Foxy 4020/10 prop choice

[Geoff S]

My current project is a 60" ws sports model built from a kit which specifies a 46 to 60 glow engine. I've built it to be electric rather than glow and I've fitted a Foxy 4020/10 with a Foxy 65 amp esc. I bought the motor from Puffin Models.

It's a 755 kv motor with a maximum current of 50 amps for 30 seconds. I'm running it on 4S and the spec suggests either 14x7 or a 13x8 propeller. However, on running the motor for the first time it's drawing 61 amps (800+ watts) on the 14x7 and 57 amps (780 watts) on a 13x6.5 (both APCE) which is well above the maximum. Obviously I only ran the motor for a few seconds - sufficient time to take the readings.

Foxy (or Puffin) recommend a 12x6 on a 5S LiPo so I tried one on my 4S set-up and that draws a safer 41 amps ( 580 watts). That's a HobbyKing electric prop rather than APCE.

The model, which is almost ready to maiden, will probably weigh about 2.5 kgs (5.5 lbs ish) but I haven't weighed it complete yet. I guess the 12x6 will fly it but it seems to be a lot smaller than that recommended. I contacted Puffin and John Emms was supposed to ring me back but didn't.

Has anyone else here used this motor? If so, what prop/current draw did you get? I used my HobbyKing wattmeter which may not be the best but it compares quite well with my old Astroflight one which cost a small fortune when I bought it.

Geoff

[andyh]

I don't have your motor, but I dare say the 12x6 you've tested @41A/580W would be ample to test-fly a 5.5lb model

regarding the 13x6.5/14x7 that you tested - if you only tested them for a few seconds, you may have been seeing the transient spin-up current draw, rather than steady-state

I'd try a 12x8 - I find they generally fall somewhere between a 12x6 & a 13x6.5 in terms of current draw

[Geoff S]

Thanks, Andy. That had occurred to me as well but I don't have an infinite supply of props to hand and a 12x8 isn't among the finite number I do have I just hoped someone here had actual experience with the Foxy, which is a mid-priced motor claiming to be the equivalent of an Axi 4120/14 which has a slightly lower kv (680) and a slightly higher max current rating (55 amps).

I suppose I could adjust the throttle range on the 13x6.5 so that I don't exceed 50 amps but I'll probably go with 12x6 for motor safety's sake. It does seem odd that the recomended prop is so far out and shows the importance of making measurements with electric set ups rather than relying blindly on the supplied data.

Geoff

[Geoff S]

I found a 12x8 APC E prop I didn't know I had (I bought a stack of props last year from a club member who was selling up) and tried it this afternoon. I got the same readings as I had from the 12x6 HobbyKing but it was the same battery and I hadn't recharged it (77%) so perhaps it was getting a little tired. I was able to measure the rpm as it was outside (optical tachos all read 3000rpm in artificial light) and got 8.900 rpm, which is quite reasonable.

I've also been able to weigh it properly and, as always, it's a bit heavier than I hoped. The airframe is a touch under 5lbs and a pound heavier with the battery installed.

Hope to test fly on Saturday as the weather looks reasonable so far. Much too windy before then to test fly an unknown model.

Geoff