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Old Dec 06, 2012, 08:14 PM
Seeker of Thermals
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Originally Posted by peterlngh View Post
Hi, RoboSerg.

A sailplane, not a hotliner, will generally use a motor with a kv from 850-1200. One wants to be able to swing a large prop to generate thrust at low speed.

A non folding prop will generate so much drag, particularly if it windmills, that soaring will be virtually impossible.

Soaring time depends on how much current one uses with the motor. A sailplane with four micro servos will be able to run the radio for well over an hour if one doesn't spend time flying under power.
Hi, peter.
Just curious why you think a freewheeling prop generates more drag than a stationary prop? I can't think of how that's possible.

--
Greg
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Old Dec 06, 2012, 08:48 PM
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Hi, Greg.

A stopped propeller blade causes drag in proportion to the blade's area. A windmilling prop causes drag in proportion to the area of the disk because it absorbs energy in order to spin. I'm having a senior moment and can't recall the technical term. It'll come to me. Eventually...
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Old Dec 06, 2012, 10:06 PM
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i think i found the reason my phoenix dropped off to the right on second launch the other day, the stinking rudder servo went nuts and locked the rudder . So i put in new servos for rudder and elevator. 17g metal gear servos,had to cut just a bit of the plywood mount to get them to fit,but man what a difference !
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Old Dec 07, 2012, 02:52 AM
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Originally Posted by botulism View Post
Hi Serg.
1. Lower RPM = more torque = ability to swing a larger prop.
Thanks for the answers guys. Couple more:

1.
So whats the downside of using smaller prop with higher KV motor? This way you have almost same torque + more speed.
Turnigy2836 1200kv has 1100g thurst.
Turnigy2836 2350kv has 800g thurst.

Ok less thrust but double more speed, right ?

1a.
What is highest KV motor one whould go? I still dont understand what downsides I will be having installing 2200KV motor ?

4.
Are the wings detachable? I can see the wings are held in play with 4 screws. Does it hold good or any further mods are required to hold the wings in place (think of AXN Clouds Fly)

5.
What shaft diameter do I need for the spinner that comes with the ARF version? I was thinking about Turnigy 2836 1200kv motor but it has 4mm shaft.
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Old Dec 07, 2012, 05:03 AM
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Nothing important to add to this discussion but I do want to say I have the Turnigy 2836 1000KV motor using a 9x6 folding prop on my Phoenix Evolution (flying with the short wings) and that motor has been working superb. Also the 9x6 folding propeller (from Hobbyking) has not come off once from the 4m diameter shaft on the 2836 motor. What a great combination those two are. I'm really thinking of trying the other 2836 with a slightly higher KV rating and see how that does. And after this motor with the 4m diameter shaft I really don't want to use any motor with a smaller diameter shaft.

http://www.hobbyking.com/hobbyking/s...dProduct=13141

http://www.hobbyking.com/hobbyking/s...idProduct=8139
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Old Dec 07, 2012, 08:00 AM
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Originally Posted by HoosierGuy View Post
I'm not sure that foam in the can I used to fix my wings would be good for more repairs. So far it has not become rock hard. I did use it to fix my wings but I also coated the top and bottom with that spackling I bought and then covered the top and bottom with shoo goo. But I'm still experimenting with it though.
H.G.,

the foam-in-a- can "Great Stuff", etc. has solvents which need to escape / evaporate; the Goop coating you mentioned may be slowing that process, or you may have the low density product version.

Manufacturers produce the foam in at least two different expansion rates / densities- one that expands a lot but has bigger gas pockets (weaker) after expansion & drying, and another that expands less but ends up with smaller gas pockets, is denser, and is likely structurally stronger after drying. The can label should specify how many cubic feet to which the contents can be expected to expand.

Snow is coming to Colorado's high country; time to swap out wheels for skis again! Here's one of my more recent designs for snow flying; the 62" span wing mounts on three different fuselages. (With all of the venting on a glider fuselage as I've done on my Phoenix 2000, landings in soft snow can result in moisture getting to the ESC, servos, receiver... not a good thing!)

I built the fuselage below with snow flying in mind; ESC & receiver protected within the solid EPP foam fuselage, & a solid 1/8" aircraft ply mount plate in the belly to which the ski /wheel gear can be mounted. (A folding prop is used.) Even with the drag of the wire landing gear and skis / wheels, this one glides power-off nicely & does some thermalling now & then too! But the snow takeoffs & landings are really a lot of fun- what I designed this fuselage to do! So If you already have a wing you like, you might consider scratch-building a fuselage for it.

