Xfsaer wants to build a fan car type FSAE car for autocross type demonstrations

[Xfsae]

As per title ... I am thinking of making it also an open source type projects for all those fsaers who are dreaming of being part in a similar project-so polite input and ideas exchange is welcome
I am searching for photos from the Cornell 1990 fan car for starts - maybe people who designed it and raced it can offer some insights since this tech has been banned for more than 2 decades from FSAE
Thank you in advance and congratulations for this beautiful forum , tool for knowledge , been a long time lurker

[rjwoods77]

I would start with looking up the Cheaparral Corvette that competed in the Grassroots Motorsports $2007 challenge. There is a nice PDF of the how's and why's of what they did along with a bunch of pictures. I was thinking of doing a similar setup with my Impreza. Anyone know where I can get a surplus Abrams blower fan or the manufacturer of one?

http://www.cheaparral.com/

[AxelRipper]

I know we had a program in our office that had a picture of it. I think it was pretty straight forward as you didn't have to try to get around any rules against it at the time. These days if you were going to play by the rules I don't see why you couldn't duct the cooling intake of your air cooled engine to the diffuser.

[exFSAE]

There can only be one...

[Pat Clarke]

There can only be one...

Sue for breach of copyright! ;-)

Pat

[Z]

Xfsae,

Here is an earlier discussion (2005) (http://www.fsae.com/forums/showthread.php?11272-Build-my-own-sucker-car&highlight=sucker) from the Off-Topic section...

Z

(PS. Xfsae vs exFSAE?
Hmmm..., an "X" is that mysterious, unknown factor, like in those "X-Men" movies. But an "ex" is just a "has-been", innit??? :) :) :))

[exFSAE]

It's true, it's true... I'm a has been.. my student racer career is all washed up. Several years in it and not a 1st place in any competition event - not even the push bar contest. It's a tough life, but guess I'll have to settle on all these wins at the pro level instead.

Back to the original post.. interesting idea. I think a variety of people have had similar concepts. Some thoughts...
Why look for inspiration from a 1990 FSAE car? Or any FSAE car for that matter. I feel like you might be better off just starting completely ground up, open mind (and also being very familiar with the rules for whatever SCCA class or whatever you intend to run)
If you intend to make a group or "open source" project - gotta think about a suitable design software (or series of softwares). That may be particularly difficult.
Likewise, need to have some lead designer to orchestrate everything. Really, I my gut feeling is that just doing the whole car yourself would be much easier than trying to do group/"open source" route.

[Xfsae]

First of all - apologies to the exFSAE - had no intent to have any confusion with you - if it really puts you off let me know and i will ask for a new account. I chose it for fun X designates an old(er) fart that has been away from FSAE for a while and misses the enthusiasm and brainstorming associated with it but also it is X-branded by Formula SAE rules - banned stuff , the what ifs that we all dream about (if FSAE was like a dream race car competition and not strictly educational tool etc - i try to sleep but i dream of a Chapparal running upside down etc and no disrespect for FSAE rulemakers or anything - i believe they have done a good job throughout the years and an immense contribution educating new engineers)

At this point I wanted do something like an FSAE car with no restriction on active aerodynamics for several reasons-i enjoy a lot the design and analysis side of things and thats why i posted here also - i like the exchange of opinions with others and the enthusiasm associated with Formula SAE

I wanted to have info on the Cornell car because it was a succesful execution of the concept

I wanted to have it opensource type of thing because i want to avoid the single acceptable solution - that there can be only one acceptable solution - if through brainstorming , design and analysis come out more than one concepts thats ok. I wanted also to provide a vehicle to people who might base thesis or class projects on it (doesnt mean i will necessary follow their solutions but i love to see what they come up with and maybe help in optimising key components such as fans, ducts, sealing skirts etc)

I liked to keep it FSAE power limited because this means that the components ,their weight etc must be more carefully selected

I also dont want to do an anything goes type of vehicle - something based on FSAE safety rules, engine limits (maybe we can simplify things by avoiding hybrids or electric solutions for main propulsion at the moment) is ok

Also i was thinking about open source because some key components such as custom designed fans or ducting could be manufactured anywhere by rapid prototyping methods - other can adapt them to karts , older Formula SAE cars or other similar single seaters of they want to do research of active aerodynamics themshelves but they see it kind of pointless or a step too far since they are banned from almost any type of formal competition

Z - i have seen the thread before thank you and great for fsae.com to have you onboard . I have been reading about the interconnected suspension stuff here-fascinating

I was thinking of the second solution you proposed in the "Build my own sucker car" thread but a slightly different way : instead of having a separate engine with ducts why not have 2 separate electric motors ducted fan style . I was thinking also about a concept you proposed somewhere else (solid axle thread please provide a link if you can because i cant recall where exactly) where you drew a solid axle car front and rear pivoting on 2 points on the chassis for each end . Maybe solid axles are a good idea for the FSAE type fan car we are contemplating as the 2 ducted fans can get attached to the 2 subframes supporting the solid axles so the problem the Bowlands had with the compression of the suspension due to vacuum is gone

What do you think? (what do you all think ?) I am thinking of how the incoming air in the front fan can be ducted away cleanly if the front axle is behind the drivers feet as in most FSAEcars -do you think it is necessary to move the axle in front of the drivers feet as the old beam axle Brown cars?

Also-multi wing optimizer link as provided by John Bucknell in the Build my own sucker car thread didnt work for me

This did : http://www.multi-wing.net/downloads/optimiser/

@ Z about the doghouse solutions around each wheel:
How are you thinking of supporting it in a double wishbone type suspension? (only one sided from the upright or also with a second bearing on the outer side of the wheel?

if solid axle Z concept is used maybe they can be molded in a single piece with each of the 2 subframes in composite with molded in ribs for stiffness

active aero is exciting....

and what about the poor tires? are they going to hold up

People from Cornell ... join in - we want to see this famous 1990 skidpad terminator machine - any photos or input?

Also Z - about the doghouse concept for the front , the outer sphericals must be bolted on the outside of the doghouse to prevent complex sealing arrangements for preventing vacuum loss inside each doghouse ? Or you have something more clever?

Scrub radius is what had in mind when i wrote the last comment about sphericals on the outside of the doghouse-but i guess old FSAE concordia with sliding pillar and karts can do well with a lot of it (speaking of high performance 4 wheelers of the size of vehicle we are contemplating)

[exFSAE]

I still feel pretty confident that if you're a graduated, working engineer now.. that you can do significantly better than a 1990-ish FSAE car. I'd still say, don't look to student built stuff for inspiration. Just blank slate it. I find that blank slating and focusing on fundamentals tends to yield better results with fresh perspective than trying to copy something else as a starting point. Often much easier too!

