Do I really need to see MAP INHG?

[Amadeus]

Originally Posted by hevansrv7a

(this part is for carbs only?) Since the mixture is automatic, fuel flow is closely correlated to power. I don't recall who pointed it out (Ross?) but the mixture (A/F ratio) is quite flat in the cruising range. For a given A/F ratio, power is the result of fuel flow. Fuel flow is the result of RPM, air density, throttle, temperature, etc. MAP is mostly the result of pretty much the same list. For non-precision flying, fuel flow, power, MAP are equally informative but RPM is perhaps a bit less so.

Went flying again today and utilized your "fuel flow" method. This is a truly remarkable method! Climb at 6.5 gph and cruise at 4.0 gph. Descent is just pull the throttle back but I did notice there was a range of fuel flows that was between 2.2 and 2.9 gph for the various descents.

This is on the fuel injected Rotax.

[bshawco]

This may be obvious to most, but wouldn't fuel flow rates vary with altitude?

If so, would that be a downside (or additional challenge) to using fuel flow values for throttle settings?

Are there different fuel flow vs. altitude implications for the ULS vs. IS?

[hevansrv7a]

Originally Posted by bshawco

This may be obvious to most, but wouldn't fuel flow rates vary with altitude?

If so, would that be a downside (or additional challenge) to using fuel flow values for throttle settings?

Are there different fuel flow vs. altitude implications for the ULS vs. IS?

I am not an expert but I've spent a lot of time studying this issue, mostly for Lyco engines.

It's pretty well known that the IS version of the engine has better BSFC but that's really all that I know about it. When you introduce full FADEC including spark timing and mixture then it's not simple anymore.

The fuel flow relationship to power should be insensitive to air density (altitude, etc.) BUT it should be sensitive to the torque curve. In addition, prop efficiency varies with RPM and TAS but not as much as you'd think. That will be more with CS than FP, oddly.

The efficiency of the prop and of the engine (with or without boost) will vary with Density Altitude. But how much it varies depends on many other things.

If the air is thinner then, at a given air-fuel mixture, there cannot be as much fuel used and, of course, the maximum available power is thus less. But the fuel flow at a constant mixture will still be the big determinant of power. We make power by burning fuel.

The air-fuel mixture primarily, and other factors less so, will have a close relationship with engine power. The power as it affects performance is engine power modified (reduced) for prop efficiency. Some engines have quite flat torque curves in the regions at which we cruise. Some less so.

To the best of my limited knowledge, the 912 ULS has a reasonably flat torque curve and a reasonably flat mixture as controlled by the carbs. That is why fuel flow reflects actual power with reasonable accuracy. I define reasonable as useful for normal flying such as XC.

All gasoline engines have BSFC (Brake Specific Fuel Consumption) which we usually see expressed in pounds per HP per hour. My 7A with manual leaning, etc. could run at about .40-.42. Lower is better. The 912 ULS runs at around .44-.45. You can compute this from the data in the Rotax specs. I'm writing this from memory. BSFC is a measurement; you can't directly control it. It's the result of many things, even, especially, compression ratio. Not that you care, but my 1975 C-150 had a BSFC of about .44-.45 over a very large range of RPM's and Density Altitudes. I calculated that from the POH.

If you think briefly about the above paragraph you will see that a given BSFC means that fuel consumption "equals" power. I was able to see computed BSFC on my GRT in my 7A but it would be useless without the ability to control mixture. My Lyco IO360 had a BSFC curve in the "book" and it showed me what I was able to observe experimentally - that my best engine efficiency was at 8GPH (lowest BSFC). Unsurprisingly, the best cruise performance was at 8GPH and 8000'.

I'm sorry if this is TMI, but I tried to take your question seriously. It's fun to study this stuff, but simple read-outs like fuel flow are very good for low performance, fun airplanes.

[Piper J3]

So, H.... Do you fly your 12 with Map or Tach?

Or maybe just wet our finger and stick it out the cabin air vent....

[Jcurry]

Originally Posted by Piper J3

Or maybe just wet our finger and stick it out the cabin air vent....

+1. Lol....

[hevansrv7a]

Originally Posted by Piper J3

So, H.... Do you fly your 12 with Map or Tach?

Or maybe just wet our finger and stick it out the cabin air vent....

I just listen to it and keep my eyes 👀 outside, of course. Wet fingers readout varies too much with ambient humidity.

Fwiw, I used % power on the 7A.

[hevansrv7a]

This point about MAP may have been missed. For a given MAP, power is a function of rpm. MAP by itself is not telling you much about power and also, it is not linear with power. You can demonstrate this by observing MAP at full throttle while static.

[hevansrv7a]

I "did the math" for clarity.

RPM is not needed. MAP is not needed. Mixture is held constant and is not used.

HP x BSFC / pounds-per-gallon = Gallons per hour.


I am very skeptical of these claims but Rotax says that the maximum continuous power is 97.3 HP and that 75% uses 4.9 gph. That requires a BSFC of 0.40 for the 912 ULS.

(97.3*.75=73) x .40 / 6 = 4.9 gph

The good folks at GAMI would be amazed. Even the best big Continentals with balanced injectors, leaned past peak would be doing well to get that.

https://www.aviationconsumer.com/unc...-than-claimed/

Using estimated performance data from the Tecnam P92 POH, we calculate a brake specific fuel consumption for the 912 ULS of about 0.44 pounds per horsepower/hour. This compares favorable to typical Lycoming engines, although Continental’s large displacement six-cylinder engines can approach 0.39 BSFC when running lean of peak.

I like .44 as a realistic number for BSFC. if I use 4 gallons per hour I'm at around 55%.

The Dynon doesn't compute % power for the 912 ULS.

(97.3*.55=53.5) x .44 / 6 = 3.9 gph

If you read the article it will explain why the 912 iS is much more efficient.

[pstraub]

Hi all, I will chime in on the original question....I don't use the MAP data. It's there on the panel but not a part of my 'scan'. I hope nobody thinks less of me (if that's possible, haha). I am not saying it's not relevant, I just don't use it. Other than full power for takeoff and initial climb, I am normally operating in the 4800 - 5300 range on the tach (2000 - 2200ish prop speeds). My scan is oil and fuel pressures, engine temps. This Rotax 912 continues to amaze me. I just did 10 hours on Thursday with a passenger and some gear, burning about 4GPH and TAS 110-115Kts (and that's with the ULS). In my 6 years of flying my -12, I am still amazed. Cudos to Van's for designing this incredible airplane. By the way, if you ever get a chance to fly the Hudson River Corridor (NYC), do it!! What an unbelievable experience for this ex-Californian. I will attach a couple pics.

[funflying]

Thanks for the trip report and don’t worry about my feelings regarding your use of MAP. GREAT to see you out flying your plane to some really cool places.
Safe flying to you my friend.