Sweep and dihedral are not the same. The behavior of dihedral is far more self-stabilizing than sweep. But sweep has its place.

Never said it was...... but sweep gives effective dihedral... with degrees of sweep having a similar effect to some degree of dihedral
There is a more in depth analysis here http://www.b2streamlines.com/EffectiveDihedral.pdf
which I must admit goes over my head a fair ways...

Whatever... with a swept wing the rudder makes the model roll ...and with dihedral the rudder makes the model roll... with a circular or seriously curved back leading edge the rudder will also roll the model.. the curved leading edge is a basically a variable sweep back...

edit....I do admit I am still puzzled as to why the circle or UFO shape behaves so much in the same fashion.. I tried a circular wing on a 3D body a while back, with rudder both above and below the wing in equal parts and the roll coupling was still there..
With the swept wing I understand the reasoning but I did think a circle might be different

In a yawing situation, sweep back presents more span and greater effective area on the 'wing forward' side and less on the 'rearward wing' side.
With dihedral in the same situation, the 'forward' wing has increased AOA and the 'rearward' wing has reduced or negative AOA.

Aerobatic designs have similar area of rudder above and below the horizontal stab and in knife edge flight the rudder effectively becomes the elevator.

Which turns a bent or swept wing into a crooked and most unhelpful rudder! (Actually, the centered fin/rudder is set up that way to make the rudder input as pure in yaw as possible over the whole flight envelope, not just knife edge, but during knife edge it's just as you said.)

A competition aerobatic design strives to avoid any sort of control coupling-- roll is roll, yaw is yaw, pitch is pitch, and throttle is power. Any crossover makes the precise maneuvers harder to perform precisely.

The location and shape of the wing, elevator, rudder, and thrust line are all set up carefully to make that happen.

Dihedral and sweep would both be avoided for the same reason.

Ahaa! That makes sense. Thank you for that.

Keep it simple. Stupid. Now I understand.

That's why this is such a good thread.

excellent!

Dave, back home, I measured the c of g position of the radius. It is 40 cm wingspan and 38cm in length, measured from the firewall to the trailing edge of elevator. At the moment, the c of g is at 8cm measured from the firewall/leading edge. I started test flights at 11 cm and it was nearly uncontrollable. Step is at 11,5cm. Hope this will be helpful.

I changed the shape of the fins, I extended them to the bottom to improve directional stability in slow ( high alpha) flight and it seems to work well - no change when going fast and more precise handling in slow flight. ( see Pictures).

Next will be a 60cm version with Turnigy D 2826/6 2200kv and Kfm 4 wing. Hope for 150 plus kph.

I like your Obelix project! Saw it many years ago in RCM&E and planned built it but did not get to it.

Hi all, hope i am not hijacking this thread. I have an OD delta flies well. Would like to add KF2 airfoil. have read the step should be aligned with LE. Can any kind gent point me in the right direction to find the answer, or would the step go right across the top of wing. Cheers from a newbie.

Originally Posted by Fly Boy3

Hi all, hope i am not hijacking this thread. I have an OD delta flies well. Would like to add KF2 airfoil. have read the step should be aligned with LE. Can any kind gent point me in the right direction to find the answer, or would the step go right across the top of wing. Cheers from a newbie.

KFm2 is normally at 40% of the root chord and runs in a straight line to the same percentage of the tip chord. So if the leading edge is swept back (not straight) the step is also.

But you can make it a straight line all the way a cross (located 40% back from the root chord) to wherever it happens to hit on the tip and that will work too.

And the 40% is not a hard and fast rule, you can move it fore and aft some and it will still work. It can be a problem if you get the step crowding the aileron hinge line but has not always been a problem for everyone every time.

Forgiving, that's the word!

"OD delta", does that mean own design? Is there a thread where it can be seen?

Jack

Cheers jackerbes, exactly what I wanted to know. OD ,yep own design, but as its a delta, its much the same as any other delta. Can't help you on an od thread, may be one around. Its probably incorrect to say my od, as its copied from all I have seen and learn't on this wonderful forum. Into ic for 25years, now you cannot even cover a model for £20. A Depron foamy can be built, plus all the gear and flown for around £25, and free plans too. Cheers and thanks for the help. FB3 ps has a delta got a tip chord, and a root cord ? Told you I was a newbie ha ha.

Hey, I "borrowed" the design for the SDXS from goldguy and massaged it into what I call "my" Longhorn. It's different enough that I can get away with it, methinks. So it you want to call your delta yours, who's to argue?

Originally Posted by Fly Boy3

Cheers jackerbes, exactly what I wanted to know. OD ,yep own design, but as its a delta, its much the same as any other delta. Can't help you on an od thread, may be one around. Its probably incorrect to say my od, as its copied from all I have seen and learn't on this wonderful forum. Into ic for 25years, now you cannot even cover a model for £20. A Depron foamy can be built, plus all the gear and flown for around £25, and free plans too. Cheers and thanks for the help. FB3 ps has a delta got a tip chord, and a root cord ? Told you I was a newbie ha ha.

You can go to this flying wing calculator and enter the dimensions of your Delta and it will show what the chord is (tip of nose to trailing edge). Enter you tip chord as 0 if there is no width there.

Enter the sweep as the distance from the nose to trailing edge.

http://fwcg.3dzone.dk/

When you get the numbers in right it will look like your delta.

For a Delta I would consider 50% of the chord to be 50% of the nose to trailing edge distance.

The CG location it gives you might or might not be right but if you don't have one it will be better than nothing.

Build a KF flying wing if you can, they are simply lovely flyers and very easy to fly too.

Jack

With special thanks to Thomas and Dave Reap

On holiday in California, I didn't want to go too far afield looking for materials so I bought balsa, skewers, paper clips from the hardware store here. I brought my electronics in my luggage. Diameter of this version is 500mm, following the drawing of Semi Speed 500 by Dave Reap. As Thomas said, a too far back CoG makes it uncontrollable. Here's my video from day 2, with the CoG set at 95mm from the LE at the root. A great plane indeed, very promising, loads of fun and loads still to learn.

So what went wrong?

What went wrong? (2 min 42 sec)

Thanks for thr COG thomas... that scales up to 10cm for a 500mm span and is exactly where Ive put it.... now all I need is some better weather.

It looked to be fine.. then that great spin in... I normally have a job to get a spin with KFm sections.. the only one of mine that went like that was caused by vertical areas.. the relationship of front to rear areas....I had deepened the fuz on a prop jet and I think it was too deep and the fins didnt have a big enough advantage in area to keep it sorted...
It would fly ok at any reasonable speed but spun out just like yours when I slowed it right down..
edit..the location of the fins also matters... by bringing them in more central to the body their effectivness has been reduced..

please show some still pictures, but for now increase the fin depth x2 and try again... an increase in fin area sorted my propjet.

I can see I will have to watch my roll rates, as yours rolls a treat....on your loops, was that as tight as it would go? If thats a yes then the COG can be taken back a bit..

Nice plane, Nick!
I agree with Dave, the relatively small fins may be the cause for this spin. I do not get the radius to spin even if I try hard. The fins are quite big. (see post 4822)