But is it enough to allow the car to drive upside down in this video we’re going to find out if it can but we’re also going to walk through a ton of testing, as well as the design and build process? If you couldn’t already tell this project is heavily inspired by the real life braebum bt46b and the chaparral 2j, which are both two cars that used fans to create massive amounts of downforce at any speed, and even though this is a much smaller scale. I faced a lot of the exact same problems i read about these cars facing which really keep me into some of the challenges these designers faced for the time, and it also helped me understand why these cars were banned by almost every racing series out there. The car itself is a modified traxxas, slash that uses two 50 millimeter ducted fans to pull air from underneath the car to create the pressure drop. These fans are mounted in a high flow 3d printed housing attached to a large under tray that is designed to maximize the low pressure area and the front wing is really only to protect the shocks in the very unlikely event that i crash and it’s cut from The exact same aluminum that’s, used on the under tray, i used screens on the air intakes to prevent any large debris from damaging the fans but went with a larger size. So i didn’t totally destroy any chance.
I get of creating a high pressure drop. The most important part of this entire build was probably the side skirts. I tried several iterations and spent way too much on weather stripping through all this testing, and this required lowering the shocks to get my initial air gap. Just right and when i finally got the downforce i wanted, i had to stiffen the shocks well, beyond anything, you would ever run on a normal rc. The fans also had their own speed controllers and batteries, which added a reasonable amount of weight and i’m. Just using a servo tester to actually set the speed for now when i finally had everything together, i was actually impressed with how well it Music Laughter looked, but none of that matters if it doesn’t actually work. I tried to characterize downforce several ways to see if it was even close to driving upside down and they were all horrible test methods, but they gave me just enough confidence to test this thing inverted and just like that, i knew i was on to something i Also realized just how incredibly sensitive this car was going to be to the condition of the side skirts, and i really needed a way to measure this impact of side skirt position so right before i was about to spend way too much money on a digital gauge. I remembered that i could just use some cheap tubing and 3d printer housing to create a pressure measurement device in the form of a youtube.
Manometer that’s youtube not youtube. I should also mention that i had a very specific pressure drop target going into this. The car itself weighs about seven and a quarter pounds, and if i wanted to drive upside down, it has to be able to support seven and a quarter. Pounds plus a little bit for wheel friction, so it can actually move, and my area of the under tray is about 120 square inches. So i was left with about 0.083 psi of pressure drop averaged across the bottom to get the 10 pounds of downforce that i needed for reference that’s equal to about 2.3 inches of water on our new youtube. Manometer. Okay, enough math let’s get an actual reading to see what kind of pressure drop we are getting. To my surprise, i was getting between five and six inches of water, based on where i measured. That is a huge number that adds up to more than three times the weight of the car and is more than twice my original downforce target and just for fun let’s see what kind of pressure drop we can get out of something like a shop vac. My confidence is now through the roof. I am the second coming of gordon murray, who i think is probably still alive, so maybe gordon murray jr, but i did still build a test plank. So i didn’t have any huge falls from the ceiling because i didn’t want to destroy this thing right off the bat and the first few tests went well let’s watch Music in the first few tests.
I realized that, even if the skirt position is perfect from the beginning moving, the car is obviously going to destroy that if the skirts are too flexible with this configuration as the cart moves forward, it folds the skirt under at which point it gets sucked in, and I have a sudden loss of pressure even when the skirt is working it’s, sucking the car up so hard that the back wheels can barely overcome the skirt friction, and this reminded me of something i had read from gordon murray regarding the side skirts on the bt46b. He described hinged side skirts that use springs to push them down and if the spring force was too low, the negative pressure would just suck the skirts up and you lose vacuum. This is exactly what i saw after the front skirt folded under and if you had the spring force too high, it would wear the side skirts out too early, or in my case it would just lock the car to the ground in the initial test. I was just using a thick vinyl tape for the side skirts, which proved to be way too flexible and the shocks themselves weren’t even remotely close to being stiff enough. So that’s when the weather stripping fiasco started, and i tried several different profiles, which all created way too much friction so the car itself, couldn’t move because again it locked to the ground or ceiling or whatever. I also tried to lock the shocks out with some plastic tubing, but that still wasn’t enough, because there was a little bit too much play at this point.
