DingleyDon

Hi All,
I have a Sig Rascal ARF electric-version that I have recently built but not flown yet. I'm using the included 20A sig ESC and 7.2 Speed 400 motor (geared 1:3) that came with the kit, along with a 7.2V / 750mah 7-cell NiMh battery pack. Propeller is an APC 9x7, though I've had the recommended 8x6 on the plane as well.

My problem is that I see no way this setup provides enough thrust to get the plane in the air. I've tried a hand launch at full throttle with no luck; the plane will tip stall right away, even with a pretty hard throw. On the ground, the plane will slowly roll on concrete, but there is no way it would be able to ROG for a takeoff.

I know the battery is a little on the small side capacity-wise, but I figure that should just show up in shorter run-times, not power. Would a Nicad pack provide the extra punch I'm looking for? I'd entertain a brushless motor/esc combo, but the cost is kind of prohibitive as I'm looking to purchase my first glow-powered plane soon and don't want to sink another ~$150 into the Rascal if I don't have to.

Any advice? This is a neat looking plane that I would love to fly, but so far I'm disappointed with it.

Thanks,
Don

Dr Kiwi

Hi Don,
I can't quite make out what battery you have at the moment - if it is the 7-cell 600 NiMH then that is 8.4V (7.2V would be a 6-cell) - but no matter - it won't give nearly enough juice. With a 3:1 geared 400 you are going to be marginal on battery power with anything less than an 8-cell 600 NiCd, or something like a 8-cell 950KAN NiMH pack. The physically smaller cells of your NiMH 750[probably 2/3AA] are going to drop voltage dramatically under load so the power [Watts = VI] you get is less than you will get from physically larger cells [say, 5/4AA or 2/3A]. Also, with that gearing an 8x6 is way too small - I have tested my Speed 400 [3:1] on an 8-cell 600 NiCd and only with a much larger prop are you going to be able to pull the 6amps or so you need to get enough thrust to get, and keep, a 20-24oz airplane in the air [Rule of thumb would suggest you need 10-12oz minimum].

Here are some figures for your interest [A: momentary peak, and B: suggested max. sustainable amp draws]:

8x6 APC E prop:
A: 5.1amp, 9.9V, 52W, 5730rpm, 216g (7.6oz) static thrust

B: 4.0amp, 9.3V, 38W, 5280rpm, 186g (6.5oz) static thrust

9x6 APC E prop:
A: 6.4amp, 9.5V, 63W, 5400rpm, 294g (10.4oz) static thrust

B: 5.1amp, 8.7V, 46W, 5190rpm, 275g (9.7oz) static thrust

10x7 Aeronaut prop:
A: 7.9amp, 9.5V, 77W, 4830rpm, 368g (13.0oz) static thrust

B: 6.0amp, 8.7W, 54W, 4500rpm, 300g (10.6oz) static thrust

11x5.5 APC E prop:
A: 7.3amp, 9.4V, 71W, 4890rpm, 402g (14.2oz) static thrust

B: 6.0amp, 8.7V, 54W, 4500rpm, 344g (12.1oz) static thrust

As you can see, the only way to get sufficient thrust is to increase your battery cell size and prop size - and I doubt an 11x5.5 would clear the ground on a Rascal.

The other alternative, which you have already thought of is to go brushless. The cheapest, yet very good, motor to get would be the MP Jet 25/35-20 with a Model Motors 18e+3ph ESC - about $120 altogether.

With that motor (comes geared 5:1) and a 9.5x6 Aeronaut prop (though with 8-cell 1700NiMH) I recorded as follows:
A: 11.3amp, 9.2V, 104W, 6900rpm, 460g (16.2oz) static thrust

B: 7.8amp, 8.9V, 72W, 6030rpm, 354g (12.5oz) static thrust

Taking about 8amps to be the maximum sustainable current draw:
With a 10x7 Aeronaut at 8.0amps (73W) you can get 382g (13.5oz), and with an 11x5.5 APC E at 8.0amps (74W), 460g (16.2oz) static thrust.

With a smaller battery such as an 8-cell 600AE NiCd you'd get a bit less than this, due to greater voltage drop under load, but it should be enough to fly the Rascal okay.

Hope this helps, Cheers, Phil

SALMONBUG

My Feedback: (1)

I builded and test flight one for a friend some months ago.
I made the setup using SIG recomandations (booklet).
I don't own the book and the plane anymore so I can't give you infos, but I remember it flown perfectly with a 7 cells /600Mah comon battery pack. prop was APC 8/6 electric

take off from grounf in 20 feet.

I remember only one thing, the test flight showed that the recomanded CG was a LITTLE bit too far backward....


maybe judicious to see if there isn't a parasite programation on the throttle of your programable TX who influence the maximum RPM

I am not a electrical power expert, all my planes are glow, so I can only give you my small experience

Dr Kiwi

Sorry Don, I meant to put in some figures using a 7-cell 600NiCd to really emphasize how little one can squeeze out of a Speed 400/8.4V pack combo (and remember this is NiCd not NiMH, which would fare even worse by comparison).

Here we go:

8x4 APCE :
A: 3.6amp (peak), 8.3V, 31W, 5010rpm, 175g (6.2oz) static thrust

B: 3.0amp, 7.9V, 25W, 4680rpm, 146g (5.1oz) static thrust

9x6 APC E :
A: 4.4amp (peak), 8.3V, 36W, 4800rpm, 232g (8.1oz) static thrust

B: 3.0amp, 8.1V, 26W, 4260rpm, 176g (6.2oz) static thrust

10x7 Aeronaut:
A: 6.2amp (peak), 8.1V, 53W, 4410rpm, 300g (10.6oz) static thrust

B: 5.0amp, 7.8V, 49W, 4080rpm, 250g (8.8oz) static thrust

11x5.5 APC E:
A: 6.0amp (peak), 8.2V, 51W, 4440rpm, 336g (11.8oz) static thrust

B: 5.0amp. 7.5V, 39W, 4110rpm, 280g (9.9oz) static thrust


It is quite obvious that, on just 7-cells, NONE of these prop choices on your 3:1 geared Speed 400 would seem to be able to provide you with the desired sustainable thrust ratio of 1:2 for your 20oz+ aircraft.

That said - questioning minds must find room for interpretation of these test-stand observations - since Salmonbug apparently, somehow, could get a Rascal to fly using 7-cells and an 8x6 but yours would not!

Hope this helps out, Cheers, Phil

thriller

My Feedback: (28)

what would happen if you were to use a direct drive 540 motor and a 7 or 8 cell pack? What size prop would you use?

ragbag

A friend got very frustrated with his Rascal and destroyed it before I could help. Stomped it, more money than brains. It was tail heavey and had O/O thrust. Tip stalled. Another friend has the same bird and flies all over with it, a very comfortable flyer.
He doesn't top off his batteries before flying so he does need a hand launch. Stock Sig set up.
George
RAGBAG

thriller

My Feedback: (28)

hey ragbag does your friend have all the electrical parts (or some) left?