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Not enough power?

Twin motor set-up

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Stephen Belshaw05/10/2019 16:19:09
111 forum posts
27 photos

This doesn't seem like enough power to me, my set-up is as follows:

2 x HobbyKing FC 28-12 1534Kv brushless outrunners

2 x 30A HobbyKing ESC's

2 x 8 x 4 Aerostar composite props

The ESC's are connected to a Y lead with the red wire cut on one of them, the battery feeds the ESC's in parallel. With a wattmeter connected I'm getting 106 watts at full throttle, drawing 16A. I'm assuming, given how the battery is wired, that this is the sum total of both motors?

The AUW of the model, a TwinStar, is around 1300g in new money so a tad under 3lbs and I doubt this is anywhere near enough power.

What am I doing wrong, how can I get more power?

Denis Watkins05/10/2019 17:03:31
4541 forum posts
123 photos

The Lipo, and the number of cells is important Stephen.

What size is yours

Stephen Belshaw05/10/2019 17:09:28
111 forum posts
27 photos

Oh, it must have gone AWOL during the edit???

2S 7.4V 2200mah

Piers Bowlan05/10/2019 17:12:30
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2167 forum posts
53 photos

Try 3s on a couple of 7x 6 props. 2s is not enough I suspect.

Denis Watkins05/10/2019 17:15:19
4541 forum posts
123 photos

As Piers says

Those motors are 3 - 4s

Makes a bit difference from 2s

Stephen Belshaw05/10/2019 17:18:02
111 forum posts
27 photos

Ok, I can plug a 3S in, would the 8 x 4 props not be suitable?

David Hall 905/10/2019 17:18:19
279 forum posts
16 photos

Given that you see 106W at 16A, you have around 6.6v at the power meter. This indicates that you are using (if LiPo batteries) a 2S arrangement. If you have a 3S battery, there is something seriously wrong....

You might try a 3S battery and ensure that it has a good enough C rating. You might to well with a 300W power draw, so as a yardstick.. a 3S battery capable of supplying 30A without breaking a sweat. I like to overspec mine, so I'd look for a battery that will give 30A at around 60% of its C rating, so a full rating of 50A minimum.. for example a 2200mAh 3s 25C battery would work, or a smaller capacity at a higher C rating...

 

oops...    more answers appeared whilst typing... 

 

Edited By David Hall 9 on 05/10/2019 17:19:11

Geoff S05/10/2019 17:24:34
3701 forum posts
29 photos

What battery are you using? LiPo? 2S?, 3S?, 4S?. Those props are quite small. You can get more power either by upping the voltage (eg going to 3S from 2S) or increasing prop diameter or pitch.

Had a look at the motors and they're only rated at 10 amps with a 3S LiPo so that's only about 100 watts each with a 7x4 prop or a bit more (150 watts) with a 7x6. They seem a bit marginal for a Twinstar weighing 1.3kg but would probably fly it.

btw your assumption is correct. The 16 amps is divided to the 2 motors so the total power is 16 x battery voltage. So I'm guessing your battery voltage is only 6.25 volts which is only a 6 cell NiMH or a 2S LiPo just about fully discharged. So I guess you need more volts.

Sorry this is a bit rambling - I was thinking aloud

Geoff

Piers Bowlan05/10/2019 17:26:19
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2167 forum posts
53 photos

These are the same motors as the Emax CF 28-12 1534kv which are rated at about 16A max I believe. With the 7x 6 prop and a 3s LiPo you should get about 150w (?) each, - without melting the motors. This is a bit hypothetical but I will try later today and get back to you, hopefully....

You could try the 8x4 props on 3s but the amps might be a little high. Best to test it with a watt meter if you you have one. If you pull too many amps for too long you will need new motors. crying

Edited By Piers Bowlan on 05/10/2019 17:48:50

Stephen Belshaw05/10/2019 19:41:25
111 forum posts
27 photos

Connected a three cell 2200 LiPo, wow, what a difference!

Now putting out 314 watts and pulling 28 amps with the 8 x 4 props (14 amps per motor so still within safety margins)?

I'm guessing the TwinStar would fly quite nicely on half throttle, if so, what would be my approx. duration?

Edited By Stephen Belshaw on 05/10/2019 19:43:25

Andy4805/10/2019 20:29:17
1550 forum posts
1 photos

Difficult to say. Half throttle equates to half thrust, but the current consumption will be a lot less than half, at a guess less than 10 amps. That should give about 8 minutes without discharging the battery too far.

Geoff S05/10/2019 23:16:15
3701 forum posts
29 photos

To determine endurance the best thing to do is fly for a set time well below the anticipated maximum (say 4 minutes), land and check the battery. That'll give you pretty good feel for how long it's safe to fly.

Geoff

Simon Chaddock06/10/2019 00:31:02
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5736 forum posts
3034 photos

Stephen

An 8 x 4 is quite a fine pitch prop for its diameter. Yes, it will deliver a good static thrust but this will fall off quite noticeably as the plane speeds up.

You may well find you get a better overall performance with 7x6 props which is what is recommended by Hobbyking for this motor on a 3 s..

Stephen Belshaw06/10/2019 05:43:41
111 forum posts
27 photos

Thanks all, some very good advice. I will look to swap the props, I'm using the 8 x 4's as they are all I have - as I'm predominantly a glider guider I don't have a stock of these things!

 

Would there be any advantage in running the two motors in opposite rotation to each other, I understand it is often done on twins?

Edited By Stephen Belshaw on 06/10/2019 05:45:59

Piers Bowlan06/10/2019 09:55:04
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2167 forum posts
53 photos

Some twins have counter rotating propellers (P38 Lightening, Piper Seminole) because of the engine out case, so that there is not a critical engine (the resultant torque/yaw/roll effect & p-factor is the same), regardless of which engine fails.

The Vought V173 had inward rotating props to reduce the drag caused by the wing tip vortices.

You could try it but if you break one you will need two spares, just in case!

Simon Chaddock06/10/2019 12:50:37
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5736 forum posts
3034 photos

Stephen

The benefit of counter rotating props really comes down to the planes power to weight ratio.

For example the props on the DH Mosquito rotated the same way but the smaller lighter and even more powerful DH Hornet had counter rotating.

It is normal to have them rotating 'over the top towards the fuselage' so if an engine fails the torque of the good engine tends to lift the wing of the dead engine making control a bit easier.

Whilst an electric motor failure is not that common all models do usually have more power for their weight than full size. To make matters worse even modest powered electric motors will produce a substantial torque from low revs, considerably more than a similar power IC engine can, so counter rotating props do eliminate the torque effects from suddenly applying full power in a panic when you are flying low, slow and near the ground.

Its a more common situation than you might think! wink 2

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