Rotax safety question

Talk about airplanes! At last count, there are 39 (and growing) FAA certificated S-LSA (special light sport aircraft). These are factory-built ready to fly airplanes. If you can't afford a factory-built LSA, consider buying an E-LSA kit (experimental LSA - up to 99% complete).

Moderator: drseti

User avatar
bstrachan
Posts: 41
Joined: Sat Mar 27, 2010 4:03 pm
Location: Cedar City, Utah

Re: Rotax safety question

Postby bstrachan » Thu Mar 23, 2017 11:16 am

drseti wrote:
bstrachan wrote:
"Bing carburetors belong on motorcycles."


Pretty strong generalization. There are advantages (and limitations) to fuel injection. There are advantages (and limitations) to carburetors. One size does not fit all.


Note I said BING carburetors. I didn't say ANYTHING about fuel injection.

SU carbs are variable venturi carburetors just like Bings, but they have a couple of significant advantages.

1. No rubber diaphragm to wear out and break

2. A vastly simpler, less troublesome start device.

And then there is the good old Marvel carburetor. People have been flying for nearly a century with manual mixture controls. It seems to be a skill most of us can master.

So why do we have Bing carbs on the Rotax? "Buy German?"

User avatar
bstrachan
Posts: 41
Joined: Sat Mar 27, 2010 4:03 pm
Location: Cedar City, Utah

Re: Rotax safety question

Postby bstrachan » Thu Mar 23, 2017 11:42 am

3Dreaming wrote:Your vents are likely tucked under the bail that holds the float bowl on. The reason for having the vents routed into the airbox or tucked under the bail when you have K&N filters, is that the vent and the air coming into the carb need to be matching pressures. If the pressures aren't matching it can cause the engine to run poorly.

You may be right.

As for the engine driven fuel pump, the engine needs to be able to run independently of the electrical system. If all you had in your low wing airplane were an electric pump and it fails you will be on the ground. By having the engine driven pump, in the event of an electrical system failure your engine will stay running.


When was the last time your 2007 Taurus left you on the roadside because the electrical system or fuel pump (which is in the tank where it belongs) failed?

Seriously, the Facet cylindrical pump which would be a good candidate for an aircraft application has a MTBF of >5,000 hours. Yes I've heard of infant mortality, that is what "burn-in" is all about. You could run one of these on the bench for 50 hours in a couple of days... that ought to take care of infant mortality.

All that said, I DID have an electrical system failure on my StingSport. But it wasn't the Rotax alternator which is fairly bulletproof, it was the cheap Ducati motorcycle voltage regulator. Motorcycle parts are lightweight and cheap, but you can generally pull over and call AAA when your bike goes TU. Anyway, I have to agree, a mechanical engine-driven pump is a good idea on aircraft that use motorcycle parts.

User avatar
drseti
Posts: 5215
Joined: Sat Nov 28, 2009 6:42 pm
Location: Lock Haven PA
Contact:

Re: Rotax safety question

Postby drseti » Thu Mar 23, 2017 11:55 am

bstrachan wrote:Note I said BING carburetors. I didn't say ANYTHING about fuel injection.


I stand corrected. I thought you were arguing against carburetion in general.

Why Bing? Because Rotax already had experience with those, on the engines they've been building for half a century, for use on -- BMW motorcycles!
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

User avatar
MrMorden
Posts: 1733
Joined: Fri Aug 17, 2012 7:28 am
Location: Athens, GA

Re: Rotax safety question

Postby MrMorden » Fri Mar 24, 2017 7:54 am

Given that the Rotax engines are relatively trouble free and low maintenance, many thousands are in service, and 80%+ of new piston aircraft engines are Rotax, I'd say Bing carbs absolutely belong on airplane engines. 8)

That said, I wish on the 912 series that Rotax had gone to a single larger carb instead of two small carbs requiring synchronization. Simpler is usually better.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

User avatar
FastEddieB
Posts: 2130
Joined: Wed Jan 07, 2009 9:33 pm
Location: Mineral Bluff, GA

Re: Rotax safety question

Postby FastEddieB » Fri Mar 24, 2017 8:08 am

Interesting historical note...

BMW motorcycles have used BING carburetors since about 1970. They did a fine job, but were always a bit problematical.

I've owned two BMW's with BINGs and had my share of issues. More than once ended up with a gas-covered boot.

Here is the original float setup:

Image

My guess is that BING came up with the dual float arrangement to deal with uncoordinated flight, something a motorcycle does not generally need to deal with.

