As a mechanical engineer, I have to say that my selection process always starts with how it's used then to look at properties. I think about stuff like, "is this part ok to fail/sacrifice to save other parts?" "Which part should fail first?" "Is rigidity a good thing or bad thing?" "Can the system be compliant?" "Is the environment harsh?" There are a lot of things to consider, but I always like to think of which part is the 'mechanical fuse' of the system.
Thank you for sharing your thought process! Just out of curiosity, what sorts of things are you designing that might need to function as a “mechanical fuse”. The closest example I can think of like that in kit aircraft is shear pins in autopilot servos which are meant to fail first.
@@DarkAeroInc I do mechanical design and engineering for large automated industrial equipment. And you're definitely on the right track about shear pins being a mechanical fuse. Besides failing, the thought of which wears first when you think about service. You don't want to replace a giant weldment when it just needs a new bushing or something. I do a lot of design with flexures (which are awesome) they can be designed to be ridged in one direction but compliant in another and at a certain point it will bend or break protecting the rest of the system. AND it's a single piece. They're so cool. You can also think about which part fails first when designing your parts. I forget the term, but it's like strain energy balance, essentially if you design a part or assembly that has a uniform stress throughout then the failure mode is the entire part or assembly. However, if you tip that balance so the stress is higher in the cheaper easier to replace part, the cheaper part is the "fuse". And you can save the more expensive part or be able to still function at a reduced capacity. There's plenty of design philosophies out there, my favorite being the Gene Kranz school, "a preoccupation with failure results in highly reliable systems."
@@DarkAeroInc It is typical in automotive engine design to make rocker arms out of relatively weak cast material that will break first if a valve hits a piston. If the valve breaks the whole engine is likely ruined. A broken rocker arm usually leads to a moderate cost repair of the engine. The rocker design is “sacrificial”.
these are the things i like about your channel, the inside/engineering thoughts in design and construction i want more videos like this, because there aren´t many who explain the way you do
It's also worth mentioning that composites are not very good at taking load concentrations (rivets, bolts). Metal being isotropic, it spreads the load much better. It's not just about the through thickness stress.
True! Are you encountering many bolted or riveted joints in composite parts on your projects? Most of our mechanically fastened composite parts sit firewall forward. Good examples would be the spinner and cowling.
@@DarkAeroInc Well where I work most cowls are indeed flush bolted or bear nutplates so although not ideal it's not a big deal. I have just seen bad things happen when holes are too close to edges because composite will tear (cleavage strength) easier than metal. But things go real ugly when you need to bolt structural elements such as spars because our structures are too large to be made or bonded in one single piece. There, you have to pad up an insane amount to make up for the lack of bearing strength of composite and aluminum starts to catch up in the weight contest !
@@maxhugen typically you add composite plies just in that area. Bonding metal is not as reliable as bonding composite, and the joint between materials of dissimilar CTE and modulus will be the weak link. Corrosion must also be prevented, whether bonded or bolted or riveted. Sometimes is is the only way, the wing spar cap of the F/A18 for instance is a unidirectional carbone laminate bonded to a titanium fitting with a scarf joint.
A great example of concentrated stress and high cyclic loads is the old way the Firewall was reinforced for the Engine Mount Bolts on the MX/MX2 aircraft. It's since been updated and I believe retroactivity addressed. The pilot walked away, but he was incredibly lucky and talented. But simply viewing pictures on the backside of the firewall where the bolts, nuts and washers were claiming the motor mount (Google it for images) you can only shake your head like how any engineer would think that was adequate for a world class Aerobatic aircraft, that specializes in the four minute freestyle is beyond me. A high schooler would have known it was sketch. Maybe thats why Walter Extra sells so many more planes. Their engines don't fall off. aviation-safety.net/wikibase/178764
I also tend to weigh in budget and weight constraints into my decision process. Carbon is lightweight but there is a point that for a small weight penalty the same thing can be made from say traditional fiberglass or aluminum for a fraction of the cost with similar strength specs. Love your content gentlemen👍 Keep- em coming!
