When you have a swept flying wing should the COG fall around 30 percent of the MAC??

Do you know where the mac is?

The CG will likely need to be forward of 25% MAC.

Thanks, for looking and answering.
To make my question a bit clearer:

Does the method of determining the MAC change with a swept wing?
Also add into this equation that it is a flying wing, tailless aircraft: pusher design.
Or would it be the same as any other high aspect wing?

Mainly I was not sure about a good start point for the cog when using a swept wing, in a flying wing type of aircraft. Although scrap balsa glider test suggest it should be way forward of where it is in a normal high aspect wing that is not swept. I have never dealt with a swept wing design before other than paper airplanes.
A fellow club member was having an issue with his build the above mentioned airplane where it has twice rolled to the left on take off. I have seen the same thing happen with a canard which had an aft cog and was wondering if he may have the same type of problem with this plane. Because his design is so light all that has happened is some damage to an wing tip and control surface, and I like to help him get it in the air.

If I may add one last question?
I have been working on a bwb ( lifting body with wing) type of concept myself and it seems here to that I want to have my cg way forward of normal 27% - 33% range of MAC that I know of in normal aircraft design. The glider test that I have done with this design is up in the 12%-20% range.
Does this seem right?
Seems to be more of a lawn dart type of range to me?

You are correct; on a tailless design, the CG will generally need to be forward of 25% MAC. How far forward depends on the airfoil. An airfoil with trailing reflex has a positive pitching moment about the AC, so it is most stable for flying wing designs.

A symmetrical airfoil is a distant second choice, since it's pitching moment is zero about the AC, but still not great for stability. They can and do serve, of course, especially on aerobatic model designs, if marginal stability is tolerable.

A simple positive cambered wing is the least stable, since it's pitching moment is negative about the AC, and is therefore the worst choice for a flying wing. To counter the negative pitching moment, the CG would have to be aft of the AC even for equilibrium, and then you would have a very short arm to the control surfaces, and poor flying qualities, instability at large AOAs, etc.

It is difficult to get flying wings to naturally (un-augmented) meet FAR Part 23 (full-scale) stability and handling quality requirements, hence the proliferation of horizontal tails. (Heading off the flying wing proponents here): Not talking efficiency above: Flying wings have a theoretical effeciency advantage, but this is difficult to realize without computer stabilization, due to the stability requirements of human pilots, i.e. they can be made stable without computers, but they generally are not more efficient or more "usable" than their conventional tail counterparts.

I suspect the (lawn) dart problems are CG and/or airfoil related.

MAC determination is the same for any wing, swept or unswept.

One way to find 25% MAC is:

.25(R+T)/2 (R & T are root & tip chords, assuming these are measured parallel to "fuselage" centerline)

Then, at half the panel span, measure back (parallel to "fuselage" centerline) from the leading edge by the distance obtained above. That will also be very close to the AC of the wing for non-compressible (subsonic) flow.

The root and tips of his design are parallel on his design and his airfoil looks to be a flat bottom with reflexed trailing edge. So I will find the mac of his wing then shoot for 25% of that for a location of the COG. I suspect he was using the 30% value to set up his cog, or the center of the mac itself, I did not really measure it, as I was on my way to work when I stopedd by the field.
I was not sure if he needed to go more forward because of the flying wing design.
When I set up a glider that looked like his design I found the best point was ahead of the 30%, but as I said I have no experince with flying wings. And from what he explained to me about the takeoffs that he tried it seemed as if it just did a ground loop.


As for my lawn dart....
It glides very good, slow sink rate enters ground effect and skims the ground for a bit then lands very gently. I was just concerned because the cog is so far forward that it has the weight distribution of a lawn dart not a airplane. This one is a test bed for a full scale design, I am one of those EAA nuts. I want to build my own aircraft.

The CG will probably need to be forward of 25% MAC, maybe 20% or so as a starting point, or possibly even farther forward.

The lawn dart I was referring to was the one that was crashing immediately after launch and rolling left each time. Different thread.

I want to home-build a full-scale plane too. Cant' do it yet, but I hope to start one in the next two or three years. I'm studying engineering so I can design one. Good luck!

I to am studying and a few experiments...

I want my design to fall under the ultralight or sport FAR's as an experimental aircraft.
Well off to work...

A plank (no sweep) flying wing can get away with a 25% c.g.
The others need to have it forward..12 to 18% is usual. And need reflex in the wing profile.... either designed in or achieved with surface deflection.
An irregular manuver on takeoff as described is usually due to a aft c.g. ....or insufficient vertical area aft of the c.g..

An old friend (dec) built a Dyke delta - over my advice not to do it -but he was an engineer-actually our cheif engineer -Togather we flew a Mooney 21 once cross the US and back . He was also an instructor. Agood one
The Delta turned out to be a thing of beauty
Unfortunately -one day - the engine failed on takeoff ------
The thing I hated about the design - - did not allow for any low speed recovery .
I put that design -for homebuilts alongside my all time favorite the The Ospry
We should give 1000 of these to Iraq to form an airforce.

That would be my reservation about a very low aspect-ratio design too. I would prefer a lifting body for interior space with real wings too, for glide.

What happened to your friend? It seems fairly obvious but you never know.

Dick funy you should mention the Dyke Delta. Sorry to hear about your pal, with his plane.
Did you see the EAA write up on the Dyke Delta a year or so ago? I always liked the concept but was a bit concerned about what would happen with a power out situation. Deltasa are not noted for making good gliders. as a mattar of fact when I first saw a F177 I said to myself, gee that reminds me of the Dyke delta.
I have it on my flight sim at home flys nice as long as you have power....
Anyhow, thanks guys I will heed your advice regarding the flying wing.

I know about the issues involved with a delta design that is why I have moved more toward the BWB concept with my design using both the lifting body concept and adding wings to it. Also thinking about multi or twin engines for power in a ducted fan arangement so I can use vectored thrus for more control.
Just a nice spring day today (TEMP 65 WIND 1-2 MPH) it was hard to drive past the field on the way in to the city for work. Should have put a plane in the car, AND CALLED IN SICK LOL.
oh well....
Better get back to work before I get into trouble....
Thanks again

He was killed outright '
There are so many good homebuilts available -I am always surprised when builders opt for the quirky ones .
In a model plane - who cares.
Most bad accidents in private craft -are pilot error- that is a given
But -designs which are more critical at landing/takeoff speeds-do not appeal to me.
call me chicken

Sorry to read about that, Dick. I lost a friend to to a mistake during low-level aerobatics. He wasn't trained for it but did it anyway. NTSB thinks he may have started a loop too low.

I will not call you chicken for that.

Nor would I call you chicken Dick, sorry about you pal.
Mesa sorry to hear about your buddy also. Low-level aerobatics, not a good phrase to be used with full scale aircraft , in my opinion. But I do know the lure that aerobatics have for many pilots (certified or not) just get the training before you try it at all. I have gotten to the age that recovery from unusual attitudes is a pain, let alone slaming myself around on purpose...
In other words I like a nice smoth ride nowadays.
My instructor told me many years ago the one thing to always keep in mind....
There are old pilots there are bold pilots but there are no old bold pilots.
Although General C.Y . was a lot younger back then...

What my goal is in spite of what you may think of the weird combo of design elements is.

A aircraft that is:
Stable
Solid control response
STOL
Low Drag
High Lift
Low wing loading
cruise 50 - 150 knots
IFR
Cheap - yea right.

But anyhow this is just a model at this point but I have tried to set goals for a design and want to give it a whirl, hopefully no spins.