G4 Powerplant Question

[doublepsych]

So, every turbofan that I have ever flown up til now (CFM56, CF34, JT15) has been N1 primary, now I'm about to go to school for the GIV and I see it is EPR primary. But I've been studying a bit, and I see that it also has LP % RPM as the second guage in the stack.

Can anyone enlighten me as to the pros/cons of EPR v. N1 primary indication and why a manufacturer chooses one over the other. I never really understood, and up til now never really cared. I know that on the A320 for example that the CFMs that I trained on were N1 primary, but the V2500s that some operators run are EPR, which seemed perplexing when I first heard it, but I never really put much thought into it. Now Rolls wants to have both on the same engine...

 

[Waldom]

All axial-flow two-spool engines are equipped wih a tachometer for each spool, but not all high-bypass, twin-spool engines use an EPR guage.

My guess....and thats just what it is.....is that with the greater mass and consequent momentum of a high-bypass fan makes it more likely to overspeed when adding power and in that scenario it is more important to monitor fan speed than pressure ratio. I also expect that fan speed (N1) is a more accurate measure of power on a high-bypass engine than the N1 on a low-pass or straight turbojet engine.

Of course the Rolls Royce Tay on a G-IV is considered a high-bypass engine, but the bypass ratio is lower than that of say a CFM-56. A big fan makes a lot of thrust at lower altitudes, but thrust suffers at high altitudes compared to a low by-pass engine. The G-IV was designed to operate efficiently at forty-five thousand feet and I expect one reason that Gulfstream selected the Tay for this airplane is that engine's fan-size compromise between fuel efficiency and high altitude thrust.

I have also heard a rumour that one reason the G-IV has EPR guages is that EPR inputs were easier to match up to the FMS performance computers / auto-throttles with early nineteen-eighties digital technology. I have no opinion on the validity of that rumour since I confess total ignorance of digital technology of any vintage.

And one last note regarding Rolls Royce's propensity for "Britishness". They couldn't bring themselves to refer to the N1 and N2 rpm. They had to refer to the "low pressure" spool and the "high pressure" spool. And you have "fuel cocks" on the power quadrant, not fuel levers. If that's not enough, the engines rotate in an "anti-clockwise" rotation, just like the Merlin engine installed in a Spitfire. That, of course is "backward" compared to an American-designed engine.

 

[TXGold]

Interesting post^^^

 

[G4G5]

Its just old school, you will notice that the older powerplant producers like Pratt and R/R use EPR and newer producers like GE use N1. As engines have evolved and the fan section has grown and by pass ratios have grown.

With the advent of high bypass engines, EPR became a less important measure of thrust. Total thrust is fan thrust and core added together.

EPR measures core thrust, not by pass thrust, and in the old days that's all you cared about. With old low bypass engines its was extremely difficult to have the tips of the N1 fan blades go beyond the speed of sound, it was more likely that you would over stress the engine turbine section (remember the old philosophy. add more fuel to get more power). Previously the fuel to air ratio was more important to monitor so that you would not blow up the engine. Now N1 (the big prop) produces the lion's share of the thrust, and that's not measured by the EPR system.

Try to remember that what you are trying to do is limit the engine from blowing apart. How fast can you safely spin it, AKA how much thrust you can get out the tail pipe. In the old days you were concerned about creating too mush internal pressure. Now a days fan sections have grown to the point where the speed at the tips of the N1 blades are limited by the speed of sound. For some mfr's, this number has become more important then the engines internal pressure ratio. Keeping in mind that the internal core is what turns the N1 fan section so if you control the limits of EPR you are indirectly controlling the speed of the N1 fan section.

If you want to get truly technical I would think that EPR is a better indication then N1%. N1 is a measure only of the rotational speed of the the fan, the actual Thrust developed will vary with the efficiency of the fan. This is in turn affected by pressure / Temperature / Density. Simply put, if the fan blade is bent by a bird strike or FOD, it may still be spinning at 100% rpm (or takeoff N1%) but will not develop the same thrust as a perfect unit.

