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Sep 21, 2005, 02:08 PM
sportytourer
Thread OP

AXI 2808-20 vs AXI 2808-24 for Quiet Storm


I am planning on getting a Fliton Quiet Storm and want to try an outrunner on it. I want to get the AXI but do not know which of these engines performs better.
It seems the 24 is more efficient and the manufacturer claims it is better suited for a larger prop, while the 20 is for a smaller prop, is slightly less efficient but can handle slightly larger current.
It seems the 20 may be more powerful but the 24 more efficient.
Anyways i would like the power to do 3D flying and I life at sea level.
Can anyone help or suggest what would be good.
tks!
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Sep 21, 2005, 02:48 PM
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Dr Kiwi's Avatar
I would assume a bigger prop would be better for 3D, so why not go for the 2808/24 (22A max)

Here are the Motocalc "ballpark" figures [if you care to believe them]

Motor: Model Motors AXI AC2808/24; 1160rpm/V; 1A no-load; 0.115 Ohms.
Battery: Thunder Power 1900 (G2); 3 cells; 1900mAh @ 3.7V; 0.012 Ohms/cell.
Speed Control: Castle Creations Phoenix 35; 0.0045 Ohms; High rate.
Drive System: Direct; 11x7 (Pconst=1.11; Tconst=0.95) direct drive.
Airframe: Anything you want.
Motor Amps = 20.0
Motor Volts = 10.3
Input (W) = 205.5
Prop RPM = 6105
Thrust (oz) = 24.1
PSpd (mph) = 40.5

Cheers, Phil
Sep 21, 2005, 03:07 PM
Registered User
darrell_f's Avatar
Those numbers are out of line with my actual results on two 2808/24's. First, 11X7 is far too big for that motor on 3S, and second, 24 oz thrust is far too low. I'm using a 9X6 folding prop with a TP3S 2100 and it will pull a 30 oz sailplane vertical. That prop is pulling about 17 - 18 max amps.


Darrell_F
Sep 21, 2005, 03:42 PM
Registered User
Dr Kiwi's Avatar
Quote:
Originally Posted by darrell_f
Those numbers are out of line with my actual results on two 2808/24's. First, 11X7 is far too big for that motor on 3S, and second, 24 oz thrust is far too low. I'm using a 9X6 folding prop with a TP3S 2100 and it will pull a 30 oz sailplane vertical. That prop is pulling about 17 - 18 max amps.


Darrell_F
Well, I did say - if you choose to believe Motocalc! I have 2808-20, but not a -24 so I have no real world data for that motor. On my test stand, on 3s, my 2808-20 pulls 18.65A with an 8x4 GWS HD for 29.8oz static thrust, and 23.9A with a 9x4.5 APC E for 37.9oz static thrust.

Cheers, Phil
Sep 21, 2005, 08:51 PM
sportytourer
Thread OP
I likely will be getting the 2808-20 as I feel it will take more amps.
I plan on using an APC 10x5e or 11x5.5e prop with this setup. This will likely be around 24amps draw when really pushing it. It should also give me the thrust needed for 3D or pattern. Cross my fingers and see when I get the plane. Tks for the input guys!
Patrick
Sep 21, 2005, 08:56 PM
Registered User
Dr Kiwi's Avatar
Quote:
Originally Posted by sportytourer
I likely will be getting the 2808-20 as I feel it will take more amps.
I plan on using an APC 10x5e or 11x5.5e prop with this setup. This will likely be around 24amps draw when really pushing it. It should also give me the thrust needed for 3D or pattern. Cross my fingers and see when I get the plane. Tks for the input guys!
Patrick
Didn't you see my previous post! 23.9A with only a 9x4.5 APC E - a 10x5 APC E or, heaven forbid, an 11x5.5 APC E on a decent 3s pack will be waaay above that.
Sep 22, 2005, 01:20 AM
sportytourer
Thread OP
I guess so. I wonder why so many are using 10x5 to 11x5.5 in their planes?
Must be pushing the limits of the motor?
Sep 22, 2005, 05:32 AM
Registered User
Dr Kiwi's Avatar
Quote:
Originally Posted by sportytourer
I guess so. I wonder why so many are using 10x5 to 11x5.5 in their planes?
Must be pushing the limits of the motor?
Perhaps they are, but then again everyone says AXI ratings are conservative. I have not run my 2808-20 enough in-flight to know whether the stated 25A is really the limit or not - all I know is that it was beginning to feel it on the test stand at close to that level of input.

