high wing vs low wing

[pilotho]

there are obvious advantages to using either design particularly to suit the aircraft's intended ops. however, despite all these clear advantages, there are still odd aircraft designs that i can't figure out.

the high wing design is usually designed for short haul props aircraft, however the BAe 146 is a short haul aircraft operating out of short runways, using jet engines, why does it have a high wing design? surely, using a low wing design would improve ground effect and further improve STOL attributes.

so since high wing aircraft are usually designed to accomodate prop aircraft, why does the SAAB 340 using a low wing design? surely, the prop blades are quite long and therefore a high wing design would seem more adequete.

[Mad (Flt) Scientist]

BAe146 ended up needing 4 engines, due to the required thrust level and availability of powerplants (as I understand it). It's a challenge to fit 4 engines anywhere except on the wings. (VC10 Jetstar notwithstanding).

Once you decide to put the engines underwing, you're forced to consider ground clearance and FOD issues, which tends to drive the wing upwards. Essentially, the bottom of the engine cowlings has the same kind of requirements as the bottom of a prop disk would have. So if a "prop 146" would be a high wing, chances are so would a "jet 146".

Bear in mind that wing position is a compromise amongst many, many factors. You can't just burn it down to a couple of factors. Even things like "design habits" of a particular design team plays a part.

[pilotho]

if porp and jet engines requires a certain amount of ground clearence, then how come the SAAB 340 still operate using a low wing design?

i appriciate that there are alot of factors affecting the desing of the aeroplane, however, i still can't think why the 340 would have a low wing since the props would be so close to the ground. :confused:

[Mad (Flt) Scientist]

They decided to accept longer gear to get the clearances.

Bear in mind that if you're a high wing, you either need gear on the fuselage (complex installation) or long stalky gear from the wing.

[pilotho]

ok, this is kind of making more sense.

however a new query now popped up. if it is disadvantagous to install high wings due to landing gear, then how come the likes of the Dash 8 havent done the same? possible longer blade span due to larger size of aircraft?

[Graybeard]

Airplane design is driven by two things: engines and landing gear.

You have to size the airplane to suit an available engine. You have to place the wing to accommodate that engine.

You have to then design the airplane so as to have the least worst location for the landing gear. The DC10-30 and MD-11 have both wing mounted and fuselage mounted gear.

A high wing plane can be slightly more efficient, as it can get by with less (stability enhancing) dihedral, which wastes energy.

GB

[John Farley]

The 146 was originally conceived by deHavilland long before BAe. Back in 1966 I was told that if the Harrier did not get ordered then I could expect to go to Hatfield and deal with it. It was designed as a hub and spoke aeroplane to open up Africa where there were no facilities at the end of the spoke. Literally none.

Because they did not want it stuck at the end of a spoke with an engine problem they decided it had to have four engines and be certificated to RTB without pax on three.

Since some strips were expected to be pretty rough the high wing was chosen to keep the intakes as far from the surface as possible.

Eventually BAe happened and decided to launch the design on the back of super quiet for parts of the US and the rest as they say is history.

[old-timer]

Thanks John for the background on the 146, very interesting.
I always liked it, I know many referred to it as the bomber with tank engines & similar unfriendly terms but I loved flying on it with Air UK (ahh, another remininescencethere..... I must be getting too old ? !)

What a shame they couldn't keep Hatfield going too, so much knowledge &
a huge skill-base there.

[Old Smokey]

........surely, using a low wing design would improve ground effect and further improve STOL attributes........

The problem is that during certification, the time that ground effect really helps, i.e. the airborne segment from Vloff to V2 and onwards to the end of the 1st segment, you're not allowed to take credit for it.

Of course, in the real world, it genuinely helps, but you cannot increase your payload by 1 gram because of it, RTOW calculations are not permitted to use it.

Regards,

Old Smokey

[Thirty Eight South]

The design and conguration - where to put the wings for example - are part of the design process and the positioning (eng/wing) is derived at by a design and config process that establishes the final A/C function

starting from a blank piece of paper, the top level requirements are established, basically, from here you figure out what it's suppossed to do - move people or freight ect, what engines are avialble, what the MTOW, MRW , what the mission requirements are (how far, high, fast does it have to go) and so on. military customers have different requirements from civil customers with corresponding different levels of structural and systems redundancy with equally frustrating weight penalties

in the early design phase weight is key driver, eg, if you have a high wing definition, putting the LGD on the wing requires the beefing up of the wing primary structure up to cope with the wing (limit/ultim') loads ect, but the ground handling is better (eg f-27).then what do the weight penalties do to the performance - fuel burn, A/C dynamic stability (particualrly at low speed): from this point on there's a massive bun fight between all contributing depts - engineering/aero dynamics ect until a compromise is arrivied at.

the easiest way to figure out why any specific config has been adpoted is to ask 'what is the plane suppossed to be doing and how do you get things (people or freight) in and out of it', from then on it's relatively simple.

[barit1]

The 146 ... was designed as a hub and spoke aeroplane to open up Africa where there were no facilities at the end of the spoke. Literally none.

Because they did not want it stuck at the end of a spoke with an engine problem they decided it had to have four engines and be certificated to RTB without pax on three...

jondc9 will tell you about how the 146 3-engine ferry was routine ops in wild & wooly Callyfornia! :cool:

[skippiebg]

Seem to recall high wing produces more lift because the fuse doesn't get in the way.

In fact, the low wing/high speed config gives loads of design problems. The DC-8 (mid-1950s) was the first such airframe to employ an inverted inboard section (flat top, curved bottom; actually the camber still follows Bernouli principles, but is skewed). This was necessitated by interference between fuse and wing. Douglas rightly assumed the fuse produced some lift anyway, so they dumped much wing lift to help cut drag.

Also look at the Convair 880/990's troubled wing/fuse fairings (et ceterea, coz there was LOADS of other issues!).

Most of these woes never rear their head in high wing configs.

Buuut...

- high wing is problematic in crashworthiness (no structural cushion beneath your feet, and loads of weight over your head);

- high wing doesn't reduce the cross sectional area and doesn't cut drag in that way (profile drag, that it; y'know -- Cd).

So mid-wing is best.

One BIG advantage of mid-wing that hits me rightaway is, stewardesses climbering over the spar as they walk the aisle. Let's get the Vickers Viking back into service again :)

[barit1]

Seem to recall high wing produces more lift because the fuse doesn't get in the way...

Some of the best-performing aircraft of the late 20s - 30s were high wing with a flat-top aft fuselage (Monocoupe, Howard, ...) Benny Howard called it a "clean downwash zone" for high-velocity airflow coming off the upper surface of the wing. He advised against placing any antennas, etc. in this area. (OK, so this addresses drag reduction, not lift, but it seemed to work well for him!)

...
Also look at the Convair 880/990's troubled wing/fuse fairings (et ceterea, coz there was LOADS of other issues!)...

Many low-wing types went through a lot of experimentation on fairings. The first Lockheed 10 (1934) had huge fillet fairings, soon abandoned. Postwar DC-3 enhancement kits included smaller fairings (you could often see the rivet pattern of the originals in the fuse & wing skin).

[skippiebg]

Interesting... thanks :)