Critical Understanding #9 -Percent of Power and fuel flow.

Builders:

Three things come from article #9 in this series:

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You know  what 100% power is and the fuel burn at that setting

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You know what conditions produce 75% power, and the fuel burn at that setting.

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You know when the engine is at “Substantially reduced power” and understand that Carb heat is required for operation at these settings. (This will be covered in detail in part #10 )

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Piston engines inhale air and fuel, and burn it to make power. If you know the displacement of the motor, the RPM it is running at, and the density of air it is inhaling, you can make a pretty accurate calculation on which setting will produce a given power output and commiserate fuel flow. If you cross reference this with several very accurate dynamometer runs, your calculations will be within a few percentage points of your flight experience.

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Mass Flow:

The number you need to know is How many pounds of air are inhaled by the engine in an hour? Let’s look at an example from a 3,000 cc Corvair:

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3,000cc = 183 cubic inches = .106 cubic feet.

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3,200 RPM is 1600 inhalations on a 4 stroke motor, 1600 x .106 = 170 cubic feet of air per minute.

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170 cubic Ft./Min.= 10,200 Cubic feet per hour.

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At seal level on a standard day, air weighs .08 pounds per cubic foot.  10,200 x .08 = 816 pounds of air per hour.

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Engines without turbos or superchargers don’t inhale with perfect efficiency, the Volumetric Efficiency or VE of a Corvair head is .95 so… 816 pounds x VE of .95 = 775 pounds of air per hour going through the motor at wide open throttle and full RPM on a standard day.

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Engines make their best power at 12 to 1, air to fuel ratio by weight so …

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2,775 cc Corvair – 716 pounds of air per hour + 60 pounds of Gasoline = 100% power

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3,000 cc Corvair – 775 pounds of air per hour + 65 pounds of Gasoline = 100% power

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3,300 cc Corvair – 848 pounds of air per hour +  71 pounds of Gasoline = 100% power

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What is 75% Power?

It is any condition that results in the engine burning 3/4 of weight of air and fuel as the 100% power rating above.

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2,775 cc Corvair – 537 pounds of air per hour + 45 pounds of Gasoline = 75% power

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3,000 cc Corvair – 581 pounds of air per hour + 48 pounds of Gasoline = 75% power

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3,300 cc Corvair – 636 pounds of air per hour +  53 pounds of Gasoline = 75% power

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Any combination of throttle opening and rpm that results in the engine inhaling the 75% weight of air and fuel per hour will yield 75% of the rated power.

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Sea level standard pressure is 29.92″ of mercury.  3/4 of this pressure is 22.5″ If you had an Manifold air pressure gauge ( MAP ) in your plane and took off at SL on a standard day, it would read damn near 29.92″. If you left the throttle wide open and climbed to 8,000 ft. it would then read 22.5″ MAP, and if you leaned the mixture so the fuel flow dropped by 25% to match the reduction in the weight of air, you would now be at 75% power.

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If you were in Denver on an 85 degree day, with a bit of humidity in the air, you are now at 8,000′ Density Altitude (DA) and guess what? you now have a motor that only makes 75% of its rated power. In cruse flight, the plane will not loose 25% of its speed because it is flying through thinner air, but it will have reduced take off  and climb performance, but the people who build airports know this and tend to build longer runways in high places. Denver’s Runway 16R/34L is 16,000′ long, the longest commercial runway in North America.

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You can get a calculator and use the values in the first part of this article, along with an on line  DA calculator, (http://wahiduddin.net/calc/calc_da.htm) and work a great number of examples about your size engine at airports and climate conditions you may flight plan for. Note that your engine will likely not turn 3,200 RPM static, so realize that planes with fixed pitch props don’t start their take off roll at 100% power.

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Rich mixture for wide open throttle, leaned for cruise, a fuel flow variable.

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Aircraft carbs are designed to automatically run rich at wide open throttle. Please read this story to learn :Air / Fuel ratios on Corvair carbs.. They can go as far as a 10.5 to 1 air fuel ratio by weight. That mean our 3,000 cc Corvair  example above could go from 65 pounds of fuel per hour to 74 pounds per hour at a wide open seal level take off on a cool day and hitting 3200 rpm.

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Aircraft carbs are designed to be leaned at cruse if desired. The Corvair at low power settings can be leaned to 14 to 1 air / fuel ratio by weight. On the chart above, a 3,000 cc Corvair cruising at 75% power can possibly be leaned from 48 pounds of fuel per hour to 42.

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How much gas is that?

If you don’t know that gasoline weighs six pounds per gallon right off the top of your head, call up your flight instructor and tell him you want your money back, and he needs to mail his CFI ticket back to Oklahoma City. Seriously, if your instructor didn’t drill that into you, how do you know how much weight you are adding to the plane at the gas pump?  How does one do a weight and balance? These are the kind of skills that have been allowed to degrade in the modern era of  the “CFI Lite”  The man who taught me how to fly got his CFI in the USAF in 1952, when the standard response to a student asking “Is this going to be on the test?” was punching the student in the mouth. ( Sensitive CFI’s with progressive attitudes just slapped people.) Be your own hard core, old school CFI, and that way you will never look stupid around planes.

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If you don’t know this stuff, it’s ok, you are on team Corvair, and I am going to make it my business that you have a fresh chance to really learn it, because I want every moron in experimental aviation to know that Corvair builders know their shit. Being a dopey moron about aircraft operations is why they make Rotax engines and Bing carbs. Set your goal in aviation that just in case Chuck Yeager shows up at your airport on his 100th birthday and says to you “Hey, somebody want to take me flying for an hour?”  and he points at a in a PA-18 super cub on the ramp, you won’t have to be like all the pilots trained on Rotax’s who will have to say “Sorry, I can’t. I don’t know how a mixture control works.”  Besides, Yeager is old school, and he is going to punch those people in the mouth. -ww.

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Note Book Section:

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Make line 9.1 in your Hand book a hand written entry, stating what displacement your Corvair Engine is in both CC’s and Cubic inches.

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Make line 9.2 in your Hand book a calculation for your engine showing the fuel flow in Gallons per hour for your engine, at full rated power,  both at 12 to 1   air fuel ratio and at the auto rich setting of 10.5 to 1. Make a note in CAPITAL letters stating the maximum fuel flow the engine is capable of, and right under that figure out how many gallons it would take for your plane to climb to 5,000′ with a 600′ per minute rate of climb, to learn just how much fuel your plane could potentially use.

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Make line 9.3 in your Hand book a calculations of the DA of your home airport on a standard day, a 25F day, a 75F day and a 100F day. 

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Make line 9.4 in your Hand Book a notation of the fuel flow in Gallons per hour for your engine at a 75% power setting both at sea level and at 5,000′ These will be numbers you use frequently to flight plan.

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Make line 9.5 in your Hand Book a notation for the actual mass flow rate through your engine at it’s static rpm. Express this as a percentage of the mass flow rate of your engine at its rated RPM. This number will be your percent of rated HP available on take off.

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