Cheater Hole design?

[dap35]

Most cheater holes I see are on the bottom of the fuselage but that seems like a great way to suck a lot of grass or other FOD into the EDF. Is there any downside to creating them on top of the wing instead?

 

[shadeyB]

Most cheater holes I see are on the bottom of the fuselage but that seems like a great way to suck a lot of grass or other FOD into the EDF. Is there any downside to creating them on top of the wing instead?

just visual cheaters on top. they do the same job on top or bottom

 

[quorneng]

Better to do without them altogether if the design allows.
Size the EDF to suit the scale inlet and then lighten the plane so it suits that size of EDF.

 

[telnar1236]

Most cheater holes I see are on the bottom of the fuselage but that seems like a great way to suck a lot of grass or other FOD into the EDF. Is there any downside to creating them on top of the wing instead?

Depending on the location of the cheater hole you also get some slightly different effects. Putting them on the bottom helps prevent the hole being blanked out at high positive angles of attack which is the big one, but because you're so slow, there is not that huge a difference. There are also a number of different possible interactions with the boundary layer on the fuselage and possibly the wing and tail, but unless you do something really crazy, you can probably ignore this.

My advice is to keep any cheater inlets symmetrical, left and right, as much as possible and otherwise to place them where is most convenient. And, as quorneng said, if you can size your EDF to not need cheater inlets, that is even better.

 

[dap35]

Thanks. I am rebuilding my Mirage2k and decided the inlets were way oversized which created too much drag. So my thought was to put smaller cheaters on each side where the fueslage meets the wing if I need to add them in.

 

[quorneng]

dap35
How much drag does an inlet actually create?
If the inlet is sized so the air is being sucked in at the forward speed of the plane there is very little drag.
The fan has to accelerate the exhaust so it is going faster than the planes forward speed to create thrust so the inlet area can actually be bigger than the fan swept area, about 1.2 times, before the frontal area of the inlet creates any significant drag.

As an example does the huge frontal area of an airliner's turbo fans create drag?

 

[telnar1236]

To add on, real fighter aircraft frequently have smaller inlets than EDFs need. If you make your inlets smaller and need to add a cheater inlet, you're adding drag. If you just want smaller inlets for looks though, a cheater hole is a good option.

To add on to quorneng's answer, you want inlets 95% to 110% FSA and a thrust tube outlet 95% to 85% FSA. Even with a thrust tube and inlets the same size the fan still produces thrust (you can tell inlet and outlets speeds are the same in this case from continuity) but that is because the fan accelerates the air external to the ductwork and it is a less efficient setup. This even goes for a larger thrust tube than inlets, but efficiency losses continue to get larger.

 

[dap35]

The problem origninated when I scaled plans up by 30% to go from a 64mm to 70mm. I suspect the inlets were close to 2x the fan area. So if I properly resize things I may not need cheater holes, but I am just looking ahead.

 

[PsyBorg]

Aren't naca ducts designed to allow airflow into an airframe without the drag issues of simple slots or holes? I always wondered why EDF planes have slots cut but not simply use a naca duct to draw more air in without affecting drag and such.

 

[telnar1236]

The problem origninated when I scaled plans up by 30% to go from a 64mm to 70mm. I suspect the inlets were close to 2x the fan area. So if I properly resize things I may not need cheater holes, but I am just looking ahead.

Ah, I see. You probably don't even need cheater holes then. 64mm to 70mm is only a 20% increase in area while a 30% increase in scale is a 70% increase in area.

 

[telnar1236]

Aren't naca ducts designed to allow airflow into an airframe without the drag issues of simple slots or holes? I always wondered why EDF planes have slots cut but not simply use a naca duct to draw more air in without affecting drag and such.

NACA ducts are better than a lot of other options. You see them on a number of EDFs as cheater holes, especially in smaller aircraft where drag is a larger concern. I think bigger aircraft use grates to reduce to chance of sucking up debris. Properly sized inlets are still better though, for efficiency. NACA ducts still produce some drag. They are just better optimized than a straight slot.

I should also add. The FT version of a NACA duct is not really a proper version. If you search for pictures of NACA ducts, you'll see what I mean.