High speed aerodynamics advice
 

Hello all,

Let me start by stating that this is an out of the normal post in what might seem like the wrong forum. Oddly enough this is a great place for some aerodynamics advice even though most of you have not seen 70+ mph since your youth :)

I am attempting to build an high speed RC car to beat the world speed record. Currently this record is held by Nic Case at 202 mph.

I do not want to get side tracked on the equipment involved as I have those portions figured out. Instead id like to focus the topic on aerodynamics methods between two ideas.

Idea 1: Building for airflow under the car to utilize the low pressure zone as the down force much like the John Cobb in 1939 went over 369 mph (later went over 400 mph) Railton Mobile special

Idea 2: Build for minimal air flow under the car. This would be much like Collin Chapman's designs from the golden era of F1 with floating side skirts sealing the under side of the car. This design also utilized minimal ride height.

The example here is Speed Demo land speed car which really didn't improve much over the Railton car given 70+ years of advancements. Their "record" was 439mph and had seen speeds much higher (462 mph) during their run outside of the official zone.

I cannot make mine exactly like example 2 as my vehicle is a standard 4 wheel car configuration, where Speed Demon is setup more like a 3 wheeled vehicle with the "pointy rocket" nose cone. I am hopeful to get some insights about aerodynamics at these velocities and near ground aerodynamics.

These RC cars like to go air-born over 70mph so they require some significant downforce. The acceleration is also very extreme as it will get from 20mph up to max speed in less than 15 seconds.

Some technical details for reference:
The weight is around 12-15 lbs. Width will be around 11" and length will be a fairly good size estimated around 30"
The tires / wheels are 4.15" and ill likely shave them down to 4" So I can estimate the total height being around 4.5"
the nets:
Frontal_area_m_2 0.03193542
Drag_N 34.49627467571459
Max power 330 amps at 34V
Electric motor power 11,000 - 15,000 watts

Thanks for any feedback / input you can provide.
This topic tends to be exciting for engineers since you can create the ideal package given there is no driver who needs visibility. (inside the car)

-Liberty

I raced slot cars in the 60's so I can tell you the 1/24 scale LSR guys were looking at 200 mph also. There is a matter of CG location that will affect the amount of down force needed at the nose. I assume you have a flat belly pan and the will be some cooling air needed for the motor in the rear. See Joseph Katz on race car aerodynamics. As a starting point h=0.25" with a half body of rotation of the 3:1 airfoil.

First, assume that you will hit a bump some day, and everything suddenly goes nose-up. It has happened to the best of 'em, but planning for it is not a performance penalty at all. What you need is a stable glider that will land just a tad nose high in ground effect. Now, once you have that basic problem solved, you only need to pay for enough down force for traction. You can design for minimum drag, and dial in a bit more traction by adjusting the rake if you need it.

If you need a lot of downforce, and are not bound by rules, a wing is probably the most efficient solution outside of a sucker car. The ideal run probably uses more downforce for acceleration than top speed.
I don't think the Railton was planned for downforce - people were still perplexed by lift on various streamliners a decade later.

Welcome to Ecomodder.

I was talking to my brother earlier today and mentioned my high school friend. We rode his father's black 1957 Buick 2-door from Monmouth to Rickreall at 120mph. If things work out I want to put an Arcimoto FUV on the Bonneville Salt Flats to 100-120 this September. I already have a world class driver['s verbal commitment].

That said though, reminds me of the 90s. I worked at a hard drive manufacturer in Beaverton/Tigard. On Fridays we'd go out in the back parking lot and set up cones. There were three teams and on ours (Tech Support plus the receptionist one hard drive was named after) the other guy took the chassis and drive train. I took the vacuum-formed body, a Mercedes racer —basically a wedge with M-B badging and left the front wheel wells uncut and unpainted but the entire back was cut out. The downforce was such that it sucked dust out of the pavement and left a visible trail suspended in the air like a dry lakes race car.

