High speed aerodynamics advice

[LibertyMKiii]

Originally Posted by gumby79

Reduce frontal aera as much as possible. 34V ÷4.2(LiPO)=8.09 cell batteries.
How many AH are needed for the run? Before the wattage drops out.
Would capacitors be a better pixie( electron) storage media, as in this instance brut force( extremely fast discharge) is more desirable than capacity?

I am working on modifying the suspension in an effort to reduce the total frontal area. At some point after some testing I may pursue smaller diameter custom made wheels to alter this further.

I have not seen much testing of batteries in the sizes that I will be running, but with smaller batteries for racing drones I have seen that max voltage and current is only good for about a 5 second burst. From memory these were 1500 mAh or less.

The setup for my car will be 8 cells at 4.2v (I might try LiHV 4.35)
The packs generally are in the 4000 to 6000 mAh range. No one currently offers a good 8s battery with high discharge amperage, so I'll likely run some 4s packs in a series configurations. (Or if I can get a sponsor who wants to make me some custom packs that would be great!!!)

I have been kicking around the idea of 4 packs total with 2 in parallel, which are in series with the other 2 in parallel. This should provide all the power needed. Capacitors are used primarily to reduce the ripple current. I don't believe the size and array needed to do anything worthwhile would fit in the vehicle. I have seen some large capacitors tested and they could only operate the motor for a second or two before discharged.

-Liberty

[freebeard]

a better pixie( electron) storage media

AvE-fan-like typing detected?

There's quite a range of scales shown. What are you constraints there? When aerohead was crafting his Baby Template, I thought about how to do something similar. It went (as far as it did) like this:




With a variety of aspect ratios, these are very light aluminum tubes. One could take three and make a base with crestline/loft line, then stretch a fabric in tension over it and have a fluted aeroform with minimal weight.

[oldtamiyaphile]

Originally Posted by LibertyMKiii

I have the traditional RC car transmitter with the wheel. I have not found any benefits to the models like Futaba with the screens. My eyes are on the car during its short 15 second run. Inside the car I have many data log sensors running. (IR Temp, internal measured temp, voltage, amps, watts, GPS and so on)

freebeard's talking about FPV. If nothing else that could help you in the accel/braking zones to make your usable distance longer.

You could even have a co pilot using FPV to get up to say 100mph, then the car passed into your view you take over, and after the finish you hand it back over to your FPV co-pilot.

I don't think I'd be trying for 200 with such a basic TX though. As much as I favor a solid mechanical set up before resorting to electronics, being able to set things like steering sensitivity as speed rises definitely helps. I think I'd want to run something like a DX4C with AVC, or possibly re purpose an aircraft auto pilot for surface use. One crash saved is the radio paid for.

[ChazInMT]

Do you have even a basic drawing of the chassis and wheel arrangement that we can try to draw a skin on? Are you front wheel or all wheel drive on this? What sorta tires are we talking width wise? Are active aerodynamics allowed?

[freebeard]

What oldtamiyaphile said. I've heard that cars that campaign at Bonneville year on year have to basically re-engineer the car for each 10mph increment.

For instance....

Thrust vectoring and traction control via an Arduino controller to keep it from going sideways. And GPS for homing on a waypoint.

[LibertyMKiii]

Originally Posted by ChazInMT

Do you have even a basic drawing of the chassis and wheel arrangement that we can try to draw a skin on? Are you front wheel or all wheel drive on this? What sorta tires are we talking width wise? Are active aerodynamics allowed?

It is an AWD chassis with a drive shaft down the center. Double wishbone configuration on all 4 corners and some very heavy duty axles to handle the power. There is allot of suspension linkage going on next to the wheel so an "open wheel" design is not ideal.

I will try to get some sort of graphic for you to look at. I finally have the post count up so I can add photos to the posts.

For some general specs that could get something started.
Width (max outside distance of wheel/tire) 296mm or 11.65 in This is an area that I have been considering reducing also. Modifying the A-Arms and cutting the axles I could see reducing this as much as 1.2 in total.

Length is to be determined I am open to any advice here, obviously length adds significant stability at speed as well as more room for a nice long flowing body. I was thinking something in the neighborhood of 24-30 inch wheelbase.

Height is right at 5 inch (I may be able to reduce this some)

wheels and tires are made by John at BSR racing. The tire itself is a special foam that has great traction but also does not expand at high rpms like the tires tend to. GRP makes some rubber steel belted tires but they tend to de-laminate at 160mph.
The BSR foams are 4.15 " but I will be shaving them down to 4" to reduce the possibility of issues. @ 210mph they will be rotating 17,656 rpm!!!
The width of the wheel/tire is adjustable. I have successfully cut one down as well as rounded the corners of the "tire" foam. Thinner is better in this area but it will need some meat there for traction also.

