Prop Size?

[Tosh McCaber]

Embarrassment – twice over!   A long winded tale!

 

A couple of weeks ago, I was flying my Little Plank – a 48 inch power glider, first built in 1972, I have flown it , mainly as a glider for many years.  It originally had space for a Cox.049 in the tail.   I recently added a leccy motor, and had several pretty decent flights.  The motor was  a 2012/10t/1400, and, from recollection, I think that the prop was a folding 8 x 4.  3S battery. Total weight 950g

 

The incident occurred as it approached the field.  An object fell from its rear end. I managed to glide in to land with no problems.  Ay this time, I discovered that, not only had the propeller fallen off, but it also took the motor, along with a small chunk of the rear fuselage!  It had been very noisy, and I suspect that the problem would have been an unbalanced prop!  (Bad boy!  Always balance the prop!)

 

I ordered a new motor via AliExpress. They told me it would arrive sometime in the early September. In fact, seven days later, it dropped through my letterbox from China (- via Leicestershire, according to the package!)

 

Repairs made to the rear fuselage, I installed the motor with a fixed 8 x 4 propeller,  took it out to the field today, to find that the prop was just hitting the swept rear edge of the fin, which hadn't been the case before!  (Second bit of embarrassment- test at home before trying to start the motor on the field!)

 Now, I can't remember the size of the original prop! 

This situation could mean that the geometry of the fixed pitch prop has placed it closer to the back of the fin.  Or perhaps the original folding prop, which has now disappeared, was actually a 7x4!

 

Before I make any modifications, I would be obliged if any of you motor gurus out there can give me some guidance as to a suitable prop for my motor setup.  Whilst I do think that the previous prop was an 8x4, I don't want to overload the motor /ESC with the wrong size!

 

Thanks

[Shaun Walsh]

35 minutes ago, Tosh McCaber said:

Embarrassment – twice over!   A long winded tale!

 

 

 

A couple of weeks ago, I was flying my Little Plank – a 48 inch power glider, first built in 1972, I have flown it , mainly as a glider for many years.  It originally had space for a Cox.049 in the tail.   I recently added a leccy motor, and had several pretty decent flights.  The motor was  a 2012/10t/1400, and, from recollection, I think that the prop was a folding 8 x 4.  3S battery. Total weight 950g

 

 

 

The incident occurred as it approached the field.  An object fell from its rear end. I managed to glide in to land with no problems.  Ay this time, I discovered that, not only had the propeller fallen off, but it also took the motor, along with a small chunk of the rear fuselage!  It had been very noisy, and I suspect that the problem would have been an unbalanced prop!  (Bad boy!  Always balance the prop!)

 

 

 

I ordered a new motor via AliExpress. They told me it would arrive sometime in the early September. In fact, seven days later, it dropped through my letterbox from China (- via Leicestershire, according to the package!)

 

 

 

Repairs made to the rear fuselage, I installed the motor with a fixed 8 x 4 propeller,  took it out to the field today, to find that the prop was just hitting the swept rear edge of the fin, which hadn't been the case before!  (Second bit of embarrassment- test at home before trying to start the motor on the field!)

 

 

 Now, I can't remember the size of the original prop! 

This situation could mean that the geometry of the fixed pitch prop has placed it closer to the back of the fin.  Or perhaps the original folding prop, which has now disappeared, was actually a 7x4!

 

 

 

Before I make any modifications, I would be obliged if any of you motor gurus out there can give me some guidance as to a suitable prop for my motor setup.  Whilst I do think that the previous prop was an 8x4, I don't want to overload the motor /ESC with the wrong size!

 

 

 

Thanks

 

Can you provide a link to the motor?

[Shaun Walsh]

If it's this motor

https://www.aliexpress.com/item/32672300112.html?spm=a2g0o.productlist.main.1.614a3b89u8ScHA&algo_pvid=32056846-a0b4-409f-809a-ed9f3d3e60aa&aem_p4p_detail=2023090108111029836197666000004908744&algo_exp_id=32056846-a0b4-409f-809a-ed9f3d3e60aa-0&pdp_npi=4%40dis!GBP!9.43!1.5!!!11.61!!%40211b88ec16935810705748319e1a76!60071035157!sea!UK!0!A&curPageLogUid=nkeAQVGWLHTn&search_p4p_id=2023090108111029836197666000004908744_1

Then a 7x4 fixed prop will draw about 10 amps at WOT, within its limits, an 8x4 would draw around 15 amps, above the motor's rating. 

