June 22, 2020

Understanding windage

When it comes to designing industrial lighting solutions, windage is something you ignore at your peril. So, we’ve asked our mechanical engineering guru, Andrea Peratello, to explain what it means and why it’s so important.

What is Windage?

Windage can mean different things to different people. In ballistics, for example, it’s the adjustment that needs to be made to keep a missile on target after taking the wind into account.

However, in our industry, we need to consider its engineering definition. This is where windage refers to the area or shape of an object that makes it susceptible to pressure.

Let me put that into an everyday, easy to understand context. If you’re walking down the road on a windy day you get pushed by the pressure of the wind. That pressure pushing you is called drag or drag force.

If you’re walking head on into a constant wind, you’ll find it harder to walk than if you were walking sideways into it – because the drag is greater. The wind speed is the same, it’s just the amount of your body you’re exposing it to – the windage – that’s changed. And if the wind speed and/or your windage goes up – the drag force goes up and you get blown over.

Why is it important and when does it need to be considered?

When you’re building something, like a large mast holding lots of lights, you want it to stay where it is – not sway around or even worse topple over. This explains why engineers have to think carefully about windage when they’re building something. They also need to take into consideration other things that can add to this drag force e.g. the weight and positioning of the mast and lights.

How do you calculate windage?

In general terms, engineers are less concerned with calculating windage itself – but more with the drag force, it can contribute to. It’s this drag force that can cause damage.

Drag force is easily calculated with this formula:

DRAG= ½ X V2 X ρ X EPA

Where:
V – is the air velocity.
ρ – is the air density.
EPA – is the Effective Projected Area.

And EPA is calculated with this formula:

EPA = Cd X FPA

What’s the difference between EPA and FPA?

Let me try to put this into everyday terms again. Imagine an oblong box with different dimensions on each side. The top, front, and end of the brick each have a different surface area, which is effectively the FPA for that side. And the greater the FPA, the greater the drag. That’s why we need to consider the largest FPA of an object as our worst-case scenario in any calculations we make.

However, we need to consider an object’s aerodynamics also to calculate it’s EPA.

Now imagine a ball and a cylinder of the same diameter. If the wind were blowing directly on the ball and the end of the cylinder, they’d have the same FPA. But because the ball is more aerodynamic the drag force against it is less than the cylinder and it has a lower EPA. That’s why we need to use an object’s drag coefficient and its FPA to get to its EPA, so we can calculate the actual drag.

So, floodlight manufacturers usually characterise the aerodynamics of their products with the maximum EPA value under the Windage tag of the datasheet as a worst-case scenario.

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