Swept wing C/G

[Skyshark]

Hi All
This is the first time I have dealt with a swept wing plane. As you can see, it is balanced right now but I don't know if it is right for flight. Anyone have any suggestions?

[Uhhuh35]

Here's a link that should keep you busy, for a few years:
http://www.mh-aerotools.de/airfoils/flywing1.htm
Or just go here for a simpler explanation:
http://www.eng.um.edu.mt/~andebo/aero/cg_loc.htm

Good luck.

[RCSuperPowers]

The CG looks right on. That's where I would put it. Always best to error on the side of nose heavy for first flights just in case.

Dave

[flypaper 2]

This one works for me. Measure your cg at the mean aerodynamic chord.

[Butch777]

Heres a center of gravity calculator, try 30% for a starting point.

http://nasascale.org/howtos.htm

[Skyshark]

Thanks Guys

[Robert May]

Before you try any of these, at best, partial methods I would do a search and find Gordon Whitehead's full calculation, failing that go and ask this same question in the Aerodynamics forum.

The full "Gordon" calculation is not hard and is far less likely to result in a pile of bits. Gordon's calculation has been covered in full several times and the threads include drawings to help you understand the method.

here's a start

http://www.rcgroups.com/forums/showthread.php?t=489711

[flypaper 2]

How do you mean 'partial methods' Been using this method for the past 45 yrs. and havn't caused any "bits" yet. :D

[Robert May]

Please mark on your drawing where you would balance the model and explain why so that Skyshark can undertsand how your foolproof method works.

I have only got 35 years experience and I know that

"That looks about right" is relying 100% on luck or massive experience.

30% of what and measured where?

A drawing just showing the MAC of the wing, with no reference to the position or the size of the tail is not a CG calculation. Unless it is a flying wing and you measure the MAC and divide by 6 to give an approximate position, which ignores the fuselage ahead and behind the aerodynamic centre


I have designed and built models of all sorts and the CG has ranged from 16% MAC to 37 % MAC. I am not trying to challenge anyones methods but I do think that having designed and built a nice looking model it would be a shame to pin a CG just anywhere and hope for the best.

[flypaper 2]

If Skyshark had any more questions he would have asked. Don't know where "that looks about right" came from. Butch said 30 % which would cover about 90% of the planes. These are foamies and sport planes, not high tech pattern planes. The 30 % is a place to start then set the CG to suit yourself for the type of flying you do. In aerodynamics nothing is writtem in stone. I doubt if anyone here can tell you accurately where the CG is on any of their planes. Like the old Chinese proverb says "It ain't rocket science"

[Sparky Paul]

If Skyshark had any more questions he would have asked. Don't know where "that looks about right" came from. Butch said 30 % which would cover about 90% of the planes. These are foamies and sport planes, not high tech pattern planes. The 30 % is a place to start then set the CG to suit yourself for the type of flying you do. In aerodynamics nothing is writtem in stone. I doubt if anyone here can tell you accurately where the CG is on any of their planes. Like the old Chinese proverb says "It ain't rocket science"
.
30% yes.. but 30% of what, and where?
Unless you determine where the mac is, putting the c.g. at 30% of the root will be wrong on a swept wing.
butch's drawing locates the mac. The c.g. is then measured at that point on the wing.

[flypaper 2]

Is there no diagram in #4.? I thought I installed it. Shows up on my screen but maybe no one elses. Did I screw up.? Should have this pic.

[Robert May]

There is a diagram but you have not marked a CG. The model shown has a very short couple and relatively huge surfaces.

30% MAC in this case will provide a pitchy ride of death. Without measuring anything my absolute guess for this model would be about 22% - 25% MAC

[Brandano]

AFAIK 25% of the MAC is the "standard", but there are too many variables, really. If you want to be totally accurate you have to consider fuselage area as well, and tail area too. The easiest way to find a good approx CG for unusual shapes is to build a small balsa or foam chuck glider and use it to try out various settings. Won't tell you how responsive the plane is on the controls, but will show you if it's stable

[Skyshark]

Wow! I didn't expect to start a war! The plane in question is a $5 chuck glider that I stuck leftover R/C equipment on. Without all this stuff the plane needed aout an ounce of lead shot in the nose to get a good glide without pitch up and stall.
Now, with everything installed, I had to position the battery just under the tail section just to get it to balance on the wing. I know the C/G as pictured is wrong. What do you guys need to help you make a determination. I can weigh, measure and post whatever you need. Just stop the arguing, it doesn't help.
Jim

[RCSuperPowers]

Without all this stuff the plane needed aout an ounce of lead shot in the nose to get a good glide without pitch up and stall... Now, with everything installed, I had to position the battery just under the tail section just to get it to balance on the wing.
It will balance in the same place you had it for the test glide when you did not have all the electronics in it.

I know the C/G as pictured is wrong.
Based on my dozens of scratch planes and flying things worth of experience, it looks about right! Somewhere between the 2 lines. Again, its always best to error on the side of being nose heavy and then work your way back...
A nose heavy plane like this will need to fly with power and some up elevator to go straight. If its tail heavy, it will flip and pitch out of control.

