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Park flier electric project, motor spec ?
29 Jan 2019 23:16 - 29 Jan 2019 23:18 #2
by Quorneng
Replied by Quorneng on topic Park flier electric project, motor spec ?
100 W/lb is a good figure to use and would give a reasonable performance but the "slow" part of a "slow flying trainer" really all comes down to wing loading and of course ones mans 'slow' is another's 'fairly quick'! 
For a gentle park flyer you should be aiming for a wing loading of no more than 10 oz/sqft which indicates the plane should have a wing area of 3.6 sqft. Is it that big?
As your motor is only going to give you 71 w/lb at your anticipated weight you could do with both shaving of some weight to keep a limit on the wing loading and achieve some 'spare' performance. The faster you have to fly the more power you must have.
On the positive side you should find a LiPo of the same capacity and voltage as a NIMh is about half the weight so there is a significant weight saving to be had there.
You could get a more powerful and heavier motor but then the slow flying bit may go out the window.
The nearest thing I have to a conventional slow park flyer is a Hobby King Slow Stick.
It is slightly modified (surprise, surprise to save a bit of weight!) and tips the scales a 22 oz with a 2200mAh 2s LiPo giving a wing loading pf 6.8 oz/sqft. With a slow revving 9x6 prop it manages very well on 120W and has power to spare.
For a gentle park flyer you should be aiming for a wing loading of no more than 10 oz/sqft which indicates the plane should have a wing area of 3.6 sqft. Is it that big?
As your motor is only going to give you 71 w/lb at your anticipated weight you could do with both shaving of some weight to keep a limit on the wing loading and achieve some 'spare' performance. The faster you have to fly the more power you must have.
On the positive side you should find a LiPo of the same capacity and voltage as a NIMh is about half the weight so there is a significant weight saving to be had there.
You could get a more powerful and heavier motor but then the slow flying bit may go out the window.
The nearest thing I have to a conventional slow park flyer is a Hobby King Slow Stick.
It is slightly modified (surprise, surprise to save a bit of weight!) and tips the scales a 22 oz with a 2200mAh 2s LiPo giving a wing loading pf 6.8 oz/sqft. With a slow revving 9x6 prop it manages very well on 120W and has power to spare.
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01 Feb 2019 12:47 - 01 Feb 2019 12:48 #11
by Quorneng
Replied by Quorneng on topic Park flier electric project, motor spec ?
Do suppliers always give the right numbers when advertising a sale?
It will pay to take the 160 W figure of your motor as being "indicative".
You can boost a motor's power quite truthfully by simply using a big capacity battery rather than one likely to be used for flight. There is less battery voltage 'sag' under load so the motor can see a bit more voltage.
As an example lets say your motor actually gives 140W on a 3s 'flight' size battery that is giving a realistic 10 V at the motor. This means 14 A will be flowing.
With a big capacity battery the motor could well see 10.5 V. The current is more or less proportional to the voltage squared. 10.5 squared is 110 whereas 10 squared is 100 so a 10 % increase. The amps will thus rise to 15.4 A. Watts are VxA so 10.5 x 15.4 = 161 Watts. Bingo!
Furthermore there is also a question as to whether the motor could sustain this elevated power level continuously.
The only sure way is to measure with a Watt meter for any particular motor/battery/prop combination.
I am not suggesting your set up will not work and there is nothing wrong with 'try it and see' but simply to be aware of the potential problems.
Yes, at 41" the wing loading will be a bit lower but 'not a lot'!
It will pay to take the 160 W figure of your motor as being "indicative".
You can boost a motor's power quite truthfully by simply using a big capacity battery rather than one likely to be used for flight. There is less battery voltage 'sag' under load so the motor can see a bit more voltage.
As an example lets say your motor actually gives 140W on a 3s 'flight' size battery that is giving a realistic 10 V at the motor. This means 14 A will be flowing.
With a big capacity battery the motor could well see 10.5 V. The current is more or less proportional to the voltage squared. 10.5 squared is 110 whereas 10 squared is 100 so a 10 % increase. The amps will thus rise to 15.4 A. Watts are VxA so 10.5 x 15.4 = 161 Watts. Bingo!
Furthermore there is also a question as to whether the motor could sustain this elevated power level continuously.
The only sure way is to measure with a Watt meter for any particular motor/battery/prop combination.
I am not suggesting your set up will not work and there is nothing wrong with 'try it and see' but simply to be aware of the potential problems.
Yes, at 41" the wing loading will be a bit lower but 'not a lot'!
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03 Feb 2019 14:23 #14
by Quorneng
Replied by Quorneng on topic Park flier electric project, motor spec ?
Klippy
You post looked coherent enough to me!
About the only thing I would ague about it a 7x6 stalling with a 1450 Kv motor on a 3s. I was hand launching just such a set up (150W motor) this morning with absolutely no problem but then its wing loading and all up weight are well below Ian proposed figures!
Ian
Reading through your posts I would point out that the kV of a motor does not determines its power output but just the speed it will turn at for a given applied voltage.
The power of an electric motor, and even more so for out runner types, is largely determined by its weight. There is a fixed ratio between the weight of the copper wire, the iron bits and the magnets. The weight of everything else (mainly the shaft and bearings) is small in comparison. So the heavier the motor the more copper, iron and magnets there are to convert electricity into mechanical power.
I am still concerned at the wing loading you are expecting to achieve. Whilst 160W (if you can achieve it) is just about enough to fly a 1.75 lb model a wing loading of 16? oz/sqft will make a hand launch marginal.
Using an undercarriage, which adds both weight and drag thus raising the power to fly a bit further and you don't have much 'spare' to start with, so unless you have a long enough super smooth take off surface it may be no more successful than a hand launch.
If only the wing area was 2.7 sq ft instead of 1.7 for little or no weight increase!
You post looked coherent enough to me!
About the only thing I would ague about it a 7x6 stalling with a 1450 Kv motor on a 3s. I was hand launching just such a set up (150W motor) this morning with absolutely no problem but then its wing loading and all up weight are well below Ian proposed figures!
Ian
Reading through your posts I would point out that the kV of a motor does not determines its power output but just the speed it will turn at for a given applied voltage.
The power of an electric motor, and even more so for out runner types, is largely determined by its weight. There is a fixed ratio between the weight of the copper wire, the iron bits and the magnets. The weight of everything else (mainly the shaft and bearings) is small in comparison. So the heavier the motor the more copper, iron and magnets there are to convert electricity into mechanical power.
I am still concerned at the wing loading you are expecting to achieve. Whilst 160W (if you can achieve it) is just about enough to fly a 1.75 lb model a wing loading of 16? oz/sqft will make a hand launch marginal.
Using an undercarriage, which adds both weight and drag thus raising the power to fly a bit further and you don't have much 'spare' to start with, so unless you have a long enough super smooth take off surface it may be no more successful than a hand launch.
If only the wing area was 2.7 sq ft instead of 1.7 for little or no weight increase!
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