# How did the F28 have such a good L/D ratio?

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:

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?

• The F 50 is a development of the F-27 and has a different wing. Jun 3 '17 at 21:25
• @PeterKämpf: Thanks, corrected. Jun 4 '17 at 15:37

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.

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.

• 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. Jun 3 '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). Jun 4 '17 at 8:32
• Indeed. The y-values seem similar as well. Jun 4 '17 at 12:54
• A little like comparing Vmin sink to Vbg, no? Oct 9 '20 at 2:52

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.