[Historical Inspirations: Dynaflite sells a built-up balsa kit design called the "Butterfly" which had a 100" span wing on a fairly streamlined fuselage with a small nose-mounted glow engine, and light landing gear. Great Planes still distributes it these days:

http://www.dynaflite.com/airplanes/gpma0090.html

Back in the early '80s, I put aluminum skis under a 100" span Bob Martin 2S glider design so that I could land on the snow of the ball fields at the foot of the Lake Dillon dam after flying the slope breezes there.

[In these days of 'homeland insecurity' they don't allow anyone to fly that fine slope any more... but I spent a lot of hours there teaching myself to fly in those slope winds in the early '80s.]

FLY!!!

VIKING
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Old Dec 07, 2012, 11:44 AM
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viking60 awesome plane dude! Where do I get those ski ?

Quote:
Originally Posted by HoosierGuy View Post
Nothing important to add to this discussion but I do want to say I have the Turnigy 2836 1000KV motor using a 9x6 folding prop on my Phoenix Evolution (flying with the short wings) and that motor has been working superb. Also the 9x6 folding propeller (from Hobbyking) has not come off once from the 4m diameter shaft on the 2836 motor. What a great combination those two are. I'm really thinking of trying the other 2836 with a slightly higher KV rating and see how that does. And after this motor with the 4m diameter shaft I really don't want to use any motor with a smaller diameter shaft.

http://www.hobbyking.com/hobbyking/s...dProduct=13141

http://www.hobbyking.com/hobbyking/s...idProduct=8139
So is the spinner that comes with phoenix 2000 is for 3mm or 4mm motor shafts ?
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Old Dec 07, 2012, 12:02 PM
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To put it very simply...

A small prop spinning fast will help a plane dive fast while a big prop spinning slower will make a plane climb fast. Since the motor in a powered glider is just a means to gain altitude fast, big slow props are better. Mostly it's due to blade area.

A prop is just a wing that spins. A bigger "wing" gives more lift which, when it's applied to a propeller, means more thrust. This is especially obvious at low airspeed and that's where performance, for anything but a hotliner, is most important. In fact, excessive speed can be a problem with any plane with a lifting airfoil.
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Old Dec 07, 2012, 12:21 PM
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United States, MT, Kalispell
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Originally Posted by RoboSerg View Post
Thanks for the answers guys. Couple more:

1.
So whats the downside of using smaller prop with higher KV motor? This way you have almost same torque + more speed.
Turnigy2836 1200kv has 1100g thurst.
Turnigy2836 2350kv has 800g thurst.

Ok less thrust but double more speed, right ?

1a.
What is highest KV motor one whould go? I still dont understand what downsides I will be having installing 2200KV motor ?

4.
Are the wings detachable? I can see the wings are held in play with 4 screws. Does it hold good or any further mods are required to hold the wings in place (think of AXN Clouds Fly)

5.
What shaft diameter do I need for the spinner that comes with the ARF version? I was thinking about Turnigy 2836 1200kv motor but it has 4mm shaft.
1. Well, think of a F1 race car engine (20K RPM) vs. a Monster truck engine. Which would you rather have to get you off the ground and out of trouble? The higher RPM might give you more top end speed, but the motor with more torque will haul you around and get you up to altitude faster. This is a powered sailplane. They are meant to be flown by powering up to altitude (200-500 m), and then turning the motor OFF while you go looking for lift (thermals). They are not meant to be flown by flying around with the motor on continuously. Buy yourself a Parkzone T-28 if you want to do that.

1b. totally depends on the prop you're swinging. You could probably go with a 8 incher with the high kV motor. But the 10 or 11 inch prop will give you more power. And if you put the larger prop on the higher kV motor, it will draw WAY more current than it should, and either blow up the motor, the ESC, or shorten your battery life to a few minutes.

4. yes, removable wings held on w/ four 3mm screws. When you crash, it will sometimes rip out the foam around the screw holes in the wing. Repair w/ polyurethane (gorilla) glue.

5. the supplied motor has a 3mm shaft. Most of the motors in that size range have a 4mm shaft, so you'll need new prop adapter and spinner if you replace the motor. I believe the stock prop is a 10x5 or 10x6.

--
Greg
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Old Dec 07, 2012, 01:22 PM
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Originally Posted by botulism View Post
5. the supplied motor has a 3mm shaft. Most of the motors in that size range have a 4mm shaft, so you'll need new prop adapter and spinner if you replace the motor. I believe the stock prop is a 10x5 or 10x6.

--
Greg
Thanks for your answers! Really helps!
So I would need the spinner - like this one? Do I need a prop adapter too? I dont have a clue how folding props are "working" / mounted.

For the motor I thought Turnigy 3632 Brushless Motor 1200kv
Has Pull: 1400-1930g but a 4mm shaft.

There is also a motor with 3mm shaft, so I dont have to buy a spinner - Turnigy 2632 Brushless Motor 1000kv
But it has thrust: 500~900g, a little bit too low isnt it?
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Old Dec 07, 2012, 02:42 PM
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Hi, RoboSerg.

