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Folding prop size

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Phil B17/03/2019 09:21:24
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I need to replace an old brushed 600 sized motor in an electric glider with a brushless motor. I want this to produce a lighter better thermal capable model. So 7.4v lipo and small folding prop for slow climbs and low drag when gliding. I have put in a 400 sized 2510kv motor. Is a small folding prop suitable? If so what size? The original prop is 8 inch diameter including its 2 inch spinner, so the blades are 3 inches each. On a power meter this gives 140 Watts at 11.1 volts, but only about 60 Watts at 7.4 volts. The recommended prop is a fixed one of about 5.5 by 5.5, but I don't want this on a glider.

David Hall 917/03/2019 09:33:25
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I too have to go through the prop selection process from time to time.. Especially when changing to 2 cell working. Most of the 2s planes that I have seen using motors around about that size suggest 2826/2830 motors of 1200Kv to 1800Kv with props around 9x5-10x5. A larger prop is generally desired.

Your motor rating of 2510Kv seem to be high for a folding prop installation. What size is your motor (diameter etc.)?

 

Edited By David Hall 9 on 17/03/2019 09:34:00

ken anderson.17/03/2019 10:10:01
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hello phil,i think also you have chose a motor with a too high KV......

ken anderson...ne..1....high KV dept.

David Hall 917/03/2019 10:41:00
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To add.... the reason for the choice of a 2s installation for smallish gliders (<2.5m) is, for many, to allow the whole system (RX, servos,esc) to run at battery voltage without a BEC. This will enable a very small ESC to be used...as saving weight and (I'm told) reducing the number of failure points in the system.

For me, the extra cost is not worth it. Running 3s suits me in gliders of that size. This allows plenty of flexibility in motor/prop choice. For my 2M class E-soarers, I've settled upon 2826/1000Kv motors and 9.5x5 to 10x6 folding props. I like the Turnigy Propdrive 2826 / 2830 series motors, but this setup tends to be more powerful than I need for F5J style climbs with these 650g gliders.  Many use even lighter 2826/2822 motors.  

I have a single 2s powered glider, my Topmodels 2.5M Avia. This weighs around 1100g. I tried a 3730 1000Kv motor and a 13x7 prop and found it to be very good for my needs, and the low revving sound is just great!

Edited By David Hall 9 on 17/03/2019 10:42:57

Phil B17/03/2019 15:44:11
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Yes thanks all for helping . I know the motor is not really suited.

It's specs are 27mm diam 30mm length, 59grams max amps 20, 2510kv. I am going to try it on a small folding prop and if the (old) glider flies well, I will change to a Turnigy or similar. I've had the motor so long I just want to make some use of it.

My other glider restoration projects are

Reichard Champion, fitted with Mega motor

Puffin Big Swift with a Phasor 15/4.

All are round 2m.

I will have similar guesswork about props for those too.

Simon Chaddock17/03/2019 17:41:20
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Phil

With a Watt meter connected you could play around with the prop sizes until the motor to draws up to its 20 A limit on 2s and then change it for a folding one.

I would agree 2510 Kv is much too high. 1000kV would be better with something like a 9x6. Serious thrust and much better efficiency so a smaller battery for the same 'power on' time..wink 2

PatMc17/03/2019 17:56:48
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Posted by David Hall 9 on 17/03/2019 10:41:00:

To add.... the reason for the choice of a 2s installation for smallish gliders (<2.5m) is, for many, to allow the whole system (RX, servos,esc) to run at battery voltage without a BEC. This will enable a very small ESC to be used...as saving weight and (I'm told) reducing the number of failure points in the system.

I don't see that using 2s without a BEC would save any weight, In fact using 2s the ESC would have to have a higher current rating compared with using 3s for the same power. It's more likely that a higher current rated ESC would increase the weight, albeit by a small amount.

Piers Bowlan17/03/2019 18:28:55
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Your motor sounds like it might be better suited to a pylon racer than a glider Phil, so why use it, just because it is what you have? Without knowing a bit more about your glider (weight/wing area) it is difficult to speculate but something like this might be suitable and as Simon says, a 9X6 might be appropriate, looking at the spec. The best investment would be a what meter, as already sugested. Personally I would use a 1300Ah 3s LiPo with this, but whether that would fit in your glider and put the C of G in the right place I have no idea. What battery pack was used in the model originally and how much did the battery weigh?