There are no rules.... I have been considering adaptive aero and have been imagining a setup with pitot tubes and Arduino.
The car currently has a little bit of tech in it for stability. It has a gyro which detects a slide scenario and automatically counter steers for you. It has yet to save the car from a crash... Once its sideways at high speed it takes off in flight.

I am in disagreement about the FPV ideas. You can youtube RC Cars running with cameras in them (GoPro etc) its a blurry mess. Being just a few inches off the ground you cannot see anything. I suspect any corrective action seen in the goggles would be too late. I do agree on adaptive steering based on speed. Given the car will be attempting to go straight I plan on setting that up for minimal steering at all times.

I have also been debating on floating rubber skirts or nylon brushes as under car aero as well as around wheel openings.

[Bicycle Bob]

Aha! You are already having crashes from getting out of shape. Try to understand that you are basically "in flight" at speed, but trying to stay on the runway. Aero stability comes first. That does not come from length, but from balance. Nor does length aid "flowing," past a certain point. The Railton is the only famous Bonneville car that does not waste surface area. Air is sticky as well as pushy.

With no rules, purity is up to you. A rocket sled would make it very easy. You could add fans for downforce, push, or both. You could use a catapult for acceleration and a catch-net at the end. Or, you could do it the hard way and use the suspension to adjust wings for sufficient traction with minimum drag. You probably want a big wing, to become effective early on when there's gobs of power for acceleration and little drag.

You are very lucky with regard to the potential for laminar flow, which can radically reduce drag. If you produce a suitable shape accurately, turbulent flow will only begin at the wheel openings, and spread at 15 deg along the streamlines.

[LibertyMKiii]

Originally Posted by Bicycle Bob

Aha! You are already having crashes from getting out of shape. Try to understand that you are basically "in flight" at speed, but trying to stay on the runway. Aero stability comes first. That does not come from length, but from balance. Nor does length aid "flowing," past a certain point. The Railton is the only famous Bonneville car that does not waste surface area. Air is sticky as well as pushy.

With no rules, purity is up to you. A rocket sled would make it very easy. You could add fans for downforce, push, or both. You could use a catapult for acceleration and a catch-net at the end. Or, you could do it the hard way and use the suspension to adjust wings for sufficient traction with minimum drag. You probably want a big wing, to become effective early on when there's gobs of power for acceleration and little drag.

You are very lucky with regard to the potential for laminar flow, which can radically reduce drag. If you produce a suitable shape accurately, turbulent flow will only begin at the wheel openings, and spread at 15 deg along the streamlines.

I agree completely. I want stability and adequate downforce most of all. I believe I will have more than enough horsepower to get me "there". It will easily generate around 15hp and could peak as high as 20. Pretty crazy for what it is!

My crashes to date were all my fault making adjustments to the suspension that negatively adjusted the rake of the car.

I do want to stay somewhat pure in that the car will be wheel driven. The adaptive aero could help in braking. The ESC has a drag brake function which does a great job of slowing the car down in a controlled manor. I'm not too worried about the slowing down part...


As a point of reference below is a guy that I have been following. He has one of the worlds fastest 1/8 scale cars, which is largely a production Traxxas Slash 4x4 with more power added along with a sleek car body. His best is this run at 163mph:

https://www.youtube.com/watch?v=8JVw0HW4xVw

-Liberty

[Bicycle Bob]

I wonder if opera glasses would help you extend the range. Anyway, it sounds as if your problem is more like a dragster than a top speed machine. You'd need miles to do the run without downforce, making that the dominant aero force. A sucker car would look tidy, at least and get off the line instantly. A wing based system would have the potential to fly high in the sky on just momentum if it didn't tumble, so it becomes more like an aircraft, upside down, with controls attached to wheels.

[LibertyMKiii]

Originally Posted by Bicycle Bob

I wonder if opera glasses would help you extend the range. Anyway, it sounds as if your problem is more like a dragster than a top speed machine. You'd need miles to do the run without downforce, making that the dominant aero force. A sucker car would look tidy, at least and get off the line instantly. A wing based system would have the potential to fly high in the sky on just momentum if it didn't tumble, so it becomes more like an aircraft, upside down, with controls attached to wheels.

Like most projects there is no perfect solution...
I'll likely come up with a design and then modify it based on what testing reveals. I would LOVE to build a wind tunnel. Safe controlled testing of theory is always nice!

I'm looking at a career change and a move this summer so a wind tunnel will largely depend on what occurs with that process.

-Liberty