Not to take anything away from students who have done well in student competitions... but I'd say that shortly after graduating, working professionally, and then looking at student cars afterward - any "wow" factor of what used to be really impressive equipment just evaporated for me. Bottom line all these cars are design and built by folks without engineering degrees [yet] and without much experience. Sometimes surprisingly good stuff emerges, but it could all be better. For what it's worth, upon quick Google search, that "active aero" Cornell car only finished 5th overall of 45. That's not even top 10%.

With regard to everything else you're covering, all these little specifics, it comes across as way too scattered. If you're serious about doing this then maybe pump the brakes a second and start from absolute basics. Start design at a high level conceptually and all the specifics of piece parts will fall into place. Much better approach than what I feel is typical of FSAE kids or young engineers of being focused on the parts and pieces and trick gizmos first.

[mech5496]

This thread looks like fun! XFsae, it seems like cost doesn't scare you away (you have listed things like 3D printed fans and composite structures, I would expect it much more "down to earth" since it is a personal build). Anyway, welcome to the forums!

[Zac C]

First of all - apologies to the exFSAE - had no intent to have any confusion with you - if it really puts you off let me know and i will ask for a new account.

For maximum confusion, I recommend the username 'Jersey Tim'

[jlangholzj]

This indeed does look like some good fun! I've tinkered with this idea myself but more-so in an effort to make a car that fits me! (I'm about 77" tall and 240lbs).

Personally, If I had to design a "weekend fun" "extreme FSAE" type car....

-dual wishbone: This is mostly because I think it would be easier to design and there's more information out there "readily available" on the design and setup. Plus there's more parts out there that are readily available as well. I think a solid axle simple car design like Z pushes would be a fun project but my big concern there is the manufacture of the rear axle. something like a quad would probably work o.k. and would be a viable option....but what about compliance? I find that design to be a bit more difficult to keep the compliance down on (could be completely wrong there)

-obvious space frame

-whatever motor you know the most about/can get your hands on easiest: After spending hours on hours on hours with our R6 and a dyno I'd probably choose an r6. Might not be the best "design" option for FSAE but I know every little trick about that motor. When its happy, when its not. Just the sheer knowledge I have about the internals of that motor would make it a better choice for ME. Plus finding a wrecked sport bike typically isn't that hard. Someone's constantly rear-ending/low side/high siding one somewhere. They're relatively cheap and are reliable if you know what you're doing.

Also I'd probably increase the FSAE restrictor imposed. Scotty's diff is rated at something like 200ftlbf IIIRC and as long as you've got the torque in mind designing around it shouldn't be too bad. The reason for the increase is just some more drivability. None of these motors are optimized to run through that small of an orifice and with the power limitations we typically will gear them deeper than the bikes to get more acceleration. Get that choke flow outside of the powerband and I'll be happy.

-Big aero: Simply for the reason that wings don't work. ;) Also running a bit stouter motor should allow you to completely neglect the drag from it.

All of these thoughts are not the most "correct" design process but in reality if you're looking for something fun to autocross its going to give you the best options. There's a lot there to optimize out but why worry about that when you can be having fun!

[Xfsae]

@ Zac C .Ok mate next thread will be Jersey Tim build active suspension fsae car and a cartoon blowing in a pitot tube and the fsae car moving up and down like mad :)

@ exFSAE all you said is appreciated. i do have a day job as engineer but lets keep the extreme seriousness on things and knit picking out of this thread please. Lets be positive, have some engineering fun , and try /learn stuff that otherwise cant be done in FSAE competitions nowadays

I want to see this Cornell car so bad :) If this is so immature and overly enthusiastic so be it

@ Harry . Thanks for the welcome mate :) I am not afraid of composites - i do some hand lay up - vacuum bagging myself - not anything extremely sophisticated by todays Formula SAE standards but gets the job done . 3d printing in nylon was what i was thinking but we will have to see what we come up with it (if blades of nylon will hold up or come back in pieces like the first versions of the BT46B blades).Maybe we can be clever and build hybrid structures combining RP parts with common fabricated parts for lower costs
I have sourced some typical stuff like brake master cylinders. callipers,rotors, sprockets ,some used moto engines(among them some small ones maybe to be used as auxiliary fan units ) , wheels 13in , steering wheel & seats and some meters of frame tubing in 3 dimensions so i am not in step zero (that doesnt mean i will rush anything - just means that i have some components that maybe be useful to work with ) .Also have my small backyard shop with a tabletop milling machines, small lathes, welders , cnc router and a manual pantograph

One thought is to build it as an 1/5 model first or similar to test the final concept but i am not in a hurry i want to enjoy the design and analysis process with you guys and gals
Who knows, maybe through personal or department contacts and associated thesis projects we can get some help for the full scale model. who knows , if i see people really interested ,helping and involved i might donate the creation to a department afterwards ( i said might :))

[Z]

If you're serious about doing this then maybe pump the brakes a second and start from absolute basics. Start design at a high level conceptually and all the specifics of piece parts will fall into place.
exFSAE (the old one :)),

That really is expert advice!

(Err..., from "ex" = "a has-been", and "spurt" = "a drip under pressure"... (Z thinks - Oh, no, no, no... you're just digging yourself a deeper hole! Quick, move on... :))
~~~~~~~~~~o0o~~~~~~~~~~

Xfsae (the mysterious, unknown one),

Taking the above very good advice, here are some thoughts.

GENERAL, BIG-PICTURE STUFF - As indicated on the "Sucker Car" thread (link back on page 1) and covered in more detail below, an FSAE sized active-aero (fan) car should be able to get AT LEAST a ton of downforce, at any speed from zero to Vmax. In fact, I think 3 tons would be quite easy, and 10 tons would be feasible!

The important point here is that with these numbers the big-picture changes radically. Things like tuning "under/oversteer handling balance at the limit" become irrelevant, because YOU WILL NEVER REACH THE LIMIT.

Let's consider roughly where "the limit" is. Take the middle figure above of 3 tons downforce (technically ~30 kN downforce). Assume that because of the extra strength that you have to add to the car (more below), the car+driver weighs 1/2 ton (technically ~500 kg mass). And assume racing slicks good for about Mu = ~1.5. The tyres thus have the capability to exert a ~45 kN force horizontally, which will accelerate your car+driver mass at 45,000/500 = 90 m/s.s, or ~9 G!. A lower mass car, with greater downforce, and maybe stickier tyres, are all possible, so even higher Gs possible...

Bottom line here is that YOUR PROBLEMS ARE MAINLY STRUCTURAL, both for the car and the driver!

So, toss those girly-boy 68xx thin-ring wheel bearings beloved of FSAE teams. You will need, at the least, wheel bearings off a largish production car. When you really start pushing the envelope you will need something off a light truck (hint - tapered-roller bearings).

Most definitely toss those Keizer wheel centres from the other thread currently running! Some stout, production-car, steel wheels might be a good start.

I am not sure what loads typical FSAE tyres will take, but you will certainly need higher than normal inflation pressures (eg. 2 bar, 30psi?). Le Mans style cars weigh ~1 ton, and develop 2 or 3 times that in aero downforce at high speeds, so those wheels and tyres should be strong enough (at your lower downforce numbers!).

Other chassis structure stuff should also be appropriately up-sized. Main point here is do NOT use typical FSAE-sized parts.

As for the driver, well cornering at 9 G would be like lying on your side while your favourite football team jumps on top of you! The sides of the cockpit should be very well padded. And lest the G forces rip the driver's head off his shoulders, I suggest a very effective "Head And Neck Restraint System" (preferably something much better than the conventional "HANS", which is just a few strings tied to the helmet).

"Centrepoint" steering would be good (ie. Offset/Trail/SAI/Castor = 0), but even then you will probably need power-steering. Think about how fast those slalom corners will be coming at you. Do NOT worry about "steering feel". Remember, you are unlikely to get close to the tyre's limits, and I doubt you would have time to notice if you were.
~~~o0o~~~

