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Nov 12, 2019, 11:17 PM
Theuns
T. van Vuuren's Avatar
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Discussion

a dumb 90mm edf question


Guys I would like to hear your input WRT 90mm edf setup VS. someting like a 105mm to 120mm setup.

Most of the affordable 90mm units I have seen advertised including the very nice JP EDF run off of 6 to 8 cells and will produce arrounf 3-4 kg (or even a little more with the high end stuff) of thrust.

The biggest issue I can see is that the running time with these fans on 6 to 8 cells is rather short, arround 3-4 min if I understand it correctly.
Most of the 105mm and larger fans run off 12 cells and therefore will have longer flight times, let say up to 6-7 min maybe.

The hassle with the 12 cell setup is that it can become rather expensive to have to buy several larger complete flight packs and the space available in something like a "smallish" F-86 of 1.45m wingspan (57") is not really enough to carry these large and heavy packs.

I have a few ideas (and very old kits) I would like to give a go but 90mm is basically the best "size wize and affordable" to do with these smaller non foamy tipes.

Is it even worth trying for a 90mm setup, or is the 105 the best way to go to extend flight times, but then the inlet duckt size becomes an issue WRT the FSA.

For the 90mm , as I understand it, the fan has to work allot harder to produce flying thrust VS. a larger fan and therefore the lipos are drained allot faster on the single pack 90mm setup.

Thanx for any advice

Theuns
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Nov 13, 2019, 04:33 PM
Registered User
You might consider Watts-per-pound as a gauge in matching propulsion components to a particular composite airframe.

For example, I'm running a 5400 Watt (12S) setup in an a jet that weighs 15 lbs AUW. That is 360 Watts per pound of total aircraft weight.
This gives turbine-like performance in a fan-powered model.
What's the minimum ratio for jet-like performance? Some pilots are content with 250 Watts per pound.
Nov 13, 2019, 04:44 PM
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turbonut's Avatar
There is many variables that effect run time...In the end how much power do you need? As posted above watts per pound will get you an idea of how much power..But you will need more info still..what airframe? a U 2 with long wings or a f104...Power and weight wont tell the full picture in this case...In my opinion your assumptions are incorrect if not just incomplete. The fan size is only a problem if its not correct for the airframe..That part you will need to figure out first..larger fans require more power..no way around it..
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Nov 13, 2019, 11:24 PM
Theuns
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Thread OP
Thanx for the replies guys.

Some more info -
I am planing a 1.45m span F-86 and about the same size F-9 panther project next year.
The intakes on both might be under the required amount needed to get away without having to use some form of cheater hole if I go bigger than 90mm.
I already have a couple of 12 blade changesun units gathering dust and would like to use them in something, or I could get a JP 90mm that makes good thrust.

The airframe I believe will requre less thrust/watts than say a modern jet due to bigger wing area/lighter wing loading.
Speed is nice but not the top factor, I enjoy "scale" speeds the 1950's jets had.

I would very muck like to use a 6 to max of 8 cell setup to not load the airframe with 12 cell weight.Space in the F-86 is also rather tight so 2 by max of 4 cell 5000 packs could still work.

Basically what I am trying to figure out is if it is actually worth spending time in building an old school non foamy jet with a 90mm setup on 6-8 cells and not be underpowered. I do understand that there are factors like weight and airfoil section ect that will factor in.

If a 90mm on max 8 cells will be to "marginal" I would rather build these kits as PSS and glide them off my local mountain ;-)

Theuns
Nov 14, 2019, 06:01 AM
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Arngeir Blakseth's Avatar
90mm will be perfect for a F-86 of that size, you won't have to cheat the intake size much at all and it will actually have space for batteries and equipment around the ducting. It will just need more than 6-8S to be useful. No problem setting up a Jetfan 90 or a Wemo Midi to run on 12S with fairly low amp draw, so you can use 3 4S 4000mAh and get good flight duration. To get close to the same performance on 8S you will be at 100A+ and that means a larger esc, more strain on batteries and so on. Forget about 6S, not going to work in a plane this size. My opinion anyways.