Figures from PropCalc.

Battery Load: 8.84 C Voltage: 10.76 V Rated Voltage: 11.10 V Energy: 13.32 Wh Total Capacity: 1200 mAh Used Capacity: 1020 mAh min. Flight Time: 5.8 min Mixed Flight Time: 7.3 min Weight: 105 g   3.7 oz

Motor @ Optimum Efficiency Current: 10.46 A Voltage: 10.66 V Revolutions*: 13065 rpm electric Power: 111.5 W mech. Power: 91.0 W Efficiency: 81.6 %

Motor @ Maximum Current: 10.60 A Voltage: 10.65 V Revolutions*: 13038 rpm electric Power: 112.9 W mech. Power: 92.2 W Efficiency: 81.6 % est. Temperature: 35 °C   95 °F   Wattmeter readings Current: 10.6 A Voltage: 10.76 V Power: 114.1 W

Propeller Static Thrust: 628 g   22.2 oz Revolutions*: 13038 rpm Stall Thrust: - g   - oz avail.Thrust @ 0 km/h: 628 g avail.Thrust @ 0 mph: 22.2 oz Pitch Speed: 80 km/h   50 mph Tip Speed: 437 km/h   271 mph specific Thrust: 5.56 g/W   0.2 oz/W

Total Drive Drive Weight: 190 g   6.7 oz Power-Weight: 124 W/kg   56 W/lb Thrust-Weight: 0.66 : 1 Current @ max: 10.60 A P(in) @ max: 117.7 W P(out) @ max: 92.2 W Efficiency @ max: 78.3 % Torque: 0.07 Nm   0.05 lbf.ft Climb Capacity: 1275 m   4183 ft

Airplane All-up Weight: 950 g   33.5 oz Wing Load: 19.0 g/dm²   6.2 oz/ft² Cubic Wing Load: 2.7   est. Stall Speed: 21 km/h   13 mph est. Speed (level): 50 km/h   31 mph est. Speed (vertical): - km/h   - mph est. rate of climb: 3.7 m/s ( ~35...40° ) 725 ft/min

[Tosh McCaber]

Here it is Shaun- cheap as chips, and great service!

 

https://www.aliexpress.com/p/order/detail.html?orderId=8173377361214137&tracelog=rowan&rowan_id1=send_all_goods_20220522_1_en_US_2023-08-17&rowan_msg_id=4137817337736121$503b8464db6a4f46919a73ad83710612&ck=in_edm_other

 

Actually, on further inspection, for 3S iAli Express seems to be recommending a 6x3 prop.  I certainly had either an 8x4 or7x4 on before though!

[Tosh McCaber]

Shaun- many thanks for the info above- it came in after I'd posted- that's an excellent set of statisics! I was going to check out loadings on my watt mter anyway.

 

I guess Propcalc is on the web- I shall check it out.  Does Propcalc give recommendations for all sizes motors?

 

Anyway- I'll try it out withn the 7x4- hope that there's enough oomph!

[Tosh McCaber]

Just checked the site out- looks very good!

[Keith Billinge]

Install some motor standoffs. I use plumbing type tap washers with longer bolts through the centre or sometimes listed on Ebay as rubber spacers/standoffs. Doing some up tighter than others can give any side-thrust etc that you need.

They have the benefit of making motors a bit quieter too.

That motor setup you quote is fine I use that on quite a few models....

Edited September 1, 2023 by Keith Billinge

[Tosh McCaber]

Hi Keith,

 

Good tip!  Did you mean that you've had success with the 2012/10t/1400 motor, with an 8x4?  Don't want overheating/ overloading, having seen the charts sent by Shaun!!?

