2 metre...no, make that a 4 metre motor glider!! - Page 5 - RC Groups
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Feb 17, 2015, 03:59 PM
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Knoll53's Avatar
each cross brace needs to be a cross brace for the CF tow approach...............do you like weaving?
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Feb 17, 2015, 04:02 PM
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Knoll53's Avatar
another approach would be to beef up the torsional capacity of the balsa D tube by added glass FRP...................make that CF cloth in the joiner area, then glass, then remainder with just balsa.
Feb 17, 2015, 04:50 PM
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Knoll53's Avatar
BTW, at the local club, we had conventional scale sailplane with 1/16" balsa sheathing over white foam and it fluttered on a super fast aerotow. He fixed his flutter (on a his non-swept wing) by adding glass FRP over the balsa.
Feb 17, 2015, 05:43 PM
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Stuart A's Avatar
How about making up some 1/16 balsa ply from 2x1/32 and sheeting the first 6 or so bays with the grains at 45* .It would add some weight,but still be lighter than a sheeted foam wing.The opposing grains ought to impart some stiffness.It would look good if you tapered it in along the spar.
Feb 20, 2015, 09:21 PM
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iron eagle's Avatar
Using 1/16 balsa ribs diagonally in each bay worked well for me in this wing.
Feb 23, 2015, 04:35 PM
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miniphase's Avatar
Kent, I like the blue struts...I'll go with that....though not in blue! I'll take it out to the half semi-span point. I'm planning on using Sig Koverall which will add a good amount of rigidity to the structure too. I may be worrying too much, but it's easy to put the struts in now for very little weight penalty....I don't fancy ripping off covering to beef things up at later stage. The carbon tow thing had also goe through my mind too, do they call that the 'tinker bell' approach where you have a network of small struts making a large, strong, light structure?

Anyway, I've had a few flights with the 2 metre version. All ok so far, bit of a pitch up when I drop the flaps. It's also the first wing that I've had to reduce the elevon differential on. I was getting a distinct pitch up when applying roll...had about a 3:1 difference in elevon throws, note they are not tapered control surfaces on this model.

Wind speed in this video was about 7mph. I also did a test flight in about 4mph....it just about stayed up, and also 20mph...not much fun!

wing test 2 (2 min 58 sec)
Feb 23, 2015, 05:18 PM
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Knoll53's Avatar
Nice GpPro work.........I guess there is a good reason they make them waterproof.

I think that concern regarding flutter/torsion/stiffness is well justified. That 4m job is a big skinny wing! I trust that your struts will be "rib material"? Both strut layouts, blue and black, have all forces concentrated at each panel point, that is at each connection. Break just one connection and all is for nothing. It is the weakness of any truss design. The cross braced carbon tow approach has the advantage of continuity, in that the carbon tow is continuous from root to tip. It creates a space frame with the ribs as the compression members. Carbon tow could easily cut through a rib so connections are still tricky. Never heard of a Tinker Bell structure.

Possibly just the Koverall would be adequate for most flight conditions, inasmuch as you have dual spars, which IMHO is the most important feature for flutter resistance.

The new 2m wing is a sweet ride. Very light on it's toes. I saw the zoom up when flaps were dropped abruptly. This is to be expected even in perfectly moment neutral flaps. What if you were to ease the flaps down over a 3 to 5 second time span, thus allowing the airspeed to slowly adjust? Would the trim setting feel the same before and after? I'm thinking that it very well might! I was thinking of putting my flaps on a switch with a 3 second delay built in.

Also the slow passes near the ground were encouraging..........no surprise nose-ins. So these flaps are considerably longer than your previous gliders. Any complaints so far. I'm guessing that this is your slowest flying plane.....with the big flaps.
Feb 23, 2015, 05:37 PM
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miniphase's Avatar
It'll not slow down quite as much as my EH3/12 wing, but there's not much in it. This wing can certainly fly faster, so should be more versatile....it'll be interesting to see how well it takes ballast.

Interestingly, if I deploy the flaps on the downwind leg, I get no pitch up. At the moment, I'm running with a bit of down compensation. This was programmed in from the start as I knew they'd not be pitch neutral, there's certainly no tendency to nose dive in. I'm sure you're right; a slower deployment would help negate the pitch up I'm currently experiencing.

Definitely need a headband btw!
Mar 15, 2015, 12:36 PM
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Any further flights and thoughts on the "pitchyness" of the flaps. (not to be confused with bitchyness).

Your initial observations seem to be that they are pitch positive. That is, for example, if you were flying as slow as possible, then slowly dropped the flaps, it would work it's way into a stall sooner or later.
Mar 15, 2015, 03:32 PM
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EdSoars's Avatar
Kent, back over on your SW-11 thread, you mentioned that flaps need to be swept to have neutral pitch. This seems like a good place for this discussion too.

I'm having a hard time getting my head around that. I thought that if the flaps' center of pressure chord-wise was on the wing's pitch axis, it would have neutral pitch input. If the flap CoP was forward of the wing pitch axis, it would be pitch positive, and if behind, it would be pitch negative.

??? Any clarification? Assuming we're talking about steady-state flight, not transition. I have no clue about the downwind - only phenomenon, unless you're at low enough altitude for ground drag to be reducing the wind speed, and even then I haven't figured out the behavior!

Ed
Mar 15, 2015, 04:27 PM
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miniphase's Avatar
Kent- the flaps are definitely pitch positive as expected, but with a moderate amount of down trim on the elevons it hangs in the air absolutely fine. Also with the compensation, if I gradually drop the flaps (with the compensation mixed in) I don't get the ballooning up.

Ed- as I understand it, the centre of the flapped area has to be in line with the centre of lift for the flaps to be pitch neutral. Think of what we are doing when we deploy flaps; we add a load of lift (paid for with a load of drag) allowing us to fly a lot more slowly. When I drop the flaps at a speed, this first translates to a boost of lift, hence the ballooning up before it settles to the new 'high lift, high drag' state/speed.

No stunning progress on the wing atm as I am currently knocking a dirty great hole in the back of my house....
Mar 15, 2015, 04:31 PM
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Stuart A's Avatar
....so you can get the 4m wing out ?
Mar 15, 2015, 04:45 PM
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miniphase's Avatar
Lol! not quite Stupot.

Though the main beam holding the floor up in the 'building room' needs to be supported by a new steel. The new steel needs to be placed on new plates set in a high strength mortar that will need 14 days to set fully, so that's two weeks of the building room being 'out of bounds'.

That does, however, remind me of the Horten brothers taking out a supporting pier in their parents' house so they could remove the Ho1 they'd built in the dining room.
Mar 15, 2015, 04:54 PM
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Quote:
Originally Posted by miniphase
The new steel needs to be placed on new plates set in a high strength mortar that will need 14 days to set fully, so that's two weeks of the building room being 'out of bounds'.
I'm sorry, but we can't have that. High early grout to the rescue. 3000 psi in a day.

Back to more serious matters.
the 2m version appears to have a 9" chord.
Have you pushed up the aspect ratio on the 4m version?
It looks quite narrow.
Mar 15, 2015, 08:32 PM
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EdSoars's Avatar
Miniphase: thanks. It looks like a straight center flap requires extreme sweep in the outer panels to get the wing's pitch axis far enough aft to make the flap pitch neutral. So Kent is right about the flaps needing to be swept.

Congratulations to your wife for allowing you to blow out the living room for a fortnight just to remove a big model glider. My wife let me do that once but it was for a canoe that we could both paddle.

This is the stuff of urban legends.

ed


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