What size prop

[Basil]

Once again I am to ask advise from the Forum.

This time it is for advice regarding the size of prop to use.

Model Auster AOP9, WS = 82", scalw 1/5. Model Wt unknown, Scatch build from plan.

Motor SKU-5065-320(Hobby King).

Trike u/c , max clearance about 10".

I have various LIpo's

Dont know what other info might be needed.

I have a Watt meter, that I know will be required at some point, but where do I start??

Thanks in advance .

Bas

Edited March 4, 2021 by Basil
additions, re watt meter

[Geoff S]

That's quite a beefy motor and probably well suited to the AOP9.  The maximum current is 70 amps but probably best to keep it around 50.  First you need a watt meter to give you an idea of what load your chosen prop in imposing on the drive system (motor and esc).  I assume you'll be going for a 6S LiPo battery which is around 24 volts which will, at 50 amps provide 1.2kw which I think will be more than enough.  I'd start out with a 14x7 (probably because I already have one) which would be very safe and see what current draw and power you get.  I don't think the type of model needs any more than 80 watts/lb for scale performance though allowing the classic 100 watts/lb wouldn't be too much over the top.

 

Just remember that the higher the voltage, the lower the current and it's current that kills electric drive trains.  You can adjust prop size to determine power consumption depending on the final weight of your model.  So 6S and an esc easily capable of 50 amps (so rated at 60 minimum IMO)

 

Geoff

[kc]

Basil, are you sure that's the right motor for that model?   Thats a 2220 watt motor 320kv for a 10S Lipo.  The Original article said a .60 four stroke ( that was 1986 type 4 stroke not modern ones ) would power it adequately.   I would expect a much smaller motor would work.

 

[Bob Cotsford]

If this is the Keith Humber plan then it was rated for .40-.61 2 stroke glow motors according to the Sarik site notes, say 600-800W equivalent.  According to E-calc your SK3 will give around 700W turning an 18*10 prop on 5S or 670W turning a 16*8 on 6S, both options giving a pitch speed of just over 50mph.  As Geoff says you shouldn't need to go near 50A (more likely 40A) so a 60A esc should be ok.

Edited March 4, 2021 by Bob Cotsford

[Basil]

Yes its Kieths design.Was for IC.

The motor selection was based upon a statement given by a well known outlet @ 5065-360, but the prices are a bit on the high side, so I whent with the 5065-320. I am no expert but this was my feeling at the time that a 5050,  or smaller, would perhaps have been  more siuted. The gentleman was adament that it required a large motor.

Is the  18x10 prop able to fit AOP9, is it not a little large?

What motor/prop would you suggest KC , I am very flexible.

Thanks Bas

 

 

Edited March 4, 2021 by Basil

[Geoff S]

4 minutes ago, kc said:

Basil, are you sure that's the right motor for that model?   Thats a 2220 watt motor 320kv for a 10S Lipo.  The Original article said a .60 four stroke ( that was 1986 type 4 stroke not modern ones ) would power it adequately.   I would expect a much smaller motor would work.

 

 

It's true that the motor could be considered overkill but, if there's space, the weight will not be a problem (better than lead ballast) and it will be very under stressed, which IMO is a good thing.  I like to use the biggest motor I can accommodate in glow to electric conversions.  A smaller motor would work but I'd stick with the 5065 - 320.

 

My 1/4 scale 9.5 lb Percival Mew Gull in on 6S and the correct prop is a 16x8 according to the motor specs (eMax GT4030 06 420rpm/v) but I test flew it on a 14x7 perfectly well because I didn't have a 16x8.  I like to over spec drive trains ?

 

Geoff

 

[Bob Cotsford]

21 minutes ago, Basil said:

....

Is the  18x10 prop able to fit AOP9, is it not a little large?

What motor/prop would you suggest KC , I am very flexible.

