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variable fuel burn

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I've got about 30 hours in my new to me 2008 CTLS.  Has a 912 ULS.  At 5300rpm I have been getting about 5.5-6 gph fuel burn.  Haven't flown in about 2 weeks and this afternoon I was getting 6.5 to 7+ gph at my normal cruise.  I had about half tanks with 91 octane no ethanol.  Flew an hour and landed wit 15 gallons.  Filled up with 100LL and Decalin.  Fuel burn dropped back to about just over 6 gph at same rpm so still not what it was last month.

The only thing that has changed was I pulled off all the sail tape from the seams around the wing roots and landing gear and had not gotten around to replacing it.  Also my max rpm at cruise was just under 100rpm less(5500 instead of about 5600) and top speed was 5 knots less.

I am due for an oil change too.

Any ideas or am I just crazy?  It's been hot and muggy.  Could my gas have been going bad?

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Funny you say this. I recently had a condition inspection done, and have noticed my fuel flow was about .5-.8 gallons higher than normal (I usually get book value for a power setting). I to am curious about this. I am going to check the grounds and make sure I don’t have a bad connection somewhere though. Might be a good place to start.

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I may be wrong, but even 5.5-6gph sounds high at 5300rpm.  I would expect more like 5gph.  My experience is the ULS sips gas until you get to about 5400rpm, then the fuel burn goes through the roof very quickly.  I think the book burn at 5500rpm is something like 6.3gph at ~3000ft MSL.  Again, I might be mistaken.

My airplane doesn't have fancy fuel flow gear, so I have to infer burn rates from some math.

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58 minutes ago, FlyingMonkey said:

I may be wrong, but even 5.5-6gph sounds high at 5300rpm.  I would expect more like 5gph.  My experience is the ULS sips gas until you get to about 5400rpm, then the fuel burn goes through the roof very quickly.  I think the book burn at 5500rpm is something like 6.3gph at ~3000ft MSL.  Again, I might be mistaken.

My airplane doesn't have fancy fuel flow gear, so I have to infer burn rates from some math.

True but variable.  If you change RPM to throttle setting then it applies no matter how you are pitched.

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24 minutes ago, Ed Cesnalis said:

True but variable.  If you change RPM to throttle setting then it applies no matter how you are pitched.

Pitch does factor in.  If you're over-pitched you'll burn high at well below 5500rpm, and if real flat you might not get to 6gph until 5700rpm.

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My WOT at 5500ft MSL has always been about 5500+.  

Ill get it in for an oil change and have them look it over.

 

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Are you basing fuel burn on the fuel flow, or buy actual measurement of the fuel level? I have seen the fuel flow transducers vary in readings before.

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2 hours ago, Tom Baker said:

Are you basing fuel burn on the fuel flow, or buy actual measurement of the fuel level? I have seen the fuel flow transducers vary in readings before.

Isn't there a "K Factor" calibration number that can throw off the calculation if off by a significant amount?

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Tom, I compared both and it they seem to match.  I set up my first oil change for Monday and described what I've seen.  The mechanic said it may be a bad sensor or a fuel leak.  I haven't noticed any drips or stains under the airplane so we'll see what he comes up with then.

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