Turbine engine limits vs. ambient conditions?

[Stuck_in_an_ATR]

Hi!

As I understand, most helicopter turbine engines have 3 limits to observe: TQ, TOT and Ng. TQ will be reached first in low ambient temp/low press alt. TOT => high temp/high alt. So far, I am correct, I hope... :}

My question - How about Ng? What ambient conditions will cause the Ng limit to be reached first and why?

Also (sort of a bonus question) - why does high humidity affect engine performance?

Cheers!

S.

[212man]

Ng limiting is reached with increasing Density Altitude. Typically smaller engines are affected at lower altitude as they spin faster e.g a Makila operates around 30,000 rpm whereas an Arriel runs around 50,000 rpm. The reason is as the air becomes less dense the turbine has to spin faster to achieve the same mass air flow. If fuel consumption is an issue then achieving Ng limiting is the aim of the game as the engine is then operating closer to its Design Point. One type I've flown would Ng limit at around 5000 ft and having taken off with full tanks and 3:00 hours endurance you could level off at 9000 ft with 3:30 endurance showing! :ok:

Humidity decreases engine performance for two reasons - one the water vapour is less dense than air so effectively you are increasing the DA, secondly it reduces the oxygen content available for combustion. Well, that's my understanding anyway. The losses are quite small though - of the order of 1-2.5% depending on the humidity.

[nodrama]

Humidity decreases engine performance for two reasons - one the water vapour is less dense than air so effectively you are increasing the DA, secondly it reduces the oxygen content available for combustion. Well, that's my understanding anyway. The losses are quite small though - of the order of 1-2.5% depending on the humidity.

and just to confuse matters....

not a technology that is used much in modern gas turbine engines nowadays, but in high ambient temperatures, adding water to engine intake air (water injection) can actually improve engine performance in two ways:

The water cools the air (latent heat transfer during vaporisation) and increases the air mass flow (increases the intake air density).

Cools the combustion gases, allowing more fuel to be added.

[JimEli]

Ng is limited to prevent stall (conditions of high high Ng/Sqrt Theta) or excessive compressor blade loads (high RPM).

Typically the torque limit is not specifically an engine limit but rather a transmission limit. Installations which incorporate sufficiently robust transmissions may limit power through collective stop or pitch.

Also consider an insidious limit could actually be maximum fuel flow, i.e. maximum power is attained without reaching an apparent limit on a gauge (Ng , TGT or Q).

[Brian Abraham]

My question - How about Ng? What ambient conditions will cause the Ng limit to be reached first and why?
Flying the 76C in our operation Ng was the usual limiting factor. Once had a particularly good engine on one side which would hit the Tq limit first, while the other was still the usual Ng limited. Talk about crossed eyeballs. Some latest helos have a single gauge with inputs from the three systems (Tq, Ng, TOT) and shows how your doing regarding limits without showing which is the limiting factor. Only one needle to watch, not three.

[Frenchrotorhead]

Yes, the FLI, First Limit Indicator, great in mountainous areas. Thank you Eurocopter !

[Shawn Coyle]

Humidity is supposed to be taken into account in civil helicopter performance. The only place I've ever seen anything to do with humidity was in Russian flight manuals, where there was a small chart at high temperatures and high humidity to show the change in performance. Can't remember exactly what the changes were, but they must have been measurable.
Maximum difference that is normally quoted is that high temperatures and high humidity can make up to 1,000' increase in density altitude. That affects power required to hover, but I'm not sure what the effect on power available from a turbine engine will be as that has some interesting variables. Piston engines depend on density altitude alone for power available.

[JimEli]

Some additional information on humidity here:

http://www.pprune.org/rotorheads/318977-effect-humidity-turbine-engine-perf.html (http://www.pprune.org/rotorheads/318977-effect-humidity-turbine-engine-perf.html)

[Stuck_in_an_ATR]

Gentlemen, thanks for great input :ok: To recap, I understand that both Ng and TOT have to rise with Alt/OAT to give the same power output and that first limit to be reached depends on engine type and also particular unit (compressor/turbine degradation, etc.). Is that correct?

@JimEli: Could you provide (or point me towards) any more info concerning engine stall at high Ng? I always thought that Eng stall is more associated with low power settings and/or sudden power changes...

Cheers!

[212man]

Stuck in, Jim is referring to compressor surge - I guess you are familiar with surge lines and margins? Helicopter engines follow the same principles as all gas turbines - there are no peculiarities other than being connected to a gearbox.

[HeliComparator]

In the types I have flown (AS / EC) , you should always reach a Tq or N1 limit first (depending on density altitude). You should not reach a TOT limit first unless the engine is dirty or has some defect such as an air leak. Of course this is achieved by scheduling the N1 limit with density altitude so as to reach that limit first, before TOT. With a hydromechanical FCU its quite easy to limit power based on N1, harder to do it on Tq and much harder on TOT. One of the advantages of this approach is that you can "pull to the droop" in the event of an engine failure on a multi, without having to worry too much about the TOT.

