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10 Answers

RPM setting vs. fuel burn rate

Asked by: 4018 views
General Aviation

I am hoping you take time to consider my question. The normal operating range on the tachometer...the green arc...on the RPM gauge displays my power setting. If I maintain a certain cruise power setting, will my fuel burn RATE change all that much during flight?  It is my understanding that my groundspeed could change due to a headwind or tailwind, but the RATE at which fuel is burned shouldn't change. HOWEVER, if I haven't touched the throttle, what would case my RPM's to increase over the "red line" on the tachometer.  A strong tailwind?  A fast descent without reducing power?  It seems to me that if some force was strong enough to cause my RPM's to "red line", my fuel burn rate would change. Also, when we say we are at 65% power...or 75% power...is this percentage based on the maximum RPM's the plane can achieve...or...is the percentage based upon the "red line" number? I hope this question makes sense via computer.

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10 Answers



  1. Jason Schappert on Nov 25, 2010

    If you “lock in” your RPM’s you may not get the same fuel burn through the entire flight. Things that impact and would cause changes in fuel burn include…
    Altitude
    Temperature
    etc…
    If you’re worried about pushing the engine to the red line just know that you’ll hear it MUCH before it even gets there.
     
    In my 150 you’d have to be full power in a dive to get the RPM’s close.
     
    Jason

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  2. Flyer on Nov 25, 2010

    Thanks, Jason! 
    I was just scouring over previous topics and read about how to use time, speed and distance (performance) charts.
    Someone responded that, “…fuel burn, being a function of the time, will remain constant.”
    I suppose your GPH doesn’t HAVE to necessarily stay the same throughout the flight.  It may not change all that much, but it can change…
     

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  3. Flyer on Nov 25, 2010

    Perhaps I should have said CONSUMPTION RATE…or how many gallons you burn per hour.
    This number shouldn’t change all that much if you lock in your RPM’s…should it?

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  4. Brian on Nov 25, 2010

    Select a power setting for a given altitude and the fuel burn rate it tells you will remain relatively unchanged, at that altitude, for the entire flight.
     
    RPM increases because your speed through the air increases for the same reason wind spins a windmill. Thus, we term this a windmilling effect.
     
    You are right to use time in your climb chart because that climb chart factors in the altitude change effects. The given fuel burn will be accurate as well. What the text you read is getting at with distance is it will change based on ground speed and you must factor this in. It is worth discussing with your instructor to ensure you understand it.

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  5. Paul Tocknell on Nov 25, 2010

    To answer the last part of your question.  100% power for unsupercharged engines is developed at sea level and is usually the maximum brake horsepower the engine is rated to develop.  As the altitude increases, air density decreases and the engine is only capable of developing so much power.  It might be that at 7000 feet you might only be able to develop 70% power even with the throttle wide open.  At 10,000 feet you may only be able to develop 65% power. 

    Interesting to note.  At 70% power the fuel consumption rate is about the same, even at different altitudes (as long as the mixture has been properly leaned out).  The difference however is that your miles per gallon increases because you True Air Speed increases.  So at sea level, 70% power might be 8 gallons an hour yields a TAS of 91 knots.  7000 feet, 70% power might still mean 8 gallons an hour but now because of the decrease in air density you are able to sustain a TAS of 101 knots. 

    Also wanted to add: A headwind / tailwind will not affect your engine performance; it will affect your groundspeed and how much total fuel you may use over the entire length of the flight but will not affect how your engine consumes fuel.

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  6. Flyer on Nov 26, 2010

    I appreciate all of the help and information.  However, some of the information provided seems conflicting to me and therefore I am having trouble gaining a better understanding.
    Brian says “RPM increases because your speed through the air increases for the same reason wind spins a windmill. Thus, we term this a windmilling effect.”
    But, Paul says “A headwind / tailwind will not affect your engine performance; it will affect your groundspeed and how much total fuel you may use over the entire length of the flight but will not affect how your engine consumes fuel.
    Can someone please help me resolve these statements which appear to me to be conflicting?

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  7. Paul Tocknell on Nov 26, 2010

    Hi Flyer,

    Sorry we confused you.  What Brian and I said are not conflicting.  A headwind or tailwind will not affect your engine performance.  If your sitting at cruise altitude burning 8 gallons an hour, you will burn 8 gallons an hour if you have a 50 knot headwind or 50 knot tailwind. What the tailwind or headwind will affect is your groundspeed.

    If you fly an airplane with a fixed pitch prop (not constant speed) when you descend (especially quickly), your indicated airspeed naturally increases because of gravity.  When your airspeed increases in a fast descent, your prop speed will increase, this is what Brian was referring to as a windmill affect (think of the wind, pushing a windmill).  Any yes, if your prop speed increases it will affect your engine performance (plus the fact that you would be descending into higher density air which should require a richer mixture)

    I hope this clarifies things.

    Paul

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  8. Flyer on Nov 26, 2010

    Got it, Paul!  THANK YOU!  That’s what I thought!!!!  I just wanted to clarify!  You helped me quite a bit.

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  9. Brian on Nov 28, 2010

    Paul hit the nail on the head, if you will. As opposed to me, I usually hit my thumb. 🙁

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  10. Cheryl on Dec 23, 2011

    can you help me solve a puzzling question that my husband and I are betting???/  neither of us are pilots or have flying experience………..   if all factors remain the same and it takes 6 hours to fly from PA to CA into a head wind, on the return with a tail wind and flight time of 4 hours will you use the same amount of fuel on both of these flights.   One of us says you will use less with a tailwind and one of us says it’s the same amount of fuel both ways. 
    THANKS

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