Old Hirobo Bell 47

[pchristy]

Stainless steel is pretty hard, which will make it more liable to fracture. When I rebuilt my first boom, I used aluminium tube from B&Q, which almost exactly matched the dimensions of the original! Of course, mine is one of the early "bolt-it-together" ones - I believe the later ones are welded?

Jim Morley used to make his 47 booms out of welding rod! This would certainly make - er - welding them easier.....! May not do much for the weight, though!

Although my original boom is in a pretty sorry state, I think many of the smaller parts are salvageable. I've kept all the bits as I think it may be possible to rebuild it. But it will be a loooong job.....!

For the tail drive shaft I just used some metre long piano wire from Sussex Model Centre. The stuff you get from most model shops is only a yard long, and a bit marginal for the Bell 47! I forget the exact dimensions, but it comes in tubes of 5, so I'll have a look later in the week. For the sleeving, I used some ali tube from the local model shop K&S metal stand.

[electricsceadu]

the g2 has a stainless tubed truss hence why i'm repairing it with stainless tube and is silver soldered together
the drive is by a stainless torque tube

i'm hoping to have a look at the morley truss jig to get an idea how to get my jig to work out
as the morley jig should be localish to me

the good old morley boom will repair in an even worse state than my hirobo one
as the one i straightend won the vintage cup for a few years

[electricsceadu]

after a boom strike today in a comp with my other hirobo bell 47 i now have two boom truss's to repair
i'm not having a good year so far
bring on next year and fingers crossed my unplanned reshaping reduces

[trillian]

Ooooooh, not good :-( That's certainly not where you want to have a boom strike! (not that anyone wants a boom strike on any heli).

[pchristy]

Sorry to hear about that! Boom strikes are not good on these machines! I speak from experience!

I'm trying to get mine ready for the British Nats. I've put the RXcel ignition back on again, and once again all the power has vanished! I think its a timing problem. I suspect the electronically controlled advance doesn't reach its maximum until well past the engine's peak rpm, so the static timing recommended is too retarded (if you get my drift!). I tried retarding it further to see if I'd gone too advanced, but that didn't help.

So, I'm making a new mount to fit the hall sensor onto the original swinging mount. This should make it easier to play with the timing. I'd really much prefer to go with the RXcel ignition, as there are far fewer connecting wires, and the ones to the timing sensor aren't constantly moving, so it should (hopefully!) be more reliable!

Back to the workshop, then........

[lawrencelkl]

Sorry to hear about the boom strikes...now I am kind of scared to fly my Bell 47G hearing from fellow pilots with boom strikes..
I think I will do a good overall checking and maybe replace some old servos just to increase peace of mind, especially my skill is not that advanced yet.

Pete, hope you will get some good results with the new ignition system.

cheers all.

Regards,
Lawrence

[pchristy]

Work progresses......

I've now mounted the Hall effect sensor on the original swinging arm so that I can adjust the timing more easily:




Its been quite tricky, getting it close enough to the flywheel to sense, without anything actually touching! Also, all the screws in this engine seem to be made of that well-known alloy, cheesium, and a number are starting to look a bit the worse for wear!

Initially, I'll use a pushrod that's externally accessible to get the coarse setting. Then I can fix it in place and use the screw-adjustable ball link to fine-tune it.

At least, that's the plan! If the weather holds, I might be able to test fly this afternoon.....

[pchristy]

Finished off the installation today, using the original "swinging arm" to mount the sensor:





By using the original "swinging arm", and a couple of ball joints, I can now fine tune the timing!

I took it up the field this afternoon, having set the timing to the recommended 28 degrees. It would just about stagger into the air at full throttle, but struggled to stay there, and never got more than about 18" off the deck!

So, I started winding in the link, and increasing the advance. The first effort showed an improvement, but still not much power. However, when I wound it in a little further 1) It started instantly (something it hasn't done for a while), and 2) The tickover speed increased, meaning I no longer needed a fair amount of + throttle trim to keep it going! Suddenly, it was airborne at about 60% stick! I did a couple of circuits, and then landed to try even more advance! The further advance didn't seem to show any significant increase in power - maybe a little, but not enough to be sure. I flew a couple of circuits, and then carried out a prolonged hover test. I was worried in case I'd over-advanced the timing, but it showed no sign of over-heating, and continued to run just fine. Its still marginal on power, but it can (just!) climb vertically in hot and calm conditions!