[Outrunner]

56 minutes ago, Tosh McCaber said:

Hi Keith,

 

Good tip!  Did you mean that you've had success with the 2012/10t/1400 motor, with an 8x4?  Don't want overheating/ overloading, having seen the charts sent by Shaun!!?

Why don't you check your setup with a Wattmeter. You have a Wattmeter don't you Tosh?

[leccyflyer]

Tosh - using stand offs as Keith shows would also have the advantage in your pusher configuration in making the model much quieter when the power is on. Just increasing the distance between the prop and the trailing edge by a few mm can make a huge difference in the prop noise generated.

[Tosh McCaber]

Put the motor on the wattmeter, with the 7x4 prop..  It measures 148W, 12.6A, on a 7x4 prop on 3S, which I hope will be within the motor limits??

[leccyflyer]

On 01/09/2023 at 16:26, Tosh McCaber said:

Here it is Shaun- cheap as chips, and great service!

 

https://www.aliexpress.com/p/order/detail.html?orderId=8173377361214137&tracelog=rowan&rowan_id1=send_all_goods_20220522_1_en_US_2023-08-17&rowan_msg_id=4137817337736121$503b8464db6a4f46919a73ad83710612&ck=in_edm_other

 

Actually, on further inspection, for 3S iAli Express seems to be recommending a 6x3 prop.  I certainly had either an 8x4 or7x4 on before though!

Tosh - that's a link  to your account, not to the motor itself. It just comes up as a Change Your Account error message for me.

[Shaun Walsh]

2 hours ago, Tosh McCaber said:

Put the motor on the wattmeter, with the 7x4 prop..  It measures 148W, 12.6A, on a 7x4 prop on 3S, which I hope will be within the motor limits??

Remember that the current draw will drop in flight, so it will probably be OK especially if the ventilation is good.

[MattyB]

@Tosh McCaber, is the new motor identical in specs (or close enough not to worry about)? The fact the new prop hit the fin implies can length is shorter, and (whilst it’s somewhat of a simplification because other factors can play into it) if the diameter is the same as the old one and the can length shorter, it’s ability to dissipate heat (and therefore it’s peak power) is likely to be lower.
 

What was the diameter, can length, kV and weight of the original motor?

Edited September 2, 2023 by MattyB

[PatMc]

4 hours ago, Tosh McCaber said:

Put the motor on the wattmeter, with the 7x4 prop..  It measures 148W, 12.6A, on a 7x4 prop on 3S, which I hope will be within the motor limits??

That's about 2.5 - 3 times the power of a Cox 049 which would give that power on 6 x 3 or 6 x 4 prop.  

[Tosh McCaber]

Here's the correct replacement motor specs page:

 

https://www.aliexpress.com/item/1005002438773237.html?spm=a2g0o.order_detail.order_detail_item.3.1959f19cRGM3oS

 

Unfortunately, the original motor, bought a good while ago, is now in the long grass somewhere off the field!  Although I'm pretty sure that it would have been the same dims- I have another couple sitting around in the workshop, all the same size.

 

I'll see how it goes on the field, in a couple of days time.

[leccyflyer]

Tosh - depends which of the 6 different Kv that the 2212 motor at that link you have, most of them have 12amps as the maximum current capacity, the 2700kv says 14-22amp peak current. Your prop will unloads a little in the air, which might get you down to 10amps WOT and if it's your flying ironing board that you flew the other night, I don't think you'll be at WOT very much. As PatMc said you're putting a lot more power into the model with that set up than the old Cox .049 was delivering, so you can throttle back a bit.

[Tosh McCaber]

Thx Leccy et al- I'll give you the lowdown when I fly it again.  (would have been yesterday, but for a very blustery wind- no fun!)

[MattyB]

Before you fly it, run the numbers on eCalc or similar to see it you are over amping anything. It only takes 5-10 mins, and may save you making an error which costs you a motor and/or ESC.