It turned out the receptionist could mentally steer herself around the back corner, where the control needs to move opposite the actual car better than anyone. I painted it hot pink and pearlescent purple. The bosses wife wanted her car painted that color. Good times.

But heed the words of Bicycle Bob, with dynamic downforce, if it breaks suction, that's all she wrote. Sweep the track!

Where is this run? How long a track?

I would at least consider the Luigi Colani 'WaterStrider' approach

http://ecomodder.com/forum/member-fr...wu5wo1-500.jpg

Otherwise, the Solarworld One Morelli body is much admired.

Thanks all for the comments so far.

I have experienced the car taking flight a few times already with traditional motor sports aerodynamics. Front splitter, flat bottom, and diffuser in the rear. It does require some significant traction as the acceleration is done in such a short distance.

One of the biggest issues for this hobby is that things are done by visibility. I have about 1/8 mile from the start to my position (in the middle of the run) and another 1/8 mile past me plus slow down room. As a ball park the runs average about 1/4 to 1/3 mile distance. Going much beyond this distance makes it too difficult to see what is going on with the car.

As far as location I have tried a few spots and dreaming of access to a air strip, but for now it is a smooth black top road surrounded by farming fields.

Currently I am in the process of building a long carbon fiber chassis for the car so I can build the streamlined aero body afterwards.

-Liberty

One of these things is not like the other. Is the controller a hand-held with two joysticks or a wheel? Upgrade to drone parts and you can have a LCD screen on the controller.

You know Speed Demon? Why are you constrained to a four-square layout? Who is the sanctioning body? My own land speed design is a reverse tadpole with a two wheel bogey in back. Speed Demon has the long nose for transonic speeds (~250+mph). I went reverse tadpole for stability.

Have you considered Reynolds number? To be consistent with a full size car 1/4 or 1/5th scale is considered the limit. and the air speed has to be doubled at that. Most are trying to extrapolate from the model to full scale. You're going the other way. One Ecomodder greybeard built and tested a scale model in a wind tunnel to test his understanding. Have a look. Err, good luck, the pictures are all behind the Photobucket curtain. Best I can find is this thumbnail

https://encrypted-tbn1.gstatic.com/i...-j1F7Spd3aUSFA
http://ecomodder.com/forum/showthrea...6-a-32673.html

What construction method do you propose for the shell? Vacuum-formed over a mold?

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)

I have looked into Reynolds values, however since I have not decided on a design method for the body which will give the best safety/stability/speed combination I cannot determine a final decision.

There is not much of a sanctioned body for this sort of thing. (ROSSA and Guinness book) Generally there are specific measurement methods to determine peak speed. Other than that it is pretty open. Power methods separate the classification much like land speed runs at Bonneville. I do plan later to build a 2 wheel "motorcycle" and fully expect that would go MUCH faster. For now I have a significant investment in this 4 wheel car and would like to see it through completion.

It will be an Vacuum-form over a mold. I expect a few crashes when getting the car dialed in. This process is best for re-producing parts.

-Liberty

I mentioned a land speed design:

http://ecomodder.com/forum/member-fr...belly-tank.png
http://ecomodder.com/forum/member-fr...11-5-38-12.png

This would be a formless construction. A flat sheet cut into a 'peel pattern' with tabbed darts. This folds into a convex shape. With edges and flats, the sharp angles are at the front and back but flat at the separation point. Else the darts could be butt-welded and the whole thing planished in an English wheel for a true compound curve.

Did I mention formless? I should probably have an example in CAD (cardboard aided design).

The enclosed wheel version is motor home sized:

http://ecomodder.com/forum/member-fr...-w-caption.jpg

Nic Case appears to have tried this sort of design. Not certain on his results. He certainly had a bigger budget for this hobby than I do.

Check out in his video from time 0:49 - 0:52 sec
https://www.youtube.com/watch?v=U4Ax3yd-1lE

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

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:

http://ecomodder.com/forum/member-fr...0-100-1101.jpg
http://ecomodder.com/forum/member-fr...6-100-1030.jpg

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.

