die-casting carbon fiber?

[dav1d]

I was told by an old and trusted member of the foundry industry to cast my fan blades in carbon fiber. He's built a Lancair and knows a bunch about carbon fiber and things aeronautical.
I had planned on designing a mold for a 2-part polyurethane foam but might as well go with carbon fiber if it's doable. I have zero experience in designing a mold so am asking for pointers.
The fan blade is based on a Cessna (NACA 2412) airfoil. It's chord is 39.9mm and height is 37.5mm. I'd originally thought I'd design the mold with the blade laying horizontally in the cavity, but since I'm putting a Sportsman STOL leading edge on it, there's no way I can think of to do it without having a parting line near the leading edge which is unacceptable (a parting line near the leading edge would encourage a stall of the blade). So, I'll likely have to design the mold with the leading and trailing edges of the blade/airfoil vertical. I don't mind the draft, tapered wings are everywhere. Why not in my fan? That way the parting line could be either at the root or the tip of the blade with zero consequence regarding a stall. Heck, it'd be more like a stall fence.
Can a mold be made using RenShape 460 Medium-High Density Modeling Board for casting carbon fiber? Your thoughts?
Thanks!

 

[Bob Korves]

I spent three decades around sailplanes and home built aircraft, and the modern composite ones are mainly made of structural carbon fiber and Kevlar, with some mostly non structural fiberglass. The molds are typically made mainly of fiberglass. You will first need to make plugs the exact shape and size of the desired final propeller blade, to a fine finish and close accuracy. They you will need to use the plugs to make the two part female upper and lower surface molds to make the propeller. They will need to be rigid, smooth, close fitting to each other, and lacking voids. Parting lines at the leading and trailing edges are the correct places for them. After laying up, curing, and then pulling the propeller from the mold the top and bottom surfaces are bonded together, adding reinforcement as required by the design. Leading and trailing edges are sanded to shape, with the leading edges back to about 20% of chord carefully matched to the book airfoil profile. Then it is polished out with progressively finer sandpaper until smooth and polished. Any small voids are treated the same way. Warning and Disclaimer: Centrifugal and other forces on propellers can easily make them explode if not properly designed and fabricated. The forces are huge. This should not be your first or your tenth composite project. If one blade comes off, your engine can easily go with it.

 

[dav1d]

Parting lines at the leading and trailing edges are the correct places for them.

Excellent information. I very much appreciate it. I've spoken with a pattern maker here in Lubbock who is in agreement with your advice. He has also suggested squaring off the trailing edge a bit, in other words, avoid the knife-edge trailing edge as I had drawn it.
Thank you.

 

[Nogoingback]

. Warning and Disclaimer: Centrifugal and other forces on propellers can easily make them explode if not properly designed and fabricated. The forces are huge. If one blade comes off, your engine can easily go with it.

I knew someone that had that happen with a wood prop on a racer he was flying at Reno. In the time that it took him to close the throttle, the engine came completely off it's mounts. He said the vibration was unbelievable. They require a secondary safety cable from the engine to
the firewall just for that reason: to keep the engine on the airframe if that happens. (And in case anyone is wondering, he successfully dead sticked it back to the runway.)

In addition to what Bob said, you should also plan on vacuum bagging any parts you make to maintain the correct ratio of resin to cloth, and minimize weight.

 

[Bob Korves]

I knew someone that had that happen with a wood prop on a racer he was flying at Reno. In the time that it took him to close the throttle, the engine came completely off it's mounts. He said the vibration was unbelievable. They require a secondary safety cable from the engine to
the firewall just for that reason: to keep the engine on the airframe if that happens. (And in case anyone is wondering, he successfully dead sticked it back to the runway.)

In addition to what Bob said, you should also plan on vacuum bagging any parts you make to maintain the correct ratio of resin to cloth, and minimize weight.

Agreed. And there are lots more gotchas that can bite you in the butt. Flutter, for one. I have flown composite aircraft at high speeds in rough air, and I was always really glad that they were designed and built by companies with decades of experience in composite design and construction. It is pretty easy to mix resin and fibers and make it look sexy. It is a lot more difficult to make something that will stay together when the going gets tough, even after being used hard for decades. The last sailplane I owned was made in Lithuania, had a 15/18 meter wing span (50/60 feet), had a wing chord of less than 30" at the roots, and was 3.5" thick at the root, tapering to 3" thick 2.5 feet from the fuselage. The wings were wet, carried 50 gallons of water in them, and more in the fin for C.G. control. Flew that ship grazing thunderstorms, in powerful mountain waves, within a few feet of ridges, and into booming Sierra Nevada thermals at high speeds. The wings barely flexed, no problems. That is not insanity, it is SOP, among experienced pilots. Modern sailplanes can do that, and well designed seat belts are not optional. Amateur designs in sailplanes are a thing of the past. I have watched an aircraft spiral down with one wing missing...

