ROTAX, Bings, CV carbs, mixture, altitude and performance

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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FastEddieB
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ROTAX, Bings, CV carbs, mixture, altitude and performance

Post by FastEddieB »

In another thread, CharlieTango said this:
eddie, are your carbs set up correctly? i find that 7,500 msl is the sweet spot for best performance and best range seems to be around 10,000'

my plugs always look good, i think the auto-mixture works pretty well.
As background, I have a fair bit of experience with carbs in general. I've installed overhaul kits in Bing carbs (on airhead BMW motorcycles), and have a decent basic knowledge as to how they work. To summarize, here's my impression:

The throttle is hooked up, not to the slide, but to a butterfly valve, only controlling the amount of air flowing into the carb. This air, via Bernoulli, causes a drop in pressure when constricted. This lower pressure air is channeled to the top of a rubber diaghram, which in turns lifts the actual slide. The flow of fuel into the venturi is metered by a tapered needle which rises in the main jet with the slide - the higher the slide, the more fuel.

Now, to my way of thinking, the volume of fuel remains constant for any given slide setting, but since the density of the incoming air decreases with altitude, the mixture gradually gets richer and richer.

So, I don't see how the Bing carb can be said to have "auto-mixture".

Here's the Cruise Performance chart for my Sky Arrow with as ROTAX 912ULS2:

Image

This is a very different situation from a "normal" aircraft engine - note what happens to range at 6,000' compared to 2,000'. In an aircraft engine with mixture, its a truism that both TAS and range increase with altitude for any given power setting/fuel flow. Clearly not the case here, and my assumption has always been that its due to a necessarily "too-rich" mixture at altitude - necessary because otherwise it would be too lean at low altitiudes.

Subjectively, my Sky Arrow does seem to "coarsen" up a bit with altitude - never rough, but it just feels like I'd reach over and lean the mixture a tad if I could. I think 8,500' is the highest I've had occasion to climb so far.

With a performance chart like mine, I tend to fly low. I'll often fly 1,500' or lower, often just following highways. I know the higher I go, the less range I'll get, significant if I'm trying to fly close to 300nm non-stop.

At the last annual my plugs looked normal.

So, is there an "auto-mixture" feature* on the aviation Bing's I'm missing? Or is it just my Sky Arrow? I'd love to see the Cruise Performance chart for a ROTAX engine that shows best range and best speed at 7,500' or 10,000'. Not arguing, it could clearly be the case - more curious!

*Cirrus SR20's DO have a real "auto-mixture" that progressively leans the mixture with altitude, so such a concept is clearly possible.
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
[email protected]
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CharlieTango
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Post by CharlieTango »

hmm,

i'm not much of a carb mechanic, i did always think that the clear tubes with the holes in them that go to the air box where involved.

doing searches i can find a number of references to the atuo-lean feature but no info on how it works.

like this one from vans http://www.vansaircraft.com/public/rv-12his_3.htm

"They also feature automatic mixture compensation for altitude changes, so there is no mixture control in the cockpit"


my ct operates from sea level to 14,000', i see a lot of evidence that the mixture adjusts. my home field is 7,100' and if i was full rich i could never get off the ground in 5 seconds. i don't run rough at altitude, my egt's are always in the green, my fuel burn at wot at altitude is lower, ... etc.
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rfane
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Post by rfane »

Eddie,

Most of us did not even receive performance charts with our LSA. Has your actual fuel burn matched what the charts tell you?

I've never seen fuel burn rates as high as those in the charts. My average fuel burn is 5 gph, and I typically cruise at 5,200 - 5,300 rpm's. Cruise altitude depends on where I'm going, but is usually at 6,500' +, on up to 11,500'. I took a cross country trip a year ago. Over 53 hours of flying, my fuel burn came out at 4.63 gph. Going east, I was up at 9,500 - 11,500' most of the way to take advantage of strong tail winds, and coming west I stayed lower when possible to minimize the head winds.

I've been over 14,000' Density Altitude with no engine roughness, still turning 5,200 rpm's, so I know the carbs are compensating. Flight Design however, does not use the standard Rotax airbox.
Roger Fane
Former owner of a 2006 Flight Design CTsw
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CharlieTango
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Post by CharlieTango »

eddie,

if your charts are correct you don't have a self adjusting mixture feature, you just get richer with altitude.

there are mixture adjustments systems available for the rotax.

roger lee says he can explain the auto-mixture but wants to do it on the phone, you can give him a call.