Weight and cost are important, especially in the realm of building an aircraft, so thank you for pointing out that these factors should be included in the discussion! It’s always a balancing act trying to create strong, lightweight parts without the cost getting out of control. We have a few spreadsheets where we track the part weights and costs. This way it’s easier to identify where to focus your effort when trying save weight by spending more money. For example, we could save a few grams by converting some of the landing gear components from aluminum to carbon, but this would be really inefficient from a cost perspective. A higher priority item might be converting the aluminum oil pan to carbon fiber which would save a few pounds without being crazy costly to manufacture and test. Also, we’ve been enjoying following the progress you’ve been making on your project! Keep it up!
LOL exactly! Just wrap some plastic in a carbon fiber skin. That's usually good for another 20hp. ;) Kidding aside, BMW has some pretty amazing composite work on their i3.
One small addition - there are alternative ways of making CFC parts for other applications, for example "forged" carbon fiber. They consist of chopped carbon fiber fragments and resin, typically 5-15mm in length. The manufacturing process is somewhere inbetween what you're used to from carbon fiber and injection molding. That means you can make a lot more intricate shapes than you could with normal CF mats. Less ultimate strength than carbon fiber mats, but still very light and pretty strong, and more isotropic material properties since all the fibers are oriented differently.
Good point! We’ve experimented with “forged” carbon a little bit to make carbon fiber tube connectors. It definitely has the potential to create more intricate parts that can handle z-component stresses. One part that would be cool to try to make forged would be our nose gear trunnion halves.
@@DarkAeroInc , and your polymer is also an issue/opportunity. You can design into your composites temperature constraints, as well as friction coefficients, hardness and other properties.
I subscribe to several aviation channels. How I haven’t ran across yours until this evening is baffling. I have watched about 10 episodes thus far and am extremely impressed! I really enjoy your episodes in all their various iterations. I believe this keeps everything fresh and engaging. So far, I am most impressed with you all being brothers and working on this amazing venture together! Very well mannered, educated, polite and kind gentleman! My brother and I would have already exchanged gunfire two or three times! Great show! i just subscribed!!! 👍🏼
Carbon fibre in general has higher toughness for part weight, compared to other materials. (given the geometry is in line with its suitable aplications). However metals are by far tougher for part size. Such as, a tough stainless steel rod hard or flexible,, (based on purpose) is going to be tougher to flex, bend ,or break, in comparison to titanium alumilnium or carbon-fibre rods. (in correct relation). In general, toughness by part weight: Carbon>titanium>alumilnium>steel tougness by part size: steel>alumilnium>titanium>carbon Obviously these materials come in their specific alloys. There is a range of allumilnium alloys, which expands further out in both directions (toughness per size and weight), than any titanium alloy does. So, there is a lot of variety between specific material alloys, but ^ this would be a general mind set. Then there are things such as toughness by part geometry, as guys in the video said, non monolithic parts aren´t very suitable for 3d force aplication scenarios, they also have a minimum size requirement (fibre bundle size). Also toughness by temperature aplication was a correct thing to point out too. Firstly because carbon fibre has much more temperature-strenght ratio variety than any of these other materials (on a scale from failure by low to high flexibility yield due to temperature) - Carbon fibre becomes brittle at relatively low negative temperatures and it melts at relatively low positive temperatures, and even then between these two fail points, it´ s strengths change drastically. Its not even isn´t fire resistant, metals and more specifically steels are. Some metals also have specific properties,, such as Titanium could be considered somewhat heat retardant, but not resistant or reflective as ceramics are. Stainless steel 303 seems to be the most tough readily available material on this planet - in general and at various temperatures across its temperatiure spectrum. All of the mentioned materials vary in these properties, with various trends. It is thus a valid concern to pick and choose which material is used for which part, as the toughest most flexible and heat resistant material, may fail in the least distinguished stress aplication, simply due to some particular condition as: "The bundle size of a material only holds strength with parts with a consistent crossection twice the width of that of the bundle." or "the lighest toughest most energy absorbant material on earth may not be suitable due to its low density, and therefore required high volume"
The honeycomb mentioned in the video is aluminum but we also use quite a bit of aramid honeycomb core. In production we are planning on using all aramid honeycomb core. Thanks for watching!
