Why are military drones shaped so strangely?

Question

Military drones, such as the Predator drone or the Global Hawk, tend to have a weird shape with a whale-like head, and engines concentrated at the back. The wings tend to be of very high aspect ratio, and v-shaped tails seem to be popular:

Why aren't there, say, Cessna-shaped drones? If Cessna 172s work fine for human pilots, why not for computer pilots? The Cessna 172's low speed and stability also seems like a good asset for military recon drones.

Answer 1

The whale-shaped forward fuselage covers a parabolic antenna for a high-bandwidth datalink. The operators want to receive the reconnaissance data in real time, and by giving the antenna the best place in the aircraft, it will be able to connect to communications satellites even when they are just above the horizon.

Global Hawk cut-away drawing (picture source)

The V-tail of the Predator is an inheritance from its origins as a Navy drone which could be folded to fit into a torpedo tube. Read all about it in this answer. If you want more info on the benefits of an inverted V-tail, make sure you read KeithS's answer, too.

In the Global Hawk the V-tail was chosen to mount the engine above the fuselage for better IR shielding from below. The two tail surfaces also shield the exhaust from the side without affecting aerodynamics negatively.

The Aerosonde's configuration was chosen after Tad McGeer had experienced in 1990 from the 1/5th scale RC model of the Perseus UAV of Aurora that a tail-mounted propeller would stabilize the aircraft to the point that it became practically uncontrollable. A pusher propeller is more efficient than a tractor propeller, which also explains the propeller location on the Predator.

If it strikes you how many of the UAVs have V-tails: This allows to use fewer components (surfaces, connectors, actuators), and since all are moved by computer control, the issue of a mechanical mixer does not exist. The lower damping of V-tails can also be easily tolerated, since the FCS can react much more quickly and commensurately to disturbances than any human pilot. The lower maneuverability possible with V-tails is also not an issue - reconnaissance UAVs fly mostly straight and level.

The high aspect ratio wings are helping in extending the range and especially the flying time of observation drones. Similar to glider wings, the long and slender wing will offer the lowest drag for a given lift at subsonic speed.

In the end, each detail has been chosen deliberately and can be well explained. There is no conspiracy to make UAVs look different from Cessnas.

Answer 2

The biggest reason UAVs look like they do is because they don't need to carry a human, or the life support and avionics displays that human would need. The designs, therefore, are built around what the aircraft does need, mainly communications and viewing/recon equipment in addition to the standard stuff any combat aircraft needs (fuel, weapons, propulsion, lift/control surfaces). The use of a V-tail in practically all of them is for drag reduction; the aircraft doesn't have to be as maneuverable as a manned fighter, so a twin tail with separate all-moving elevators is superfluous, and the lower drag increases flight time for more range or loitering capability.

A key design consideration in any aircraft is weight balance. The 172, a fairly conventional aircraft design, is designed for a high degree of flexibility in its payload and distribution; the high wing keeps the center of gravity below the center of lift for stability, and by the same token the cockpit and cargo area are under the wing so changes in CG don't cause quite as drastic a change in handling.

In an unmanned aircraft, weight distribution is fairly static; any ordnance is right under the wings and thus the center of lift (fuel is also placed similarly), while the contents of the fuselage besides fuel load hardly change at all between flights. That allows the aircraft designer to allow form to follow function; the front of the aircraft has most of the avionics and communications, counterbalancing the propulsion system at the rear, with the fuel in the middle. The relative size and shape of these three elements is at the designer's discretion, and then the wings are simply placed at or slightly behind the balance point.

The RQ-1/MQ-1 Predator is probably the weirdest-looking, because in addition to having a V-tail for the weight and drag reasons, the tail is inverted:

The inverted V-tail has several advantages in a plane the Predator's size and with the complexity of the total flight system:

• A rudder surface placed beneath the CG, as on the Predator, will roll the plane toward the direction of yaw instead of away from it as a top-mounted rudder would, which allows for coordinated turns using only the rudder.
• This "rolling rudder" behavior is also pretty much all the RQ-1/MQ-1 needs in terms of roll control, so the rudder surfaces can be the only control surfaces on the plane, dramatically simplifying the control layout of the craft. Theoretically the lack of ailerons allows the wings to be removed or folded easily, but the MQ-1 has weapons pylons with firing circuits that complicate this. Larger drones with longer wings have higher MOI in the roll axis, more than the rudders could overcome by themselves, so these designs have ailerons (either wing-warping or hinged), which allows the V-tail to be upright for better ground clearance on landing.
• A crosswind on an inverted V-tail will cause the plane to pitch up and roll into the wind instead of pitching down and rolling away from it with an upright V-tail. Given the high comms latency between Nevada and Kandahar, this is an advantage, as the plane usually has plenty of room above it to fly while the pilot notices and reacts to the disturbance, but can only fly downward for so long before it runs out of air. Again, larger planes have higher MOIs and so disturbances are more minor.
• The inverted tail masks a little of the plane's engine noise in flight from specific forward angles. Not much, but every bit helps when you're trying to stay hidden.
• Lastly, the inverted tail protects the more expensive propeller if the pilot scrapes the back end on the ground. The larger Reaper has more ground clearance to allow for the weapons pylons, and so it can flare more on landing without involving the prop.

