Drag Factor, Deceleration Question

[aerostar 42]

HiMy first post here,,, I've had the 737ngx for over a couple of months now and I love it , to me the finest example of an aircraft in FSX... though the Flight 1 Cessna Citation Mustang is up there with it...Having flown it a lot now , I'm starting to get a niggle about the deceleration speed rate in descent and more significantly the rate of deceleration when putting in large rudder deflection and dipping a wing to create a side on component to a descent in order to gain more drag from the fuselage to aid deceleration.I have no facts, figures or otherwise to support my feeling, it's just a kind of gut feeling, I fear I may but talking rubbish but would like some of the more experienced and knowledgable pilots to give their opinions, facts etc....Thanks... and just to reiterate, the PMDG 737 ngx is the the number one aircraft in my hangar at the momentIan Whiteside

[AirbusCG 52]

Hi Ian,I'm also not a real world NG pilot, but I understand what you're talking about. But for the 737 it's just normal that it is not so easy to get it down. It'll take some time and you have to plan ahead when flying it. If you enter everything in the FMC normally then you won't have much problems to get it down in LNAV/VNAV.By the way, you shouldn't do the slip (where you push the rudder in the opposite direction of the ailerons) to decelerate because for the passengers a slip feels like you'd be loosing control of the aircraft and also the aircraft is shaking very much which is very uncomfortable. Such manovers are only used in gereral aviation and even there mostly only in gliding and even there only as the last option.

[davierosoft 13]

What is your idle N1 (and what altitude) when you find it is difficult to slow down?Do you have cowl AI ON or OFF?

[aerostar 42]

Hi Andrea,That is something else I forgot to mention in first post - question of N1.When descending I was getting somewhere between 37 - 31 % N1 with throttles full back usually I'm more focused on the altitude when nearer an airport therefore under 5000' , say. With regards to AI ... I haven't really noticed relationship but I would say usually off....The point about the N1 that I wished to add is ... when throttling back why is there a thrust registered my idle is approx 20%. If no thrust then the engines would act as a huge 'airbrake' and slow the plane... but with the residual thrust of 35% (approx) is that being negated ? I have a feeling though that the engine flight idle is high for quick spool up response and anti stall of the turbine and that's designed into their operation.Crabbing or slipping in a descent doesn't seem to alter the overall drag of the aircraft though....

[stweaver321 0]

One thing that I've found helps quite a bit is right after you begin your decent, reduce your speed to around 0.70M or so. Up at the higher altitudes, I've found that it will slow down much easier than if you wait to slow down until lower altitudes.

[scandinavian13 5,192]

The 737-800 is notoriously 'slippery', meaning it is difficult to slow, and picks up speed pretty easily.

Having flown it a lot now , I'm starting to get a niggle about the deceleration speed rate in descent and more significantly the rate of deceleration when putting in large rudder deflection and dipping a wing to create a side on component to a descent in order to gain more drag from the fuselage to aid deceleration.

For what it's worth, you're talking about a slip, and you'd never do that in an airliner like that.

[davierosoft 13]

Hi Andrea,That is something else I forgot to mention in first post - question of N1.When descending I was getting somewhere between 37 - 31 % N1 with throttles full back usually I'm more focused on the altitude when nearer an airport therefore under 5000' , say. With regards to AI ... I haven't really noticed relationship but I would say usually off....The point about the N1 that I wished to add is ... when throttling back why is there a thrust registered my idle is approx 20%. If no thrust then the engines would act as a huge 'airbrake' and slow the plane... but with the residual thrust of 35% (approx) is that being negated ? I have a feeling though that the engine flight idle is high for quick spool up response and anti stall of the turbine and that's designed into their operation.Crabbing or slipping in a descent doesn't seem to alter the overall drag of the aircraft though....

The idle N1 is correctly simulated, flight idle is higher than ground idle, but, flight idles in the 737 are 2, one is called approach idle wich is greater than flight idle. Engine antiice, gears, flaps trigger that higher idle thrust.As mentioned the aircraft itself is slippery.I was talking about idle because there are few users experiencing approach idle where it is supposed to be only flight idle, this will result in a descent that cannot be done normally, but with the help of the speed brakes.However, from your post, your values are correct, so you don't suffer about this problem.

