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Let's say you have a seagull and a drone of a comparable size. Pretend either one was going to be ingested by large turbo fan engine, like the one found on a 737 (eg., the CFM-56)

Which would cause more damage and why? I'm particularly interested in how the fan blades and compressor stages will be affected by the difference in materials.

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  • $\begingroup$ Related: Can a drone hitting a plane be mistaken for a birdstrike? $\endgroup$ – fooot Mar 16 '16 at 19:35
  • $\begingroup$ @fooot Agreed. This question is kind of a focused continuation of that question. $\endgroup$ – Jay Carr Mar 16 '16 at 19:38
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    $\begingroup$ @Andy Given the numbers of birds out there compared to the numbers of drones, you're odds are probably far higher of taking damage from a bird. The stats out there for annual costs of bird-strikes are some prett big dollar (Euro) amounts, while for drones, the stats are pretty minute. But, if you get one bird in "this" engine, then one drone in "that" engine, of equal size & weight, the mechanical effects will take over, and that's where I suspect that the drone will really tear things up far worse than the bird flesh will. $\endgroup$ – Ralph J Mar 18 '16 at 7:57
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    $\begingroup$ Not a test but there has at least been a study. $\endgroup$ – fooot Mar 18 '16 at 20:14
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    $\begingroup$ While I have done significant research related to UAS bird impact UAS Safety report I have not been able to find specific data related to this particular import question. High speed impacts are a complex phenomena, and detailed data is necessary. If it were easy to characterize them based on just a paper analysis, engine manufactures would not do actual bird, water and hail testing I have heard many claims about "soft" versus "hard", but anyone who has done a belly-flop from a h $\endgroup$ – Adam Mar 20 '16 at 16:11
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There is a numerical study on the topic at VirginiaTech, where they simulated a 5kg drone being ingested by a turbofan, some pictures related to the study below:

From Aerospace America November 2015, graphic Modeling by PhD Students Yangkun Song and Kevin Schroeder of VirginiaTech Crashworthiness for Aerospace Structures and Hybrids lab.

On the same Aerospace America Article it can be read that:

  • FAA will begin testing of drone ingestion next year
  • The metric for danger is the density of the material, so metallic parts are more critical than the bird bones/tissues. Modern drones have more aluminum parts, but the most critical component of a Drone ingestion would be the batteries.

Comparing different sizes, a commercial quad copter is around 1.4kg, while the most frequent bird impact (from FAA database) is around 1.8kg.

This is not the worst case scenario since larger drones are commercially available, and a goose, which can weight up to 6kg.

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    $\begingroup$ Thanks for finding this. They have the article online too (though there isn't much more than the illustration shown). link $\endgroup$ – Andy Mar 22 '16 at 11:12
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    $\begingroup$ @Andy Thanks for the feedback, I included the link you provided into the answer to improve readability $\endgroup$ – GHB Mar 22 '16 at 11:51
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Birds are soft; drones tend to have at least some parts that are hard metal. If a small bird goes through the fan blades of an engine, it gets pretty cleanly sliced, and often causes no damage at all. (A bigger bird going into the core is a different matter -- ask Captain Sullenberger!) I'd suspect that the metallic parts in a drone would cause far more damage to the fan blades than the bird would -- not necessarily anything immediately catastrophic, but nicks in the leading edge of the fan blades would probably be visible when looking at the shut-down engine, and those take maintenance action to resolve.

Anything going into the core is more involved than if it went only into the fan blades, and the difference between bird flesh cooking and getting spit out versus the metal & plastic of the drone melting & causing clogs & damage is probably another point of difference -- the bird may cause damage, but the drone would probably be more likely to cause damage, and would probably cause greater damage.

To what extent the crew could tell the difference in flight, is hard to say. (The experience with bird-strikes to the engines is pretty wide-spread, at least in terms of corporate knowledge; the number of drones-in-engines is far, far fewer.) Knowing that something is wrong with a motor, and dealing with it, isn't the same as knowing the cause -- at least until you're back on the ground and the engine gets inspected.

