Dave Hopkin Posted December 17, 2015 Share Posted December 17, 2015 Anyone got any "rules of thumb" for thrust to weight ratios,,,, Using the scales with the fan pointing down I get ad addition pressure of 660g with the fan at full pelt.... what sort of weight can I expect that to fly? Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted December 17, 2015 Share Posted December 17, 2015 Dave It is going to depend on both how you want to fly and the aerodynamics of the airframe you are using. 1:1 is a good target and you could expect to fly even an 'aggressive' looking plane with that. On the other hand a scale U2 with its 'glider' wings can manage with a lot less although you cant then expect a 'rocket' climb. Remember you will loose a bit of thrust when the EDF is 'buried' in an airframe. My small (32" span) DH Venom flies very nicely with a maximum thrust to weight of just 50%. Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted December 17, 2015 Author Share Posted December 17, 2015 Thanks Simon - those measurements were taken with the EDF installed in the fuselage and blowing into the "jet pipe" The airframe is a cartoon scale Jet Provost of 40" span and at the moment the AUW is just around 2.5lb - so its not looking good Quote Link to comment Share on other sites More sharing options...
Ian Jones Posted December 17, 2015 Share Posted December 17, 2015 Now this is an interesting thread. Dave has around 1/5 Simon's suggested target and I'd really like to know what the remedy is. Actually I wish EDF fans had some sort of rating that offered a comparison between units. They only need to be simplistic just like propellers, got a .40 well stick a 10x6 on it is the rule of thumb but not set in stone. So got a 2lb EDF then....well I don't know. Wouldn't be nice to have a table of suggested power trains to weight, again just as a rough guide. I have a couple of EDFs that sruggle to get ground speed for take-off so I'll be intersted to see how to squeeze a bit more thrust out of the back. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted December 17, 2015 Share Posted December 17, 2015 The other way of looking at it is the power to weight.. You will need in the order of 150W/lb for a modest performance so does your EDF draw at least 375W? If you can get the EDF out and do the same test it would give an indication how much thrust the ducting is costing you. Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted December 17, 2015 Author Share Posted December 17, 2015 Simon its one of these **LINK** Running on a 4s - and its pulling a 58Amps so 974W electrical -W/lb its going to be around the 220w/lb when its finished - sp from that perspective it should have bags of power Yet when I weight the fuselage with a lipo but the fan off then power up the fan to full I get an extra 660g of extra weight (which is what I am calling thrust) Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted December 20, 2015 Share Posted December 20, 2015 If we assume you testing method itself does not impeded the air flow only getting 660 g thrust is a bit of a concern. Your electrical power figures are reasonably close to the specification (maximum 1000W) at which they claim no less than 1.4 kg of thrust. I am always a bit suspicious of claimed figures (like the claimed hp with IC!) but it does look like your ducting is costing a lot of thrust. I would have thought 1 kg thrust would a reasonable target which would fly a 2.5 lb plane pretty well. Inlet duct losses in particular can be a killer to static thrust as the fan has to do work just to restore the pressure back to atmospheric before even beginning to do work to force the air out the back! Edited By Simon Chaddock on 20/12/2015 16:03:05 Quote Link to comment Share on other sites More sharing options...
Ian Jones Posted December 20, 2015 Share Posted December 20, 2015 P'haps EDF dynamics is an overdue magazine article? Quote Link to comment Share on other sites More sharing options...
PatMc Posted December 20, 2015 Share Posted December 20, 2015 Posted by Simon Chaddock on 20/12/2015 16:01:56: ... as the fan has to do work just to restore the pressure back to atmospheric before even beginning to do work to force the air out the back! How can that be if it's located in the atmosphere ? Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted December 20, 2015 Author Share Posted December 20, 2015 The model in question was tested with the wings and top hatch fitted, so air intake was via the two (over scale) cheek inlet ports.... when static running there is considerable suction on the top hatch (which opens up into the void in front of the midships fitted EDF). So Simon's point about low pressure in front of the fan causing a drop in output pressure may well be an answer The fan duct runs about 14" and goes from 68mm ID down to 50ID - its relatively smooth inside but as it was build around a chuck away mandrel the amount of sanding possible is very limited! So Simon point about low pressure in front of the fan causing a drop in output pressure may well be an answer Quote Link to comment Share on other sites More sharing options...
PatMc Posted December 20, 2015 Share Posted December 20, 2015 Surely the air throughout the whole duct starts at atmospheric pressure, it's just the resistance to the air movement of the ducting causing the fan to create lower than atmospheric pressure in front & the resistance to air movement in the rear ducting causing higher pressure behind the fan ? But that's bound to happen however good the ducting is, it's just a question of reducing the resistance as much as possible. Quote Link to comment Share on other sites More sharing options...
Stuart C Posted December 21, 2015 Share Posted December 21, 2015 Maybe repeat the test without the top hatch fitted. If any significant increase in thrust noticed, then think about a cheat hole. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted December 21, 2015 Share Posted December 21, 2015 Dave The reason I suggested the fan had to restore atmospheric pressure on its inlet face is that is almost certainly the condition (no inlet duct just the bell mouth) of the EDF when producing its claimed static thrust figure. It is always instructive to test an EDF in 'free air' as it is the only way you can truly ascertain how much thrust has been lost by the ducting. Not only their area but the path shape of bifurcated inlets can be critical. Any significant change in direction can cause the airflow to 'break away' and create turbulence which locally increases the duct resistance. How smooth a path do your inlets have? The only true bifurcated inlet I have is on my Fairey Delta 2. Although the path is reasonable if I were to do it again I would concentrate at bit more on the point marked where the two ducts become a true cylinder. The duct is long at 30" and it is all inlet as the EDF is right at the back. It costs about 20% of the EDF's 'free air' thrust. The whole of the duct including the actual inlets themselves are 125% of the fan swept area so duct losses should be recovered by 'ram effect' as the air speed increases. Quote Link to comment Share on other sites More sharing options...
Tony K Posted December 21, 2015 Share Posted December 21, 2015 Posted by Simon Chaddock on 20/12/2015 16:01:56: If we assume you testing method itself does not impeded the air flow only getting 660 g thrust is a bit of a concern. Indeed. Even with a less than perfect duct one could expect to draw 90W per Newton of thrust. So for 974W you should have about 11N or about 1,1 Kg thrust. Quote Link to comment Share on other sites More sharing options...
Keith Simmons Posted December 21, 2015 Share Posted December 21, 2015 So it's change of direction within the duct that loses thrust? I have seen a model of a vampire which the inlets are 60 degrees to the fan as to avoid the wing spar. The inlet ducts ends at a wall which a bell mouth duct sticks out proud, the duct walls don't taper into the fan mouth. The set up works fine & to me less work to get it as smooth and tight fitting as possible. I agree to test the fan first without the ducting & experiment with various duct set ups. Was there a thread here in a few years back? & was that to test different EDF fans rather than duct loses? Quote Link to comment Share on other sites More sharing options...
Keith Simmons Posted December 21, 2015 Share Posted December 21, 2015 Ducted fan theory and practice from Richard Sharman. I would like to see it updated to view how EDF's improved over the years. I know that batteries have got lighter & more power's available. Quote Link to comment Share on other sites More sharing options...
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