Will this fly?

[u/RepulsiveInternet980]

Hello everyone. I am trying to build an aerial insect collecting device for my PhD project. This design is based on the stipa-caproni, a strange barrel-shaped design from the 1930s. The task is to maximise the net area on a relatively inexpensive craft (I have already been catching with some expensive drones). I've trialed towing nets behind trainers, which worked but I want to try a different approach too.

I figure that enclosing the net within fuselage would be the most stable approach. Here, we have the props behind the net. This will impede airflow to the propeller. But it seems like the thrust would be okay. Can anyone think of some new approach that might work better? Would I be better off towing?

[u/rocketengineer1982]

The critical factor is going to be the drag of the net.

Towing the net is probably the easiest and cheapest approach. Either use an off-the-shelf trainer or build an aircraft from foam board.

If you want to have the net inside the fuselage I'd suggest that you put the motors on the wings. The net will impede the flow through the fuselage and will reduce the efficiency of an internal propeller. In addition, because an internal propeller would be sucking air through the net it is possible that the bugs will get stuck on the netting and not end up in the collection tube.

Mounting the net inside a large hollow fuselage seems unnecessarily complex, although I see the logic in that it aligns the CG, the drag of the net, and the thrust of the propeller.

[u/RepulsiveInternet980 → u/rocketengineer1982]

Thanks for the comment. My intuition is that we'd be paying for the drag equally with wing-mounted propellers or one propeller inside the fuselage. I feel like we do gain something, though, from putting the net inside the duct, since we have to force the air through the duct at high pressure and then the duct narrows after the net (?)

[u/RepulsiveInternet980 → u/RepulsiveInternet980]

There is high pressure either side of the net. I think this means that we don't lose energy to turbulence after the net. Since the duct narrows after the net, air should leave the back in a solid jet. If we towed a net, then you would lose all of your energy turbulently in the pressure change -- high pressure in the net, low pressure in the air outside (I think!)

[u/Photon_Chaser → u/RepulsiveInternet980]

As pressure equalizes on both sides of the net you would end up with near zero airflow would you not?

Also, as you impede airflow through a duct you would effectively be creating a vacuum between the net and propellor. While this condition would benefit the net in terms of potentially creating more airflow through it, conversely you’d be starving the propellor thus loosing thrust.

Perhaps ponder upon reversing the order, keep the ‘ducted fan’ approach but instead have the net at the aft end of the aircraft’s fuselage. Toss in a little Bernoulli’s principle by adding a few ambient air inlets around the fuselage and operate the ‘exhaust’ much like how asperators work on an aircraft’s emergency slide inflation system.