UPDATE: This Popular Mechanics article discusses these sorts of issues, with some simulations linked, such as this video.

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    $\begingroup$ I think this is a good framework for an answer, but I really was hoping for more specific details into how the materials would interact with the internals of the engine... $\endgroup$ – Jay Carr Mar 17 '16 at 17:39
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    $\begingroup$ This answer is mostly speculation - one could easily argue that birds are mostly sacks of water (like most organisms) and as such are made of a heavy, incompressible(!) medium that may cause bending and cracking rather than piercing or scratching damage. I think any answer not reporting on field tests (whether intentional or accidental) is indeed not very useful to @JayCarr as he indicates. $\endgroup$ – Sanchises Mar 18 '16 at 21:05
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    $\begingroup$ Take a meat cleaver and chop through a chicken 50 times. Inspect kinfe for damage. Now chop through 50 drones & inspect. Bet you find more nicks on the blade from metal in the drones. Until P&W or GE is willing to write off an engine or several by flying drones into them, we probably won't have perfectly comparable engineering data, but intuition informed by experience can tell us what the qualitative results are most likely to be. We DO know the damage that FOD does to motors, and pound-for-pound, birds do far less damage per incident than metal & rocks do. $\endgroup$ – Ralph J Mar 18 '16 at 21:19
  • $\begingroup$ @RalphJ Sanchises is suggesting that at the speeds involved, water doesn't work the way our low-speed intuition suggests, and I don't think a low-speed thought experiment is helpful. On the other hand your suggestion about FOD seems very relevant -- anything about aluminum or other lightweights in particular? $\endgroup$ – Charles Oct 31 '17 at 16:55
  • $\begingroup$ @Charles There is ample experience with birds going through engines, and they tend to get neatly chopped up & spit out without significant damage to the engine (as long as they missed the core). We know from experience with FOD that metal -- any type -- is far, far worse on motors, even at low speeds. For somebody who doesn't think a fan blade will fare far worse trying to slice through metal than it does slicking through bird at the same speed, regardless of what that speed is, I don't have anything else I can offer for enlightenment. $\endgroup$ – Ralph J Oct 31 '17 at 21:26
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While a collision with anything during flight would be bad, please keep the fear of drones in perspective. the OP's scenario has never happened.

There have been more than a million hours of flight of small drones, yet there is not one verifiable report of a collision between a personal drone and a manned aircraft. Not one.

There is also not one verifiable report of a drone crash in the U.S. that resulted in a serious injury as defined by the NTSB to someone not connected to the flight. Not one. It is a safety rate that all other segments of aviation would be jealous to have.

The only significant metal in most consumer drones is the motors, about the size of a large hailstone. The batteries feel dense, but they are mostly a liquid polymer electrolyte.

Yes, the FAA plans to include a drone in their engine ingesting tests next year. The problem I see is which drone? The333.org has examined 5469 Exemptions through 7/12/2016 and the data contains 902 Models of UAS from 256 manufacturers. So, which one is "typical"?

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    $\begingroup$ The batteries come with a stainless steel container. THAT, and the mass inside, makes them more dangerous than birds in a collision. $\endgroup$ – Peter Kämpf Oct 1 '16 at 17:02
  • $\begingroup$ The SS wrap is not much thicker than your kitchen aluminum foil, and not all batteries have that. As I said, the batteries are mostly liquid. $\endgroup$ – user3573562 Oct 3 '16 at 13:35
  • $\begingroup$ Citations or sources? This may be right, but currently reads like opinion. $\endgroup$ – nexus_2006 Dec 24 '16 at 14:52
  • $\begingroup$ There are 10-30 billion birds in the US and so I'm not particularly impressed with a million hours of drone flights compared to a quadrillion hours of bird flight since the FAA was founded. 13,546 bird strikes were reported to the FAA in 2015; if only 20% are reported (Khan, Kapania & Johnson) we'd need more than ten billion hours of drone flight before we'd expect to see a single collision, if birds and drones flew similarly. $\endgroup$ – Charles Oct 31 '17 at 18:09

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