@@DarkAeroInc if memory serves you made sandwich panels for some parts to be cut out of. Are you able to find commercially available stock for those applications in production?
Hi Tim! We make two main types of flat panel stock: honeycomb sandwich panels and solid carbon plate/billet. We wanted to use commercially available honeycomb panels but we couldn’t find any standard panels that were right for our needs so we built our own. We plan to stick with our own in-house panels for production. The solid carbon plate we make is more ubiquitous so we might offload that in production. Some of the parts we currently make from solid carbon plate could be converted to phenolic plate which is readily available, inexpensive, and easy to machine.
Another great video - succinct while having enough detail and technical content. I’m looking forward to the stage when we can see how the finished product performs against specification.
Machinist here. I use aluminum for random stuff with loose tolerancing, for soft vise jaws that don't have to last long, and as a non-maring barrier when clamping things. I hate trying to hold tolerance with Al. Cold rolled steel is my go to for stuff that has to last, like soft jaws for longer production runs and general fixturing. Hot rolled steel gets used whenever warping must be kept to a minimum as it has far less internal stress than cold rolled. Medium carbon steel, especially 1144, is my favorite for anything with tight tolerancing. You can hold 5 microns all day with that stuff. Case hard is our go-to for tooling that must last in long production runs. Brass is shockingly good at absorbing vibration, which is occasionally very useful when designing fixturing for parts prone to tool chattering.
👍 As an amateur interested in racing sailboat design, such as the America's Cup AC75 and the SailGP F50, I've been really enjoying your series. Thanks for taking the time to make these videos, and best wishes for your project. 😎 🇦🇺
One thing you might want to consider is chemical resistance. I recently found out you can make gas tight 3d prints on a home resin printer, only to realize that EPDM o-rings aren't suitable for propane gas. Things like this don't come up very often, but finding out your fuel dissolves your carbon fiber matrix in a wet wing design is something you wouldn't want to learn the hard way.
Man you guys are really hitting this thing out of the park. The whole process/determination/professionalism. Speaking of shapes, what was the reason for the large-tall-narrow vertical stab vs small-low-wide style (lancair / glasair). I imagine there were discussions. Between you guys and Mike Patey we're covered.
Very nice presentation! One thing I noticed when Doing ramp inspections in Stansted was the frequent flap delaminations because of the cfm56 exhaust in 737NG.
Nice summary. Other points to consider are susceptibility to damage, repairability, strength vs stiffness, fatigue cycles, and even direction of stresses. Bare carbon fibre is vulnerable to impact damage, so very expensive bicycle frames can be broken by small pieces of gravel. Fokker ran into this problem with forward-facing partrs like radomes, and came up with fibre/metal composite structures. If a part does get damaged, how do you repair it? In an aluminum structure, small patches get riveted on, large panels get rivets drilled out and the whole sheet replaced. Metal can be welded. Composites might seem to be fixed with glued on patches and bondo, but that would probably compromise the entire structure. Direction of loads (tension vs compression) can be significant, especially if a structure has to be thin and light. An empty aluminum beer can may be crushed between the fingers, but I defy anyone to tear off the entire top with both hands. A structure can be strong enough to withstand loads, but not stiff enough to hold its own shape; think of a balloon. That may be more a matter of structural form, rather than material strength, per se. Think corrugated cardboard, (basically paper.) The examples he shows in the video also illustrate that. Cycles of load application, leading to fracture, may be an issue. Steel can resist repeated modest loading better than aluminum (which may harden and crack), while composites may de-laminate. The Lotus Elite sports car had glass-fibre cones in which the suspension struts hammered away until the tops came off, usually in mid-corner, rather spoiling the handling. How things are joined may differ, depending on the material. In a composite, a tension load would require a splayed fibre joint, (like a duck's foot), where a metal to metal one might work well with a bolt or pin. Finally, there's corrosion. Is it going to be subject to water, oil, (both hot and cold), UV, or bird poo? All affect choices. "Stuctures" by J.E. Gordon www.springer.com/gp/book/9781461590767 and its companion on materials www.goodreads.com/book/show/230510.The_New_Science_of_Strong_Materials are great introductions to the topic. Don't pay the Springer price, there are plenty of cheaper sources.)
