Properties of a Downforce Fan

[MIKEY_!]

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?

[Tozza Mazza]

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?

This is massively pointless. For any real effect you need a diffuser or full ground effect and the ride height must be minimal.

The most important design areas of a rally car are the 4WD system, an engine with a flat torque curve and SUSPENSION DESIGN. Aerodynamics are not too vital.

If you want to innovate with a rally car look to hybrid systems.

I do wonder if a fan above or below a wing in a serious downforce formula could change pressure.

[MIKEY_!]

Can't see this is pointless at all. We can run the car very close to the ground since the stages are on good public roads and we have sliding skirts lined up or special brushes if that is to hard to engineer. Have you never seen a BT46B. It sticks like glue, just as effective as the ground effect of the day.

See no diffuser.
Hybrid (battery electric) is too heavy and expensive we will use 4x4 of course, suspension can be taken from an existing road car or track car if needed. What we are looking to achieve is increased grip at all speeds. We do not seek to innovate just for the sake of it we just see this as the best way to get extra grip.

[Tozza Mazza]

Can't see this is pointless at all. We can run the car very close to the ground since the stages are on good public roads and we have sliding skirts lined up or special brushes if that is to hard to engineer. Have you never seen a BT46B. It sticks like glue, just as effective as the ground effect of the day.

See no diffuser.
Hybrid (battery electric) is too heavy and expensive we will use 4x4 of course, suspension can be taken from an existing road car or track car if needed. What we are looking to achieve is increased grip at all speeds. We do not seek to innovate just for the sake of it we just see this as the best way to get extra grip.

Your car is going to handle like a pig. The low ride heigt will kill it, and if you just take suspension from any old car, the handling will be.awful.

The fan doesn't just make downforce. The Brabham car made use of full ground effect, although primitive. Actually research aerodynamics before making a thread, there's a load of info already on the site and on google waiting to be found.

Is there a minimum ride height for a WRC car? Are you even sticking to any regulations? It is possible to have a 4WD electric or hybrid car. Why not design an ERS system which feeds back into the throttle pedal for more power?

If you see this as a way to 'increase grip at ALL speeds' do some reading. At the low speeds of a Rally car, downforce has very little effect.

[xpensive]

An interesting engineering xcersice indeed, when I currently work with surface-effect coastal corvettes in Norway.

http://www.umoe.com/subsidiary/umoe-mandal/

First of all, at such low pressures as this, 2-5 kPa perhaps, you can behold the air as incompressible, why the fan's power will always be flow times pressure differential to the atmosphere, which simplifies calculaton.

The pressure-differential over the fan will basically be downforce over pressure-xposed area, 10 kN (1 metric ton) over 2 sq meters means a pressure diff of 5 kPa (0.05 Bar), just as an xample.

The flow needed is more difficult to estimate, when that will largely depend on what ground-clearance you can maintain as this will effectively act as a restrictor, creating the under-pressure as the air flows in. A near zero clearance, as with sliding shirts all around, flow can be kept at a minimum as long as the fan can produce the needed pressure-differential to the atmosphere.

But if you imagine a 5 kPa dynamic pressure drop over the ground clearance, pd = Rho * air-speed^2 / 2 (simplified);

If Air-speed is 100 m/s, it gives the desired 5 kPa.

If the two sq. meter area has a circumferance of 6 meters and ground-clearance is 5 mm, flow will be 3 m^3 per second.

Conclusively: Fan output will be 5 kPa times 3 m^3/sec, which means 15 kW. At a 75% efficiency, fan motor should be 20 kW.

[thisisatest]

this was for a $2000 autocross car, but it may still be relevant. they used a snowmobile engine for the sucker fan.

www.cheaparral.com/files/GRM-2-Dist_Std.pdf

[MIKEY_!]

Thats really helpful x could you tell me roughly (cause even I don't know how effective the skirts will be) how much DF (in kg) this could produce or are there too many variables.

(You might have even said but my maths is not yet as advanced as yours)

[xpensive]

If you take my draft above from the top, I began with 1000 kg of downforce and a 2 square meter area, giving 0.05 Bar pressure, which in turn called for a 3 m^3 per second flow at a 0.5 cm gap-height and 6 meter gap-length.

You can play with these numbers at your leisure, they are simplifications of course, but they should be close enough.

[mep]

Sure it is possible to add a fan to the car. Well you could even mount a jet engine pushing the car into the ground but first think if it is reasonable for a Rally car. This Brabham car was operated on an old fashioned F1 track which had very long and sweepy corners with at least some run off area. Even so it got banned after 1 race and there is very good reason for it. Even compared to the old F1 tracks are Rally tracks very dangerous. They run trough narrow tracks trough forests or along cliffs with zero run off. If you put a fan on your car and let's say double the downforce then you will increase your cornering speed a lot. The car will be far above what the track is able to handle. It is a very good way to kill yourself.

