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prop selection help


Phil 9
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4s 3600mah battery. I have 2 props that I just tried out

12x8 produces 500w at 35A

15x6 produces 600w at 45A

The 12 inch prop is too small. It feels like it would fly well on the 15x6 however the diameter is to large for the model and would hit the floor when the model is level

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Posted by Denis Watkins on 29/06/2016 19:52:01:

Hi Phil, there is only one item different on the web, from your actual build, and that is the weight. The web flys the Sig with your prop and set up on a 5lb model. But you are using 800w to fly a 6-7lb model. Are Sits figures misprints ? And does the model weigh nearer 7lb?

my mistake, I just weighed the model and it is 5.2lbs. Now things make a little  more sense. my 12x8 should be good enough for the maiden flight. because of clearance I am probably limited to a 13 inch prop

Edited By Phil 9 on 29/06/2016 20:12:29

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Thats good Phil, you are well inside the flight envelope as you know. Ground clearance is always a problem with low wingers, so it has to be your decision to go 13 inch or 14 inch. From the motor drive shaft, measure to the floor, and take off at least one inch. If there is not enough prop left, then consider a 3 blade.

But, USA has flown these at 1000w on a 12 x 8 prop, on 4s 3500 range, there is just the ESC to consider

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Posted by eflightray on 29/06/2016 20:58:45:

790kv ?, the HK Turnigy D3548/6 790kv ?

If so, then the Customers Reviews on HK can often be useful.

Personally I don't always trust 'calculations' given out, compared to real world user information.

Ray.

it is a 4 max purple power 3548-790 is this the same motor re badged?

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Phil,

For what it’s worth, and that’s perhaps not a lot, I’d forget about the 15 and also 13 inch props and go with the 12 x 8 inch for a start. Beg, borrow or buy a tacho, like this one, and find out how many revs per min the prop is turning when flat out; that will give some indication of how it’s going to fly. To get a reasonably honest indication I’d use a pack that’s down a little bit for this caper, it’s very easy to use one that’s fully charged, but in flight that doesn’t last long!

At a guess, on a half decent 4 cell pack and a 12 x 8, I’d say you’d clock around the 10 thou mark, although perhaps less rather than more; that gives it a straight and level speed of around 75 mph. But even 70 is not that desperately bad though, in my opinion anyway, it should still be fairly lively at that I’d have thought.

Then just to be a bit more perverse, I’d bolt a 11 x 8 ACP i/c prop on. This will let the motor, (and prop..) spin a bit faster but hopefully the draught would be more or less the same as before so it might not reduce the performance enough to notice, but it might reduce the the current enough to make the extended flight time more enjoyable.I feel it’s always worth while to have a little experiment with prop sizes, particularly as these smaller ones are generally not that expensive; plus the tacho test is always going to let you know that the first flight speed is going to be fast enough. Taking off on a maiden flight with really marginal power can soon result in a disaster… I suspect more than a few pilots have probably proved that statement in the past.

Good luck!

PB

Edited By Peter Beeney on 30/06/2016 14:05:48

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Phil,

To be quite honest, the 8k doesn’t really seem to be quite right there somehow. I’m speculating here, but if you were using a fully charged 4S pack for that measurement I’d have thought that the current would have been rather more than the thirty five amps you noted on the power check.

I usually start at the beginning, so to speak, by checking the tachometer calibration against a mains lamp, 3000 rpm on a two blade setting, then check the no-load revs of the motor. Normally this reading is very close to what it should be; then when trying a prop the revs will always be less, but by not too much, the less the better, in fact; the proportional difference in the different propeller speeds always directly governs the amount of amps that are flowing. In other words, if the motor speed changes, (by changing the load), the current flow will change by an exactly corresponding amount. I’d have thought that the 12 x 8 prop is very near the best for that motor, so in my opinion there shouldn’t be such a big discrepancy in the revolutions readings.

I’m sure it will fly ok at that prop speed, the wing loading is nice and light, and if the 8,000 rpm figure was what I expected to see then I’d certainly give it a go. But under these circumstances I think I’d be investigating a little bit further to try and find out exactly what is going on first. To me, the projected no-load speed on a fully charged 4S of 13,000, the actual 12 x 8 loaded speed of 8,000 and the 35 amps all together seem to be slightly at odds…

But, of course, maybe it’s just me reading it all wrong again; I very frequently do…

PB

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well I gave the model its maiden today with mixed results. The wind was too strong for the model and not a good day to test things out, it flew well enough but did feel lacking in power so I added a 13x8 prop to the front. vertical performance was ok but not unlimited by any means but I think there is enough power there for now buy I may want more later on. with the wind picking up I got myself into a little trouble when at full throttle the model was not moving forward. I tried to land it but with the wing gusting it was tricky. the landing was heavy, It broke the battery tray within the model, But this was a lite ply made to hold a fuel tank and not strong enough to hold a battery. I will need a calm dat to be able to judge it properly

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I flew it again today in much better conditions. I also found a taller U/C that allowed me to fly with the 15x6 prop, there was not a great deal difference between the 13x8 and 15x6 in the air. the 13x8 was faster and the 15x6 climbed better but both flew it well. I went back to the smaller prop and original u/c only because the taller u/c was swept back and made the model prone to tipping forward. I was a little disappointed with the rudder authority as it lost a lot of height when in knife edge. some weight off the nose helped but I think I still need to move the CofG further back yet.

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  • 1 year later...

Slightly off at a tangent here, but here goes. My question is mainly for electric flight, although it could just as likely apply to IC.

Am I right in thinking if I want to reduce the current draw (increase duration), then changing a 24 x 8 prop to either, for example, to at 23 x 8 or 22 x 10 will make much difference?

Secondly, am I right in assuming that carbon fibre props of equivalent size to wood are in fact lighter in weight? Most stockists don't seem to list weight in the specs. Also whilst I understand the need to use an electric style wooden prop as opposed to an equivalent I/C wooden prop due the weight difference, does this similarly apply to carbon fibre props? The reason why I ask is that I am finding it difficult to source the larger size carbon fibre props with an electric use label on them. There is much more choice in CF props for I/C use. Thanks.

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Dropping from a 24 to a 23" dia with the same pitch will drop the amps by around 8%, but dropping the dia to 22" and increasing the pitch from 8" to 10" wouldn't (my quick calc shows this might result in a slight amp increase)

There is no problem using IC props on electric, but electric props are slightly more efficient as they have a thinner section (especially nearer to root), but they aren't strong enough for torque pulses on an IC engine, so an electric prop of the same size may give more thrust for slightly less power than an IC prop which is built thicker due to the additional loads on it.

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