Electric Ducted Fan Jets Discuss electric ducted fan jets here including setup tips, power systems, flying techniques, etc.

"Need Some Numbers"

Old 12-18-2008, 07:34 PM
  #1  
Brner
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Question "Need Some Numbers"

OK guys...just looking for some guidelines to go by here.

What should a person be looking for in terms of watts, amps, thrust, etc. per lb/oz/gram to get some reasonable flights on these EDF's?

Also, how do you measure thrust, short of sitting a digital scale on its side, against a wall and throttling up with the nose against same?

Any help appreciated here,

Don
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Old 12-19-2008, 01:06 AM
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wcc963
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well it depends on drag of the airframe, altitude of flying field, weight of model, the type of model etc. what EDF do you want to use, and what kind of performance r u looking for out of it?
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Old 12-19-2008, 02:00 AM
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Bill G
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Exactly. What I've come down to for a basic guileline is this:
If you want the 50% thrust-to-weight or the 100W/lb for reasonable performance rules to work, you have to be at or less than a 15oz/sq-ft wing load. In other words, it only works well on lighter foam designs without a ton of epoxy, reinforcing, or whatever added. Often when you get into scratchbuilding, you end up heavier, and the guidelines go out the window.

For many jet flyers, they don't bother getting highly technical with their setups. They've experimented with enough different airframes of different weights, designs, and sizes, that they have a really good off-top-of-head idea of what's needed. I think that' a lot more valuable than trying to work out numbers without the value of practical experience. That's what the forums are best at. Finding what has worked for others with various planes.
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Old 12-19-2008, 08:23 AM
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I agree with Bill and wcc963 - it depends

In general you will need at least 1.5 to 2.5 times the watts per pound to get the same performance you could get from the same identical prop driven model.

Those are very general numbers though. Its highly dependent on the airframe, the fan/motor combo and the ducting and as Bill pointed out - the wing loading.

The only thing you can count on 100% is that you will need more power
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Old 12-19-2008, 04:15 PM
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Originally Posted by Larry3215 View Post
The only thing you can count on 100% is that you will need more power
that is so true so when i plan out my motor, fan, battery, and ESC combo i always aim for around 400w per pound, becasue when you account for lack of proper ducting, drag, and flight time you still get good performance.
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Old 12-19-2008, 06:03 PM
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when working with duct fans... everyone here is completely right... power goes out the door... its prob best to do what wcc963 says... aim for 400w per pound... edf's need alot... plus higher quality battery's usually come to play a big part here... I've seen alot of puffed batterys from edf guys...

on prop planes, there are alot of stuff you can play with to make it easy on your electronics/batterys... unfortunately... on edf's once you have the fan unit... you usually play with battery size (weight) motor kv, and cell count (more/less lipo cells)

SK
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Old 12-19-2008, 07:09 PM
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Brner
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Thanks guys for all the info....I did realize that there is no one answer that fits all, didn't expect that. But you've all given me some guidelines to start with and I thank you for that...I'll go play with my new WattsUp now and see if I can understand what it's feeding me....lol

Thanks again,

Don
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Old 12-19-2008, 07:51 PM
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Originally Posted by ministeve2003 View Post
plus higher quality battery's usually come to play a big part here... I've seen alot of puffed batterys from edf guys...

on prop planes, there are alot of stuff you can play with to make it easy on your electronics/batterys... unfortunately... on edf's once you have the fan unit... you usually play with battery size (weight) motor kv, and cell count (more/less lipo cells)

SK
thats another thing, battery quality plays a big part in performance, 30C batteries are ideal but more expensive and you get less flight time... most guys go for 25C which gives you more flight time with plenty of power, and they are cheaper.

example:
a 70mm EDF with a good quality 25C 3300mah lipo gets you 800watts, and 6min flights.

a 70mm EDF with a poor quality 25C 3300mah lipo gets you 550watts and 4min flights

even though the C rating and the mah are the same the quality of the pack is the deciding factor.

now i know that some people get away with cheaper lipos but if im putting 800.00 into a EDF jet an extra $15.00-$30.00 per battery for a good quality name brand lipo is a no brainer and worthy investment

the battery plays a huge role in how your EDF performs, probably the best are FlightPower, their 25C and 30C batts are IMO the best on the market... just about every Thunderpower batt i've had has puffd or loss significant flight time and power. i've never had a flight power puff on me. and i truely punish my batteries... i really push them... even their EVOlite 20C batteries are great, between them and the 25C i havent noticed and power differences
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Old 12-20-2008, 03:37 AM
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it sounds like your pretty new to EDF Jets, I would but a decent RTF combo and possibly upgrade to get your feet wet and go from there...and don"t be afraid to do some internet searchs to get as much info as possible...when I started out gathering info I had a few all nighters....LOL
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Old 12-21-2008, 01:25 AM
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Originally Posted by azjc View Post
it sounds like your pretty new to EDF Jets, I would but a decent RTF combo and possibly upgrade to get your feet wet and go from there...and don"t be afraid to do some internet searchs to get as much info as possible...when I started out gathering info I had a few all nighters....LOL

You got that right....this is my first EDF...in fact my first electric that I'm having to do anything to. I'm not new to modeling but it's been a while. Actually I'm doing exactly what you stated...got an RTF A-10 (cough, cough)...lol It did get off the ground with the original equipment but was very doggy and I got it down or maybe should say it got itself down very quickly...no damage though.

