CG, decalage, and trim - Page 2 - RC Groups
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Nov 10, 2012, 07:02 AM
ciurpita's Avatar
here's some basic theory that may help you better understand how decalage and elevator trim come into play

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Nov 10, 2012, 10:28 PM
cityevader's Avatar
Well, I decided to approach this more scientifically than just taping a rock to the nose or tail and see what happens. So I rechecked cg and marked it, then added nose weight to one plane and removed nose weight from the other two, enough for about a 2mm shift and made marks at that cg. Lather, rinse, two are flying so much better!! One went from barely flyable to really smooth, the other no longer porpoises!!! (and not the right conditions to fly the third plane).

Turns out they were so far off (despite right per the plans) that my taped rock experiments weren't enough weight, and with one plane I was adding it to the wrong end

I should have listened to what I thought was right before I mixed it all up in my head with other factors involved... and also waited too long before asking for help for fear of sounding like a dum dazz.
Nov 10, 2012, 11:05 PM
Red Merle ALES VI
Curtis Suter's Avatar
Great news! I am glad it worked out.

In the early 90's I crashed a sloper on the first flight due to the plans having an inaccurate balance location, way too far aft. Then I read Martin Simons book on Model Aircraft Aerodynamics and he had formulas there to figure the balance point for a model. Thus Sailplane Calc was born. For over 20 years it has proven 100% successful. It gives a slightly nose heavy initial balance which is a good thing and then I fine tune it during flight testing.

Nov 11, 2012, 12:07 AM
cityevader's Avatar
How many ridiculously-often-posed questions abound in any one thread (of any particular model) asking "where is the CG of this plane?" Many seem to be lucky to even get instructions at all (I'm presuming this an abroad-based-company problem in general).

Now I'm wondering how many other folks are having an even worse time than I...those who have little experience at all. I'd consider myself a competent pilot- being just below "good". A decade+ absence- and a recent return to- flying has made me realize just how many painfully obvious areas in life that were oh so better 20 years ago. Goodness, I need a beer now.

Where was yeah, the instruction thing leaves the build up the the individual, which leads to a heck of a lot of variances between any two planes- obviously the result is differently flying planes. Hence all you wonderful folks step in to help someone better understand the path through this wonderfully garbled interweb we're in.
Nov 14, 2012, 12:50 AM
cityevader's Avatar
Dangit I need to fly!!
It's only been a couple days since last flights and I'm dying because It's only every other weekend that I even have available to fly, period...regardless of any other factors in life that may happen on the same weekend. And at best, every other night to perform repairs/build/other things in life...

Needless to say, I get very little practice actually flying since a million years ago as a young punk and flying/building/repairing every stinking day. My skills have degraded to merely... reacting. However!! I'm hoping that my fairly smooth flying with all of my dozen+ planes that are out of whack has sharpened a new set of skills- keeping a plane on a track that I'm aiming for. I've felt that skill increase over about the 25 or so (slope- which is a brand new experience for me) flight experiences to date. Once I get some stick time with a trimmed plane I'm hoping I'll at least look better!
Nov 15, 2012, 10:16 PM
Registered User
Originally Posted by CloudyIFR
Would you mind expanding on what you mean? Do you mean balancing a model in the horizontal axis i.e. pitch?

What specific issues are you having?

You could read "CG Position" and "Decalage" here:

Definitely read the articles by Mark Drela. He is a chaired professor of Aeronautics and Astronautics at MIT. In addition, he is a superb designer, builder and sailplane competitor.