I was at an impasse with skirt design. Do i redirect and go the direction of the chakra 2j, which use sliding skirts, or do i dig further down the bravo bt46b path, but use a stiffer side, skirt that’s not going to deform to the ground too easily, and i want the brave morale mostly for Simplicity, but also because it looks cooler which led me to design a 3d printed nylon skirt. That was somewhat flexible. If i were to hit a rock or something. But then i added teflon tape to extend it down a little further so that i had a little bit better seal to the ground and this created sort of a dual stiffness skirt. That was intended to keep the car from locking too tightly to the ground, and it worked a little bit too well, because now i couldn’t keep the car inverted okay, so this is getting really frustrating and a little boring so i’m going to take a quick break To just drive this thing around see what happens after the run as you might expect, this thing gets absolutely filthy it’s, acting exactly like a vacuum cleaner, as we probably already expected. Oh and i crashed it, but the fake wing on the front actually did do a great job, det er, saving my shocks and a arms. Åh, you noticed the two entirely new ducks on each side of the car yeah yeah. Those were added to improve the pressure distribution on the bottom of the under tray.
I noticed that the front commonly fell first and it did have less of a pressure drop than the rear. When i was testing with the manometer, i was also only running 3s batteries on the edfs and they’re technically rated for 4s. So i upgraded that which i really didn’t want to do because of the added weight. But more power is always better and i also added a die spring as a second spring on the shocks to increase the spring rate quite a bit, so it wouldn’t bottom out, let’s see what we got Music. Finally, we are able to drive and stop and resume driving again without the risk of falling. We can move on to real projects now, but why would anyone go through this much trouble when they could just add huge wings on the car, like i’ve done in some of my other projects or modern day f1 cars, and the main reason i took this approach – Is because it gives me tons of downforce at very low speeds, actually zero, which i really want, because i want to create a car that has tons of lateral acceleration right off the line and the second reason is aerodynamic drag. If i wanted the same amount of downforce with a conventional wing that these fans generate, i would basically have to double my frontal area, which would effectively double my drag, and it would probably also make my drag coefficient worse. So i would be worse off all the way across the board.
This testing did help me understand why fan cars are banned in just about every racing series. Ever i got loads of sand and dirt to the face and legs whenever i was testing pretty much anywhere, because these cars are like literal vacuum cleaners and in the actual racing series they were said to throw debris and trash at every racing car behind them, which You would expect because they just exhaust the air right out of the back of the car, and the second reason is because of how rapidly you lose downforce. When a skirt fails. I usually had almost no warning, and now there is a big difference between when a car is inverted and when it’s right side up. But even when i was testing as soon as i hit a bump and the skirt would break, it would lose down force right away and the driver pretty much has no warning that this is happening. So they’d shoot off the track and it was a huge safety hazard. While i focused most of this video on driving upside down that really wasn’t the goal of all the testing, the testing was done to understand two things. The first was: can i get a pressure drop equal to about two and a half inches of water, and i definitely accomplished that goal and blew it away. The second was to understand side, skirt design a lot better so that i could carry it forward into the actual build that these fans are intended to go into, and i think i know enough now to really make the serious project much better than if i just Jump straight into it, i know a lot of y’all are saying i’m, a guard.
Why put all that effort into a brushed motor and that’s, because the speed run for this car was never the point. It was just to learn about pressure, drop and side, skirt design. Car still had a slipper clutch, it could have never even used that much downforce. My long term goals with these fans and skirts is to make a lateral acceleration monster a 0 Til 60 Bil, and to do that, i would ideally use a 4×4 chassis, but i don’t want to spend the money i’m still going to have to buy all new Driveline components: a new motor and esc that can support the amount of torque required for sustained lateral gs and a bunch of other smaller stuff i’m, also going to need to do a lot of testing on the tires. So if you have any recommendations on how to improve the grip of tires, tire, prep, motor combinations or driveline components for the slash, please let me know i am open to all the feedback i can get, so we can set an awesome 0 Til 60 time. Oh one more thing: i know the edfs are backwards, they look cooler that way and it distributes the mass more towards the center of the car, and it was good enough thanks everyone for watching.