Overwing engines: why don't we see this more often?

Modern passenger jets seem to have two options for engine placement: tail mounted below a T-tail, or hung in nacels below the main wings. The two alternative designs offer tradeoffs, with wing-mounted engines beeing the universal choice for larger aircraft, while tail-mounted engines are favored by smaller business jets.

As I understand it, Tail mounted engines require additional plumbing (fuel lines) and increased structure to support the weight and thrust they generate. And they require T-tails, which are heavier and more complex to build. However smaller craft need to mount their engines high enough fo avoid sucking up runway debreis (FOD). And the high tail helps with pitch control, which is needed with shorter aircraft.

Larger commercial planes can keep their engines higher above runway, and mounting the engines to the wings reduces the structural requirements - it's near the fuel supply and wings/landing gear that support it's weight. And you can use a low-mounted horizontal stabalizer (lighter, less complexity).

The trade-off seems to occur at the ~100 passenger limit; anything significantly below mounts the engines in the tail, anything bigger will mount the engines to the wings (with some exceptions, of course).

The new Hondajet offers an interesting alternative: mounting the engines over the main wing. This isn't the first jet to do so, but it's interesting as it offers wing-mounted engines in a jet that would normally be too close to the ground to use them. Honda claims the design offers a 5% fuel efficieny increase. It seems like an ideal solution, which makes me wonder if there aren't other tradeoffs involved that make the design less favorable(i.e. why we haven't seen it used more often).

So... I think thats the end of my ramblings. I guess this is where I say, "what does everyone els think?"

Link to the Hondajet: Link


P.S. Of particular note, both Boeing and Airbus are working on their 737/A320 replacements. They're both (reportedly) waiting for a high-bipass (high efficiency, low noise) engine that can fit below the wings of a smaller jet. Mounting the engines over the wings seems like one possible solution to (or short-cut around) the problem...

there are many ats'ers who will be able to help you out more in depth, but I think that one of the reasons is because mounting the engine on top will make for harder maintence, especially in a huge commercial jet.

how does a top mounted engine effect the airflow over the wing?
Maintenance issues shouldn't be a problem, since this is marketed as an tradeoff for the tail mounted engines, right?

A couple of reasons I can think of right of the bat why they will not do this on commercial aircraft are:

A) It would obstruct the view of a large amount of the passengers.
B) It would make the cabin noisier.
C) It would bring the engines more in line with the windows in the case of a catastrophic failure.
D) It would raise the center of gravity and would most likely be less stable (good for maneuverability but bad for a smooth flight).
E) Requires fuel pumps (but so do tail mounted engines).

Also there are a number of aircraft that have tail mounted engines, or both tail and wing mounts, DC-9’s, MD-80’s, 727’s, L1011’s, DC-10’s, and MD-11’s to name a few…

Just my 2 cents, but I am not an aviation engineer.


[edit on 7/27/2006 by defcon5]

Originally posted by northwolf
how does a top mounted engine effect the airflow over the wing?

Badly. It will be mounted in the middle of a local supersonic bubble and causes all sorts of drag and lift distribution problems.


Probably the most comparable PAX jet to the Hondajet is the Fokker VFW-641:





Overwing engines dramatically reduce noise propagation to the ground, but do have other drawbacks as mentioned by defcon and acura [apart from centre of gravity and fuel pumps - they aren't really issues].

The Russians also tried a semi-overwing engine with the An-72/74, and tried to use the engine exhaust to "blow" the air over the upper surface of the wing, increasing lift.



However, the latest version, the An-74-300 hangs the engines underwing as they get increased fuel efficiency from the aircraft [some 20%, although I'm sure not all of that can be attributed to the engine placement]. They have compromised in terms of STOL performance though.



[edit on 27-7-2006 by kilcoo316]

Originally posted by defcon5
Also there are a number of aircraft that have tail mounted engines, or both tail and wing mounts, DC-9’s, MD-80’s, 727’s, L1011’s, DC-10’s, and MD-11’s to name a few…

I haven't been able to figure out why an arrangement like the MD-80s (one engine in the tail, two under the wings) is adventageous. I know all design options are looked at during development by people who studied this stuff in far greater depth than I, so there's a good reason they went with that design. I just don't know what it is (it's entirely possible my assumptions in the first post re: engine placement are wrong).

