• Head-2-Head Feature XIII: Full Versus Empty

    Head-2-Head Feature XIII: Full Versus Empty

    By Ron Blehm

    Something a little different this time out: same plane, same model, same weather, same speed, same altitude, same flight, different weight.

    I got to wondering how much weight factors into aircraft performance, specifically mileage, when it comes to flightsim. So this month we are flying several different aircraft empty and at max take-off weight to see how they perform.

    As we have discovered in this series, FS modelers sometimes have their own ideas about how things work (like an L-1011 that ran out of gas after 750 miles). We've also learned that we can tweak the CFG files a bit to get some very different (perhaps more accurate?) numbers. But that was like comparing sim models to the real thing. We've also looked at various versions of the same airplane (like Premier Aircraft Designs versus AI Aardvark). This Head-to-Head Challenge pits several different aircraft against ... themselves.

    Set-Up:

    I loaded a flight plan from Beirut across the Mediterranean (one of my favorite places to fly) with several random waypoints along the way. Next I took a plane and loaded it with more cargo than gas. Then I flew at relatively low altitudes* and constant speed measuring mileage and fuel burn. Finally, I took the same aircraft, loaded the fuel previously burned and nothing else! Then I repeated the flight to compare efficiency. I would have thought that same weather, same altitude and same speed would give me the same times but I was in for some interesting data on that bit too.

    * A lot of a flight's efficiency can be gained or lost in how one climbs or descends as well as by what altitude you fly at. I wanted to nix all of those variables, so all flights were flown at 3,000 feet ASL.

    Mooney Acclaim Type-S:

    This is the default Mooney modified by Mark Rooks of RDG Aircraft to the specs of the Type-S turbo (a great little plane!). The MTOW listed is 12,100 pounds. I really don't care what the real-world aircraft's stats are because we are pitting this model against itself. In this case I loaded up 4,650 pounds of cargo and 450 pounds of gas. As noted above, cruise was at 3,000 feet and 200 knots. I flew 396 miles and burned 398 pounds of gas (about a pound per mile). So, next up I took an empty aircraft, that's 7,000 pounds, and added 398 pounds of gas. Then I flew the same 396 miles at the same 3,000 feet and 200 knots. I arrived two minutes faster. How is that possible? I had a whopping 20.2 pounds of gas left.

    Hmmmm, let's do it this way:

    Weight:12,100739861%
    Fuel Burned:398377.895%

    So, for the Acclaim Type-S, 61% of the weight still burns 95% of the gas. Not much savings for 396 miles!

    [Note: the weights for the Mooney Acclaim are obviously way off for a plane of this type. The Mooney web site gives Gross Weight as 3368 lb. However, the values used for this test are those actually found in the CFG file for this flightsim aircraft (image attached).]

    C-46 Commando by Mike Stone:

    Weight: 44,000 34,000 77%
    Fuel Burned: 5798 5096 88%
    Distance: 523 miles

     

    DC-4 by Greg Pepper:

    Weight: 73,000 48,898 67%
    Fuel Burned: 5585 4900 88%
    Distance: 726 miles

     

    F-4 Phantom II by Kazunori Ito and reworked by Mark Rooks:

    Weight: 74,350 35,545 48%
    Fuel Burned: 5543 3017 54%
    Distance: 295 miles

     

    Dash-8 Q400 by Dreamwings:

    Weight: 64,800 41,500 64%
    Fuel Burned: 3782 3518 93%
    Distance: 473 miles

     

    AIA EMB-190 modified by me:

    Weight: 115,000 66,549 58%
    Fuel Burned: 10,475 9420 90%
    Distance: 691 miles

     

    A318 by iFDG:

    Weight: 146,400 98,864 68%
    Fuel Burned: 14,254 11,551 81%
    Distance: 639 miles

     

    B739 by Project Open Sky:

    Weight: 189,700 110,712 58%
    Fuel Burned: 15,432 13,117 85%
    Distance: 527 miles

     

    DC-10-30 by SGA:

    Weight: 578,000 287,412 50%
    Fuel Burned: 74,631 70,783 95%
    Distance: 759 miles

     

    Boeing BWB by William Ortis:

    Weight: 748,240 300,785 40%
    Fuel Burned: 47,974 46,092 96%
    Distance: 883 miles

     

    Well, if I were an airline CEO I certainly would want those planes flying FULL because the fuel savings on an empty aircraft are less than I expected! (The exception to this was the F-4 and my only thought was that a loaded military plane has all that drag hanging off the wings and fuselage so the fuel savings on a clean aircraft are much more notable. The external size and drag of all the others is the same despite the weight so fuel savings are less noticeable.)

    "Pounds Per Mile" Winners And Losers:

    Well, pretty much the lighter planes burned fewer pounds per mile. The Mooney was in single digits. The C-46 did better than the Q400 or DC-4 but the DC-10 and Boeing BWB burned dozens of pounds of gas per mile; then again, their MTOWs are about three times more than the 739!

    Ron Blehm
    pretendpilot@yahoo.com

    Tags: head-to-head

    1 Comment
    1. SunnySaratoga's Avatar
      SunnySaratoga -
      To make this test a bit more realistic you might also need to look at the CG calculation at each weight.
      At least in a real aircraft that has a lot to of impact on how fast or how fuel efficient you can fly the same bird.
      If you end up with a CG that is particularly nose heavy in the lighter configuration you lose some efficiency to the fact that your tail surfaces have to work harder.

      Cheers
      Stefan