When the gas in the balloon expands due to heat and its relative density decreases, the mass of the gas in the balloon will decrease to a very small extent, so that the buoyancy of the air to the whole system of the hot air balloon is greater than the gravity it receives.

Before making, weigh the whole system, including plastic bags and candles, and then check the atmospheric density to see how much buoyancy is needed to make it float.

When the gas in the plastic bag is heated, its mass remains zero, but it is very small, so it is necessary to judge how much volume the plastic bag needs to increase according to its heat capacity and make it as large as possible. It is made of high melting point material and will not be melted by hot air. As for other accessories, the lighter the better. Carbon rods as rigid materials and rubber as flexible materials are all cheap and applicable materials, and solid alcohol is more suitable as heating fuel.

I have done it before, and the effect is not very good, but I can still achieve the effect of suspension or slow descent.