A number of threads on RC Groups have discussed "decalage" without being clear as to what that means or what is desirable. However one defines decalage, the key idea is that the COMBINATION of the stab and elevator set the angle of the wing relative to the airflow. Paul Naton's video has caused some confusing emphasis on the angle of the stab alone. As Mark's articles emphasize, fly the plane and adjust CG and elevator trim to achieve the desired flight characteristics. Don't worry if the stab and elevator are not aligned at the end. As Mark says, "decalage is way overblown".
Nov 15, 2012, 10:27 PM
cityevader's Avatar
The main reason (for me right now) to have the stab/elevator aligned is for "maiden flight" characteristics. When the flaps/ailerons are flat with the wing, they are in perfect trim. But the elevator gives a ridiculous roller coaster ride trying to set the trim. If not careful, the first throw will nose it straight into the ground! I've had many many "maiden flights" due to new plane/old radio, new programming, then new radio/new programming...then switching to 2.4, and my goodness, that first 30 seconds is hairy with it not in trim...It would just be super nice to know for sure that 0-0-0-0 throw is "neutral".
Now, the v tail is removeable, so I've started with a 10mil strip of plastic as a shim, and will continue to adjust for that one single end goal. Right, wrong, or indifferent, it's still my goal.
Nov 15, 2012, 11:46 PM
Registered User
I dont think there is anything wrong with that as a goal. For esthetic reasons if nothing else
I do it myself when the degree of trim offset is noticeable.
Nov 15, 2012, 11:59 PM
cityevader's Avatar
yeah, a good 3/32+" is pretty noticeable! Especially when the elevator is "inset" in the stab (vs the whole T.E. cut off for elevator) with a solid reference for a "visual neutral".
Dec 04, 2012, 12:28 AM
Registered User
Gordysoar's Avatar

You guys can't be reading what you're righting

Dive testing...? really in this day and age?

First of all, ALL our sailplanes balance just fine at 45% of root chord, DLG, 4M F3J, doesn't make a difference...why?

Because we balance for communication, not performance.

Dr Drela wrote :
The importance of decalage is way overblown.
First of all, if you have an all-moving tail, then obviously "setting
the decalage" has no meaning. You just set the elevator trim to
whatever it has to be for your CG position and desired trimmed speed.

The problem with this quote is that its out of context. IF decalage isn't important WHY did we all go to full flying stabs?...because fixed stabs fixed us into a certain incidence, and the only way to compensate for the wrong decalage was to hobble the model with nose lead. You don't fix porpoising with nose lead, you fix what causes the porpoising.

Okay read this a few times guys till you understand what does what on a sailplane in flight....airspeed empowers tail feathers. Read and repeat.

When you put a sailplane in to a dive, you are putting into a test environment that it WON"T BE FLYING IN during its task. Why would you test a model at a speed it won't ever be flown at during thermaling? It is thermaling that we are after right?

(I had some crayons, the box said the melted at 130 degress, so I put them in my garage, started the garage on fire and yep they melted.

And lets go on record for one other offered this fella advice yet never asked if his model had a fixed stab or full flying?...ready for a repeat?

Airspeed empowers tailfeathers....and by the way its the tail that directs the nose. IF you load up enough lead in the nose a model, the only way it will fly level would be to add some up ELEVATOR trim. THEN when its put into a dive, the tail gets more powerful and can force that excess nose lead away from gravity's grasp. (the tail gets stronger and can direct the nose up).

You don't need PHD's to use common sense but you do need to take some time to think about what causes what.

Task sailplanes have zero incidence...why? Because their pilots need them to tell them about lift and sink...not airspeed changes! They definitely should pull out of a dive! See Mike Smiths explanation of the dive test on Radio Carbon Art's Masters of Soaring video...he starts his dive at about 1,000 feet and if it pulls out before it hits the ground, its balanced properly.

Airspeed empowers tail feathers and if the tail is being forced to carry extra nose weight, then it will be set with up trim....a crooked sailplane...that when sped up, raises its nose and when slowed down drops its nose....that means the pilot can't tell if the model is indicating conditions or airspeed. He can't trust the information.

We don't balance tasks sailplane for some mystical stability or feel, we sure don't balance them for some kind of performance least the guys who get their times don't.

A task sailplane only needs enough lead in the nose in order to make it fly forward, if there is one dot more lead in than is needed to make it fly forward, it will be flying forward and nose can fix that with some up trim AND by flying at one single specific speed - increase the speed and the nose goes up, reduce the speed and gravity reaches up for that extra nose lead.

Most guys never understood the reason for balancing a sailplane, so full flying stabs got popular, that way each guy could adjust his stab incidence to compensate for the amount of unbalance each pilot wanted to fly with.

Change the CG on a plane with a fixed stab and you have an unflyable plane. Change it on a plane with a full flying stab and you have a crooked plane that if flown a single specific speed, flew pretty well....and that's partially why you would hear pilots say things like "you're flying it too slow".