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.

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?

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.

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.

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

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

Why not go scale? Do a Chaparral 2J and get the record too. The other nice thing about a sucker car is the constant effective weight and ride height.

That is a valid point, I'll have to do some research into some ducted fans.

-Liberty

Here is a quick sketch in MS Paint that I came up with. I'm open to any thoughts/suggestions.

http://i66.tinypic.com/jzd7x4.jpg

The battery behind the front wheel would be level with the chassis like the front lower battery is. That was drawn in incorrectly.
-Liberty

Do you follow the other threads in Aerodynamics? This is kach22i's latest post in the smoke and wind tunnels thread:

http://forums.pelicanparts.com/uploa...1521672529.jpg
http://ecomodder.com/forum/showthrea...tml#post564458

It appears there is an underbody tunnel to the height of the axle line. What is it's width? Could you group the batteries in between the wheels on each side, in a catamaran body for less frontal area.

The fan could pull 10-15% of it's flow off the top or sides of the tail to eat any attached turbulence at the last moment before it's added to the wake.

I had poked around in that thread but had not made it to page 70 yet. I love wind tunnel tests, I hope to have my own scaled tunnel at some point in the future with load cells, pitot tubes, rotation for crosswind etc...

The Catamaran design is more inline with what I had hoped to build. I want to utilize the air flow under the car to my advantage and should have a much lower drag on the car with that design.
There is a large space between the front and rear wheels for the battery packs.
The width of this area is not 100% defined at this point. I have not determined how narrow I want to make the wheel/tire combo. It would range any where from 1" up to 2.5"

-Liberty

-Liberty

http://oldconceptcars.com/wp-content...lver_Fox_6.jpg
http://oldconceptcars.com/wp-content...lver_Fox_6.jpg

I was thinking while out to lunch how someone might accomplish this shape with a standard road car. Thanks for sharing the photo.

My design for this RC would be much more like how the EV solar teams do theirs. The only significant difference would be that they design for neutral lift at speed where I would be adding around 5 degrees AOA of the airfoil for some downforce.

-Liberty

Sucker cars were designed to hold the car to the road, including around corners. They had virtually unlimited HP to work with because their racing class allowed it. That suction costs power. You need to consider the tradeoff. My money is on it being a non-starter, else other LSR cars would have already done it.

It certainly adds some complexity, cost, and risk to the car that I'd rather avoid.

A small pebble in the road is like a 5" tall rock to a real car. Breaking suction is too high of a risk given the scale and scenarios possible.

I will have way more HP than necessary to reach my speed goal so I believe the downforce / drag penalties will be just fine.

After doing some quick sketches with the catamaran type designs I am liking what I see. I can do some things here that LSR guys probably cannot given there is no occupants in the vehicle.

-Liberty

That sucker skirt should sweep off any pebbles the suspension can't absorb. Given the non-lethal nature of accidents, the excess power, and the course length, I'd go for a pure sucker for downforce, and anything like a widened half-bullet on top.
I'd still argue for a stable glider, except any launch is likely to cause a roll as well, and roll stability is a slow response.

If we're back to The Template, I'm in favor of a autofellater with a squircular cross-section:

http://ecomodder.com/forum/member-fr...14-1-42-00.png

Here's my take on it, stretched version of the Aero Template, bottom does angle up slightly from front to back, have not been able to get my head around how to create some down force during max acceleration then have it go back to neutral nearing top speed to allow maximum speed. But it's a start. The Catamaran thing looks like problems to me. What I have drawn is more a test sled than a mini "Car" since we aren't going to be putting anyone in it, why bother making it look like you could. A starting point perhaps in the direction of a clean small aero shape.

http://i67.tinypic.com/2zeyqed.jpg

A free weight on a slide with a return spring. It's connected by a bellcrank to your active aero.

