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Old 03-01-2015, 04:42 PM   #1
FlyWheel
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Question Calling all Skimmerphiles!

Yup, I'm back, and I'm planning to build another Skimmer electric glider. However, this is not the build thread, it's more of a planning thread.

I have built this bird before, but that was her smaller sibling, the 400. This time I am doing the 600 (and hopefully my dog won't eat this one!) The 600 has some differences, and I intend to bash in some of my own in as well. I am waiting on the bird purchased to arrive, and I want to build a few accessories and build tools as well as workout the 'neededs' first.

First of all is the powerplant. On the 400 build I chose to go with a brushless outrunner for the obvious reasons of it's slower RPM and greater torque. It is, after all, easier to climb a steep hill in third gear than in overdrive, right? However the appropriate mill was so big around that even after widening the front of the fuselage I still had barely a millimeter of clearance between the spinning bell and the fragile 1.5MM (1/16") balsa! Not only did this make me uncomfortable, it also doesn't allow for very efficient cooling. I don't want to make the same mistake this time, so if the correct outrunner doesn't fit to my satisfaction, I want a back up plan. I was thinking of a (preferably inline) geared inrunner as that plan. That way I don't have to worry about rapidly spinning metal right up against my fuselage walls.

I don't need unlimited vertical at Mach speeds, obviously, this isn't a hot or warm liner, it's just a "liner". But I do want a descent climb, say around 45, without having to worry about flirting with a stall. So I plugged in the known info into MotoCalc 8.08 and allowed it to make it's own choice of direct drive or geared (brushless and 3C Lipo) and this is what it gave as it's #1 recommendation:*
Originally Posted by MotoCalc
Motor: Kontronik KIRA 500-26 (12V); 2600rpm/V; 0.5A no-load; 0.0153 Ohms.
Battery: Kokam 910SHD (15C); 3 cells; 910mAh @ 3.7V; 0.0275 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: Generic 13x9in Prop w/6:1 Gearbox; 13x9 (Pconst=1.31; Tconst=0.95) geared 6:1 (Eff=95%).
Airframe: Skimmer 600; 532sq.in; 43oz RTF; 11.6oz/sq.ft; Cd=0.043; Cl=0.43; Clopt=0.63; Clmax=0.99.
Stats: 42 W/lb in; 38 W/lb out; 18mph stall; 22mph opt @ 67% (27:51, 60F); 27mph level @ 79% (19:54, 61F); 702ft/min @ 21; -171ft/min @ -5.

Possible Power System Problems:

The full-throttle motor current at the best lift-to-drag ratio airspeed (10.2A) is lower than the motor's maximum efficiency current (18.3A). A higher current level would improve system efficiency.
Current can be increased by using more cells, a larger diameter or higher pitched propeller, a lower gear ratio, or some combination of these methods.

Possible Aerodynamic Problems:

The static pitch speed (37mph) is less than 2.5 times the stall speed (18mph), which may result in reduced performance at typical flying speeds and a low maximum speed. This situation is usually acceptable for an electric sailplane or other slow-flying model.
Pitch speed can be increased by using a higher pitched and/or smaller diameter propeller, a lower gear ratio, a higher cell count, or some combination of these methods.

Aerodynamic Notes:

With a wing loading of 11.6oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (29.7oz) to weight (43oz) ratio is 0.69:1, which will result in short take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).
At the best lift-to-drag ratio airspeed, the excess-thrust (15.2oz) to weight (43oz) ratio is 0.35:1, which will give strong climbs and rapid acceleration. This model will most likely readily loop from level flight, and have sufficient in-flight thrust for many aerobatic maneuvers.

General Notes:

This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.

Does anyone else have any suggestions? Tobydogs, you built this bird do you have any suggestions?? (EXCELLENT build thread, BTW, more informative than any three others I have found on this plane combined)







(Who the hell is "Kontronik"? I have never even heard of them, let alone if it is a descent motor. And why is Kontronik always number 1 of MotoCalc's choices? "Kokam" is always their choice of battery too. Hmmmm...)

