Bing carby altitude compensation

[moroa]

high fuel flow

 

Why at altitude as you say power out put is low are people reporting at full throttle fuel flows of aproaching 30 litres per hour. If you assume that the SFC of most engines is .5 lb/hp/hour you can estimate the actual horsepower being produced by looking at the fuel burn.

 

 

[facthunter]

Fuel burn/power relationship.

 

Fuel burn does give a pretty good idea of power output in the medium power ranges (and overall for jets), but piston engines are enriched in the high power settings deliberately, to prevent detonation.

 

Some figures I have extracted from the Citabria POH (Lycoming engine) gives a power figure at 10,ooo ft (RPM being constant), which is 63% of the sea level figure. I would expect that to be typical for a normally aspirated engine. The fuel flow virtually matches it.(after manual leaning).

 

If the fuel flows reported for some Jabs are accurate then something is quite wrong although extra RPM could account for some of it.

 

The shape of the taper in the needle in the bing does most of the metering, (except when it is fully up, which will only occur at sea level, with a good condition engine and the specified prop), when the main jet has a lot to do with it as will in most of the CV carbs I have dealt with. (stromberg CD, SU, Keihin). You could oversize the main jet and leave the needle to do the whole job.( Don't do this yourself, I am only talking concepts.... Nev..

 

 

[moroa]

leaning at altitude

 

I agree with what you have said power drops off whith altitude, so you need to apply full throtle, or near to it so you stay off full enrichment ,which is for cooling (thats why they call it full throttle height) usually above say approx 6000 feet power will be below 75% thats what i do in a lycoming or continental you manually lean it and fuel consumption settles down to the same regardless of altitude for a constant power output

 

As you state power at 10000 feet is approx 63 % less than sea level (poh citabria ) so a 3300 jabiru would burn say 40 od litres/hr at high revs down low & 63 % of this is say 15 l/hr but this does not happen.

 

I have spent thosands of hours cruising at altitude in light twins we do this to get a high tas (because the air is thinner) for a constant fuel burn for that percentage power.

 

you are correct stating in the fixed pitch propellar revs are higher (say leave the revs at max in a constant speed propellar all the time) to achieve percentage power but it should not make that much difference the mixture should be approx 15/1 this clearly does not happen if you where to cruise a bing carbed jabiru at say 2850 rpm at 10000 feet why?

 

 

[facthunter]

Continuing...

 

Full throttle at the butterfly will NOT cause enrichment with a Bing at altitude, as the taper on the needle does this, and at height the slide will not go full open, so the needle does not get into the fuel enrich position.

 

The power is reduced BY 37% TO 63% of its sea level figure.(unless the revs increase).These figures are with a fixed-pitch propeller.

 

Full throttle height has more to do with supercharged engines. (direct driven supercharger). This is where you make the supercharger more efficient by flying high enough (or by reducing engine revs,with a controllable pitch propeller) so that you can fully open the throttle and not have the supercharger compressing air that you have already impeded by part closing the throttle upstream..

 

An interesting fact emerges from perusing the POH cruise charts. The MAX RPM (2800) chart gives a range of 445 statute miles at 2500' and 700 statute miles at 10,000 feet. If you set max endurance figures the range at all heights up to 10,000' is 880 statute miles.( There is no variation ). This principle should apply to any aircraft with similar characteristics, I would imagine... Nev

 

 

[moroa]

bing carb

 

sonex 3300 owners regularily push the revs up at higher altitude and confirm TAS of up to 147 kts (170 mph) at aceptable fuel burns ,but most of there owners use either aerocarb or rotac TBI (not without there own problems either) these are posted on yahoo groups sonex talk.

 

jabiru j230 owners do not post as high TAS cruising at higher altitudes ,maybe not because the aircraft isnt capable of it, but probably because fuel burn is to high.

 

maybe the carburation could be improved ? ,jabiru have revised there jeting several times ,maybe the 50 year technology in the old marvel schebler carburetor of the lycoming/continental suits aircraft better?

 

http://www.airplanefactory.co.za/world/press.asp SOUTH AFRICANS fly round the world

 

but then this guy uses a bing carburetor in his rotax 912 & has just flown the pacific so it must be a good carb or he would of got wet on the california to hawaii leg ,a very impresive achievment.

 

would you trust the jabiru with your life & fly it around the world with 2 pilots, spend 5 days at oskosh, with only scheduled maintenance and only be away a MONTH could the j230 be economical enough on fuel to do it ??????