Midair wing fold avoidance!

[Hai-Lee]

I have like most FB builders had a model or two that have folded their wings in flight. In my case thankfully they were older builds and often destined to be salvaged soon anyway.

After examining the points of failure I noticed that most were due to wing joint failure, (this was most prevalent on wings joined using Hotmelt - I live in a hot climate). The failures were generally just the glue letting go of the foam and not an actual foam board failure.

Initially I added extra bracing including wooden dihedral bracing and the like but this brought another aspect of the failure into focus. I started to get wings that would still break along the central glue line and change their incidence angle in flight.

The fitting of dual spars, (still my preferred method of construction), solved this side issue but the eventual failure of the wing joint was still a nagging problem, (and an added maintenance issue).

I hate weak structures and so I undertook a quick investigation of the stresses in the wing joint itself and I found something interesting. The FB wing has all the same structural failings of a shaped tube of FB. The spars themselves help a great deal with the maintenance of the wing shape and really cannot handle much stress and provide little resistance to folding when the central glue joint fails!

In order for the wing to fold the tube structure must fail, and for this to happen one of the wing joints needs to fail/break! Normally the lower seam/joint which has the greatest forces pulling it apart in level flight and in high positive "G" maneuvers! Rather than just adding large and heavy amounts of CF/Wood/Aluminium or the like I wanted a simple and very light method of securing the joint against the forces involved.

It just so happened I had a roll of very high strength tape, (I think it has woven fibres and a strong adhesive). I cannot rip the tape and I must use a sharp knife because even a blunt knife has problems cutting it. I now fit the tape across ALL wing joints on the inside of the wing before I fold the wing and glue!

Since using the tape across the wing joints on the inside I have not folded a single wing, (including my Explorer which lasted long enough to be gifted away). I use only lengths of around 150mm and one at the LE, one at the TE and one either side of the wing spar, (top and bottom panels).

I am sure that simple packing tape would suffice for most FB builds which I intend to start doing soon to all but my largest builds which will still get the strongest of tapes.

I still wrap the wing joint on the outside of the wing in a strong tape but now that is mostly aesthetic rather than a necessity!

My current rebuild of an Ugly Stick will show such tape being applied to the wing in the next few days, (it is not a really rushed build).

Just something that has improved the life and strengths of my builds, (and help keep the weight down).

Have fun!

 

[FDS]

That’s very interesting. I put a strip of “lite ply” across the top of my TT wing as it had begun to crease. A fibre tape on the inside would have been easier, the ply did improve the fixing area where the rubber bands sit.
I am making another TT wing this month so will incorporate that idea. What about wrapping the FB spar before insertion? Would that be worth doing?

 

[Hai-Lee]

That’s very interesting. I put a strip of “lite ply” across the top of my TT wing as it had begun to crease. A fibre tape on the inside would have been easier, the ply did improve the fixing area where the rubber bands sit.
I am making another TT wing this month so will incorporate that idea. What about wrapping the FB spar before insertion? Would that be worth doing?

I still use paddle pop? sticks across my TT wing spar joint but more from habit than anything else. besides I line the Peanut shaped sticks you get on some ice-creams and shave them to give the proper dihedral angle.

I just finished and delivered a new TT with the Wooden brace but not the tape as I built both wings separately and not as one piece. The main reason is the wing build method and the internal tape work best when building a FLAT wing, (no dihedral or anhedral).

Having said that My next scratch FB design and build will be designed and built so that the added strength of internal tape can be included along with dihedral.

Have fun!

 

[CapnBry]

As always good information from you @Hai-Lee. Quick question about the direction you apply the tape to the inside, do you mean a strip of tape running left to right on the leading and trailing edges or a strip running on the inside front to back?

 

[Hai-Lee]

As always good information from you @Hai-Lee. Quick question about the direction you apply the tape to the inside, do you mean a strip of tape running left to right on the leading and trailing edges or a strip running on the inside front to back?

It depends on the tape you select and which direction the non-stretch fibres run. I use a tape where the fibres rung along the tape and not across it. Anyway I use the tape so that the non-stretch fibres run across the wing joint.

As for the placement I put the tape in the inside of the wing joint BEFORE I fold the wing and glue. After the wing is sealed/glues I use standard tape, sometimes cloth tape as a standard wrap around the wing joint.

If it is a large wing span model I will also use a cloth tape along the wing LE but mainly to reduce hangar rash than impact damage. Cloth tape on the LE can make repairs even more difficult if you still crash frequently.

When building KFM wings or other wings where there is no internal space and a plastic backed tape will cause a weakness if used to join sheets together I use a very open weave fibre tape, (without backing as used for "DRYWALL"), to reinforce the joints. This open weave allows the glue to work on both sheets/layers of the FB and the tape is effectively embedded in the glue layer on each side of the joint. So strong that failures due to impact are almost NEVER in the area where the tape exists!

