Jet a/c design: why not top-mount pylons?

Aside the fact pylons mounted on top of wings look bizarre, is there a reason not to? Top-mount would seem to reduce FOD, & shorten gear legs (& so weight).

I understand keeping the engine away from wing turbulence is good, so a strict over-wing might not be ideal. What about a forward-swept? Or back-swept? That is, more/less a standard Me-262 or 707 pylon, mounted on top, instead.

I also realize keeping jet efflux away from the horizontal stabilizer is desired;:rolleyes: T-tail might be needed.

So, beyond esthetics, why not overwing instead of underwing?
 

Driftless

Donor
Ease of maintenance? Early years especially, ground crew would need more specialized work platforms to do repair work.
 
My first reaction was: top mounted bomb racks??????
Only a couple of military jets have tried this configuration. English Electric mounted massive fuel tanks above the wings of their Lightning fighters.
SEPCAT Jaguars mounted air-to-air missiles (AIM-9 Sidewinders?) above wings to free underwing bomb racks for ground attack ordinance.

Only a couple of small, civilian jets have reached production with wing top, pylon-mounted engines: VFW and Honda Jet.

The primary reason for conventionally hanging engines under leading edges is to reduce the risk of flutter. Boeing learned this when building B-47 and B-52 bombers with high aspect ratio, flexible wings.
 
Aside the fact pylons mounted on top of wings look bizarre, is there a reason not to? Top-mount would seem to reduce FOD, & shorten gear legs (& so weight).

I understand keeping the engine away from wing turbulence is good, so a strict over-wing might not be ideal. What about a forward-swept? Or back-swept? That is, more/less a standard Me-262 or 707 pylon, mounted on top, instead.

I also realize keeping jet efflux away from the horizontal stabilizer is desired;:rolleyes: T-tail might be needed.

So, beyond esthetics, why not overwing instead of underwing?
It's been done a few times, most notably that I recall with the HondaJet (which indeed sweeps the pylons aft). There's a few reasons to go underwing rather than overwing, though: underwing you can access the engines easily from below for ground service, and even removing the engine entirely is mostly a matter of opening the nacelle cowling, bringing a a cart up to the required height, picking up the weight, unbolting it, and lowering away now carrying the engine. With a top-mounted engine, you need an overhead crane (bridge crane or mobile crane). You'd similarly need a lot more complex work platforms to get to the engines

If you were going to have a T-tail and complex access to the engines, you might as well just tail-mount them like the DC-9. Indeed, if I don't look carefully at the HondaJet, I often find myself thinking it looks like the engines are on tail pylons, not wing pylons:

maxresdefault.jpg
 
Ease of maintenance was (is) an excellent reason, & I should've thought of it.:oops:

And no, I don't mean leading-edge mounts like the YC-15. I was thinking more like the HondaJet. (I recall a '50s design with swept-forward pylons, too.) I also recall the Lightning missile pylons, which also made sense to me...except loading from a trolley has to be easier to an underwing point...

I didn't know about the flutter issues.
 
I believe the main obstacle for top-wing mounted engines is the fear that at certain steep angles of attack the wing would move in front of the engine intake and cut off its air.
 
I believe the main obstacle for top-wing mounted engines is the fear that at certain steep angles of attack the wing would move in front of the engine intake and cut off its air.

Pretty much this. Top mounts have been used on some aircraft, usually tactical airlifters that both want to reduce FOD risk on rough fields and arnt expected to pull aggressive manuevers.

For more agile combat aircraft the risk of the air flow to the engine being cut off is deemed too great.
Commercial jets are expected to operate off of well maitained airports so FOD is not as much of a concern, and hanging the engines under the wing gives better ease of access for maintenance crews.
 
A bonus to under-slung loads is it's far more easier to man handle bombs, missiles, drop tanks etc... into position if lifting gear isn't available, with over wing pylons you can run the risk of damaging the aircraft if something should drop. However I would love to watch armourers fitting missiles to the top of a wing or even a fin :p
 
Gentlemen,

The Antonov and the Boeing aircraft employ overwing (not pylon mounted) engines to substantially improve takeoff and landing performance by entraining high velocity exhaust along the upper surface of the wing and over the depressed flaps. In effect a portion of engine thrust is diverted downward. This is an example of applied "Coanda effect".

