PROP SIZE

What would be the max prop size for;

Inrunner,1100Kv max amp 18,Battery 3s 2250 mamh x20c ESC 30 amp max

Running a 8x4.5 folding could I use a 11" folding ?

is there some equation for working out the prop size?

does it not give you some indication in the motor docs

I reckon 11" is too big for 1100 Kv motor on 3s, especially if its only rated at 18A !
Of course, you could always try a 2s, depends on what power you are after.
Equation ? Yes, its called a wattmeter .
Various online calculators such as motocalc may put you in the ball park, but frankly - even same kV motors from different suppliers will give different figures....so TBH a wattmeter and a few props to try is the absolute best way to match up components in a powertrain.

Rich,motor is from Giantrcod.A Black Mantis, B 2846 no details given for prop size


Shaft Size:￠3.175X12.0(mm)
Motor Dimensions:￠27.8X46（mm）
Weight: 130g
RPM/V (KV): 1100
Idle Current (Amps): 0.4
Resistance [Ri]: 0.072
Max Amps:18

I suggest around 9 X 6 would be maximum on 3s with that motor John, which will likely pull about 15-16A.
And be aware that when they state "maximum current" its often for only short bursts, and the constant current is lower.

Yes, a wattmeter is pretty essential!

What type of plane is it going in? And what's the all up weight? (inc battery)

I know this is slighly off topic but is this the same motor you had trouble with in a previous post, if so what was the matter with it

Rich,its a different motor.The previous one had an open circuit.PDQ tested it for me.Problem was I opened the motor up,before Paul tested it, so invalidated the Guarantee.

Alan, the AUW is nearly 2 lb Its a Glider,Perkins Pretty.

if as Tim says it pulls 15 amp x say 10v it should be ok for a Glider ? 150 w ?

The Pretty is supplied with a plastic gearbox and inrunner motor installed.

I've run it through Motocalc and can't find a combination that works with the replacement motor using a direct drive and therefore tried a few options of props and gearboxes. Always suspicious of this program, but it does seem to work quite well for me.

One of the results is here below :-

MotOpinion - JOD_Pretty
Sea Level, 29.92inHg, 59°F

Motor: Hacker B50 22L; 1098rpm/V; 0.75A no-load; 0.0496 Ohms.
Battery: Kokam 360SHD (20C); 3 cells; 360mAh @ 3.7V; 0.0688 Ohms/cell.
Speed Control: Generic Brushless ESC; 0.006 Ohms; High rate.
Drive System: Generic 12x9in Prop w/3:1 Gearbox; 12x9 (Pconst=1.31; Tconst=0.95) geared 3:1 (Eff=95%).
Airframe: Jp pretty; 331sq.in; 20.3oz RTF; 8.8oz/sq.ft; Cd=0.039; Cl=0.28; Clopt=0.64; Clmax=0.99.
Stats: 42 W/lb in; 33 W/lb out; 15mph stall; 19mph opt @ 65% (19:20, 63°F); 29mph level @ 95% (9:19, 65°F); 658ft/min @ 23.1°; -140ft/min @ -4.8

and later on.......
Possible Aerodynamic Problems:

The static pitch speed (30mph) is much less than 2.5 times the stall speed (15mph), which may result in reduced performance at typical flying speeds and a low maximum speed. This situation is usually acceptable for an electric sailplane.
Pitch speed can be increased by using a higher pitched and/or smaller diameter propeller, a lower gear ratio, a higher cell count, or some combination of these methods.
The diameter (12.0in) to pitch (9.0in) ratio is less than 1.5:1, which will result in reduced propeller efficiency at low speeds (the propeller is stalled). Although this is not likely to affect flying characteristics, it may make take-off or hand launching difficult.

Aerodynamic Notes:

Due to some of the potential problems listed above, this model may require an experienced pilot.
The static thrust (14.8oz) to weight (20.3oz) ratio is 0.73:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (7.7oz) to weight (20.3oz) ratio is 0.38:1, which will give strong climbs and rapid acceleration. This model will most likely readily loop from level flight, and have sufficient in-flight thrust for many aerobatic maneuvers.

So bottom line
It needs a gearbox too.

Full version sent on to John

thanks Paul, though my version was supplied with no gearbox and the inrunner was actually an outrunner inside the fuz,

The Pretty does have a gearbox (still in the fuz) in the form of a built in internally toothed ring gear. If you still have the pinion gear, then that should be fitted to the new motor you bought.
That motor has a fixed coil and an internal rotating sheathed magnet

The difference

Inrunners (like yours) have magnets (in a can) spinning INside a stationary coil.

Outrunners have multiple magnets-glued to the inside of a bell attached to the output shaft. This spins around (OUTside) a wire-wound fixed stator.

In both cases the coil does not rotate and allows easy connection to the speed controller
Inrunners are designed for high rpm (>20,000rpm) - they are often geared to drive bigger props.

Generally outrunners are lower Kv and higher torque can be used as direct drive with large props. Even then, outrunners can be geared.

