to mind.

The crankshaft thrust line is determined from the centerline or datum line drawn thru the center of the plane from front to back. Usualy found on the building plans. You don't get that picture with an arf.

There is an old RCU thread about the device:

http://www.rcuniverse.com/forum/m_70.../tm.htm#708060

Dave,

As suggested above, I would verify any radio mix for the throttle first; second, I would make sure about the CG location.

The rig for CG is very precise, but it is useless if the location of the CG is wrong to start with.

Specifications:
Wingspan 73"
Wingarea 1025 sq in (66sq dm)
Length 68"

By looking at the pictures of the manufacturer's website and going back over the dimensions and proportions that we used earlier here, it seems to me that we are still off about the proper CG location.

http://redwingrc.com/planes/mx2.html#mx2_photos

What about contacting the manufacturer to double check what the manual reads?

"For Support and Technical Questions
Email us at - support at redwingrc dot com
Call us at 1-636-600-8735"

If you read the old thread on this device...you will understand how it works. I will post a photo when i get time to take one &amp; post it here. A line level can be used to check for airplane being close to level. Capt,n<hr />http://www.rcuniverse.com/forum/m_70.../tm.htm#708060

Dave,

Using a picture form the manufacturer's website, I have re-calculated the CG at 25% of MAC.

I did it graphically at scale, resulting in 4-7/8" measured aft from the point where the leading edge of the half-wing intercepts the side of the fuselage.

That is a conservative location and a safe point to start testing a more aft location in little increments (1/8" at a time), until you feel the airplane as responsive as you like it.
Once the elevator becomes twitchy, it is time to return one step back and leave it there.

If you verify that your MX2 balances approximately at 4-7/8", you should find the problem somewhere else (mix or thrust angle).

Another idea, if everything else fails:
Verify that the thrust angle doesn't change when throttle is fed.
That could happen if there is any problem with the structure or engine anchoring.

Again, best luck!

John, Jim Archer called it vanessa after his ex wife. It depicts the hate/love relationship he had with the device. It is very accurate, even when the plane is not exctly level. By twisting the dowel you can even determine the vertical CG location. The plomb will always be the line on which the CG is positioned, regardless of plane attitude.

Pe, I see what you mean. I think perhaps the best use for it,is getting the batteries and radio gear, ect placed in the right places to work out a final balance. If you had the dimention for the CG from the leading edge, you could get evrything in place for a good balance job. Also if the CG was not know off your plans, you could find someone online that has the same airplane and get the CG point from him. Thanks for the reply, and have a nice day. Capt,n John

I guess once again I'm going to have to be the one to do it. Something completely avoided so far is knowing a little bit about what you intend to do before jumping in. Take the time to do some research and learning before divinginto the pond. It just might be too shallow to go head first.

Most of the planes we fly will have an effective CG for most people between 25% and 33% of MAC. 25% is very stable. 33% is less stable. Some planes can go as far back as 37% of MAC in the right hands. Note I said in the right hands. Unless you're flying a delta winged craft those numbers work 99% of the time.

Next is to always start out with a clean radio. By clean I mean it hasn't been used to set up another plane on that memory channel, or if it has that memory channel was cleared out and in factory default stage.

Next is to learn how to trim out a plane, and what actions are intended for what response. Several websites devote some attention to this activity, and many articles have been written in magazines to explain this to people.

Just because one wants to do something does not absolve them of investing the time and effort in learning about the activity before getting started. There might even be a requirement to invest in a few tools that will let you accomplish the task. Because some of this stuff may cost more than you want to spend or afford doesn't make it someone elses responsibility to fix for you. Nobody is entitled to fly anything. They earn the right and ability through personal effort and experience. You don't start at the top.

If I have a plane, and put it on my workbench with stab absolutely level, it is quite easy to roughly check all other angles, and even the horizontal stab displacement relative to the wing. If something is out of whack, I can spot it right away. The wing with 3D foils seldom has more than 1° incidence. (cambered foils have 0° or negative incidence).  Probably closer to 0° if the CG is aft enough. In the old days with marginal flight power, even cambered foils had positive incidence and were nose heavy for added stability. The extra incidence angle would also result in extra engine downpull angle.
Motor down angle in modern powered planes would be very close to 1/2 airfoil camber % in degrees.
Only then would I turn to CG issues. A plane can be quite tail heavy before it becomes non-flyable. It also can be very nose heavy and still land decently with full up elevator.

Yep. a level plane on a bench and a tape measure will usually give all the surface incidence angles. Also the engine thrust line if you have a long straight edge. All tools of the trade.

A plane can be quite tail heavy before it becomes non-flyable. It also can be very nose heavy and still land decently with full up elevator.

That is sorta true...buttail heavyairplanes with too muchelevator throw can and most likely will end up in a very bad landing, most often a crash. I just flew my aircraft again this afternoon. I put it into inverted flight and let the stick go to center. The airplane just barely dove....that is the way I like it. I can also fly it hands off and it goes strait &amp; true. The landings are super fun....I love it! Capt,n PS Tom, I am ready for a bigger one already!