3D Printed Planeprint Jetwing - STRUCTURAL FAILURE WARNING

[EvilC57]

A warning for anyone who has, or is thinking of building this model.
 

I built the EDF powered version last winter, and had two successful maiden flights with it recently.

Unfortunately during a third flight yesterday it suffered a major (and quite spectacular) structural failure.

I had completed one fast low pass, and had just gone around for another one when the whole model seemed to blow apart in the air.

 

Studying the recovered remains on the ground afterwards, it became evident that the fuselage had failed just forward of the wing. I suspect this to be due to a potentially weak point at the rear of the PLA skid where it joins to the carbon fibre tube on the bottom of the fuselage. During flight the weight of the heavy lipo battery with additional g-forces is effectively cantilevered from around this point. I would suggest strengthening the area by glueing CF strips to the inside of the fuselage, and/or soaking the area with thin cyano, as Planeprint suggest for other areas in the model which need local reinforcement. Despite carefully following their instructions regarding print settings and Cura profiles to ensure sufficient adhesion between the LW-PLA layers, I suspect the stresses in the area which failed were just too great for the material.

The failure point was around the area I’ve marked build instruction extract below, and not at the joint between parts.

 

During its short life the Jetwing flew beautifully and I feel deprived now of the fun I could have had with it in future, so if I do decide to build another one I shall certainly be paying more attention to strengthening the structure where it seems it’s needed, and suggest that anyone else building the model do the same.

 

 

 

Edited May 28, 2023 by EvilC57

[J D 8]

  Makes one wonder how much test flying the designer's do or do they rely on the sums. 

[John Stainforth]

2 hours ago, EvilC57 said:

A warning for anyone who has, or is thinking of building this model.
 

I built the EDF powered version last winter, and had two successful maiden flights with it recently.

Unfortunately during a third flight yesterday it suffered a major (and quite spectacular) structural failure.

I had completed one fast low pass, and had just gone around for another one when the whole model seemed to blow apart in the air.

 

Studying the recovered remains on the ground afterwards, it became evident that the fuselage had failed just forward of the wing. I suspect this to be due to a potentially weak point at the rear of the PLA skid where it joins to the carbon fibre tube on the bottom of the fuselage. During flight the weight of the heavy lipo battery with additional g-forces is effectively cantilevered from around this point. I would suggest strengthening the area by glueing CF strips to the inside of the fuselage, and/or soaking the area with thin cyano, as Planeprint suggest for other areas in the model which need local reinforcement. Despite carefully following their instructions regarding print settings and Cura profiles to ensure sufficient adhesion between the LW-PLA layers, I suspect the stresses in the area which failed were just too great for the material.

The failure point was around the area I’ve marked build instruction extract below, and not at the joint between parts.

 

During its short life the Jetwing flew beautifully and I feel deprived now of the fun I could have had with it in future, so if I do decide to build another one I shall certainly be paying more attention to strengthening the structure where it seems it’s needed, and suggest that anyone else building the model do the same.

 

Looks like flutter.

 

 

[John Stainforth]

Looks like flutter.

[Shaun Walsh]

17 minutes ago, John Stainforth said:

Looks like flutter.

I can't believe it's not flutter.🤣

Edited May 28, 2023 by Shaun Walsh

[Don Fry]

Why flutter lads, I thought, I certainly have, heard flutter in the past? And I struggle to see why flutter should break it there. 

[leccyflyer]

2 hours ago, J D 8 said:

  Makes one wonder how much test flying the designer's do or do they rely on the sums. 

In viewing many reports and videos of fully 3D printed aeroplanes that is certainly something that springs to mind, along with the level of flying experience of some of the pilots. No small number of examples where the 3D printed aeroplane is the first time some of those users have ever tried to fly RC.

[EvilC57]

4 hours ago, leccyflyer said:

In viewing many reports and videos of fully 3D printed aeroplanes that is certainly something that springs to mind, along with the level of flying experience of some of the pilots. No small number of examples where the 3D printed aeroplane is the first time some of those users have ever tried to fly RC.

1. It had had two previous flights without any problem (both included several high speed passes).
2. At the time it broke up I had just pulled out of a turn back in towards the field, and was at the start of a high speed diving pass, but was at nowhere near the full speeds previously attained without issue.
3. I have been flying RC for over 20 years and many of the aircraft I fly are high performance EDFs, and ‘Reno Racer’ type models.
4. As Don Fry says above, why would it break where it did if flutter was the problem? Several observers saw the short fuselage (with the battery inside) fall away from the wings separately, in other words it didn’t break off in the impact with the ground.
5. Looking again at the design, there is clearly no reinforcement and no measures taken (like increasing the 3D infill) in the area to take account of the not inconsiderable weight of the 3700mAh 4S lipo it has to support in flight.
6. In my opinion, and the opinion of several people there at the time, it was structural failure, not flutter.

