Old Hirobo Bell 47

[JohnyTDR]

My Bell 47 Mk1 with the RXcel ignition in http://www.youtube.com/watch?v=VgvQXSWaD9Y
good to see you've sorted yours Pet

[JohnyTDR]

Better video of my bell 47 from the weekend
Bell 47 Mk1 1982 - YouTube

[pchristy]

Very nice! I'm still struggling with marginal power on mine. I re-fitted the Rexcel ignition after the Scottish Nats, and managed to get it flying, but its very marginal. At the British Nats, it took full throttle just to get airborne at all, and a climb-out was only possible with translational lift! What is your ignition timing set to? Mine is a whisper over 30 degrees - any less and I can't take-off at all! I don't think I'm getting enough RPM for the ignition to fully advance when running. It may also be that your carb allows better breathing than my WA12.

Any chance of some close-up pix of your engine / carb for a comparison with mine?

Cheers,

[JohnyTDR]

If I can remember timings set at 28.5 degrees, the inside ofthe carb is 11mm diameter.
2012-10-11 19.00.32.jpg2012-10-11 19.00.53.jpg2012-10-11 19.06.21.jpg2012-10-11 19.06.41.jpg

[pchristy]

Thanks for that, John! I wanted to confirm that yours is exactly the same motor as mine, which it is. Next step will be to investigate a new carb, as I suspect that mine is unduly restrictive. I haven't measured the bore yet, but will do after the week-end.

I've been suspicious of the carb for a while, as it seems to be a very early one. It could be that the mixture is not as consistent throughout the range as it should be, and that is what's causing the misfire! If that doesn't fix it, the next step may be to convert it to electric, but that's a big (and expensive) step!

[JohnyTDR]

Got mine from the US, see link
Walbro Carburetors and Parts - 2-Cycle - CARBURETOR WA1671

[JohnyTDR]

Got mine from the US, see link
Walbro Carburetors and Parts - 2-Cycle - CARBURETOR WA1671

Pete, got my old WA167 if your interested? it came with the bell from R Stocks.

[pchristy]

After finally emerging from my winter hibernation, I've got around to having another go at the Bell 47. I must confess I was getting quite dispirited about it, as nothing I tried seemed to improve it! However, chatting to Steve Roberts down at Charmouth, he came up with a possible explanation. I tried it, and glory be, today - for the first time ever - it was airborne at half throttle on the Rexcel ignition! So what was the magic trick? Simple: change the NiMH cells I was using for the ignition for NiCADs!!!

I had been using a brand new NiMH cell pack to power the ignition, and another identical one for the flight-pack. These are rated at 2000mAH, which falls well within the range of suitable cells specified for the ignition. However, Steve (correctly) pointed out that although they might have been able to supply sufficient *average* current, perhaps they couldn't cope with the *peak* current the ignition was demanding. And he appears to have been right!

Unfortunately, with Nicads no longer available, I'm not sure where to go from here. I suspect I'll end up with a 2-cell LiFe pack with a diode in series to reduce the voltage down to 6V - the maximum specified by the ignition manufacturers. In the meantime, I do have some old NiCAD cells that are even now being cycled to see how much capacity they have left! Hopefully enough to keep me flying for a while!

Its still not exactly a ball of fire! It could be the blades are a bit much for it (designed for the TAS24 powered version?), or it could be the carb being restrictive. I might take you up on that offer of your old carb, John, if its still around. But at least I can now fly it around again! Oh! And the misfire on landing approach is now gone!

[electricsceadu]

it sounds good
i need to get my finger out of my bottom and finish my rebuild on the mk2 that i boom stuck
and then start on the other one

[JohnyTDR]

Sounds like your getting there, what size blades you using ?.Ill dig the carb out for you Pete. send us your address.

After finally emerging from my winter hibernation, I've got around to having another go at the Bell 47. I must confess I was getting quite dispirited about it, as nothing I tried seemed to improve it! However, chatting to Steve Roberts down at Charmouth, he came up with a possible explanation. I tried it, and glory be, today - for the first time ever - it was airborne at half throttle on the Rexcel ignition! So what was the magic trick? Simple: change the NiMH cells I was using for the ignition for NiCADs!!!

I had been using a brand new NiMH cell pack to power the ignition, and another identical one for the flight-pack. These are rated at 2000mAH, which falls well within the range of suitable cells specified for the ignition. However, Steve (correctly) pointed out that although they might have been able to supply sufficient *average* current, perhaps they couldn't cope with the *peak* current the ignition was demanding. And he appears to have been right!