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Old 03-01-2015, 10:41 PM   #2
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can't think of anything to add from my build. but i wouldn't skimp on the power. i get unlimited vertical to speck out in a flash and then the search is on for lift.

the skimmers are discontinued as a kit,you'v got a rare bird to build. have a blast,she easy to mod for ailerons. having many flights on the skimmer and belly landing in thick grass i am glade the servos are topside of the wing,but using some sort of plastic fairing to protect them would be better on the bottom of the wing.

the next skimmer like kit might be a sig nija. one of my greatest weaknesses is completing a build and not getting around to maidening for years. i still have to maiden my bird of time elec conversion. it has the same motor and basic setup with the esc and battery all up front in the nose. only difference is the hatch has a lot of lead in it to get cg. i'v considered a custom hatch cover built to hold a go pro and fpv gear to fly a distantance flight using thermals to stay aloft. an osd module would be needed to track altitude, distance,speed,level of plane and direction........[dream on]

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Old 03-02-2015, 01:45 PM   #3
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Originally Posted by tobydogs View Post
can't think of anything to add from my build. but i wouldn't skimp on the power. i get unlimited vertical to speck out in a flash and then the search is on for lift.
As the kit isn't scheduled to arrive until next Monday, I don't have access to the plans; so if I may ask, what are the actual inside dimensions of your Skimmer's motor compartment? All of the acceptably powerful motors MotoCalc is giving me range from 40-45MM Dia. (I don't even bother with anything larger, I don't plan on doing 3D with it).

Also, did you ever weigh your completed bird without the motor and battery? One of the parameters MC wants is the weight of the plane without these components. Since weight has a lot to do with this, as the motor will mostly be used only for going up, my guesses may be screwing me up.

Originally Posted by tobydogs View Post
the skimmers are discontinued as a kit,you'v got a rare bird to build. have a blast,she easy to mod for ailerons. having many flights on the skimmer and belly landing in thick grass i am glade the servos are topside of the wing,but using some sort of plastic fairing to protect them would be better on the bottom of the wing.
Discontinued, yes, but not as hard to get as I thought, at least not yet. I put a WTB up on RCGroups and had a response within 30 minutes! Re: your strip elevator mod, I am planning on doing something similar with mine, not ailerons though. However I am toying with an idea that may let me hide the servos and linkages entirely. But that's an topic for another day. Motor now, mods later.

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Old 03-02-2015, 02:39 PM   #4
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flywheel,

i used a turnigy 35-30-1400kv sk motor and 40amp plush esc from hobby king and they don't sell these motors any longer . the nose of the fuse is left as my pictures show it before covering as unfinished since i to had to wait for the motor to arrive. then i mounted the motor to my custom fire wall and custom finished the fuse sides to fit the firewall/motor. once i knew how big the firewall needed to be for the motor to be inside the fuse,i knew how much shaping would be required to mount it and sand the fuse round. so you need to custom design your skimmer to fit the motor. note there is very little space for motor wires to fit in that space and i used a plastic wire management clip to keep them off the spinning motor bell. you can see the bolt head in the fuse side where i bolted the clip in place.

also don't overly worry about cooling the motor,it runs for 20 seconds at wot and get shut down for gliding. i could have left the hatch scoop off but thought it looked cool. a couple of small vent holes would have been plenty.

lastly,i always fly in windy conditions at the club field and enjoy the ailerons very much,i get flat level turns if i fly steady without aggressive aileron use. when landing, the skimmer loves to glide so make sure you have plenty of room to setup and power enough left in your battery to do go arounds. i always seem to go around a few times till i am 100%[or 90%] sure i won't have to walk to the end of the field to p/u the skimmer..lol. feel free to ask any questions along your build and would you consider doing a build thread here to share with us any mode you do? that would be great

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Old 03-03-2015, 12:29 AM   #5
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Originally Posted by tobydogs View Post
flywheel,

i used a turnigy 35-30-1400kv sk motor and 40amp plush esc from hobby king and they don't sell these motors any longer . the nose of the fuse is left as my pictures show it before covering as unfinished since i to had to wait for the motor to arrive. then i mounted the motor to my custom fire wall and custom finished the fuse sides to fit the firewall/motor. once i knew how big the firewall needed to be for the motor to be inside the fuse,i knew how much shaping would be required to mount it and sand the fuse round. so you need to custom design your skimmer to fit the motor. note there is very little space for motor wires to fit in that space and i used a plastic wire management clip to keep them off the spinning motor bell. you can see the bolt head in the fuse side where i bolted the clip in place.

also don't overly worry about cooling the motor,it runs for 20 seconds at wot and get shut down for gliding. i could have left the hatch scoop off but thought it looked cool. a couple of small vent holes would have been plenty.

lastly,i always fly in windy conditions at the club field and enjoy the ailerons very much,i get flat level turns if i fly steady without aggressive aileron use. when landing, the skimmer loves to glide so make sure you have plenty of room to setup and power enough left in your battery to do go arounds. i always seem to go around a few times till i am 100%[or 90%] sure i won't have to walk to the end of the field to p/u the skimmer..lol. feel free to ask any questions along your build and would you consider doing a build thread here to share with us any mode you do? that would be great
I most definitely intend to do a build thread, and mirror it on RCGroups. Although if this thread is any indication "us" may consist of only thee and me.

