Ideal ratio of inlet to exhaust length?

[Allan Bennett]

Having just about completed a couple of TN projects, I've been mulling over the idea of restarting, perhaps from scratch, an own-design EDF F16 which has been partly built out of foam and is taking up space in my workshop. It has a Wemotec mini fan with I don't remember what motor.

What stopped work on the project a few years back was the realisation that the whole thing was becoming unrealistically heavy, plus the fact I was having problems getting the front fuselage to look right. I had made a nice, but rather heavy, fibreglass intake duct and exhaust, with the fan mounted on the main spar. What I'm thinking to do is to do away with the intake duct other than the 'scoop' part that protrudes below the fuselage, and have it feed into as big a void as I can create within the fuselage, from which the fan would get its air. That would give me the advantage of being able to incorporate cheat holes if necessary.

But my main query is, apart from weight disribution issues, which can probably be sorted by battery placement, would I be best off putting the fan further back from the efficiency point of view? I know I can probably replace the fibreglass exhaust tube with one made up from rolled up laminating pouch, so that will be a handy weight saving even if I leave the fan where it is.

[Martin McIntosh]

I would ditch the Wemotec to start with if you are thinking of using it and get a Power Fun or FMS. The power increase should be considerable.

[SIMON CRAGG]

Agreed, just done some comparisons Powerfun V FMS.

FMS all day long!.

[Allan Bennett]

Interesting comments, thanks. Wemotec was state of the art when I got it! I suppose I can keep it as a spare for my Learjet, which has flown very nicely for years on two of them.

My question about fan positioning was triggered by the pictures of the MIG Fulcrum in this month's magazine, where the fans seem to have no exhaust duct at all.

[Engine Doctor]

All of my EDF models including a built/ balsa HET Tomcat have the EDF unit mounted in the rear fuselage. The battery , ESC etc are arranged to give correct balance along the fuz . Re fan exhaust or efflux tubes , they can increase thrust tremendously . A few summers ago a friend and I were test flying his EDF powered flying wing . The fan was simply mounted ontop of the wing with no inlet ring or efflux tubes fitted .It didn't have enough power to maintain flight. I suggested a temporary fix of a plastic coffee cup trimmed to fit the rear of the fan . The cup had about one inch cut off the bottom and was pushed onto the rear of the fan and taped in place. Thrust improvement was excellent ,probably doubled giving good rate of climb . The length in this case was found by trial and error until the best results were obtained. I don't know the formula for working out the optimum length or taper of the efflux tube although seem to remember the exit area should be around 90% of fan intake area. No doubt someone will come along and give us the formula.

Also fan intake needs a radius on it as tthis improves the fans ability to draw air into it . Once an intake ring was fitted the model really had decent powerv. Fans usually come with an intake ring ,if they do try and fit it unless buried in a fuselage. 

There are some very reasonably priced 12 bladed fan units on eBay that run on 4s not bad but do need a bit of balancing/ fettling to get that turbine sound.

Edited By Engine Doctor on 28/01/2021 11:06:11

[Allan Bennett]

Thanks E.D. A picture is emerging. I'll redesign with the fan positioned as far back as I can whilst leaving room for an eflux tube. I know Wemotec give recommendations for size where, IIRC, the outlet diameter as a percent of the fan diameter was more important than its length, within reason. I'll certainly fit an intake ring, as I have done with my Wemotecs.

[john davidson 1]

Re using a coffee cup, I have used one to boost the speed of the model but it certainly made the motor work harder and heated the battery quite a bit so might be good idea to check with a watt meter

[Allan Bennett]

Thanks John, I never trust the manufacturers' figures, so always use a wattmeter.

[Roger Jones 3]

A few tips from doing years of EDFs, not always successfully.

My comments are using 90mm multi-blade fans (not the old ones with only 4 or 5 blades).

1) Comparing the thrust of one make of fan unit with another without using a a wattmeter is a total waste of time.

2) At the same watts input the difference between the Schubeler, Wemo and Jetfan is less than 5% (I suspect the higher quality 'Asian' ones. will be much the same, but some of them are rubbish, both in efficiency and physical strength.)

