Needs some help in calculating the Center of Gravity for a tandem wing airplane. Any help would be appreciated.

Jeff

Use the CG calculator in the Aerodyanamic Calculators and Online Resources sticky thread. Just input the front wing sizes as the main wing and the rear wing sizes as the stabilizer. It'll give you the CG location you need. If you're not the trusting sort then make up a small 12 inch scale model of your tandem wing design and put the CG in the same scaled spot and try some test gliding to check that it shows a proper degree of positive pitch stability.

Thanks for the C/G calculator. I did not realize that it could be used to find the C/G of tandem wings.

Jeff

Hi jefff
I believe there is a calculator named something like Canard Wing. I would use that one also.

did use that calculator and got totally different results. I was told that the tandem wing is not the same as a canard. I would like to know the difference between a canard and a tandem wing setup.

I would think that they are the same. I wouldn't trust any CG when there are contraditions between various calculation methods.

Jeff,

The principle is the same: find the center of pressure where the lift of all the surfaces is considered applied, then select a stability margin and locate a safe CG.

This article may help you:

http://www.geistware.com/rcmodeling/...calc/index.htm

Now, the incidence angle of each surface is another matter and is related to longitudinal dihedral.

Andy Lennon;s book has a good description of designing a tandem plane.

As Mark Drela wrote here one time there is no difference between a regular wing and tail layout, a tandem wing and a canard. They are just different spots on a continuous line of transition from the one extreme to the other. So if you got different results from the two calcualtors you either entered the wrong or different information in one or the author of the calculators used a wrong equation in one of the applications.

Or it could just be some build up of tolerances. What where the results from the two calculators? Perhaps you could give us the measurements of your design and we can try it ourselves to double check?

Thanks for all the help with finding this CG. A gentleman asked me to help him get his experimental plane ready for flight. I did not design it or build it. He has changed the CG a couple of times and I figured I better find the correct CG.

Here are the numbers.

Front wing: 18.25" straight chord, WS 90"
Back wing: 22.5" straight chord, WS 90"
Sweep 0"
distance between Leading Edges: 53.5"

Results from Reg. CG Calculator: 26.85" from LE
Results from Canard Calculator: 29.41" from LE

Sorry I did not include the Static margin. 10%

Oh great! It seems we have some discrepency in what equations or some other issues that the various calculators are using.

Using http://www.geistware.com/rcmodeling/cg_super_calc.htm with your numbers I get 30.1 inches back from the front wing's leading edge for a 10% stability margin. Now that is certainly close enough to what you got from the Canard calculator. But it's a pretty big jump away for both of those from the 26.85 you got from the other one.

I'd suggest to your buddy that he makes a little 12 inch or so scale test glider from simple 1/16 sheet wings with a 1/4 inch square stick fuselage. One of the wings shoudl be adjustable for incidence angle. Then play with the CG location. What you're looking for is how far back he can get the CG and then trim the adjustable wing's incidence to get a stable glide where it doesn't try to diverge from stable flight. This means that a gentle push should glide in a flat stable glide. A hearty push should show some signs of raising the nose up to a stall. Even if it doesn't recover before it hits the ground the key is that the model MUST nose up even with the CG as far back as you can get it. When it gets to where the model does not nose up with a fast push or even arcs down then he went too far and the CG needs to move back forward a bit and the wing re-trimmed for a stable glide. This final setting is the most rearward position he should put the CG. But don't think that putting it WAY forward is good either. Like the baby bear's porridge you want to get it "just right".

It'll be interesting to see what the test glider says about all three numbers.

I was really surprised to see the numbers all over the ballpark when the results are to be the same.The closest to the 26.85 figure is when you take 25% of the chords.The distance between the two 1/4 chords divided by 2. I get 26.2" from the FW leading edge. I believe your right about making a small scale model. I will see if he would like me to make one for him. This plane has twin motors mounted on rotating post that are on the upper sides of the fuselage. The motors will be rotated for short field take off, very short take off.
Is my thinking correct that first you find the CG per the wing layout, then you position the motors on the CG or behind it?

If the motor thrust will be used to provide some lift as well as thrust then the line of thrust must be through the CG of the overall plane. So yeah, you'd want to do a test glider and find out just where the CG is best to be placed and then arrange the motor thrust lines to pass through that point in a manner that keeps the thrust line thru the CG at all points of rotation. So the rotational axis would need to be at or very close to the CG position.