I imagine that most motors feel better if run at only ~80% of their max, just the way LiPoly packs do better at ~80% of max C rating.

It depends a lot on the left stick - if you don't use WOT a lot, the average draw is only a small percentage of max, and if you never reach WOT, what the motor draws at WOT is completely moot!

Cheers, Phil
Last edited by Dr Kiwi; Sep 22, 2005 at 01:54 PM.
Sep 22, 2005, 11:06 AM
Glow Free Since Aug, 2005
tommy321's Avatar
I'm using the -24 in a Groove. I've settled on a 10x5APC E prop and I get roughly 22-25A full throttle static (depending on the condition of my battery pack :P).

I initially used an 11x4.7 APC SF prop on the motor (because I saw others using it on here) with 3S, but ended up blowing the motor during a static run-up. It did teach me how to rewind and AXI though It's had 100s of flights since then.

I'd say go with the -24 for 3D. The higher winds will let you swing a slightly bigger prop, which (IMHO) is better for the slower flight speeds of 3D flying. If you wanted a pattern plane I'd suggest the -20.

Tom
Sep 22, 2005, 11:42 AM
sportytourer
Thread OP
True! The 24 I have read is better for a larger prop. But then I get confused when I read that the 20 can handle slightly more amps. If the 20 can handle more power than the 24 with a 10x5 prop than it may be more versatile in the end.
I heard from another guy who has friends using both 20 and 24 and he said the 24 is slightly slower than the 20. But the 20 can swing a 10x5e prop and has the ability to hover.
I feel it may not be as efficient doing this (using a larger prop) but still can and has a higher Amp rating that will allow more power to be used. So the 20 likely would not be as good for 3D but still can do it but would also be good for pattern.
I still think I will try the 20 mainly based on the higher Amp rating.
tks for all the input!
Last edited by sportytourer; Sep 22, 2005 at 07:30 PM.
Sep 22, 2005, 03:08 PM
Glow Free Since Aug, 2005
tommy321's Avatar
Fair enough… I’m sure that motor will work fine with it…. I know of lots of SA Grooves that are flying on the –20 wind and the people flying them seem to like them as much as those on the –24 wind. The quiet storm is probably the same.

However, simply to continue the discussion, remember that the reason the –20 is rated for more amps is probably simply because with the fewer winds they’re able to fit thicker diameter wire into it. Thicker wire can carry more current before melting. The downside is that fewer turns means less torque at a given speed, which generally means a smaller prop is used.

I don’t have any first hand experience with the –20 (this is all just theory speaking), but if you were to put the same 10x5 prop on a –24 and a –20, the –20 would draw more current, but turn the prop slower. Where the –20 gains power on the –24 is by spinning a smaller prop a lot faster. (which lends itself to faster flying). You might find that you hit the current limit of the –20 with a propeller that the –24 can swing without hitting its (albeit lower) current limit (again, never had a –20, so I’m just guessing).

I tend to think of it like gears in a car. High winds (low kVs) are like first gear in a car. Lots of oomph to turn a big prop, but they loose torque quickly as propeller speed (and aircraft forward speed) increases.

As you reduce the winds the motor is more like 2nd or 3rd gear in a car. They produce power efficiently by turning a smaller prop faster.

Assuming you want to keep the setup running in its efficiency band, you should match the winds to the style of flying that the airplane does.

