validator

Hello guys,

I have a question that has been bugging me for some time now, but i couldnt find a satisfying answer.

What are the adventages and disadventages of using an axial or radial air intake on a turboshaft engine?

Seeing that arriel series uses an axial air intake, where ardiden uses a radial air intake, made me wonder, which one of these intakes is better, and why?

Do you have any ideas looking from a rotorhead point of view? (for better noise absorbtion? for better low speed mass flow, where the probability of an engine failure is less? is it easier to filter air? or is it completely for aerodynamic considerations of the compressor? etc)

Thank you =)

Dagw00d

Arriel - Single stage Axial and single stage Centrifugal
Ardiden - Two stage Centrifugal

The Ardiden is a simpler and more robust design and meant for much larger A/C applications.

Both are very reliable and will withstand more ingress than engines with multi stage axial compressors.

Gemini Twin

Doesn't really matter as long as you design the inlet plenum correctly. Axial inlets usually take more room and must by designed to ensure that the airflow is smooth and evenly spread around the inlet area. An inlet distortion survey must be performed to ensure that the in coming air pressure and temperatures are evenly distributed. Ram effect is not so important as max flow is usually in the hover. Radial inlets are more compact and not so critical with respect to inlet distortion. They also allow the opportunity for "handed" inlets.
The design of the compressor also effects the inlet plenum design, axial compressors can be greatly effected by uneven inlet air flow.

Dennis Kenyon

I appreciate this may be a slight drift, but I've been asked to produce an article on 'Compressor Stall/surge' which is very much associated with engine intakes. There have been some well reported accidents (B407 - B206 OH) which the NTSB have put down to compressor stall. Any pilots on here had actual experience? Thanks Dennis Kenyon.

Dennis Kenyon

PS ... you won't be quoted by name. DRK

Gemini Twin

Yes, very much associated with engine inlets but not limited to just the inlet. We have during tests experienced compressor stalls due to faulty exhaust stack design (causing a little to much back pressure), faulty flow fences and bleed valves. Incorporating an inlet barrier filter usually smooth things out and testing produces good inlet disortion results.
If the inlet design passes the test for surge, stall charcteristics, stalls in service are usually realted to dirty compressor, flow fence / bleed valve problems or sometimes a faulty FCU.

ATN

Does anyone have an idea of the quantity of air sucked in by the engines of a medium twin, let's say on the ground at ground idle, flight idle, and in hover ?
Sorry for highjacking the thread.
Cheers

ATN

Gemini Twin

I can give you one example:
Sea level 59F shp 732 5.23 lb/sec.
59F shp 500 4.66 lb/sec.
59F shp 300 3.92 lb/sec.
Not sure at lower power, never worry too much at flight or ground idle.
Flows are uselly quite close from make to make, depends on the efficiency of the design.

riff_raff

validator- This is an excellent question.

First, as others pointed out, there is a fundamental difference between the type of low pressure compressor used on the two engine examples. The Arriel uses an axial type and the Ardiden uses a centrifugal type. The current design trend in turboshaft engines (up to about 2500hp) is to use centrifugal compressors, since they give a better combination of packaging, pressure ratio, and efficiency. Centrifugal compressors are also somewhat less sensitive to the changes in flow velocity/direction that naturally occur in a radial inlet duct.

However, there are also other considerations when selecting the type of inlet duct shape. The Ardiden design does not allow for anything other than a radial inlet. Then there are other engines like the PT6 that locate the compressor inlet facing opposite the PTO end, with the PTO shaft exiting through the center of the turbine outlet. The reason for this is that given the small cross section, long length, and high power turbine speed of the PT6, a PTO shaft running from the power turbine to the opposite end of the engine would be so long and slender that it would operate at super-critical conditions. A super-critical shaft would create all sorts of nasty problems, so with engines like the PT6 or TM Makila it was easier to run the PTO out the exhaust end.

With some military helicopter applications, there is also the need for an inlet particle separator system. These devices commonly turn the flow radial just ahead of the compressor face to mechanically separate the heavier solid particles and direct them into a discharge.

helicopterray

A centrifugal compressor has a higher compression ratio than an axial.

A number of engines, RR250-C20 series, Arriel series, have an axial at the initial inlet stage to provide an increased pressure at the inlet to the centrifugal compressor, and then the centrifugal takes that pressure much higher.
The reason for this is that these engines have a bleed valve, and too high an initial pressure will lead to a compressor stall. So the compression is done in stages.

The RR250-C30 in the Longranger and 250-C47 in the 407 have just a big centrifugal and no axial.
These engines do not have a bleed valve, but a bleed port instead.


Dennis,
I'm not a pilot, but have seen and fixed many compressor stall issues.

On the 250-C20 series, it was any number of issues. A sticky or slow bleed valve not reacting quickly enough. A worn compressor impeller, really dirty compressor, and once, really badly worn turbine blades.

The Arriel, usually a dirty bleed valve or plugged filter in the bleed valve.

ericferret

Badly erroded compressor case linings will also do the trick on an RR/Allison C20 series.