Something else for him to consider is that if the model lifts off in a level attitude but climbing to some extent due to the thrust then the wings are operating at a negative angle of attack. To get the very best short takeoff performance the model would either need to be set up with landing gear that presents the wings to the airflow at a high angle of attack or the wings shoudl be set up to pivot to some extent along with the motors. Otherwise the lift of the wings during the negative angle of attack portion will be fighting the lift of the motors. And without some sort of zero airspeed controlls he won't be stable in taking off vertically. The best he can hope for would be a slow speed liftoff. And again that would imply that the wings should be angled such that the model can either have the wings at a high angle of attack or rotate the fuselage to get the wings to such an angle quickly and as part of the wheels leaving the ground.

I talked to the gentleman and he wants to use his figures for the position of the CG. I am still going to make scale model for testing the CGs. I have the four different results from the calculations that I will be using. Hopefully we will able to try the CG position that is determined form test gliding the stick model. Thanks for all your help.

May we know what are the figures of your friend for the location of the CG?

Does he have any values for the incidence angles?

The book that I referred to in post #7 explains that, for tandem wings, the lift effect of the aft wing can be considered as 80% of what it can be.
That is due to the shade effect of the forward wing.

Similar thing happens in the canard layouts, with the difference that the forward stab does not shade the full span of the aft main wing.
Note that the values shown in the on-line calculator for canards have a reduced area value for the main wing in order to compensate for the shading effect.

I think there is a lot of crap around about these things. "Shading the rear wing" is a good example. Most canards have the front plane raised up to negate most of this effect so how can the canned calculators really take this into account properly?
When I was building a canard a number of years ago, I checked out every way to calculate the cg that I could find and most were just dumb guesses, some of which would mislead you into very poor choices.
One of the worst was Martin Simon's otherwise excellent book on model aerodynamics. The cg calculated with his method would give you a crash for sure.
There is a lot of stupid ideas ("static margin*" is one) in these theories. The only sure way to know what your model needs is to make a chuck glider. However throwing it is not enough - you should attach a string to it so you can swing it around your head. Not only does this make you look foolish but it gives you a longer time to observe the characteristics with a particular CG location.

*The fancy term for FUDGE FACTOR

Allan,

Most of the calculations start from assumptions and they involve compromises.
Being no rocket science, the estimation of a safe location for the CG previous to a first flight may be in error, but is always better than a non-calculated or a subjective guess.

As long as there is enough authority of the surface that balances the main wing(s), the CG can be located at a safe or exaggerated distance ahead of the aerodynamic center for those first flights.
The shading of one surface with other may be a myth, but it introduces harmless error toward the safe side.
Later on, when any other problems are solved, we can slide the CG toward its limit and test until the plane and us are both happy.

Tail heaviness, however, is not a myth.
It is an unstable condition far from which you want to have your plane at least for its maiden flight.

I guess my point is that it is kind of silly to plot aerodynamic centers to three decimal points, then apply a broad fudge factor and think we are being "scientific".
Of course it is important to calculate a starting point if you don't have a suggested CG from a plan. I have always just used the 25% of chord and that has worked fine for the average airplane. Many folks will complicate this effort with things like "tail volumes" and "static margins", etc. and I suspect that is a waste of time.
When it come to canards, you can sense I have actual hostility toward some theories because they have often proven to be very misleading (you can ask me how I know...) and personally caused me much grief.
I will attach a pix of a canard which I rebuilt three times before I got a reasonable cg location. I was NOT favorably disposed toward the authors of the first two calculation methods. Probably the on-line calculators today are better than what I was using back then - at least I hope so.

Here are the numbers that he wants to start off with. I just finished making a test glider.to do my own testing.
CG from LE 19 1/4"
Rotation axes of motors from LE 23 1/4"
Incidence Angles from fuse. FW- +1*, RW- -1/2*

I did not do the calculations, but it seems to be way too forward; that will be 1" aft the trailing edge of the forward wing.

The incidences look OK.

Let us know of the results of your test glider, please.

I don't know where he got those numbers but given the results of the CG calculators that sounds very nose heavy. And even my gut opinion from considering the layout that I see in my mind's eye I'm sure he's way too nose heavy to fly well. But it's his model.

Besides, the whole concept of pivoting the motors seems flawed to me anyway since it results in the wings both trying to lift off the ground with negative angles of attack.