airframefixer wrote:
Why not..
This is all my opinion and it should be treated as such..
...the reason why it just doesnt make sense in that it our projects (7 type home built cars) just wont see the true benefit of what an aluminum semi monocoque offers, extreme stiffness beyond of what a seven requires.
Dig into the history of racing dating back the late 70s. Tubs and underbody aero were introduced by lotus. To keep the car suspended properly after the additional several tons of effective weight were added to the car via downforce, spring rates had to be massive. The prosche 956 used a variable diameter and variable rate tiatinum spring to help conteract the force, as well as offering a wheel rate near equal to spring rate.
Essentially chassis improvements were born as a result of higher suspension loading. A chassis not stiff enough would deflect prior to the suspension.. largely making it impossible to tune..
Again my opinion. As our cars arent nearly as highly sprung as an aero car, the need for additional stiffness just isnt there justifying the need for alternative/advance chassis construction. Dont get me wrong, theres always a benefit of adding stiffness to a point.
As far as the engineering goes. I just dont see it as big as a hurdle in our purposes. The reality is that we most likely would overbuild it and incurr a weight penalty, where as a experienced p eng would build it lighter to the same strength.
Keep in my mind that no airplane I have flown or worked was an engineering home run. They are still subject to the tests of daily abuse. You can apply FEA models all day long but there is no substitute for validating your design through a trial or test. Theres a good book I read about the test flying of the dehavilland otter and twin otter. Lets just say airplanes were crashed and people died as a result. Specifically calculating the flutter on the tail at high speed. It was eventually improved and put into service but remains the limit for the planes, and most aircrafts performance.
This trend continues today as new materials are brought into use on modern aircraft. Just ask airbus how there aluminum wing rib replacement program is costing for the A380. Bassically it was engineered to be light and flex properly, but the ribs cracked early into the service life and had to be replaced by a directive.
Anothet example is the A350 wing tip flutter... and easier fix, it was dialed out with stability augmentation.. stuff that didnt show up in design.
Im rambling here, but hopefully this illustrates that you can design all day long with best practices but you still be in " your own no- mans land" untill you put it through enough stress that the weak spots finally show. Unfortuneatly public road are the testing ground for self designed cars, and your life and others depend some safety factor, the good news is that we tend to overbuild imho.
The porsche 956 and 962 are great examples of how a design evolved. Originally the tubs were sheet metal. Over time as weakness showed up, the front bulkhead was replaced by a milled one and the sheet stock in the tub was replace with aluminum honeycomb.
At best a home designed semi mono tub constructed from inferior materials with poor methods is going to yeild either an underbuilt structure somewhere between a maintenace pig to abrupt catastrophic failure to an overbuilt overweight expensive structure that defeats the purpose all together..
I was watching a seminar on additive manufacturing recently, not because it has anything to do with my job, but just because I am a dork. The engineer summed it up well.
"Anyone can build a bridge, but only an engineer can build a bridge that will barely stand".
I think we covered why welding is not ideal on aluminum. Bonding of aluminum and its alloys is out of the locost league and too risky, im sure all of the aerospace/aviation people would agree.
That leaves mechanical fastening.. a whole other topic, and one I believe in if your making parts fro aluminum.
Lets keep this rolling.
Andrew
agree
even I still haven't build my own kit car, I had doing research about monocoque for few years, and try to design it---include 7 style & Mid engine, also elan backbone chassis
what I feel is, if u have no calculation, u always feels the chassis is not strong enough, after u think it is ok, it is too heavy, too much supporting component, but if u see other chassis designed by engineer, u think: OMG it is to simple, looks like very week, is it really work?
maybe it will be easier after we have a fully open source monocoque chassis design just like 7