Abrupt end at 4’

[Steve Fitton]

Sorry to see those losses Joe.  We've all been there but that doesn't really help.

[Ken Culbertson]

Ken,

A few thoughts:

Even if you have a "flexible wing" the full force is always reacted in the wing center section. This is easy to understand for quasi-static flight loads. The only thing you might gain by intentionally building a "soft" wing is that sharp buffeting loads are absorbed over a slightly longer time period as the flex occurs, potentially reducing peak stresses.

As to the stiffness added to the wing in bending from the fuselage tube--don't count on much. (I assume everyone was hollowing out the solid fuselage blocks.) The fuselage is not the right shape to provide much additional help. If the fold failures you see are occuring at the side of the fuse, then the fuse was assisting (and perhaps a stress concentration was built in.) If they are folding where the center section planking ends, then we need to avoid that abrupt change of section modulus. Ultimately, putting the shear webs into the spar is in exactly the right place to directly help. And, if you used 5/16" square balsa spar caps, and felt they weren't up to it, then glue on some .007" carbon strip facing the inside of the wing before you glue in the caps. It doesn't have to go all the way out to the tips, but at least run it out past the landing gear bays. It adds weight, but also adds strength and confidence that you will have structural margin....

I'll take your other question as rhetorical--no one wants a wing to fold because they were overly zealous about saving 1/2 to 2 ounces of weight depending on the model size. Certainly not if it is relatively new and isn't oil-soaked or damaged from a previous incident.

Dave

We may have to politely agree to disagree.  Maybe I am clouded by my experience with the unlimited sailplanes of the 80's.  A stiff wing could not stand the zoom launch and would often bend the wing rod.  In fact, the flex was needed for a really great one. I am not talking about making the wings weaker, just not so stiff that all of the force of a corner becomes concentrated on the center section.  I also think that the block mounts we use for wing wheels add to the weakening of the center section. On a hard landing there is no distribution of the impact. I use shear web out to the landing gear but leave the rest of the wing able to flex if needed.  My ribs are hollow for the last three which really helps.  I used to use light spruce for spars and I-Beams.  I have never seen a properly made I-Beam wing fold.  The center spar on those puppies would support a small house and they flex.  The wings I have seen fold the most at the center section - "D" tube.

Question to those of you who still have a Nobler ARF, I lost mine in the fire - is the Center Section double sheeted? 

Just one man's opinion - Ken

[Dave Hull]

Ken,

There are a few different things going on with a sailplane wing compared to a stunter. First, the flex helps absorb the surging force at initial release to keep that peak force down. And once the climb angle is established, you can modulate the winch to keep the loading below the limit load. Second, the wing in climb flexes and reduces the projected area and therefore the total lift and thus the maximum moment load at the wing root. Finally, once the line is nearly overhead, energy that was stored in the deflected wing enables a zoom launch, not just coasting upwards while bleeding off excess airspeed. Think of the wing as a deflected spring with energy stored in it like a diving board. At least that's how I see it.

If you draw a free body diagram of a stunt wing going thru a hard corner, you should find that the entire weight of the suspended fuselage (less any body lift of the fuselage itself) causes a bending moment in the center of the wing. Making the wing more flexible doesn't change that. To get the total lift, integrate the area under the lift distribution. To get the total moment load on the wing root integrate the lift distribution as a function of semi-span with the origin at the center of the fuselage.

The two Nobler ARFs that I have cut into had a single layer of 1/16" center planking.

[Phil Spillman]

Hi Joe, So sorry for your loss. I no longer will use crimps for anything! I use a Gerry Phelps to wrap lead outs as well as lines and, knock on wood, am avoiding similar troubles. As for lines I do not solder the wraps any more but do use shrink tight tubes to squeeze the wrappings. As for lines I use whit tubes for "up" and black or some other color such as pink or red for "Down" helps to maintain correct attachments when hooking up at the plane. Hope you ar able resurect this bird for more hours of pleasure!

Phil Spillman

[Doug Moon]

Well I didn't want to be left out of the Joe Daly party. I folded a wing today as well.

Sorry for your losses Joe.

[Ken Culbertson]

Finally, once the line is nearly overhead, energy that was stored in the deflected wing enables a zoom launch, not just coasting upwards while bleeding off excess airspeed. Think of the wing as a deflected spring with energy stored in it like a diving board. At least that's how I see it

We may have had slightly different winch procedures.  I launched with 1/4 flaps and took it pretty easy up to about 60 degrees.  Then I started pumping the winch, raised the flaps and put in a small amount of up elevator.  The goal was to have it going as fast as possible as it went tight over the top.  Just past direct overhead you could see it gaining speed and I would hit the winch hard till the wings cried help give it a quick down elevator, release the line and point it upward at about 30 degrees till it ran out of boost.  I could get an early round max without any lift using that method.  Learned it from one of the masters of the time, Don Chancey.  We were about the only ones releasing well after it passed over the winch anchor at the time but using that method and a shallow climb after we usually got 50 or more yards further upwind and the same height. 

Enough about sailplanes.  The pictures are of two wings that are nearly identical except for construction.  Both are the same airfoil/chord and roughly the same area.  Both were about 11oz with controls and no flaps.  The Warren Truss one is pretty standard for today's thinking and has shear web to 1 bay past the LG block and the full TE.  It does not have a double planked center section.  The other wing has center planking and no web for the Spar or TE.  It does have a wider TE.  Both are covered with MonoKote.  If I take the planes and balance them on one wingtip and hold the other, I can flex both wings.  The Warren Truss has the most flex and you can feel it right at the fuselage joint.  I estimate that it flexes about a 1/2" each way.  The other one had no flex at the fuselage joint with a total flex just about the same as Trussed wing.  There could be hundreds of reasons why this is not a scientific observation but given time, I do think I know which one will fail first, if at all.  I repeat, I have never folded a wing on a double center section planked wing.

Ken

[Ken Culbertson]

Well I didn't want to be left out of the Joe Daly party. I folded a wing today as well.

Sorry for your losses Joe.

Ouch, I loved that plane.

Ken

[Joseph Daly]

After sometime has passed, I finally got around to repairing the red one! Are structural repairs are complete, now on to making it pretty!

[FLOYD CARTER]

You only need to go as far as www.flyinglines.org and click on the tab "Many Fragments" to see the result of some of my "many" mishaps.

Why is it that someone is always around with a camera when I crash?

[Doug Moisuk]

Sorry, always sad to see.
This has been long post hope someone else hasn’t asked this. We pull test our ships at contest but not on a regular bases. A pull test might have saved the plane. How often does everyone pull tes?

[kevin king]

The road to success is littered with broken  stunt planes.