Why is it possible to describe an airship as a lighter-than-air craft though the materials used to make airships are heavier than air?

When you conider the total volume of the aircraft, the weight within that volume is less than the same volume filled with air. It is the average density that determines whether it will float or not.

I don't get it still, can u explain further?

Why is the weight within the volume of the aircraft less than the same volume filled with air?
Thanks in advance

"Airship" is this case something like a blimp.

The weight within the airship is filled with helium, which has only 4/29 the density of air. That is why the airship is "lighter than air" when you average over the entire volume.

Hot air balloons work because the hotter air inside is lighter than the air outside.

The buoyancy force on an airship depends upon the density of the air OUTSIDE.

Did u mean that the airship can float because the density of the airship is lower than the density outside?

yes.

An airship is referred to as a lighter-than-air craft because it achieves buoyancy in the atmosphere by utilizing a gas that is lighter than the surrounding air. While the materials used to construct the airship, such as the framework and the envelope, may indeed be heavier than air, they are not responsible for generating the lift. Instead, it is the gas contained within the envelope, typically helium or hydrogen, that makes the airship lighter than the air it displaces.

To understand this concept more thoroughly, let's go through the steps to calculate the buoyant force of an airship:

1. Determine the volume of the airship: The buoyant force is directly related to the volume of the airship. The greater the volume, the more buoyant force can be generated.

2. Calculate the weight of the displaced air: The buoyant force is equal to the weight of the air displaced by the airship. This can be calculated by multiplying the density of air (which is relatively constant) by the volume of the air displaced.

3. Compare the weight of the displaced air with the total weight of the airship: If the weight of the displaced air is greater than the weight of the airship, the airship will experience a net upward force, resulting in buoyancy.

Therefore, even though the materials used in constructing the airship may be heavier than air, it is the combination of the gas lighter than air, the volume of the airship, and the weight of the displaced air that allows the airship to achieve buoyancy and be classified as a lighter-than-air craft.