We all dream of lighter sailplanes and sailplanes so clean that they glide forever on the lightest thermals...yet you have no qualms about advising guys to make their models crooked and heavier....remember airspeed empowers tail feathers and the only way a sailplane will pull out of a high speed dive is if the tail directs the nose to do so...a crooked airframe.

And one other observation... CG was jumped on as a topic even though every newbie porpoises his model, because he flicks the sticks or over controls. Which has nothing to do with his models incidence or nose weight.

No one asked about if he had his dual rates set and used. His throws could be and inch up and down.

I happened to be present when Smarty Jones first showed up at Churchill Downs before winning the Derby...the trainer told the jockey to take him for a hot lap. When the jockey returned, the trainer asked "How was he?" The jockey said, "Kind of twichy", the trainer said," I read on RC Groups how to fix that!" he grabbed a short piece of rope, tied it between the horses legs and said, "That will make him more stable".

Dive till you are blue in the face but how does the lead in the nose direct the nose?

Its the tail not the nose that is affected by the high airspeed in those dives.

So I think there are some questions have to be asked about the model in question before advice is given.
PS, you'll also want to note that Dr Drela's designs use full flying stabs.
Latest blog entry: Check out my YouTube Channel
Dec 04, 2012, 12:49 AM
Registered User
Thanks Gordy!
Dec 04, 2012, 01:05 AM
cityevader's Avatar
For what it's worth, I haven't even flown the plane in question (with 3/32" up-elevator trim) since last post. With two days a month available to fly that haven't been when wind is blowing (slope flying).

The best thing about this thread is that my OH SO RUSTY brain has gotten a bit of WD-40 in it!
One plane improved because I was going the wrong way with CG and needed reminding of how to interpret "dive tests".

But that was for ballpark settings. I haven't flown at all since last posting.
Dec 04, 2012, 01:23 AM
Registered User
Gordy - now that you have shot down one of my favorite methods for checking cg, how do YOU do it with a new plane?

My "technique" if you can call it that, with a new model is to trim it to hands off at what I think is going to be a good glide speed. Then I fly it a while to get a feel for how its handling. Somewhere in there I do a dive test. Then land, adjust the CG (usually back) rinse and repeat. I stop when it gets to where the model is indicating lift well and responds well to the controls - is light on its feet - AND has a shallow pullout from a dive test.

My current go-to model is a Mirage wing on a Bubble Dancer fuse (the BD wing is taking a while to finish). Its balanced at just a hair more than 50% back at the root. I really like how it handles.

However, it still pulls out of a dive test a lot sooner than 1000 ft!

How do you do it?
Dec 04, 2012, 10:00 AM
Registered User
AMBeck's Avatar
Something to consider is that there are several different kinds of pitch stability. Each one requires a different approach to arrive at good handling, and they interact. Three in particular will make life interesting.

The one everybody knows is static pitch stability. It is the ability of the aircraft to hold a trimmed angle of attack. This is controlled by CG position and tail volume. Moving the CG back, or making the tail smaller, gets you less and less static pitch stability.

The other pitch stability you are sure to have to deal with sooner or later is dynamic pitch stability. This is proportional to tail length squared. It is the aircraft's ability to return to a trimmed AoA after a disturbance. A dynamically unstable plane will fly normally until you stall or dive, then it will tend to stall-dive-zoom-stall-dive-zoom...until you get on the stick and force it to fly level, at which point it will behave nicely, until it stalls (or dives) again. You can have a plane that is statically stable and dynamically unstable. Unless the dynamic instability is bad, it's more of an annoyance than a problem, but it can make speck-out climbs interesting. Moving the CG forward tends to make dynamic problems worse. The best way to increase it is to lengthen the tailboom.

The third of the big three is phugoid instability. This is the tendency of a plane to hunt in pitch around the trim attitude. Most planes have phugoid instability to some degree, but it is rarely an issue and the pilot doesn't even notice. It is more of a curiosity than a problem, but it can be going unnoticed in the background and increase the pilot's workload. You see it occasioanlly in a long hands off glides in very smooth air. The plane will gently pitch the nose up and down, and your straight glide will turn into a gentle sine wave flight path. Airplanes with large horizontal tails seem to be more suceptable to this. P-51s reportedly had some interesting phugoid stability issues, especially with the long range tanks full.