Example : Wemo Midi with HET 650-68 1130kV on 12S : 3300W 74A and 4.5kg thrust. Would be a good setup, running a HET 650-68 1600kV on 8S will be a 100A 3000W setup, 4.2kg thrust. Also fine but will give shorter flight times on 5000mAh, would need 6000mAh to have similar duration. And 4S 6000 is a tight squeeze in a F-86 I think.
Last edited by Arngeir Blakseth; Nov 14, 2019 at 06:26 AM.
Nov 14, 2019, 08:34 AM
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jjantti's Avatar
I'd choose 90mm for this size. 145cm span is about ideal for it. I'm basing this opinion of my heavily modified FlyFly and newly printed 3Dlabprint Sabres that both are 140+ cm in wing span. Intake area needed os FSA should be easy to achieve and like said earlier nowadays pumping up the power with quality 90mm is no problem. Printed plane is about double in weight compared to 10 years old design...

I always choose the fan size by intake area available and start design /builduing. When I have rough guestimation of flying weight of it choose motor battery voltage and equivalent kv to get power needed.

/Jyri
Nov 14, 2019, 10:17 AM
Registered User
T.van Vuuren
Technically for a given power output there is no significant difference in weight between 6s and 12s.
A 5000 mAh 6s pack has the same energy capacity and weight as a 2500 mAh 12s.
To give the same power the 5000 mAh 6s pack will have to deliver twice the Amps of the 2500 mAh 12s but using the appropriate kV motors the flight duration and performance will be more or less the same.
Nov 14, 2019, 12:31 PM
Theuns
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Thread OP
OK, that seems like that 90 mm can workh well, and I didnt even think of using 4 by 3cell packs, that may just be the way to go. Nice to ave a different view from someone else to solve a problem.
Also if a pack goes bust it is not as hard a blow as losing a single 6 cell pack.

Thanx, I will give it a go, however I dont think my GRP changesun will stay together very long on 12 cells ! I will have to look at something elce that is safer at the high power input.

Thanx ;-)

Theuns
Nov 14, 2019, 01:02 PM
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turbonut's Avatar
Best is to put money in to a good fan..Get a Jet fan or Wemo 90 and you wont have to worry about it coming apart..I have a Jet fan 90 in my Mystere jet that weighs in at 15lbs and it is pulling 4000 to 5000 watts depending on the set up on 12s
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Nov 14, 2019, 01:04 PM
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Arngeir Blakseth's Avatar
Good 90mm fans that does not cost too much are Jetfan 90 or Wemotec Midifan EVO.
Nov 14, 2019, 10:58 PM
Theuns
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Thread OP
I agree, I wil not try to push more than 6 cells through my changesun, I will use it for some or other foamy tipe in the future and rather get a carbon or alluminium 90 setup for the sabre and panther.

Thanx

Theuns
Nov 15, 2019, 11:25 AM
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turbonut's Avatar
Good luck with the projects..Please start a build thread. What kits do you have? Or will they be your design?
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Nov 17, 2019, 11:34 PM
Theuns
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I will start a thread on the F-86 early next year.
Mirage project - https://www.rcgroups.com/forums/show...e-3-EZ-project. Hoping to fly it on the new JP alloy 105 fan by latest year end. I am now working on getting the elevons hindged.

The other kits are a mix of old 1980's highly brittle FG fuselages and a mix of own designed projects.
I usually make molds of the fuselage as the FG is no longer suited for the job and new composites are allot better.
All the other parts are usually missing or not really usable like wings and stabs, these are also molded in full stressed dkin composite like on the Mirage III kit.

It does seem from all the reply that an avridge flight time of about 3 to a max of 4 min is what I can get from the above mentioned edf/lipo setups....if that is the "industry standard" that that is what I will have to work with :-)

Maybe a nice U-2 pruject could get better flight times but may be less "exiting" to fly unless I can get a friend with a little RC chace car to help out on landings LOL

I do have a 6 kg turbine but must say that the "ease of operation" of an EDF is what I like ,once all the little issues like duckting and cooling havs been sorted . I dont really mind the tinkering if I can get better results now and then .

Theuns
Nov 18, 2019, 08:15 AM
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Arngeir Blakseth's Avatar
5 min. is what I would expect, with mixed throttle use. If you have enough power you should be able to cruise at 50% power or less and that helps a lot on your flight times.
Nov 18, 2019, 10:38 AM
Theuns
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Thread OP
That wuld be cool. 3 min is just a little shprt but then my concentration levels can only handle about 3 min of high intencity flying anyway LOL

'Theuns


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