Just my 2 cents, but I am not an aviation engineer.

Neither am I, but I do like figuring out how things work...

The MD-80 DOESN'T have one in the tail and two in the wings. The L1011, DC-10, and MD-11 were the only ones that had that configuration.

Michimasa Fujino, HondaJet project leader and vice president of Honda Research & Development Americas
said in an interview that the patented wing-mounted engines were one of the keys to the jet's superior speed, fuel efficiency and cabin space.

The aircraft does get far better fuel efficiency (5% more fuel-efficient than its competitors) and apparently the over-wing configuration gives the jet far more room on the interior since there is no carry through structure required in the aft fuselage to support tail mounted engines.

It would therefore seem that this design is at least efficient and spacious, negatives brought up by other posts notwithstanding.

I'd love to see the CL/Cd figures for it and the weight comparisons with something similar to see where their savings come from.

I'm slightly dubious of the 5% assertion - 2 reasons;

- its a very "handy" figure
- its from the man that built it, he has to say something like that [not half skeptical me ]



Overwing engines could provide a big improvement in load paths for the engines, as they will be directly linked to the wingspar, they will be over the wing, meaning bending moment from winglift is reduced at wing-root. However, bending moment at landing will increase somewhat - depends what way that trade-off goes. If its not a critical factor then they'll be saving weight - and it'll multiply weight savings.



semi-edit: Actually, looking at this pic, the main undercarriage is directly below the engines - meaning no bending moments - reducing structural weight significantly. I'll bet thats where all their savings have come from.

Oh, and looking at the wing sweep - or lack of - I assume the aircraft is strictly subsonic [not even transonic] so the upper surface won't be quite as sensitive to obstructions as would be the case for a Mach 0.8 airliner.


Bizjets traditionally tend not to have underwing engines as they don't have the room underneath for fitting them in - thus they've all had tail mounted engines. The undercarriage length is limited in height by the fuselage diameter - to ensure the steps are big enough to reach the ground. For a traditional aircraft, they wouldn't have to concern themselves with that.

Originally posted by Zaphod58
The MD-80 DOESN'T have one in the tail and two in the wings. The L1011, DC-10, and MD-11 were the only ones that had that configuration.

Gah... you're right. Should have double checked before posting.

Originally posted by kilcoo316

Oh, and looking at the wing sweep - or lack of - I assume the aircraft is strictly subsonic [not even transonic] so the upper surface won't be quite as sensitive to obstructions as would be the case for a Mach 0.8 airliner.

Supposedly it cruises at 450 mph

Im sure that steps aren't just the only thing effecting the hight of the landing gear, but it is a good point. Also i just noticed how "badly" (i use this term losely) the wings are to the fuselage.
From the pic kilcoo linked you see how short the landing gear is and how the fuselage bends with th nose pointing down and the body going up only to have the wingroot join on what seem a box attached to the bottom of the body. I'm not a engineer but it seems sorta .... how do i say lacking.

There are 3 main reasons for choosing under-wing engines over over-wing engines.

1) Maintenance - By having under-wing engines ground crews can easily check the condition of the engine. Also bringing in scaffolding for maintenance costs time and money both of which airlines hate to use. The cost of maintaining over-wing engines, at least on larger plane, is greater than any advantage. Airlines also would need to re-equip with new, and probably more expensive, equipment.

2) It's the way it's always been done - Over-wing engines mean shorter landing gear (economics). This makes things difficult for airports whose walkways are designed for taller jets. It also means new maintenance, ground handling and emergency procedures all of which costs time and money.

3) Noise - While passenger comfort isn't the highest thing on the requirement list when designing an aircraft it still has some relevance. On under-wing engines the exhaust is blocked from that cabin by the wing. Noise from over-wing engines goes directly into the cabin.

Overall it comes down to money. Any benefits from over-wing engines just don't out-weigh the cost.

Originally posted by RedMatt
I guess this is where I say, "what does everyone else think?"

