Best prop for AXI 5330/18 259 on 8S

[Applewood]

Hi there I’m finishing setting up a tony nijhuis 72” spitfire (6500g) with an AXI 5330/18 v1 259kv running 8S. I’ve put on an 18x10 pro which im sure will work but doesn’t pull massively well when testing on the ground. Does anyone have access to an online calc or uses this setup that could advice prop setup on 8S. Cheers

Edited By Applewood on 08/05/2020 23:42:31

[Chris Walby]

I am sure others more knowledgeable will chip in, but I would say a 10 inch pitch is really big. My experience with high pitch props is a significant loss of low speed performance, A bit like driving your car in 3rd of 4th only. Bit of a devil at low road speed, but goes like the clappers once up to a reasonable speed. This just increases you workload for landing, just when you want something to behave nicely.

If it was me would go for less pitch and up the diameter if you still need the total performance, (My 80 ic Hurricane has a 18x8).

There is software to cal, but some advice from the old timers is invaluable (I have found) in the past.

[Frank Skilbeck]

Just run this through ecalc, I assumed 5000mah 45c lipos and got 1400 watts, 6.7kg thrust at 52amps on a 18x10 APC E, 6500 rpm.

Going to a 19 x 10, gives 7.6kg, 1700 watts, 61 amps, 6400 rpm. Estimated level speed on both props is 68 mph.

An 18 x 8 drops the power to 1240watts, 5.6kg, 44amps.

I would say based on this that if you think you need a bit more thrust then a 19 x 10 prop would do the job (subject to ground clearance).

BTW we are just refurbishing a Mick Reeves Hurricane, 80", and using a 19 x 10 prop on a 400kv motor on a 6s battery. Haven't weighed it yet but it's no where near 6.5kg.

 

 

Edited By Frank Skilbeck on 09/05/2020 08:23:52

[Dickw]

Just checked the data using MotoCalc and come up with numbers remarkably close to Frank's ecalc figures, apart from top speed which MotoCalc suggests will be in the mid 50s mph. That sounds fine - unless you are planning on pylon racing with it

Both ecalc and MotoCalc suggest a thrust to weight ration of around 1:1 or better, and since you say it doesn't seem to pull that well on the ground it might be a good idea to check rpm and make sure you are getting something like the numbers in Frank's post.

Dick

[Applewood]

Just checked the plane weight. It’s 5.35kg without 8S lipo or 6.35kg with . I assumed you include lipo weight.

[Applewood]

Current prop is a cheapy hobbyking Aerostar wooden job, So probably not best to gauge best results. If I’m getting a pucka apc-e prop which is best to start with?

[Applewood]

And thanks for the help Btw. I’m happy with IC and smaller electric setups, less confident with electric as it gets bigger.

[Applewood]

Just done some rudimentary tests in the garden (rigged up luggage scales to the tail). It’s pulling 6kg with the 18x10 wooden Aerostar, And 5.5kg on an apc ic 18x8. Interestingly the wooden sounds so much quieter. Unfortunately I’ve not got any other lager props to try.

ive got a watt meter but it’s got the wrong connector type, Not got a rpm meter.

Edited By Applewood on 09/05/2020 12:32:43

[Frank Skilbeck]

Those numbers are not far off the Ecalc ones, the electric props are thinner than the Aerostar wooden ones so you will get a little more thrust for the same power input.

What size ESC have you got, if you have enough head room, say an 80 amp ESC then a 19 x 10 prop would give you more take off ummph,

[Applewood]

Hi Frank, thanks for the response. It's an OS HV 70a (85a peak) esc. I might try an apc19x10e. Like you say it would give more ummph for short periods. Should get more of a linear throttle response.

[Frank Skilbeck]

Posted by Dickw on 09/05/2020 10:54:37:

Just checked the data using MotoCalc and come up with numbers remarkably close to Frank's ecalc figures, apart from top speed which MotoCalc suggests will be in the mid 50s mph. That sounds fine - unless you are planning on pylon racing with it

Both ecalc and MotoCalc suggest a thrust to weight ration of around 1:1 or better, and since you say it doesn't seem to pull that well on the ground it might be a good idea to check rpm and make sure you are getting something like the numbers in Frank's post.

Dick

My mistake Dick, I didn't change the aircraft characteristics apart from the weight, so probably underestimated the drag and frontal area etc

[Dickw]

Frank. It remains to be seen which system produced the best top speed estimate, but I was pleased to see how close my results were to yours in all other respects.

I know some people don't trust motor calculators, but 12 years of real flight data from on-board data-logging and/or telemetry compared with predictions from MotoCalc, Ecalc, and DriveCalc, confirms how good the calculators are if you give them the correct information.

Dick

[Peter Jenkins]

One thing worth considering is that the worst case for checking power and current draw with electric is on the ground and restrained. I recorded 3,600 watts at 99A on a Hacker Q80-14 tethered (10S pack). In the air, I've only once seen 2880 watts and 80A and usually see around 2600 watts max. I use on-board telemetry and it really surprised me how different the tethered and "wide open throttle going vertical situations" differed. The Q80 is rated at 2880 watts max and 85A max. The motor is in a 5 Kg F3A aerobatic biplane. In normal level flight, I only use 1/2 throttle and rarely have to use full throttle for vertical manoeuvres unless it's a very windy day and I need to make progress against the wind in a big looping manoeuvre. The prop is a 3 blade 22x13 carbon prop.

On the basis of my experience, I would say that going 10-15% above the max wattage when tethered will be fine once the aircraft is airborne but best to use on-board telemetry to be absolutely sure!

[Frank Skilbeck]

Peter, couldn't agree more, onboard telemetry is the way to go. If your system supports it then the SM Modelbau Unisens sensors are great, measuring current, volts, mah and rpm in one small neat unit.

I often set up high current and low voltage alarms, as well as a mah alarm, had a couple of magnets come loose in a motor and rub on the back plate, this set the high current alarm off so I landed and checked. If I had been using just a timer then the first I may have known about this was when the LVC kicked in.

[Applewood]

Quick Qu, with lower speed flying characteristics more challenging, would more pitch give you ability to throttle up and gain speed quickly if needed? How does the 18x12e lookout the calcs if you don’t mind gents? It would give more pitch speed without sacrificing too much thrust, amps are still ok.

[Denis Watkins]

High gear or Low gear ? Or compromise

Low gear, good acceleration getting off the ground and control in the air if in trouble, more response

High gear, long ground run, slower to get off, but fast in the air, though less breaking effect

A good compromise is half the pitch of the diameter, in round numbers

So 18 x 10 high gear or a better overall run on 18 x 8

Edited By Denis Watkins on 10/05/2020 11:17:39

[Applewood]

Suppose I’m trying to work out for a warbird, is the thrust to weight more important, pitch speed or pitch to stall ratio. Or the combination. I might be in the realms of over thinking this now . I like to be able to fly comfortable, not massive speed , but enough power to get out of trouble. I like to have reasonable throttle range response, throttle mid to 3/4 for average flying.

[Frank Skilbeck]

18x12e on Ecalc gives 59A, 1630 watts, 6440 rpm, 7.8kg and estimated speed 75mph. Looks quite a good match.

Note Ecalc estimates the battery voltage sag under load and in this instance the batteries I've chosen drop to 27.8v at this load. If you batteries are better you'll get more power (more amps) and if they are a bit tired results won't be as good.

If you pm me an e-mail address I can send you a PDF of the output.