[Mike T]

On 03/09/2023 at 11:16, Tosh McCaber said:

Here's the correct replacement motor specs page:

 

https://www.aliexpress.com/item/1005002438773237.html?spm=a2g0o.order_detail.order_detail_item.3.1959f19cRGM3oS

 

That's the bog standard motor people use when brushlessifying their Twinstars.  It's what I've got on mine and with 20A ESCs they run just fine on 8 x 4 props.  BTW - folding props always tend to fold back a little when running, which may have been enough for your original prop to not hit the fin.

[Tosh McCaber]

OK guys- took the Little Plank out to the field earlier, having installed a 7x4 prop.   (Once again- a blustery wind- but I went for it this time)

After an initial check , an initial chuck at full throttle,it blasted its way heavenward!  Well- not quite- but- a very spritely climb!  Big loops no bother from level flight, easy rolls, quite sensitive, but positive controls.

Interestingly, the 7x4 definitely performed as well as the 8x4.  Which I guess proves that it probably was overpropped before.

Unusual design- I can definitely recommend it for either power or slope soaring!

 

Thanks for all your interesting input!  (Hope that the motor doesn't part with the model again!)

Edited September 9, 2023 by Tosh McCaber

[Tosh McCaber]

Part 2

It's been flying really well, since it's been trimmed out.  However, on launching, it really needs a good 45 deg throw by the person who's chucking it, to avoid an initial contact with the grass! He has to have a really good strong arm!

 

The structure would take it, (see my initial photo at the top of the thread) so I'm considering upgrading the motor a bit from the 2212 10T 1400kv with a 7x4 prop, to a 28?? -????kv motor.  Next motor family up.  Now, unfortunately,  I'm pretty useless at filling in the question mark spaces for the specs.  I'd be obliged if any of you motor gurus out there can help?  Along with prop size (preferably 8" max due to the prop clearance on the wing and fin?)

 

Here's the range of motors that I am looking at from Ali Express-  free 7 day delivery:

https://www.aliexpress.com/item/1005005796051525.html?isdl=y&aff_fsk=_oBXVPn9&src=Connexity2023GBchannel&aff_platform=aff_feeds&aff_short_key=_oBXVPn9&pdp_npi=2%40dis!GBP!6.71!6.71!!!!!%40!12000034385635562!afff&dp=16965144744380026599815511159008005&cn=326576&cv=90dc9d53ad3a459934a2d8241df6ed58

 

Any suggestions/ discussion welcomed!

[MattyB]

48 minutes ago, Tosh McCaber said:

Part 2

It's been flying really well, since it's been trimmed out.  However, on launching, it really needs a good 45 deg throw by the person who's chucking it, to avoid an initial contact with the grass! He has to have a really good strong arm!

 

The structure would take it, (see my initial photo at the top of the thread) so I'm considering upgrading the motor a bit from the 2212 10T 1400kv with a 7x4 prop, to a 28?? -????kv motor.  Next motor family up.  Now, unfortunately,  I'm pretty useless at filling in the question mark spaces for the specs.  I'd be obliged if any of you motor gurus out there can help?  Along with prop size (preferably 8" max due to the prop clearance on the wing and fin?)

 

Here's the range of motors that I am looking at from Ali Express-  free 7 day delivery:

https://www.aliexpress.com/item/1005005796051525.html?isdl=y&aff_fsk=_oBXVPn9&src=Connexity2023GBchannel&aff_platform=aff_feeds&aff_short_key=_oBXVPn9&pdp_npi=2%40dis!GBP!6.71!6.71!!!!!%40!12000034385635562!afff&dp=16965144744380026599815511159008005&cn=326576&cv=90dc9d53ad3a459934a2d8241df6ed58

 

Any suggestions/ discussion welcomed!

 

There is a whole forum dedicated to this with specialist threads on motors, ESCs, prop choices and wattmeters - worth a read...

 

 

[Tosh McCaber]

Many thanks for that Matty, I shall study these articles.  I am -very very gradually- delving into the intrecasiesof electrics, but it's taking some time to master these.  The articles are great, in the broader sense, but, right now I'm trying to sort out this particular question!

Oh, that I still dabbled with IC (for the past 60 years!).  However, in my old age, and this new fangled electric power, I'm looking to present experts who (hopefully) might give me some specific advice!