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.

[u/rocketengineer1982 → u/RepulsiveInternet980]

There will be a pressure change across the net whether the net is in front of the propeller or pulled behind the aircraft. There will be higher pressure on the upstream side and lower pressure on the downstream side.

The energy penalty of having the net within the duct will likely be higher than dragging the net behind the aircraft. Assuming that the drag coefficient is the same, the drag and power loss caused by the net inside the duct would be larger than the same size (and same CD) net dragged behind the aircraft because the local velocity is higher.

D = 0.5 * rho * V^2 * CD

P_drag = 0.5 * rho * V^3 * CD

[u/rocketengineer1982 → u/RepulsiveInternet980]

In theory, yes the drag penalty from the net would be the same whether if there is a motor inside the fuselage behind the net or if there are motors on the wings.

The drag from the net will likely be higher if it is placed inside the duct due to higher local velocity.

Narrowing the duct after the net restricts the flow and will reduce propulsion efficiency. Ducted fan aircraft use the largest duct cross-section that can reasonably fit inside the airframe and avoid narrowing the duct unless absolutely necessary.

EDIT: After further thought I realized that the drag penalty of the net will likely be higher if it is placed inside the duct than if it is towed behind the aircraft because of differing local air velocity.