Old Hirobo Bell 47

[trillian]

'Looks really nice!

[pchristy]

.....May I know if anyone can help on the part number and /or place to find a spare for
(1) metal flybar see saw centre hub

I've probably got one of these! Along the way I've acquired a couple of spare heads. One has a broken feathering shaft and can be stripped for spares - everything else on it looks in good order! I'm not sure from your description exactly what it is you are after, but if you can post a pic - or give a more specific description - I'll strip the parts off the broken head for you.

(2) main helical gear.

I have got one of these, but want to hang on to it for the moment. I'm pretty sure Dave Nieman has some. I'm at work at the moment, but next time I'm home, I'll pm his contact details to you. I'll also look up my throttle curves for you.

My rebuild is finally making headway! The broken pushrod (and its opposite number!) has been replaced, and the new boom is now attached to the chassis. It took quite a bit of "fettling" to get it to fit, and as soon as the weather improves, I will have to remove it again to spray it! I'm making a new horizontal stab. The old one was salvageable, but quite heavy. I'm hoping that if I can reduce the weight at the rear end, I will be able to reduce the ton of lead up front quite substantially! I also need to make a new fin!

The gear train has never run particularly smoothly on this model, and the ensuing vibration may have accelerated the demise of the snapped pushrod! So whilst its in bits, I've been having a good go at the alignment. Strangely, it seems to bind if the clutch shaft is too loose just as it does if its too tight! There is a "sweet spot" where it runs smoothly, but at this point the mesh is slightly looser than I would like. Since these are helical gears, it may be fine like that, but that's why I don't want to part with my spare gear at the moment....!

It looks as if the binding could be caused by the clutch shaft not running true to the crankshaft. But even slackening off the engine mounting bolts whilst setting the mesh doesn't help. Most strange, but it has to be something to do with keeping the shafts in line.

Once the boom is sorted out, the next job will be to replace the ignition system. Just Engines have something that looks promising, and I'll probably go down that route.

Onwards and upwards.....!

[pchristy]

Here's the throttle curve I use on my 47:
L 0%
1 40%
2 45%
3 50%
4 inh
5 inh
6 100%

Pitch curve is:

L 0%
3 45%
H 80%

Bear in mind that I've fitted the alternate mixing levers to give more pitch range! On hold, it goes up to 100% at the top to give a bit more safety factor should I ever have to auto it!

I'm told they auto very well, but I haven't had the bottle to try it!

The idea behind my throttle curve is to get the rpm up to flying speed as quickly as possible, and keep the head-speed constant under load whilst flying. Mine has always suffered from a misfire when throttled back for landing, which causes the tail to kick a bit, and the raised bottom end helps with this too! I'm hoping a new ignition system will finally cure this problem!

Will send PM with DN's contact details!

[lawrencelkl]

Hi Pete,

Thanks for the info. Sorry for my late reply. Been away home in Malaysia for vacation and jsut back to UK 3 days ago.

The spare flybar seesaw holder that i mentioned is the middle metal piece that holds the bearing and the fly bars are screwed on. mine has the half circle that holds the bearing broken. please kindly let me know if you can spare me one.

I have brought my MK1 bell 47G to Malaysia (tail taken apart, in two big boxes) last month and while everything was seems fine (as in the video) in the UK jsut before i bring it back, the helicopter refused to lift in Malaysia! I think main reason is that my previous setting might not be optimum but still can hover in cold weather and lost power in much warmer weather in Malaysia (30 degree Celcius!)

I tried to lean out the needle (as i thought warm air means less dense and hence require less fuel to get the same mixture). finally it kind of can lift to few feet but basically the top of the throttle is useless...so few question from me again, thanks for being so patient to help!

(1) is yours MK1 with the original hirobo 20 cc engine? if so, what is your L and H needle settings? my last OK-ish setting is L(1-1/4 turn out) and H(1-3/8 turn out)

(2) what is your hovering head speed rpm? i searched online and i get figure 1200-1400 rpm but want to be sure...i did not have tachometer in Malaysia but will get one when i go back again next time.

