12
$\begingroup$

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:

the SHIELD modified Globemaster(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?

$\endgroup$
6
  • 13
    $\begingroup$ putting an engine right in the exhaust of another engine seems like a bad idea to me... $\endgroup$
    – falstro
    Dec 2, 2014 at 21:58
  • $\begingroup$ there's also shock bodies on the rear wing $\endgroup$ Dec 2, 2014 at 22:46
  • 5
    $\begingroup$ 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. $\endgroup$ Dec 2, 2014 at 22:56
  • $\begingroup$ @ratchetfreak couldn't they just mount the rear engines straight on the fuselage? $\endgroup$ Dec 2, 2014 at 23:00
  • 1
    $\begingroup$ See this question for more biplane tails. $\endgroup$ Sep 25, 2022 at 20:16

8 Answers 8

13
$\begingroup$

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:

B-52 Bomber

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

Anatov AN-225

$\endgroup$
2
  • 22
    $\begingroup$ 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!" $\endgroup$
    – rbp
    Dec 2, 2014 at 22:51
  • 2
    $\begingroup$ 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! $\endgroup$ Mar 17, 2019 at 17:29
5
$\begingroup$

@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.

$\endgroup$
5
  • $\begingroup$ 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? $\endgroup$
    – fooot
    Oct 5, 2016 at 21:10
  • $\begingroup$ @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. $\endgroup$ Oct 5, 2016 at 21:22
  • $\begingroup$ @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? $\endgroup$
    – fooot
    Oct 5, 2016 at 21:31
  • 1
    $\begingroup$ @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. $\endgroup$ Oct 5, 2016 at 21:35
  • $\begingroup$ 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. $\endgroup$
    – Meower68
    Oct 5, 2016 at 22:10
4
$\begingroup$

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.

$\endgroup$
1
  • 1
    $\begingroup$ There's no nefarious super-weapon, just a couple of cars. $\endgroup$ Dec 3, 2014 at 18:35
3
$\begingroup$

Double horizontal stabiliser have been recently introduced also in the helicopter world with the H160 but for a slightly different reason.

H160 tailboom Horizontal stabiliser on the H160. Source https://en.m.wikipedia.org/wiki/Airbus_Helicopters_H160#/media/File%3AH160_Tail.JPG

Horizontal stabiliser in helicopters mainly provides pitch stabilisation like in a conventional airplane plus some other useful function like, for example, limiting the fuselage's pitch attitude in forward flight, giving less drag.

The standard position of the horizontal stabiliser is at the far end of the tailboom since this position maximize its structural and aerodynamical characteristics; anyway it has also some drawback. In hover, the wake of the main rotor doesn't reach the stabiliser since it contracts and moves downward:

main rotor wake in hover Main rotor wake in hover. The horizontal stabiliser would be at the far right of the tailboom, just before the tail rotor and out of the wake.

However, as the forward speed increases, the wake is more and more skewed backward and at a certain speed it impinges on the horizontal stabiliser:

Main rotor wake in forward flight Main rotor wake in forward flight. Both images from this presentation which, in turn, uses images from J. G. Leishman, Principles of helicopter aerodynamics.

At that point the stabiliser, which is already generating a stabilising downward lift, generates an even bigger download which makes the fuselage more or less suddenly pitch up. The opposite is true when the helicopter slows down and the stabiliser exits the wake. This phenomenon is called... well, pitch-up.

So, a horizontal stabiliser should have:

  1. a big surface to generate enough lift to stabilise the helicopter but
  2. a small (ideally zero) surface as well to minimise the pitch-up.

A smart solution to this contradiction come from noticing that:

  1. the stabilising lift is proportional to the (mainly horizontal) flying speed while
  2. the pitch-up force is proportional to the (mainly vertical) main rotor wake.

Splitting the stabiliser surface into a biplane configuration solves this contradiction: the stabilising force in pitch is unchanged since the total stabilising lift is the same but the rotor wake sees now just the upper half of the stabiliser since the lower half is in the shadow of the upper one. Smart indeed.

$\endgroup$
2
$\begingroup$

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.

All in all the design is mad, but not barking mad.

$\endgroup$
1
$\begingroup$

A biplane elevator was useful in improving the arc of fire for a gunner, where the gunner is located near the wing - such as with the Hannover CL.II, III and IV, but it was a very rare solution, and even on their CL.V they abandoned it. Hannover CL.IIIa on its nose, showing how narrow the tail was On a transport aircraft, it might result in fewer collisions between ground vehicles and the tail, but offers no flight or structural advantages.

In any case, it would result in more drag and weight and less aerodynamic efficiency. Any time an airfoil is superimposed over another, the low pressure area above, and the high pressure area underneath interfere, reducing lift dramatically, while also increasing drag. Staggering them doesn't help much, and the gap necessary to reduce this needs to be huge compared to the chord of the airfoils. Worse, it also adds two extra tips, where vortices are generated when the high and low pressure air spills over the ends, and mixes, hence why in many cases the rudders are placed to act and end plates.

$\endgroup$
0
$\begingroup$

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.

$\endgroup$
0
$\begingroup$

A double horizontal stabilizer has been useful during early days of aviation, for structural reasons regarding heavy bombers.

Airplanes like Vickers Vimy or Handley Page Type O, had a biplane horizontal stabilizer roughly the size of a Nieuport 11 fighter, and it used the same buiding principles as other thin airfoil wings of that time which is a light rigid truss structure.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .