Looks like a good method for making larger buoyant flies. As you get smaller you will soon reach a point where the tube weighs more than the bubble of air can lift.
From tying lots of foam beetles I found that lower density foam had to be used. Craft store foam was too heavy to be buoyant on a size 14 1x long medium wire hook. You will find a similar point with this method.
Buoyancy is a very specific thing. When a body is totally, or partially, immersed in a fluid it experiences an upthrust equal to the weight of fluid displaced. When the upthrust exceeds the weight of the body the body it is buoyant (Archimedes Principal).
The other thing to remember is that traditional hackled dry flies don't float! They stand on the meniscus. They displace only a very tiny weight of water.
It is a good idea when designing a fly to consider the "float line". the line through the fly that the surface of the water will make. Any material that sits above the float line cannot add to the buoyancy of the fly. It is not immersed in the water when the fly is presented, so it isn't displacing the water. It is though immersed in a fluid... Air! Any material that sticks up above the water when the fly is presented ideally, has to be lighter than air to contribute to the buoyancy. It is worth remembering that, as adding tubes increases the weight of the fly. If they are not in the water they do not add buoyancy.
A little complex but if you think it through you'll see how it works.
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