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Need to Change Kit Specified Prop?


Chris King 3
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Hi, I hope someone can help with this question regarding prop size.

I took a 20 year odd break from rc planes and recently dug out an old (Astra) fomie spitfire that I never completed.

In carrying on with its construction I have now remembered why I shelved it (apart from kids!). The recommended prop is a 10x7 APC. The motor is a brushed speed 400, the kit came with a reduction gearbox which has 'MPJET' on it.

The problem is that when I test tun it the motor quite quickly becomes very hot. Even when backing off to 50% throttle. The way it sits in the plane it wont get any cooling air so im concerned it will start to melt foam!

Tried a spare 10x6 wooden prop and that doesnt make the motor anywhere near as hot. Should I use a smaller/different pitch prop or go with the manufacturers recommended prop and keep the revs down?

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Ooo OK a little bigger than I was expecting & quite large for a Speed 400 hence the gearbox & 10" prop....

OK let's begin....

The well worn formula says we need around 100 watts per lb of all up weight (apologies for the mixed units but's a simple "rule of thumb"; for a Warbird I might be tempted to go a little bit higher so we'll "round up" when it comes to the maths....if we use a LiPo & a brushless motor in place of the Speed 400 & gearbox I think it will come out a good bit lighter anyway.

800g is about 28 oz or 1 3/4lbs so 1.75 x 100 equals 175 watts (BTW that's a lot more than your Speed 400 was ever going to give you.....wink 2). A 3 cell (3S) LiPo battery will be ideal for this sort of application….a 3S LiPo is nominally 11.1V but all batteries sag a bit under load so lets say it’s holding 10V as that makes the maths easier.

Applying Ohms law (or a derivative of it anyway) we know that Power in watts equals Voltage in er Volts x Current in Amps….rearrange that slightly & substitute the numbers we get Current equals Power (175W) divided by Voltage (10V) so our full power current is 17.5A. We can use this figure to determine the size of speed controller we need…a 20A or a 25A unit will be ideal. Similarly this gives us a good idea of battery size. Personally I like to use a discharge rate of around 10x the capacity of the battery (commonly known as 10C) as this equates to around 6 minutes of full throttle flying to fully discharge the battery (60 minutes divided by 10 equals 6 mins). Now two points to bear in mind here 1) we never fly at full throttle all the time & 2) you should never fully discharge a LiPo…aim for around 80% to keep your batteries happy. This two factors mean that our 6 minutes of full throttle flying will equate to around 8-10 minutes of general flying with a bit left to keep the battery happy.

So…if 10 x the battery capacity equals 17.5A then the battery capacity must be 1.75Ah or more commonly 1750mAh. That’s a bit of an unusual size so probably look for 1800mAh or you could go down to 1500mAh…obviously a smaller battery will offer less duration but will also be a little lighter.

We now have the size of ESC (20-25A) & the battery (a 3S 1500-1800mAh unit) defined so we are well on the way to a working set up…to the most difficult bit…the motor. We already know a bit about it…we know it must be able to handle a 3S battery & a current of around 18-20A (it’s wise not to spec your electrical components to their very maximum ratings….always leave a little bit of headroom) so what else do we need?

We need…… The kV figure… This is probably the hardest thing to get your head around. It represents the rpm the motor will spin at per 1V applied so a 1000kV motor will spin at 1,000rpm if we apply 1V or 10,000 rpm if we apply 10V. So how do we know what to go for? Difficult one to answer….experience does play quite a large part here as does the type of model. A large slow flying model will need a slowly turning large prop (low kV) whilst a small fast model will want a small fast turning prop (high kV). The thing to remember about the kV figure is that this is the speed your motor will always try & turn at; it is in effect the speed it is designed to run at & is a function of number of windings, number of magnets & a few other things that don’t concern us here) so whatever sized prop you fit, the motor will always try & reach it’s design speed. If we have a high kV motor & fit a big prop this represents a high load for the motor & it will draw more & more current until it reaches its design speed or melts & dies in the process. Similarly if we fit a small prop to a low kV motor this represents a low load so the motor will reach its design speed no problem but won’t generate much thrust in the process.

Back to your Spitfire then…..originally designed for a 10” prop because that was the only way the poor thing would stagger off the ground with it’s Speed 400 up front but I think we would be better going for a 9” prop & spin it a little faster…..lets aim for about 9,000 rpm. 9,000 rpm divided by our battery voltage (10V remember) gives us a kV figure of 900.

And that’s pretty much it….we need a 20-25A ESC….a 1500-1800mAh 3S LiPo & a 20A motor with a 900kV. Pop a 9x5 or 9x6 prop on the front (check with a Wattmeter & go up or down a little in diameter/pitch to get the desired current) & you’ll have a very nice Spitfire with a pretty good turn of speed.

I'm sure you'll have a few questions after reading that lot so fire away & I'll try & answer them for you...thumbs up

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Wow - Many thanks Steve - thats a lot of info!

I'll work my way through that and see what questions I have.

TBH I dont think the model will last long, the foam is very brittle and the glue on some of the seams is breaking down. But none of this gear is really expensive so as long as the bits survive any catastrophe Ill use them as a basis for another kit. I enjoy building as much as flying. I'd rather fly it once than leave it to rot in my loft for another 20 years!