THE FAN - You have the choice of axial-flow fans (like an aeroplane propellor, or a domestic fan, or on the Chaparral 2J, or the (two-stage) Abrams-tank fan linked to by Rob on page 1) or radial-flow fans (like the compressor on a "turbo", or many industrial units). Either of these can be compounded (put in series) for greater pressure difference, and some fans are partly axial, partly radial-flow.

Briefly, I suggest a radial-flow fan for the following reasons. Generally, for pumping a given amount of air, axial-flow fans are smaller and more energy efficient (ie. less input power required). BUT (!!!), once the airflow is reduced past a certain limit (say, your "skirts" start to seal really well), the fan "stalls" and the pressure difference disappears! Also, the blades must be accurately "aerofoil" shaped, and the incoming airflow must be "straightened" for good operation (ie. to prevent stalling).

On the other hand, radial-flow fans are "STALL-PROOF". They operate by "centrifugal force", not by subtle "aerodynamic" means. As a result they can be made very simply, such as from fabricated sheet metal, like the majority of industrial "blowers". Even with very simple design (eg. just radial, sheet-steel vanes inside a sheet-steel, snail-shaped housing) they still have reasonable efficiency, and generally higher pressure difference capability than the axial-flow fans.

As confirmation of above, I have just conducted an extensive series of tests (err, while the jug boiled for my next cup of coffee...). My oldish domestic vacuum cleaner has a small radial flow fan in it (not sure which way the blades are curved, probably radial or backward). It has a hose nozzle of ~34 mm diameter, so about 9 sq.cm area. The suction from this vacuum-cleaner comfortably picked up a plastic drink bottle with ~1 litre water in it.

So the "stalled" suction is about 1 kg/9 sq.cm, or 0.11 bar, or 11 kpa, or 1+ ton per square meter. Keep in mind that a column of air 1 metre square at its base, and reaching from ground level up to the edge of space, weighs about 10 tons (or "atmospheric pressure" = 10 tons/sq.m). I put a few 1+mm thick toothpicks between the suction nozzle and the drink bottle, to simulate skirts that are "off-the-ground", and I could still comfortably pick up 0.5 litres of water. So still ~0.5 ton/sq.m lift (or downforce).

