Engine restart problems!

So, crossfeed OFF is the normal flight condition, right? I'm struggling to justify why that is though in the light of what crossfeed does beneficial when it's ON. Help me out here.

Actually, having one of your tanks leaking is exactly why you DO open the crossfeed valve. Also, when one tank gets the 100kg alarm, you should also open the crossfeed valve.

 

The tanks do not crossfeed into each other. The crossfeed valve only allows fuel from either tank to feed either engine. Even if the crossfeed valve is open, fuel will not transfer from one tank to the other. The APU and the right engine feed from the aft tank. If the crossfeed valve is closed and your aft tank is empty, you cannot operate the APU or the right engine.

This pretty much explained why...and a little more detailed info.

Thanks for that AOS.

The DSC manual say that the front tank supplies fuel to the port engine and the rear tank supplies the starboard engine and APU. page 1-4

So, crossfeed OFF is the normal flight condition, right? I'm struggling to justify why that is though in the light of what crossfeed does beneficial when it's ON. Help me out here.

My guess, the main purpose of separating the fuel tank like that is the KA-50 balance. I do not work on helicopters, but in aircraft, fuel weight is a big part of balancing the aircraft. The F-16 for example, does not have a direct X-feed valve switch. It does have the valves but basically you tell the system if you want to use forward/right, Aft/left or normal fuel(A/L and F/R equally to try and maintain balance). The aircraft normally has more fuel on the front (C models) and to maintain proper balance you want the front to be heavier (in the F-16)

So my guess, the x-feed valve is normally kept close to maintain balance of the KA-50.

taken from FAA/library

http://www.faa.gov/library/manuals/aircraft/media/FAA-H-8083-1A.pdf

Helicopters are affected by lateral imbalance more than

airplanes. If a helicopter is loaded with heavy occupants

and fuel on the same side, it could be out of balance

enough to make it unsafe to fy. It is also possible that if

external loads are carried in such a position to require large

lateral displacement of the cyclic control to maintain level

fight, the fore-and-aft cyclic control effectiveness will be

limited.

Left external tanks feed the aft tank, right external tanks feed the forward tank. The switch in the cockpit turns on inner or outer tanks, you can't pick left/right individually. So if you have two external tanks, one will feed each tank.

Ok maybe im misunderstanding it but then why are there 4 pump switches labled forward/aft/left/right?

Ok maybe im misunderstanding it but then why are there 4 pump switches labled forward/aft/left/right?

There are not 4 pump switches labeled forward/aft/left/right.

There are 4 pump switches labeled forward/aft/inner/outer.

OK, so has the topic question been answered already? What's the reason behind original poster's problem to restart engine?

Hard to say unless he makes a second visit.

A16, any chance you could explain why crossfeed-off is SOP?

The x-feed valve on every multi-engine aircraft being fixed or rotary wing is normally closed. It's purpose is to supply fuel from one tank (or group of tanks) to all engines or fuel from all tanks to one engine. I.e. in BS you open it when you run low on fuel or you have an engine out.

As to what the user's problem is - we can only guess until he provides a track. One possible reason is when you hit the Start button while the compressor is still spooling down - especially on the ground, in flight you'are allowed to start at up to 7% compressor RPM at windmilling. That will cause the starter's driveshaft to shear and it's modeled in BS.

What starter's drive shaft? Iren't engines started by compressed air from APU?

http://www.digitalcombatsimulator.com/img/technology/HelModelDesc_02.gif

The x-feed valve on every multi-engine aircraft being fixed or rotary wing is normally closed.

What's the time frame on the compressor assembly spinning down below 7%? Is there any instrumentation that can help?

P.S. I think he means that by the "starter driveshaft" that the "drive shaft of the engine being started"?

There is a small turbine fan in the starter. When the starter operates, air from the APU blows through this turbine, forcing it to turn. There is a very small shaft that conects this turbine to the accessory gearbox on the engine. Normally, the compressor section runs the accessory gearbox, but during engine start, it's the other way around...the accessory gearbox (itself being powered by the starter) spins up the compressor section. The drive shaft mentioned by AirTito is the small driveshaft inside the starter that connects the air-driven portion of the starter to the accessory gearbox of the engine.