Then with the help of SK on this forumn, got me a decent charger, a WattsUp meter and some Lipo's and am currently doing some testing on the A-10. Changed from Nmh to the Lipo and that made a world of difference in the sound of the fans....not sure the rest of the power train can take that for long but if not, I've added another A-10 to my collection and now have a back-up with brushless in it. First one I'll chaulk up to a learning experience....it has been fun trying to decipher all this stuff but I think I'm slowly but surely getting there.

Thanks again for all the help guys...really do appreciate it,

Don
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Old 12-21-2008, 01:30 AM
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I got a kick out of these typos last night at the HL site, where they're talking about the Vasa fan:
http://www.hobby-lobby.com/ductfan.htm

"It is possible to use this ducted fan in models which weigh up to 3 times more than the thrust (30 oz. airplane), however, the optimal ratio between thrust and total airplane weight is 2:1. This would be an airplane that weighed 20 oz."

First they talk about optimum ratio as 2:1. Realisitically that weight:thrust ratio is the minimum, for reasonable performance, and definitely not optimum. The optimum would be more like the typo they wrote towards the bottom. I know what the really meant to say, but taken literally, it's a kick. Two times thrust to weight for an airplane weighing 20oz is 40 oz thust. Man that sound's pretty nice!

I'd like to see what a 20oz jet would perform like with 40oz thrust. Not sure how I would go about doing it, if even possible, but would be fun. Could build me a Bachem Natter.
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Old 12-21-2008, 02:27 AM
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Bill,

If I'm gonna need those kind of numbers, I might as hang this thing up as a ceiling ornament....lol

The thrust reading I've got so far (just by placing a digital scale up against a wall and the nose of the plane against the scale) are...with the Nmh that came with the plane, 7.5 oz. With the Lipo, 10.8 oz. Neither of these batteries were fully charged and I'm not sure if that method gets a very accurate thrust reading or not. I just got the new battery charger the other day, got some connectors on it today and have all batteries fully charged now. I'll redo my thrust readings tomorrow and see how much they change. BTW, it looks like I'll be right at 27-28 oz all up.

I'll be posting more on this I'm sure....

Don

Originally Posted by Bill G View Post
I got a kick out of these typos last night at the HL site, where they're talking about the Vasa fan:
http://www.hobby-lobby.com/ductfan.htm

"It is possible to use this ducted fan in models which weigh up to 3 times more than the thrust (30 oz. airplane), however, the optimal ratio between thrust and total airplane weight is 2:1. This would be an airplane that weighed 20 oz."

First they talk about optimum ratio as 2:1. Realisitically that weight:thrust ratio is the minimum, for reasonable performance, and definitely not optimum. The optimum would be more like the typo they wrote towards the bottom. I know what the really meant to say, but taken literally, it's a kick. Two times thrust to weight for an airplane weighing 20oz is 40 oz thust. Man that sound's pretty nice!

I'd like to see what a 20oz jet would perform like with 40oz thrust. Not sure how I would go about doing it, if even possible, but would be fun. Could build me a Bachem Natter.
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Old 12-21-2008, 06:08 AM
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Bill G
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Originally Posted by DEG View Post
Bill,

If I'm gonna need those kind of numbers, I might as hang this thing up as a ceiling ornament....lol

The thrust reading I've got so far (just by placing a digital scale up against a wall and the nose of the plane against the scale) are...with the Nmh that came with the plane, 7.5 oz. With the Lipo, 10.8 oz. Neither of these batteries were fully charged and I'm not sure if that method gets a very accurate thrust reading or not. I just got the new battery charger the other day, got some connectors on it today and have all batteries fully charged now. I'll redo my thrust readings tomorrow and see how much they change. BTW, it looks like I'll be right at 27-28 oz all up.

I'll be posting more on this I'm sure....

Don
I have a few of those.

The postal scale works well, since you can make a plane holder to hang off the scale's table. I put the setup on the edge of a desk/bench, and hang the plane over the edge.
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Old 12-22-2008, 09:06 PM
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OK guys...ran some bench tests on this unit as follows:

The Wattsup meter reads as follows (only ran up the system for 4-5 secs, just enough time to get the readings):

Amps = 19-20 WOT

Watts = 185 WOT

Volts = 11.9

I tried two batteries and got roughly the same reading from both. An 1100 mah and an 1800 mah, both 3S and both 15C. The 1100 warmed up a bit (and this was only in a 4-5 sec run at WOT) so I think I'll be using the 1800 (it stayed cool) and live with the 2 oz weight gain. Now looking at approx. 26 oz AUW.

