As mentioned before, I'm basing my full scale quad design around a 4m rotor diameter. That makes each rotor blade 2 m long. I've decided on a NACA0012 airfoil which is commonly used for full scale helo's. This is a symmetrical airfoil with a glide ratio of about 60:1. I'm planning to build the rotors of composites with E-glass fiberglass cloth, epoxy resin and extruded polystyrene foam core. My budget is a meer trickle so I am selecting lower cost (and lower performance) materials than might be desired if my budget were larger. For example, I might use urethane foam and carbon fiber cloth for the rotors. However, I believe I can meet the needs using these lower performing materials. The symmetrical airfoil will allow me to build all 8 rotor blades essentially the same which should simplify things a bit. I have purchased some 6 oz/yd standard weave fiberglass cloth, and Series 2000 epoxy resin with 2 hour working time hardener from http://www.fiberglast.com/ . I am currently experimenting with the fiberglass layup process as this is a new area for me. The 2 hour cure hardener requires elevated temperatures to cure, so I glued up a small part and put it under a work lamp to cure. (suggested temperature is 120-130 Deg F for 12-14 hours. I guess I put it a bit close to the lamp because it melted a crater into the polystyrene foam. In hindsight, I learned the working temp of the Owens Corning Foamular 150 polystyrene board is 165 F. I exceeded that.
I'm working towards building a small section (12" long) of rotor spar comprised of a foam core, 5/16" diam fiberglass rod leading edge (for impact durability and rotor CG management) wrapped in two layers of 6 oz glass at +- 45 deg to hopefully give it strength against shear as the web of the rotor "beam" formed by the upper and lower skins.
The chord of the airfoil is 20 cm, and this spar is the front 30% of that chord, or 6 cm. In my research I've seen this approach (a wrapped leading edge acting as a rotor spar) used for rotor blade construction. After building this small section, I will test how much bending moment it can withstand before breaking or kinking.
Comments
Nickolas,
Well, "really doing this" is yet to be seen. I've already learned a tremendous bit about how a rotor system works, and the myriad forces that act on the blades, bearings, mast, etc. But yes, I intend to continue work on this project in logical steps. Since my last post I've been educated on flapping and lead/lag motion of a rotor blade and learned that my approach with a fully rigid rotor blade is likely doomed to either failure, or at least intense vibration. Ironically, in a single rotor helo, the rotor disc moves, then the airframe follows. I realized that in a quad copter, it's exactly the opposite. The airframe tilts as a result of dissimilar lift among the 4 rotors, then the rotor discs need to tilt to follow.
For this reason, I'm now working on a pitch horn design that will allow the rotor disc axis to follow the mast axis using either fully articulated blades, or a "bearingless" (composite) hub design. Working out the details of attaching fasteners to composite elements so that I can bolt the blade (composite) to the hub (also composite).
I've also obtained some Foamular 250 polystyrene foam which will hopefully withstand the pressure of vacuum bagging with less shrinkage. I've also ordered a digital temperature controller to help control my epoxy resin cure temperatures so they are high enough to cure the epoxy, but not so high as to excessively soften the foam core.
I'll add a post to my blogs when I have results. Thanks for your interest.
Oh, and this is the only place I am posting any info regarding this project.
So, your really doing this, huh?
Do you have a blog or web page somewhere I can follow?