Thanks Bas

 

 

Basil, what would a scale sized prop be?  It all depends on what cell count you use, the 18" would be for a 5S pack while 16" would be for 6S.  I gave you the figures for a 5055 motor by mistake, your 5065 will actually draw something like 80-100W more on the props that I quoted.  If you want smaller props you might have been better off with a higher kv motor, say 430kv.

Edited March 4, 2021 by Bob Cotsford

[Chris Walby]

I think your biggest issue will be prop diameter selection without having a really big pitch and ground clearance as its a low KV motor. Not so bad if you select over 6S and probably getting on for 10S (but that makes ESC's expensive).

 

IMHO, pick the largest diameter prop with plenty of ground clearance (allowing for a bounced landing on long grass) and a reasonable pitch (say 6 to 10 inch) and see where that gets you in total watts. I would avoid very high pitch props unless you have a need for ultimate  speed with a sacrifice of low speed control.

 

If you are having trouble then you could (if appropriate) try a 3 blade prop as size for size they consume proportionally more power. 

 

 

  

[Dickw]

If it is any help, I use a 17x8 prop on a slightly smaller 365kv motor on 6s to fly a 3 kg biplane with a very spritely performance.

The lower kv of your motor would mean slightly less power (say around 800 to 900 watts) , but that should still be more than adequate for your Auster with a max pitch speed of about 35 mph.

 

Dick

[kc]

The AOP9 should be a slow flying plane relying on it's large wing area to fly in a scale manner.  Too much power with lots of wing area could be a problem.   Can be even worse than too little power. 

The same designer ( Keith Humber ) used an OS .25 LA in a later 54 inch version, so I think his comment that  a  ' . 61 four stroke would be adequate '  ( for the 82 inch) should confirm the amount of power he thought desirable in such models.

 

If you have already bought that motor I suppose the most sensible thing is to continue with the construction and see what the final weigh turns out to be - allowing for any lead that might be needed to get CG correct.    Perhaps  50 to 100 watts per pound would be the power to aim for.  A 10 pound slow flying model might be well suited with just 700 watts. 

  If it were my model I would build in an alternative mounting for a smaller motor just in case!   Either way I would make sure that there was plenty of space to get the Lipo well up front or slide further back as needed.  

 

Edited March 4, 2021 by kc

[Danny Fenton]

1 hour ago, Basil said:

 

I agree the 5065 is a great motor, but a bit much for this.

In my two Chipmunks, both on 5S, one at 85" and one at 80" I am using Turnigy 4258 500kV motors turning a 15 x10, they pull around 50A max.

The larger Chippy is shown with a similar 4260 500kV but when that wore out it too got the 4258 and same prop.

 

 

 

I start with the size of the scale prop and work from there. The 4258, and the 5065 come in a variety of kV ratings to match up to your scale sized prop.

 

Cheers

Danny

Edited March 4, 2021 by Danny Fenton

[Trevor Crook]

Basil, I think you are in the right ball park. I have an Avios Spitfire that flies on 6s with a 380kV motor turning a 15x10 3-blade prop, and pulls about 1200W. If you start with the 15x10 2 blade that Danny uses, you should pull less than 1kW, which should be somewhere close for the Auster. You can tweak the power by changing props once you have an auw, I agree with KC that 70-80 W/lb would be about right for that kind of model.

[Basil]

The real thing sports a 6' (1.8m) prop, @1/5 =14.4". The problem is that the model will touch the ground @18" prop, angled at take off.Thats without ANY allowance for anything so even allowing 1" cleance thats 16".

I am waiting for a 70a ESC to arrive from HK, they are impicating that because I did not pay for tracking that if it does not turn up thats my loss, surely that down to them!!!.

I'll have to see what Lipo's I've got then Props and go from there. Pity the club's 'Shop' is closed, as then I could try all manner of props.

So much for some one like me to take in so  !!!!!!

Thank you all for now, need to 'Codgekate' on all this info. I will report back, thanks again.

Bas