Jet engine versus internal combustion engine: helicopter edition

June 24, 2011 12:56 PM

How might an observant person be able to tell, without being inside the cockpit, whether a helicopter has a jet engine or an internal combustion engine? Difficulty: no markings on the exterior of the chopper.

(A follow-up to this question I asked a couple weeks ago.)

I'm trying to figure out ways someone from the ground could identify an unmarked helicopter flying overhead, as having a jet engine versus an internal combustion engine.

Are there any visual or audible external characteristics (such as rail guns, or a specific kind of propeller for example) that would signify to a helicopter expert what kind of engine the helicopter uses?

Thanks in advance for any help.

posted by np312 to Travel & Transportation (11 answers total)

Exactly the same way a car expert is going to be able to tell you about the engine of a particular car just by looking at it: models are distinctive. It needn't be anything inherent about the outfitting of internal combustion/turbine powerplants. Informed persons would know that a Bell 407 runs with this engine while a Sikorsky S-76 runs on that one. Or whatever. Just like someone who knows about BMWs will be able to tell you just by looking at it what's under the hood.

Again, it needn't involve any knowledge about flight characteristics or visual/aural cues as much as just being able to identify different chopper models on sight, by profile.
posted by valkyryn at 1:02 PM on June 24, 2011

Best answer: Today, all but the smallest helicopters (like the Robinson family) use turboshaft engines. It's been that way for a long while.

On preview, what valkyryn said: pilots know which aircraft is which on sight & sound. I've worked with helicopters for a few years and can identify a startling range of machines in the air. It might be difficult to pick out specific models or configurations (ie: the Eurocopter AStar/Squirrel line: could be a B1, B2, B3, BA, FX, FX2, SD, SD2, etc...they all look very similar if not identical), but airframes are easy.
posted by t_dubs at 1:09 PM on June 24, 2011

Best answer: As stated above, only small helicopters will have piston engines (which are more fuel efficient), larger ones will have the turbine/turboshaft engines (which are more powerful).
posted by HLD at 1:12 PM on June 24, 2011

Yeah, seconding there aren't little signs that tell you, mainly the make and style of Helicopter. Basically, at anything like the range that you can tell that a car is (for instance) a Pontiac or Ford, a Helicopter person can tell what Helicopter it is and so know which engine it is. There aren't really any physically similar helicopters with the two engine options in them to the level of sameness as you seem to be assuming.
posted by Brockles at 1:29 PM on June 24, 2011

Best answer: Just hearing the sound is probably enough.

Piston engines operate by means of a sustained series of physically constrained explosions.

Turbojet engines suck in air, mix it with fuel, and burn it continuously so that a shaft spins around.

I'm fairly sure that the sound is completely different. I'm pretty sure that different size engines sound different as well, as would different propeller configurations and sizes, and the machinery and hull enshrouding the engine will also change the sound signature significantly.
posted by krilli at 1:56 PM on June 24, 2011

Best answer: I was thinking the same thing about the exhaust noise, but I considered that maybe you'd have to be able to hear the exhaust to distinguish it reliably - so you'd have to hear it from at least the side or the rear (as it passes you maybe) as from the front the rotor noise would be most prevalent. If it passes you, you'll hear the turbine whistle rather than just the noise (or lack of, I guess) of a piston engine (which doesn't stand out much above the rotor noise).

Memory fails if this is correct, though, and I am fresh out of test 'copters flying past my house.
posted by Brockles at 2:20 PM on June 24, 2011

Best answer: One visual "clue" that is shared by most turbine engine designs, that is absent from all piston engine models that I can remember, is that nearly all turbine engine helicopters have large air intakes, and comparatively large exhaust shrouds near the upper part of their fuselage, just below the rotor. Military models of such helicopters, and some specialized civilian models, have extra baffling around the exhaust to mask the heat signature of the engine(s), and/or to quiet the exhaust noise.

Piston engine machines, being generally lower powered, and having higher engine vibration signatures to damp, usually mount their engines lower in the airframe, and don't have the large air intakes/exhaust shrouds of turbine engined machines, just below the rotor.
posted by paulsc at 3:08 PM on June 24, 2011

Just came in to say the exact same thing that paulsc mentioned.
posted by saladin at 3:10 PM on June 24, 2011

Best answer: Aside from the large air inlets, the turboshaft helicopters sound different from piston-driven ones. Aircraft piston engines produce a lot of power at low RPM, so you'll get a constant, low-pitched sound in addition to the sound of the rotating blades. Jet engines have a higher-pitched whine that is very distinctive.
posted by backseatpilot at 3:50 PM on June 24, 2011

Response by poster: You guys are awesome. Thank you!
posted by np312 at 10:11 PM on June 24, 2011

Best answer: I'd just add (as someone who works on both piston and turbine powered helos, at one of the aforementioned companies), the typical Jet-A fuel used with turbines has a pretty distinct odor from piston engine gasoline. If at an airport you've ever had to deplane onto the tarmac (without a jetbridge), you've smelt it.

Also, a jet/turbine/turboshaft is definitely a type of internal combustion engine. It's just not a reciprocating one.
posted by mnemonic at 11:36 PM on June 24, 2011