They sure would be a long, hard pain in the butt to work on and replace if they were mounted high up on the wing. But you go right on ahead and build em way up there if that's what you want. Just don't expect me to fix them.

Placing engines in interesting places isn't a new idea. Anyone remember the DeHavilland Comet? Nice plane. Engines were actually IN the wings. This gave it a characteristic "clean" look and it looked a lot more futuristic than our current commercial jets . No kidding.

Originally posted by Darkpr0
Placing engines in interesting places isn't a new idea. Anyone remember the DeHavilland Comet? Nice plane. Engines were actually IN the wings. This gave it a characteristic "clean" look and it looked a lot more futuristic than our current commercial jets

. No kidding.

The Comet wasn't the only british plane to use such a setup. The V-bombers (Victor, Valient, and Vulcan) all used burried engines. The problem with this arrangement, is that switching engines (to a newer design) is just about impossible, short or redesigning the wing.

Another layout is the "Sport jet": www.sport-jet.com...

Single tail-mounted engine inside the fuselage.

Originally posted by intelgurl

Supposedly it cruises at 450 mph

Hmmm - on the website it says that @ 43,000ft - thats near Mach 0.7 => pretty damn high for such a wing


semiedit:



Guesstimating from the planform, I get a nose/wingtip angle of around 45 degrees - which would correspond to just over Mach 0.7. So thats what it does, I have to say unless that wing has a really low t/c, I would have expected there to be transonic problems with the engines.

[edit on 28-7-2006 by kilcoo316]

In my opinion this is all old news, commerical airliners are in the configuration you see today for a reason; they've tried and tested other configurations years eariler and deceided that the configuration you see today is best, for all manner of reasons.

Tri-engined planes are planes that are too big for simply two engines (that were available at the time of design) and it was too costly to design in extra strengthening to wings to mount more engines on wings, it may also of been the case that the designers only requested 3 engines in the first place and the only other position that is subject to 'fresh' air without need for major strengthening would be the tail root.

TBH this is really all just a gimmick - Honda is lauching a new biz-jet it needs to put its new product in the limelight and distance it over its rivals - its worked we're all talking about it.


*The Vulcan had various engine changes throughout it's lifetime btw - for those that didn't know

One of the reasons that underwing engines are popular is that there is less of a chance of the engine coming into the passenger cabin in the event of a crash. You figure that the engines are the heaviest components in an aircraft. That Honda jet is pretty but I'd hate to be sitting in the cabin when it crashes. Both of those engines are going to come into the cabin with you.

Canada_EH.

>>
I'm sure that steps aren't just the only thing effecting the hight of the landing gear, but it is a good point. Also i just noticed how "badly" (i use this term losely) the wings are to the fuselage.
>>

While I agree that the step from the fuselage to the wing is less than attractive, especially in relatively short jets, it seems to be 'all the rage' with everything from the Citation X to the later Hawker 800s and even the new Bombardier/Lear crossovers having it.

I have to assume that it is an aerodynamic feature (similar perhaps to the gull 'step' on the Spitfire), but it may also be related to getting a deeper wingroot for a unified wingbox mount with a composite fuselage, added fuel (quite a few bizjets are not in the 3K+ legs range and some of the Gulftreams etc. are up to 6 I guess) or added baggage stowage reasons.

>>
From the pic kilcoo linked you see how short the landing gear is and how the fuselage bends with th nose pointing down and the body going up only to have the wingroot join on what seem a box attached to the bottom of the body. I'm not a engineer but it seems sorta .... how do i say lacking.
>>

What bugs me is that the jet looks like an early Citation. The location of the wings and the engines suggests a very narrow CofG displacement range and so if they 'at some future time' decide to increase the wingsweep or extend the cabin length, I can't help but wonder if they aren't gonna end up shooting themselves over the engine placement and wingchord/sectional profile issues anyway.

Other things to be asked:

Does the keel effect of the engine pylon help or hurt in terms of stability and empennage drag? Do you get any adverse fencing effect with roll at yaw or similar coupled maneuver modes beyond straight and level?


KPl.