I have a Holy Smoke Delta that I'm converting from aileron/elevator to elevons. The roll rate is slow with the stock ailerons and they entered a flutter condition due to the sloppy cable/guide tubes. The hinges started to go away but I got it down OK. I'm planning on the using 1/8" "lite ply" as before and add a bit of feathered balsa veneer near the elevons hinge point to make it a tad thicker than the T.E. of the wing. There will be two used Hitec HS-5985 MG's (172 oz/in. and zero backlash) in the wing moved aft (about where seen) to move the CG more to the rear. The elevon will run the full wing length and cut/separated at center.

My question is, should I run the elevons full width or cut the taper as seen? Is there an aerodynamic reason for the taper at the wing tips like tip stall?






PS I'ts getting new cooler covering too.

I believe that there is no aerodynamic reason for the taper.

the shape as is is fine - the problem is in flying fast - the ailerons can get into a flutter from flex.
the taper reduces this potential problem
Most problems on these planes is not an "airfoil/"problem but simply flexing in the wing or control surfaces
keep everything as smooth yet slop free as possible-And do not use flexible control surfaces!-

The delta was designed before computer radios, the elevator and aileron setup was to allow you to have enough elevator throw without having too much aileron. Deltas are soft on the elevator and fast on the ailerons normally. The tapered tips of the aileron would be a anti-flutter tweak to the design.

Where you have the servo is good, nice direct stiff linkages. I'd glass the elverons, they can not be too stiff. I would also put a 1/2" tip on the wing so the elveron doesn't go right to the tip, it would do two things:
First it protects the elveron from being knocked during transporting.
Second it makes the tip vortex leave the tip, which is fixed and not going to flutter, rather than the end of the elveron which might flutter. I don't think you would have to taper the elveron if you put a tip on the wing.

Ok, thanks guys. You gave confirmation with what I was thinking. The subtleties in design can come up and bite you in the rear, I just don't want to do things twice.

Nothing much 'subtle' about deltas, but the elevon idea may result in some loss of pure elevator movement. One of the likeable things about deltas is the ability to land at speeds and AOA's that normal airplanes cannot reach. Losing the elevator as a separate surface, and reducing the total movement in 'elevator' mode because of the responsive ailerons may mean you won't be able to use this feature any more. Consider, instead, separate aileron servos (instead of the snakes), and maintaining the current elevator set-up. Best of both worlds, that way.
Evan, WB #12.

The taper used in the elevons reduces their mass near the tips, where their mass is most likely to induce flutter because simple wing flexing vibratory excursion is highest near the tips.

Control surfaces should be as light as practical while retaining sufficient strength to do the job. What they need in order to minimize the likelyhood of flutter is high stiffness to weight ratio, not maximum possible stiffness at the expense of weight. I would be strongly inclined to employ mass balancers at the tips of the elevons. I employ them on all of my airplanes that are expected to fly fast enough to make flutter a possibility. The mass balancers, if mounted at the tips, should statically balance about a third of the static unbalance of the elevons.

Excessively heavy mass balancers can be more dangerous than none, since they may induce flutter a flutter mode where the wing or stab flexes in a more complex manner than simple flexing of the tips up and down.

Such higher harmonics usually only appear at rather high airspeed, where flutter can be extremely brutal, and break up the airplane in a fraction of a second.

I have found that models built from EPP -cut foam- will flutter -but this stuff simply refuses to break- I have been playing with it - recently, doing overpowered ,extremely light aerobatic designs
I wish this stuff had been around years ago - - with a little carful use of carbon fibre rods and tubes and strips - you canmake areally GOOD model which will fly nice straight aerobatic sequences .
The beauty of it is IF you can see flutter - simply strengthen or brace the offending section and go back to flying.
The foam has a slightly spongy texture and the beads are bonded together far more solidly than the stiff foams such as expanded styrene bead or the Depron type stuff.
The rigid materials we have all used for years, will ALL flutter- then fracture or break given the right circumstances

If you want to make this with elevons one way to couple the aileron and elevator function is to make the elevons taper in all the way to a wider center width. Make them about twice as wide at the center as they are now. Connect the servo at the mid point to gain as much from stiffness as practical. And using C grain or glassing the elevons as mentioned would not be all that bad an idea.

