Recently, I have started my graduation internship at the
Netherlands Aerospace Centre (NLR). During my internship I am trying to map the
severity of erosion on a helicopter engine. In this article I will briefly
explain why erosion is funest for gas turbine engines, especially for
helicopters.
First let me start off by giving a definition on erosion
from found literature: Finnie, Wolak and Kabil defined erosion as “The removal
of material from a solid surface by the action of impinging solid or liquid
particles” [Source 1]. Sundararajan and Roy defined erosion as “the removal of
material from component surfaces due to impact of hard particles travelling at
substantial velocities” [Source 2], which is a more applied definition to
erosion. Erosion is a broad term which
also is used in the geology. The picture below is known as Árbol
de Piedra, an eroded rock formation in Bolivia.
The rock is eroded due to grains of sand that have been
carried away by the wind and impacted into this rock formation. One grain of
sand does not make a difference, but if this happens often enough, one could
see the effect: erosion.
This is also a problem for gas turbines, especially the ones
operating in sandy environments. Sand that has been ingested in the engines
will pass all the blades and vanes inside the gas turbine and will erode them on
long term. Especially helicopter engines suffer even more from this erosive
effect due to blown up sand due to the downwash of the rotors, also called a ‘brown-out’.
The picture below shows a V-22 Osprey demonstrating a brown-out landing.
Brown-out landings are mostly notorious for the hazard of
the loss of situational awareness of the pilot. An example is the crash of a
Sikorsky MH-60K Black Hawk of the US Army in Afghanistan, 2001 [Source 3]. The
secondary effect of a brown out is excessive sand ingestion in the engine, with
more erosion as a result.
What is the effect of erosion on gas turbine components? The
late Professor Widen Tabakoff did extensive research into erosion and the
effects on gas turbines. He described in his researches, along with other researchers
that for the compressor, erosion resulted in (see also the illustration on the right):- · Blunted leading edges (1);
- · Sharpened trailing edges (2);
- · Reduced blade chords (3);
- · Increased pressure surface roughness (4).
What is the effect of these four erosion results? Overall,
the efficiency of the compressor will degrade. What is the effect of compressor
efficiency degradation? Higher fuel flows. To explain this, let me start by
explaining how a turboshaft engine works. The illustration below is a basic
schematic of a turboshaft, a gas turbine engine type applied on helicopters. The
green part is designed to deliver a high energetic gas to the purple part, the
free power turbine (3). The Free Power Turbine extracts the energy from the gas
and transfers it to a power shaft which is coupled by gearboxes to the rotors
of the helicopter. The engine is designed in such a way that the Free Power
Turbine always delivers the same amount of work on the Power Shaft.
If it happens, due to erosion, that the compressor
efficiency drops, it means that the compressor (1) requires more energy which
is ‘harvested’ from the compressor turbine (2). This requires more energy from
the compressor turbine and, compared to a more efficient engine, would deliver
less energetic gas to the Free Power Turbine. To keep delivering a constant
amount of work to the Power Shaft, the Free Power Turbine requires more
energetic gas and the engine will adjust this by injecting more fuel. More fuel
usage means less range or bringing along more fuel which means more costs. Either
way, no positive effect for the operator but no hazardous conditions.
A more hazardous effect is the potential of an engine surge,
or compressor stall. A compressor is designed to do a very unnatural thing:
moving air from low pressure to high pressure. To achieve this, the compressor
is designed very precisely. When the compressor blades are off-design, for
instance as result of erosion, disruption of air could occur within the engine
and the high pressurized air will flow out of the front of the engine: an
engine surge.
This video [link] is an example of what happens during an engine
surge. The GE90 engine was mounted on the first 747 during its test program. You
could see (and hear) a violent bang with flames coming out of the front of the
engine, which is the high energetic gas mixture from the combustion chamber. An
engine surge damages the compressor section and will result in an engine
shutdown. For a Boeing 747 with three other fine working engines this does not
have to end catastrophically, but for a low-flying helicopter it certainly
could.
To summarize: brown-outs cause excessive erosion to
helicopter engines. Blunted leading edges, sharpened trailing edges, reduced
blade chords and increased pressure surface roughness is the effect of erosion.
This results in higher fuel usage due to compressor efficiency detoriation and
could lead to the potential of engine surges which could end catastrophically
to a low-flying helicopter.
Sources:
[1] Finnie,
I., Wolak, J., & Kabil, Y. (1967). Erosion of Materials by Solid Particles.
Journal of Materials, Vol. 2, No. 3, 682-700.
[2] Sundararajan,
G., & Manish, R. (1997). Solid particle erosion behaviour of metallic
materials at room and elevated temperatures. Tribology International Vol.
30, No. 5, 339-359.
[4] Hamed,
A., & Tabakoff, W. (2006). Erosion and Deposition in Turbomachinery. Journal
of Propulsion and Power, 350-360.
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