Are there cases where a double horizontal stabilizer is useful?

I was watching Agents of Shield recently, and in it the team fly a plane that's claimed to be a modified C-17 Globemaster. It has two extra engines, rotary turrets, and, according to the wiki, a pair of half-length wings at the rear of the fuselage, seen here:

(image courtesy of Agents of SHIELD wiki)

To me, it also kind of looks like a second horizontal stabilizer. Certainly some of the modifications to this fictional plane are pure flights of fancy, but is there any basis behind this one? Is there any reason why this feature would be useful? Have there possibly been any experimental or production aircraft with something like this? Or is this pure "cool factor," with no real basis in aviation?

• putting an engine right in the exhaust of another engine seems like a bad idea to me... – falstro Dec 2 '14 at 21:58
• there's also shock bodies on the rear wing – ratchet freak Dec 2 '14 at 22:46
• it apparently has VTOL capability as well, by rotating the engines forward, I guess that's why there needed to be an engine in the back. – ratchet freak Dec 2 '14 at 22:56
• @ratchetfreak couldn't they just mount the rear engines straight on the fuselage? – raptortech97 Dec 2 '14 at 23:00
• @raptortech97 then they wouldn't have room for the cargo hatch – ratchet freak Dec 2 '14 at 23:02

The main aerodynamic purpose of the horizontal stab (or certain canards) is to provide longitudinal stability.

If the rear wing with the 5th and 6th engine flies "up," like the main wing, then it will counteract the longitudinal stability of the horizontal stabilizer. If the rear wing flies down, like the h-stab, then it is just extraneous, since the h-stab can be made as large as necessary.

If all you want to do is to add extra engines, they can be mounted on the main wing, like in the eight-engine B-52 bomber:

or the six-engine Antonov An-225 cargo jet:

• a joke> A reader wrote us, retelling the story about the military pilot calling ATC for a priority landing because his single-engine jet fighter was running "a bit peaked." ATC told the fighter jock that he was number two behind a B-52 that had one shut down. "Ah," the pilot remarked, "the dreaded seven-engine approach!" – rbp Dec 2 '14 at 22:51
• Oh, but this question was posted right after the aircraft was initially seen on the show. In a later episode, we discover the aft positioning of the extra engines is essential. Spoiler alert! – Harper - Reinstate Monica Mar 17 '19 at 17:29

@rbp has a good answer. I'd like to add something to it.

For most aircraft, the horizontal stab provides stability and drag but relatively little lift. The wing provides 100% of the lift and everything else out there provides stability. And, of course, everything in the airflow (aside from the engines, which provide thrust) provides drag.

Contrast that with a canard, which provides stability and lift (and some drag, coincident with the lift). The canard typically provides 10 - 20% of the lift, with the main wing providing the rest. By putting the canard in front, and designing it to stall before the main wing does, the canard will be unable to lift the nose high enough to cause the main wing to stall. It's not 100% safe; there are still cases where a canard aircraft can stall but they're really obscure. Dick Rutan, who served as test pilot for Burt Rutan's canard-based aircraft designs, once joked that he could take one of Burt's planes up and try like to crazy to make it stall but "no joy; all I ever got was exercise."

Back in the late 1980s, Airbus started designing the tailplane to provide significant lift. After takeoff, they shift some of the weight aft (usually by moving fuel around) and take advantage of that. Airbus has been using this for over a decade to achieve greater fuel efficiency from their aircraft. With improved fly-by-wire flight controls, they've gotten to the point where they don't have to wait until after takeoff. The C-17 uses this idea, too (including the fly-by-wire). But it's my understanding that the tailplane provides no more than 10% of the lift of the main wing.

For the fictional aircraft, they wanted the aircraft to be able to hover. So they have main engines which can pivot downward. When in a hover, though, you need some lift forward and aft of the center of gravity (CG) to provide forward / aft stability and translation. Putting engines on the tail provides that. Making the tailplane a 1/2 span wing, with appropriate amount of lift, guarantees that approx 1/3 of the total lift will be provided by the tailplane. Which means that, when the engines pivot downward to hover, 2/3 of the total hovering lift is provided by the wing and 1/3 is provided by the tailplane. In this fashion, the plane is balanced in normal flight and in hovering flight.