The speed of the compressor is your N1 RPM. In the manual, it's called Ngg, for gas generator turbine speed, but since the gas generator turbine is physically connected to the compressor turbine, they have no choice but to spin at the same speed. In fact, the N1 tachometer-generator (the device that sends the RPM signal to the gauge in the cockpit) is attached to the engine's accessory gearbox, which is driven by the compressor section.

The crosfeed valve is normally closed in case there is a problem downstream of the fuel tanks. Here is one scenario: Let's say you take battle damage to one of your engines, and it starts leaking fuel. First off, it will probably catch on fire, but for the sake of this example, let's assume that the fire is put out by the extinguishing system, and you quickly turn off the fuel shutoff valve and the fuel cutoff lever for the affected engine. If your crosfeed was OPEN, then your other tank would still be pumping fuel all over the engine deck of the damaged engine.

The only time you want your crossfeed valve open is if you take battle damage to one of the actual fuel tanks, or one of your fuel tanks is close to empty.

TV3-117VM air starter:

Ah, so the shaft mentioned by AirTito is after the APU's bleed air is again transferred back into a mechanical form. The bleed air itself doesn't start the engine moving directly, how could we know.

On yet another side rail you say that the RPM gauge in the cockpit is driven by the compressor/compressor-turbine side and not the free-turbine/gearbox/rotor side? Seems a bit strange since the generators and rotors are what you're usually interested in.

As to what the user's problem is - we can only guess until he provides a track.

Is this really necessary? Problem is easily reproducible: start engine; once it is started, immediately cutoff the fuel; wait until it spools down; try to start it again. Although EGT will rise slightly, RPM needle won't move at all.

AirTito, A61

Thank for the info, it makes since to me:thumbup:

What starter's drive shaft? Iren't engines started by compressed air from APU?

http://www.digitalcombatsimulator.com/img/technology/HelModelDesc_02.gif

Using this diagram you can see APU does not provide air directly to engine

13-5: Engines and power train

1. Right engine impingement starter

2. Right TV3-117VMA engine

3. Main gearbox

4. Left TV3-117VMA engine

5. Left engine impingement starter

6. Intermediate gearbox

7. APU

8. APU bleed air pipeline

9. Right generator

10. Turbine drive

On another note:

The only time this occurs is if you wait less than one minute after start to shut down. Is the only time I was able to duplicate this error. Even shooting the engine down from operational/auto throttle setting did not have ill effects unless I waited less than one minute to shut down.

Edit: Already said in post one, Douh!

On yet another side rail you say that the RPM gauge in the cockpit is driven by the compressor/compressor-turbine side and not the free-turbine/gearbox/rotor side? Seems a bit strange since the generators and rotors are what you're usually interested in.

The rotor/gearbox has it's own tachometer-generator. It's attached to the main gearbox and it sends its signal to the rotor rpm gauge.

There is no gauge for the free turbines on the engines, but they do have overspeed protection (FT/PT 1 and 2). If your free turbine overspeeds to 118%, it will shut the engine down, assuming your governors are on.

It seems like it's longer than 1 minute. I'm now investigating it deeper but it seems to me that we have a spool down time bug. This is exactly starter driveshaft shear but it shouldn't be happening when the compressor RPM reads zero.

I'll bring that to the devs. Thanks for the finding, good catch.

Could it be a bug related to the rotor brake?

Just guessing, based on that this is exactly the same behaviour that when you try to start engines with rotor brake set.

No, when the rotor brake is set, pressing the start button doesn't do anything at all, the start sequence never starts, you never get an EGT rise, and the starter light doesn't come on. As soon as you turn off the rotor brake, it works. There is a switch which prevents engine start with the rotor brake on.

With this problem, the start sequence DOES start, you just never get a rise in N1.

Ok :book:

It seems like it's longer than 1 minute. I'm now investigating it deeper but it seems to me that we have a spool down time bug. This is exactly starter driveshaft shear but it shouldn't be happening when the compressor RPM reads zero.

 

I'll bring that to the devs. Thanks for the finding, good catch.

Well, that's what i was trying to tell you all about, and this, that you may have found as well!:D

Making the most realistic sim in the world requires time and sweat to clarify all these things..., but everything worths it in the end!:D

Thank you!

I brought this engine start (re-start) problem up a long time ago, I am glad it has been officially noted..

:thumbup: