Allan Bennett Posted August 21, 2021 Share Posted August 21, 2021 My depron F-16 is progressing, and I'm wondering if I shall need cheat holes for more air for the fan. The scale air intake's cross section area is exactly the same as the swept area of the Wemotec 70mm fan (good luck rather than good judgement!), and from there the air travels in a rectangular box, which has a cross section area about 50% greater than the intake, to the fan. The pictures below show the head-on view of the underside, and a top view (with the top sheeting and the top half of the fan mounting yet to be added) of the box leading up to the fan. So my question is, given that the intake area equals the fan area, do I need cheat holes. If so, is there a general rule for their placement -- should they be close to the fan or close to the front end of the box? Quote Link to comment Share on other sites More sharing options...
J D 8 Posted August 21, 2021 Share Posted August 21, 2021 It is usual for cheat holes found near the fan and are there to enable the fan to get more air in at the critical low speed part of the take off. Once the plane is up to speed they do little as you now have ram effect on the air into the duct. It may be your model may have enough power/thrust and not need any cheat holes. My old EF16's duct is pretty much the same diameter from intake to just before the fan where it opens out some in the position of the cheat holes. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted August 22, 2021 Share Posted August 22, 2021 Airflow theory would suggest that changing the shape, area or direction of a duct incurs additional losses beyond those resulting from its length. The ideal would be a straight circular duct the diameter of the fan and as short as possible. Note a fan duct is greater than the FSA by the area of the fan hub which can be as much as 20%. To minimise additional losses any changes to a duct should be as gradual as possible. The same criteria to reduce losses apply to both the inlet and exhaust sections of the duct. IMHO a cheat hole is a solution resulting from a poor inlet duct configuration usually resulting from trying to maintain a scale appearance. ? Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted August 22, 2021 Author Share Posted August 22, 2021 Thanks Simon. My initial idea was a moulded fibreglass duct the same diameter as the fan, with a transition to the intake shape at the front end. But this would add weight, so I decided to use the fuselage box structure itself which is needed anyway. I suppose the obvious answer is to finish off the exhaust end of the setup (a tapered tube rolled from laminating pouch material), then install the fan and motor and run it without the front top sheeting (i.e. as per the second photo) and measure the thrust. That'll give me the target to acheive when everything's closed up, and will show me whether my setup needs cheat holes or not. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted August 23, 2021 Share Posted August 23, 2021 If you really want to dissapoint yourself measure the EDF thrust stand alone in free air and with its bell mouth on. This would represent the best achievable static thrust. The aim is to get as clsoe as possible to it but a drop of 20% is not unknown. My Hawker Sea Hawk with both its inlet and exhaust bifurcated looses a good 30% static thrust compared EDf in free air but nevertheless still flies nicely. It just a penalty from sticking to true scale.? Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted August 24, 2021 Author Share Posted August 24, 2021 Thanks Simon, I see what you're saying. My exhaust tube tapers as recommended by Wemotec, and looks scale, so I'm not going to modify that. So, to my way of thinking, a test in free air only gives me a target that will be impossible to reach. At least, with the exhaust tube on I'll be able to see how much putting the top on the inlet box affects the thrust and, hence, whether cheat holes are needed. I'll check with and without bell mouth as well, for that may be unproductive extra weight. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted August 24, 2021 Share Posted August 24, 2021 A friend at our club was trying to launch a flying wing fitted with an EDF. No bell mouth or efflux tube fitted. The model just gradually sank to the ground. He eventually listened to advice and fitted the bell mouth and thrust improved enough to just fly. We then fitted an improvised efflux tube made from a plastic coffee cup ( the old plastic type, ) it was a nice tight push fit on the rear of the EDF unit and was cut down about half its lenght. The thrust increase was much higher and the wing flew really well after these quick mods. Just proves that induction and the efflux are very important considerations. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted August 24, 2021 Author Share Posted August 24, 2021 Thanks Engine Doctor. I think I'll make the setup with bell mouth my default for testing. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted September 1, 2021 Author Share Posted September 1, 2021 Well, I've got my benchmark now. With the tail end structurally complete, the bell mouth installed, and the top off the intake duct as per my second photo, I got 1.236kg thrust (2lb 11oz) at 70A using a 3S 3300mAh 20-30C LiPo. The model weighs 460g at the moment with the EDF installed, and ESC, battery, servos, Rx, and some more depron will take it to about 910g. Sometime next week I should be able to close in the intake duct, and then see what thrust I get. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted September 2, 2021 Share Posted September 2, 2021 Allan Just remember 70A will discharge a 3300 mAh pretty fast, like 2 minutes to not over drain the battery. Hopefully with such a thrust to weight you will be able to throttle back some during a flight. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted September 2, 2021 Author Share Posted September 2, 2021 Yes, that's exactly what I'm hoping Simon. I also have a few 3S 2200mAh packs, and there's room in the battery bay for 2 of them in parallel to give me about 50% more capacity -- though extra weight will reduce that advantage. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted September 8, 2021 Author Share Posted September 8, 2021 Wow, what a difference! With the inlet duct totally enclosed now, I did another thrust test: 345grammes at 62A, compared with 1,236grammes at 70A with an unrestricted inlet. So, even though my inlet opening is the same area as the fan, looks like I'm going to need quite a large area of cheat hole(s) ? Quote Link to comment Share on other sites More sharing options...
J D 8 Posted September 8, 2021 Share Posted September 8, 2021 That is the problem with ducted fan. It is like driving a car in top gear all the time, hard to start off, slow to accelerate but top end performance is still there in the end. EDF does have the benefit of ram air at the inlet helping boost the thrust the faster the model goes. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted September 9, 2021 Author Share Posted September 9, 2021 Yes, I was aware of that. But I was a bit taken aback by the massive reduction in thrust caused by the intake duct -- no other changes made since the initial test. Quote Link to comment Share on other sites More sharing options...
J D 8 Posted September 9, 2021 Share Posted September 9, 2021 Others have experimented with methods to improve the flow with things like two stage fans [ motor with extended shaft and fan each side,] the second having greater pitch but any improvements do not seem worth the hassle and extra weight. Quote Link to comment Share on other sites More sharing options...
Allan Bennett Posted October 5, 2021 Author Share Posted October 5, 2021 (edited) Almost there! I've cut two cheat holes about 15 x 46mm each, in the belly right in front of the fan, and a test today gave me 1175g static thrust. Not far from the 1236g I got with the top of the fuselage completely open. I'm going to lengthen the holes another 5mm or so to see if I can get any more thrust, but if not, I'll be happy with what I've got. The black you can see in the picture is the front end of the fan. Edited October 5, 2021 by Allan Bennett 2 Quote Link to comment Share on other sites More sharing options...
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