Google radial-flow fans and you should see that much higher pressure differences are possible with commercially available, off-the-shelf, relatively inexpensive fans. 3 tons per square metre (0.3 x atmospheric pressure) should be a feasible...
~~~o0o~~~

THE VACUUM-BOX - From the above, a "vacuum-box" of about 1 to 2 sq.m plan-area should suffice. This can have "skirts" that are either held just off the ground, or else allowed to lightly touch the ground and gradually wear away (say, made of something plasticky, like polycarbonate, UHMW-HDPE, or even plywood). For some comformability of the four wheels and the vac-box to uneven ground, and also for a smoother ride for the driver, I reckon it would be best to mount the vac-box "unsprung".

The easiest way I can think of to do this would be by using beam-axles front and rear (http://www.fsae.com/forums/showthread.php?1324-Beam-Axles-Front-Rear-or-both./page3). Longitudinal beams running down each side of the car (just inboard of the wheels) would hang off the beam-axles and form the side-skirts (possibly with flexible plastic extensions, like "side-splitters", for better sealing). Similar cross-beams at front and rear would seal the ends of the vac-box. The front might best be V-shaped (like a ship's bow) to help sweep away gravel, etc. In fact, "broom-like" skirts at the front might be a good idea.

The whole, roughly rectangular, vac-box should be allowed to twist to give more equal wheel loads on uneven ground (ie. a soft "twist-mode"). The several tons of down load on the roof of the box will help this, and should also be considered when building said roof! The rest of the car, namely chassis, engine, driver, etc., can now be softly sprung above the vac-box. So at least you get some vertical ride comfort, if not horizontal.

This whole arrangement is very similar to UWA's 2012 car, just with the active-aero added. Oh, and another couple of hundred kilos of structure, to keep it all together...

Enough for now...

Z

[Xfsae]

Z thank you for your post :) i will need some time to go through the info you mention in detail

just 2 quick questions the 1-3 ton of downforce you start your line of thought with is based on a fan(s) of what kind of diameter and what type? Also how much power you allocate to running the fan? unlimited ( 45 hp you mention somewhere else in the sucker car thread i think is not acceptable for an FSAE type car - it is like having a second engine equal to the main one for driving fans ,packaging & space available, weight and weight distribution - i think the vehicle gets heavier larger etc )

The Cornell car fans were driven by the engine with clutch like the BT46B?

[exFSAE]

http://cdn.meme.li/i/pcjtc.jpg

Consider the extra weight of everything. Consider packaging compromises. Consider that FSAE cars don't put out much mechanical power to begin with, and that current draw can be a challenge to manage when only using fans to help cooling.

To respond to an earlier post, I really don't think I'm nitpicking here, nor trying to be negative. I'm trying to be pragmatic and consider all the alternatives and consequences at an early stage here. That's IMO is good engineering. Now I will be blunt, and say that going into the design process with a singular vision of, "I need to have this gizmo on the car" before laying any of the high-level groundwork is.. poor engineering at best, and dangerous at worst. As I believe I mentioned earlier, that trap is not uncommon at the FSAE level and is a bit of a tough habit to break (I used to do it myself).

Back to a positive tone. If you want to make a "tech demo" car of sorts outside of any legal SCCA class and this concept has really grabbed your attention - then hey, go for it and CAD it up. But if you want to make a good racecar I think it really warrants taking a clean slate and more objective approach.

[Xfsae]

exFSAE - thanks again for your post. At this point the final product in my mind is a show/demo car that can be used in autocross events maybe in breaks between classes - car or other xtreme sports type events shows etc. But as i said this is not my primary concern now, i just want to enjoy the thinking , analysis and design process and learn something in the way now in company with the big community here

It is more about opening the door of exploring something that there is no point in researching normally because it is banned- that doesnt mean though that it is not interesting from an engineering point of view (please dont start me up with it is poor engineering etc...we all know it works in several type of vehicles - in the sucker car thread you see it worked in everything from karts to autocross monstrous prototypes - the physics are there - specifying the power you want to allocate to powering the suction system is key)

As i said also , for safety reasons i might do a model car first or even a kart next before arriving to the fsae type so that i can check things like failure of sealing etc even Deltawing or Lunar Rover followed that route-its good , exciting and good engineering practice in my opinion.Yes a lot of things can be simulated but it is only when you do a practical experiment like Z did with the vacuum cleaner and the bottle that the forces involved hit you

CAD work i do slowly because i have a normal work too. I post here because i believe FSAE community is a great think tank of talented and enthusiastic people (yes you need enthusiasm it is not bad) and more people can get involved and contribute in several levels (they might do their own version too - the sucker car club :))

[exFSAE]

please dont start me up with it is poor engineering etc...we all know it works in several type of vehicles

Well to be fair, the "Oh c'mon everybody knows 'X'..." is poor engineering. It's hand-waving. Alternative approach here is that if it's such a no-brainer huge performance gain which easily trumps all the challenges that come along with it, then it should be quite easy to demonstrate that objectively - no? The reason I make such a sticking point of this is because the hand-waving stuff is so prevalent in FSAE kids and young engineers, and it has no place in industry. Big pet peeve of mine, and a hard habit to break.

On a related note, enthusiasm can be great. Can be. I'd rather bring someone into a team that's over-enthused than under-enthused. But there is such a thing as too much enthusiasm. That drive has to be reeled in a bit by pragmatism. Again I find it prevalent among FSAE kids being so excited or interested in something that they just dive in before asking... (a) is this the most effective use of my time? (b) is this the best approach? (c) have I exhausted all alternatives? etc. When that comes into the professional workplace it can be "negative value added."

With all that said, this is your baby and if it's just your spare time and money - go for it in any manner you please. Probably worth thinking about making a little website or blog about it, some place to organize some thoughts and materials. I'm just taking the opportunity to point out that some of this drives ya nuts at the professional level, and I wish more students coming out of FSAE didn't have bad habits!

[Xfsae]

exFSAE you are just the fuel a grown man wants to spend time in the garage/shop after work...Keep it coming the you cant do it its a waste of time, grow up etc works for me well (i bet deep inside you is the 21 yo buried that wants to build the baddest FSAE car out there)

I keep the blog idea though but at the moment want i want is more interaction with others, brainstorming ideas ,examining concepts etc

[Xfsae]

9090909090

[Xfsae]

90

Cornell 90 sucker car let's reverse engineer it

Suction area doesn't seem to extend under the engine
Hope i had these image clean up skills to have a better look
What do you guys and gals make out of it?

Also how does the suction work?I see no fans?
cant see fans parallel to the floor or ducting leading to a remotely placed fan .Car seems pretty clean as a design
BUT i see this aluminium hose leading to the floor ( look low in front of the rear wheel) .Could it be there are 2 vacuum hoses one on each side leading to the vacuum box in the floor?
Could a standard vacuum pump or 2 as used for refrigeration work ? I mean for a Cornell type solution and not an all out sucker car as Z described above (like for a first attempt attempting to get the benefits of big wings seen today in FSAE from the sucker layout - first target- max 1 ton of downforce and no more than 2.5-3 gs lateral )

Is this Cornell guy blipping the throttle or flipping a switch to feel the downforce in a standstill just before the start of the skidpad?

[Z]

On the issue of "how much performance gain is possible from active-aero (= "vac-trac")?", there is no doubt at all in my mind. There is, in fact, SO MUCH PERFORMANCE GAIN possible, that everyone and anyone that has ever come near it finds it quite terrifying.

It is instructive to read the history of the Chaparral 2J. Very briefly, (IIRC) Jim Hall was recovering from a major racing accident during the 2J's development, so he couldn't provide much motivation. For the most part, only one engineer and one mechanic worked on it, because the rest of the Chaparral team hated it (it was the unloved, ugly-duckling, "skid-pad racer"). It was entered in four races, but never won any, because of various little teething problems and its lack of resources (only the two guys...).

But its performance in the four races was enough to terrify the whole of the supposedly "anything goes, let's really push the envelope" Can-Am field. It was banned by unanimous pressure from the whole field, except for Hall himself. In fact, the banning of the 2J was the tipping point that pushed Hall out of motorsport.

Personally, I see the "fear" of vac-trac coming mainly from its apparent open-endedness. Where is the limit? Is there a limit? Is the limit when the driver's body, or what is left of it, has to be drained out of the bottom of the cockpit via a sump-plug? Essentially, IMO, the potential limit of vac-trac cars is so far in excess of what any driver could take (or would enjoy taking!) that very few people want to play with this particular toy.

Because of above, I reckon the future of vac-trac is limited to demonstration type events. And because the potential horizontal G forces are so hard on drivers, the demo-cars may have to be remotely controlled (ie. "play-station" drivers, tele-connected to the cars).

However, here is an easy-on-the-driver alternative. During a break in the 800+ hp "real racecar" racing, a small, lightweight, perhaps go-kart sized, car is rolled out in front of the grandstands. Four men grab a corner each and pick the car up to prove that it really is lightweight. A stout rope is tied to the rear of said small car, with the other end tied to the rear of one of the real racecars (V8 Supercar, Nascar, F1, whatever...). The two cars start their engines, and ... engage in a tug-of-war.

The real racecar has smoke billowing from its rear tyres, but it is going backward! The little vac-trac car has no tyre spin, and only a small engine, but, in an appropriately low gear, pulls the screaming monster the length of the main straight...

Well, I would find that entertaining! :)
~~~~o0o~~~~

Xfsae,

Regarding the "45 hp" engine to power the fan, that figure was for the Chaparral 2J, which had a 700+ hp (?) main engine (8-litre V8), and lots of skirt length. For an FSAE-sized project I figure you would need less than ~10 hp. The power requirement is directly proportional to the amount of skirt leakage, and the less of this, the less power you need.

I suggest the following development path. Take four 1 metre lengths of 50 mm x 100 mm (2" by 4") pinewood, and join as a 1 m square "picture frame" placed on a level concrete floor (frame is 100mm high). Take a 1 metre square by 10 mm thick sheet of plywood, and glue&screw to the top of the frame. Drill a ~30 mm hole in the plywood, duct-tape your mum's vacuum-cleaner to the hole, and switch on.

This vac-box will quite obviously have downforce. Seal the floor-to-box edges with some duct-tape (or plasticine, blue-tack, etc.), and the downforce will be more obvious (the plywood roof will bow further downward). So time for some measurements.

Screw some more lengths of pinewood to the top of the box so that they extend outwards and can be placed on top of some weight scales (bathroom, or professional racing ones...). Try different levels of skirt clearance (1mm - 10mm), and also with the skirt-gap completely sealed with a flexible membrane (eg. duct-tape). Based on my earlier tests your fully sealed vac-box + vacuum-cleaner should have about a ton of downforce, but less when there is significant leakage.

