High speed aerodynamics advice

[aerohead]

Originally Posted by LibertyMKiii

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.



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

There is a concept in fluid mechanics called dynamic similarity,or,verisimilitude.It has to do with maintaining a super-critical Reynolds number,as would be encountered with a full-scale vehicle.
As you scale down with models you must increase air velocity accordingly,in order to maintain a constant Reynolds number,or else the boundary layer conditions will be totally wrong,giving false measurements.
With your RC racer so small,in order to maintain the proper Reynolds number you'd have to have an airspeed in the supersonic range;which introduces compressibility effects you wouldn't normally encounter.
If I were you,I'd write a letter or email to Cal Tech,addressed to the folks who operate the GALCIT wind tunnel,and see if they could offer some advise.
They've done the testing for many land speed record vehicles,motorcycles and cars.
Sometimes,grad students will take a race car on as a graduate research project and help underwrite expenses.

[LibertyMKiii]

Originally Posted by aerohead

If I were you,I'd write a letter or email to Cal Tech,addressed to the folks who operate the GALCIT wind tunnel,and see if they could offer some advise.
They've done the testing for many land speed record vehicles,motorcycles and cars.
Sometimes,grad students will take a race car on as a graduate research project and help underwrite expenses.

I will certainly keep this in mind, for later on.
First I need to make some prototypes and confirm as many details as possible so that I do not waste their time should they consider taking on such a project.

-Liberty

[Bicycle Bob]

There are flow visualization programs that might answer many of your questions. I'd also think about low-speed water tank testing as a possibility. People also do things like running a model with a fresh coat of paint on it, to see where it dries first, and with paint spots, to see where they run.
However, it sounds as if you are going to brute-force your way on acceleration, so aero drag becomes a minor component, except for the efficiency in downforce.

[freebeard]

A water tunnel breaks the Reynolds number conundrum.

http://ecomodder.com/forum/showthrea...ies-31047.html shows how it's done. Upside down, with hydrogen bubbles.

It does involve large quantities of water. Here's my design. Version 0.2:



The opposing paddlewheels are interleaved so they move a large volume with minimal turbulence going into the diffuser. Version 0.4 would have a single return and a single paddle wheel at the far end from the diffuser. Big return channel, small test section.

[LibertyMKiii]

Originally Posted by freebeard

A water tunnel breaks the Reynolds number conundrum.

http://ecomodder.com/forum/showthrea...ies-31047.html shows how it's done. Upside down, with hydrogen bubbles.

It does involve large quantities of water. Here's my design. Version 0.2:



The opposing paddlewheels are interleaved so they move a large volume with minimal turbulence going into the diffuser. Version 0.4 would have a single return and a single paddle wheel at the far end from the diffuser. Big return channel, small test section.

I really like this, however the cost to build would be as much as the project itself. I'll probably stick with software and best practices.

-Liberty

[LibertyMKiii]

I am doing version 1 of the body design as a catamaran.
Looking around on the inter-webs I found some various things that were quite interesting on the topic of near ground aerodynamics. I know that Solar EV college teams try to engineer their cars at speed to have neutral lift. We can see the top of the airfoil's shape would normally generate some lift except for the accelerated air speed under the car "squished" between the bottom and the ground creates the low pressure zone.



This image is quite common on this site which indicates an ideal shape for low drag, but my application needs some significant down-force.



Given that using larger diameter wheels will help reduce the rotation speed at 200mph I have the ability to build in some air gap under the car where I could take advantage of the ground effect principles.
This image is less common when searching the topic and appears that noticeable effects require the car body to be a significant height off the ground.



In this image the top version has the axle line and lowest part of the body as the highest drag of the various models... It seems to conflict with the above image.



This image is a bit more scientific.



And finally the manta-ray


I have done some significant changes to the car where the main chassis is very thin for making this section into a slim airfoil. The tallest part is the top of the 4.15" tire. The bottom of the chassis is exactly half way at the axle line around 2.15"
What are your thoughts on how effective this design idea might be?

[LibertyMKiii]

I plan on making the airfoil similar to this sketch which helps give an idea of what I am describing the current shape to look like, where the tires at 4.15" and bottom of the chassis is 2.15" off the ground:



I appreciate any feedback and comments.

-Liberty

[freebeard]

Is that the section through the middle, with pontoons?

Hey, a Colani vehicle I hadn't seen before: 1991 Colani Stingray - Studios

Same guy I mentioned at Permalink #4. Did you see what he did with the wheelwells?

[Bicycle Bob]

If you fill in the sides, and improve the curves it should give decent downforce. I still think that you'll waste a lot of track getting it up to the speed where it starts to bite, and then wind up with far more downforce than you need. The taller wheels can stick up or down for the same penalty.

[LibertyMKiii]

Originally Posted by freebeard

Is that the section through the middle, with pontoons?

What I had drawn would be just showing the airfoil shape of the center chassis. Adding lines for the sides over the wheels makes the image confusing unless it becomes a 3d image at an angled view.

I would have the side pontoons be fully enclosed all the way back. Not much turning going on in my application.

I agree I may have too much down force at high speed but that is a problem that I am ok with initially. That can be experimented with later.

-Liberty