Anyone know of an online calculator to derive aero drag from top speed and HP?

[classicalgas]

I'm trying to figure out (roughly ) the drag coefficient of an old car, where all I know is top speed and peak HP. I can estimate the weight within 10% or so, as well.

The mission is to get a rough Idea how good or bad the aero was on cycle cars of the 1920's-30's

 

[amberwolf]

Well, if any of them give the math showing how to get one of the things you know from the thing you don't know and one of the other things you do know, you could transpose the equation to get the thing you don't know from the ones you do.

I'm not a math person so can't tell you exactly how to do that, but I know that it is a common thing to do.

A quick websearch finds numerous sites that purport to give HP to get a certain speed given a certain drag; this one

Bowling's Aerodynamic & Rolling Horsepower Computation

also gives the math below the calculator, if it's helpful.

 

[ferret]

Hi.
The weight does not matter but you will need the car's frontal area:

The vehicle's horsepower is equal to the maximal drag it can overcome.

From: The Drag Equation.

One way to deal with complex dependencies is to characterize the dependence by a single variable. For drag, this variable is called the drag coefficient, designated "Cd." This allows us to collect all the effects, simple and complex, into a single equation. The drag equation states that drag D is equal to the drag coefficient Cd times the density r times half of the velocity V squared times the reference area A.

D = Cd * A * .5 * r * V^2

Avner.

 

[classicalgas]

Well, if any of them give the math showing how to get one of the things you know from the thing you don't know and one of the other things you do know, you could transpose the equation to get the thing you don't know from the ones you do.

I'm not a math person so can't tell you exactly how to do that, but I know that it is a common thing to do.

A quick websearch finds numerous sites that purport to give HP to get a certain speed given a certain drag; this one

Bowling's Aerodynamic & Rolling Horsepower Computation

also gives the math below the calculator, if it's helpful.

I'm very poor at math. For the number of sample cycle cars I want to check, I need an online calculator where I can just plug in speed, HP, weight, (or frontal area estimate instead of weight ) numbers and get a drag number out. An equation I need to manually run through isn't going to work.

 

[classicalgas]

So I'm getting somewhere, but numbers look strange. If I pick 5 square feet as the effective frontal area, and use claimed top speed and engine output for two similar three wheel cycle cars (1929 JAP v-twin Morgan Aero, and Sandford with the Jewel enclosed inline engine) the drag numbers calculated are radically different. .179 for the Sandford, .5 for the Morgan. I'm using a calculator that seems to include frontal area and aero drag (Cd ).

The big twin out front can't be THAT bad for drag, can it? Those numbers put the JAP Morgan as draggy as a bike and rider, and the Sandford more like a modern streamliner, and that can't be right.

 

[E-HP]

I'm trying to figure out (roughly ) the drag coefficient of an old car, where all I know is top speed and peak HP. I can estimate the weight within 10% or so, as well.

The mission is to get a rough Idea how good or bad the aero was on cycle cars of the 1920's-30's

You’ll have to make some assumptions. Top speed won’t be at peak HP. You need torque to accelerate to a higher speed, and then the HP to maintain that speed. You’ll probably also need to assume what type of rolling resistance (Cr) the car has.

 

[classicalgas]

I asked my son (an engineer and math wizard ) if he could find me a calculator I could plug numbers into. He came up with this...

Car Drag Coefficient Calculator

www.desmos.com

He insists it was just... " assembled out of online math Lego bricks"


Playing with that a while, using just trikes, I figured out that...

mistakes in actual frontal area will make any calculation worthless.

that old HP and top speed numbers were often not particularly accurate (wildly overestimated) The modern Morgan three wheeler has a lower top speed and higher HP than you find quoted for trikes of the thirties)

that using a 10% as a HP loss number (to rolling friction and drivetrain loss) brings the Cd numbers into a the realm of possibility.

Trikes with big V-twins out front have Cds like a brick ( or a Jeep Wrangler)