 

[Nogoingback]

Honestly, if you have no experience designing highly stressed composite parts, and have never worked with them, it might be prudent to start with something a little less ambitious... Bob is
completely right: propeller design and fabrication should not be a first composite project.

 

[dav1d]

Good advice. Thanks to you all, but I left out one very important detail in my original post, Sorry! This blade is going in a scale-model proof-of-concept prototype of my VTOL aircraft concept. The chord of the blade is under 40 mm and it's "length" is under 38 mm.
The model is under 1000 grams all up weight as best I can estimate at this point in the design phase and the rotor having 17 copies of that blade is in the neighborhood of 305 mm in diameter. It is not a propeller. If you'd like to know more, I'll be happy to supply details after a NDA has been properly executed. That said, I have gotten the cold shoulder for mentioning NDA in other forums.

FYI: I hold an FAA Airline Transport Pilot certificate in Airplanes Single and Multiengine Land, was certificated as a flight instructor in April, 1972, towed sailplanes off of Taos Municipal Airport at the foot of the Sangre de Christo mountains in Citabrias and 182s, was first-officer, line captain, director of training, chief pilot, check airman and director of operations for a scheduled regional air carrier in West Texas flying Twin Otters, Piper PA31T3 and Beech Airliner (99). I flew Jetstream 31 for American Eagle out of DFW. I flew a corporate King Air C90 for an oil well service firm in the Permian Basin of West Texas.
I lost my airman medical as the result of being crushed by a 40 ft long joint of 13-3/8" diameter well casing (2000 lb of steel pipe) in an industrial accident which cased total occlusion of the right internal carotid artery and a stroke while doing grunt work in a pipe factory when not flying the company's Cessna 340A. Since then I obtained an Advanced Instrument Ground Instructor certificate and taught Airman Knowledge Test (written) curricula at a community college.

 

[Bob Korves]

Good advice. Thanks to you all, but I left out one very important detail in my original post, Sorry! This blade is going in a scale-model proof-of-concept prototype of my VTOL aircraft concept. The chord of the blade is under 40 mm and it's "length" is under 38 mm.
The model is under 1000 grams all up weight as best I can estimate at this point in the design phase and the rotor having 17 copies of that blade is in the neighborhood of 305 mm in diameter. It is not a propeller. If you'd like to know more, I'll be happy to supply details after a NDA has been properly executed. That said, I have gotten the cold shoulder for mentioning NDA in other forums.

FYI: I hold an FAA Airline Transport Pilot certificate in Airplanes Single and Multiengine Land, was certificated as a flight instructor in April, 1972, towed sailplanes off of Taos Municipal Airport at the foot of the Sangre de Christo mountains in Citabrias and 182s, was first-officer, line captain, director of training, chief pilot, check airman and director of operations for a scheduled regional air carrier in West Texas flying Twin Otters, Piper PA31T3 and Beech Airliner (99). I flew Jetstream 31 for American Eagle out of DFW. I flew a corporate King Air C90 for an oil well service firm in the Permian Basin of West Texas.
I lost my airman medical as the result of being crushed by a 40 ft long joint of 13-3/8" diameter well casing (2000 lb of steel pipe) in an industrial accident which cased total occlusion of the right internal carotid artery and a stroke while doing grunt work in a pipe factory when not flying the company's Cessna 340A. Since then I obtained an Advanced Instrument Ground Instructor certificate and taught Airman Knowledge Test (written) curricula at a community college.

Well, I guess that changes things a bit! Best of luck on your prototype. Sounds interesting. Impressive resume as well... Well, not the injury part, sorry to hear that. Good thing you can still do ground instruction. I had to give up flying for medical issues as well. Keep us informed on how the project progresses.

p.s. I also towed gliders for a while, at Byron, Calif. with a Bellanca Scout.

 

[Nogoingback]

Ah, that clears up some questions. I was wondering what the part was for: sounds like a very interesting project. Sorry to hear that you lost your medical, but at least you stayed in aviation.
Let us know how that part turns out.

 

[dav1d]

I was fascinated with flying machines from an early age. At one point I quit flying and started using a skill I learned in High School: mechanical drawing. I seemed to have a natural aptitude for visualizing different views of objects in my head. I started out by laying out printed circuit boards for an audio/video mixing board manufacturer, then as an electro-mechanical designer/drafter at an electro-surgical generator firm in Englewood, CO, then was offered a contract (temp) job with Martin Marietta Aerospace, and then a stream of high-tech temp drafting gigs at IBM, Draper Labs and others. Then I went back to flying, got hurt and took up IT work since I'd started learning to write code to do the math modeling for this project before the injury. I fell back on my computer hobby to make a living working for Shell Oil as an IT nerd, CompUSA as an instructor and Iomega Corp as a Software Test Lead. I've served as telephone technical support for Intuit, Convergys Corp and others.