David Hall 917/03/2019 18:42:35
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Posted by PatMc on 17/03/2019 17:56:48:
 

I don't see that using 2s without a BEC would save any weight, In fact using 2s the ESC would have to have a higher current rating compared with using 3s for the same power. It's more likely that a higher current rated ESC would increase the weight, albeit by a small amount.

Yes, it is a fine difference.. As I have been advised, the ESC and combined BEC might weigh around 25g whilst an ESC only, even at a higher current rating, can be around 6g (I am using one with a separate BEC in my 2M). Added to this, the servos, running at a high voltage, can be a little lighter due to their higher torque output. I reckon that if I were to refit my 2M esoarer for 2S, I could save 40g.

It is the case that many in the 2M class comps run 2s for these and possibly other reasons.... my own 2S just uses a standard ESC/BEC and servos.. I just liked the sound.

As I said, I'm not about to try to cut the odd gram from my installations at extra expense (the range of servos is more limited and they tend to be more expensive), but lots of people see this as necessary.

 

Edited By David Hall 9 on 17/03/2019 18:43:44

Phil B17/03/2019 18:49:27
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The original was a nicd or nimh pack of 8 or 10 cells I think and it was under the wing as the huge 600 motor up front balanced the model. I think a 60g motor plus 2200 3 cell lipo up front will get the cg right. The glider is old, 2.4 m span, scratch built rudder and elevator only and I got it to renovate as it reminds me of my first, a precedent electrafly. I might just use fix props of 5x5 or 6x4 and have spares for breakages on landing.

PatMc17/03/2019 20:18:51
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Phil, is there any reason you want a slow climb ?

I mainly fly E-gliders & always try to limit the power on time to 25 - 35 secs then hunt for lift, more or less emulating tow line launching. The climb is fast enough to get the model to useful thermal detecting altitude in that power burst time. This way if any signs of lift are spotted before launch the model can be sent up to the area quickly. I usually use lipos with sufficient capacity for 6 - 8 power bursts simply to save messing about changing batteries too often,

The motor you have seems a bit small for a 2.4m glider (unless it's extremely light) & as has been said the high KV means it can't use a prop that will provide much thrust.
The latest glider I've built is a vintage Graupner Amigo that I modified to take a nose mounted 71g motor. The Amigo is 2m span, RTF weight is 930g with a 3s 1400 lipo or 900g with a 3s 700 lipo fitted, prop is a 9x5 folder, initial power 165W. The motor I use is this 1050 kv one the spec is given here.
One incidental benefit was that I had a couple of folding prop with spinner sets left over from 600 can brushed motor days & the shaft dia (3.17mm) of the Keda motor was identical. Simply changing the dubious original prop blades for Graupner 9x5 blades saved a bit of cost.

Phil B17/03/2019 22:31:04
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Yes that motor looks good but how do you mount it in the nose ? The existing models nose is for a can motor held by 2 screws through the front just behind the spinner.

Phil B17/03/2019 22:45:32
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115 forum posts
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If I mount the KD2217 outrunner from the front the wires have to pass by the rotating case. Or would I install an internal bulkhead to take the rear mount?

PatMc17/03/2019 23:05:03
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  The Keda is mounted the same way as a can motor but the screw centres will be different as it's smaller. Here's my build blog of Amigo which includes some pics of the motor mounting details.

When I bought the motor I also got the KD2217/20 860kv version, this is intended to power one of these , which I haven't got round to building yet. The lower kv will allow me to use a bigger dia prop which should produce more thrust. Although the Introduction is a lot bigger than the Amigo at 2.8m span it should be about the same RTF weight therefore a lower glide speed.
IMO the 860kv motor will probably suit your 2,4m better as well.

This photo from the blog shows the channel I made in the nose block to accommodate the wires past the motor. The channel is in a bottom corner where there's more space as the nose width is quite narrow. 

023.jpg 

Since your model had a 600 size motor there should have more room than the Amigo. 

Edited By PatMc on 17/03/2019 23:20:36

Phil B17/03/2019 23:26:19
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115 forum posts
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Thanks that's most helpful and I think my models nose disc with 17mm centres M3 screws may work with the keda. I'm struggling to find a 45mm spinner with 3.17mm shaft size for 9x5 blades though.

PatMc17/03/2019 23:46:08
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The 3.17mm shaft size is often stated as 3.2mm. Robotbirds have a range of spinners that might be worth looking at - they are catalogued in quite a haphazard manner though.

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