Pylon mounted overwing engines have the disadvantage of operating in the higher flow velocity present over a wing developing lift. (Circulation theory "Lambda") Smaller low wing jets, like the German 614 have used this engine location successfully despite the increased drag.

Dynasoar
 
IIRC English Electric Lightnings of the RAF did have an optional above the wing missile pylon

RAF Lightnings only ever mounted fuel tanks above the wings. Saudi and Kuwaiti Lightnings were displayed with double SNEB JL-100 rocket/fuel tank pods above the wing but I'm not sure they ever used them in service.
 
My first reaction was: top mounted bomb racks??????
Only a couple of military jets have tried this configuration. English Electric mounted massive fuel tanks above the wings of their Lightning fighters.
SEPCAT Jaguars mounted air-to-air missiles (AIM-9 Sidewinders?) above wings to free underwing bomb racks for ground attack ordinance.

Only a couple of small, civilian jets have reached production with wing top, pylon-mounted engines: VFW and Honda Jet.

The primary reason for conventionally hanging engines under leading edges is to reduce the risk of flutter. Boeing learned this when building B-47 and B-52 bombers with high aspect ratio, flexible wings.
Here is a picture of a Jaguar with Sidewinders mounted over-wing, for those who are wondering what that would look like:

AA581597-C230-4013-B3E6-8F65868D252D.jpeg
 
On the other hand, as Boeing learned with it's 737 Max, having the engine under the wing limits re-engining possibilities. The re-engining effort on the B-52 i think has the same problem.

So... Suppose company X wants to introduce a jet early on. They want to offer something NOW, but right now turbojets are the only thing available, and turbofans are coming 'soon'.

To allow room for the larger engines, it's decided to put the engines on to. This also provides better rough and short field performance, so the plane is marketed to places that don't have full sized jet airports.

(I remember in the 60s when Saskatoon had to upgrade to handle jets.)
 
Suppose company X wants to introduce a jet early on. They want to offer something NOW, but right now turbojets are the only thing available, and turbofans are coming 'soon'.

To allow room for the larger engines, it's decided to put the engines on to. This also provides better rough and short field performance, so the plane is marketed to places that don't have full sized jet airports.
If you do this early enough (say, TCA management shows even a lick of sense,:rolleyes: which might be ASB:rolleyes:), & the C.102 enters service with this feature, you might butterfly the 727/737, Caravelle, &/or HS.146 entire; you might also butterfly some Antonovs (tho they might be getting too high an export subsidy, IDK).

You've made the C.102 the best-selling jetliner in history.:cool::cool::cool::cool::cool::cool: And saved the Canadian aerospace industry.:cool::cool::cool::cool::cool::cool::cool: (You may've dealt Apollo a crushing blow, tho.:eek::eek: Unless it's Canadair that builds the LEM, instead of Grumman.;))
 
Yes, Honda Jet engines are located very close to where they would be if mounted on the aft fuselage, but they are far enough outboard that they do not interfere with airflow around the aft fuselage. Honda Jet has a simple, circular aft fuselage compared with the complex curves on business jets with engines mounted to their aft fuselages.

Hansa Jet was a German-built business jet with forward-swept wings. The primary goal was passing wing spars aft of the pressurized cabin, to allow the longest possible cabin. In all other respects, Handa Jet was conventional with two jet engines mounted on (horizontal) pylons extending from the aft fuselage.

VFW 614 was another 1960s vintage commuter jet - designed in Germany - with engines mounted on pylons over the wing. The wing was supposed to mask engine noise to reduce noise pollution. Otherwise it had a conventional airframe with a conventional horizontal tail. Both German jets were only built in small numbers.

I suspect that most of the engine noise - from conventional airliners - is actually fan noise. Many turbo fan inlets have sophisticated sound-dampening insulation lining their intakes.

Yes, Boeing 737 and Avro Jetliner suffered from engjne configurations that limited growth. Engines that close to the ground are more susceptical to FOD damage.
Boeing has done some weird and wonderful aerodynamic tricks to upgrade the basic 737 airframe to accommodate turbofan engines. Since they are much larger in diameter, engine nacelles had to be moved forward of the wings' leading edge and up a bit. They also got weird intakes that are not round .... to prevent them from dragging on the runway or sucking in FOD.
Boeing has repeatedly re-engined 737 without having to significantly modify the landing gear.
 
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