## Key calculations in the Aerodynamics of the BT46

Post here information about your own engineering projects, including but not limited to building your own car or designing a virtual car through CAD.
Smokes
3
Joined: Tue Mar 30, 2010 4:47 pm

### Re: Key calculations in the Aerodynamics of the BT46

try a skirted flat floor and suck the air out from behind. it may give enough surface area to generate some DF with out going to complex shapes.

alj
1
Joined: Wed May 23, 2012 10:14 pm

### Re: Key calculations in the Aerodynamics of the BT46

Well it looks likei should put wings on front and back and skirt the whole undertray and forget GE. Would that be most effective?

Greg Locock
159
Joined: Fri Jun 29, 2012 11:48 pm

### Re: Key calculations in the Aerodynamics of the BT46

A fan car doesn't need a complex underfloor shape, in fact the profile is irrelevant.

marcush.
268
Joined: Tue Mar 09, 2004 3:55 pm

### Re: Key calculations in the Aerodynamics of the BT46

not complex -but it gets complicated ....you need unobstructed flow to the fan to achieve maximum efficiency...

Greg Locock
159
Joined: Fri Jun 29, 2012 11:48 pm

### Re: Key calculations in the Aerodynamics of the BT46

Well I guess the crucial parameter there is the ratio of the average underfloor height to the skirt gap height, in my case we were looking at 120mm ground clearance for general use, and maybe 10 mm of gap, so the velocities under the floor were very small.

alj
1
Joined: Wed May 23, 2012 10:14 pm

### Re: Key calculations in the Aerodynamics of the BT46

Greg Locock wrote:Well I guess the crucial parameter there is the ratio of the average underfloor height to the skirt gap height, in my case we were looking at 120mm ground clearance for general use, and maybe 10 mm of gap, so the velocities under the floor were very small.

My ground clearance is 50mm, and I was hoping for a minimal gap utilising the 2J style skirt.

If we are analysing total downforce. Is it better for me to have GE side pods and fan controlled underfloor, or would there be higher negative lift by utilising the total floor pan for the fan?

Greg Locock
159
Joined: Fri Jun 29, 2012 11:48 pm

### Re: Key calculations in the Aerodynamics of the BT46

That could go either way depending on the specifics of your design and the circuit you are on. You could estimate the downforce vs speed, and power required vs speed for each alternative. I think Smokes idea of using a fan to assist a GE floor sucks completely as without front and rear skirts to reduce the flow rate the fan has to shift a lot of air.

alj
1
Joined: Wed May 23, 2012 10:14 pm

### Re: Key calculations in the Aerodynamics of the BT46

I only have 70hp. So utilising some of that for the fan is not feasible. I was thinking to use a Electronic Ducted Fan (EDF). I saw one that does 24kgs but it was a HUGE price. I have found one that does 7kh and is reasonable price. I was thinking to use two of them.

Specification:
- Rotor Diameter: 120mm
- Outer Diameter:125mm
- Weight: 894g
- Working Voltage:22.2V-45V 6S-12S
- Motor : Brushless Motor 5075 650kv
- Maximum Current: 150A
- Max Surge Watts: 6300W
-RPM: 14625
Efficiency (G/W): 1.14
- Recommend ESC: HV160A
- Thrust: 7kg

Tommy Cookers
464
Joined: Fri Feb 17, 2012 3:55 pm

### Re: Key calculations in the Aerodynamics of the BT46

how are you going to run these motors ?
unlike brushed motors there must be some unit(s) between these motors and your electricity supply
such units would also allow fine control but you don't really need control
each 160A ESC unit costs maybe \$400

alj
1
Joined: Wed May 23, 2012 10:14 pm

### Re: Key calculations in the Aerodynamics of the BT46

You are right no control needed over a simple on/off switch. I would most likely run a completely separate power supply and required electronics.

I can get secondhand complete edf’s for \$100 to test the theory but I’d rather have a better idea first.

Tommy Cookers
464
Joined: Fri Feb 17, 2012 3:55 pm

### Re: Key calculations in the Aerodynamics of the BT46

ok if you get secondhand ESCs to go with the EDFs that's a good thing
the earlier EDFs are a bit less clever but that may help secondhand availability
sounds good !

I've been waiting years to make the smartass point that brushless doesn't work 'online' ie directly off the battery leads
EM interference is why brushes have mostly gone
complicated modern stuff eg brushless and PWMs give interference but it's narrow band and easy to attenuate or avoid
Last edited by Tommy Cookers on Wed Nov 07, 2018 4:30 pm, edited 1 time in total.

alj
1
Joined: Wed May 23, 2012 10:14 pm

### Re: Key calculations in the Aerodynamics of the BT46

Haha well I only just learned that with my boys electric ride on “car”.... I know little about electronics.

The edf says 12 blade 7kg thrust. But no other details given. Mostly due to its role as an RC engine I would guess.

I was after a rough estimate of downforce using two of these engine to see if the idea is even feasible, but the lack of data makes it neigh on impossible.

Tommy Cookers
464
Joined: Fri Feb 17, 2012 3:55 pm

### Re: Key calculations in the Aerodynamics of the BT46

hovercraft lift calculations seem to be relevant (DF fan being a reverse hovercraft) and encouraging
there's a Bosch educational project for a hovercraft powered by 2 handheld type battery leafblowers
these have a high exhaust stream speed (210 km/h) - seeming similar to or higher even than your EDFs

this was a (UK) royal academy of engineering project 17 02 15
raeng.org.uk
Last edited by Tommy Cookers on Wed Nov 07, 2018 5:57 pm, edited 1 time in total.

alj
1
Joined: Wed May 23, 2012 10:14 pm

### Re: Key calculations in the Aerodynamics of the BT46

Very interesting. Battery leaf blowers would be an extremely easy test to conduct.

I will try that route first.