EPR is a measure of the real thrust produced. ie. Input vs output pressure expressed as a ratio. As both input & output pressures are equally effected by ambient pressure / temperature & density variations thus the indication is not affected.

Really you are just splitting atoms here, both indications work well

http://www.aircraftenginedesign.com/RRTaypics.html

 

[Beechjetr]

EPR can also give you a really bad reading if fouled by a bird...or ice.
Air Florida...

 

[AA717driver]

This is interesting. Every jet I've flown has used EPR as the primary thrust setting measure: RB211, JT8D, JT9D, PW2037, BR715, BR710 and the Tay.

I'd heard about the GE's on the AA 767's using N1 and figured it was just an oddball.TC

 

[gutshotdraw]

Doublepsych,

Just to throw another log on your fire, you'll also find that when you fly approaches with autothrottles off (which any good examiner will require you to do at least once on the type ride), fuel flow will be the best indicator for managing your speed. With full flaps and gear down, try 1800 pph / side to start.

Good luck with the training. Once you get past all the new acronyms and abbreviations, the airplane is well-designed and largely bulletproof.

 

[CallmeJB]

EPR can also give you a really bad reading if fouled by a bird...or ice.
Air Florida...

True, but it wouldn't have been a problem if they had ever turned the engine anti-ice on.

 

[Beechjetr]

whatever shu...
"check"

 

[Ralph Cramden]

Doublepsych,

Just to throw another log on your fire, you'll also find that when you fly approaches with autothrottles off (which any good examiner will require you to do at least once on the type ride), fuel flow will be the best indicator for managing your speed. With full flaps and gear down, try 1800 pph / side to start.

Good luck with the training. Once you get past all the new acronyms and abbreviations, the airplane is well-designed and largely bulletproof.

Just use the speed lolipop with the AT off. It is easier to see and right on the PFD (while I agree that FF vs Speed is a good reference).

 

[doublepsych]

good postings...

thanks for the informative posts. I've done an inhouse SIC checkout, and am going to school in November. I have maybe 50 hours in the thing, so I'm starting to feel fairly comfy in there, but for the last 3 or 4 legs one thing I have been trying to figure out is WHERE is the whiskey compass in a g4? I have been too embarassed to ask the guys I'm flying with...

and whlie I'm throwing questions, here's one more. Into ASE the other day, loading an approach (the VOR), I notice the fixes in the box dont agree with the plate. It was good weather, so I loaded the fixes from the plate one by one, but I know that Honeywell box must have the approaches for ASE by procedure name, but I was stumped at the time. Any input on that one?

many thanks for the info...

 

[Partyfoul]

There is no Whiskey compass. You have 2 DBDI's that serve that purpose. As fas as ASE you probably loaded the wrong VOR approach. There are usually 2 in the database. Try loading the other one and see if that fixes the problem. Have fun drinking from the firehose in SAV.

 

[gutshotdraw]

Psych,

Party is correct. The Digital Bearing and Distance Indicator is what everyone else calls an RMI. It's just one of the Gulfstream-isms that'll be spinning around your head soon.

Due to limitations of the database in the Honey-not-so-well, any approach that REQUIRES a circle or has too steep a descent rate for straight-in TERPS criteria and is therefore coded with a letter (VOR-A, NDB-B, etc) is not in the database. You can manually build the approach but the box will only navigate to those waypoints in en-route and not approach sensitivity on your blue CDI. The box only shifts to approach sensitivity when the approach is extracted by name from the database. Consequently, the aircraft is not certified to fly lettered approaches using only "blue data" and you MUST tune the appropriate navaid and reference the "green data" for the approach. The most common examples of this are the VOR approaches to SMO, ASE and TEB.

(Although our company specifically prohibits it, most G-IV operators build the approach, put the "green data" up in preview mode to monitor, and fly in "blue data." Even in en-route sensitivty, "blue data" is more accurate than any navaid except localizers.)