So time to get a bigger fan! My quick googling (for ~ "industrial high pressure blowers") found a smallish, moulded plastic, centrifugal fan, about 0.3 m outside-diameter and about 0.1 m in axial length. The pressure (P) vs flow-rate (Q) diagram for this fan had a smoothish curve from P = 20 kPa (= 2 tons/sq.m) at Q = 0.056 m^3/s (almost stalled), to P = 10 kPa (= 1 t/sq.m) at Q = 0.25 m^3/s (near max flow). Pressure actually dropped slightly at stall, and these numbers are for "suction".

Power required is simply = P x Q x Efficiency, so given that efficiency (while not stalled!) is usually above 50%, the above fan can be powered with a ~5 kW motor (as was claimed). So get a ~10 hp industrial IC engine (Chinese copies of Hondas about ~$500 here in Oz), or borrow from your lawn-mower, and arrange appropriate belt drives, suction-ducts to the vac-box, etc., then re-test for downforce vs skirt gap...

Depending on skirt arrangement (which could be a lot more development, though fairly easy if done as above), I reckon the above vac-box should easily see ~1 ton of downforce. Or maybe 2 ton? Anyhow, bolt under your FSAE-sized car, and enjoy! :)

Z

[Xfsae]

@ RJWoods77 reading through the cheaparral website now - thank you for the link
They used a fan from M1 tank ...It is one of the intake fans at the back of the tank

i was trying to see if there was a specific reason for them choosing this fan-12000cfm at 70 pounds weight must be the reason ... It is a 2 stage vane axial design in alloy91

Some useful info from the corvette "cheaparral" fan car project:


"Extensive calculations and testing with a proto sled identified the ammount of airflow and vacuum required to stick the car to the ground with 1500 pounds of force (6675 Newton or like 680kg extra) ,the ammount necessary to boost the Vette's rating from 0.9 to 1.4g lateral "

I wonder how they found that they had to add this ammount of downforce to get this specific increase in lateral acceleration given that the ultra low budget of the project should not allow them to buy tire data - not even new tires- Did they start loading the car with weight and driving around with an accelerometer ? Any idea or suggestion?

The Abraams intake blower seems to be about 0.4 m in diameter and 0.4m in height judging from the photos of it placed in the Vette and on the tank and was powered by a 33hp snowmobile engine in the cheaparral project

95 (placed in the codriver seat place on the Vette)

At 10 inches of water operational vacuum (that is -14.33 psig relative to atmosphere ) they got 1000 pounds (454kg) of downforce
They measured it with a vacuum meter on the car - not sure how they were deriving the downforce amount

To maintain traction throughout the suspension travel a 2 piece skirt frame connected by bellows was developed and installed under the car - shopping trolley type of caster wheels attached to the lower frame would maintain a 1/2 inch gap between the skirt and the ground

96 (skirt- trolley wheel photo)

Z- as always interesting , informative (and entertaining in a cool manner in this case!) post .Thank you

I was looking in the internet for performance curves ,dimensions and weights for plastic industrial high pressure blowers of the dimension you suggested

http://www.mkplastics.com/documents/...MARCH_2008.pdf
http://www.plasticair.com/dat/files/28.pdf

is what i come up with but they dont seem very promising (in terms of space, weight and power required) ,in comparison to the cheaparal project figures i found and mention above

can you post a link to the high pressure blower you found?

97

FSAE fan car boy-George Bowland's leaf blower vacuum assisted B8R A-Mod

leaf blower said to have been running WOT

Skirts seem to be of polyethylene or nylon variety -someone mentions them as rubber skirts but i dont know they dont seem like that to me- they seem to be actually touching the ground on the sides - i don't know how they seal front and rear and in the corners

[rjwoods77]

XFSAE,

Forum etiquette would have you edit a post with addendum vs creating a new post for each addendum. You can do this by clicking on the "Edit Post" button. Consolidating your thoughts before you write as well as combining topical information will keep us all from getting an eye beating.

[Francis Gagné]

XFsae,

First, I have the same comment as rjwoods. Please do not double post, it is not a higly regarder behavior on any forum that I know of. (Plus this thread really ought to be in the Off-Topic section)


Back on topic,

I have not read much about the Cheaparral, so I have not verified your input values. But if we do the maths :

10 inH2O = 0.36 Psi = 2500 N/m^2 (not 14.33 Pascal as you have mentionned) so nowhere near full vaccuum!

They say it pulled 1000 lbf, so about 4500 N. Assuming constant vaccuum under the skirt, we would arrive at 4500 N / 2500 N/m^2 = 1.8 m^2 of skirt area. Which is plausible.

Using the 680kg supplied in your quote, to obtain the 0.9g to 1.4g they would have made their calculations using a curb weight of 1225kg. The C4 Corvette curb weight (from Wikiped) is 1469 Kg, so again plausible. assuming the stock car goes 0.9g around a skidpad (the 0.9g as probably been measured on a skidpad with a clockwatch). Assuming all else equal, adding 460kg of downforce would results in increas of (1225 + 460) / 1225 = 1.38x This means the car would now pull 0.9g x 1.38 = 1.25g

[Xfsae]

@ Francis - Hello and thanks for stopping by to contribute :)

= As far as i can see, i never double posted (posted the same thing 2 times)

about this being in the off topic thread - i dont see it being a huge deal (given the number of views we are getting here i dont think people care that much) but anyway if the administrator here say it should be moved , they can do it (at least in other forums i have been to they seem to be moving threads without too much fuss all the time)

http://www.cheaparral.com/
(in the first page and the links to the documents in the right are all the info)

Never mentioned pascals anywhere
I headed here - in the second category that is about vacuum:

http://www.convertunits.com/from/inch+of+water+column/to/psig

There seems to be a general confusion regarding psi vs psig but this convertunits.com seems right-according to them 10inches of water column (relative to vacuum)= -14.33 psig (relative to atmosphere)

Also i am aware that 10inH20 =0.36 psi if you are measuring differential pressure, such as the difference in psi between two points. It also gives the correct answer for absolute pressure, assuming you are measuring psia, which is the pressure relative to absolute zero vacuum.