Paul, the Pretty came with an outrunner but fitted inside the fuz.The Bell rotated and came very near the three wires from the motor.There was no gearbox or ringed whatever.It was a outrunner with the shaft turned round.

you tell him John

My head's starting to hurt

John,

If it was me with that motor on a power assist glider of that type I would try a 7 x 6 prop used direct drive

Then with the power meter check I wasn't either overloading anything or taking too little power for useful flight and either prop up or down accordingly

Then flight test to check the pitch is ok

If the 7 x 6 was fine I would change to a folding prop for longevity of the motor shaft and the prop as well as drag reduction

A 7 inch prop wouldn't be the most efficient in the world but does that matter? The max motor power would be in the region of 200 watts and a glider the size and weight of the Pretty would need not much more than 100 - 125 watts for a reasonably relaxed climb. So, personally I would forget the added complexity of a gearbox and stick to direct drive. If you needed more power then just prop up a bit as long as you stay within the 18 amps

thanks guys for all your help.I really do appreciate the time and trouble you go to. ( I don't understand half what you said,just glad Andy didn't contribute ) Now, whether its giving me advice or helping members like Dave S or doing the thousands of other necessary jobs that makes the club what it is.

Mmmm suffice to say I don't trust motocalc any more !
John's duff inrunner motor is shown stripped down here.

The rotor single magnet is 2 pole within a metal sheath. This magnet has phenomenal power. See the "highly scientific" test pictures of it sopporting a 3", 10lb Vice just by it's magnetism.


The aluminium case contains a potted complexly wound winding with 3 external wires. These are all open circuit to each other.
Strangely, this lump is not magnetic itself but sticks like glue to the rotor. Anybody got any idea why?

The new motor John bought £12 from Giantcod is very similar and is one of of a new breed (to me) of brushless motors using inrunner technology. It has a kv of 1100.
Giantcod give some reviews

Chose this as a direct replacement for a brushed 400 in a 600g electric glider. Motor weighs 120g and comes with mounting screws (3mm) : you have to supply connectors. Front mounting holes are 16mm and 19mm : also has four rear mount holes at 19mm? Wires are a little stiff not as flexible as others. Tested using 30 amp esc and 1300 3s Turnigy h@y 8x4.5 folding prop and spinner. 145 Watts @ 12amps on fresh battery falling to 120 @ 10amps Motor ran cool and smooth with no problems. Good value and excellent service

Have been looking for an inrunner like this for some time.This will replace the Jamara 480 in my Pico Jet It is almost exactly the same dimensions but about 30-40% more power. It did sound a bit gritty on start up so I put oil on the bearings both ends whilst running hoping the oil soaked in under capilliary action anyway it soon got smoother.Have not had time to fly with it yet but it seems ok. and well made. And as a change it only weighed 120 gm.instead of the 130gm.as stated in the spec. Seems amazing for the money well recomended.

Excellent motor for "s400" class warbirds or slim nosed birds where fitting a outrunner is hard to impossible. APC 8x6E, 25A ESC, 1300 3S lipo @11.0V - 13.5A 140W A good buy this far.
In the past inrunners were very high revving and had to be geared down to be of any use with propellors. That was resolved by the surge of outrunner motors that appeared in the last 4-5 years.
The big bonus with this motor is it is slender, can fit in the narrowest of fuselages. No gearbox required and can be a drop in fit in any model that would have flown just OK with a conventional 400-480 motor. And now 70% more power.
Have a look at giantcod black mantis range of brushless motors

johnoxleydean wrote: I don't understand half what you said,just glad Andy didn't contribute .

Glad I could be of assistance However, you might want to look away now John:

Paul, brushless inrunner type motors generally fall into two catergories. Slotted, and slotless. Slotted motors use iron (usually silicon steel) laminations which have little prongs on that poke through the windings. It looks a bit like the stator laminations on an outrunner, but inside out. Slotless motors still have the laminated steel laminations, but don't have the prongs to poke through the coil windings. Both constructions have advantages and disadvantages, but generally slotless motors are used for low rpm, high torque motors. It can be thought of as the equivalent to cored and coreless brushed motors....

The point is that the windings still have steel laminations in there somewhere, and they are there to complete the magnetic "circuit" (think of it like an electrical circuit, just with magnetic flux flowing from North to south, instead of current flowing). That is probably why the magnetic rotor is "sticking" to the windings.

Cheers
Andy

PS. Double check you have removed enough of the winding insulation before testing for continuity between motor coils, and also check the wires are not fractured between where they exit the windings, and the connectors. The cheap motors usually just extend the winding wires, which is not really a good idea for applications that might be subject to flexing/vibration.....

adding to the topic.Ordered a 10x6 from Giantcod yesterday came this morning but though the package said 10x6 and on the prop was stamped 10x6 the prop was 9x6.They have put a replacement in post.

Decided on the 10 " after testing with 11" Ran ok with 11" though motor did get warm .This was due to no inlet ports on the mounting wall to allow for cooling.,Inlet holes now in,no over heating.

Thanks again for all your help.