Hey Fellas, if I may preface by saying I love and admire what you guys are doing. I’m not sure how you guys take advice or ideas but from experience aluminum doesn’t like that circular or radial movement. Industry usually adds stainless bushings to it (never stainless on stainless) but since you’re using Cad plated NAS bolts CRES would work. On another note, CAD plated fasteners should never be in any contact with carbon something to keep in mind. I hope this helps and doesn’t get dogged in the comments just trying to help. Super excited to see this project fly. Wish it had two brothers to do something like this 💪🏻👍🏻🤙🏻
have you seen the new ul520ist? Having a turbo normalized motor is intresting. Not sure if it will fit,but if it would the extra 30# for an extra 20 hp at all flight levels would be nice
When i was building formula student car; for the upright, along with the strength i was trying to reduce unsprung mass on the wheel,that was one of many criteria,and finally set on 7075.so weight in the application also servse very important roll.
Excellent point! Thank you for bringing that up! Cost is often another important variable that factors into the material selection process, and it's definitely something we look at. Many of the aluminum alloys we work with are relatively inexpensive for the raw stock, but when you start looking at titanium alloys or even certain stainless alloys, the cost of material can get crazy! Carbon fiber material cost often lands somewhere between aluminum and titanium. Once you start factoring manufacturing costs like labor or CNC machining, the cost comparison starts to get pretty muddy. Thanks for watching and for the comment!
Weakness in the Z direction might apply to CFRP billet, but you could assemble such a flange with reasonable strength if you applied the layers in a non-planar fashion.
You mentioned heat and headers. Something to look into is having them ceramic coated. It can lower engine compartment temps by a lot simply by keeping heat inside the header until it exits the exhaust rather than heat soaking the whole engine compartment. Another route is header wrap.
That's a good point. I once came across a guy experimenting with aluminium headers containing a ceramic internal coating. No idea how it was done but just incredible the way he could hold his hand on them with minimal noticeable heat.
I love the attention to detail of this company, and this specific project is very well organized. Can't wait to see the real world flight performance and handling qualities. I think the estimates are absolutely spot-on for top speed and range. Stall speed might be cutting it close, as the empty weight target is pretty aggressive and builders like stuffing in stuff they dont weigh.
I know exhaust headers have become something of a sacrosanct industry standard, but it seems like if you’re trying to reduce weight, larger manifolds would provide more volumetric flow throughput with fewer components. In other words, you can get more flow through a single 3” manifold than through two 2” headers.
Good piece on material design considerations. Just to add...carbon fiber structures do not fatigue like metal structures-an important consideration in wing design. This recent video release teaser looks like it was actually captured a year ago. Am I correct? What’s happening now?
Yes, corrosion is an important consideration as well! Composite parts, in general, are mostly immune to the corrosion processes you see with metal alloys, so composites are well suited for corrosive environments like coastal or marine applications with salt spray. When we do use metallic components we try to select alloys that have good corrosion resistance like stainless steels, titanium alloys, or 6061 aluminum. Thanks for watching and for the question!
