Low wing vs high wing.

[Boustrophedon]

The 737 Max problems stem from having to mount a huge engine on pods beneath a low wing. So why are passenger airplanes always low wing?




On the other hand almost all military transport planes are high wing



Obviously mid wing would be stupid, who wants a wing spar going through the passenger compartment, but given that, why is the low wing configuration preferred for passenger craft?

[dyqik]

I think there's some noise reduction in the cabin from having the wing between the cabin and the engines. Also some ground effect gains in fuel efficiency for takeoff and landing

Military aircraft are generally required to have some level of rough/unimproved field capability, which requires higher ground clearance, and they aren't generally as sensitive to fuel efficiency.

[shpalman]

I think there's some noise reduction in the cabin from having the wing between the cabin and the engines. Also some ground effect gains in fuel efficiency for takeoff and landing

Military aircraft are generally required to have some level of rough/unimproved field capability, which requires higher ground clearance, and they aren't generally as sensitive to fuel efficiency.

Also, a transport aircraft needs a low and uninterrupted floor.

If you're doing absolutely fantastic you can watch Mentour Pilot: https://youtu.be/ODP9efQBddA

[Holylol]

This topic seems to be discussed here : https://www.quora.com/Why-do-most-passe ... high-wings

[Matatouille]

Its a while since I had to look at this in much detail, so this is probably not exhaustive, and some of them have already been noted.

The biggie: For mil transports or anything operating off unpaved strips that also have engines hanging off the wigs, the additional debris protection offered by high wing is usually the determining factor. As has been pointed out, another advantage for military transports or specialist cargo aircraft is that they are easier to load/unload with a cabin floor as close to the ground as possible, so having the spar going through the ceiling rather than the floor is a plus.


Otherwise, for large aircraft, low wing generally wins on the tradeoffs, so long as you can fit the engine that you want without excessively long landing gear legs (hence multi-engine prop aircraft almost always have high wings).

1.Stability. Low wings aren't inherently as stable due to the centre of lift being lower than the centre of mass, but having a few degrees of dihedral sorts that out sufficient for most purposes and keeps the wingtips well clear of the ground when landing in high winds. Conversely, high wings are often excessively stable, and need anhedral to reduce it enough to make the aircraft sufficiently responsive, so the wingtip clearence usually ends up about the same.

2. Low wings get greater ground effect so takeoff performance is improved by a small degree.

3. A big thing is ground handling. Fueling a low wing aircraft is much easier. Similarly if you can get to important items within the engine nacelles without a long ladder your costs will be lower. High wings are harder to bump your truck into though!

3. Landing gear legs are exceedingly heavy, so the shorter the better. You either have a low wing and use the root volume there to put short legs in, or you have a high wing with heavy landing gear legs, or need to have bulges on your fuselage for the wheels which is an aerodynamic compromise. This is a big one. The F-4 Corsair had its iconic crank wing shape purely because they wanted the tinyest wheel legs they could with the biggest propeller manageable. Similarly the Boeing 737 MAX issues stem directly for the aircraft being initially designed for the narrow diameter engines of the 1960s, and so getting very short main landing gear legs. This became inflexible as engines became wider and wider in diameter for efficiency gains.

4. It affects the tailplane too. A high wing aerodynamically favours a high tailplane too, which is a maintenance headache you'd avoid if you can. High tailplanes can also get you into a nasty condition called "deep stall", which is usually unrecoverable.

[TimW]

The low wing makes the plane look better, and gives a better view out of the windows.

[Pucksoppet]

I think there is a safety benefit from low-wing in the event of uncontained turbine failure - there is slightly more hardware between a disintegrating engine and passengers than when the engines are slung below a high wing, parallel to the passenger cabin.

[bolo]

And with a low wing passengers can exit onto the wing in an emergency.

[dyqik]

The low wing makes the plane look better, and gives a better view out of the windows.

Not if you are sat over the wing.

[Matatouille]

I think there is a safety benefit from low-wing in the event of uncontained turbine failure - there is slightly more hardware between a disintegrating engine and passengers than when the engines are slung below a high wing, parallel to the passenger cabin.

Kinda. There should never be an uncontained fan or turbine failure, it shouldn't be possible, of course it does happen very occasionally because we are bound by a cruel universe and sometimes we get things wrong.

The most recent time that I can recall this occurring with a low wing aircraft was Quantas flight 32. From the wiki:

On inspection, it was found that a turbine disc in the aircraft's No. 2 engine (on the port side nearest the fuselage) had disintegrated, causing extensive damage to the nacelle, wing, fuel system, landing gear, flight controls, engine controls, and a fire in a fuel tank that self-extinguished.

