Tips on making a REALLY strong spar (25G)

[soholingo]

I am looking at making a spar so that I can make a fully molded plane. I have seen the technique described here...

How to build a spar CST Sales (https://www.cstsales.com/How_to_Articles/SparBuilding.htm)

Does anyone have any techniques? Thanks

Jay

[davidfee]

The one on the CST page is the easiest way to make a really strong spar. Usually in fully molded planes, as you have seen in Marcus' thread, as well as posts by Jan and others, the spar caps are molded into the wing skins to maximize the spar depth (strength to weight). This second method can be a lot of work, since it uses raw carbon tow which must be measured, handled and wetted carefully.

BTW, my tow-wetting machine is nearly finished now... just need to add a spring tensioner on the pinch roller. Then I will post pictures. It is quite similar to Marcus' machine.

-David

[SoarNeck]

The problem with the CST method is that it’s very difficult to achieve a dimensionally correct spar, since no mandrel is used during the bagging portion. I used to use these spars quite a bit, with a couple of slight modifications:
http://www.geocities.com/pipeline/halfpipe/4579/spars.html
or my old heavy-lift pages here:
http://www.geocities.com/sunbirdz/2002_AeroDesign_Home.html

In these, a heavy-duty mandrel/form is used to maintain the spar along the critical dimension.

The couple of modifications include appropriate wrapping with kevlar thread or tow to replace the glass, and elimination of the spruce for weight-sensitive applications.

Remember that the spar has to be designed for stiffness…not strength. A spar can be designed to be strong enough in the air with little difficulty, but may prove to be too flexible in use.

I really dislike the tow spars because the easiest failure mode is delamination of the shear web from the caps (bursting). You end up having to use excessive amounts of bonding “splooey” to maintain adhesion...especially if you introduce stress risers with wing tubes for multi-piece wings.

On the other hand, it would be a lot more work to build a spar for a molded wing that fits properly. It’s what I would do if I ever build one, however.

[davidfee]

The way I've done Free Flight spars (F1B) is a combination of all of the above, with a twist. The twist makes it more complicated, but stronger, lighter and more dimensionally correct.

The spar caps are tow. The shear web is Rohacell or vertical grain balsa. There is also a carbon sleeve which goes over the web. Now for the twist. Instead of using vacuum to press everything together, a mold is built from aluminum plate and channel.

You lay the pre-wetted tows for the lower spar cap into the bottom of the mold. Then insert the web into the sleeve, wet out and insert into mold. Then lay the tows for the upper spar cap in place. Add the upper mold piece and screw together, giving several hundred PSI. The foam web core will crush slightly to shape... the balsa must be cut more precisely. The result is a virtually unbreakable spar, with precise final dimensions.

Of course, you first need to build the mold.

-David

[Tony D.]

David,

I have tried the technique you are talking about here but I used 12.5mm ID aluminum channel to lay the tow, balsa tow up in and a piece of maple that I had cut to 12.5mm on a table saw which I placed over the tow balsa and then bagged it.

Unfourtunatly even though I lined the channel with sandwich wrap I could not pry the finished spar out of the channel and in the end I tossed it.

I have been thinking for a long time about making up a set of moulds for laying the spars up just as you mention but using hardwood with "keying" for different spar depths and widths.

I have seen a video of the moulds used in full scale glider manufacture and they are nothing more than highly polished and waxed hardwood sections that fit together to make a spar and depending on the wing the mould pieces can be assembled to make a variety of spar widths and depths in a true
I-Beam configuration.

The Drela spars are a lot of work but they are the cats meow as far as strength to weight.

On a related subject, when I build a wing with a Drela type spar I cut the foam section out of the wing where the spar will be glued into and I use this foam cutout as a "guage" to do the final dimensioning of my spar. This usually means a lot of sanding and messing about.

Does anyone know of an easier or faster way to determine finished spar height so that one can just do a simple measurement and go straight to building a finished spar?

If so would you take a moment to post your technique here?

Thanks.



Tony Dempsey

[soholingo]

The way I've done Free Flight spars (F1B) is a combination of all of the above, with a twist. The twist makes it more complicated, but stronger, lighter and more dimensionally correct.

The spar caps are tow. The shear web is Rohacell or vertical grain balsa. There is also a carbon sleeve which goes over the web. Now for the twist. Instead of using vacuum to press everything together, a mold is built from aluminum plate and channel.

You lay the pre-wetted tows for the lower spar cap into the bottom of the mold. Then insert the web into the sleeve, wet out and insert into mold. Then lay the tows for the upper spar cap in place. Add the upper mold piece and screw together, giving several hundred PSI. The foam web core will crush slightly to shape... the balsa must be cut more precisely. The result is a virtually unbreakable spar, with precise final dimensions.

Of course, you first need to build the mold.

-David

Fascinating... any pictures of the the molds. So it seems the basic light/strong spar construction is as follows...

1. Some sort of light center material (foam/balsa)
2. Wrap the spar in carbon.
3. Coat the spar in epoxy.
4. Wrap the spar w/ kevlar thread or fiberglass.
5. Compress

Is this correct?

[davidfee]

Yes, that's basically it. The core material needs to have good compression strength, in addition to light weight. The "spar" is not just a tube... it's the carbon caps, separated by the shear web core. The carbon sleeve (or other fabric wrapping) is for increased shear stiffness. Kevlar thread wrapping (if used) is to prevent the spar caps from delaminating from the shear web/core material under extreme bending/shear loads.

I'll post a picture later... I'm at the lab now and don't have access to my stuff.

-David

[davidfee]

Actually, I think the notebook with the pictures may be at my parents' house. I can make a sketch, though. This should give you the basic idea:

-David

[davidfee]

Ok, I found the article online. It's in German, but the pictures should be good enough. Use bablefish or ask me if you have specific questions.
http://www.iag.uni-stuttgart.de/people/thorsten.lutz/thermiksense/t_jeder.pdf

-David

[nuevo]

Neat document. Thanks for posting. You're resourceful.

Won't be able to use any web translators on it. It contains scanned pages only.

[davidfee]

Just let me know if you need help reading it. I may translate the whole thing at some point, but not today. ;)