• Welcome aboard HomebuiltAirplanes.com, your destination for connecting with a thriving community of more than 10,000 active members, all passionate about home-built aviation. Dive into our comprehensive repository of knowledge, exchange technical insights, arrange get-togethers, and trade aircrafts/parts with like-minded enthusiasts. Unearth a wide-ranging collection of general and kit plane aviation subjects, enriched with engaging imagery, in-depth technical manuals, and rare archives.

    For a nominal fee of $99.99/year or $12.99/month, you can immerse yourself in this dynamic community and unparalleled treasure-trove of aviation knowledge.

    Embark on your journey now!

    Click Here to Become a Premium Member and Experience Homebuilt Airplanes to the Fullest!

Why expensive composites can be cheaper; an in-depth comparison of weights and cost.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

autoreply

Super Moderator
Staff member
Joined
Jul 7, 2009
Messages
10,824
Location
Rotterdam, Netherlands
https://www.dropbox.com/s/3d673dtzj...rious composite construction methods.pdf?dl=0

Data in the link above. An in-depth discussion with all the factors at play, assumptions and relations would take way more time that is feasible in this format, so just some random conclusions:
  1. The difference in production methods (Cf, fiber volume) is enormous and so is the weight difference of a laminate because of it. Yet the actual weight difference between an infused sandwich skin and a vacuum bagged one is pretty limited (37 vs 46 kg). Why? Both the adhesive joint between foam and laminate and the foam itself are about half the weight.
  2. Note that rather spectacular weight reduction due to infusion. Most of this is because a thick gelcoat primer is no longer needed to avoid pinholes. In either a production environment or a homebuilders environment, using infusion or autoclaved prepregs is realistically the only way to get rid of a heavy gelcoat.
  3. By now, hopefully the theory that "longer wings are heavier" has been trashed. If not, look at the results. Both the Rutan style wing and the Warren-truss wing get lighter as they get more slender. The foam core of the Rutan-style wing becomes lighter, while for the Warren-truss wing the smaller chord drives thinner required skins since it's mostly driven by buckling.
  4. To further put the "long wings are heavy" to bed; I've split spar weight in two components: everything outboard of the root rib and below the extra weight of fitting to spars together and dealing with all the concentrated loads. Numbers neatly agree with data from multiple certified aircraft. Spars using pultrusions will be lighter. NLW=350 kg is the underlying assumption. Doubling that for the 18M span results in about 2 kg more in spar weight and about 5.5 kg more in "root spar" weight.
  5. We often hear that carbon is "too expensive". See the summation of cost. Then realize that with all the hardware required (pushrods, fittings, gelcoat or PU etc, etc, etc), total cost of just the wing is going to be at least a few grand more than mentioned. Now look at the weights of the carbon wings. The realized weight savings means for the same performance you can use a smaller and thus cheaper engine. That price difference is way bigger than what you'd ever save on not using CFRP.
  6. The Rutan method (solid foam cores) is a really heavy way to build wings if your aspect ratio is low. Once you elongate the wing, it quickly gets competitive (weight-wise) with molded fiberglass wings.
  7. I have added the weight of the systems, being ailerons and flaps, control system weight and drag brakes (like on sailplanes). The reason for the latter is simple; I have data on a lot of composite wings and this way it's pretty easy to cross-check.
  8. We can put all our efforts in designing an optimal structure. But while doing that, systems weight is not changing and is a significant part of the total weight. Sometimes it's better to focus on these for weight savings, especially since there are a few multipliers in there (ballast weights for flight surfaces, pushrod mass for ballast weight)



No doubt, later more.
 
Back
Top