This was a very severely wounded aircraft, and it is fortunate that the pilots managed, with considerable difficulty, to safely land it. My point is, that the wing shouldn't be considered as a shrapnel absorber; we intend to catch shrapnel in the engine casing where it is most mass-efficient to armour the structure against failures. If a wing recieves a significant blast of shrapnel, particularly in the vicinity of an engine pylon where there are lots of control and fluid lines converging, there is a very good chance of a total loss of aircraft and occupants. The result of such an event would be the same for a high wing aircraft, but the safety benefit in this scenario for passengers in low wing aircraft is marginal at best.

Additionally, this aircraft should have been a write-off due to the damage recieved. It was only because Quantas wanted to keep their schtick of never having lost an aircraft to an accident, that they took the more expensive step of repairing it rather than the cheaper option of buying a whole new plane.

For my money, I'm happy to get a seat in the shrapnel zone, its often the best place to see vortices streaming off strakes on the nacelles, or condensation over the wing at takeoff/landing. Now that is a benefit of low wing!

[greyspoke]

...
4. It affects the tailplane too. A high wing aerodynamically favours a high tailplane too, which is a maintenance headache you'd avoid if you can. High tailplanes can also get you into a nasty condition called "deep stall", which is usually unrecoverable.

Is a "deep stall" where the tailplane is in the wingwash so doesn't provide enough lift to point the nose down? (But wouldn'nt the same occur if you lowered both by the same amount?) But given that disadvantage, what are the aerodynamic advantages of a high tailplane/high wing?

[shpalman]

...
4. It affects the tailplane too. A high wing aerodynamically favours a high tailplane too, which is a maintenance headache you'd avoid if you can. High tailplanes can also get you into a nasty condition called "deep stall", which is usually unrecoverable.

Is a "deep stall" where the tailplane is in the wingwash so doesn't provide enough lift to point the nose down? (But wouldn'nt the same occur if you lowered both by the same amount?) But given that disadvantage, what are the aerodynamic advantages of a high tailplane/high wing?

Sometimes there's a high tailplane anyway, because the engines are at the back: Mentour.

[Matatouille]

Is a "deep stall" where the tailplane is in the wingwash so doesn't provide enough lift to point the nose down? (But wouldn'nt the same occur if you lowered both by the same amount?) But given that disadvantage, what are the aerodynamic advantages of a high tailplane/high wing?

Yes, thats the one. I don't really have specifics of why the low tails aren't susceptible (or as susceptible) to deep stall, so this is mainly my thoughts rather than authoritative fact.

Tailplanes at the bottom of the fin tend not to be nearly as high above a low-wing as those at the top of the fin are above a high wing. This is because unless your fuselage is very long to give you a really good moment arm, the fin tends to be a very tall surface. Therefore when a low wing is stalling, a low tailplane will still tend to be flying in undisturbed air below the wing's wake, whilst a high wing with high tailplane might be in the stall wake of the main wing.

As for aerodynamic advantages, for the main wing height I can't think of any really overwhelmingly significant ones that would outweigh the mission specific circumstances that would influence you to pick one over the other. I'd speculate that a high wing could be marginally more efficient, as it is the top surface of a wing that usually generates the most lift (circa 70%), so having an unbroken top surface from wingtip to wingtip can be beneficial.

Aerodynamic advantages of T tails can be significant, aside from keeping clear of propeller wash a high tailplane tends to have a slightly longer moment arm than a low one, so can be physically smaller, and a swept fin will increase the moment arm further. Secondly having a "fence" structure at the top of a flying surface will increase its effectiveness, and having a tailplane at the top of a fin provides this, where the fuselage usually still has just enough width to it to do this adequately at the base.Therefore you can often shrink the fin also. Shrunk surfaces can mean a T tail is a lighter option than a conventional arrangement.

I think this might be why you see relatively few tailplanes mid-way up the fin, a sort of a jack of all trades master of none configuration, so probably only employed when there are specific aerodynamic quirks or mission reasons for having it.

[greyspoke]

Thanks matatouille, and shpallers. So for some applications, engineers may end up with a configuration that is capable of a deep stall anyway (which is a bit worrying). But... as compared to the model gliders I flew in my youth, which stalled regularly and often deliberately, how often in its life would a large jet aircraft stall?

[Matatouille]

Very rarely indeed. It is one of those things that shouldn't happen. Stall recovery on a jet airliner could absorb 3000,5000 feet of altitude, so considering that most of the time that they fly at low speed is when approaching airports for landing, at or below 5000 feet a stall would be .

It can happen at high speeds too, in the event of loss of situational awareness (eg AF447, which is the last big airliner stall I recall off the top of my head), so being theoretically able to recover from the stall is still valuable. I don't think that stall recovery is actually flight tested in airliner certification, that is how marginal it is considered both in terms of test flight crew safety and likelihood of occurrence.

[shpalman]

There was a crash in 2013 but that was caused by shifting of the cargo just after takeoff.