But if you are measuring relative to vacuum and want to resolve the pressure relative to the atmosphere, then:

10inches of water column (relative to vacuum)= -14.33 psig (relative to atmosphere)

so i dont agree with you Francis that they were nowhere near full vacuum ( full vacuum= -14.7psi)

again you can use this for less confusion:

http://www.convertunits.com/from/inch+of+water+column/to/psig

Anyway , in relative difference terms your calculation still stands - pressure difference they achieved=10 inH2O = 0.36 Psi = 2500 N/m^2

Given the mentioned 1000 lbf (or apx 4500 N) of downforce achieved & assuming constant vaccuum under the skirt, we would arrive at 4500 N / 2500 N/m^2 = 1.8 m^2 of skirt area. Which i agree is plausible and the ideal skirt dimensions for this type of pressure difference would be those IF there are no leakages ( vacuum box sealing perfectly to the floor)

In fact i found today this spreadsheet in the 3rd pdf in their website GRM-3 :98

From which i see :

target = 1000 lbf of downforce
Skirt size = 48in x 115in =5520 sq in (or 3.48m2)
By leakage under skirt they mean the distance from skirt to road =0.5in (as i mentioned above)
Calculate pressure required to achieve the target downforce
= 1000lbf/5520sqin= 0.1812 psi (5in H20)
(no consideration about leakages)
In another article as i wrote above they mentioned that they actually needed DOUBLE that ammount of pressure difference (10in H20) to achieve that ammount of downforce in practice
i guess this is due to leakages mainly




@ rjwoods77 - the thing that i can make out from your comment is that i should try to put in the same post the info of mine?there is some time difference between my posts always ( i dont deliberately break up my posts ) - someone else might post in the meanwhile and he/she might not have noted i changed something above - i personally prefer to see posts separate (as done in a messenger twitter type situ) so that i know what was posted and when in retrospect

I moved some consecutive posts to the same one- hope its better for you (dont know why most of the links to my pics were not working though after some hours-initially i checked them and they were all working) - for me it was not visibly disturbing to see the small blue bar between posts but i did for you guys and gals to enjoy it :) Hope its more pleasant for you now :)

Also excuse me for my ignorance but if i edit a post by adding info ,how does a third party reader know ? how is he/she notified ?- i cant believe they will easily remember the last edited date after some weeks). Also does editing a post make the thread surface ? ( i mean show i some way that there is activity in this thread comparable to making a new post so that people understand there is movement there?)

For example if i want to ask a question at the end of a post that i have done before that did not include a question , how is somebody else to know if i consolidate it with the older post if he/she is not just regularly opening the thread lets say out of their own curiosity? Thanks

[Z]

Xfsae,

As exFSAE said a few pages ago, your posts do come across as being "way too scattered". And as Rob said, you ARE repeating yourself. Quite often. To keep people interested in the subject it would help if you prepared your thoughts a bit more clearly before posting them.

Francis's numbers above are in the ballpark. I guess the Cheaparral crew managed to lower the mass of the car a bit more (just stripping out stuff), and hence got a somewhat higher predicted cornering G for their 1000 lbs vac-downforce.

Based on the link to Cheaparral's "Estimated fan requirements" (some funny/wrong numbers there!) I would strongly suggest ditching imperial units (inches, pounds, psi, etc.), and stick to the following units, mainly because these numbers are so easy to remember and work with.

1 x "atmospheric pressure" is (approximately) equal to;
1 bar,
1,000 milli-bar,
1 kilogram (taken as a force!) per square centimetre,
10 Newtons per square centimetre,
(1 ton per square foot <- ok, this imperial unit is easy...),
10 tons (force!) per square metre,
100 kilo-Newtons per square metre (100 kN/m^2),
100 kilo-Pascals (100 kPa),
10 metre high column of water (which, if cross section = 1 metre square, would weigh 10 tons).

So, from above, if you want 1 ton/sq.metre downforce, then you have to lower the atmospheric pressure under the car by 1/10th (= -10%), which means finding a fan that gives a pressure difference of ~40 inches of water (because 40 inches = ~1 metre = 1/10th of that 10 metre column of water).

Here is a fan that comfortably delivers this suction (and twice as much...) from a motor of less than 10 hp.

http://www.aircontrolindustries.com/userfiles/file/01._ep10a.pdf

Note that this is a "backward curved blades" radial-fan of aluminium construction (I can't find the plastic one?), and most of the quoted weight would be in the three-phase electric motor. Also, I would not trust their quoted RPMs (20,000 rpm!, I reckon they must get the tea-lady to prepare these data sheets!!! :().

Z

(PS. The image of the Cheaparral's vac-box appears quite a bit smaller than in some of the above calcs (eg. the "fan" ones). Hence, I guess, their move from 5" to 10" w.g. to get the required downforce.)

[Xfsae]

Hi Z and thanks for the comments i will go through your latest info as soon as i can .I agree 20000RPM from this set up doesnt seem likely - more like 1500 to 3000 RPM max... 20000RPM only if it was a super precisely balanced brushless RC ducted fan for giant scale RC aircraft

And people loosen up a bit please - i understand that you are tired of people asking "can you tell me how to design a suspension" or similar but you see that this i not the case here - i do my research, take the time to share it for all the people that might not have the time to go search for individual resources and I read through all the input i get here as carefully as i can despite this being not a thesis project for me or similar , so please people dont bite ... Want to make clear once again that i am doing this for fun - i dont have to write an essay or report here

I am running through the cheapparal numbers so would help me to check together the thought process behind them:

- I get a fan flow rate of 166,98 m3/s (or 170m3/sec as they mention in the spread sheet) through a combination of the Bernoulli and continuity equations if i use as suction area the sum of:

plan area of the skirt suction area (top view area of the vac box) =5520 sqin

plus the leakage area (the sum of the 4 rectangular pieces of gap formed between the bottom of the skirt and the road :

2 x 115 x 0.5 + 2 x 48 x 0.5= 163 sqin

Do you think that this is correct as strategy? What happens if for example we use skirts touching the road ? we just ignore the leakage area? - doesnt seem very correct to me ...What are your thoughts on that?

- Fan flow rate of 166,98 m3/s (or 170m3/sec as they mention in the spread sheet) is not equal to 10192cfm but 360209,6 cfm wherever i checked - i think that is a mistake on the spreadsheet - if there is something else happening let me know

- Bernoulli equation is used for incompressible flows mainly - do you think that it is good practice to use it here for the twin stage Abraams fan ? I mean if it was a slow rotating non ducted propeller/fan we can make those assumptions ...but here? I understand that there must be a simple method to get the ballpark cfm but is this the best?

I want to make a similar spreadsheet for baseline calculations so any input on my questions/remarks above is welcome

Z yes you are right - the dimensions of the vac box in the spreadsheet are definitely not the ones of it in the finished car .Judging from the comments on the spreadsheet they must have been going for the max possible area (as is reasonable to rip max benefits) but probably when they started designing in detail the (4) panhard rod type system that allows the skirt to move with the suspension travel they saw that the kinematics of it would be difficult to achieve in practice eg when the car is pitching a lot. The Corvette C4 wheelbase is 2440mm and in this99 side view the length of the box seems to be apx half the wheelbase (1200mm)

Also in this photo100 the width of the skirt seems to be about equal to the track width (1500mm). This results in a 1.8 sqmt area ,apx half that mentioned on the spreadsheet (3.5 m2) and due to the linearity of the equations above , must need twice the pressure to give the same downforce

[exFSAE]

I still feel like this would be best served in a different sub forum, or better yet in a blog.

