Twin Star with 11" props?! What do you think? - RC Groups
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Aug 05, 2001, 04:38 PM
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Bill Glover's Avatar

Twin Star with 11" props?! What do you think?

MotoCalc has a lot to answer for

It occurred to me that as I always fly my Twin Star with the trike u/c fitted, it would be possible to swing quite large props. Looking at readily available props & gearboxes and plugging them in to MotoCalc, here's what I've come up with for my 8x2400 packs:

Graupner Speed Gear 400 6V (inline 4:1) motors, with APC-E 11x10 props.

The numbers look pretty good (even with "heating effects" switched on) .. reasonable pitch speed (41 mph), static thrust to weight 0.52:1, good climb (543 ft/min), fair excess thrust to weight ratio at best l/d airspeed (0.28:1), and low amps (8.6A per motor). 11" props would easily clear the fus. (in fact you could fit 12" ones!) and are well clear of the ground with my u/c. Only downside I can see is that the props are not working very efficiently (stalled) up to 10 mph.

This wouldn't be an expensive upgrade and (being S400 based) would drop straight in. Performance looks to be approximately double that predicted for Permax 480s turning 6x4 props (climb rate, power to weight ratio, etc.), and with lower motor currents too!

What do you reckon? Seriously tempted to try this
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Aug 05, 2001, 08:01 PM
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I think that you should give it a shot.

I've been tempted to do something similar with my TS. I thought about putting on three s400 7.2v motors, 2.33:1 GB, and 9x5 props on a big ten cell pack. Three you ask? Yes. I have been contemplating putting a third motor in the nose and putting my tail dragger gear back on.
Aug 06, 2001, 12:38 AM
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please post pictures if this is done Dave!! i owuld very much like to see this!
Aug 06, 2001, 04:32 AM
ChrisP's Avatar
Go for it Bill. And let us know the results.
Aug 06, 2001, 07:18 AM
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Bill Glover's Avatar
OK, I've gone for it! Only flaw in this cunning plan is that I couldn't find any 11x10E props in the UK (even spoke to the importers - MacGregor). But you can now order online direct from APC at:

So that's what I did - will have to wait & see how long they take to cross the Atlantic
Aug 06, 2001, 09:37 AM
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Bill Glover's Avatar
Got the email confirmation of the order from APC ... 2 props at $3.95 each, plus shipping at $22!
Aug 06, 2001, 11:22 AM
ChrisP's Avatar
That hurts !!!!!!
Aug 06, 2001, 11:54 AM
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Bill Glover's Avatar
Just have to make sure I don't nose it over and break one! I expect the shipping would still have been $22 for 4 props ... oh well, I know for next time! Any good European dealers for APC-E props??
Aug 06, 2001, 05:39 PM
CMFC Secretary (UK)
BioHazard's Avatar
Bill - I wish I'd known. I'm going stateside this week. I could have probably have picked some up for you. Looking forward to bringing back loads of goodies...
Aug 06, 2001, 08:11 PM
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Bill Glover's Avatar

Oh well, it's only money ... sob ...
Aug 07, 2001, 05:01 AM
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Bill Glover's Avatar
Well the geared motors and prop adapters arrived this morning ... less than 24 hrs since I ordered, normal superb service from Gliders Distribution (and they don't charge any postage or packing on orders over 10).

The motors look good, the inline gearbox is spaced a couple of mm away from the front of the motor can so the cooling inlets are not blanked off ... nice touch. Cost is 32 in UK, weight of motor/gearbox is 95g (just 0.75 oz more than the stock Permax 6v S400). Output shaft is 3.17 mm.

Graupner's instructions say from 7-10 cells and props 9x6 up to 12x10 ... so my 11x10s should be OK. Just need the props now, though if I get desperate I might try folders .. 11x8, 12x9 & 12x10 are all easily available. With the BEC brake disabled folders should be OK, you wouldn't want them to fold as the wing would get in the way when they tried to open up again!

Of course, with folders you could do this mod even without the u/c

Will post results as soon as I've flown it.
Sep 05, 2001, 09:44 AM
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Bill Glover's Avatar
Well the rest of this topic disappeared in the 'great Ezone meltdown'

Executive summary: it flies GREAT with APC-E 11x8.5 props ... static current draw is about 20A. The 11x10s weren't very satisfactory (despite what MotoCalc said), I also tried CAM 10x8 folders and APC-E 11x7s and 10x7s. The 10x7s would be my second choice, they gave low current draw (about 16A I think) and good speed, but much lower climb rate than the 11x8.5s.

With this geared setup the model ROGs quicker from grass, climbs better, pulls less amps (so flies longer), and is still aerobatic (in fact it will now do stall turns, which it wouldn't before). It's easier to land (ESC brake is off so the props windmill with the throttle shut, causing some useful drag) and it sounds fantastic ... sort of like a turboprop.

I removed the upthrust when mounting the Speed Gear units - this had no bad side effects and must help efficiency. I left the right sidethrust alone.

Incidentally, the 11" props clear the ground even without the u/c fitted (wings level).

Excellent service from APC, I got two lots of props from them ... in each case I ordered online on Monday and they arrived by the weekend. And the actual shipping charges were much less than stated on the automatic email confirming each order.