(3) with the throttle curve you posted above, is 100% refer to the butterfly valve fully open? i.e. 90 degree from the fully closed position?

(4) may i know the pitch curve range you are using/ i.e. -3 degree at bottom and +10degree at top?

also some more bad news....the tail boom pipes that are holding to the round tube connected to the tail gear broke and i am yet to think of a way to repair them....any idea and suggestion? thinking of using epoxy and fiber glass thread to wrap around but not sure if that can handle the vibration at the tail..

(5) any progress on the ignition?


sorry for so many question and thank you in advance first!

Regards,
Lawrence

[pchristy]

The spare flybar seesaw holder that i mentioned is the middle metal piece that holds the bearing and the fly bars are screwed on. mine has the half circle that holds the bearing broken. please kindly let me know if you can spare me one.

Yes, I've got one of those. The bearings feel a little sticky, but will probably be OK with a good clean!

I have brought my MK1 bell 47G to Malaysia (tail taken apart, in two big boxes) last month and while everything was seems fine (as in the video) in the UK jsut before i bring it back, the helicopter refused to lift in Malaysia! I think main reason is that my previous setting might not be optimum but still can hover in cold weather and lost power in much warmer weather in Malaysia (30 degree Celcius!)

I tried to lean out the needle (as i thought warm air means less dense and hence require less fuel to get the same mixture). finally it kind of can lift to few feet but basically the top of the throttle is useless...so few question from me again, thanks for being so patient to help!

(1) is yours MK1 with the original hirobo 20 cc engine? if so, what is your L and H needle settings? my last OK-ish setting is L(1-1/4 turn out) and H(1-3/8 turn out)

(2) what is your hovering head speed rpm? i searched online and i get figure 1200-1400 rpm but want to be sure...i did not have tachometer in Malaysia but will get one when i go back again next time.

(3) with the throttle curve you posted above, is 100% refer to the butterfly valve fully open? i.e. 90 degree from the fully closed position?

(4) may i know the pitch curve range you are using/ i.e. -3 degree at bottom and +10degree at top?

The original Hirobo 20cc engine (yes, that's what I've got!) was always regarded as a bit marginal. Mine seemed to have "adequate" power, but it certainly isn't over-powered! And high temperatures coupled with high humidity are not conducive to high power output! Even full size machines suffer a serious performance drop off in "hot & high" conditions. I'm not surprised you were having problems!

From memory, both my needles are around 1 1/4 turns out, so yours sound about right.

I've never measured the hovering head-speed, but I'd be surprised if it was as high as 1200 rpm! It certainly seems slow, and these engines work best when loaded. Maybe you are over-revving it a bit, and getting past the power peak?

My throttle curve was set so that at 100%, the link was just touching the mechanical stop on the throttle barrel.

Again, I'm not sure what the pitch range was the last time I flew it. When I got it at first, even with the swashplate travel adjusted to an absolute maximum, I was only getting about 8 or 9 degrees of range. I changed the mixing arms in the head (to Dave Nieman's disgust! He reckons I shouldn't need it!) so that I got a small amount of negative - probably about 2 degrees, certainly no more than three. At the top end, I backed the top end pitch off so that it was just starting to bog at full pitch. Then on the "auto" setting, I left it at 100%. The idea was just to give me a fighting chance should (Heaven forbid!) the engine stop at altitude!

also some more bad news....the tail boom pipes that are holding to the round tube connected to the tail gear broke and i am yet to think of a way to repair them....any idea and suggestion? thinking of using epoxy and fiber glass thread to wrap around but not sure if that can handle the vibration at the tail..

Not sure which tubes you are referring to, but try finding some hard dowel or carbon rod that is a tight fit up inside the tubes. Smear some 5min epoxy on before sliding the broken tubes over the rod. Then you should be able to make a reasonable cosmetic job on the outside, rather than have unseemly bulges!