Just throwin' it out there... if anyone's had a -24 and -20 side by side, free to tell me if I'm wrong

Tom
Last edited by tommy321; Sep 22, 2005 at 03:14 PM.
Sep 22, 2005, 07:39 PM
sportytourer
Thread OP
Ohh you had to tell me. I was not aware that the power consumption of a 20 would be higher than a 24 for a similar prop (say 10x5e). If that is really the case than I likley would be better off with a 24 cause I know I will be using a minimum 10" prop in order to do both 3D and pattern. And if so then I would use less power with a bigger prop in the 24 than in a 20.
BTW how much does your plane weigh and how is it's ability to hover and pull up?
Sorry for the questions but I have lots of theory but lack the actual flight results....
I heard from another guy who has a 26oz plane and uses a 2808-20 with a 10x5e. He says that he can fly fast, hover at half throttle and pull up with ease.
I went to the model motors website and they have a chart to use but it seems to fall short with reagards to 11.1 v Lipos. It appears the efficency of the motor falls off substantially the larger the prop is used. However when looking at an 8 cell config with 10x6 prop the 20 puts out more watts at a slightly higher rpm though at a lower efficiency. It is no wonder both engines work.
Motor Prop. w/o gearbox Battery I/A RPM U (V) P- OUT (W) P- IN (W) Efficiency (%)
2808/20 10x6 Aer CAM 8xRC 1700 21,4 8080 8,6 121 184 66
2808/24 10x6 Aer CAM 8xRC 1700 19 8010 8,8 118 166 71
My interpretation is that they both can swing a 10x5e prop with 11.1V Lipos but the 20 may be less efficient doing so. It however may offer more thrust in the end.
I also pulled these stats on the two motors:
Model 2808/24
Cells: 6 - 10
RPM/V: 1160
Max Efficiency: 82%
Max Efficiency Current: 6 - 13 A
Current Capacity: 22 A / 30S
Model 2808/20
Cells: 7 - 10
RPM/V: 1250
Max Efficiency: 80%
Max Efficiency Current: 10 - 18 A
Current Capacity: 25 A / 30S
This indicates to me that the 20 can handle more current at more efficient rates than the 24 and also has a higher overall capacity.
It seems the more I read the more I realise how similar both motors are in the end though they accomplish it in different ways.
I hope more people share their experience with how they perform....
I think I will try the 20 cause I wish to do both pattern and 3D. Will see, I can always try my park 400 4200Kv with a 12x6 and 6:1 if I have problems..
tks,
Last edited by sportytourer; Sep 23, 2005 at 01:19 AM.
Sep 22, 2005, 11:44 PM
3d and other
kevbeer's Avatar
nice...I will go with the 24 for 3d also.
Sep 23, 2005, 01:37 PM
sportytourer
Thread OP
I read this in another post. "A friend of mine has the 2808-24, there is not that much more of a difference between the 2808-20. It does run a lower amp draw though. I think it was an amp or two with the same prop. Power wise the 20 has more speed. Vertical is about the same for both motors."

So I guess in conclusion if you want more speed and 3D with more power consumption get the 20. If you want mainly 3D with less power consumption and slower flight speeds use the 24.

The 20 it seems can do it both well as long as you have an esc and battery that can take a 30A draw. If not or if you are looking at efficiency than the 24 may be better though you likely will not get good pattern flying.

Because I have Lipos that are 10c 2200mA and can take 22A continuous I will be forced to get the 24 in order to stay in a lower Amp range.

tks for everyone's input!
Sep 23, 2005, 04:03 PM
Clinger, MAGA
rclark's Avatar
FWIW, I didn't see it above, but The Great Motor Test is very useful real world results . I have both motors. Nut shell /20 for pattern, /24 for 3D . I routinely fly my /24s (have 3) on a 11x5.5E no problem (flying one on my Jitterbug right now in fact) . On my late Groove I used an 11x7E . This is using 3S1P 2200/2600/3100 packs. Note that I am at 5000 ASL too so at SL a smaller prop may be more appropriate... Currently pulling my /20 power Acrocub with a APC 9x6E. The motor stays cool too, so I must not be pushing it too hard and I have excellent performance on the 25oz plane. I suspect I could go to a 10x5 no problem if I actually needed the power . This has been my experience ... your's may vary with your usage of the throttle stick ...

Here is a link to some data that I meaured for a couple of my planes Some Data that may be useful ...


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