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The second link given on a Google search for "Breguet Equation" is to a PDF document from MIT. On page 7 of this document is a chart showing L/D ratios for various commercial aircraft (turboprop, regional jet, and "large aircraft") plotted against the year the aircraft entered service. Below is an image of this chart:

enter image description here

What appears to stand out (at the top of the chart) is the Fokker F28 aircraft.

From an engineering and operational perspective, what enabled that aircraft to have such a high L/D ratio?

Also, I thought that the F50 / F70 / F100 aircraft had the F28's wing (or an evolutionary version). Why didn't these aircraft have an equally good L/D ratio?

Vikki
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pr1268
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3 Answers3

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They just got this wrong. I have flown F28-1000 and -4000, the F100 and the B767-300. The most feet per nautical mile lost of these with idle power at any given Indicated Air Speed (say 270-280 below 25000 ft) is the F28-1000 (around 475 fpnm) then the F28-4000 at about 425 fpnm, then the F100 at about 350 fpnm and the best is the B767 at about 300-330.The 425 fpnm figure for the -4000 was measured by me around 2004 to get data for MS Flight Sim. All the rest are educated guesses.

FRoad
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Taken as I am with the little Fokkers because I used to walk around the flight line and see them being built, I reckon that's a mistake. The F-28 having a better L/D ratio than the F-27 with its long slender wings doesn't sound right to me. The F100 with its supercritical wing definitely should have better L/D than F28.

The two entries for F-28 in figure 1.4 seem very similar to the entries in Figure 1.9 of the MIT document that you mention.

Koyovis
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    The supercritical wing only helps at higher Mach numbers, but the F 100 had a higher aspect ratio than the F-28, so I agree with your verdict. The low sweep angle helps in comparison with other jets, but not enough to make the graph look credible. – Peter Kämpf Jun 03 '17 at 21:46
  • I agree with both Koyovis and Peter... The data look suspect. Of course, the horizontal locations of aircraft are the same on both charts (x-axis is year of introduction in service). – pr1268 Jun 04 '17 at 08:32
  • Indeed. The y-values seem similar as well. – Koyovis Jun 04 '17 at 12:54
  • A little like comparing Vmin sink to Vbg, no? – Robert DiGiovanni Oct 09 '20 at 02:52
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There is one possible explanation in the analysis of the airfoil and the airspeed it is flown at. A cambered wing, such as the Davis or the DAE-21, will produce superb lift to drag ratios, but at much lower airspeeds than supercritical wings fly at.

The Breuget Equation is as follows:

Range = Velocity × Lift/Drag x Specific Impulse × ln(W2/W1)

Specific Impulse is Thrust/fuel flow rate, Weight = Lift, Thrust = Drag, therefor:

Range = Velocity × Weight/fuel flow rate × ln(W2/W1).

The faster, heavier plane with the lowest fuel flow rate becomes "tops of the charts" in any year. The graph, though interesting, seems to only point out the F-28 lifts more weight/drag at the speed it flies at per given unit of time, but not per given mile. I would ask more of MIT.

Robert DiGiovanni
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