Not me about aircraft performance in air ;)However, most 737 pilots here reported that the aircraft is close to the real in terms of drag. PMDG also programmed it using real simulators, so they did it as close as possible.Many threads here from the release of the aircraft, pointed to how slippery the plane is, pilots confirmed. But, wait their answers ;)

Sorry guys if I'm not being too clear... I kinda understand the descent management etc but.....Is the 737 acting like real world in terms of Drag reactions and Deceleration rates .... ?Also if the engines are put to idle in flight then those engines become effective drag (?) Is this modelled ?Thanks for your input, appreciated...Ian Whitesideooops, I didn't notice last post before I posted my last one.....Ian Whiteside

From thrust in cruise and pull back throttles to idle there should be a noticeable quick change in speed initially ?

YES! you will even go down to 0kt in the speed tape :( and then you will come down in altitude REALY fast and then you push forward the yoke and pick up speed and simply make the final approach and land, come out from the cockpit in your sunglasses and you will be a super pilot :Love:In real life you trade the speed loss with gravity to buy altitude loss with constant speed, sell altitude loss and you will lose speed.

Also if the engines are put to idle in flight then those engines become effective drag (?) Is this modelled ?

A engine that still rotates by combustion will never have more drag than trust, even if the combustion stops the engine will be aerodynamicly like a holow tube since the fan is still moving the air through the engine and fan.The most effective drag you will ever have on a plane is induced drag=you need lift to slow down (do you have winglets you have even less induced drag and you will have a bigger problem to slow down).

Edited February 8, 201214 yr by flygarn1

In a nutshell, "NO". The speedbrake only spoils lift, so it's not the magic problem solver when your decent planning goes awry. We're too used to unrealistic aircraft that are easy to tame.

I'm not saying you don't get it Zach, I just quoted your post because it's on the subject I'm addressingSo many people don't understand that spoilers and speedbrakes are two slightly different issues. Note in the 737 and the MD-11 and most transport aircraft, the checklist item says SPOILERS -> ARMED, not SPEEDBRAKES -> ARMED. The reason is exactly as Zach mentioned: spoilers are there to spoil lift. That spoiled lift requires a higher angle of attack for the same descent rate (or in order to not descend), which causes more drag as a result of the whole process. While most spoilers could be considered speedbrakes in and of themselves, they're normally limited in flight (in terms of how far they extend) in order to only spoil lift, and not too much of it, either, because the wing still has to create some lift.Speedbrakes on the other hand (think Fokker 100, or RJ85/100) are there to simply produce drag, and that's it.

Also if the engines are put to idle in flight then those engines become effective drag (?) Is this modelled ?

Sure, they produce some drag, but in order for them to keep running, they must be at least idled, and there's a decent amount of thrust still coming out the back end. The fact that you slow down when you reduce thrust is essentially illustrating your point.

I really would like to see video on how to perform such subject on different scenery mostly with ATC in Vatsim.Another aspect is the balance between the cost index and descent profile, I have been watching the video of a pilot using speed to burn altitude and stay under the path then bleed the speed right before 1° decel point.But if you want to use CI 36 and save fuel and you fill all info on the FMC like WIND in CRZ and landing forecast, how do you archive an economy path and don't be higher at the approach phase.I know that can be a matter of technique but i still blind and i need to use speed brake to always stay on the path.

What's that supposed to mean? Even with ATC, it's possible. Simply tell us the emergency and the nature of what you're doing and you'll have a clear path, and likely a very large one because your course deviations to manage the descent could be very much non-standard.

What it means if you had a double engine failure, last thing you'd do unless you had no choice (water) were to try to glide to your destination airport if there's a suitable airport close by..

Don't forget parasitic drag. Increases with TAS, so the faster you are, the higher the drag initially. As you slow, this drag reduces, so the effective deceleration rate reduces too. That is why it seems easier to decel at higher altitudes at higher speeds than lower altitudes at lower speeds.BTW - the engines can still have a net drag at idle.Best regards,Robin.

I'm not saying you don't get it Zach, I just quoted your post because it's on the subject I'm addressingSo many people don't understand that spoilers and speedbrakes are two slightly different issues. Note in the 737 and the MD-11 and most transport aircraft, the checklist item says SPOILERS -> ARMED, not SPEEDBRAKES -> ARMED. The reason is exactly as Zach mentioned: spoilers are there to spoil lift. That spoiled lift requires a higher angle of attack for the same descent rate (or in order to not descend), which causes more drag as a result of the whole process. While most spoilers could be considered speedbrakes in and of themselves, they're normally limited in flight (in terms of how far they extend) in order to only spoil lift, and not too much of it, either, because the wing still has to create some lift.Speedbrakes on the other hand (think Fokker 100, or RJ85/100) are there to simply produce drag, and that's it.

