sim vs real lifte

[gura75]

When simulating a 737 -800 PMDG flight I have noticed one strange thing comparing with real flights data given by flightradar24.com. When looking at data given by flightradar24.com it looks like 737-800s fly at much higher speeds at cruise altitudes.In PMDG the red warning dots indicating speed restrictions shows "danger" at about 280kts (PFD)....looking at 737-800s ( flightradar24.com) the cruise altitude speed is much higher ....about 400kts. How come I can not get these speeds in simulation.gura75

[Foghorn]

First impression is the 280 knots refers to airspeed, 400+ knots refers to ground speed. Cruise airspeed is different as, at altitude, the air is thinner. Others may be able to either correct me or give you a more detailed explanation but this should give you some idea.Cheers

[PHXMD-11]

I thought it would have been another bloody FSX limitation...

[Superpilotv2]

First impression is the 280 knots refers to airspeed, 400+ knots refers to ground speed. Cruise airspeed is different as, at altitude, the air is thinner. Others may be able to either correct me or give you a more detailed explanation but this should give you some idea.Cheers

He´s right on that. when you for exapmle, fly with teh 747-400 in 36.000ft with arround 300 KIAS cruise speed, you´ll find yourselfe flying with arround 500 KTS GS. In that point FSX is right within certain limitations.

[jeffhunter]

When simulating a 737 -800 PMDG flight I have noticed one strange thing comparing with real flights data given by flightradar24.com. When looking at data given by flightradar24.com it looks like 737-800s fly at much higher speeds at cruise altitudes.In PMDG the red warning dots indicating speed restrictions shows "danger" at about 280kts (PFD)....looking at 737-800s ( flightradar24.com) the cruise altitude speed is much higher ....about 400kts. How come I can not get these speeds in simulation.gura75

Next time you are cruising at altitude, have a look at your groundspeed. You'll see the 400+kts you're talking about.cheers,Jeff

[gura75]

Thanks for your answers guys. You were right. Ground speed and airspeed show different values...during my simulation: air speed 270kts ......ground speed: 0,8 Mach which is about 533 kts gura75

[sekos]

Thanks for your answers guys. You were right. Ground speed and airspeed show different values...during my simulation: air speed 270kts ......ground speed: 0,8 Mach which is about 533 kts gura75

Well .8 mach is not a ground speed it is used at high alt. because of IAS inaccuracy. Ground speed is showed in upper left corner above wind speed.Jiri Sekerka

[MorsAbAlto]

There seems to be some confusion about airspeeds in this thread.The speed scale on the PFD always shows the Indicated Air Speed (IAS). Then there is True Airspeed (TAS) and Ground Speed (GS). The latter also contains the current wind component at the current cruise altitude.

[3-2-1-Now]

Well .8 mach is not a ground speed it is used at high alt. because of IAS inaccuracy. Ground speed is showed in upper left corner above wind speed.Jiri Sekerka

The airspeed indication on aircraft that use ADCs display Calibrated Airspeed (CAS), which is not the same as IAS as found on a Cessna, but is derived from IAS corrected for position and instrument error, and at higher speeds is further corrected for compression effects actually making it Equivalent Airspeed (EAS). The compression effect starts at Mach 0.3.Mach is the measure of the local speed of sound. It is dictated by temperature. The aircraft measures SAT (Static Air Temperature), and computes the Mach of the aircraft upon it. e.g. if SAT is -40 degrees Celsius, the local speed of sound is 596 kts. If the aircraft has a TAS of 500 kts, then it is flying at Mach 0.839.I used an E-6B but the ADC just uses algorithms instead.Note that FSX has issues with density altitude, so the TAS/Mach you see for a given pressure altitude/temperature might very well be wrong for the conditions, and thence not match what you see real aircraft achieving.Best regards,Robin.

[Nickdj]

Well .8 mach is not a ground speed it is used at high alt. because of IAS inaccuracy. Ground speed is showed in upper left corner above wind speed.Jiri Sekerka

IAS indeed has an inaccuracy (compressibility) at high level but that's not the reason to switch to Mach. If you keep climbing at a constant IAS your Mach no. will keep increasing because as you climb the local speed of sound (decreasing temp) will decrease hence the need to switch from IAS to MACH so that you wont reach Mmo. (During air acceleration the wing will reach the speed of sound well before the local speed of sound. This is Mach Critical and aerodynamic characteristics (controllability/stability) start to degrade. The limit point of the aerodynamic characteristic still being tolerable is called Mmo= Mach max operating speed)cheersNick

[3-2-1-Now]

Mach Critical (M_CR) is the speed at which airflow over any part of the aircraft reaches Mach 1.0 (usually the tail plane).By climbing at a fixed Mach number you ensure the airflow doesn't reach Mach Crit. (if we are talking only subsonic aircraft). In supersonic aircraft you fly at a speed that gives a good margin before Mach buffet/loss of control. Aileron reversal often occurs first (the downward deflecting aileron increases the camber of the wing, causing separation of the airflow resulting in loss of lift, and a roll in the opposite direction of the aileron input as a result).Reaching Mach Critical may not immediately result in loss of control/Mach tuck. Mach tuck is the result of boundary layer separation over the tail, severely reducing the effectiveness of the tail and resulting in the nose dropping. Pulling the nose up may stall the tail further and exacerbate the problem. The only solution is to reduce speed and increase drag, then recover from the dive. Note that it may be impossible to recover if left to develop. Rolling the aircraft may help unload the tail and regain control.The altitude at which transition from IAS to Mach (and back again) occurs depends in large part upon the aircraft, and desired climb/descent speed. It is the point at which a given IAS matches a given Mach number. The altitude at which it occurs is further affected by atmospheric conditions, primarily temperature.Best regards,Robin.