It seems to me that the vast majority of civilian medium and large sized airliners are of a low or mid wing design. All of the Boeings and Airbuses that I know of, for instance, are low-wing. A handful of commuter types seem to be mid or high wing, but certainly none of the big jets are.
On the other hand, almost all of the military transport aircraft that I know of are a high-wing design. C-130 Hercules, An-22 Cock, C-17 whatchamacallit, etc. Actually, other than dual-use KC-10 and KC-135, I can’t think of any low-wing military transport designs, and those two are adaptations of civil designs. The last low-wing transport the C-54, methinks (?)
Why the difference? Is it as simple a reason as FOD from rough runways, or is there something more to it? I am guessing here, but I would think that civil counterparts would almost always be more efficient, and lower support costs, so there has to be a good reason…right?
One of the prominent differences between High Wing and Low Wing design is the following: High Wings have better lateral stability, and Low Wing has somewhat better maneuverability. There aren’t many Mid Wing Planes, (none that I can think of at the moment) since it is difficult to manufacture. Military Cargo Aircraft have high wings to keep the engines clear of the ground. This helps the aircraft take off from unprepared airfields to keep the engines clear of the ground, gravel and other debris. If you think of it nature has favored the high wing design, there aren’t any birds with low wings.
Actually, the high-wing design of military cargo planes is because it makes loading and unloading cargo easier.
“Drive-on/Drive-off” capability makes the airplanes more useful, and was most likely in the design requirements when the DoD proferred the contracts. Lockheed and others simply designed around the requirement, which results in a high-wing airplane.
The KC-135 and KC-10 are variations on an already-existing commercial design. They are used to carry cargo when needed, but their primary role is aerial refueling.
As an example, take something simple like a Humvee. You have to move it from point A to point B, and you have a KC-10 and a C-17 available. To move it on the KC-10 you need a loader that can lift the Humvee up to the level of the cargo door (probably 25 feet or so). You also need a loader at the destination to get the Humvee off the airplane.
With the C-17, you open up the back doors, lower the ramp and drive the Humvee on. When you reach the destination you drive the Humvee off. Since the military often goes to places where there is little or no infrastructure such as loaders, drive-on/drive-off capability means you can still go there and get the job done.
Oh, and the official name of the C-17A is the Globemaster III.
To anticipate the follow-on question of why commercial freighters don’t have high wings too, for the reasons pilot141 just gave:
They don’t need roll on, roll off: They carry big containers, not vehicles. They always go to civilian airports with dedicated loading equipment. They’re not getting shot at (with the exception of a DHL A300 at Baghdad, whose Belgian pilots landed it with a missile hole and a burning wing but no hydraulics, in a case that ranks with UA232, but I digress), so they don’t have to be in such a hurry.
It’s cheaper: The ramps and doors and such, not to mention the low-ground-pressure landing gear with lots of wheels that let military planes not need asphalt, weigh a lot and would take a lot of costly fuel to carry. Modifying a military design would be a huge job. There are passenger planes of all sizes in production, and they can have freighter derivatives designed and certified easily and cheaply. Too, there are a lot of grounded passenger planes with remaining fatigue life that are no longer economical as airliners, but, since they can be had cheaply, can still pay off after conversion to freighters (a lot of DC-8’s and -10’s have found second lives that way). There have been a number of clean-sheet commercial freighter design studies, but they’ve never been able to compete with the derivative/conversion market.
This is obviously something of a hijack, but the thread is about wings and I can´t seem to get a new thread to open.
(I never knew there were aerodynamics experts on the SDMB)
Can somebody explain the airfoils on the latest aerobatic planes? I went to Oshkosh (EAA convention) last August and had a chance to see Patty Wagstaff´s Extra. Never managed to coordinate my visits with Ms. Wagstaff´s interviews so I couldn´t ask her.
Anyway the aircraft has a wing that´s flat as an ironing board, top and bottom. Basicly it´s just a rather thick wedge with a large round leading edge, nothing like I´ve ever seen before. The Zlins and Yaks and CAPs etc. all had reasonable looking airfoils, as did three other Extras that I saw. (One brand new one did have the same wing as PW´s.)
What´s going on?
From what I’ve heard, acrobatic airfoils are like that because it’s symmetrical. A wing like that works exactly the same when the plane is flying upside down.
Have you ever heard about how a barn door could fly provided there´s a powerful enough engine hauling it?
Well, this is it.
There´s no need for a refined airfoil on those planes, they don´t need it; there is a whooping lot of horse-power under the cowling; there´s no need for nice cruising or high speed characteristics neither. So why bother then?
Keep in mind that the flight envelope of this planes is not the wind-tunnel laminar flow, tidy smoke streams situation as in most other planes, they flip, tumble and spin like crazed flying monkeys, the important factors are lots of thrust, large control surfaces and that the plane goes where it´s pointed at. A simetrical airfoil is enough for that, and I don´t see why a flat trailing edge section would change things, aerodinamically wise (keeping in mind that this planes are not particulary fast). But, on the other hand such an airfoil makes construction much easier, so there: the triumph of functionality
The Extra aerobatic planes have a symmetrical airfoil but it is very different from than on other aerobatic planes. I’m sure Walter Extra chose it for specific reasons but I can only make a WAG that it may be for sharp and predictable stall characteristics. As Ale said this plane is made to do everything but fly in a straight line.