[Spin737 102]

One thing that I've found helps quite a bit is right after you begin your decent, reduce your speed to around 0.70M or so. Up at the higher altitudes, I've found that it will slow down much easier than if you wait to slow down until lower altitudes.

Unless you planned your descent at .70, slowing up high will make it harder to get down. You are changing your speed enough that you probably need to report it to ATC for a Speed Bump Alert. It's probably not a technique you'd see RW.

[3-2-1-Now 126]

Go down or slow down. Pick one. :)If you plan your descent well there is no problem decelerating. The term missing here is "energy management". Another way to look at it is as if you suffered double engine failure, so the throttles are no use to you. Can you glide to the airport at idle thrust and still make the flap/gear speeds on the way down without using the speed brake? If the answer is NO, you simply aren't planning your descent properly.Best regards,Robin.

[CaptBillyBob 51]

Go down or slow down. Pick one. :)If you plan your descent well there is no problem decelerating. The term missing here is "energy management". Another way to look at it is as if you suffered double engine failure, so the throttles are no use to you. Can you glide to the airport at idle thrust and still make the flap/gear speeds on the way down without using the speed brake? If the answer is NO, you simply aren't planning your descent properly.Best regards,Robin.

In a world without ATC that would be possible, but you'd find the nearest suitable airport and spiral down to land if that happened..

[scandinavian13 5,192]

In a world without ATC that would be possible, but you'd find the nearest suitable airport and spiral down to land if that happened..

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.

[aerostar 42]

Thanks guys for your input and advice....Still wondering on the point of Drag Factor as modelled. Does any one think it might be a bit too 'slippy' compared to real ?Would be helpful and put my niggle to bed if any real world pilots could confirm the deceleration rates as per real... there doesn't seem to be any noticeable immediate reduction in speed to the extent that maybe expected in real... From thrust in cruise and pull back throttles to idle there should be a noticeable quick change in speed initially ? Does the speedbrake mimic real also... seems very little difference ? Ian Whiteside

[scandinavian13 5,192]

Would be helpful and put my niggle to bed if any real world pilots could confirm the deceleration rates as per real

These guys are:

The idle N1 is correctly simulated, flight idle is higher than ground idle, but, flight idles in the 737 are 2, one is called approach idle wich is greater than flight idle. Engine antiice, gears, flaps trigger that higher idle thrust.As mentioned the aircraft itself is slippery.I was talking about idle because there are few users experiencing approach idle where it is supposed to be only flight idle, this will result in a descent that cannot be done normally, but with the help of the speed brakes.However, from your post, your values are correct, so you don't suffer about this problem.

Unless you planned your descent at .70, slowing up high will make it harder to get down. You are changing your speed enough that you probably need to report it to ATC for a Speed Bump Alert. It's probably not a technique you'd see RW.

[PIC007 82]

The single biggest reason for difficulty slowing is your descent rate. Anything over about 1900FPM and you need to use the spoilers to help slow her down. I plan my descents at no more than 1500FPM and she slows just fine. That usually requires starting down 30 miles or so before the FMC computed TD. Of course if you are using ATC and they give you a descent, crossing altitude, etc, then use what you need to comply, or tell em "unable' :-)

[ZachLW 37]

Thanks guys for your input and advice....Still wondering on the point of Drag Factor as modelled. Does any one think it might be a bit too 'slippy' compared to real ?Would be helpful and put my niggle to bed if any real world pilots could confirm the deceleration rates as per real... there doesn't seem to be any noticeable immediate reduction in speed to the extent that maybe expected in real... From thrust in cruise and pull back throttles to idle there should be a noticeable quick change in speed initially ?Does the speedbrake mimic real also... seems very little difference ?Ian Whiteside

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.

Go down or slow down. Pick one. :)

That sums it up. My friend, a 737-400 captain, says you can descend in a 737, and you can slow down. But you can't do both.Finally, I have no issues getting the aircraft on speed and stabilized using the FMC, VATSIM vectors, etc. That being said, it comes down to technique or lack there of. (not a slight on you, just a thought!) Edited February 8, 2012 by ZachLW