[marcush.]

obviously you need something serious to propell that fan and stones and debris will be a serious topic to answer even on a tarmac stage ...As tarmac rallying is all about hooking the inside wheels into the edge of the tarmac ...so either you accept to drive more conservative orr the proble of stones shreeded or pinched by the fan is something to solve right in the original design (succing through a grid will seriously lower your fan performance -depending on distance from the fan blades).

I think you also need some possibility to reduce the level of downforce under certain circumstances lower vehiclespeeds and something to show you the downforce magnitude (a target rideheight vs actual rideheight comparison would be a great help).as with anything in the world more is not always better..

As for the fan efficiency - luckily all the fancy curving backwards and forwards sweeps of modern fanblades is not helping efficiency but is just for noise reduction purposes...hopefully your rallye project has no issues there.
electric fan drives are ruled out unless you are prepared to carry a trailer for the batteries /generator.

[MIKEY_!]

O right sorry thanks again x

Yes stones present a major problem although the stages are all good quality sealed roads. The fan will also blow across the radiators so some solution needs to be found in that area. I am thinking brush skirts will help reduce this problem. Very coarse wire brushes sandwiching a softer but very dense layer. Still struggling with my main worry that a fan blade might shatter when debris hits it. Bits could go into the fuel tank or driver compartment (scary though) any suggestions?

[marcush.]

sucking through the rads will allow you to minimize the radiatorsize considerably and aloow for closing of all openings in the upper surface ....I think the best idea is to place a deflector grid in front of the rads .Any stone getting succed up will hit the deflector and not reach the cooler matrix .As you rely on fan operation for air movement you don´t really need a "open" path to the matrix when you suck the air through the rads .Sealing all the components for efficiency will of course be a challenge .

[MIKEY_!]

Flexible fan blades or strong rigid ones? With a tough ring to protect the driver. Whats best do you guys think?

PS - So where the stones go after they hit this matrix. Any opening will result in considerable flow loss to the rads surely?

[Pierce89]

Can't see this is pointless at all. We can run the car very close to the ground since the stages are on good public roads and we have sliding skirts lined up or special brushes if that is to hard to engineer. Have you never seen a BT46B. It sticks like glue, just as effective as the ground effect of the day.

See no diffuser.
Hybrid (battery electric) is too heavy and expensive we will use 4x4 of course, suspension can be taken from an existing road car or track car if needed. What we are looking to achieve is increased grip at all speeds. We do not seek to innovate just for the sake of it we just see this as the best way to get extra grip.

Your car is going to handle like a pig. The low ride heigt will kill it, and if you just take suspension from any old car, the handling will be.awful.

The fan doesn't just make downforce. The Brabham car made use of full ground effect, although primitive. Actually research aerodynamics before making a thread, there's a load of info already on the site and on google waiting to be found.

Is there a minimum ride height for a WRC car? Are you even sticking to any regulations? It is possible to have a 4WD electric or hybrid car. Why not design an ERS system which feeds back into the throttle pedal for more power?

If you see this as a way to 'increase grip at ALL speeds' do some reading. At the low speeds of a Rally car, downforce has very little effect.

The Brabham fan car did not use full ground effect tunnels as is obvious from the picture. It's only source of underbody downforce is the fan. The fan would destroy the flow under a car ruining the ablity of tunnels to create large amount of DF. When flow comes under the front it would be sucked up by the fan instead of accelerating through the tunnel. The way you treated Mikey's idea is unconscionable on a site about ideas just like his.

Edit: Not that wiki is the best source but here you go :"It had not been clear to other designers just what Wright and Chapman had done with the Type 78, but by early 1978, Gordon Murray had grasped how the Lotus design was achieving its remarkable levels of grip. He also realised that the Alfa Romeo flat-12 engine used by Brabham that season was too wide to permit the venturi tunnels needed for really significant ground effect.[12] In the meantime, Murray's idea was to use another way of reducing the pressure underneath the car."

[Tozza Mazza]

Sorry, from a page I looked at it suggested the car had underbody aerodynamics (showed a large diffuser cut away).

Obviously this was wrong, I do apologise.

How 'good quality' are these roads.

You really need to have a good think.

What is the elevation difference, 5mm or 100mm between the tyres.

How fast are these roads, tight and twisty, or mostly straight.

If it's rally I'd be inclined to say the former.

Downforce will always vary with speed.

You need to work out if the fan will work, if it's possible to use, and how to use it.

If you are going to use it to cool radiators also, surely the aerodnyamics will be compromised (downforce from the fan).