Chris Walby Posted December 27, 2021 Share Posted December 27, 2021 I have a foamboard model using a 70mm fan and two 35x76mm inlets with an inlet tract of 460mm and think I have an issue with fan performance. In short although the inlet is 27% larger than the fan area I think the inlet is restricting the fan and was thinking of adding some cheat holes. I have some options as to where the cheat holes can be located and assume as close to the fan inlet is preferable, but could do some in the fuselage sides or top/underside depending of the effect these will have on hand launching the model? It has a very narrow wing span so relies of gaining airspeed ASAP + has a flat underside, hence not to sure if big cheat holes on the underside would impact this. Any advice on cheat hole location and size would be welcomed, cheers Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted December 27, 2021 Share Posted December 27, 2021 Chris If your inlets are more or less a rectangular 34x76 then each has an area of 2660 sq mm giving 5230 sg mm total. 70 mm fan has a gross area of 3849 sq mm. Say it has a hug of 28 mm dia gives 615 sq mm meaning the Fan Swept Area (FSA) is 3849-615 = 3234 sq mm. Your inlets thus have an area 160% of the FSA suggesting they are more than adequate. Obviously if you inlets are no rectangular their area may less. What shape are they? Any pictures of the inlet and the duct? Provided the duct after the inlets changes smoothly to an ideally a circular duct of the same total area then I doubt cheat holes will do much as the EDF will not be unduly 'staved'. On the down side such generous ducts may mean the air is entering slower than the plane is flying so they actually create some drag particularly if your EDF is 'slippery' and wants to fly fast. With a long duct, inlet or exhaust, its cross section shape. surface smoothness and 'straightness' are as equally important as the area of the inlets. Quote Link to comment Share on other sites More sharing options...
Chris Walby Posted December 28, 2021 Author Share Posted December 28, 2021 Simon, Thanks for your reply and yes you are correct the inlets are rectangular and sweep into the joining section of the Y where the fan is located. There is plenty of room around the fan (approx. 15mm) but no rectangle to round transition section so its a bit of a plenum box. It might just be the structural design of the fuselage, but I can see the sides deflect in at WOT with it just sitting on the bench and my thinking was that the fan inlet must be quite negative (air pressure wrt to outside the fuselage). I don't mind loosing some straight line speed at the expense of max thrust at launch as its a bugger to hand launch without it meeting the ground after 2 seconds. I see your point about the size of the inlets, but don't think big inlets will make much difference at point of launch and the guys at the field are good at chucking hand launch models. Although you may have a significant point around the fan in a box so will have a look at making a proper inlet manifold to feed the fan. The fan exhaust is a tapered tube so I don't think there are any issues there. I was hoping it would be an easy self hand launch, but I might resort to a small bungee ramp if it just needs an extra push to get it to min air speed. I have some ESC/Battery issues, but will discuss with George first to see what the issue might be (numpty behind the programming card most likely) which will sort out one of the other problems I have. ? Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted December 28, 2021 Share Posted December 28, 2021 Chris If you have a rectangular inlet that is bigger than the fan it is important to retain the fan bell mouth otherwise the airflow to fan is restricted. A bit stylised but it shows the effect. The airflow around the square edge actually reduces the effective fan inlet area and thus creates a low pressure area that the fan has to make good before it even starts to speed up the airflow. It may be only a small loss but the outer 1/3 of the fan does most of the work so any restriction at the circumference has a disproportionately large effect on the fan performance. The circulating air zones created by any right angles in a duct also represent wasted energy. It is amazing how little pressure differential is required to distort flat areas whether inlet or exhaust. Obviously the distortion depends on the stiffness and area of the flat surface but an obvious distortion may not actually be restricting the fan performance that much. It is perhaps worth noting that over large inlets can "super charge" the inlet duct when flying at speed. If this happens a cheat hole can let out any excess pressure. I hope this helps. Quote Link to comment Share on other sites More sharing options...
PeterF Posted December 28, 2021 Share Posted December 28, 2021 Should the rectangular inlets also have rounded edges. Quote Link to comment Share on other sites More sharing options...
Christopher Wolfe Posted December 29, 2021 Share Posted December 29, 2021 7 hours ago, PeterF said: Should the rectangular inlets also have rounded edges. The inlet lip does affect turbine efficiency, thick and well rounded seems to be accepted practice. More info here NASA and inlets Chris Quote Link to comment Share on other sites More sharing options...
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