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Hi Chris...that motor ticks a lot of boxes...the max current & power are all about there but I think I would go for the 1000kv version. It will take a bit more current & power & it's always useful to have a little headroom with electric components...running anything flat out or close to flat out will inevitably shorten its life.

The main reason though is the kv.....1000 rpm per volt will give us a something over 10,000 rpm at full throttle & that "feels" about right for the size of model we are talking about....the 800kv motor will spin at little over the 8000rpm mark which feels a bit slow for the model to me. For sure you can increase the prop size & this will increase the thrust generated & the power produced by the motor but IMHO a slightly smaller prop spinning at a high-ish speed will suit the model best....something around a 9x5 should do it.

The thing is to get somewhere in the ball park using the maths & then fine tune it with the size/pitch of the prop used to get the right current/power/flying characteristics. You quickly find that in electric flight you need a lot of props of slightly different size on hand for that final testing.

Ooo & another thing....buy a wattmeter....you'll need it to make sure all the parameters are within the limits of the components & that nothing is going to melt.

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It does depend on the way the battery is mounted but a 1800 mAh 3s LiPo will be a quite a bit lighter, about 3/4 the weight of a 1600 mAh 9.6V NiMh, so you may have to re-position it a bit further forward to maintain the correct CofG.

I can only reinforce Steve's suggestion to buy a Watt meter.

LiPo are very powerful batteries capable of delivering a huge current so are easily capable of burning out all the components (including themselves!) if unduly over loaded.

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  • 3 months later...

I have my motor and Esc - had to wait a while as the 1000kv motor was out of stock.

I think I'll have to go with a 1500mah battery (Turnigy) as the maximum length the spitfire battery compartment will accomodate is 73mm and the only 1800mah batteries I can find of that size are on Bangood or shipped from Hong Kong on eBay!

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Shame it won't hold a 3S 2200 mah LiPo as these seem to be ubiquitous in so many sensible models, readily available from all the known suppliers. E.g I can use exactly the same batteries in my Multiplex Heron, tiddly Great Planes Sukhoi parkflier and the new Explorer F3A trainer.

If the frame is tired and there's enough spare foam to go at, I wonder whether there's anything to lose by cutting out to squeeze a larger battery in, and ending up with a flight pack which will recycle very readily? Maybe up-spec the esc another 10 A on the same basis?

Just a thought.

BTC

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Chris, rather than wait for stock you could get one of these motors. I think you will find it useful for your next model too, if you are not expecting a long service life from your Spitfire wink 2.

I also tend to agree with Bruce and I think you will see the benefits of 'standardising' on the 2200mAh LiPo as it is a very useful size for a variety of models and often available at knock down prices. It may also be helpful as it will be a little heavier, so you may not need so much lead in the nose after replacing the old NiMH batteries, (which the model is designed for). It only takes a moment to cut a larger bay for the battery and you may have to do that anyway if you need to move the battery aft to sort out the c of g.

just my 2p worth!

ps. I would get one of these too.

 

Edited By Piers Bowlan on 01/02/2019 07:20:33

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I do agree about having a standard battery to use across multiple models. I have a few other models from my flying past that I'll probably upgrade to brushless and Lipo in the future but theres no way a pack of that size will fit in the spitfire.The battery compartment sits upright in the fuselage in front of the wing and theres no where else that a space could be opened up that I could get access to especially as the wing isnt removable.

Im not too bothered having to go with a 1500mAH cell for it TBH. They will work in my Miss Moravia park flyer when that goes brushless and they are only a tenner each.

Probably would have gone for that motor if I'd spotted it but have already bought the AirMax one!

Gradually getting the other bits that are needed now like wattmeter, prop tacho etc. Lipo charger is next!

Also going to join the local club I was a member of way back so i can use their field and benefit from their insurance.

Many thanks for the advice, always good to get other angles on things!

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  • 3 weeks later...
Posted by Chris King 3 on 17/02/2019 12:08:23:

I've trying out my setup. Motor is this one **LINK**

with the 30A Flycolor ESC, Turnigy 1500mAH 3Cs with a Master Airscrew Electric Prop 9x5.

When I run it flat out it pulls around 15.5A and nothing gets hot but the rpm only gets to 8800 max. With 11.1V and a 1000KV motor shouldnt it be reving to 11,100 rpm?

No that,s the no load speed, with a load on it will be less, plus the battery volts will sag under load as well.

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The motor’s kV figure is the unloaded speed at which it turns, Chris, as soon as you load it, put a prop on, it slows down proportionally to match that load; but the current flow also increases proportionally too. That’s what you are seeing. A heavier load, bigger prop, will further decrease the revs and increase the current.

I always check the unloaded rpm in the very first instance to get a benchmark starting point, it’s not always what it says on the label, and go from there.

All other things being equal, and in little or no wind conditions, your model will fly at around at up to about 40mph.

Hope this makes sense…

PB

Edited By Peter Beeney on 17/02/2019 13:27:59

Edited By Peter Beeney on 17/02/2019 13:30:27

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