Bad news is I fried the ESC. Sort of figured that would happen. I tested all the other components this morning and they all seem to be ok. No markings at all on the fried ESC but, if I understand all this right, I'll be looking at a 25 Amp ESC (brushed motors) to do the job. A bit over amp requirements but as I read these threads that's a safer way to go.

I've looked at the Electrifly and also at one of the Chinese imports...big difference in price. Not sure if I want to save bucks or the plane...lol

Hoping the motors will hold up until I at least get one or two flights in...lol

Any and all comments greatly appreciated,

Don
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Old 12-22-2008, 09:14 PM
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Poked around and found this one too.....price is pretty good...any comments?

http://www.bphobbies.com/view.asp?id...6&pid=NBA06105

Don
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Old 12-22-2008, 09:26 PM
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Bill G
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Originally Posted by DEG View Post
Poked around and found this one too.....price is pretty good...any comments?

http://www.bphobbies.com/view.asp?id...6&pid=NBA06105

Don
Out of stock. They're not good at restocking. I like BP and buy a lot there, but I don't know why they won't just remove half of their website, that they probably will never stock again. Especially some of the jets, which they waste your time clicking on, keeping them up just to try to sell spare parts they still have. Electrifly stuff is overpriced junk. Great Planes thought they could sell garbage at high prices, by abusing their name recognition. Their still are some who haven't flown the demands of EDF to realize that the stuff will burn. HobbyCity SuperSimple ESCs and dirt cheap Rhino batteries are easily as good as any Electrifly stuff. Their bl motors were actually a reasonable deal, until they got greedy and raised the prices. Now I'd say they have nothing that is even worth looking at.

Eflite brand name is overpriced too, and you can find as good for much less. At least in Eflite/Horizons case, they were smart enough to sell halfway decent stuff, as they actually seem to have some concern for overall quality, especially true when you are selling a lot of PNP stuff at top dollar. If you are newer to rc, this stuff is so important to get straight. I would love to have back the $500 I spent on garbage like Electrifly lipos/ESCs, and other stuff like that. No need to throw away money. Search the forums fully, and you will get a good feel of the best deals.

For whatever reason, I have not had good results with ESCs in high power brushed setups. Even the supposedly proper rated. Keep in mind that ratings are a joke. That said, I'd probably get one of those low cost 60A Polk's ESCs, simply because they have so many FETs on them.

About the only brushed motors I'd use on EDF would be the something like the 2900kv Maxx 4011 or I believe 4009 cobalts. The kv is misleading in the sense that they really do reach close to peak rpm with their immense torque, unlike bl that doesn't really see the rpm under load. These motors not only have to fit in 400 class fans, but probably would only work well in fans over 64mm.

One quality brushed ESC but not dirt cheap is the FMA Super. I haven't killed one yet, and was running well over 30A on a twin, using their 30A, without proper cooling to boot. Must just be higher grade power components.

Their is a real satisfaction in getting brushed EDF to fly, but it's more work. You will be intentionally building much lighter than if you had power to spare, and have to build to spec. I have a little brushed EDF50 twin that flies well, but the build was 100% to spec. Once I had the airframe/tail completed, I had to come up with a very good estimate for the AUW, assuming the wing and whatever else might go on the plane. At that point, I literally drew the wing profile to give me a wingloading light enough that I knew would work. It's that kind of attention to detail that will make lower power brushed work.

BTW, I would not pay new for anything brushed, at this day and age. I just gave away a 20A Sig brushed ESC a week or so ago in the ESC Sale forum. Folks will sell you them for almost nothing here. I have a few Supers at this point, but they're keepers. I given away all my other larger ones. I also burned a few up experimenting with my 109 scratchbuild, when it was Cobalt powered. That got old, I'm lucky to still have the plane, and yes, it went BL.
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Last edited by Bill G; 12-22-2008 at 09:44 PM.
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Old 12-22-2008, 10:35 PM
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Originally Posted by Bill G View Post
Out of stock........

lol....wouldn't you know it...I missed that. Oh well, guess I'll keep poking around.. Thanks Bill

Don
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Old 12-23-2008, 02:46 AM
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30amp brushed esc $8... I've purchased 2 of them and they seem to do fine... no issues with them at all...

http://www.hobbycity.com/hobbycity/s...0A_BRUSHED_ESC

SK
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Old 12-23-2008, 05:29 AM
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Originally Posted by ministeve2003 View Post
30amp brushed esc $8... I've purchased 2 of them and they seem to do fine... no issues with them at all...

http://www.hobbycity.com/hobbycity/s...0A_BRUSHED_ESC

SK
Thanks....I'll order them tonight...
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