Dave

[Skyshark]

OK, That is good to know. Atleast I am in the ballpark.The pencils are almost within the lines you have posted and I can move the battery back to get them where they should be. That is good, it means I dont't have to add weight to the tail.
The only thing left to do is hook up the ailerons and take it for a spin. That is if the wind ever stops blowing out here, (two weeks so far, no letup).
Thaks RCSuper Powers and to everyone who took the time to help.
Jim

[Robert May]

Wow! I didn't expect to start a war! The plane in question is a $5 chuck glider that I stuck leftover R/C equipment on. Without all this stuff the plane needed aout an ounce of lead shot in the nose to get a good glide without pitch up and stall.
Now, with everything installed, I had to position the battery just under the tail section just to get it to balance on the wing. I know the C/G as pictured is wrong. What do you guys need to help you make a determination. I can weigh, measure and post whatever you need. Just stop the arguing, it doesn't help.
Jim

You haven't started a war and there certainly isn't any argument. The link I gave you, takes you to one of the many threads where CG position has been properly discussed.

The CG range shown in RCSuperPowers post seems to put the CG at 0% of MAC which is well forward. Your plane will fly but you will run out of elevator.

To give you some idea of CG on a swept wing, a B-52 balances where the root trailing edge joins the fuselage. That is 100% root cord and about 27% MAC

This is not an accurate method but without wing/tail dimensions and areas it is will be closer than anything you have got so far.You should read up on Gordon Whitehead's method, which in itself is a simplified version of a much longer calculation but it does work.

Use the diagram supplied by flypaper 2 fo find your MAC. Measure the cord at that point and divide by 5, then divide the MAC by 4 this will give you 2 measurements 20% MAC and 25% MAC

Measure these distances back from the leading edge at the MAC and mark a starting CG range, this will give you a flyable starting position.

I have guessed at the 20%-25% range because the photos show a very close couple, about 1x root chord. This has the effect of shifting the CG forward.

[Skyshark]

Thanks Bob

[Robert May]

Here is Gordons calculation laid out in full and easy to follow

http://www.fennisflyers.org.uk/id66.htm

[Sparky Paul]

Using the photo...

[Skyshark]

Sparky
That is about the best anyone could ask for. Should the "tip chord" and "root chord"
be reversed in the photo?
And does that mean that where the lines cross, the plane should balance on those points? And that is my starting point for fine tuning the plane once I get it to fly? Or should the point of balance be at the "30% of MAC" line?

[Sparky Paul]

To use that method, the tip chord is added to the root chord, at both ends.
The root chord is added to the tip chord at both ends.
Where the extended lines cross when connected root to tip is the mac.
Measure that chord, and the 30% (or whereever you want the c.g.) point is positioned on that chord, and extended perpendicular to the fuselage center line to the wing root for its balancing location.

[Skyshark]

Thanks again Sparky.

[finishman2000]

I was just about to post a question on a swept wing drone so I might as well ask right here. It is the Goldberg drone that the wing comes right off as a one piece unit.
I have cut it and reglued it in a more swept wing shape.
My question is......is the CG of the plane on the wing? Can I balance the wing on my fingers then mark it, place it back on the model and adjjust the weight front and back to match that wing CG?
Thanks

[nmasters]

My question is......is the CG of the plane on the wing? Can I balance the wing on my fingers then mark it, place it back on the model and adjjust the weight front and back to match that wing CG?
Thanks

No. If you just changed the sweep without changing the area then the centroid (the point where the diagonal lines cross in the method shown in post #21) will be in the same place but the MAC will be rotated by the change in the sweep angle and it will be a bit longer. BTW the chord found by the graphical method is actually the average geometric chord NOT the true MAC. The length of the AGC is close enough to true MAC but the spanwise position is always wrong. On wings with a lot of sweep and taper the AGC is to far aft and if you do your stability calculations based on the 1/4 chord point of AGC your static margin (AKA stability factor) will actually be far smaller than you expect. The main result of too small a static margin with a swept and tapered wing is that the tips end up carrying to much weight and that contributes to tip stall. This chart (http://www.rcgroups.com/forums/showthread.php?p=6190131#post6190131) is for flying wings but here's a method to adapt it for tailed airplanes.

Take the AGC as your reference length.
Put the 1/4c point of the reference length on some point of the 1/4c line in the blue box
30% of the length of this line NOT THE ACTUAL CHORD OF THE WING AT THAT SPANWISE POSITION is a reasonable place to start adjusting your CG

[Sparky Paul]

I was just about to post a question on a swept wing drone so I might as well ask right here. It is the Goldberg drone that the wing comes right off as a one piece unit.
I have cut it and reglued it in a more swept wing shape.
My question is......is the CG of the plane on the wing? Can I balance the wing on my fingers then mark it, place it back on the model and adjjust the weight front and back to match that wing CG?
Thanks
.
What you need to do is diagram your wing per the diagrams presented here.
What's so hard about that?

[finishman2000]

thanks, thta's what i'll do will all the sizes as well.