Edited May 28, 2023 by EvilC57

[MattyB]

4 hours ago, EvilC57 said:

1. It had had two previous flights without any problem (both included several high speed passes).
2. At the time it broke up I had just pulled out of a turn back in towards the field, and was at the start of a high speed diving pass, but was at nowhere near the full speeds previously attained without issue.
3. I have been flying RC for over 20 years and many of the aircraft I fly are high performance EDFs, and ‘Reno Racer’ type models.
4. As Don Fry says above, why would it break where it did if flutter was the problem? Several observers saw the short fuselage (with the battery inside) fall away from the wings separately, in other words it didn’t break off in the impact with the ground.
5. Looking again at the design, there is clearly no reinforcement and no measures taken (like increasing the 3D infill) in the area to take account of the not inconsiderable weight of the 3700mAh 4S lipo it has to support in flight.
6. In my opinion, and the opinion of several people there at the time, it was structural failure, not flutter.

I don’t think @leccyflyer was questioning your skills or experience, only that many of the 3D printed aircraft designs that are available are not designed by experienced aeromodellers. Certainly based on your own description it seems the designer of this model has not strengthened it sufficiently for flight loads.

[J D 8]

              Agree that looks like a structural fail as the result of not enough meat to resist bending moments on the nose/battery section going into and out of high Gee turns 

[Don Fry]

That’s a about right, 3 flights, 15 minutes of stress across that sheet face, and it cracks. 
Respect to the materials, just needs a tweak. 

[leccyflyer]

5 hours ago, EvilC57 said:

1. It had had two previous flights without any problem (both included several high speed passes).
2. At the time it broke up I had just pulled out of a turn back in towards the field, and was at the start of a high speed diving pass, but was at nowhere near the full speeds previously attained without issue.
3. I have been flying RC for over 20 years and many of the aircraft I fly are high performance EDFs, and ‘Reno Racer’ type models.
4. As Don Fry says above, why would it break where it did if flutter was the problem? Several observers saw the short fuselage (with the battery inside) fall away from the wings separately, in other words it didn’t break off in the impact with the ground.
5. Looking again at the design, there is clearly no reinforcement and no measures taken (like increasing the 3D infill) in the area to take account of the not inconsiderable weight of the 3700mAh 4S lipo it has to support in flight.
6. In my opinion, and the opinion of several people there at the time, it was structural failure, not flutter.

To clarify, I wasn't speaking of your own experience, which is well established by your posts in this forum, or even that particular design. I was pointing out the all too common case of guys on the various 3D Printed models groups doing what I described, namely building a 3D printed airframe and that being their first experience of attempting to fly an RC model. I also said nothing about flutter - I'm perfectly fine accepting your explanation that the design of the model was not robust enough in that critical area.

[EvilC57]

9 hours ago, leccyflyer said:

To clarify, I wasn't speaking of your own experience, which is well established by your posts in this forum, or even that particular design. I was pointing out the all too common case of guys on the various 3D Printed models groups doing what I described, namely building a 3D printed airframe and that being their first experience of attempting to fly an RC model. I also said nothing about flutter - I'm perfectly fine accepting your explanation that the design of the model was not robust enough in that critical area.

Yep, OK thanks leccyflyer. I would add a point 7 to my post above in relation to this particular aircraft, Planeprint are an established company (in Germany I believe) with a range of around 24 different models. There are also a number of videos on YouTube from around the world of various people flying their Jetwing, and none have reported structural failures or other problems with the design. I was unlucky I guess, and as I say, if I build another one I’ll pay more attention to reinforcing the structure around where it supports the battery.

[Simon Chaddock]

Now if you were 'laying up' such a structure you would not put the strands side by side but along the line of the stresses. The printed filament has many times the tensile strength along its length. LW-PLA does have better 'side to side' adhesion but it is still much stronger along the filament.  

Unfortunately the way the majority of structures are printed - rising out of the bed - means the result, at least in one plane, is bound to be relatively weak for its weight.

 

 

[Keith Billinge]

Looks like that model has a carbon tube that supports it logditudonally.
But the carbon could do with being longer, if possible, to support the nose setcion.

The failure seems to be in front of the carbon longeron. Maybe that the stress point?

 

KB

 

[EvilC57]

1 hour ago, Simon Chaddock said:

Now if you were 'laying up' such a structure you would not put the strands side by side but along the line of the stresses. The printed filament has many times the tensile strength along its length. LW-PLA does have better 'side to side' adhesion but it is still much stronger along the filament.  

Unfortunately the way the majority of structures are printed - rising out of the bed - means the result, at least in one plane, is bound to be relatively weak for its weight.

 

 

I agree. I wasn’t really happy with the way the fuselage and wing parts have to be printed with their layers running perpendicular to the direction of maximum stress. Unfortunately because of the necessarily flat ends of each section of the structure (where they interface to the adjoining part), there is no other way to print them unless you use a lot of supports, which when removed would leave a pretty unsightly surface.