As for widening the fuse, yeah, I had to do that just to get the AXi 2212 to fit into my 400 Skimmer, and it's wires barely made it around either (I actually had to route them along the inside wall of the air scoop). Widening this one wouldn't do much good though, as it is already the same size both horizontally and vertically, and I can't widen it vertically without making 2 entirely new fuselage halves.

As for your motor, I don't think my version of MC 8.08 even lists it. However some of the motors it does list don't exist either. One brand, "Medusa Research", doesn't even make motors anymore! Methinks an update may be in order...

The model came today, so I got to measure the space inside the nose and came up with about 37MM*. So much of what MC has recommended so far for for "quick takeoffs and steep climbs" may be out of the picture. I'll keep looking though. EDIT: I still need to know what this bird weighs sans battery and motor.




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Old 03-03-2015, 12:16 PM   #6
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Watching here as well still have my skimmer 400 with its brushed s400 motor! It is my new radio test bed.
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Old 03-03-2015, 10:56 PM   #7
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Angry

Does anyone here know what the weight of a completed 600 Skimmer is, without the motor, esc and battery pack? I would like to be able to plug in something better than a guess so I can refine my results.

-------------------------------------------------------------------------------------
OK, now here comes the rant...

I spent hours last night going through all the suggested motor/prop combinations MC has given me. Every combo it listed as "ideal" was simply too big (the fuse nose measures out at 37MM I.D. by the way. According to the plan at least). Of the "acceptable" ones that would also fit, they were all either listed as discontinued, were not sold by anyone Google could find or (according to Google at least) didn't exist, period!

HOWEVER (comma...) I found that when it is given free reign to choose amongst it's library MotoCalc seems to be highly biased against certain brands. Model Motors (AXi) for example. I say biased because not only was there not one AXi motor listed, but when I forced MC to select only from only the AXi motors in it's library it came up with truck loads of possibilities! Most of them were acceptable, many were ideal, and ALL of them fit!!! Instead of scratching and clawing for something that might work, I was actually having to cull the results from the plethora of "ideal" choices.

Axi Rules again!


So why does Moto Calc invariably leave this brand out of their general search?

Is there an RC motor calculator that is not only unbiased in it's selections but can also be periodically updated to stay in touch with the real world? While AXi has so far seemed to be able to bail me out, I really don't wan't to limit myself. There very well may come the day when some other brand works better. There may be one right now but I don't feel like conducting seperate searches of each and every individual brand using MC.

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Old 03-04-2015, 12:46 AM   #8
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[QUOTE=FlyWheel;969692]Does anyone here know what the weight of a completed 600 Skimmer is, without the motor, esc and battery pack? I would like to be able to plug in something better than a guess so I can refine my results.

-------------------------------------------------------------------------------------
sorry i can't help ya as my skimmer is tucked away on a shelf 1000+miles away from where i'm sitting. i could dig it out when i get home.

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Old 03-04-2015, 06:28 PM   #9
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Default The Final Five

Thanks Toby. I eagerly await your info. Anybody else, please speak up...

I got my plethora narrowed down to five. Well, four actually, as the highest powered combo is obviously too powerful; I want it to climb good, but I'm not looking to take out any satellites! Here are the stats for the combinations I thought best from the lowest to the highest selection. As I stated the highest is probably too powerful for my needs, so I'm hoping the correct power output will fall somewhere between it and the lowest. Please look over the facts and give me the combo you think would work best.

Basic stats:
70" WS - 7.6" Avg. chord - 532Sq" wing area - Selig 3021 airfoil.
All data based on the estimated (best guess) weight of 26 ounces without battery, ESC and motor. Reference only.

1.
Model Motors AXI AC2820/10; 1200rpm/V - 2 cell LiPo - 11x7 Prop.
AUW: 37.5oz RTF; 10.2oz/sq.ft
Stats: 77 W/lb in; 56 W/lb out; 17mph stall; 21mph opt @ 52%; 25mph level @ 60%; 963ft/min @ 31.7; -160ft/min @ -5.

