Q about EPR and taxiing on the ground

[Guest Ricardo Sevarant]

When using the RB211 engines on the cargo version, the N1 values respond instantly on the EICAS in response to subtle throttle changes, whilst the EPR readings seem to lag. It is therefore much easier to set taxi thrust via N1 rather than EPR. Is this consistent with real-world prodecure?Thank you.

[Tabs 2,790]

You'd have to ask one of the pilots, but my guess is that taxiing is something that's done more by feel than by setting a specific throttle setting using N1 or EPR.

[Guest Ricardo Sevarant]

I'm sure you're right about having a 'feel' in regards to most taxi power settings in real-life. The trick with FS2004 and PMDG is that you have to use considerably more power to taxi large jets in the sim than in real life, due to the 'wet cement effect' taxi bug with FS2004. I guess I need to rephrase the question: Is it normal for EPR reading to remain the same whilst N1 values change in response to subtle power setting changes?

[Guest D17S]

For taxing this big beast, just watch ground speed. Once you get it rolling, idle thrust is generally too much (at least at the GWs we mx dogs taxi with). Do not ride the brakes. If you are on a long taxiway, let it roll on up to 20 kts than bring it back to 15 with one brake application, then release. Repeat as necessary. Setting 'taxi thrust' just isn't the deal. Watch ground speed. EPR? Something in the back must have happened if the fan is now spinning faster. There was some pressure ratio adjustment back there somewhere. That

[cowpatz 243]

The EPR on the rollers is a fairly coarse power indication when compared to N1. Technically EPR it is supposed to be a more precise indication of thrust, however with a large bypass ratio engine that is debatable as the Fan provides most of the thrust. In real life there is a noticeable difference in residual thrust when taxiing. The rollers seem to need a bit more to keep rolling. I can recall, with the rollers we had on the 747-200, that they were a pain to start. They quite frequently needed the use of standby ignition to get going. A failure to light off is quite often the source of that foggy vapour that surrounds an engine from time to time....raw unburnt fuel.I agree with you Sam those JT9's were a shocker, primitive and dirty.Thrust setting is more by feel "thrust lever angle" than looking at an indication although when applying break away thrust (initial application to get going) it is normal to refer to the N1 (more so at at high weights) to make sure it is not too high as jet blast can cause a lot of damage. I saw a piper cub get airborne one day behind a DC10 that had spooled up for a cross bleed start. The cub weathercocked around and fortunately the pilot managed to apply thrust and keep it "flying there" until it was noticed by the ground crew. Generally I find that the sim does not reproduce the taxy momentum particulary well.CheersSteve

[Qavion 19]

"Technically EPR it is supposed to be a more precise indication of thrust, however with a large bypass ratio engine that is debatable as the Fan provides most of the thrust."Not sure I get your point, Steve. EPR on the RB211 is looking at the fan inlet/exit pressures as well as the core exit pressure, too.During idle descent, EPR's can go below 1:1, showing the engine is producing drag rather than thrust. Do N1's show this?Steve.... As CF6's get older... I'm sure you'll see just as much oil coming out the bottom of a CF6 as you will with a JT9. I have oil stained white shirts to prove it.BTW, what do you mean by internal pressures? EPR is simply front and back pressures.I work on CF6's and Rollers.... Unfortunately, we don't have electric cowl openers on our CF6's so they are a pain in the butt when it comes to basic maintenance. With a Roller, most of the engine accessories are on the outside of the engine (under the hand-opened fan cowls) and easy to get to.I guess everyone has their favourites ;)Cheers.Q>

[michal 44]

>The EPR on the rollers is a fairly coarse power indication>when compared to N1. None of us here is probably qualified to make statements on this subject. We could settle this issue if someone provided some view graphs showing relation between N1 and thrust and EPR and thrust. The one where this relation resulted in more straight line would "win" as being a better representation of thrust. I am actually curious how such curves look like.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[cowpatz 243]

"During idle descent, EPR's can go below 1:1, showing the engine is producing drag rather than thrust. Do N1's show this?"I am not quite sure why I would want to know this. I have not yet seen an EPR below 1.1 but I guess it is possible. Setting TO thrust is a good example of what I mean by coarse indication. A typical RR TO is somewhere between 1.54 and 1.76. The same corresponding N1's at 15c would be 89 to 103. The only pilot indication of EPR is to 2 decimal places so there would only be 23 possible TO EPR's but 140 N1's (as N1 is indicated to 1 decimal place)so it is a little more accurate to set. EPR is also prone to icing and hence the need to confirm an N1 in icing conditions. I too have worked on the RB211 and the CF6 (many moons ago). I can recall the RR being a pain in the A if you had to change any of the plumbing as it is all stainless and has no give. Everything had to be loosened off to take out one section of plumbing.Generally in operating them there is little difference, just in our case having no autostart on them makes starting a little more labourious. A little more noisy in the cockpit too.These are only minor issues and make for good discussion at the bar with a beer and pretzels!Have a look at this:http://www.pprune.org/forums/archive/index.php/t-10159.htmlCheersSteve

[Qavion 19]

"We could settle this issue if someone provided some view graphs showing relation between N1 and thrust and EPR and thrust."That's the problem... No one but engine designers have them. There aren't even any in the Boeing Maintenance Manual. There are just too many variables. Temperature, ram air effect, etc...Cheers.Q>

[michal 44]

>No one but engine designers have them.And doesn't PMDG have them in order to come up with this lovely simulation ;)Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest Boeing747-430]

I have been meaning to ask the same question as Rico. I have noticed that the EPR indication smoothness is more laggy than N1. N1 has smooth bars(the white bars that indicate engine setting) while the EPR bars are laggy. And i have also noticed how if i slighty move the thrust N1 will change while EPR will stay the same.Adam

[cowpatz 243]

Read my last post 4 up and you will see why this is.Steve