Dennis Kenyon

Thanks for all the info lads ... much appreciated and I have plagiarised some of your words for the upcoming article. Dennis K

RVDT

Compressor stalls.

A250's rarely stall in flight. Normally it is when accelerating off idle.

In a B206 it feels like the input pinion has stripped the gear teeth off!

In a H500 you wonder who the guy is with the semi-auto 50 cal in the back seat!

You should be able to slam the throttle open on a RR250 from idle as much as you dare without exceeding limits and the friction of the surface under the skids without it stalling. Allison Tech Rep recommendation to establish all is in order!

Stalls in a B205/UH-1 Lyco T53 are more interesting and normally associated with VIGV setting too wide. It can create havoc with the TR power train. Due to the sudden power surge on and off the TR drive train can overrun and damage the rear side of the gears in the 42 degree and TRGB amongst other parts that need to be inspected and/or thrown away.

205's/UH-1 can be a little discerning about large skidding pedal turns with the flow across the inlet and can hiccup as well.

Regardless of the type compressor stall/surge will get your attention!

I think this is from a simulator for AH64D -

riff_raff

Dennis- Compressor surge is basically the operating condition at which the compressor cannot impart enough energy to the fluid to overcome the system resistance or backpressure. In this regard, centrifugal compressors have an inherent advantage over axial compressors, since they can achieve much higher single stage pressure ratios. This is due to the way in which the compressor airfoil imparts momentum to the airflow. In short, the centrifugal compressor with its longer radial flow path performs this task more effectively than the axial compressor with its shorter axial flow path.

As for stall, the outboard tip of the leading edge of a centrifugal compressor runs at a lower velocity than that of an axial compressor. The outboard tips of an axial compressor airfoil leading edge tend to operate much closer to transonic.

Good luck with the article.

Ian Corrigible

In terms of compressor design, two main benefits of a dual-centrif (or axi-centrif) design are shorter overall length and better FOD/erosion characteristics. The 14-stage all-axi compressor on the V-22's powerplants have suffered badly from erosion during deployment, resulting in a frequent need for low-power repairs; the simpler axi-centrif design on the CH-53K's engine should - on paper - provide better robustness.

In terms of inlets, radial inlets tend to be easier to 'protect' than frontal intakes, resulting in far lower inlet barrier filter costs.

I/C

ericferret

BO 105

Both engines start normally at sea level a/c flies to landing site 6000' and shuts down.

Return trip engine one starts normally, hit starter button engine two and number one engine stalls. Pilot and engineer exchange puzzled looks and try again. Same result as soon as starter button for two pressed number one engine stalls. A cursory inspection due to location performed with no obvious defects.

Another attempt made this time starting two first then one. No problems!!!

Aircraft returns to sea level and the problem disappears and is put down as altitude related although no obvious answer.

A couple of weeks later the problem reaccurs this time at sea level.

Investigation shows that the number one compressor casing is very badly erroded.
The penny drops.

Due to disturbed airflow the stall margin on one is reduced.
Normally starting one first is on battery and when stabilised generator is switched on prior to starting number two. The additional load on the number one generator caused by starting two caused a drop in N1 sufficient to drag the compressor in to the reduced stall band and induce the stall. This explains why there was no problem starting two first as the compressor was in a far better condition.

Moral of the story, if operating in a sandy environment minus sand filters keep a good eye on you compressor case errosion. Also dont ignore P.A checks!!!!!!!!!

riff_raff

Compressor blade and case erosion are indeed a huge problem for any turboshaft engine operating in a desert environment, especially military vehicles. Military rotorcraft usually try to avoid using air filters since the reduction in installed power would not be acceptable, but they do use inlet particle separators like the one shown below (which appears to be a T700 series engine):



Regarding the issue of compressor blade/case erosion, the turboshaft engine facing the most difficult operating environment, by far, is the AGT1500 engine in the M1 tank. I've seen pictures of eroded AGT1500 compressor blades that were so "fish-hooked" it was hard to believe the engine would even run, let alone be able to fire up.

Interesting topic!

Squirrel

Had a compressor stall on a 350B3 in flight, very strong tail wind & updraft, vertically out of a mountain top site, very light AUW, relatively low power setting +25c. Can provide more detail if you are interested.

Dennis Kenyon

Thanks Riff Raff .... I think I've managed to produce something that will provoke further subject discussion and the article is now winging its way to the FLYER magazine's desk. (April edition)

I'd be pleased to hear of any technical errors I may have included. Hopefully none, but occasionally I do leave in a minor mistake, just to see if my readers are awake. Wake up at the back there will you!

Regards & thanks. Dennis K.

validator

Thanks for all the answers guys,

So can we say a radial inlet + 2 stage centrifugal compressor leads to a better particle seperation (with a particle seperator of course) and better stall/surge characteristics? (since centrifugal compressor is less dependent on uneven airflow and performs better with its longer radial flow path)

btw dennis i would really like to read that article but FLYER magazine is not published here. is there any way that i can reach it?