Aerodynamic Notes:

The static pitch speed (42mph) is within the range of approximately 2.5 to 3 times the model's stall speed (17mph), which is considered ideal for good performance.
With a wing loading of 10.2oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (30.6oz) to weight (37.5oz) ratio is 0.81:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (19.8oz) to weight (37.5oz) ratio is 0.53:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.
2.
Model Motors AXI AC2826/10; 920rpm /V - 2 cell Lipo - 13x9in Prop
AUW: 38.6oz RTF; 10.5oz/sq.ft
Stats: 77 W/lb in; 57 W/lb out; 17mph stall; 21mph opt @ 49%; 25mph level @ 57%; 1136ft/min @ 37.7; -162ft/min @ -5.

Aerodynamic Notes:

The static pitch speed (40mph) is within the range of approximately 2.5 to 3 times the model's stall speed (17mph), which is considered ideal for good performance.
With a wing loading of 10.5oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (36.1oz) to weight (38.6oz) ratio is 0.93:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (22.9oz) to weight (38.6oz) ratio is 0.59:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.
3.
Model Motors AXI AC2820/8; 1500rpm/V; - 2 cell LiPo; - 11x6in Prop
AUW: 38.5oz RTF; 10.4oz/sq.ft
Stats: 110 W/lb in; 78 W/lb out; 17mph stall; 21mph opt @ 52%; 25mph level @ 61%; 1278ft/min @ 43.6; -162ft/min @ -5.

Aerodynamic Notes:

The static pitch speed (43mph) is within the range of approximately 2.5 to 3 times the model's stall speed (17mph), which is considered ideal for good performance.
With a wing loading of 10.4oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (40.1oz) to weight (38.5oz) ratio is 1.04:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (25.4oz) to weight (38.5oz) ratio is 0.66:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.
4.
Model Motors AXI AC2826/10; 920rpm/V; - 3 cell LiPo; - 11x6in Prop.
AUW: 41oz RTF; 11.1oz/sq.ft
Stats: 106 W/lb in; 85 W/lb out; 17mph stall; 22mph opt @ 56%; 26mph level @ 65%; 1368ft/min @ 45.6; -167ft/min @ -5.

Aerodynamic Notes:

The static pitch speed (45mph) is within the range of approximately 2.5 to 3 times the model's stall speed (17mph), which is considered ideal for good performance.
With a wing loading of 11.1oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (44.5oz) to weight (41oz) ratio is 1.09:1, which will result in extremely short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and vertical climb-outs. This model will probably be able to perform a hover or torque roll.
At the best lift-to-drag ratio airspeed, the excess-thrust (28.7oz) to weight (41oz) ratio is 0.7:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.
Overboard combination:
5.
Model Motors AXI AC2826/12; 760rpm/V; - 3 cell LiPo - 13x8in Prop
AUW: 41oz RTF; 11.1oz/sq.ft
Stats: 112 W/lb in; 81 W/lb out; 17mph stall; 22mph opt @ 47%; 26mph level @ 55%; 1586ft/min @ 56; -167ft/min @ -5.

Aerodynamic Notes:

The static pitch speed (43mph) is within the range of approximately 2.5 to 3 times the model's stall speed (17mph), which is considered ideal for good performance.
With a wing loading of 11.1oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (49.3oz) to weight (41oz) ratio is 1.2:1, which will result in extremely short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and vertical climb-outs. This model will probably be able to perform a hover or torque roll.
At the best lift-to-drag ratio airspeed, the excess-thrust (32.9oz) to weight (41oz) ratio is 0.8:1, which will give very steep climbs and incredible acceleration. This model can easily do consecutive loops, and has sufficient in-flight thrust for any aerobatic maneuver.

( Not bad for little motor that's only 35 MM in diameter!)

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Old 03-05-2015, 02:22 AM   #10
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Lightbulb While I was waiting....

I thought, although the motor and prop aren't yet set in concrete, and the ESC will depend on that choice I do know what receiver I plan to use in this bird. So I did a Google on Berg RXs and discovered that Amazon has 3 MicroStamp 4channel receivers in stock, since these aren't being made anymore I decided to get all three. I also found some Berg Microstamp crystals and snagged three of them too. I do the same thing with bike parts when I see that something I really like is looking to go extinct.

