FT Explorer Spar Problems

[voyhager3]

I have been flying my explorer a lot recently and I have had problems with the wing folding in on itself. So I build myself a revised wing with a sizable length of mystery wood and that just snapped on my last flight. No crazy G’s or anything like that. I don’t care about dihedral, so would like a carbon fiber arrow shaft be good to reinforce it?

 

[whackflyer]

I have been flying my explorer a lot recently and I have had problems with the wing folding in on itself. So I build myself a revised wing with a sizable length of mystery wood and that just snapped on my last flight. No crazy G’s or anything like that. I don’t care about dihedral, so would like a carbon fiber arrow shaft be good to reinforce it?

Arrow shaft would be great, any fiberglass, carbon, or wood works well. Make sure the wood is oriented so it can't snap easily across the grain.

 

[voyhager3]

Arrow shaft would be great, any fiberglass, carbon, or wood works well. Make sure the wood is oriented so it can't snap easily across the grain.

Ok thank you I’ll probably see if I can find a carbon rod at a hardware store or something. I think the wood was for alignment when we did flooring, and it was a crappy not that strong wood, or something else it is strong another way but it is thin so it would have been hard to stand upright. Thanks for the info!

 

[clolsonus]

I recently built a 2m foamboard wing for a plane that weighs in around 2kg. I glued in two 3/8" square wood sticks (1m long) I bought from the local hardware store. So far so good, but I have some extra avionics and an action cam on board so I haven't pulled/push g's at all. I have flown in some pretty turbulent/windy conditions at a wide range of airspeeds, and so far I haven't found any signs of any structural issues.
The challenge is that the structural requirements are so dependent on size/speed/weight and maybe a lot on flying style. It's possible you did everything right, but had bad luck with a sketchy piece of wood that had a hidden flaw.
Other things to consider is that the spar doesn't necessarily only carry the bending load. The top of the wing compresses, the bottom of the wing stretches. (when pulling g's) The bending causes the top and bottom surfaces of the wing to get closer to each other. There is a lot of knowledge and good practices developed in traditional balsa-wood modeling, but it's hard to translate that directly over to a folded foam board wing. Even in the recent me262 episode on the FT YT channel, our beloved FT team continues to struggle with upsizing their wing structures for larger models and higher loads. (So it's not just us.)
An idea? If we have some aero or mech. engineering students on here they could build different structures and test their strength and come up with some data-driven structural recommendations for different size models. Right now a lot of us are doing a lot of guessing when we are building our own designs or running into weaknesses in the FT designs.
All of this is a big balancing act, right? We generally build foamboard models because they are quick and inexpensive to put together. If we start over engineering things and requiring exotic materials and construction techniques, we start missing the whole point of foamboard. So it's good to find ways to beef up our structures using inexpensive materials that are easy to source locally, and that may mean some additional compromises.
It's all fun though right? Even at NASA levels (especially at NASA levels), available materials drives many of the design decisions, so it's good engineering practice to find ways to do the best you can with what you have access to.

 

[JasonK]

Not sure if you can tell from the damage, but if you can tell if the failure was the bottom separated or the top crushing, that would tell you where the improved strength in your wing is needed.

now, me trying to remember some college class related to load calculations.... and failing to remember the equations... however I am remembering something about 2x the distance needs 4x the strength (or something like that). Also, FT is focusing on an internal spar, however you get the most flex strength from the surfaces (so for the weight, an I-beam is stronger then a solid object), so there are specific shapes that will be stronger then others.

 

[Merv]

I agree with JasonK, an I beam type of spar is far more effective than a solid spar.
I use barbecue squires as spars in all of my planes. I arrange them as an I beam, 4 in the top & 4 in the bottom. I have been unable to beak it.

 

[dap35]

Arrow shaft would be great, any fiberglass, carbon, or wood works well. Make sure the wood is oriented so it can't snap easily across the grain.

You can find inexpensive carbon fibre arrows at Wallmart, very strong and lightweight

 

[joelspangler]

Carbon fiber aarow shafts do work well and are fairly light. I recently started using flat carbon strips to drop weight even further. Here's a post I recently made about this https://forum.flitetest.com/index.php?threads/carbon-fiber-strips-really-useful.66948/

After my son folded his explorer's non-aileron wing, I built the aileron wing with no dihedral using the flat strips (I used two - one in each half of the wing directly inside the spars, but overlapping through the center however far they overlapped - i don't remember exact details). The resulting wing is very stiff, and has held up my son's "aggressive aerobatics" and quite a few crashes.

 

[voyhager3]

Not sure if you can tell from the damage, but if you can tell if the failure was the bottom separated or the top crushing, that would tell you where the improved strength in your wing is needed.

now, me trying to remember some college class related to load calculations.... and failing to remember the equations... however I am remembering something about 2x the distance needs 4x the strength (or something like that). Also, FT is focusing on an internal spar, however you get the most flex strength from the surfaces (so for the weight, an I-beam is stronger then a solid object), so there are specific shapes that will be stronger then others.

Do you have any articles on the I beam because I think I could use them in some of my other up and coming projects

 

[voyhager3]

Carbon fiber aarow shafts do work well and are fairly light. I recently started using flat carbon strips to drop weight even further. Here's a post I recently made about this https://forum.flitetest.com/index.php?threads/carbon-fiber-strips-really-useful.66948/

After my son folded his explorer's non-aileron wing, I built the aileron wing with no dihedral using the flat strips (I used two - one in each half of the wing directly inside the spars, but overlapping through the center however far they overlapped - i don't remember exact details). The resulting wing is very stiff, and has held up my son's "aggressive aerobatics" and quite a few crashes.

I think I will go with that, it seems a nice solution for this kind of plane. Thank you!

 

[JasonK]

Do you have any articles on the I beam because I think I could use them in some of my other up and coming projects

an I beam is just a shape that you can get a beam because a cross sectional cut of it looks like an I, it isn't a flight specific thing, it is a material science/construction thing.

here is a metal I beam. when you look at the deflection resistance of an object, most of the deflection resistance comes from the top and bottom, so you can cut out the sides like in the picture and have almost as much strength, but have your object be almost as strong. [I first learned about this when I was a kid via Mr Wizard's World] .

If you need deflection strength in both axis and not just 1, changing to a box shape is likely the best thing to do. Again, you can remove the middle and still have most of the strength.

If you look at balsa builds, you will notice that the spars go down the outer edges of the wing, not down the middle of the wing, that is were they give the most strength for the weight in the wing. This is the same thing that I am talking about here.

 

[voyhager3]

an I beam is just a shape that you can get a beam because a cross sectional cut of it looks like an I, it isn't a flight specific thing, it is a material science/construction thing.

here is a metal I beam. when you look at the deflection resistance of an object, most of the deflection resistance comes from the top and bottom, so you can cut out the sides like in the picture and have almost as much strength, but have your object be almost as strong. [I first learned about this when I was a kid via Mr Wizard's World] .

View attachment 198707

If you need deflection strength in both axis and not just 1, changing to a box shape is likely the best thing to do. Again, you can remove the middle and still have most of the strength.

If you look at balsa builds, you will notice that the spars go down the outer edges of the wing, not down the middle of the wing, that is were they give the most strength for the weight in the wing. This is the same thing that I am talking about here.

Oooohhh, ok. I was just over complicating it. Thanks!