The checklist on FCOM NC.3 has Speedbrake...ARMED in the landing checklist and the FCTM also only disucsses speedbrakes and not spoilers. The point you make about speedbrakes vs spoilers is quite correct.To confuse the issue though, this is what Boeing has to say on the matter [FCTM 6.38]: " The speedbrake system consists of individual flight and ground spoiler panels which the pilot can extend and retract by moving the SPEEDBRAKE lever. When the SPEEDBRAKE lever is actuated, all the spoilers extend when the airplance is on the ground and only the flight spoilers extend when the airplance is in the air."

I'm not saying you don't get it Zach, I just quoted your post because it's on the subject I'm addressingSo many people don't understand that spoilers and speedbrakes are two slightly different issues.  Note in the 737 and the MD-11 and most transport aircraft, the checklist item says SPOILERS -> ARMED, not SPEEDBRAKES -> ARMED.  The reason is exactly as Zach mentioned: spoilers are there to spoil lift.  That spoiled lift requires a higher angle of attack for the same descent rate (or in order to not descend), which causes more drag as a result of the whole process.  While most spoilers could be considered speedbrakes in and of themselves, they're normally limited in flight (in terms of how far they extend) in order to only spoil lift, and not too much of it, either, because the wing still has to create some lift.

The checklist on FCOM NC.3 has Speedbrake...ARMED in the landing checklist and the FCTM also only disucsses speedbrakes and not spoilers. The point you make about speedbrakes vs spoilers is quite correct.To confuse the issue though, this is what Boeing has to say on the matter [FCTM 6.38]: " The speedbrake system consists of individual flight and ground spoiler panels which the pilot can extend and retract by moving the SPEEDBRAKE lever. When the SPEEDBRAKE lever is actuated, all the spoilers extend when the airplance is on the ground and only the flight spoilers extend when the airplance is in the air."

Nomenclature!  The spoilers are used as speedbrakes.  Or put another way, the speedbrake system uses spoilers (flight spoilers specifically).  I was simply using the terminology that Boeing uses, as pointed out be John.  "Speedbrakes".

Edited February 9, 201214 yr by ZachLW

I set a flight in steady cruise at 250 knots on autopilot/throttle then disconnected the A/T with throttle levers full back.....The aircraft very gently started to slow... but not at a deceleration rate that would be expected (?)I'm basing this on real life flights where even as a passenger you feel the deceleration very noticeably and even your body mass feels the effect as it leans you a bit forward in your seat...

 

I'm basing this on real life flights where even as a passenger you feel the deceleration very noticeably and even your body mass feels the effect as it leans you a bit forward in your seat...

:( Seriously? Don't take the real pilots words for it that developed this sim or fly it. Base your info on riding passenger, with no airspeed/altitude/etc data. I believe you've gotten your answer. Drag is correct, and that's seconded by real practicing 737NG pilots!

Edited February 9, 201214 yr by ZachLW

Just waiting pilot answers on the specific questions, I tried it at different altitudes, all tests made with the same airplane, loading before starting the test 4000Kgs of fuel so the weights are the same for all the tests.I choose 3 altitudes, FL360, FL200 and 10000ftI used a costant IAS of 250Kts (I've not used Mach number)All tests were made by using autopilot and no autothrottle, speed before starting the test was costant 2 5 0 KIASTest time 60 seconds. Pictures are taken after that time (as shown on the clocks)Time started when moved throttle back to idle, but consider that engines spooled down not immediately and maybe with few difference in time between each test altitude.To reach each test altitude (descending) I used LVL CHG (throttles idled) and the vertical speed @250kts never went over 1500Ft/min. Below the last test altitude VS was around 1000ftThese are the results:

Andrea, in that first pic, your idle N1 is 44.2%. I think that number should be closer to 38% at that altitude. That's a considerable difference.As far as slowing, down low I expect approximately 1nm to slow 10knots. So, for the last pic, it seems pretty close. If you were doing around 4nm/minute at 1 minute, that should be 250-40=210 and that's what you show.So, that 10k pic looks good. Not so sure about the FL360 pic.

MSFS is a questionable in this area (AoA modelling is definitely broken), but if you are trying to fly at 250 kts IAS at FL360 you are flying at a higher angle of attack with increased drag as a result. Also parasitic drag should be higher due to the higher TAS, so I'd expect you to decelerate quicker at this altitude than at 10000 ft. As it is, you show to decelerate slower than at 10000 ft, even allowing for the thicker air at lower altitude.Forgetting the N1 value, the value that counts is N2, or core RPM. This is what keeps the engine running and must not fall below a minimum value otherwise it becomes unstable. It is fairly consistent at each altitude compared with N1:N2 vs. N1:* 65.3% vs. 44.2% (FL360)* 63.8% vs. 35.2% (FL200)* 64.1% vs. 32.6% (10000 ft)I wish they had EPR instruments as it shows the relative thrust generated by the engine rather than RPM which doesn't tell you much. An older engine needs running at higher RPM to get the same power out of it.Best regards,Robin.

I will check the idle at fl360 but I'm sure I retarded the throttles.I will update the picture if find differences.In turbofan engines with high bypass ratio what is important is the N1 value that in the case of the 737 is more than 75% of the thrust. Idle control is different, the EEC controls the idle to reach N2 targets that are calculated by the aircraft and air condition.