Saw this plane on an episode of top gear (not their 747) and it looks unusual.
I haven’t seen very many planes with two engines per wing pylon (think that’s what they’re called), and none with that jagged edged outlets. Just heard about those on a Mentour video the other day which described them as a fairly recent innovation.
Yep, that’s it. Other shots in the episode show the blue tail, the external fuel tanks on the outer engine pylons, and the 747’s distinctive hump. And of course, the fact it’s currently at Dunsfold Aerodrome.
Depressing to discover it’s just a movie prop, though.
Not well. The tandem engine fairings are based on the B-52s, which uses 8 17,000 lb/ft. engines. 747s use four much more powerful engines - in the 50,000 lb/ft. range. That’s more than 50% extra thrust versus eight engines, or more than double what you’d get from 4 B-52 engines. An empty 747 weighs about three times as much as a fully loaded B-52, so it’s going to make a big difference. Now, if the tandem arrangement is for JT9 or CF6 engines that are actually mounted on 747s, it might work but the weight would be prohibitive.
Aerodynamically, there probably isn’t any major problem with mounting both engines on one pylon, but I suspect that it would be more efficient to have that pylon farther outboard (since that’s where they are on the wings of twin-engined widebody aircraft).
I’ve wondered that as well. I think the main problem with that configuration is that if one engine has a catastrophic failure and starts energetically shedding parts, they’re more likely to damage the engine right next to it. To lose one engine may be regarded as a misfortune; to lose two looks like carelessness.
It does give me a chance to post this. During the development of the JT9D and TF39 engines (which would go on to power the 747 and C-5A, respectively) they were tested in flight on a B-52. The inboard right engine pod, containing two engines, was removed and a single test engine put in its place. If you can replace a twin-engine pod with a single engine, seems like the reverse could theoretically work, even if there are good reasons not to do it.
Makes for a good trivia question for aviation geeks; I think it’s the only plane ever made with 7 engines.
How about torque, both from the thrust, and from mass downward or outward… ( as inertia or gravity would).
With two larger engines, the centre would be further away from the wing.
Thats why the previous poster said that he’s going to assume the B52 engines were producing B52 engine thrust… The two engines paired like that were only good for that thrust… The small size, meaning air flow considerations, and energy density (which leads to higher temperature… ) is one reason… but just the practicality of sending higher thrust down each side of the pylon… What happens if you want to run off one of those engines ?torque.
To make that much higher thrust, you make the engine larger… (no amount of design will get that much more thrust.)… So more thrust at a larger offset from the various axis of rotation… much more torque.
I’m not sure I’m entirely following your torque argument. When you say “larger offset from the various axis of rotation”, which axis do you mean? The engine thrust is not along the center line of the plane, but that’s true in all airliners. If one engine in a pair were to fail, the thrust from the remaining engine would put a twisting force on the engine pylon, but I don’t think that’s an insurmountable problem. Or did you mean a twisting force along the axis of rotation of the engine?
Since the airplane is the fictional Skyfleet S570, I assumed the engines could be fictional as well. Just because they’re in double pods like a B-52, I didn’t assume they were the same engines as a B-52. In fact, since the plane is still the size of a 747, and still has 4 engines, they’d have to be roughly equivalent to the originals (although the ones in the picture look a bit smaller).
As for the weight, the S570 also has those fuel tanks (what else could they be) on the outboard pylons, so that would distribute the weight along the wing a bit. To me, those look sillier than the engines. I think the designers would fill every nook and cranny of the plane with fuel tanks before they’d hang those big, drag-inducing tanks from the wing.
Other older large aircraft have had protruding external tanks (Vickers Valiant, B-52) but it does raise a valid point about drag penalties when you could just shoehorn more gas internally with a wet wing or conformal tanks to smooth airflow. I suspect the SkyFleet’s “designer” had the Valiant in mind when they made it. As far as the engine pylons, having them further out would induce a large torsion moment so tucking them in would make sense as the wing box is very stiff and the spars are stronger the closer to the fuselage you go.
Conversely, imagine for a moment that you take a B-52H (extensively re-engineer it!!!) and re-engine it with 4 GE90s (115000 lb thrust each!) instead of the current proposal of 8 regional jet engines (roughly 20000 lbs thrust) to replace the TF-33 it has. At an max TO weight of 484 000 lbs and 460 000 lbs thrust it would have almost the same power to weight ratio of an F-18! Or it would go full throttle and pull the wings off…
RNATB, according to Wikipedia an empty operating weight of a 747-800 is roughly the same as an MTOW B-52H, and roughly double a B-52 when at MTOW.
I thought I read somewhere that the 747SP (a shorter, long-range version of the 747) had a fuel tank in the vertical stabilizer for greater range. I wasn’t able to confirm that, but I did find that the 747-8 carries fuel in the horizontal stabilizer. I think external tanks might decrease a planes range, unless the airline plans to jettison them once they’re empty.
I think the SkyFleet’s designer was stuck with the fact that he was starting with a 747. Had to hang something off that outer pylon or it would have looked very wrong.
On something the size of a 747, an external tank or two would add negligible drag. On something the size of a fighter or tactical bomber, it adds a lot, which is why they are jettisoned (in addition to the maneuvering penalty). Commercial airliners don’t use external tanks because (1) they create an unnecessary accident risk with all the small tender aircraft buzzing around them at airports, and (2) there’s plenty of internal fuel capacity. That, and they aren’t designed with hardpoints to mount tanks on in the first place.
Partly that, but mainly to permit control of the CG by internal fuel transfers from the tail to the wing. That allows the elevators to stay at neutral position and reduce their drag etc. Airbus widebodies have that feature too. The Concorde’s system was especially complex.
Yep, lotsa drag, and lawsuits from the people the tanks fall on.