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Low Wing Skimmer 400 w/not fly

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Low Wing Skimmer 400 w/not fly

Old 09-16-2004, 12:37 PM
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hercules
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Default Low Wing Skimmer 400 w/not fly

I was able to get out today and try to fly my low wing skimmer 400, I built the acft to specs, checked the balance several times, right on the money according to the plans. I tried taking off from home plate heading to 3rd at about 3rd base it would start to lift, maybe a foot ar so after about 10 attempts, then drop like a rock....I tried one hand launch, it went about10-15 feet and again fell like a rock. I snapped the 1/8 dowel in the L/E but no other damage.....Thanks in advance for any help!
Old 09-16-2004, 10:53 PM
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Dr Kiwi
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Default RE: Low Wing Skimmer 400 w/not fly

If you have the recommended 6-cell NiMh pack - no wonder it won't fly! [NOBODY uses a 6-cell NiMh]. You have a huge 54" span, 22oz plane, and your 6v 400 motor, on 6 NiMh (not even NiCd) cells, with a 6x3 folder, won't get you more than about 7-8oz thrust. You need at least half as much thrust as you have weight for a park flyer (so you probably need ~16oz: well, sailplanes might get away with ~12oz, I suppose), and way more than that for real performance. I looked at some reviews and at least one of the guys was using high output 7-cell 500AR NiCds - he could get his to fly because he was probably pulling twice the amps/watts you were, and getting twice the thrust. At the very least I would get a 7-cell 2/3AA KAN 650 pack [motors are cheap - even an 8-cell!] and see how that improved things.

Cheers, Phil
Old 09-17-2004, 07:11 AM
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Matt Kirsch
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Default RE: Low Wing Skimmer 400 w/not fly

The plane was most certainly stalling, probably from that near-irresistable urge to pull back desperately on the elevator stick when the plane is underpowered and/or isn't ready to fly. Chant to yourself, "The elevator is not the magic up stick! The elevator is not the magic up stick!" A plane needs power and airspeed to climb; up elevator bleeds off airspeed and consumes power. If you don't have enough of one or the other, or both, up elevator makes the plane go DOWN.

You definitely need more power. Heed Dr. Kiwi's words on this issue. More cells is like putting a bigger displacement engine in a car.
Old 09-17-2004, 07:40 AM
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Dr Kiwi
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Default RE: Low Wing Skimmer 400 w/not fly

Well put Matt - I forgot to mention the "magic up stick".

I apologize for the math error - at 1:2 thrust to weight you need perhaps 11oz for a 22oz plane (not the 16 I calculated from a 32oz plane) - and for a sailplane you might get away with a bit over 8oz.

Another way of looking at the power issue, is to use the old rule of thumb - 40W/lb. Your 1.4lb plane needs something like 56W. Allowing for voltage drop under load your 6-cell NiMh pack might be giving 6.0-6.5v - that means more than 8.5A draw to get the required 56W - and the simple fact is that 6x3 folder can't pull that much at such a low voltage. If you go to 8.4v or 9.6v (~8-9v respectively under moderate load) you can see how the math gets better - so you only need to pull 6-7A. On 8-9v a 6x3 folder can certainly do that with a decent pack - on 7-cell KANs I've pulled 12.0A and 80W, and on 8-cell KANs, a ridiculous 13.6A (100W) [though even that still only gets you 10oz thrust!].

Indeed, direct drive 400's are only good for very small, fast, slippery planes. The best way to power a big sailplane is to use a geared motor and a big folding prop - lots more thrust at much lower amp draw.

Cheers, Phil
Old 09-17-2004, 09:58 AM
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hercules
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Default RE: Low Wing Skimmer 400 w/not fly

I did a check and you were right, I'm only getting about 7.5-8 oz of thrust, I ordered a 8 cell pack and will try again next week. Thanks Dr Kiwi and Matt! Eric
Old 09-18-2004, 02:28 PM
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Jim Finn
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Default RE: Low Wing Skimmer 400 w/not fly

8 cells! You better use just 7 nicads if you want to keep that motor alive for long. I have flown hundreds of flights on a skimmer 400 (not the low wing version) with 7 cells and a speed 400 motor and 6.3x3 prop. 8 cells may be too many to use full throttle with unless they are nimh's.

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