I have one of these motors, a KD A22-20L, in an 800gram glider and it will keep accelerating in a vertical climb till it's out of sight! I have used a 10X6 prop the most but have also used a 9X5, a 9.5X8, and an 11X6. The motor runs nice and cool with all of them and I really can't say a single bad thing about it. I would not hesitate to use one in a glider that weighs 1500 grams or even a bit more. It has a 3mm shaft and even comes with a spare. I'm assuming that you are somewhere in Europe so, considering that shipping from GiantShark was very reasonable to the U.S., shipping shouldn't be a problem.

http://www.giantshark.co.uk/a22l20l-...otor-p-49.html

The KDA has very similar performance to the Turnigy 2836 that HoosierGuy mentioned. I also recently got a Turnigy SK3 2836-1040kv. I have only bench tested it so far but it turns a little faster than the KD A22-20L without drawing more current. In other words, it's a little more efficient but I haven't actually tested it in the air so I can't give an unqualified recommendation. Here's a link for comparison and note that it has a 3.17mm shaft just to keep things confusing.

http://www.hobbyking.com/hobbyking/s...dProduct=30779

Most spinners use a cone shaped collet that clamps down on the shaft when tightened. There's one in the picture of the 2830 motor you linked to. Some spinners come with an assortment of adaptors while others only fit one size. As one collects different props and spinners one will eventually find that some adapters are interchangeable or can be reworked if you have a little experience with basic metalworking. I'm afraid that I can't help you on finding a good European supplier but someone should have the entire Graupner line available and they are of very good quality so they probably end up being cheaper in the long run because you won't end up with mismatched parts.

I am using a couple of spinners from HobbyKing and they work well but I received some dangerously defective prop blades from them. It's only the one size of the one brand but I can not recommend a HobbyKing prop in good conscience. You can read about my experience here.

http://www.rcgroups.com/forums/showthread.php?t=1779139

Also note that those motors with a 4 digit code like the 2830 and 2836 tell you the approximate, relative, power level. The first 2 digits are the can diameter and the second 2 are the length. All other things being equal, the longer motors have more power and are a little heavier. Then you get a company like KDA that uses a different code. Once again, they do all they can to keep things confusing!

Cheers!
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Old Dec 07, 2012, 04:27 PM
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United States, IN
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Originally Posted by peterlngh View Post
To put it very simply...

A small prop spinning fast will help a plane dive fast while a big prop spinning slower will make a plane climb fast. Since the motor in a powered glider is just a means to gain altitude fast, big slow props are better. Mostly it's due to blade area.

What's more important on a prop - the length or the pitch? If you get a bigger/larger motor with a higher KV then you want a smaller prop, right? When you get a smaller prop, what's more important - a smaller length or pitch?
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Old Dec 07, 2012, 04:44 PM
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Originally Posted by HoosierGuy View Post
What's more important on a prop - the length or the pitch? If you get a bigger/larger motor with a higher KV then you want a smaller prop, right? When you get a smaller prop, what's more important - a smaller length or pitch?
this will help you decide,it will also let you know if the prop size and motor will work together ..

http://www.ecalc.ch/motorcalc_e.htm?ecalc
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Old Dec 07, 2012, 05:28 PM
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Hi, HoosierGuy.

Length is more important than pitch for everything but maximum speed. Think of it this way.

A prop is a wing. The lift of this wing is thrust. A bigger wing has more lift so a bigger prop has more thrust. The pitch is the angle of attack. A greater angle of attack, to a critical point, creates more lift but, when the critical point is reached, it results in a stall so lift (thrust) quickly falls towards zero.

Let's stay with the concept of a prop as a wing. It doe not screw itself through the air. The combination of area and airfoil and speed creates a lifting force defined as thrust. The lift of the airfoil can actually exceed the "pitch speed" possible if the prop is running at 100% efficiency. At the speeds where most gliders fly; a big blade area with a low pitch and a high RPM will give more thrust than a small blade with a high pitch and the same RPM. The large blade and low pitch will allow one to climb at greater efficiency while the small blade and high pitch will only be an advantage in level or diving flight.
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Old Dec 08, 2012, 03:10 PM
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peterlngh thanks for extensive answers!

I am now actually thinking between phoenix 2000 OR durafly dynamic-s.
What I like about dynamic-s is that its more durable, has higher quality and just looks cool The downside is 1560mm wingspan vs 2000mm for phoenix.
Price difference isnt that much 78.72USD vs 111USD. Another pro for dynamic-s its available in german warehouse as PNF.

So I woudnt write all this if I didnt want to know your opinion. Does anyone has both of them? Which flies better? Does dynamic-s has really better quality? I cant decide...
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