The RQ-2 Global Hawk actually doesn't look that farfetched; it bears some resemblance to the A-10 Warthog:

The two aircraft have vastly different mission profiles (the Global Hawk is intended as the U-2's replacement for high-altitude surveillance and reconnaissance while the A-10 is a venerable armor-killer close air support craft), but there's some common design elements, such as the high engine masked by the tail surfaces to reduce IR signature, and the big low wings producing a lot of lift (for a high ceiling in the GH's case, for payload and survivability in the A-10's).

What's not similar between the two is program costs; the original A-10As had a unit cost of just \$450k in the '70s, and with new wings and a glass cockpit upgrade the A-10Cs are still just \$11 million a frame. Unit costs of the Global Hawk, including R&D, make the program the most expensive small aircraft to date at \$222 million per unit, eclipsing the "cost is no object" F-22 (\$182m).

Other drones are much more cost-efficient; the Predator is just \$4 million each while the larger and more sophisticated Reaper is \$16 million. Losing one still hurts, and the USAF has lost plenty, mainly to operator error (the communications latency inherent in piloting one of these remotely from a ground station half a world away is considerable), but the total costs of these programs is still a pittance compared to any manned airframe in service today, with dramatically reduced maintenance to flight hour ratios and total costs per flight hour. The A-10 is again the cheapest manned combat aircraft in service, costing about \$18,000 per flight hour to operate (no word on what all that price includes; likely some combination of fuel, pilot pay, parts, maintenance and ordnance). The Predator costs just \$3,600 per flight hour, and the Reaper about \$4,800. Again, the Global Hawk program is fairly expensive to operate (about \$49,000 a flight hour) and that's a big reason the U-2 Dragon Ladies that Global Hawk was supposed to replace are still flying (the U-2 is only about \$30,000 a flight hour).

Answer 3

Who says they are weird shaped? Maybe the 172 is weird...

The short answer is that it's the best design for the mission and the specific application. The 172 is designed to carry people and train young pilots so its characteristics reflect being a good fit of that mission. Drones are required to carry computers and surveillance equipment and often a weapons payload and very specifically, not people. Drones can be of a different shape than a GA plane since pilot ergonomics don't really matter. For what it's worth there are planes that are shaped like the drone you mention.

The Bonanza had a long run with the V tail design: (source)

The new Cirrus Jet has a V tail and a bubble airframe much like the drones you pictured: (source)

Bottom line is that planes are designed for specific applications and some airframe aspects lend themselves to a given mission better than others. Planes will reflect these constraints over looks 100% of the time.

Side note: low speed is by no means necessary for good recon. The SR-71, which was arguably one of the most successfully spy planes, did plenty of recon from 80000ft at Mach 3 with out any problems.

Answer 4

Another factor: Cessnas, like virtually all manned aircraft are optimized to get to the destination.

Military drones, however, are more interested in dwell time over the area of interest than in how fast they get there. This results in a very different optimization than for manned aircraft.

They also have a very different view of safety.

You have an aircraft that costs \$500k. There's a safety measure that has a 5% chance of saving the airframe over it's expected life but which costs \$50k. On a manned plane that's almost certainly going to be done. On a drone it wouldn't make sense in most cases.

Answer 5

For single engine pushers with a canopy, when a pilot bails out of a manned aircraft they might hit the prop or intake at the rear of the aircraft. This requires either special provisions for ejecting, like falling out the bottom or side of the aircraft, or the installation of an ejection seat powerful enough to guarantee the pilot clears the propeller. This all adds weight and cost. A lot of pilots don't like this idea.

An unmanned drone has no such problem, aircraft designers are free to choose designs without considering how a human will get out of it.

Answer 6

A big part of the reason for the bubble shaped head is reduced drag. The area does hide a parabolic antenna but the could be done with many shapes. This shape is designed to have the smallest flat surface area possible to allow the air to flow smoothly across the front of the aircraft instead of becoming disrupted and turbulent by striking a flat surface. This leads to reduced drag and a longer flight time per gallon of fuel.

Cessna's and similar aircraft have a windscreen that is basically a large flat plate for the wind to slam against, and while they work great for seeing through, they actually add quite a bit of drag to the aircraft. The cessna's designed mission, however, makes this acceptable.

Answer 7

If you look at the front bottom side of a typical single engine Cessna you will find that it is black and oily from exhaust fumes and oil residues.

On reconnaissance UAVs, placing an engine at the nose (although there are a few examples - IAI Hunter for one) cause engine residues to accumulate on optical payload lenses during operation and degrade payload performance.

This is the main consideration for placing (piston) engines at the rear of reconnaissance UAVs.

Answer 8

high aspect ratios are provided for recon activities like lockheed martin U3 which had aspect ratio of 15 but this types of manned aircraft, pilot should be very high skilled