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Archive for the ‘Aerodynamics’ Category

Do you know your airplane's MPG?

“What is meant by the term ’specific range’?”

This is the question that I asked David Wyndam of Conklin and de Decker after coming across this term while looking at some aircraft comparison performance tables.

The performance tables listed the specific range for various airplanes in both long range and high speed cruise.  For instance, the specific range for a Cirrus SR22 was listed as 2.051 for long range cruise and 1.810 for high speed cruise.  But what exactly do those numbers mean and what formula was used to devise those values ?

Specific range is the distance an aircraft travels per unit of fuel consumed. Typically this is expressed in nautical mile per pound of fuel.  So for instance, the Cirrus SR22 has a high speed cruise specific range of 1.890.  That means for every pound of fuel used, the Cirrus SR22 could fly 1.890 nautical miles.   Here is another example.  The Beechcraft Premier that I fly has a SFC of 0.557 NM/lb.   If 1 lb will take me 0.557 NM that means with 1,000 lbs of fuel the Premier can fly 557 NM.  This is the aviation equivalent of MPG.

One purpose of SFC is to help determine the efficiency of an aircraft.  This can be used when comparing aircraft for purchase.  Fuel efficiency is important for knowing what your direct operating costs will be as fuel is one of the largest components of DOC.

Now we don’t typically use pounds for the fuel unit in piston aircraft, we use gallons.  We also know that 1 gallon of 100LL weighs approximately 6 pounds.  So if the Cirrus can go 1.890 nautical miles on 1 lb, that mean on 6 lbs (or 1 gallon) the Cirrus can fly 11.34 NM.   On 60 lbs, or 10 gallons, the Cirrus SR22 at high speed cruise can fly 113.4 NM.  Here is the high speed specific range cruise figures (and some corresponding ranges)  for 3 different single-engine piston aircraft:

To find the values in the lower row I took the specific ranges values and multiplied them times 60 to find the range on 10 gallons of fuel.  Here is a map with range rings representing those values:

How far can you go on 10 gallons?

So how can you figure the specific range (SFC) for your aicraft?  It’s actually quite simple.  Just take your TAS (true airspeed) and divide it by your fuel flow in pounds.  So, let’s say your TAS is 130 knots and you are burning 11 gallons  an  hour (or 66 pounds an hour):That is a specific range of 1.969.  On 10 gallons of fuel, this airplane could fly 118 nautical miles.

So is the higher SFC number always the more efficient airplane?  Not always.  For instance, the Boeing 737 has a long range SFC of 0.090 and a CJ3 has a SFC of 0.606.  But, if  a 737 can carry 135 people and the CJ3 can only carry 7, which aicraft is really the more efficient airplane?  There are many other factors to include in efficiency comparisons as well such as fixed and non-fixed maintenance costs.

Thanks again to Conklin and de Decker for their helpful input to this post.  If you are looking for professional aicraft comparison software or handbooks, Conklin and de Decker have a wide range of products for a wide variety of aircraft both single engine, jet and rotorcraft .

Thought I would post a recent thread from our forum:

Here was the question that was asked:

Could you please give me a detailed explanation of maximum structural cruising speed. I’m confused as to why it is higher than manuevering speed and doesn’t change with weight like Va does if they both are turbulent air penetration speeds?

Thanks for your help.

And here was the answer:

Hi! Great Question! I actually had to do some “research” for this one!

Va. Known as maneuvering speed. This has to do with the MAX speed at which you can safely stall an airplane. During certification, this has to do with forces measured on the elevator (see image below). Any speed above Va you would exceed structual limits before reaching a stall (not good)

Vno. Maximum structual cruising speed. This has to do with wind gusts measured on the wings. I have to do a little more research but my source told me below the yellow arc it is 20 in. per second gusts and in the yellow arc it is 15 in per second gusts. This has to do with the wings bending and twisting. Not structural limits on the elevator.

Some instructors teach students that Va is turbulent penetration speed which is really not completely accurate and may confuse things. It also may have to do with the fact that Va wasn’t a required certification until the 50’s or 60’s.

Hopefully this helps some.

Instructor.
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