Properties of a Downforce Fan

[hollus]

Tozza, downforce based on the speed at which air is moving under the floor (i.e. based on Bernoulli), does depend on the speed, obviously. So does downforce from wings.
But downforce coming from forced extraction of air from under the floor does not have to depend on speed. With a fan you can achieve a constant pressure differential acting on a constant amount of surface. It is like a hovercraft working with the inverse principle. If the sealing is perfect (which it never is), it won't change with speed.

So, if skirts for this rally car are allowed, I am wondering what would be a better way of sealing the floor, considering that the car will see wild changed in ride height with bumps and lateral inclinations in the asphalt of a normal road. Consider as well that the ride height will be rather large, compared to F1. Would sturdy brushes of movable skirts be up to the task, or could it be better to use something more fluffy like a thick field of a feather-like material and change it after each stage?

Edit: I am a bit with other people here in thinking that floor based downforce could be a dangerous thing in a rally car.

[MIKEY_!]

No doubt about dangerous. Higher cornering speeds with next to no run off. But more grip also = more safety if you don't drive as close to the limit.

I think we should stop thinking of this as a rally car as that implies gravel and big air which will not be the case.

Anyway I did mention using a 'fluffy' soft but dense skirt but with coarse brush material around it to 'clean' the road of debris to some extent and to help it maintain shape. The brushes or skirt will have to be quite long as it will be difficult to get the suspension rigid enough and because of the road surface itself.


Roads no worse than this although sometimes a little frayed around the edges. Don't worry we may never get past the design stage.

[thisisatest]

did you check out the Cheaparral? it used simple casters to keep its vacuum skirt off the ground. their fan was a cooling fan from an army surplus tank. i'm not saying it directly applies to your situation, theirs was for autocross so they knew they would be in flat parking lots and at relatively low speeds, but i'd never seen anything like it before.

http://www.cheaparral.com/

[MIKEY_!]

Yes good stuff cheers.

[hollus]

My worry is about corners just like one you posted above (the one where the car is carefully avoiding the apex). It is not about handling the higher downforce, but about dealing with sudden and random losses if it.
In the picture above, if a car were to just clip that apex, the asphalt has a large drop in the vertical direction. Skirts or not, going through that at high speed, you are going to lose a lot of the vacuum under the car and hence lose downforce.
So in one pass, you miss the apex and downforce is just fine, in the next pass, you clip the apex, and all of a sudden you lost your downforce when you need it the most. It is this type of sudden change of behaviour that makes floor based downforce dangerous. It makes it dangerous for F1, where the cars pass over and over again over the same bits of track and the drivers know every kerb and every bump. It was banned for that reason, not to limit the total downforce which could also have been achieved by removing the wings.
It would only be worse for a car in real roads with no escape areas.

[MIKEY_!]

You are exactly right and upon hitting the apex the gravel around those edges will get taken up by the fan. Possibly damaging the rads, fan blades and other components. This is why i came here and asked this question. Cause you can get better results when you get people who are smarter than yourself to help. Is there any way around these problems (other than driving very carefully of course).

[hollus]

You'll take a lot of air in together with that gravel...

Just another thought, for that floor to work, you'll need to seal all 4 sides of it. And with the ride height of a rally car, that is a lot of extra frontal area in your car... you know what that means.

[xxChrisxx]

Firstly, cars aren't designed like this.

You don't latch onto an idea, or something you've seen then try to make it work no matter what, especially when you clearly don't know why it works. Start by outlining:

a: the regulations.
b: what you want from a car.

I can tell you now, you don't want downforce on your car. Because downforce adds.... drag! The fan car worked becuase it exploits the sealed venturi tunnels and sealed underbody. It also worked becuase those cars had upwards of 600 BHP to accelerate. This is ignoring that fact that these cars basically has no suspension, they were sat on springs that didn't really compress at all.

If you are talking about taking suspension from a road car, one can only assume that you will be taking an engine too.

1: you'll add weight and you'll need to power the fan. Leaving you with 1 of the 5 BHP a road engine produces for acceleration.
2: You can't seal a tin top's underbody effectively unless it was designed for it.
3: In the unlikely event you did produce some meaningful downforce, it would just compress your springs and not push the tyres into the road.
4: In the miracle event you produces F1 levels of downforce, your springs and dampers would break.

IF you want to look at stuff that works:
A: reduce weight.
B: reduce drag.
C: Improve roadholding.
D: Improve power output.


Don't get sucked in by tricks and gimmicks and ---. Look at what you want to achieve, the methods for achieving it. There is not a one size fits all solution.