When I got home I checked the static timing and found it to be 34 degrees. I've now backed it off slightly to 32.5 degrees, as I've no wish to burn a hole in an irreplaceable piston, and my last trial advance didn't seem to make much difference. Its amazing the difference that last 2.5 degrees has made! It is now just as flyable as on the original ignition system, but with far less wiring, and fewer moving parts!

Now all I have to do is fix the "hangar rash" it suffered in Scotland prior to Barkston!

[electricsceadu]

good news then



lawrence
don't worry obout a boom strike mine was a one off
i was just unlucky

[pchristy]

good news then

Fingers crossed.......! (As well as many other things!)

[lawrencelkl]

Pete,

Well done, I was tempted to upgrade the old electronic ignition box too but looking at all the hard work involved, i might stick with the older unit for quite some time ....

[lawrencelkl]

lawrence
don't worry obout a boom strike mine was a one off
i was just unlucky

Thanks, I will at least try to be more careful next time flying mine, cheers.

Lawrence

[pchristy]

Pete,

Well done, I was tempted to upgrade the old electronic ignition box too but looking at all the hard work involved, i might stick with the older unit for quite some time ....

If I'd done it this way in the first place, I would have saved myself a lot of aggro! I was trying to do away with as many moving parts as possible, to improve reliability! In fact, the "swinging arm" is quite robust, and now its locked in place it should be as solid (if not more so) than my original idea!

Basically, all I've done is make a small fibre-glass plate to bolt onto the "swinging arm", and mounted the sensor onto that. The trickiest bit was getting the clearance right. As well as being clamped in place, its also held with a good dollop of silicone sealant. Set the static timing to 30 degrees initially, then advance it in small steps (if necessary) until you get sufficient power. There was no-one else up at out field yesterday, and it doesn't take long to remove and replace the cooling duct to carry out any adjustments. You don't even have to detach the recoil starter from the cooling duct, just remove the whole lot as one unit!

When you reach the "critical point" for timing, its a very sudden transition! The engine just suddenly comes alive - at least mine did!

In short, now I've done it, it would be easy to repeat, hence this thread! If you avoid the mistakes I made initially, its a straightforward conversion! And it should improve the reliability (touching wood!), as there are far fewer wires flying around the place!

[lawrencelkl]

In short, now I've done it, it would be easy to repeat, hence this thread! If you avoid the mistakes I made initially, its a straightforward conversion! And it should improve the reliability (touching wood!), as there are far fewer wires flying around the place!

Hi Pete,

Many thanks for the sharing which will save lots of trial and error for future conversions! I am really quite new to electronic ignition on gas engine. So my question is, do i still need to drill the flywheel and fit a new magnet or I can use the existing one? if no drilling on the flywheel is required, i am quite tempted to follow your steps in the ignition system conversion!

cheers.

Regards,
Lawrence

[pchristy]

Initially I tried to use the original magnet, but it was too weak. Just Engines can supply a drill the right size for the magnet, and its a simple job to drill the flywheel to take the magnet. I had to make a "puller" to remove the flywheel from the engine, but other than that, it was straightforward. The drill is under-sized, so the magnet needs to be pressed into the flywheel. I achieved this with my bench vice, then "peened" the edge slightly with a centre-punch, just to make sure it stays put!

I installed the magnet close to the edge of the flywheel - far enough from the original magnet for the two to avoid interaction whichever system is used. But with 20-20 hindsight, I would recommend installing a new magnet with the polarity reversed compared to the original, just to be sure. The hall effect pickup will work with either polarity simply by mounting it with the other side facing the magnet.

The most difficult part of the whole job was removing the flywheel, which necessitated making a special "puller" for the job. But with this tool, it is dead easy to get the flywhel on and off!