NACA 66 Special A/BGS

[robfrey]

Okay, I think I fixed a few problem by adding some bars and moving stuff around a little.
Did I miss any bars? Any help here from experienced cage builders would be appreciated.

[saltfever]

You do have a rule book?  Not enough clearance between the helmet and the bars. In addition padding has to be added. This is concept time so I don’t know how close the model is to reality. Could be you have enough clearance but I can’t see it.

I know why you have the tubing divots around the shoulders. But I don’t like it. Tubing that is bent has already gone beyond its yield point. Would be stronger if straighter but would mess up aero. Why no vertical at the knee joint? Yeah, its 3 triangles . . . just wondering.

Nice job of cleaning up the “finger” work on the model. 

[Rex Schimmer]

Rob,
I would add a diagonal from where the front main roll bar meets the top rail back to rear bar where it meets the second rail, I would also add diagonals to the bays between the second rail and the bottom rail. And remember to do the diagonals such that their center line intersects the center lines of all of the tubes that join at each node, this is called "eccentricity" in tube fabrication jargon and will have a tremendous effect on the strength of the structure. I would also add a vertical member between the top rail and the second rail where you have the two diagonals coming together but not quite meeting, this joint also should have good eccentricity. I would also add some diagonals to the structure behind the driver and would plate the main roll cage bars with 1/8 plate all over. This car is designed to go 400 mph it needs to be strong.

Interesting how you can blend the NACA shapes using your CAD system. Sure easier than my "Pencil CAD"! I have a chance to get a copy of Solid Works maybe I get it and give it a spin. What is the difficulty to learn it? I have lots of drafting board experience but have never done CAD.

I like your idea of using an accumulator for an air spring, which can give you a pretty constant spring rate if it is large enough. I think you may want to consider also adding some sort of dampener to the flap control system to provide some stability. Possibly a very small orifice in the line from the accumulator to the actuator may work.

I thought I could see some "Blue" influence!

Rex

[Glen]

What Rex said about the accumulator and the dampening effect, make sure there is no flutter, this could cause problems. All air craft builders run flutter tests on control surfaces.

[saltfever]

Rex, check the local JC for CAD courses. Almost all of them have a Solidworks and AutoCad component. Could be a lot of fun.

[robfrey]

Rex,
Can any of the diagonals or bracing be smaller than 1-5/8 tubing? Weight is starting to be an issue. Target weight is much lighter than conventional designs although the front of the car is a good place to put it. On the lakester that Mike Cook built that I had at my place last year, I think I remember seeing some smaller bars. I really like your idea of a dampener on the flap. it should be easy enough to incorporate. I was thinking about .060 or .093 thick material for the outside skin. If I skin the whole thing do I still want to add all those braces? Should I skin it on the inside and leave this as my floor and inner panels?

[desotoman]

Rob,

What is your target weight?

Tom G.

[1212FBGS]

all tubing surrounding the driver (rider) needs to be 1 5/8... that means from back of helmet to tip of toes.... including diagonals... outside of that area you can change size and wall thickness.... read all of section 3.B.1
Kent

[saltfever]

The flaps will be spring loaded via air cylinder and accumulator tank. The faster it goes the more flap will level out. If it levels out too soon, we will add air pressure to the accumulator and if it never levels out, we will decrease air pressure. The idea is to get a constant down force whatever the speed. At least that is the plan now.

This is a very intriguing idea. But I need some help with this. It is my understanding that compressing air is nonlinear. However, drag, even though exponential, it is still linear. So as speed increases you have a linear force (the flap) reacting upon a nonlinear device (the accumulator) yet you want a constant down force. Sounds like an exercise in frustration. Don't get me wrong. This is not a criticism of the idea at all. I think it is refreshingly creative. I'm just trying to understand the fundamentals in order to judge its merits. As Rex mentioned, a large volume will tend to be less nonlinear. . . . is there such a thing?  I'm wondering if that is the way to go? A spring's force is constant. And with CAD couldn't a linkage be designed that could exponentiate that force to exactly equal the force you want to react? I'm just throwing out ideas here. I've lost a few neurons over the years . . . but I'm still trying learn from y'all.  

