Cruise Speeds - 0.81 mach overspeed?

[taguilo 33]

John,My pleasure:-)Just to avoid boring people I'll give you some rough numbers for "standard" cruise values from 35 to 39 k feet.M80, for the RR211 535E4 :EEC EPR limit (amber bug) - 1.79 (two engines on)EPR CRZ (green bug and digits) - 1.75Nominal EPR range (white pointer and digits) around 1.60-1.65, but can vary depending on winds.N1 - 84.5 to 87.5EGT - 530-600 (with strong tailwinds could be 650+)N2 - 76-83 N3 - 79-84FFlow 1.6 ton = 3.52 k pounds Regards,Tom

[Rainero 3]

Seems to be an issue even with the final release version. I can't exceed 0.807 mach at FL330. Can anybody confirm?Rainer

[kevinh 409]

Tom,How are winds going to affect cruise engine performance? The aircraft is flying along relative to a moving body of air. For a given flight condition (mach, altitude, TAT) the engine performance will be the same, regardless of wind. Wind only affects ground speed and track.Kevin

[taguilo 33]

Kevin,I was refering to nominal EPR cruise values, average M80 and ground speed of aprox 460 knots, for an aircraft weighting 190-210 K lbs. Headwinds would require a bit of higher EPR to maintain ground speed, and vice versa.Tom

[kevinh 409]

Tom,No, when you cruise you maintain a Mach number, not a ground speed. All the performance charts and the FMC are set up that way. Tail wind will not affect EGT either, apart from on the ground during an engine start.Kevin

[Guest kaysee10man]

I'm getting the same thing. This needs to to be fixed. The plane should be able to acheive the VMo of .86M at FL350 at standard day temp. One more thing to ask....where is the ILS tuner that is supposed to be on the center pedestal just aft of the throttles? Do you have the normal NAV1 radio programmed to accept ILS localizer frequencies? In the real plane the ILS frequency and inbound course is set in a receiver that is on the center pedestal. Just curious. Execellent aircraft.

[taguilo 33]

Kevin,""No, when you cruise you maintain a Mach number, not a ground speed.""Of course, but as a result of the need to follow a flight plan.Airliners use Cost Index calcs for each flight phase (clb,crz,etc). One of the vars employed is time of flight, which depends totally on ground speed, which is affected by winds. The FMC computer determines the best Mach needed at any time to comply with the CI entered, and command the engines towards that number, BUT as a consequence, not a cause. What is primary important is Time of flight (=ground speed) and Fuel flow supervision (among others). ""Tail wind will not affect EGT either, apart from on the ground during an engine start.""This is something that you won't find in the books for sure :-)Tom

[Guest Steve_Park]

Seeing overspeed above .810 with the "Final" release here also.Steve Park

[Guest N555TZ]

Bump

[Joe L 62]

I have fixed this myself just by changing the max_mach to 0.86 under Reference Speeds in the aircraft.cfg files. Once done everything seems fine.I have not updated to the latest version, yet, but it seems from what others have said it was still not fixed.Joe Lorenc

[B777ER 2,484]

Says max_mach 0.82 in my cfg's. So overspeed would be above that figure. I believe this is in line with what the 757 does.

[Joe L 62]

According to this site -- http://www.757.org.uk/limits/lim1.html -- limiting speeds are Vmo @ MSL - 350 kts / Mmo - 0.86 Mach.Janes lists mmo as 0.86 http://www.janes.com/aerospace/civil/news/...g_757-200.shtml(PSS lists mmo as 0.84 in the manual.)Not saying anyone would cruise there, but 0.82 would seem possible with a 0.80 normal cruise. As it stood 0.80 seemed to leave an unsafe margin from the incorrect 0.82 mmo.Joe Lorenc

[kevinh 409]

Tom,If you are quoting EPR figures for a cruise speed of Mach 0.8 then wind is irrelevant. Repeat irrelevant. If you need to make up time against a headwind then you will cruise at a higher Mach and so EPR will also be higher. It's the Mach number that is important to the engine model. Never wind velocity.Not sure what you mean by not finding this in the books. EGT will not be affected by a tailwind in cruise, but it can be affected by a tailwind during ground starts.I have been modelling engine performance (and flight dynamics) for full flight simulators since 1977. I think I might know a little about engine models work.Kevin

[B777ER 2,484]

Yes, you are correct..., .86Mmo

[Guest brand_da_man]

Any updates with this? Its very annoying as i try and fly my flights at the same time as the realworld and using the same weights, speeds etc (from what i can get from certain people, sites, anyway)ThanksBrandon