Engine Starter

[Caldera]

Hey All,

Rooky question here...

What actually is used to start the AH-64 engines.  Is it an electric or a pneumatic starter?

I notice that I can start both engines simultaneously on the APU or on ground electric power.  Seems like allot of load either way, but it does work.

Caldera

[Grennymaster]

after a quick overread in the Flight manual it seems to be electric. But im not 100% sure not an native english man here

also considre that the Module is in early access, it mabye changing in the future.

hope there is a real mechanic here that could give us a bit more info always glad to learn more about this Bird
 

[Raptor9]

The engine starters are pneumatically powered using pressurized air from the APU (or a ground power unit called an AGPU, which is just a power cart with an AH-64 APU in it). Technically, the engine starters are intended to be engaged one at a time since the APU is supposed to only have enough pressurization to reliably start one engine at a time. However, I know at least one pilot that decided to test this theory and started both engines once. It apparently worked, but I don't know how quickly the engines accelerated or if the engines were closer to hot starting because of it, nor do I know what environmental conditions existed at the time this occured (which plays a big factor as well).

Edited May 31, 2022 by Raptor9

[Caldera]

Raptor,

Thanks!  That makes more sense to me.  From my testing in DCS, it appears that a two engine start is identical to a single engine start as far as spin up rate, but I have not timed them.  Overall it is faster.

More questions I have been wondering about if you have a minute.

 

Are there separate electrically (pneumatic?) and mechanically driven engine oil pumps?

I notice that on an initial cold start, that in DCS, the engine oil pressure rises rapidly into the green.  This occurs really prior to spin up.  This indicates to me that there is a non engine powered oil pump in use.  After an in mission repair and engine re-start the oil pressure rise seems to mimic the rotor speed rise.  This indicates that an engine driven oil pump is being used.  I do not know how closely this relates to the real world.

 

You mentioned a hot start, are you referring to an inflight or ground start?

I am clueless as to how well the rotor rotation can be used to re-start or if the APU must be used.  Or if it can even be done as I have not tried as of yet...

 

Is there a requirement for a purge prior to a ground hot start?

In my experience with natural gas fueled aeroderivative engines in power generation, hot starts can sometimes be unreliable.  I believe that the DCS A-10C module requires purge and some cool down for a successful ground hot start.  That is my only experience.

Caldera

[Leg2ion]

Hi @Caldera . I think you may be getting some of your terminology mixed. When a 'hot start' is referred to it means just that - in that the temperatures that are monitored - normally at the turbine end of the engine - are too hot and have been exceeded on start up - hot start. Normally occurs when the engine power lever is advanced too quickly or too much load/power is demanded at start up before the engine is working correctly, and caused by insufficient airflow through the engine before a large amount of fuel is dumped into the combustor can. Once the engine is up and running properly bleed air is used to maintain/shape the flame in the combustor can to provide clean gas flow into the turbines to drive them, and then also provide cooling air across the turbine discs. IRL a hot start can result in something simple as a few boroscope inspections in the can and on the discs, or dependent on temperature reached engine rejection.

As to your questions: 

Oil pumps - engine driven. Overall fairly low pressure so a quick rise is to be expected. In older aircraft types you would expect to see the pressure rise higher than normal (into the yellow) then as the pressure relief valve in the system kicks in start self regulating and come back down to the green. In terms of mimicking the rotor speed rise - assume you are monitoring No2 engine - so there is a direct relation between engine acceleration/speed (and subsequent drive to the engine mounted oil pump) and Nr.

Rotor rotation - shouldn't be able to be used to restart an engine as the freewheels should decouple the rotors (spinning faster) from the engines (spinning slower). Normally either an APU is used to go through a 'standard' engine start or there is an ability to use bleed air off the one good engine if there is one. 

Purging - also called venting. If you have had a real hot start (but within limits allowing normal ops to continue) or aborted start for whatever reason then (dependent on engine manufacturer requirements) a shut down is required, sometimes a cooling off period of time, and sometimes a purge - so run/vent the engines for 30 seconds with fuel/ignition turned off as an example.

Looking at some of the other questions/comments I believe that both the GE T700 (US) and the RR RTM322 (UK) are spun up with a pneumatic air starter powered by bleed air off the APU.

 

[Raptor9]

@Caldera, everything Leg2ion said.

Regarding the indications during start. Take everything with a grain of salt since it is all still early in EA and everything is WIP. (This broken record has been brought to you by Raptor9 )

[Caldera]

Legion,

OK Thanks! 

A hot start to me is (was) starting an engine that is hot.  Raptor was also kind enough to explain my initial question (APU and GAPU).

I now understand that the oil pumps are engine driven.  That is what I had thought they were, except for the wacky oil pressure indication on the initial cold start threw me off.  Like I said above, that would indicate oil pumps driven by another means.

EA / WIP --> Copy

 

The rotor bearings themselves are very likely to be roller bearings then? 

My experience is that roller bearings generally have drain cavities with overflows.  The cavity always stays full even with the pumps off and bearing is always at least partially submerged in oil.  This design allows safe roll down with the loss of oil pressure.  In effect, the oil pumps do not supply oil pressure for lubrication.  But function primarily to supply cooled oil to bearing so that the oil in the cavity does not coke up and cause the bearing to seize.  The oil itself is a high temp synthetic as the bearings are in close proximity to high temperature compression / combustion.

 

Thanks again!

Caldera

Edited June 1, 2022 by Caldera