(5) any progress on the ignition?

I'm looking at the one from Just Engines, about which I've heard people say good things. But I've still got a lot of other work to do before I get that far......

Best of Luck! And keep me informed of progress......

[pchristy]

PS: Its worth making sure that the mechanical ignition advance is working properly. When I inspected mine, I found that the link had broken inside the fan shroud, so although all looked well, the timing was just flopping around all over the place! This didn't help getting a consistent engine run or power output!

[lawrencelkl]

Hi Pete,

Thanks for the useful information. I kind of getting more confidence again with your reply. It was very frustrating when I tried so many things and yet it failed to lift last time in Malaysia.

with that I do think that i might have over revved the engine. next time I will adjust the pitch curve so that more pitch to load the engine.

regarding the meachanical ignition advance, can I inspect by just taking off the fan shroud or do i have to dismantle the engine cover etc? I know nothing about this mechanical ignition advance, will appreciate if you can tell me mroe or I will look up on the net for info.

the tubes that are broken are at the last tip , which is flattened for screw so cant really slide any tube into it. I dont mind cosmetic issue as long as the repair can do the job holding tight..I just hope that with good epoxy and some fiber glass thread, it will be up for the job...

cheers!

Lawrence

[pchristy]

Its not difficult to make new tubes as long as you have the dimensions. The biggest problem is that they are prone to fracturing where you flatten them at the ends. I've found that super-glueing some tight fitting dowel inside before crimping the ends in a vice helps enormously. Also don't crimp flat across the tube, but at an angle - and do it twice to form a "V". This also helps prevent fracturing.

Just drop the fan shroud off and you should be able to see the advance mechanism. On mine the throttle link from the servo attaches to a bell crank. One end of the crank goes to the throttle barrel, and the other end to a cable that vanishes into the shroud. Inside the shroud, it ends in a ball link that pulls the sensor coil around on an arm concentric with the crankshaft.

Sounds complicated, but when you see it, its quite crude, really. On mine, the plastic ball end had fractured and come adrift, leaving the arm holding the sensor coil flopping around freely.

Removing the shroud involves releasing the recoil starter. Not a problem, but just watch out for things dropping off as you dismantle it, and make sure you put it back the same way it came apart! Mine seemed to be missing a washer which stopped the cord retracting properly after starting, but one from a Halfords washer pack fixed that quite easily!

Whilst you've got the shroud apart, check carefully the wiring to the ignition coil which is also on the shroud. On mine, the earth wire fractured due to it not being supported properly, resulting in the engine stopping without warning and my first boom-strike with it! Ouch!

The Just Engines ignition system seems to have its own internal coil, so hopefully when I upgrade it, another potential point of failure will disappear!

[lawrencelkl]

Hi Pete,

Thanks for the useful info and tips. Yes, maybe it will be better if I sleeve over some new tubes with dower inside and then flatten the new ends as you described. This will be much better than trying to glue the broken flat parts back to the tubes.

As my heli is in Malaysia now, can you help me to check what is the diameter of the tubes? I am refering to the two long tubes from the top of the main frame , straight to the tail tube. I will then try to get some aluminium or brass tubes of same or slightly bigger size for repair.

for your info, mine did not come with the tail 'loop', the one preventing the tail blades from hittign the ground. I made the tail 'loop' bending from a brass tube and filled the ends with epoxy before flattening them and it seems ok so far.

Cheers,

Lawrence

[pchristy]

Repairs are now all coming together, following the "incident" at Charmouth nearly a year ago!

The new boom is now sprayed and in place:



and it looks reasonably straight:



At least compared to the old one:



!!!



New (and lighter) tailplanes have been made, and are just awaiting the glue to dry so I can start trimming them, then I can put the bubble back on.

Then its see if I can replace the ignition system, which I suspect is the source of the misfire......