The reason explained for CF parts not able to take up loads in Z direction can be better explained by the fact that the fibres run along one direction or are planar therefore if force arises parallel to the fibres then the component of force parallel to the fibres creates shear stress in the matrix of the composite, rather than fibre(reinforcement) rendering it less resilient to these forces
There is a means of making some fine and or dimensionally precise carbon fiber parts. Depending on the weight strength heat requirements the carbon fibers are either mixed with a high strength plastic and injection moulded or mixed with a resin as a type of slurry then pressed into a mould and subjected to high pressure clamping forces to compact it to the correct size etc.Then there is Carbon Carbon ceramic
NOTE!!! Carbon fibre does not like lightning strikes one little bit ! A mitigation technique used in aviation is to run a fine copper mesh over the outer layer with special conductors in areas where that is not possible or to channel the electricity to emitters. Fuel tanks wiring and structural areas like wing roots are especially susceptible to damage that could result in loss of the aircraft. I would glue that plate to the carbon paneling as well as bolt it on as the bolts are very likely work loose due to the different flex patterns on the materials so by using a moderate (removable) strength panel bonding agent the flexing will be made uniform across the whole surface reducing point loads.
I think this is so great you three are building an airplane. Not just an airplane, but a fantastic one. I love watching you. I would love to fly one of these if the Feds would let me. Too old. Oh well, I can still dream watching you.
I have a question. Carbon fibre is not good for lightning strikes (in essence, lightning will travel through and not around like metal... bad news for passengers!), large aircraft manufacturers are investing heavily in research into copper mesh to cover carbon fibre used on aircraft exterior. Has this been considered in the Dark Aero, or is it just an acceptable risk? I'd imagine a one man cockpit would be relatively easy to insulate sufficiently. Enjoying the videos guys! 👍🏻
Akrapovic might be able to make you a titanium exhaust. Cost will be high, but they do beautiful welding. I have a race exhaust on my BMW HP4 and it is gorgeous, And light as a feather.
Thanks for the video. I wanna add a few things, carbon fiber and fastener interaction (mostly made of aluminum) can cause corrosion. So you need to seal the fastener. Then, carbon is sensitive to machining, you may damage to part and weaken to composite. Lastly, carbon has high cost :D. I have 2 question as well. How about their fatigue life performance? What precautions you take for the lightning protection of carbon? Best Regards :)
I think the failure mode is also a very important criteria. Something like glass may be hard and stiff, but when it breaks it breaks catastrophically. Metal will bend while CF composites will splinter and fail suddenly and more or less completely. Glass fiber is a little bit better in this regard because it will bend and stretch a little more before failure. Also RF properties are a think, as you see when you hide the antennas behind it.
I'm really curious how much torque approximately the bolt can be tightened before breaking or crumbling the carbon fiber? That's a very crucial point when comparing to steel righr?
It would be great to have an explanation of your evaluation for a petrol vs electric engine. E.g sample configurations for different battery sizes with performance and range estimates. E.g with Ampruis 450w/kg batteries that are commercially available. And motors like those from H3X, MITs motor, kite magnetics etc. MIT's motor apparently packs 17kW per kg of power.
surprised you didn't discuss toughness for choosing metal versus carbon fiber. Or stiffness when choosing carbon fiber versus metal. Also the spinner was probably the worst example for compound curves because that shape can be done very well with a metal spinning process. Non uniform compound shapes carbon fiber has an advantage.
True! Metal parts can often be manufactured using very cost effective processes. Looking at examples from kit aircraft, a sheet metal wing skin is almost always going to come in at a lower cost than an equivalent carbon fiber wing skin. In our design we are prioritizing structural optimization over cost which results in a tradeoff of a higher cost kit relative to a riveted aluminum equivalent.
I want to build a kitfox made out of carbon fiber. So a carbon fiber shell without a metal frame. Just the carbon fiber shells ridgity to support the load. I can only imagine the weight savings.