The spreadheets from Curtis (CloudyIFR) work great. If you use those and set up carefully, the first flight will not be too stressful. Last spring I flew a competely new design and it worked right off the board, needding only minor trim tweaks. The only adjustment I make to the spreadsheet is to increase the tail efficincy to 80-90% for airfoiled stablizers. (Leave it at the initial setting for flat surfaces.)
Last edited by AMBeck; Dec 04, 2012 at 10:54 AM. Reason: added info
Dec 04, 2012, 11:51 AM
Red Merle ALES VI
Curtis Suter's Avatar
So this begs the question, what the heck is the tail doing back there anyways?

Here's the point I'll try to make as I ramble on.
If a model is nose heavy, she'll require up elevator trim, in relation to a neutrally balanced model, to maintain straight and level flight. No big deal if it's a full flying horizontal tail or an articulated one (one that has a fixed horizontal and moveable elevator).

When a model makes a turn the lift vector is tilted thus the model will descend slightly, thus after inertia is done doing its thing the pilot needs to apply some up elevator. Now if the pilot is slow to react the model will descend and pick up speed.

Now this is the reason the dive test works. Gordy is right, we don't fly our models at too high a speed unless we're getting out of sink and need to move quickly from one part of the sky to the other. I'll touch on that more in a minute.

So here's a couple of examples:
Nose Heavy:
When the model turns she'll will pick up speed more quickly if the nose is allowed to drop, but when she does so she speed seeks the original elevator trim thus will automatically climb on the pilot. Thus the pilot will bank the wings, see the nose dip, apply up elevator about the same time the model is self correcting and then the model balloons or overshoots level flight then the pilot has to input down elevator. In other words the pilot has a very high workload!

Properly Balanced:
When the model turns and the nose drops, it'll be less dramatic and since she's closer to a neutral balance when she self corrects ie. seeks her trim speed she'll have a less abrupt nose up thus the pilot has less of a workload and may not even need any down elevator input.

This is the mistake beginner's make by flying a nose heavy model, it actually takes more pilot input than one properly balanced.

But what is that dang gone tail for? A wing with a cambered airfoil has a pitching moment this is called Cmo. If air is blown across this airfoil such as inflight or in a wind tunnel the airfoil will pitch nose down at approximately the quarter chord point. So we need a horizontal tail that applies some down force to equalize that pitching moment. So in effect the horizontal tail is lifting down. Albeit very little on a neutrally balanced model.

Now if you have an articulated horizontal tail the amount of elevator deflection necessary to counteract the negative pitching moment of the main wing doesn't matter, unless it's grossly in error than the tail may run into issues where there could be airflow separation and that would be very ugly! So if the tail stalls which way would the nose pitch? That's happened to me in icing conditions in a real plane. Yuk!

Now, with a full flying stab it's easy to setup as there are no incidence or decalage issues with the main wing. Now what difference would that make?

We set our wings on a fuselage with the idea to get the fuselage to fly at a certain angle. Let's say that angle is zero. If you draw a straight line down the fuselage from the center of the nose to the center of the aft end of the fuselage. This is a line we want to fly straight. So if the wing is placed with some positive or negative incidence the tail doesn't care, the tail will still work in relation to the wing, but now the fuselage will fly at an angle up or down. You'd have to wind tunnel test your fuselage to see what angle has the least amount of drag and place your main wing in relation to that. So now that the wing is set to fly with the fuselage level, mount your tail, if when flight testing there is a little up or down elevator trim left over, who cares. That's what Dr. Drela is trying to say!

Now, back to the dive test.
This is where you trim your model in straight and level unaccelerated flight for what you believe is the best L/D speed; push the nose over and then release the sticks and observe what she does? This is testing the relationship between the main wing and the horizontal tail down force. If she's nose heavy she'll have a greater tail down force thus she'll climb more rapidly because she's trying to speed seek her initial trim speed.

That's enough rambling for now.

Watch for some very good info in the next year or two on this exact subject, I hope. I'm awaiting some product use licensing agreements to get approved.

Curtis Suter

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