The dual floats were alleged to be an improvement, even on bikes. Here I am installing them at a BMW Rally in Missoula, MT, where BING had a booth and was selling them.

Image

As an aside, I guess I'm not the only one who's aged over the past 25 years!
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
FastEddieB@mac.com

roger lee
Posts: 685
Joined: Tue Dec 08, 2009 11:47 am
Location: Tucson, Az. Ryan Airfield (KRYN)

Re: Rotax safety question

Postby roger lee » Fri Mar 24, 2017 8:17 am

Hi Andy,

The reason for two carbs vs one is power. This is why many 2 stroke 2 cylinder engines in the earlier days went to two carbs over a single one. Bing carbs aren't an issue. It's usually the owner. leaving old fuel too long, not syncing once a year, leaving them on for many years and no service, ect...


As a rule the worst thing that happens to a Rotax is its owner.
Roger Lee
Tucson, Az.
LSRM-A, Rotax Instructor & Rotax IRC
(520) 574-1080 (Home) Try Home First.
(520) 349-7056 (Cell)

User avatar
FastEddieB
Posts: 2130
Joined: Wed Jan 07, 2009 9:33 pm
Location: Mineral Bluff, GA

Re: Rotax safety question

Postby FastEddieB » Fri Mar 24, 2017 8:36 am

To expand just a bit...

Two additional failure modes I encountered on the bikes.

1) The little "e-clip" that holds the needle in the float broke. The needle then dropped down and blocked the main jet, with obvious effects.

2) One of the four screws holding the slide to the diaphragm backed out. This resulted in a bike that would run satisfactorily nearly wide open, but very roughly just off idle.

Plus the usual trash in the float needle and seat. Inline filters just before the carb is a typical BMW mod.

Both of the two mentioned first were on relatively new bikes where the carbs had never been touched. Really hard to blame the owner for those!
Fast Eddie B.

Sky Arrow 600 E-LSA • N467SA

FastEddieB@mac.com

User avatar
MrMorden
Posts: 1733
Joined: Fri Aug 17, 2012 7:28 am
Location: Athens, GA

Re: Rotax safety question

Postby MrMorden » Fri Mar 24, 2017 8:46 am

roger lee wrote:Hi Andy,

The reason for two carbs vs one is power. This is why many 2 stroke 2 cylinder engines in the earlier days went to two carbs over a single one.


But...isn't it all about getting enough air, properly mixed, into the the cylinders? One larger carb could do the same thing with less complexity. From my car racing days I know that multi-carb setups only really buy more power of there is insufficient airflow into the engine with the largest single carb you can fit.

For example, if the biggest single that can fit on the intake flows 500cfm, and the heads can take a total of 750cfm, then you have a problem, and dual 400cfm carbs can fix the problem. But if the heads are flowing 750cfm and you can fit a single 750cfm carb, going to dual 750cfm carbs buys no power and creates added weight and complexity.

Of course, Rotax would need a different intake to do a single carb. But if I was designing it, I would have tried hard to get to a single carb solution. Only a single throttle cable, no sync issues, probably lighter weight... This is all just a quibble though, the setup works pretty well as is.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

3Dreaming
Posts: 1971
Joined: Sun Jan 10, 2010 6:13 pm
Location: noble, IL USA

Re: Rotax safety question

Postby 3Dreaming » Fri Mar 24, 2017 9:02 am

MrMorden wrote:
roger lee wrote:Hi Andy,

The reason for two carbs vs one is power. This is why many 2 stroke 2 cylinder engines in the earlier days went to two carbs over a single one.


But...isn't it all about getting enough air, properly mixed, into the the cylinders? One larger carb could do the same thing with less complexity. From my car racing days I know that multi-carb setups only really buy more power of there is insufficient airflow into the engine with the largest single carb you can fit.

For example, if the biggest single that can fit on the intake flows 500cfm, and the heads can take a total of 750cfm, then you have a problem, and dual 400cfm carbs can fix the problem. But if the heads are flowing 750cfm and you can fit a single 750cfm carb, going to dual 750cfm carbs buys no power and creates added weight and complexity.

Of course, Rotax would need a different intake to do a single carb. But if I was designing it, I would have tried hard to get to a single carb solution. Only a single throttle cable, no sync issues, probably lighter weight... This is all just a quibble though, the setup works pretty well as is.


Andy, While 2 carbs seems complex to you, the vision of the intake manifold and ducting for one carb seems complex to me. I could also see the placement of the carb being an issue with the compact size of the engine.

