[Peetwo]

On the subject of drones, I came across this Popular Mechanics article about what *really* happens when drone meets aircraft.
I always figured they'd be nothing like birds..

Drones strikes are not like bird strikes—they're much, much worse.
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By David Hambling
Dec 22, 2016

The skies are getting crowded.

The FAA says that reports of near-misses between drones and planes have surged since 2014, with as many as 650 cases as of August 2016. Last month an airliner narrowly avoided hitting a drone near London's tallest skyscraper. Dubai airport has been repeatedly shut down by drone activity, and low-flying drones are increasingly disrupting firefighting aircraft in the western U.S.

Sooner or later, those near-messes are going to become hits. So experts from the U.S. and U.K. are recreating these deadly scenarios before they wreck real-world consequences, hoping to nip this 21st century problem in the bud. New studies by the FAA and its European counterpart, the EASA, are looking at computer simulations and running physical tests to understand the problem and prevent disaster.

Mechanical Geese From Hell

What actually happens when an aircraft runs into drone? While some continue to believe drones are no more of a danger than one stray goose, new studies are showing that drones are more like mechanical geese from hell when it comes to commercial and low-altitude helicopters.

"Impacts from drones are not the same as impact of birds," says Javid Bayandor, associate professor and director of the Crashworthiness for Aerospace Structures and Hybrids (CRASH) Lab at Virginia Tech.

Drones are more like mechanical geese from hell when it comes to commercial and low-altitude helicopters.

Bayandor's simulations show exactly what happens when drones and birds meet a choppy death by jet engine. An airliner's engine would dice up the smallest commercial drones without much of a problem. But as these mechanical birds grow bigger, things get more dangerous.

Drones are made of materials much denser and stiffer than soft tissues and muscles of geese and other airborne animals. Simulations show that when a bird goes into an engine, it's essentially liquidized by spinning blades, like a macabre episode of "Will It Blend." Not so with a drone.

"Birds can disintegrate relatively easily...you get something like a very viscous bulk of fluid on the other side" says Bayandor. "A drone can be like a rock going through the engine."

That can mean immediate damage, leaving an engine blade deformed, broken, or completely fragmented, as shown in the above animation. Even where there is no initial damage, the sheer momentum of a larger drone can cause the engine to become unbalanced. This unbalance can escalate, and the blades may start hitting the casing that contains them. The engine ricochets back and forth inside its casing and the damage grows into a real problem.

Luckily, engines are designed to withstand some damage from stray objects, and the high-speed fragments thrown out by such events would likely be contained within the engine. "In general, the casing is designed so that it can also act as a shield between fuselage and engine," says Bayandor.

However this protection is not a given. Modern engines are not designed to gobble up drones, and models have already shown some unexpected results, such as the dramatic difference that the location and angle of impact can make.
But it's not just engines we need to worry about.

Break on Through

Across the Atlantic at Cranfield University in the U.K., Professor Ian Horsfall spends a lot of time "throwing things at other things" using different kinds of exotic cannons. He leads the Impact and Armor Group, and recently began simulating the aerial battle of drones versus aircraft.

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Impact and Armor Group testing facility at Cranfield University with specialized cannon, left, and mock nose cone.
Cranfield University

To test a drone impact with an airliner during take-off or landing—the most likely times for such a disaster, since it's when planes are closest to the ground—Horsfall developed a 4-inch-caliber cannon with a ten-foot barrel, powered by compressed air and capable of firing projectiles at 200 mph. While some other tests fired real dead birds–leading to an aviation urban myth all its own–Horsfall's project used blocks of gelatin as stand-ins, like the ballistic gelatin used to test the effect of gunshots on flesh.

To simulate drone impact, Horsfall created a projectile which mimics the physical properties of a drone, with components of the same size and weight (as seen in the top GIF). There are four grape-sized steel cylinders instead of motors, nylon blocks standing in for circuit boards, a camera, and drone batteries. These components were then encased in a Styrofoam block to recreate the similar weight and structure of an average commercial drone.

Tests showed that mock drones won't damage an airliner's windshield. However, the radome, the circular cover over the radar at the aircraft's nose, isn't so lucky. While simulated birds of the same weight just bounce off, a drone can become embedded or even tear right through the radome. It is the hardness of some of the components rather than the weight that matters.

Fire Danger

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Cranfield University

The other big issue is a battery's annoying tendency to burst into flames when damaged. To recreate this effect, Horsfall used a gun designed to fired a chisel blade at a drone battery. After a few seconds the damaged battery heated up and soon started burning fiercely. If the battery was lodged inside a radome or embedded in a plane, a fire could be the catastrophic icing on a tragic cake.

While most attention remains focused on airliners, thanks to the dramatic near-misses at the airport, other aircraft are in greater danger of drone strikes. Horsfall says helicopters and light aircraft have far more to fear from drones because their windshields are not as strong and are more likely to be flying at the same altitude as drones, especially when fighting fires. While airliner windshields are unlikely to be broken by bird impact, there have been cases where helicopters have been lost because of a run-in with a flock of geese. Drones will only make matters worse.

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Aftermath of a 200mph drone strike.
Cranfield University

Bayandor and his CRASH Team will be presenting two papers giving the full results of their research – including a detailed study based on their review of a hundred and fifty different types of commercial drones – at the [url=https://www.aiaa.org/EventsLanding.aspx?id=79]AIAA in January. Meanwhile, Horsfall is planning to upgrade his laboratory set-up, including a dedicated range for testing drone and bird strikes full time.

Many new measures, such as flight regulations, restrictions on drones, and sense-and-avoid systems are needed to ensure safety of shared airspace. But accidents will happen, and Bayandor says that drone impacts will need to be a part of future engine design, just like bird and hail strikes.

"Drones are here to stay," says Bayandor. "This is just the beginning."

Excellent catch Wannabe, choc frog is in the mail...

A rather more 'independent' approach to UAV/RPA safety issue research, than that of CASA with their fully funded interned boffin's reports - see Drone deaf and bone idle. - which they apparently based their whole 'safety case' for the amended CASR Part 101..
 
Reference CASA submission pg 8 & 9:

The Part 101 amendments consultation process

38. CASA consulted with the public and industry on the amendments that are now in effect between 14 May 2014 and 16 June 2014. Revisions to the draft regulations were endorsed by the UAS Standards Sub-committee (UASSC), which includes representatives from the RPAS industry as well as CASA, Airservices Australia and major Australian airlines.50 In developing the amendments to Part 101, CASA commissioned two research studies by Monash University—

(a) Potential damage assessment of a mid-air collision with a small UAV.51 This report analysed the damage potential to manned aircraft from a mid-air collision with a small unmanned aircraft. The scenarios of engine ingestion and impacts into fuselage and cockpit windscreen were considered.

(b) Human injury model for small unmanned aircraft impacts.52 This report described an injury prediction model for the impact of small Remotely Piloted Aircraft (RPA) into a person on the ground. The model provides estimates of injury severity as a function of the RPA’s mass and impact velocity.

39. CASA took studies such as these into account in developing the amendment regulations including the Standard RPA Operating Conditions. CASA considers the regulations, supported by published guidance and safety educational material, assist in minimising the likelihood of a person or another aircraft being hit by an unmanned aircraft.

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51 https://www.casa.gov.au/files/potential-...all-rpapdf

52 https://www.casa.gov.au/files/human-inju...impactspdf

MTF...P2