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JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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 Posted: Sun Apr 15, 2018 10:52 am Post subject: Question re carb balance tube |
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Perhaps Bud would be kind enough to give a definitive answer to this, but anyone else who has certain knowledge regarding this topic might also be willing to enlighten me. A friend who also has a Europa sent me some links, as follows:
https://www.rotax-owner.com/en/rotax-forum/4-general-discussion/4509-balance-tube
www.zenith.aero/forum/topics/flygas-intake-manifold
https://www.flygas.info/2017/06/18/311/
My friend believes that the earliest Rotax 912 engines had no carburettor crossover balance tubes and ran roughly, particularly at low speed. The solution was the balance tube, but perhaps Rotax tried out the diameter of tube we all know and found that it did a good enough job of smoothing things out, whereas they could have got a better result if they'd experimented with larger diameters. Who knows?
Anyhow I'm not rushing to buy the Italian mod kit costing £700+ because it's not approved by the UK LAA and any unapproved modification fitted to aircraft in this country renders aircraft insurance invalid.
Thanks in anticipation.
Jonathan
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ploucandco
Joined: 16 Sep 2007
Posts: 69
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| Posted: Sun Apr 15, 2018 11:24 am Post subject: Re: Question re carb balance tube |
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Jonathan, ROTAX recommends warming up oil at 2500RPM. When the oil is warm enough, the engine is typically OK from 1800RPM.
I don't like the flygas soluition as
1. it adds weight for no improvement -> no value if you follow ROTAX recommendations
2. an engine is vibrating and you are hard linking the 2 manifolds! The rubbers connecting the carbs and the ones connecting the airbox are taking way more vibration under this setup than when you have a flexible crosstube like on the ROTAX setup.
With well balanced carbs and no leaks on the carbs and airbox, you should be good to fly.
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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| Posted: Sun Apr 15, 2018 12:03 pm Post subject: Re: Question re carb balance tube |
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Ploucandco
"2. an engine is vibrating and you are hard linking the 2 manifolds!"
I'm with you on everything else, except your point 2. above, because the two manifolds are already hard linked by being bolted solidly to their respective cylinder heads. The flygas solution doesn't link them any more solidly than they already are, but I take your point about unnecessary extra weight.
My experience of 3 different Rotax 912 engines which were expertly carb and propeller balanced is that they all exhibited a little vibration at some rpm a bit above 3000.
Thanks for your swift reply.
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Nigel Graham
Joined: 23 Mar 2017
Posts: 36
Location: Winchester - UK
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| Posted: Mon Apr 16, 2018 2:21 am Post subject: Re: Question re carb balance tube |
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Jonathan,
There are some simple clues to judging whether a forum poster knows anything about engines or not. If you spot the keyword “RPM’s (with an “S”) …… you know straight away they don’t have a clue! You see it all the time on boy-racer forums for whom “stage one” tuning involves installing a Mega Base, putting their baseball caps on back-to-front and describing everyone as a “mud flicker” ( ….I think that’s what they’re saying).
The links you posted are classic examples.
To understand what the balance pipe does, you need to know how the engine works. A petrol engine is an air pump – at anything less that wide open throttle (WOT), it is working at below atmospheric pressure. The RPM (no “s”) and hence power, is regulated by the throttle butterfly which acts to close off the inlet tract (that’s why it’s called a “throttle”) for idle, and successively open for full power (atmospheric pressure). When you fly in the cruise, you typically use about 75% power – meaning the throttle butterfly is partially closed and the manifold pressure is below atmospheric.
The Rotax engine is a “Boxer” configuration meaning two cylinders are on the left and two on the right and the designers decided that the simplest way of feeding fuel mixture to them was to use two separate induction manifolds and carburettors.
Whilst the “packaging” issues were simplified, this introduced the problem of ensuring that both systems were perfectly synchronised so that all cylinders produce equal power. Any difference and the engine will vibrate.
When you balance the carburettors, you measure the (partial) mean pressure in each manifold and adjust them to be the same. If the mean pressures are the same than there will be NO FLOW through the balance pipe, so there is no need for a 1.5” drain pipe between the manifolds.
So why do they fit a balance pipe? Carburettors have been developed over the years to feed the right amount of fuel throughout the changing rev range and changing atmospheric pressure – but small differences in airflow mean that the two manifolds are almost never exactly matched throughout the flight profile. The balance pipe allows the small pressure differential to equalise, and smooth out any vibration that would otherwise result.
The contributor of the second of your referenced forum postings, Billy-Bob P Sledgehammer III, is frantically calculating how much extra flow he could get though his balance pipe, not realising that he should be aiming to minimise the airflow through it!
My advice would be to leave it exactly as Rotax supplied it – the power unit is surprisingly powerful for its size and very reliable.
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JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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| Posted: Mon Apr 16, 2018 3:17 am Post subject: Re: Question re carb balance tube |
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Nigel,
Thanks for the detailed explanation, which sets out amusingly what I generally understand. Those discussion forums are populated mostly by contributors with only a smattering of knowledge and as you point out, their poor English usage can be a good indicator of how seriously they should be taken. My personal gripe on that subject is "your" versus "you're" plus a lot of other misuse of apostrophes.
My mechanical knowledge might be described as "passable" but questions on electronics, for example, would leave me "dead in the water".