Perhaps I was a squirrel in a past life? Whatever, it's worked for me; I get to keep something's working the way I want it to work!

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Old 03-06-2015, 12:45 AM   #11
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Default Another question:

Is there a table or a formula for calculating the servo torque required for a particular control surface? I need to figure out what size mech's I'm going to need for the feathers on this bird.

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Old 03-06-2015, 12:52 AM   #12
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Originally Posted by FlyWheel View Post
Is there a table or a formula for calculating the servo torque required for a particular control surface? I need to figure out what size mech's I'm going to need for the feathers on this bird.
Yes....

http://www.mnbigbirds.com/Servo%20To...0Caculator.htm

This bird will take next to nothing....

You will find the values are low - but don't forget the servo gear size matters too. But again about any servo will work for this as the flight speed and loads are tiny.

Mike
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Old 03-06-2015, 01:01 AM   #13
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hobbyking hxt900's work great in the skimmer.thats what i used.

my now favorite small servos are EMAX eso8mgd's. they work smooth,have nice torque and don't break the bank. you can get them at headsuprc and shipping is cheap.

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Old 03-06-2015, 11:55 PM   #14
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Thanks Mike. I'll check that out then go shopping for servos. And thanks Toby for a 'place to start'.

EDIT: I just remembered, I'm 87.3% sure have two HiTec HS-35's, They were scheduled for ailerons in another plane but I never used them. I also have a couple of bigger ones; HS-55s or 85,s I'll have to check. if the torques are good enough on an of these obviously I will just use them.

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Old 03-08-2015, 01:34 AM   #15
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OK, I used the servo calculator rcers gave me, and plugged in the largest possible sizes of the control surface and greatly exaggerated estimates of how far it will move and at what aircraft speed and it gave me a torque estimate of just under 9 oz./in. The HS-35's in my drawer, according to HiTec are rated at 11 oz./in. So I think they'll work!

Now, one last dumb paranoid electrical question, which I probably already know the answer to but as I said; "paranoid": If two ANALOG servos are hooked up to the same receiver channel via a y connector both will move the exact same amount at the same time, right?

Or do I need digital servos to achieve this?

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Old 03-09-2015, 01:56 AM   #16
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That will work just fine for you.

Just remember while the HS-35's will be enough "torque" but sometimes those tiny gears strip easily. I personally would use the much cheaper HS-55's.

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Old 03-09-2015, 02:30 AM   #17
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I dont know the model you have chosen, but you wont need all that much power for a strong climb. My Bubble Dancer and Supra both climb very nicely - more than enough for contest flying - on about 75-100 watts/pound.

Im running a 28mm A20-22L Hacker outrunner with a 3S 850 pack and a 11x6 prop that peaks at about 200 watts in my 42 oz Bubble dancer. Thats right at 75 watts/pound. It gets it to 200 meters/700ft in under 30 seconds.

My Supra weighs in at 55 oz and has a 330 watt power system for about 96 watts/pound. It gets to 200 meters in about 20 seconds.

Your model should come in much lighter, so you certainly dont need any more power.

Keep in mind too that your power system will only be running for short periods of time. You dont need to worry about cooling all that much when run times are under 30 seconds with long cooling off periods in between.

Most guys seem to want to over power their sailplanes. More power equals more weight which equals less flight time and less ability to core small thermals.

If soaring is something you want to do, then dont penalize yourself with to much weight/power.

On my 30 oz Mirage, I ran it with a 75 watt peak power system for quite a while. Thats about 40 watts/pound. It was more fun when I bumped it up to 50 watts/pound though.

On servos - I agree the HS35's will be iffy. Probably strong enough in the air most times but fragile gears. You're going to be landing with no gear so the wing and tail servos can take some abuse from tall grass, weeds, etc. Id go at least HS55's or preferably HS65HB's for more durability.

Good luck with the build!

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Old 03-09-2015, 03:29 AM   #18
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Originally Posted by Larry3215 View Post
I dont know the model you have chosen, but you wont need all that much power for a strong climb. My Bubble Dancer and Supra both climb very nicely - more than enough for contest flying - on about 75-100 watts/pound.

Im running a 28mm A20-22L Hacker outrunner with a 3S 850 pack and a 11x6 prop that peaks at about 200 watts in my 42 oz Bubble dancer. Thats right at 75 watts/pound. It gets it to 200 meters/700ft in under 30 seconds.