Unfortunately, with Nicads no longer available, I'm not sure where to go from here. I suspect I'll end up with a 2-cell LiFe pack with a diode in series to reduce the voltage down to 6V - the maximum specified by the ignition manufacturers. In the meantime, I do have some old NiCAD cells that are even now being cycled to see how much capacity they have left! Hopefully enough to keep me flying for a while!

Its still not exactly a ball of fire! It could be the blades are a bit much for it (designed for the TAS24 powered version?), or it could be the carb being restrictive. I might take you up on that offer of your old carb, John, if its still around. But at least I can now fly it around again! Oh! And the misfire on landing approach is now gone!

[pchristy]

Hi John,

Blades are 80cm SABs, lifting section. These are supposedly designed specifically for the Hirobo scale gassers, but might be for the later TAS24 powered machines, which had a bit more grunt! It might also be that the engine could be leaned out a bit more. I'm running it just on a clean 2-stroke at the moment.

I'll take you up on the carb offer, if I may, and have e-mailed my address.

Many thanks!

[jimmyhorns]

Unfortunately, with Nicads no longer available,

Hi Pete

Although Ni-Cd batteries are not supposed to be available, there are still a number of ways to get hold of them. Ebay is one, and the other is to buy replacement drill batteries. Dewalt are still using NiCd in some of their packs....

[Steve Roberts]

After finally emerging from my winter hibernation, I've got around to having another go at the Bell 47. I must confess I was getting quite dispirited about it, as nothing I tried seemed to improve it! However, chatting to Steve Roberts down at Charmouth, he came up with a possible explanation. I tried it, and glory be, today - for the first time ever - it was airborne at half throttle on the Rexcel ignition! So what was the magic trick? Simple: change the NiMH cells I was using for the ignition for NiCADs!!!

I had been using a brand new NiMH cell pack to power the ignition, and another identical one for the flight-pack. These are rated at 2000mAH, which falls well within the range of suitable cells specified for the ignition. However, Steve (correctly) pointed out that although they might have been able to supply sufficient *average* current, perhaps they couldn't cope with the *peak* current the ignition was demanding. And he appears to have been right!

Unfortunately, with Nicads no longer available, I'm not sure where to go from here. I suspect I'll end up with a 2-cell LiFe pack with a diode in series to reduce the voltage down to 6V - the maximum specified by the ignition manufacturers. In the meantime, I do have some old NiCAD cells that are even now being cycled to see how much capacity they have left! Hopefully enough to keep me flying for a while!

Its still not exactly a ball of fire! It could be the blades are a bit much for it (designed for the TAS24 powered version?), or it could be the carb being restrictive. I might take you up on that offer of your old carb, John, if its still around. But at least I can now fly it around again! Oh! And the misfire on landing approach is now gone!

Glad it worked Pete.

With Ni-Cd's being unavailable I have been using sub C sized Ni-Mh's and these have been fine as the internal resistance is far less with the larger case size. These are similar to the NiCd that you currently have fitted. I have one fitted in a 120 size FW190 which has 9 servos and it works all day long...

The main problem with the current commonly available Ni-Mh's is Eneloops. These are great as they do hold there charge but they cannot deliver any kind of current under load as the voltage plummets. This is what I susspect the problem was, so either fit the Li-Fe pack or if you dont want to risk it fit a sub-C size Ni-Mh.

Steve

[pchristy]

Hi Steve,

On paper, the eneloops ought to have sufficiently low internal resistance. Clearly, however, they don't! One of the issues we (the modelling fraternity) have with battery manufacturers is that most of the world wants the maximum capacity from the minimum physical size. What WE want is primarily, the lowest internal resistance - and hence the highest current delivery. NiMHs will never have as low an internal resistance as NiCADs, but they ought to be low enough for most purposes.

I think the issue here (having had chance to ponder it!) is that the original ignition system was a simple "transistor replacing point" system. The new one is a capacitor discharge system. In a CD system, a capacitor is charged by and oscillator and step-up coil, which is then discharged through the coil. Of course, as the revs rise, the capacitor has less time to charge between strokes, and it looks as if this is where the eneloops couldn't cope. It may be that a big reservoir capacitor across the eneloops would help, but at the moment the old NiCads seem to be working!

I can see a move to LiFes in the not-to-distant future....!

[JohnyTDR]

Old Hirobo Bell 47

Hi Pete
I've today sent you the carb, may be with you tomorrow if not Friday. I was never quite happy with this carb that's why I got a new one, so do check it out, I purchased a service pack for it and changed the gaskets and other parts.
Please do let me know how you get on with it.
Regards john