I hope that helps and if not just ask away!

Have fun!

 

[CapnBry]

I hope that helps and if not just ask away!

No all of that is great info, above and beyond! My planes are starting to last a lot longer than they used to and I was a little crushed when my Explorer's wing folded. I'm starting to need improvements to prevent them from failing on their own rather than me smashing them into things (although I still cause the most damage carrying models around my house). Thanks for the tips, I'll start including these techniques into my new builds. You're a real treasure on these forums.

 

[Hai-Lee]

No all of that is great info, above and beyond! My planes are starting to last a lot longer than they used to and I was a little crushed when my Explorer's wing folded. I'm starting to need improvements to prevent them from failing on their own rather than me smashing them into things (although I still cause the most damage carrying models around my house). Thanks for the tips, I'll start including these techniques into my new builds. You're a real treasure on these forums.

Just one other tip which may be of use if your wing centre section keeps getting damaged when you impact the ground.
Before you fold the wing press down the foam in the bevels, (Using a skewer?), around the wing LE join or the wing centre section. Cut a piece of BBQ skewer to length, (I use around 200mm) and fit it onto the depressed bevel foam equally either side of the wing centre line.

Now apply glue and fold the wing ensuring that the skewer remains in place and the centre. When the wing is folded and ready to go the centre section of the wing, (where it mounts on the fuselage) is stronger and reinforced with the BBQ skewer. The only real issue is that any LE wing tears will tend to occur at the end of the skewer but the wing LE and the Wing LE joint will be much stronger and able to resist some serious impacts. I find an outboard repair easier to accomplish anyway!

Have fun!

 

[Erind]

So i just had my sport wing on a tiny trainer fold in mid air. i am running a 2306 motor though with a 6045 prop. (SO MUCH POWAH) so i build a new sport wing and this time i put one of these before the spar about 10 inches long (from servo to servo) and they're about 1/3 of an inch thick

Btw it flies like a dream and you cant feel the weight even on a tiny trainer and they are really strong

 

[DamoRC]

I have like most FB builders had a model or two that have folded their wings in flight. In my case thankfully they were older builds and often destined to be salvaged soon anyway.

After examining the points of failure I noticed that most were due to wing joint failure, (this was most prevalent on wings joined using Hotmelt - I live in a hot climate). The failures were generally just the glue letting go of the foam and not an actual foam board failure.

Initially I added extra bracing including wooden dihedral bracing and the like but this brought another aspect of the failure into focus. I started to get wings that would still break along the central glue line and change their incidence angle in flight.

The fitting of dual spars, (still my preferred method of construction), solved this side issue but the eventual failure of the wing joint was still a nagging problem, (and an added maintenance issue).

I hate weak structures and so I undertook a quick investigation of the stresses in the wing joint itself and I found something interesting. The FB wing has all the same structural failings of a shaped tube of FB. The spars themselves help a great deal with the maintenance of the wing shape and really cannot handle much stress and provide little resistance to folding when the central glue joint fails!

In order for the wing to fold the tube structure must fail, and for this to happen one of the wing joints needs to fail/break! Normally the lower seam/joint which has the greatest forces pulling it apart in level flight and in high positive "G" maneuvers! Rather than just adding large and heavy amounts of CF/Wood/Aluminium or the like I wanted a simple and very light method of securing the joint against the forces involved.

It just so happened I had a roll of very high strength tape, (I think it has woven fibres and a strong adhesive). I cannot rip the tape and I must use a sharp knife because even a blunt knife has problems cutting it. I now fit the tape across ALL wing joints on the inside of the wing before I fold the wing and glue!

Since using the tape across the wing joints on the inside I have not folded a single wing, (including my Explorer which lasted long enough to be gifted away). I use only lengths of around 150mm and one at the LE, one at the TE and one either side of the wing spar, (top and bottom panels).

I am sure that simple packing tape would suffice for most FB builds which I intend to start doing soon to all but my largest builds which will still get the strongest of tapes.

I still wrap the wing joint on the outside of the wing in a strong tape but now that is mostly aesthetic rather than a necessity!

My current rebuild of an Ugly Stick will show such tape being applied to the wing in the next few days, (it is not a really rushed build).

Just something that has improved the life and strengths of my builds, (and help keep the weight down).

Have fun!

In the early days of build videos FT used to recommend extreme packing tape for this purpose. It has fibers running cross and lengthways and is really good gear. I use it for wing joins (almost always) and for holding ply motor mounts to foam.

 

[Hai-Lee]

The tape I use for standard folded wings is very similar though a different brand of course. For laminating wings as in the making of KFM wings I use this tape embedded in the glue between layers;

Have fun!