Yeah, metal has a uniform strength, composite is weak on delamination, maybe also compression but trying to bend a CF plate is certainly not easy. Maybe also weak on knock fatigue? If you just have a small mechanical part I completely agree, just use metal but it makes me think about your landing gear. Ultimately I think just welded tube is the way to go, simple, fast, controllable, and I would never do 'billet' CF, but if you want the gear to look interesting then why not do an arm with similar shape, just as a CF shell. You get the stiffness via curvature, it's not all that big a load you have to carry, it's brief load and it's fast to do an interesting shape with CF unlike CNC. It can be complexly organic and look fancy and a shell is light. Why not? You might be able to do a single piece 360 mold if it's conal. Strong and fast to make. You wouldn't shape it like metal, shape it to its strengths. Could even be aerodynamic to boot.
I have seen some 3axis carbon fibres with fibers going 0°,90°,45°,-45° and Z 90°, But well, puncture strength of CF isn't very good for a landing leg that will take some high velocity impacts.
And it would also mean that you need to use dry fabric in a process like RTM I think... I'm not sure that the resin could flow well in a infusion process...
Titanium is not a suitable metal for exhaust pipes, it suffers from thermal fatigue and oxidization at high temperatures, 321 is aircraft grade exhaust material, or if you want lighter, inconel is what is used. Another benefit of an aluminium frame is it does not burn as easily as carbon fibre. The resin is flammable.
Usually we use hardpoint, which is really just a small area that is locally reinforced. In the example from the video, (the drag link mount with six bolts) each bolt travels through a hardpoint in the honeycomb sandwich panel that the mount is bolted onto. The hardpoint is a cylindrical puck of phenolic plate that replaces the honeycomb core to prevent the core from being crushed by the bolt clamping loads. Over the puck are additional layers of carbon fiber that cover the puck and overlap onto the surrounding sandwich panel skins. This transfer the loads from the puck into the skins of the honeycomb sandwich panel. Thanks for watching and for the good question!
@@DarkAeroInc In the production kits would you cut out the honeycomb BEFORE you layup the sandwich panel, insert the phenolic plate into the honeycomb layer as part of the layup? This would eliminate a lot of steps in the build process wouldn't it? Just need to drill out the hole and the hardpoint is already there!
Hi Richard! We would like to integrate the hard points into the honeycomb panels but the manufacturing process we use right now prevents that. Luckily the installation of the hard points isn’t too challenging, even though it does add another step. This is something we are still sorting out but a good compromise might be installing the hard points in the bulkheads when they are still laying flat at the factory rather than when they are installed in the fuselage and positioned more upright.
One specific question, I don't know whether you'll choose to answer that. What are different strategies and best practices do you guys have for bonding and bolting metal parts with carbon fiber and carbon honeycomb internal structures?
We use a couple different strategies. For fastening small items we just use Click Bond studs and nutplates. These work for attaching loop clamps to constrain wire bundles or fuel lines. If the bolted joint is going to have large loads we install hard points. The hard point consists of a puck of material that replaces the honeycomb locally and then this is capped with additional layers of carbon fiber. A hole is drilled through this assembly for a bolt to pass through. The puck prevents the sandwich panel from being crushed and supports bearing stresses from shear loads on the bolt and the extra layers of carbon transfer loads from the puck into the panel skins.
Naveen we appreciate the feedback, and we are very thankful for all our RU-vid community members. :) It's challenging to find time to make more videos. We hope to release more videos to Mach 1 in the future.
How does carbon fiber skin compare to aluminum skin with regard to radiation? Specifically, how will it hold up to UV radiation if it was to live out on the ramp instead of in a hanger? Will it last as long as conventional aluminum skin aircraft?
Thanks for watching and the comment! 3D printing is something we have been exploring as well, and there are many good use cases for it. We did a video on how we use 3D printing for the DarkAero 1 a little while back if you're interested: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-F4a-ZQoHnFM.html
