Comment by Andrew Rabbitt on October 24, 2012 at 10:10pm

It'd be very illuminating to plug the foils and geometry into XFLR5 to see how well theory corelates with your practice.

I have been revisiting my flying wing ideas and have a similar spec, albeit 1.4m span with MH64 section and a fuselage. I have thought of knocking together a simple model like yours to validate the geometry first.

One interesting thing that my calcs in XFLR5 are telling me is that downturned winglets offer significantly more yaw stability than upturned. I am also using my winglets to generate stabilising downforce in lieu of washout, so they are set at 60deg instead of 90deg
FWIW.

Moderator Comment by Gary Mortimer on October 25, 2012 at 2:42am

Andrew you are bang on right about down turned tips. I think the Elevons might be suffering with the outward mounting of the servos, I would bet the section close to the centre is being blown flat in flight therefore having less effect. Get some balsa wood and try making them stiffer. You should have a forward CG with a wing as they like it! 

Did you offset that thrustline any?

Comment by Ted Van Slyck on October 25, 2012 at 5:46am

So you guys are thinking I might try having the winglets angled outward and downward to make a downforce on the wingtip? Kind of similar to a CRJ900 wingtip? My understanding was angling them IN (toein?) will help with yaw stability. What exactly do you guys mean by "downturned winglets"? 

I specifically mounted the servos far out in the wing so the wide part of my elevons would not flex down in the airstream. However, you are right. The inboard part of the elevons definitely flew down when deflected. I stiffened the elevon with 1/16" balsa, but it still does. I would probably have better elevon authority and stall characteristics if the roots had better deflection. 

The thrustline is perfectly straight. 

Moderator Comment by Gary Mortimer on October 25, 2012 at 9:40am

Quite simply having a vertical portion under and over the end of the wing, it can also act as a bit of a prop saver.

Not seeing any reflex on those flaperons although it might be just the photo. You should have at least 2 or 3 mm of up on both.

Comment by Dez Socks on October 25, 2012 at 6:47pm

I have built wings similar to yours with not much luck.I had the same problems as you. Battery had to be within the tiniest of tolerances or it became uncontrollable.I moved to delta wings.

What helped me was winglets and vertical stabilizer,although it takes from the minimalist wing design.

The vertical fin i have mounted is thick and swept back, center and beneath the wing.This serves three purpose's.It protects motor and prop as with what Gary mentioned, provides a grab spot for launches and also helps stability.

The winglets i tried on top and agreed with andrew the angle.The back angle of the winglet is swept back behind the wing.In theory this gives it more of a "tail" adding to stability.I think :-)

Comment by Ted Van Slyck on October 26, 2012 at 7:15am

Interesting  comments about the winglets guys, I appreciate it. Maybe better yaw stability will help with the spin characteristic too. I've got this wing flying really well except for three things: 

1. The yaw stability at slow speeds. After aileron input during slow flight things get a little dicey. This could have something to do with the spins. It does spin, but its an easy recovery. Always to the left, so thats likely some kind of error in the wing. The link I posted below mentions a relationship between induced and parasitic drag that would apply in this situation that is quite interesting.

2. C.G. / Weight issue: In order to make it track like an arrow the CG has to be forward, but to land slow it has to be a little aft. I am flying with a large battery. I'm going to buy a smaller battery and fly with the slightly aft CG for control effectiveness and slower landing speed. The stall speed is lower when it weighs less and the controls are more effective to actually get to that speed. 

3. Overbanking: During tight turns (how I like to fly) there is a moderate overbanking tendency that requires a fair amount of opposite aileron to compensate for. I am thinking about adding a few degrees of dihedral to compensate for this. Apparently too big a tip fin can cause this overbanking tendency. 

For those interested I found an easy read on tip fins and dihedral in flying wings.

Yes I do have some reflex, or it nose dives! 

http://www.djaerotech.com/dj_askjd/dj_questions/fwingdihedral.html

Comment by Brent on October 26, 2012 at 8:22am

Lower the motor some to be more inline with the center of the wing section and not sitting above it, wings don't like the motors up to high and will not handle well.

Comment by Ted Van Slyck on October 26, 2012 at 12:28pm

I plan on lowering the motor by completely changing out the mount to mimic the design at the link. Any ideas on this?
I am on my phone and can't seem to insert a picture.
http://db.tt/HtytVORa

Comment by Andy Geppert on October 26, 2012 at 9:14pm

Great work so far! What calculations did you use to come up with the 2 degrees of washout? I'm working on a similar wing, that has not flown yet, but I was coming up with recommended washout in the range of 4 to 8 degrees with the MH60. I'm using a few different design references that I've found around the web. I think I ended up using 6 degrees as a compromise between stability and spar architecture.

Comment by Ted Van Slyck on October 27, 2012 at 4:32pm

Hi Andy,

I used the information found on this site to estimate washout, I however did not follow the advice. I wish I had. Thats OK though, now I have a base point of learning. A little reflex will take care of not enough washout. Don't get too bent out of shape on washout, it will probably still fly fine. I believe it called for 3 degrees or so, but that just felt like too much, so I settled on less from a gut feeling. I think my wing needs either another degree or so, a wider chord at the tip, or a different foil at the tip. My next wing will probably have a symmetrical tip foil that is negative slightly. Also, I don't think it is necessarily prudent to have the elevons span the whole trailing edge. I think it only needs to go from the MAC to the tip, but I have no references for that other than my own drawings. I am going to cut out a few wings from pink foam and make a few gliders to test some theories. I think with a properly designed wing you can bring the CG way aft and still be stable. Its really all a balance act between the CG and CP. Also, I don't think winglets are required. But they must help. 

I learn by trying things, Ill post my findings here. 

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