Limits could be based on TOT in a FADEC'ed engine, but the integrity of TOT measurement is typically less robust than Tq, and much less robust than N1, since a number of sensors in series is normally used, with associated connectors and wiring in a hot and dirty part of the engine. I think that's why the manufacturers prefer not to use TOT as a primary limit, instead using eg N1 and a model of the engine's behaviour at differing density altitudes to keep TOT within safe limits.

Just to disagree slightly with JimEli (even though he probably knows more than me!) surely stalling is a feature of low N1, not high N1? I would say N1 is limited to prevent excessive blade loads, but primarily to limit TOT by association.

Regarding the point about insidious fuel flow limit, I had that on a 332L, couldn't do the Max Ng check as the engine would only run a little above normal AEO power - turned out that the internal fuel piping was choked with carbon!

HC

[212man]

You should not reach a TOT limit first unless the engine is dirty or has some defect such as an air leak.

Or it's a 76A (Allison engined) in which case hitting the T5 limits is pretty normal :ok:

hydromechanical FCU its quite easy to limit power based on N1, harder to do it on Tq

Bell manage it successfully with their wonderful torque limiting system in the 212 AFCU. Governor reset air (Pg) is bled off to limit further engine acceleration once 104.3% Tq is reached, to save the MGB and trash the aircraft :ugh: I think Shawn has some views on that system!

Just to disagree slightly with JimEli (even though he probably knows more than me!) surely stalling is a feature of low N1, not high N1? I would say N1 is limited to prevent excessive blade loads

As I said above, I think he's referring to crossing the surge line at high Ng which results in the airflow being choked. However, normal scheduling would not allow the Ng to go past the surge margin (the buffer that is set before the surge line,) so it would not be an issue. That said, I believe that structural loads are more of a consideration when the limits are set.

[Brian Abraham]

Looked up an old piston flight manual for the Wright Cyclone and had this to say with respect to humidity. Produces 1,425 HP in standard conditions.

Assume OAT 32°C, relative humidity 70%, 6.3% loss of power due to humidity (90 HP), 2.8% loss due to temperature (40 HP), total loss (9.1%) 130 HP.

With respect to gas turbines I found the following in some training notes.

The reason for not considering humidity in thrust calculations is that 65% to 75% of the mass airflow through a gas turbine engine is used for cooling the combustion mixture. The presence of moisture suspended in the atmosphere has a negligible effect on either the cooling process or the remaining 25% to 35% of mass airflow used for combustion. This situation holds true for all types of gas turbines.

[marchse]

Ventilation or cooling methods are the most essential one when efficiency comes into picture...

Power Plant Development (http://www.sterling-energy.com)

[blackhand]

A mechanics thoughts:
Jet Ranger with RR250C20 will Temp out before it reaches max torque, so is viewed as temp limited engine.
Long Ranger with C30P reaches Max torque before it reaches max temp, so is viewed as torque limited.
Bell 407 with FADEC47B will reach max temp before max torque - in fact has a mod for aural warning on temp limits.
This is when operating at altitude, if you do your power checks at altitude, say around 7 - 8 000 feet you should see it.

These factors are a great troubleshooting tool.

N1 or NG fuel topping, when that is it, there is no more. Is an indication of the engines health. If you reach Max N1 (topping speed) before you make tabulated torque there is a problem.

Always learning - your thoughts?

Cheers
BH

[John Eacott]

Jet Ranger with RR250C20 will Temp out before it reaches max torque, so is viewed as temp limited engine.

blackhand, I would preface that with a big "some C20's", as it depends on the engine and the conditions! Moderate temperatures and altitudes, my 206BIIIs would be torque limited (both max continuous Tq and TO Tq), but give it a hot and/or high day then the TO Tq may be temp limited. Other machines I've flown were, as you said, TOT limited in all conditions: so it depends on the state of the engine. A C30 in an S76 was always T5 limited, but the same engine in a LongRanger would be Tq limited.

The LTS101 could be Ng limited in some conditions; not for cruise power, but operating at altitude in hot conditions the Ng could top before temp or Tq limits are reached. Dirty bleed bands would contribute, of course!

[blackhand]

A C30 in an S76 was always T5 limited, but the same engine in a LongRanger would be Tq limited.

John, is the because the different airframes have different torque limitations, that is the engine is capable of reaching max temp in a longranger - don't we know it!!!, but is limited by the transmission maximum torque?

BH

[JimEli]

@JimEli: Could you provide (or point me towards) any more info concerning engine stall at high Ng? I always thought that Eng stall is more associated with low power settings and/or sudden power changes...


It came from my notes. I suspect usntps document. I'll research.