My Supra weighs in at 55 oz and has a 330 watt power system for about 96 watts/pound. It gets to 200 meters in about 20 seconds.

Your model should come in much lighter, so you certainly dont need any more power.

Keep in mind too that your power system will only be running for short periods of time. You dont need to worry about cooling all that much when run times are under 30 seconds with long cooling off periods in between.

Most guys seem to want to over power their sailplanes. More power equals more weight which equals less flight time and less ability to core small thermals.

If soaring is something you want to do, then dont penalize yourself with to much weight/power.

On my 30 oz Mirage, I ran it with a 75 watt peak power system for quite a while. Thats about 40 watts/pound. It was more fun when I bumped it up to 50 watts/pound though.
Yeah, I have gotten a few responses like that. I think the 1st one on the list will meet my demands best. It's also the lightest. I'm still not really sure whether I should be going by the watts in or watts out, however they both fall within the "best estimates" I have been getting:
Model Motors AXI AC2820/10; 1200rpm/V - 2 cell LiPo - 11x7 Prop.
AUW: 37.5oz RTF; 10.2oz/sq.ft
Stats: 77 W/lb in; 56 W/lb out; 17mph stall; 21mph opt @ 52%; 25mph level @ 60%; 963ft/min @ 31.7; -160ft/min @ -5.

Aerodynamic Notes:

The static pitch speed (42mph) is within the range of approximately 2.5 to 3 times the model's stall speed (17mph), which is considered ideal for good performance.
With a wing loading of 10.2oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (30.6oz) to weight (37.5oz) ratio is 0.81:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (19.8oz) to weight (37.5oz) ratio is 0.53:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Originally Posted by Larry3215 View Post
On servos - I agree the HS35's will be iffy. Probably strong enough in the air most times but fragile gears. You're going to be landing with no gear so the wing and tail servos can take some abuse from tall grass, weeds, etc. Id go at least HS55's or preferably HS65HB's for more durability.

Good luck with the build!
Really? The HS-35's have "Metal Karbonite" gears and the HS-55's gears are just nylon. Is it the size of the gears that make the HS-35 more fragile? I was only going to use the HS-35's for reflex/camber on the wings anyway, I was already thinking of going with the HS-65HB for the rudder and elevator, as they will be the ones to control the plane under power (and be close to or touching the ground on landing). At 25+ oz./in. they should be sufficient for just about anything they're going to encounter on this plane short of an actual crash.

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Old 03-09-2015, 05:27 AM   #19
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I think that Axi you have listed is still way over kill. Thats a 400+ watt motor that weighs 161 grams and is 36mm diameter.

The motors Im using in the Bubble dancer and Mirage are 50 gram motors that are 28mm in diameter. You could easily save 100+ gms/3.5 oz in weight and have a lot more clearance in the fuse with a 28mm motor and still have more than enough power for a strong climb.

The Hacker I mentioned above would be plenty of power on 3S with an 11x6 prop. You could also use one of the Hyperion outrunners of similar size and kV. They tend to be less $$ than the hackers and Axi's and I have had excellent results with them over the years.

The HS35's may have carbonite gears but they break very easily in my experience.

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Old 03-09-2015, 05:37 AM   #20
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Here is a Hyperion thats on sale for a rediculous price that should work for you.

208 watts with a 12x6 prop on 3S. Or you could go with a 13x6 on 3S for 228 watts.

59 grams, 28mm diameter.

On sale for $16

http://www.aircraft-world.com/en/p10...2213#/pkl-part

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Old 03-09-2015, 05:42 AM   #21
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It occurs to me that I dont know squat about your proposed model. If it is a "warmliner" then it may have been designed for a much larger/heavier power system.

If you take my advice on a small power system, you may run into trouble with balancing unless you're willing to mod the fuse a good bit.

No point in going with a light weight power system if you just have to add back in the saved weight in the form of lead shot.

Do you know what power systems were originally recommended in this model?

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Old 03-09-2015, 05:48 AM   #22
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ok, after a quick search I see guys have flown them with 55 gram motors so you should be fine.

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Old 03-09-2015, 11:30 PM   #23
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I guess I should have placed the basic specs up rather than leave them buried in the MotoCalc readout.