 

With the wings stress across the layers isn’t really an issue because they’re supported by the two 8mm carbon fibre tubes, the longer of which runs almost to the wing tips. However, particularly for the part below which supports the weight of the battery, if I build another one I’ll increase the gyroid infill density, and fit some strips of CF as longerons inside.

 

There is some longitudinal support provided by the ‘Fuselage protector 2’ PLA parts as shown below. However these were obviously insufficient.

[EvilC57]

18 minutes ago, Keith Billinge said:

Looks like that model has a carbon tube that supports it logditudonally.
But the carbon could do with being longer, if possible, to support the nose setcion.

The failure seems to be in front of the carbon longeron. Maybe that the stress point?

 

KB

 

Yes, I said in my original post that it appeared to have failed at the interface between the back end of the skid and the front of the carbon tube which reinforces the fuselage. Unfortunately the underside starts to slope upwards towards the nose just forward of the front end of the CF tube (which is why I guess they ended it where they did). As you can see in the pic below, the skid also has to support the wheel. I agree though Keith, it’s a stress point.

 

I could however edit the .STL file for the skid, to make the glued lap joint longer where it overlaps the front to the tube.

 

 

Edited May 29, 2023 by EvilC57

[Simon Chaddock]

I did experiment with printing a wing with the filament line along the wing like this.

 

Single wall with a Gyroid infill.

What appears as separate lines when printed in LW-PLA actually merge together to create a solid top and bottom spar flange.

 

In effect it creates a "D" box wing structure with all the filament running along the wing

The rear section of the wing is printed in the same way but with no spar flanges and an lighter infill.

The two sections are simply glued together along the spar line.

Maybe still not strong enough for a wing but still amazingly stiff for it weight.

A printed fuselage done in the same sort of way divided down the centre line with left and righthand  sections most likely would be.  

 

     

[Keith Billinge]

I have printed a couple of simple 3D Printed planes and the problem is always that the material is brittle in a hard landing or in a stressed or bending situation. I haven't tried LW-PLA yet and I have an STL file of the Mitsubishi Zero to try with LW-PLA, But I think most of the models I've seen would benefit from more carbon reinforcement.
I hope you can reach a solution with this one as It looks like a nice model.

 

Best wishes

KB

[EvilC57]

I've been having a look at what can be done to reinforce the area, and it seems to me that increasing the gyroid infill from the suggested 6% to around 10% in the affected section of the fuselage, and adding a couple of 5mm x 1 mm CF strips either side should be more than adequate.

 

In the screenshots below:

Pale blue is the hatch

Pale green is the existing PLA reinforcing strip (which proved to be inadequate!)

Grey shows the additional CF strips

The red slice shows the approximate area of the previous structural failure.

 

 

[bbuzz]

Sorry to hear about the failure.  I'm in the process of printing this plane now and I will definitely take heed of your suggestions.  My prints are considerably heavier than the weights given for each part in the manual, despite carefully following all the PlanePrint profile settings.  I've printed with ColorFabb LW-PLA (active foaming) and Polymaker LW-PLA ( pre-foamed ) and they both are heavier than listed.  The pre-foamed parts actually print lighter than the active foaming parts. Do you have the same issues with weights?  Thanks

[EvilC57]

Strange that your LW-PLA parts are coming out heavier bbuzz. Mine with ColorFabb LW came out the same or lighter than suggested for each part in the instructions. Like you, I was careful to spend time setting up the profiles as per Planeprint’s instructions. However I also did a series of test prints, and established that with my Ender 3 V2 I got the best expansion at about 230°C (can’t remember the other parameters now without looking them up).

 

I’ve subsequently built a second Jetwing encompassing all my suggested strengthening changes, but with winter weather etc. it hasn’t flown yet.

 

Edited March 26 by EvilC57

[PDB]

4 hours ago, bbuzz said:

Sorry to hear about the failure.  I'm in the process of printing this plane now and I will definitely take heed of your suggestions.  My prints are considerably heavier than the weights given for each part in the manual, despite carefully following all the PlanePrint profile settings.  I've printed with ColorFabb LW-PLA (active foaming) and Polymaker LW-PLA ( pre-foamed ) and they both are heavier than listed.  The pre-foamed parts actually print lighter than the active foaming parts. Do you have the same issues with weights?  Thanks

Have you calibrated your temperature and flow rate? https://colorfabb.com/how-to-print-with-lw-pla

[EvilC57]

20 minutes ago, PDB said:

Have you calibrated your temperature and flow rate? https://colorfabb.com/how-to-print-with-lw-pla

Yes, I’m familiar with that page.

 

The graphic on the page regarding the variables of temperature, speed and flow, and how they all interact is interesting and always worth bearing in mind when printing.

 

Edited March 26 by EvilC57

[bbuzz]

Thanks for the feedback, everyone!  I'll check out that page and see what I can do.