Still feels like this is very much "stream of consciousness" style or scattered writing.

[Xfsae]

@ Z - looking at the last unit you posted above , if the 7.5 kw motor is anything like that:
http://www.motorcontrolwarehouse.co.uk/ie2-7-5kw-three-phase-aluminium-motor/prod_1213.html

it would weight around 40 kg (out of the 70kg of the complete unit)

if the motor is replaced with an industrial type diesel engine like that:

http://www.amazon.com/10-Air-Cooled-Diesel-Engine/dp/B008FQU6T6

which weights approximately 50 kg (shipping weight is mentioned as 120 pounds)
and add sealed floor, skirts, ducts and other arrangements we get to more than 100kg for the suction system ( 1/10th of the required downforce is just found and it works from a standstill :))

There is the option of connecting to a smaller engine but what about its efficiency/performance?
I am looking at the curves on the unit Z posted above

I guess the 2 separate diagrams are for the vacuum exit/entry103 and the pressure exit/entry104 of the unit .
Typical textbook information suggest that a centrifugal fan performance curve should look something like this :102

Looking at Zs attached document i can identify the static pressure curves in bold black but not the horsepower curve(s) - we need at least one Static Pressure curve and on Horse power curve (for the same rpm) from which we can determine power requirements and efficiency at our decided operating point. What are inclined dotted lines representing ?

ATM looking for a good book/source on radial fan design as there are several design variations of them but to be honest i am in deep thoughts about why 2J, BT46B & Cheapparal all went for axial flow fans.Could it be that in practical terms the seal of the vacuum box is never close to perfect so you have to move a large ammount of air/min (CFM rating high) to achieve the pressure difference needed ,something that could possibly lead to a big and heavy centrifugal fan of conventional design?
I know that there are several types of centrifugal fan variations, but Z the CFM values mentioned in the diagrams of the unit you attached above are low compared to what i should look for if i follow the thought process of cheaparal (as i described and asked for people her to double check in my post in the previous page)

This106 is the vacuum test rig of the cheaparal project the size of the centrifugal fan & the 3phase motor (which seems pretty hefty -30hp?) they used to get the pressure difference necessary leads me to the same thoughts.The vacuum box on the test rig seems very close to the one used for the finished car

I am in thoughts also about what was happening to the BT46B at low speed corners(did they have so much downforce that they didnt care or the RPM variation was not big enough to vary downforce hugely?) ? The fan was connected directly to the gearbox main shaft via a single gearing stage , if there is no other trick the downforce must have varied widely according to the level of throttle. Also everywhere i read it they say that they used clutches to prevent over reving of the fan (and possible destruction due to centrifugal forces) during gear changes. What kind of clutch arrangement could that have been ? something directly opposite in principle with a centrifugal clutch? Or just a smaller conventional plate clutch also connected to the main clutch actuation ? What do you think?101There is also the gearing from the main gearbox shaft to the fan which was calculated by an expert alongside with fan design-given that this was not a variable pitch fan - this was optimised for a single operating point - i wonder how they defined this point...
Though i cant find official figures of the levels of downforce achieved , i did dig out that they had to change from 1000lbs to 3000lbs springs from BT46 to BT46B !!!! (same suspension design for both- no rockers or anything similar introduced between the versions , just conventional outboard springs and dampers at the back and the same pullrod setup in front).Ermanno Cuoghi, Lauda's chief mechanic said that they had to do that otherwise the car bottomed out to its belly when revving up with neutral!!!

What do you people think?

[Xfsae]

@Z I did your water bottle experiment with a shop vac cleaner (a big one) - could pick up up to 0.8 lt of bottle without any gap .Using tooth picks i found (apx 2mm thk - not 1mm like in your experiment i couldnt lift it and went for 0.5 lt which i could not lift either -settled for apx 0.35lt which i lifted

Upon close inspection there is something important that happens to the bottle as the vac hose approaches - they deform making the leakage smaller so the suction stronger (try to approach the hose slowly to the bottle and see the bottle deforming (at least my 0,4-0,5 thk bottle did that

So the gap between the road and the skirt seems to be a super important factor

Curiosity led me to ask a friend to look at the back of his vaccum former/thermoforming machine where he found a radial flow snail type fan - as Z proposes but in this application we have no gaps. Try leaving a thermoforming polycarbonate sheet to stay a bit more over the oven of a vac former, during the upcoming forming process it is possible that due the extra sag from the extra unwanted heating that we introduced , the sheet will become so thin that upon vacuum over the mold a small hole will open up somewhere and suddenly all pressure is lost and we possibly have to bin the part...

So why leave ANY gap if it drops the vacuum performance so much , why did Chapparal 2J as well as the Cheapparal Vette leave apx 12mm distance from skirt to road? (the 2J went to great length to keep the movable lexan skirts at constant gap with the road with a specially designed mechanism) , why not just leave 2-3mm and form the front part of the skirt V shape as a ship (or put a V shaped rubber sweeper in front of the skirt ) as Z commented so that the debris is not stuck in the minimal gap ?

The answer (in my opinion) is to let the actual engine of the vehicle propel the thing forward and not stay stationary!! Remember the road & track competition mentioned on the "Build my own sucker car " thread ? Someone mentioned there the same thing happening to a modified vacuum kart - had difficulty moving! I guess since he had no suspension or adjustable height skirt he actually had to play with the vac engine throttle or openings in the vacuum floor to get the thing moving ....

Try to put the vacuum hose of your vac cleaner on the ground and moving the vertical hose across the ground and then putting some distance and moving it - it was much easier for me

I guess when introducing a gap the benefits from the radial fan evaporate as you obviously have to draw much volume of air to create the needed pressure difference - i dont know obviously the characteristics of a compounded radial fan set up but the gut feeling i am getting is that there is significant airflow taking place in the case of the 2J and BT46 so that their axial fans are not stalled
What do you people think?

[Z]

Xfsae,

Your posting is still "way too scattered". You are unlikely to make progress by randomly plucking ideas and numbers out of thin air. You have to think a bit harder about these things. Here are some examples:

1. The "Bernoulli and continuity equations" are all but meaningless here. I know the Cheaparral crew used them, but copying stuff off the web is not a well-reasoned engineering approach.

2. A 50 kg, 10 hp industrial diesel is good enough for static testing, but rather expensive. On a directly linked webpage I saw a cheaper, ~30 kg, 16 hp petrol engine. For use in the actual racing car I am sure you could find an engine that is a lot lighter again, and possibly with even more horsepower.

3. A fan (any type) can have atmospheric pressure at its inlet, and positive (above atmospheric) pressure at its outlet. Or it can have a "vacuum" (below atmospheric pressure) at its inlet, and atmospheric pressure at its outlet. Or a bit of both. You should be looking at the second type of curves.

