Aero Question

[Frank]

Sorry if this information is posted previously and I couldn't find it...

For the teams blessed with a wind tunnel...

I was wondering what sort of values you get for drag for your FSAE car WITHOUT WINGS / AERO DEVICES.

Is the drag proportional to velocity squared, and therefor a constant Cd*A value, and what sort of value have you arived at? (0.4m^2?, 0.6m^2?)

or is the drag a function like D(V^0,V^1,V^2,V^3?)

thank in advance if you have time to respond

Frank

[Frank]

Sorry if this information is posted previously and I couldn't find it...

For the teams blessed with a wind tunnel...

I was wondering what sort of values you get for drag for your FSAE car WITHOUT WINGS / AERO DEVICES.

Is the drag proportional to velocity squared, and therefor a constant Cd*A value, and what sort of value have you arived at? (0.4m^2?, 0.6m^2?)

or is the drag a function like D(V^0,V^1,V^2,V^3?)

thank in advance if you have time to respond

Frank

[Z]

Here are some numbers suitable for back-of-the-envelope calculations.

Cd parachute = ~1.5
Cd flat plate = ~1.0
Cd formula car = ~0.5 - 1.0
Cd streamliner car = ~0.1 - 0.2
Cd min-drag shape = ~0.05

Cd*A for a "typical" FSAE car maybe (??) closer to 0.8m^2 than 0.4m^2.

Aero-Drag-force = (rho/2)*V^2 * CD*A
(with rho=1.22kg/m.m.m, V in m/s)

Total-drag = Aero-drag + Rolling-drag + Slip-angle-drag

Rolling-Drag = Cr*Fz
(Cr = 0.01(hard tyres) to 0.05(soft tyres), Fz = car weight)

Slip-angle-drag = Sum(Sin(Alpha)*Fy)
(Sum for 4 tyres, Alpha = slip-angle of each tyre (=~0.05 - 0.1 radians during cornering, = static-toe on straights), Fy = axial force of each tyre).

Over a typical autocross course the slip-angle-drag is probably the biggest, on average. This is not counting driveline drag losses.

Z

[Frank]

thanks for theory Z!!

any monash people able to help me out

Scott / Al / Roan / Bob?

ive got a log of our car going full throttle from 0-160, then clutched back to 40

was done at a drag strip

im having a go at curve fitting, and im guessing Cd*A is about 0.45m^2

for aero force = 1/2*p*V^2*Cd*A

and "other resisting forces" f=V^c1*c2*mass

with c1 small, about 0.15, and c2 about 0.3

i was just thinking that these aero gurus would have measured an "unwinged" car before

am very interestd if the aero force is truely a function of V^2 , or is it changing with velocity

[Scott Wordley]

Drag is closely proportional to v squared.

For our 2002 car with no wings we measured values for CD.A of 0.624 with a standard deviation of only 0.004 between 10 and 80km/h so it stays pretty constant. So you were in the ball park.

We found that both our CD.As and CD.Ls with wings dropped off a little more with speed due to the need to make the wings as light as possible and therefore less stiff.

I cant post the graphs right now so give me your email address Frank and I'll send em, cant remember it off the top of my head

Just saw your last post, as you will see when i send the drag (force) versus windspeed (km/h)graph, if you fit a quadratic you get a squared coefficent of about 0.03, a linear coefficent of about 0.05 and a very small (0.5) intial y value. This is for our wind tunnel data obviously.

[Frank]

wicked thanx scott

s354079@nospam@student.uq.edu.au

frank

[Z]

Frank,

Aero forces are NOT functions of V^2 only. They are non-linearly dependent on Reynolds number, with an ugly transition occuring somewhere around Re=10e5 to 10e6 (ie. at typical FSAE Re numbers). Plots of Cd vs Re (over large range of Re) are very bumpy, not something to fit with a polynomial (except over small Re range).

This is based on practical knowledge (millions of wind tunnel tests, etc.), although there are theoretical explanations. I guess you knew that?

Anyway, the Re number effect on drag is very small for an FSAE car (maybe a few extra Newtons at low speed). It can make a big difference to lift/downforce.

At low speeds rolling-drag (+ static-toe-drag) dominates. It is, to a first approximation, proportional to V^0 (it increases rapidly at high speeds). You can measure it with a "fishing scale".


Z

[BigJake]

A great book is one by Hoerner "Fluid Dynamic Drag", it is old but (1958 ish) but it is a classic to get you going.