I would've liked to see a large aircraft with the main wing aft of CG and canards, with engines on the canards (or maybe fuselage mounted, near the canards). But they went with a more "familiar" look; there's no heavy-lift aircraft out there with canards like I'm describing. The Tu-144 and Valkyrie both have canards, but nowhere near that large.

An F-35 has the tail of the engine pivot downward, aft of CG, and has a "lift fan" forward of the CG. A Harrier has a total of 4 downward columns of air coming off the engine when hovering, two forward of CG, two aft.

• the horizontal stab provides stability and drag but relatively little lift Actually it provides negative lift, which is how it provides stability. Do you have a source for getting 10% lift from a horizontal stabilizer? – fooot Oct 5 '16 at 21:10
• @fooot: Being subject to AoA changes, the amount of downforce at the tail varies and might even change to lift at high AoA (depending on cg location). With relaxed stability, a lift-producing tail is normal (on gliders, for example) and still provides stability. With about 15% of the area of the wing, 10% as the upper limit of lift on a normal tail and with relaxed static stability sounds reasonable. – Peter Kämpf Oct 5 '16 at 21:22
• @PeterKämpf I won't disagree that it can vary, or provide lift for applications like gliders, but does this also apply to airliners in cruise? – fooot Oct 5 '16 at 21:31
• @fooot: No, the tails of airliners in cruise normally produce a downforce, unless the cg is shifted back like in some Airbus types. This can be seen by the negative camber of the horizontal tail. – Peter Kämpf Oct 5 '16 at 21:35
• My understanding: the horiz stab is normally designed to provide drag and no lift; this provides static longitudinal stability Drop the elevators and the tailplane provides just enough lift to raise the tail. Actively flying the elevator in a neutral condition, with less h stab drag (by design), would require a very light touch and very active control; very tiring for the pilot. That would require more dynamic stability. A FBW system can provide that without exhausting the pilot. Most aircraft aren't FBW, so most aircraft are designed for greater static stability. Modern Airbus are FBW. – Meower68 Oct 5 '16 at 22:10

Do they have a nefarious super-weapon in the rear of the fuselage? That could necessitate the extra lift and, more importantly, the shifting of that lift further back to keep the centre of lift near the centre of gravity. That would also explain the extra engines, as the extra power is needed.

It also needs the extra engines to maintain the pretence of balance when performing VTOL maneuvers (although this would again suggest the weight has been heavily shifted backwards behind the "regular" wing)

Overall, though, it's a bad design which wouldn't be the ideal way to handle any of those things - it's mostly for cool-factor.

• There's no nefarious super-weapon, just a couple of cars. – raptortech97 Dec 3 '14 at 18:35

My only thought is that if that rear wing is truly generating a lot of lift, and if the front is more bulbous (and thus more heavy) that plane is just going to pitch forward in flight (if it ever got off the ground) and go tail over head until it crashes.

Basically, the center of lift (between the two wings) is going to be way behind the center of mass (well in front of the wings), and that usually does not end well.

To add to the answers of @Meower68 and @rbp The craft's configuration is basically a conventional wing with a biplane tail. Such tails date from the pioneer days of aviation. The exact variant here is a cantilevered inverted seqsuiplane; both cantilever biplanes and inverted sesquiplanes have since flown successfully.

Another concept dating from those days is the lifting tail. Provided it does not work so hard as to destabilize the plane, a lifting tail can usefully offload the wing. A lifting lower stabilizer would certainly have been useful in the fictional design to carry the weight of the extra engines without upsetting the trim.

Someone said that the engines move forward when they pivot to vertical mode. Extra engines in an aft location would indeed be necessary to maintain trim then, as well.