Skimmer
Speed 600 powered sailplane
Wingspan: 70"
Wing Area: 532sq. in.
Length: 40"
Flying Weight: 44 oz.
Wing Loading 12oz. per sq.ft.
(These specs are assuming the originally intended 600 Speed motor and the 7-8 barrels of nickel that served as batteries in those days)

Do you have a lead* on that 50 gram Hacker that works with the 11 x 6 prop? 9 grams isn't a stellar weight reduction but the smaller prop will add less drag to the formula. I plan to shed as much weight as I can on the @$$ end of this bird, so hopefully I will need a minimal amount of 'snot' in the 'nose'. Cost isn't the deal breaker, obviously; I was intending on using an AXi,remember?)

*(or at least the model number so I can search for one?)

My receivers came today, and I also got a second HS-55 and two HS-65's at the HS. All they had were HS-65MG (metal gear). Over kill for the overkill, I know. But better to have than to need, right?

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Old 03-09-2015, 11:48 PM   #24
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Which ZS 2213?? I told MotoCalc to restrict it's search to Hyperion motors and use a 11x6 prop and it didn't exactly give me a rave review...

Motor: Hyperion HP-Z2213-20; 1025rpm/V; 0.65A no-load; 0.102 Ohms.
Battery: Kokam 910SHD (15C); 2 cells; 910mAh @ 3.7V; 0.0275 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: Generic 11x6in Prop; 11x6 (Pconst=1.31; Tconst=0.95) direct drive.
Airframe: Skimmer 600; 532sq.in; 31oz RTF; 8.4oz/sq.ft; Cd=0.043; Cl=0.43; Clopt=0.63; Clmax=0.99.
Stats: 43 W/lb in; 31 W/lb out; 15mph stall; 19mph opt @ 68% (23:32, 74F); 23mph level @ 80% (17:45, 78F); 459ft/min @ 16; -145ft/min @ -5.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (10.7A) falls approximately between the motor's maximum efficiency current (6.6A) and its current at theoretical maximum output (33.4A), thus making effective use of the motor.

Possible Aerodynamic Problems:

The static pitch speed (29mph) is much less than 2.5 times the stall speed (15mph), which may result in reduced performance at typical flying speeds and a low maximum speed. This situation is usually acceptable for an electric sailplane.
Pitch speed can be increased by using a higher pitched and/or smaller diameter propeller, a higher cell count, or some combination of these methods.

Aerodynamic Notes:

Due to some of the potential problems listed above, this model may require an experienced pilot.
The static thrust (18.9oz) to weight (31oz) ratio is 0.61:1, which will result in short take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).

At the best lift-to-drag ratio airspeed, the excess-thrust (8.9oz) to weight (31oz) ratio is 0.29:1, which will give strong climbs and rapid acceleration. This model will most likely readily loop from level flight, and have sufficient in-flight thrust for many aerobatic maneuvers.
I'd stlll like to look at that Hacker.

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Old 03-10-2015, 12:54 AM   #25
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The Hackers are actually 57 grms, so only a 2 gm difference.

The Hyperion I linked to above is the 2213-22 model which has a lower kV and will need a larger prop.

Are you planning to use a folding prop or regular prop? If your not going to use a folding prop, then there is no point in going over board. Any non-folding prop is going to screw your chances of soaring pretty badly.

If you're going to use a folder, which you really should, then bigger is better as it will be more efficient in the climb allowing you to get by with a slightly smaller battery and less power over all.

Take those motoCalc predictions with a very very large grain of salt. They apply to "normal" planes much more than sailplanes. Plus they are just a wild guess anyway based on motocalc's best guess about the aerodynamics of your model.

You can find Hackers almost anywhere that sells good stuff and doesnt only carry Hobby King level goods.

Here is one of my favorite sellers, but there are many out there.

Oh, and this is for the A20-20L. I mis-remembered the model number. This one should do best with an 11x6 or 12x6 on 3S depending on how good the packs are. I would start with an 11x6 and check the amps. You want to keep peak amps under about 25 amps or so hot off the charger.

My setup with 3S ThunderPower 70C 850 packs pulls right at 24-25 amps for about 270 watts for the first couple of seconds then drops to about 19-20 amps for around 210 watts or so by the end of the 25 second motor run.

Using cheaper battery packs, I was pulling much lower power levels - 190 watts peak dropping to 140 watts with some crappy 1500 GensAce packs. Good batteries make a big difference.

http://www.espritmodel.com/hacker-a20-20l-motor.aspx

As long as your all up weight comes in less than that 44 oz listed in the spec, you will be fine with a 200 watt peak power system.

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