Hi Ed,

Give me a call and I'll explain. Just too long an email.
Roger Lee
Tucson, Az.
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(520) 574-1080 (Home) Try Home First.
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Re: ROTAX, Bings, CV carbs, mixture, altitude and performanc

Post by FLA-CFI »

FastEddieB wrote: The throttle is hooked up, not to the slide, but to a butterfly valve, only controlling the amount of air flowing into the carb. This air, via Bernoulli, causes a drop in pressure when constricted. This lower pressure air is channeled to the top of a rubber diaghram, which in turns lifts the actual slide. The flow of fuel into the venturi is metered by a tapered needle which rises in the main jet with the slide - the higher the slide, the more fuel.

The drop in pressure is due to an increase in velocity of the air as it travels through the carb. Q = V x A (where Q is the airflow through the pipe, V is the velocity, and A is the cross sectional area of the passageway). And as we all know as velocity increases pressure drops.


Now, to my way of thinking, the volume of fuel remains constant for any given slide setting, but since the density of the incoming air decreases with altitude, the mixture gradually gets richer and richer.

So, I don't see how the Bing carb can be said to have "auto-mixture".
Now as the air thins out with altitude the flow/volume of air through the carb slows/decreases, and the needle doesn't rise as high yielding the necessary leaner mixture.

When the engine is running at ground level, the fuel in the float chamber, is subject to the ordinary atmospheric pressure...but the fuel in the jet...is subject to a reduced pressure because of the lower pressure in the carb throat. Thus there is greater pressure on one end of the fuel than the other; consequently, the flow of fuel from the float chamber to the jet is due to differences of pressure. As an airplane climbs, the air drawn into the carburetor becomes less and less dense with increasing altitude. Unless the fuel supply is proportionately reduced, the mixture will become too rich; the engine will begin to slow down, and will consume an abnormal amount of fuel. The diaphragm control system reduces the flow of fuel at high altitude by weakening the pressure on the float chamber. The lid of the float chamber is made with an airtight joint, and a passage/line references with the airbox. At ground level and at low altitude this passage/line governs the pressure inside the float chamber, which therefore corresponds to that of the airbox...The pressure is reduced, in proportion to the altitude reached. The flow of fuel from the float chamber to the jet is correspondingly reduced.
Adam

CFI-SP, Light Sport Repairman - Maintenance, Rotax Cert.
Orlando, FL
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Post by CTflyer »

Gentlemen - is the Bing HAC (High Altitude Compensating) automatic mixture control what you're referring to?

http://www.greenskyadventures.com/bing/hacArticle.htm

Tom
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FastEddieB
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Post by FastEddieB »

Guys,

Thank you all so much for the responses and links.

I was out all day, so give me just a while to digest them and I'll report back on my understanding of the new info (or lack thereof!)
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
[email protected]
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FastEddieB
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Post by FastEddieB »

Just got started on the links.

One quick correction to the RV12 article:
The Bing carbs are equipped with chokes for starting rather than primers (older drivers will remember chokes; they were common way back when cars had carburetors.)
The Bings do NOT have chokes, at least none I've ever seen. They have an additional starting circuit which is cable operated and provides a richer mixture by increasing the amount of fuel, NOT decreasing the amount of air, which is what a choke does and how it got its name.

Admittedly, the lever controlling the enrichener circuit may be marked "choke" on both airplanes and motorcycles, but that's not technically what it is.

May seem pedantic, but I think its important for an owner/operator to know the difference - it may help in trouble-shooting problems.
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
[email protected]
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CharlieTango
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Post by CharlieTango »

you are correct, the choke on my ct, as well as on my harley is really an enricher.
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FastEddieB
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Post by FastEddieB »

BTW, I just came across this photo of the "enrichener disk" in a Bing (from my old R100GS/PD):

Image

I took the photo to make sure I got the orientation right on the reinstall.

And here's me at a BMW Rally in Missoula, MT (1999, I think), installing the "split float" option on my R100GS, to correct ongoing float/needle valve issues:

Image

Boy, do I wish my arms were still that size!
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
CFI, CFII, CFIME
[email protected]
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