You also want to use a medium long arm on the servo and equivalently long control horns. Then use the outer holes or only one or two holes in from the outer to achieve your max deflection angles. This reduces the effect of any slop in the system.

The idea of using the wider elevons with the strong taper to the tips is to avoid the ailerons becomeing too sensitive while still not having enough elevator authourity. The wider center will ensure that you get the elevator control you require.

You didn't say how much aileron throw you're currently using. If you reduced it in an attempt to reduce the flutter issue then you may just not be using enough deflection. More info on this regard in terms of amount of deflection either as angular degrees or X up and down over Y width would assist.

Thanks for all the suggestions guys.

Looking at other deltas and weighing input, I went with elevons full width. Hatch mounted servos were used instead of surface mount to keep the push/pull forces lined up since the servo and control surfaces are so close. The C/G dictated moving the servos aft. The advantage of surface or capstan mount is being able to use full servo travel (see red delta) and this was the problem I had to overcome with my mod. The problem shows up when using "delta' wing configuration in the transmitter and you put the stick in one of the corners. The result is elevator plus aileron and the servo travel is extreme. I can reduce the max travel at the expense of elevator and/or aileron independent full travel (see yellow delta). I don't want to give up these large defections because of the thrill of high roll rates or abrupt pull outs. I tried all kinds of mixes and tricks, even a servo programer, but there was always a sacrifice. My old JR 8103 transmitter just cant set absolute servo travels. So I had to cut slots to let the servo arm and push rod disappear into the wing.

For test purposes I spit the elevator at center and joined the ailerons with 1/32 ply on both sides. So surprised buy the stiffness that I'm going to throw some quick covering over the bear spots and test flight before pulling off the old covering and controls surface for revamp.

I think the taper is done to desensitize the roll rate and to also have less unsupported flap area / mass out where they can get buzzed by the dirty air at the wing tips.
For a plane this size I like the inboard chord of the flaps to be 1 3/4" and the outboard to be about an inch.
I glue hardwood end caps on both ends of each flap to help eliminate cupping, and inbed hardwood dowel chunks to support the control horn hardware

taper is nice
This little porker is 27 grams inc battery (113 sq in)

I like it..!

How did you cut the ditch in the bottom of the fin..?

And the answer is....
Fantastic!

At first I ran into an unrelated problem, the plug in the third stopper hole squirted out and I lost tank pressure which resulted in a dead stick landing. No harm done, but that nice glide let me know the CG was dead on and the trim was only a few clicks off perfect. After wringing out the tank's foam rubber wrap and letting the wind dry it out good, I put it back together. I noted from the first short flight that the roll rate was crazy twitchy so I add expo, one hundred percent to be exact and that turned out not to be too much. Elevator was not bad, but I put in 25% to tame it a tad.

The next flight was fun/crazy with blur fast rolls and hit a wall verticals pull ups. The only thing that was a mild distraction was after banking then pulling you needed to hold a tad of opposite aileron to keep it from rolling inverted, like a top heavy boat wanting to capsize. Keep in mind it's really hauling in the turns so it's over in a second. I refrained from putting the stick in the corners because we have a ceiling at that field and I wanted more altitude/time to get out of the flat spin that would most likely result, the CG is further aft than before and may take a bit to pop out of it, so I'll try that elsewhere. Not a hint of flutter!


The third and last flight was a bit shorter when the two screws that held the header on, unscrewed and parted company. There was more vibration on this flight because the engine was turning much higher in flight due to upping the nitro to 30% and a cooler plug for the fuel. It also didn't help that the new Dave brown spinner was well out of balance. I added six 1" strips of masking tape to the inside light side of the spinner to balance, the prop was perfect. I use to think the Dave Brown spinner was as good as a Tru-Turn, looks like I got what I paid for.

It's ready for next weekend.