[marcush.]

the idea is good and it is entirely feasible.
first you need to know what level of downforce you are looking at to being able to exploit ..depending on propelling power of the car ,tyres available and charecteristics of the tracks you are navigating.

It´s a bit like Monza all are telling you it´s all about top speed and then the slowest car on the straights wins by 15 seconds...
As I said before you need a possibility to throttle downforce having a bit more than necessary will keep you in a position you can add downforce all the time you need it but you can reduce it to move quicker on the straights .Of course this implies a little headache in terms of cooling you might need some sort of auxilliary duct or live with maximum cooling only combined with maximum downforce...(could be not the worst of ideas ,depending on the length of the straights and the time you spend braking and caornering ).
I think the way how you suck the air from the bottom is going to decide if you pick up stones and debris .You could well build something like a maze sucking air horizontally or vertically up and down again to separate the big stones or widen the area of suction facing the ground enough top not end with a vaccum cleaner ,but having a slow entry speed and gently converging duct towards the fan.
The sealing is just an efficiency thing .You got the skirts to seal the outer perimeter of the thing and from there you need to find out just where your pressure drop is bleeding off . so either you increase the size of the fan and power or work on the seals .A first step could be to mount a flat sandwich panel to the floor ,creating a sealable surface.These bords are readily available and if you don´t use alumimium /carbon sandwich but nomex paper board the cost are low .
I would look deeply in f1 skirt technology to sel the stuff to the ground .The brush thing never really worked with ´wing cars .A crude ways of sealing was teams using strips of lexan attached to the outer perimeter these strips wore off during the race so the downforce created reduced over usage.Good enoughh for a first step.
Another possibility would be a frame hinged to the flat bottom like a paralellogramme and just clearing the ground ..The gap of the frome to the bottom could be sealed by some inflated rubber hose keeping it in tension just enough to keep it from getting kicked up to0 much .

[mep]

No doubt about dangerous. Higher cornering speeds with next to no run off. But more grip also = more safety if you don't drive as close to the limit.

The speed of a rally car without any downforce at all is already dangerous enough. Not just because you might run out of grip but because it demands ultra high reactions from the driver especially on road tracks. What is the sense in putting a fan on a car and then drive around slower than the non fan cars?

I would go the classic way:
Build a small car, light car, with low CoG, good suspension. Then make the car 100% reliable. Get out and drive it and build up high confidence with it.
Confidence in the car is very important for this you must be very sure how the car reacts in every given situation.

[mx_tifoso]

Safety worries aside... could anyone help him out further with what he asked for? It's interesting and I'd like to see what comes of it even if it's not practical.

[xpensive]

Safety worries aside... could anyone help him out with what he asked for? It's interesting and I'd like to see what comes of it even if it's not practical.

I tried to do that yesterday esteemed mod, if you just bother to scroll back a bit.

[marcush.]

what is your problem with safety?
-Motorsports is dangerous !- printed on every ticket you buy.people get killed driving bicycles,and fans can be used to produce downforce.
I see no difference between some guy fiddling around with brakes or suspension or cars to be driven íntentionally at the limit of adhesion and adding downforce by wings , fans or difussers.
Not better or worse than glueing up your steering column in my book.

Let the guy have a little fun.Still he needs a serious fan drive to produce downforce ..

[HampusA]

Doing a project for a road rally car and I'm planning to use a fan like the Brabham BT46B. I want to use a separate engine (at a constant speed) for this but I have no idea how powerful it needs to be to produce about as much DF as an F1 car (the goal). Assuming I use the best fan blade angle possible.

Can anyone help me on this as it is a bit above me?

for the safety of the driver i would encourage you to go with giant wings instead.

[MIKEY_!]

OK in order:

hollus: Of course you seal all sides. It will mean 1200mm x height (never more than 100mm) extra frontal area.

chris:

you'll add weight and you'll need to power the fan. Leaving you with 1 of the 5 BHP a road engine produces for acceleration.

so your saying the fan will use up 80% of engine power? I must misunderstand you. We already have settled on 20kw (credit to x for that one). + fan car won't add much drag. used right there could be a small thrust benefit.

marcush: The chassis and bodywork is planned to be from scratch so sealing is only needed around the edges. I like the hinged layout you mention.

mep: I mean not at the limit of the new car of course. That way its safer (when controlled) and faster.

x: you were much help.

marcush: thank you. someone had to raise safety but this is getting out of hand. get out of bed in the morning and you take a risk. so long as i am not putting myself or others in ridiculous danger then i'm ok with that.