[trillian]

Have you looked into the logistics of putting a different engine in there? I have a Laser 150 fourstroke (nitro) that would be perfect :-) Or just a Zenoah 260 or something.

[pchristy]

I've actually "acquired" a complete central chassis section that has been modified to take one of the big Super-Tigres. The problem with fitting anything other than a spark-ignition would be where to put the fuel! The tanks aren't anything like big enough to supply a glow engine!

The Tiggy would be good, as they can be modified to run on petrol / sparks, but then I would have to rebuild the whole central part of the model!

I'm going to persevere with the Hirobo engine for the time being. The power is adequate, its just the misfire when throttled back that is annoying - it makes the tail kick in the landing approach in an un-scale like manner! If I can fix the misfire, I'll be happy with it!

Front runner at the moment is a JustEngines ignition system....

Just Engines Online Ltd Other Ignition Systems

[lawrencelkl]

Good Job on the tail Pete, you make me really keen to repair mine now...too bad I have to wait till I get back to Asia next time. are the Aluminium tubes 5mm or 6mm? or a combination of both?

Keep it going, I would like to see it in the air soon! no pressure...

Lawrence

[pchristy]

Another major step forward, and a small one backwards!

The heli has now been completely re-assembled, and had its first run in the back garden this morning (minus its main blades!).

I've now fitted it with an RCEXL ignition system from Just Engines. The original Hirobo unit didn't have a good reputation, and Dave Nieman's advice was to chuck it and replace it with a strimmer unit from a garden centre!

I went for the RCEXL system because its micro-processor controlled, working out the ignition timing electronically, and enabling me to junk the rather Heath-Robinson timing system of the original! Its also designed to run off 4 NiMHs, and is a Capacitor Discharge system, which should provide a decent spark!

The problem is that the original used an inductive pick-up, whereas the RCEXL uses a Hall effect device, so some major work was required to adapt the engine!

Here's the original setup:



The coil is the black box in front of the cylinder. Its mounted on a pivoting arm, concentric with the crankshaft. The Bowden cable pulls the pickup more "advanced" as the throttle is opened, via this linkage:



The throttle linkage comes from the servo via the link on the left of the bellcrank. The small "T" stub on the top goes to the carb, and the right hand link carries the other end of the Bowden cable! A Real Mickey Mouse setup!

First job was to devise a mounting rigid enough to carry the sensor without riking it coming into contact with the flywheel under vibration. I settled on a piece of aluminium tube, threaded to 5mm in one and, and with a 3m cross hole in the other. These bolt to the cooling duct, and the cross hole stops the tube rotating. I super-glued some tight fitting dowel inside the tube to reinforce it. I then bolted a small piece of fibre-glass plate across the tube to mount the sensor:





I had intended to use the existing magnet, but this wasn't strong enough! The sensor had to be virtually touching it to trigger. So I drilled another hole nearer the edge of the flywheel for the new, more powerful, magnet:



Its sufficiently far away from the original that it isn't affected by it. The "dots" around it are centre-punch marks - an attempt to "peen" it in, so there's no chance of it coming loose (I hope!). The magnet had to be inserted in a big vice - its an interference fit - so hopefully it will stay put!

The static timing has to be set to 28 degrees advance (a maximum for this type of engine). The ignition will then automatically retard it at lower rpm. Moving the magnet a little further around the flywheel made it easier to mount the sensor in a manner where nothing fouled the flywheel:



The engine fired up for the first time in over a year this morning, and seems to run well on the new ignition. I've not tried it under load yet!

Th brief test run indicated that the main-shaft wasn't as true as it should be (the one step back!). I've got another one, but its even more bent! The bend is only tiny, but on a beast this size, with heavy rotors, you want it running as true as possible. I'm currently awaiting a call from Dave Nieman to tell me if he has one. If so, I'll be off to pick it up, and hopefully test hopping tomorrow! If not, I'll have to try and find a friendly mechanic with a lathe.......!

[electricsceadu]

can't you get bud at quick uk to make a shaft for you?