4. The "power lines" in my earlier fan link indicate the outer envelope (upper-right boundary) of Pressures and Flow-Rates that such a sized motor can deliver. Engineers are expected to be able to read such diagrams (even when such diagrams are drawn up by the tea-lady...)

5. As I have been suggesting since the 2005 thread, a VERY IMPORTANT part of this problem is SKIRT DEVELOPMENT. Specifically, it is beneficial to minimise skirt leakage, thus lowering power requirement, and increasing downforce. So, once again, (hopefully more clearly):

5a. If you MAKE THE DECISION that you want SKIRTS-OFF-THE-GROUND (for whatever reason, say, personal preference, or "no-scraping" Rules), then you will have a HIGH LEAKAGE FLOW-RATE, and consequently you will (most probably) need a LARGE DIAMETER AXIAL-FLOW FAN powered by a HIGH HORSEPOWER MOTOR.

5b. However, if you AIM TO MINIMISE SKIRT LEAKAGE, then you should be able to get MORE DOWNFORCE from a SMALLER RADIAL-FLOW FAN with a LOWER HORSEPOWER MOTOR.

6. Note that if you take option 5b, but then PUSH THE SKIRTS ONTO THE GROUND with the FULL VACUUM DOWNFORCE, then you are being REALLY STUPID (!!!) because, of course, you will find it very hard to move the car!

7. So look closely at the Bowland skirts for inspiration... Or, more plainly, the SKIRTS CAN SEAL QUITE WELL even if they ONLY TOUCH THE GROUND LIGHTLY! Or, even more plainly, DO NOT HAVE A RIGID CONNECTION BETWEEN THE SKIRTS AND THE ROOF OF THE VACUUM-BOX!!!

8. I can't do all of this for you...

Z

[Xfsae]

Z- you writing in capitals like shouting is not appreciated ,your thoughts come across as scattered too in many threads but i still do appreciate your contributions ,as a matter of fact i am looking closely at a Z....... suspension for a future project :) I thing it can be good for aero vehicles and i hope to see some implementations of it in the future

The "Bernoulli and continuity equations" are not meaningless if there is significant airflow (and at least in the case of the BT46B it is)- they are used in the initial design stage of all types of axial flows - been there done that before (aircraft props - non ducted) .Maybe i am coming across as a bit uncertain and wishing for participation of others because i am not a know all type of guy ... I like to hear what others want to say .I have never built a fan car before. I am honest

In any case if you don't use Bernoulli and continuity equations to find the CFM , how are you going to find the ballpark CFM of your system? Which other method there is? (You need static pressure and CFM to predict BHP required properly according to typical methodology as seen on the document attached below)

Regarding your point (4) - i am an engineer

(though i dont appreciate your comment - engineering is very broad and i could have been an engineer with no knowledge in fans - that doesnt mean someone cannot point me in the right direction without my status as an engineer being shaken - knowledge can be acquired , the willingness for knowledge is the hard thing to find)

and i think the horsepower curve being a straight line did not make sense and also its inclination and point of origin seem strange - i cannot make anything useful out of it . Also the vertical scale showing BHP ratings (which we need) is missing and instead of it we get another vertical scale of static pressure (we now have 3 when we need only one) - so we cant use the curves even if they are correct.

Another thing that seems to be missing totally also are system lines of which we need several according to the methodology i attach in the document below:

Just for clarity (and please correct me if i am wrong) i follow these steps

1) I start from the static pressure and CFM calculations (which i asked a second opinion-double check here because those data determine all the rest)
2) I need at least one static pressure curve (we get many for different rpms in Zs attached document: http://www.aircontrolindustries.com/userfiles/file/01._ep10a.pdf) with its corresponding horsepower curve (none provided)
3) I need several system lines that show how the blower performs inside systems of differing impendance which is also necessary (according to the methodology below-last pages) to predict the hp ratings for our system (none system line provided)


Methodology described Here: ftp://www.nyb.com/Letters/EL-03.pdf


Also i posted all my calculations and results for MY project for anyone - not only you - to check
The fact that i followed the guidelines of Cheapparal ( in any case if you read about them they are all engineers and they did contact the manufacturer of the fan so the whole process they followed is bound to have some ground) does not subtract anything from my effort to learn - we all have to start from somewhere


Your comment :" if you take option 5b, but then PUSH THE SKIRTS ONTO THE GROUND with the FULL VACUUM DOWNFORCE, then you are being REALLY STUPID (!!!) because, of course, you will find it very hard to move the car!" might refer to me trying to move the vacuum hose across the ground - remember the 1979-1980 ground effect cars ? they were using the harder springs they had to push the skirts to the ground -same thing - they just used Bernoulli effect to create the suction instead of the fan

My gut feeling is that there many ways to implement suction/active aero and looking at the pictures the BT46B team switched from one to another one in a matter of 1-2 days during the qualifying and race days of the Swedish GP 1978 ( they must have been living a nirvana of active aero explorations that weekend) .By the way the Swedish GP 78 is all in the youtube (German commentator) - funny to see all other team principals like Patrick Head running to take a look at the fan in any way they could after Lauda stopped the car :)

And one more engineering question (i think i posted several others good ones and well thought out that were unanswered): why are the final version of the BT46B side skirts bent inwards? ( they are the wrong way around if you are just looking to seal the floor)

There is a lot to be learned on the subject from anyone- it is not just redneck black and white stuff- i hope more people with access to specialized Uni /industry facilities like wind tunnels get involved - i will be happy to participate even as a tiny part of a movement of resurgence of active aero on ground vehicles... (how cool would be to take internal pressure and drag values for an active aero car model on a rolling road :))

As long as we live we are all students
(By the way what we are discussing here is also useful to people designing FSAE car cooling systems -air cooled single for example or choosing a bigger fan for the radiators of water cooled engines with minimal impact on aerodynamic drag and max efficiency)

[Z]

Xfsae,

1. My capitalised shouting is an attempt to get the message across, so that I do not have to keep repeating myself.

2. The "continuity equation" is applicable here in a limited, though very trivial, sense. The conventional "Bernoulli equation" (= static pressure inversely proportional to velocity squared) is most definitely NOT applicable. If you think it is, then please give a detailed example of such a calculation. More appropriate equations can be found somewhere on this Forum (or on the web, or in a library...), relating to "the restrictor" (which behaves like a small skirt gap).

3. Regarding fan horsepower, you are comparing documents from two different manufacturers and wondering why they look different. It is quite common for different companies to present their information in different formats! Anyway, I have already explained how to estimate the required horsepower (either from the curves, or from first principles), so I won't repeat myself here. Yet again.

4. The idea of "SLIDING" skirts is that they move up and down INDEPENDENTLY of the car body. This is so that they can be designed to push down against the ground quite lightly (say, with only a few kgs of force), even while the body itself is being pushed down with many tons of force. This is a VERY IMPORTANT CONCEPT to understand (hope I don't have to repeat it...).

Z