Modern practice seems to be a trimaran approach.
http://images.thecarconnection.com/m...00351544_m.jpg
http://images.thecarconnection.com/med/2011-nissan-lmp2-race-car_100351544_m.jpg

Mount the , for lack of better term wing /downforce device to the suspension to apply downforce directly . This allows for a softer springs -> better handling.
Im thinking somekind of sail on the bottom of the car that when you getting excess of airflow underneath of your air dam indicating lift, the airflow would push on the cell pushing and turn pushing a lever that would add pitch/AOA to the front wing. Kind of like a self deploying countermeasure. kind of like the roof Flaps in NASCAR but in a whole different context.
Mechanical force under the car versus the force on the wing above the car maybe too far away from each other. this can be solved with two servos. the first Servo can be the smallest one you can find you're only interested in the potentiometer and the ability to add a linkage. the second Servo would control the wing receiving its commands from the first servo having its potentiometer wired in place of the second servo. Considered an electro mechanical leverage.
another potential means of controlling this undesired vertical movement is with another pair of gyros to control the front wing and rear wing independently

Sounds complicated. I agree with applying downforce directly to the unsprung weight.

I just think it should be a mass free to slide fore-and-aft. Stupid simple. In fact I was thinking about pointing to the Deltawing config as a trimaran.

http://i.wheelsage.org/image/format/...ce_car_25.jpeg
http://i.wheelsage.org/image/format/picture/picture-medium/nissan/deltawing_experimental_race_car/autowp.ru_nissan_deltawing_experimental_race_car_2 5.jpeg

60s gassers used to ride nose high to get weight transfer off the line. You could have a Delta wing configuration where the front nacelle shortens to put weight on the drive wheels and then lengthens for longitudinal stability as the acceleration slackens.

If your course spoils many runs by breaking the vacuum of a sucker car, you could fire small rockets upward from a contact on the droop stops.

Reactive aerodynamics!

Rc lsr
 

Just now caught your thread.
*With 1:1 scale racing,you'd want as much weight as the chassis and tires could survive,and use 8-12-miles of run-up to achieve your terminal velocity.
*Liquid fuel would have thousands of kW-hr capacity.
*You don't have either luxury.
*1:1-scale race cars don't have any suspension at all.They don't want any induced drag to alter the inclination of the body at any time.
* The scale of your racer may necessitate some wheel travel,as at 30" overall length,and small diameter wheels/tires,the smallest track deformity could cause the car to be thrown out of control.
*Really spongy-gummy tires might deform enough to absorb some surface irregularities.Don't know.
*If so,then you'd want a body shape incapable of generating lift,most likely would want to intentionally generate downforce,of which the motor /battery will have to overcome this additional drag.
*At 30-inches,instead of 30-feet length,you'd have to test your model at 2,400 mph,to properly simulate 200-mph full-scale effects.
*That's impossible,due to compressibility effects.
You have some really unique challenges. Quite a project! I'll do some thinking.

The 2,400 mph comment is that in relation to wind tunnel testing? I was hoping multiple 150mph related leaf blowers might do the job.

I have had some issues with suspension, which I will not get into. The world record holder did not have suspension.
My current setup which I believe is great uses aftermarket A-Arms which are high in nylon content and solid links in replacement of the shock/spring combo. The Nylon A-Arm acts lick a spring itself yet is very stiff also. There is also some give in the BSR foam tire. I previously had some very stiff springs rated around 25lbs each and with the down force of my front splitter the front chassis was scraping.

Here is an image of the current setup. I need to get over to my friends house with a TIG welder and adapt the "shock tower" to have the shock mount be lower and more directly over that location. This should help reduce the frontal area.

http://i65.tinypic.com/33wb1au.jpg

I had a great image showing c/d ratings related to the bottom of the car's height off the ground but at 3.5 cm I do not believe there is much room there to take advantage of significant air flow?

-Liberty

We're back to Idea #1 and Idea #2.