The reason for me posting this topic was to find out whether increasing the diameter of the manifold balance tube would significantly improve engine smoothness, given that the carburettors and propeller are already well balanced. I was under no illusion that it would give any increase in power.
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budyerly(at)msn.com
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| Posted: Tue Apr 17, 2018 11:14 am Post subject: Question re carb balance tube |
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Nigel,
I couldnt agree more, but I dont think I would have been as kind.
Here in the States we have many Joe Bag of Doughnuts modifications being made and I am glad to see there are others in Europe seeing the same stuff I get to see.
In a nutshell, a crossover tube on twin carbs is interesting. You Jaguar/TR owners from the 60s had many experts commenting on the dual carb setups.
Personally, if it aint broke, dont fix it applies here to the Rotax. Learning to balance the engines carbs to a fine tune is more important in my opinion than throwing hardware at it that is dubious. Worn carbs do go out of sink. Gauges are not matched sets (always calibrate a gauge) and frankly, I learned to balance carbs (once initially set by mechanical/vacuum) by ear/vibration and sound. I had to put new carbs on 12AY because one of them had a piston that just wouldnt work smoothly. I built a vacuum setup for my Shop Vac out of PVC/Plastic and built an intake with manifold with Rotax rubber to carb supports/seals for both carbs to test side by side. I operated the dry carb throttles until I could see exactly what was happening. The pistons would not operate in sync at any setting but idle and full. This gave a mid range vibration or shaking problem and barely made a 300 RPM mag check drop and only after the piston on the right carb would finally move to a near proper position due to the vibration.
My new carbs now can be balance by ear/vibration observation of the ignition boxes. The mag check coming back from Sun n Fun was 100 instead of the normal 50 drop from the annual in January. The 912S mag drop will be slightly higher than a 914 because of the plug firing difference of the types from being top/bottom L and R vs L and R top then bottom.
The large tube, in my opinion, is the American technique of just make it bigger (high rise manifold with three duces) type mentality. Not really efficient normally, just easier. So much air could flow through the carbs it just muddled around the intake. It may allow some extra unmetered fuel air to cross over (as the fuel mixing is done low in the manifolds and the tube is on top) and give you some leaning potential. How much, who knows. They dont validate their claims. With no validation, I dont throw my money or time at it.
We have the HacMan over here for leaning. It pulls intake manifold air and applies it to the float bowl to reduce the fuel air mixture by decreasing the bowl fuel pressure vs the venturi pressure ratio. It can work, is very sensitive, and a sharp eye on the EGT (properly calibrated) and patience is needed to lean the carbs in flight. It does work, kinda, sorta to lean the engine and allow more efficiency for cruising with the 912 at 7500 to 10,000 MSL. But it not a simple leaning knob/lever as on a late model Cessna/Piper you could pull and lean by RPM/EGT/Kill the engine method.
Best Regards,
Bud Yerly
Sent from Mail for Windows 10
From: owner-europa-list-server(at)matronics.com <owner-europa-list-server(at)matronics.com> on behalf of Nigel Graham <nigel_graham(at)m-tecque.co.uk>
Sent: Monday, April 16, 2018 6:21:41 AM
To: europa-list(at)matronics.com
Subject: Re: Question re carb balance tube
--> Europa-List message posted by: "Nigel Graham" <nigel_graham(at)m-tecque.co.uk>
Jonathan,
There are some simple clues to judging whether a forum poster knows anything about engines or not. If you spot the keyword “RPM’s (with an “S) …… you know straight away they don’t have a clue! You see it all the time on boy-racer forums for whom “stage one tuning involves installing a Mega Base, putting their baseball caps on back-to-front and describing everyone as a “mud flicker ( ….I think that’s what they’re saying).
The links you posted are classic examples.
To understand what the balance pipe does, you need to know how the engine works. A petrol engine is an air pump – at anything less that wide open throttle (WOT), it is working at below atmospheric pressure. The RPM (no “s) and hence power, is regulated by the throttle butterfly which acts to close off the inlet tract (that’s why it’s called a “throttle) for idle, and successively open for full power (atmospheric pressure). When you fly in the cruise, you typically use about 75% power – meaning the throttle butterfly is partially closed and the manifold pressure is below atmospheric.
The Rotax engine is a “Boxer configuration meaning two cylinders are on the left and two on the right and the designers decided that the simplest way of feeding fuel mixture to them was to use two separate induction manifolds and carburettors.
Whilst the “packaging issues were simplified, this introduced the problem of ensuring that both systems were perfectly synchronised so that all cylinders produce equal power. Any difference and the engine will vibrate.
When you balance the carburettors, you measure the (partial) mean pressure in each manifold and adjust them to be the same. If the mean pressures are the same than there will be NO FLOW through the balance pipe, so there is no need for a 1.5 drain pipe between the manifolds.
So why do they fit a balance pipe? Carburettors have been developed over the years to feed the right amount of fuel throughout the changing rev range and changing atmospheric pressure – but small differences in airflow mean that the two manifolds are almost never exactly matched throughout the flight profile. The balance pipe allows the small pressure differential to equalise, and smooth out any vibration that would otherwise result.
The contributor of the second of your referenced forum postings, Billy-Bob P Sledgehammer III, is frantically calculating how much extra flow he could get though his balance pipe, not realising that he should be aiming to minimise the airflow through it!
My advice would be to leave it exactly as Rotax supplied it – the power unit is surprisingly powerful for its size and very reliable.
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