3) The Schubeler HDS  is the lowest load, the Jetfan the most, with the Wemo in between. You can use this to your advantage. On a given number of cells  if the thrust and  wattage using a Schubeler  fan is not what you  hoped  for replace it with a Jetfan.  Alternatively increase the number of cells by one, 

4) Using ANY motor, loading it right  up  to its claimed maximum wattage results in wasted power. . It just get hot and you are not trying to make a room heater.

5) From 4 above putting a heat sink on it just 'disguises' the energy you are wasting by overloading the motor by carrying it away.. It's still being generated in the motor coils. The heat sink may stop it burning up but the efficiency loss is still there.

6) Duct losses are very low, even with long ducts,   unless you are doing  something badly  wrong. On my planes, mostly from balsa kits or plans,  the difference in static thrust   from putting the fan unit on a test rig with no extra intake  or efflux ducting  to using it in a plane is negligible - hardly measurable. Even with the outlet only 80% of the fan swept area (area of the overall  fan diameter minus the  area of the hub).

7) If your intakes collapse so that you need to put carbon or whatever rings around them  the intakes are too small and you are wasting power trying  to collapse the ducts.

Edited By Roger Jones 3 on 29/01/2021 05:55:22

Edited By Roger Jones 3 on 29/01/2021 06:05:48

Edited By Roger Jones 3 on 29/01/2021 06:09:35

Edited By Roger Jones 3 on 29/01/2021 06:21:14

[Allan Bennett]

Thanks RJ3. My three Wemotec 69mm units (two in my Learjet plus the one in my unbuilt F16) were all updated a few years ago to 11 blades. The pair in my Learjet use Mega 16EDF motors with no extra heatsinks; the one for my F16 has a HET motor which I read at the time was 'more powerful' than the Mega option, and which came with a clip on heatsink.

I'm a bit surprised at your item 6; I thought the idea of a tapered eflux tube was to increase thrust and, possibly, power (maybe only power consumption). Do you taper it in your models, or just leave it parallel at fan diameter?

[Roger Jones 3]

Posted by Allan Bennett on 29/01/2021 06:28:28:

Thanks RJ3. My three Wemotec 69mm units (two in my Learjet plus the one in my unbuilt F16) were all updated a few years ago to 11 blades. The pair in my Learjet use Mega 16EDF motors with no extra heatsinks; the one for my F16 has a HET motor which I read at the time was 'more powerful' than the Mega option, and which came with a clip on heatsink.

I'm a bit surprised at your item 6; I thought the idea of a tapered eflux tube was to increase thrust and, possibly, power (maybe only power consumption). Do you taper it in your models, or just leave it parallel at fan diameter?

Hi,

Efflux area is a trade off. A small exit increases the speed of the exhaust (can make the plane faster) but may reduce the static thrust (longer take off).

Also too big intakes cause increased drag and additionally may cause the fan to stall at high speeds due to just too much incoming air for it to cope with. In a plane, unlike on a stationary test rig with no ducting the incoming  air  has nowhere to go except  through the fan, 

Interestingly I have read a lot about fan RPM increasing in the air as it 'unloads' the fan. My OS 1100HV ESCs can record the maximum RPM achieved on any given run. I have found that nor even a full throttle near vertical dive makes any significant difference to the RPM so I suggest that the guys who say that have never actually tried it. Though I am not sure why it doesn't speed up. Perhaps because at full speed on a properly set up system in level flight at full throttle the motor has reaches its max RPM per volt (Kv) and so just won't rotate any faster in a dive.

PS: Note  that in my point 6  I said my planes are  from kits or plans. I said this on purpose so as not to imply I had some amazing   design  'expertise' that others didn't have. 

Edited By Roger Jones 3 on 29/01/2021 07:04:28

Edited By Roger Jones 3 on 29/01/2021 07:10:26

[Allan Bennett]

Ok, that bit about eflux trade-off makes sense. I use a grass runway so I'll need maximum thrust, rather than speed, I suspect. I used Wemotec's recommendations in this respect for my Learjet, and have no more problem with our grass than prop-driven models.

Thanks for the comment about too big intake. Presumably the effect might not show up until the model is in motion, thus causing more air to be forced in rather than simply allowing the fan to suck in what it needs. So it sounds like starting with the same intake area as the fan itself (which my F16 intake achieves) then adding cheat holes gradually if flight tests suggest the unit is not giving its best, is the way to go.