[robfrey]

I'm not even sure I want it perfectly linear. It's just an idea to get more down force at lower speed without getting too complicated.
Here are some more pics of the additional diagonals Not sure I got em all.

[1212FBGS]

you may want to move the cross bar above the riders knees forward a couple of inches... it will help the driver entry and exit...
kent

[robfrey]

Kent,
Can I arc that bar up for additional clearance or does it have to remain straight?
BTW thanks for the help.

[desotoman]

Rob,

Do you still have your website? I cannot find it.

Tom G.

[Rex Schimmer]

Rob,
Your cage is really starting to take shape, I like the additional diagonals that you have added, if you are going to sheet the structure I think that I would do some inside the cage specifically where the driver is to provide him with a large surface area but if you decide to weld in the sheet I think that you could be making a car that is very hard to work on. I would still just sheet over just the roll structure over his head with 1/8 to make the gusset requirement and provide additional strength. Regarding the bar at he drivers knees you could easily make this a curved tube or even a tube with a double bevel bulkhead style structure then add additional diagonal bracing . Also regarding being able to work on the drivers area, the bracing that covers the top of the drivers area could be made to be bolted in. There are some very good methods of bolt together structures that use piloted spools and high grade bolts at can rival welding for strength and stiffness. There is a web site which has a thread on some off road trucks that were being built for the Herbst brothers that show methods of "bulk head" weld joints, I found it thru Landracing, I will try to see if I can dig it up as these trucks and their construction are truly the state of the art for welded tube structures. (It may be a challenge to get the SCTA inspectors to believe that a bulk head welded joint is stronger than a bent tube.)


Saltfever, thanks for the idea about looking for a CAD class at the local Jr College, I will be checking the Santa Rosa JC to see what they have.

Rex

[Interested Observer]

Rob:

Ref. Your reply #30 - Updated chassis tube layout

Your overall vehicle concept seems to be quite well conceived and mercifully would appear to be aero stable which is a significant advantage.  However, since you are still in the early stages of frame/roll structure layout and are open to comments, here are a few more for your consideration.

1)  Further to part of Saltfever’s reply #31, it appears that the roll structure is completely supported on kinked or curved sections of tubing.  Such a structure is terribly less rigid and strong compared to one with straight legs.  If I were an inspector, it would not be accepted on the basis of poor design.  I assume the side tubing is kinked to accommodate the juncture of the “fuselage” and the canopy.  You need to prevail upon Blue to allow a generous fillet, or better yet, a straight line tangent to the fuselage side and the canopy, so that the kinks can be eliminated.  This may add marginally to the frontal area, but would also marginally reduce the wetted area and possibly complicated flow patterns at the existing joint between the two.

It is not clear why the two central rear tubes are curved down and forward instead of tying directly into the frame rails, but these, too, would support next to no load if called upon to do so.  The priority should be to make a proper protective structure, not just make room for stuff behind the driver.  P.S.-- having just looked at reply #39, the added bracing makes this situation is not as bad as in #31, but for reasons given in 2) below, it is still a concern.

2)  The “three side rail”  layout looks stout in the side view and is probably there to more closely conform to the rounded fuselage contour.  However, as shown, the mid-height joint represents a severe Achille’s heel in the structure.  The angled joint, with no ties across the frame to the other side, much less proper triangulation, constitutes another kink in the load path and is effectively a hinge between the top section and bottom section of the frame.  (Try building a hexagon out of toothpicks or soda straws or something then load one side against the opposite and see what happens.)  This arrangement would also probably produce rather soft torsional stiffness in the chassis.  A speed wobble at 400 mph might be more exciting than necessary.

It would be better to just eliminate the middle rail, or leave it there as “side impact” protection, but for chassis structural integrity tie the top and bottom rails to each other with vertical and angled members, as well as across to the other side as is the classic approach.

3)  Is there enough internal volume in the car to accommodate all the various ancilliaries that will be needed?

4)  Just curious--how are the rear wheels tied to the main frame?