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LibertyMKiii 03-19-2018 04:19 PM

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

Grant-53 03-19-2018 05:58 PM

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.

Bicycle Bob 03-19-2018 06:46 PM

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.

freebeard 03-19-2018 09:43 PM

Welcome to Ecomodder.

Quote:

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 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.

LibertyMKiii 03-20-2018 09:40 AM

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

freebeard 03-20-2018 11:56 AM

Quote:

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.
Quote:

One of the biggest issues for this hobby is that things are done by visibility... Going much beyond this distance makes it too difficult to see what is going on with the car.
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

Quote:

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

LibertyMKiii 03-20-2018 12:18 PM

Quote:

Originally Posted by freebeard (Post 564301)
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.

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)

Quote:

Originally Posted by freebeard (Post 564301)
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

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.

Quote:

Originally Posted by freebeard (Post 564301)
What construction method do you propose for the shell? Vacuum-formed over a mold?

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

freebeard 03-20-2018 01:36 PM

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

LibertyMKiii 03-20-2018 02:05 PM

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

gumby79 03-20-2018 02:41 PM

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?


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