Hello and welcome to the next entry in our series on micro gas turbines! If you’re new to this series, be sure to check out our earlier blog where we: introduce the concept of the micro gas turbine; look into the history of it; and discuss some advantages and disadvantages that come with this technology.
This time, we’ll be looking at micro gas turbines in the Aviation industry (if you couldn’t guess by the title). Believe it or not, the concept and configuration of a micro gas turbine has been present in this industry for decades. We’ll get into that in a minute.
Gas turbines are certainly no stranger to the aviation industry. As a matter of fact, when many of us hear the term “gas turbine” we immediately jump to the image of a jet engine powering a massive airliner carrying us to our next adventure.
Yes, these mighty turbines are indeed a staple in the aerospace industry. But did you know that micro gas turbines are also making a rise in this industry?
Although micro gas turbines first made an appearance as an alternative to traditional piston engines in the automotive industry, they have actually been present in the aviation industry for some time.
On a vast majority of the commercial aircraft flying today, a ‘scaled down’ gas turbine can be found in the form of the auxiliary power unit, or APU. This machine is typically a small turboshaft engine that is utilized to provide on-board power and HVAC when the main propulsion engines are not running. Additionally, the bleed air and power from the APU is typically used to start one of the main engines for the plane, after which the other engine can be started with the bleed air and power from the other engine. Once the main engines are running, the APU can then be shut off since the main engines provide all the power needed for on-board HVAC and electricity. These engines are typically hidden in the tail of the plane and provide no thrust or propulsion power.
Want to tell if your plane has an APU? It’s quite easy, just look at the tail of the plane for the small exhaust port, and use your ears when you board the plane and see if you can hear what sounds like a small jet engine running in the background; it’ll be quieter than the main propulsion engines.
The first uses of micro gas turbines in aerospace were in remote controlled airplane models, primarily in the recreational market with model airplane enthusiasts. At the time, this got the job done so to speak, however the parts used weren’t designed for this purpose. In fact, they were often recovered components from automotive turbochargers that were reconfigured to be the primary driver for the models. From there, the use for UAV/Small aircraft propulsion expanded past the recreational consumer market into the military industry. As a result, a fair number of development projects are classified. However, with increased R&D into hybrid propulsion systems for airplanes both large and small, we expect to see advancements in UAV applications in the future.
So how are microturbines being implemented into aircraft in new ways? There are actually a variety of different configurations and setups for how they can be used to propel aircraft through the skies; have a look at the diagram below.
As you can see by the diagram, there’s no shortage of different ways that microturbines can be used in an aircraft outside of their role as an APU.
An example of this can be found in electric propulsion aircraft. There are, however, various challenges that need to be accounted for, such as power, weight, and reliability which are required for a successful flight. Currently lithium, which is used for making the batteries in electrically propelled cars, can be expensive to produce.
Keep in mind too, that lithium batteries get heavy when you put them in packs. For example, Tesla Automotive’ s famed Model 3, a 4 door electrically powered sedan, can weigh up to 4,100 pounds! Meanwhile, a fossil fuel-powered Mazda 3 of comparable dimensions, weighs just 3,255 pounds fully loaded. A Ford Escape, Ford’s compact crossover, weights just 3,500 pounds fully loaded. No, this blog isn’t about car weight, but it does a good job of showing just how much weight can be added to a vehicle by utilizing battery packs for propulsion. And while weight matters when you’re trying to make an efficient and nimble car, it really matters when you are trying to make a plane with enough power to get off the ground!
Conversely, microturbines are relatively light weight, and can provide more power and range with a comparatively lighter load for the aircraft than hundreds of batteries in packs spread throughout the plane.
As far as range-extending technology goes, there are advantages and disadvantages to using micro gas turbines in aircraft, some of them include:
- Turbines present a high power-to-weight ratio.
- A significantly increased range vs. pure-electric vehicles.
- Fewer moving parts vs. a piston driven range extender.
- Significantly lower emissions vs. standard aircraft powerplants.
- Retooling for machinery is not as challenging as it would be for, say, the automotive industry.
- Manufacturing cost – turbine component metals are expensive.
- Manufacturing complexity – small turbine parts are extremely difficult to make.
- Microturbines are not efficient – they have high losses and need recovery systems to negate the losses such as a recuperator.
- More moving parts than a purely electrical powerplant, leading to more potential failure points.
There is, however, one more configuration that is important to think about and consider, and that is having the micro gas turbine be the primary driver. I already mentioned that they were used in small unmanned aerial drones for the military as well as RC planes in the recreational market, but what about as the primary mover in other applications?
Well, when it goes toe-to-toe with an electric propulsion system (or a hybrid system) or a traditional reciprocating engine, some serious fundamental issues come up. We’ve already discussed efficiency problems when running a turbine and manufacturing costs, but that’s not where the disadvantages end when a microturbine is your primary mover.
Microturbines have a very, very narrow power band, with very poor low-end torque and poor throttle response, making it undesirable for propelling any kind of large airplane. Low end torque is a problem among all gas turbines. For example, when you are in a commercial airliner on the tarmac, the jet engines may need to rev up quite a bit, just to get the airplane to start rolling and begin taxiing. Keep in mind this is in a large engine application; it’s exacerbated in small power plants. As a result, they are not incredibly versatile, or powerful outside of one specific operating condition. Piston driven engines, on the other hand, have very good torque delivery when used as the primary mover, and electric motors deliver all of their torque at any operating speed, giving them a massive advantage over micro gas turbines as primary movers in aircraft and other vehicles.
We’ve only just scratched the surface of microturbines in aerospace applications and there are companies all over the world including SoftInWay Inc. working hard to discover where improvements can be made to air travel with turbomachinery and microturbines.
That’s all for now, but we’ll be back soon with an entry on microturbines and their potential roles in the automotive world!
If you have a project you’d like to discuss with us, or are looking for an easy to use solution for your turbomachinery project, drop us a line, we’d be happy to discuss the best ways of reaching your goals! Reach out to us at firstname.lastname@example.org, or leave a comment below!