User avatar
drseti
Posts: 5215
Joined: Sat Nov 28, 2009 6:42 pm
Location: Lock Haven PA
Contact:

Re: Rotax safety question

Postby drseti » Fri Mar 24, 2017 9:08 am

An advantage to dual carbs that nobody ever mentions: you've all probably noticed that the Rotax is relatively immune to carb ice, and that if it does occur, you get lots of warning before engine stoppage. With two carbs, they are unlikely to both ice up at exactly the same rate. So, you start to lose fuel/air flow into just two cylinders first, the engine runs rough (but doesn't yet quit), and you get some time to recognize the problem and turn on carb heat.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

User avatar
drseti
Posts: 5215
Joined: Sat Nov 28, 2009 6:42 pm
Location: Lock Haven PA
Contact:

Re: Rotax safety question

Postby drseti » Fri Mar 24, 2017 9:12 am

One of my motorcycles, an antique Honda CB350F, uses four carbs, one per cylinder. Tricky to balance, but once set up, it just purrs - and develops noticeably more power than the 2-cylinder CB350. They did the same thing on the original CB700 Superbike.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

User avatar
drseti
Posts: 5215
Joined: Sat Nov 28, 2009 6:42 pm
Location: Lock Haven PA
Contact:

Re: Rotax safety question

Postby drseti » Fri Mar 24, 2017 9:16 am

BTW, I had dual carbs (SU side-draft) on all of my 50s and 60s vintage British sports cars - for some reason that was the gold standard.
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof. H. Paul Shuch, Ph.D., CFII, LSRM-A/GL/WS/PPC, iRMT
AvSport of Lock Haven
fly@AvSport.org
http://AvSport.org
http://facebook.com/SportFlying

User avatar
FastEddieB
Posts: 2130
Joined: Wed Jan 07, 2009 9:33 pm
Location: Mineral Bluff, GA

Re: Rotax safety question

Postby FastEddieB » Fri Mar 24, 2017 10:45 am

Fuel and air are atomized in the carburetor. Ideally you want keep the distance from the carb to the cylinder as short as possible. The ROTAX setup accomplishes this nicely - just a couple inches.

With a single carb and manifold, the distance will be both greater and less direct, causing some losses.

Of course, the losses can be minimized by various methods, including smoothing and polishing an intake manifold.

As an aside, as delivered my 1966 MG Midget seemed down on power, and they were not known for blistering performance in the first place. An inspection revealed the linkage between the twin SU's was not correctly set, so I was running on just one carb. Fixing it made a world of difference!
Fast Eddie B.

Sky Arrow 600 E-LSA • N467SA

FastEddieB@mac.com

User avatar
MrMorden
Posts: 1733
Joined: Fri Aug 17, 2012 7:28 am
Location: Athens, GA

Re: Rotax safety question

Postby MrMorden » Fri Mar 24, 2017 3:09 pm

FastEddieB wrote:Fuel and air are atomized in the carburetor. Ideally you want keep the distance from the carb to the cylinder as short as possible. The ROTAX setup accomplishes this nicely - just a couple inches.

With a single carb and manifold, the distance will be both greater and less direct, causing some losses.


You also want all runner lengths to be equal. Look at the Rotax intakes -- the runner distance to the rear cylinders (front for pushers) is very short. To the front cylinders? Not so much. this will cause leaner operation to the front cylinders, and makes tuning harder and robs power. a centrally-placed single carb could easily have equal length runners to all cylinders, keeping tuning and temps more consistent across all cylinders. Moving the dual carbs to where the runners would be the same length would achieve the same thing, but not sure how having more mechanical devices in the way of the airflow helps power, all other things equal.

There may be reasons for dual carbs, but I'm not fully buying the more power/efficiency line, at least as these engines are currently configured.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

User avatar
FastEddieB
Posts: 2130
Joined: Wed Jan 07, 2009 9:33 pm
Location: Mineral Bluff, GA

Re: Rotax safety question

Postby FastEddieB » Fri Mar 24, 2017 3:19 pm

I was actually going to mention equal intake tract length as desirable, but forgot.

Frankly, I was thinking about the BMW setup when I spoke of the short distance - I had overlooked that on the ROTAX they had to feed through longer manifolds, and had not thought about their unequal lengths.

What I'm trying to say is, good point!
Fast Eddie B.

Sky Arrow 600 E-LSA • N467SA

FastEddieB@mac.com


Return to “Light Sport Aircraft”

Who is online

Users browsing this forum: No registered users and 4 guests