B77W Lower Cruise Performance

[aceridgey 330]

Hi guys,

 

The 777-300 is restricted in it's cruise altitude and isn't able to climb to the altitudes of it's smaller sister the -200.

 

Quoted from this months Airliner World:

 

'Even the best aicraft have some aspects of the flight envelope that aren't optimal, and when I pressed Bill he admitted that - due to the higher wing loading of the 300ER - the altitude capability is not as good as the -200's.'

 

"With so much more weight, we are usually limited to the mid and lower - 30,000ft flight levels. That reduces your choices if the rides are bad, and there's also a lot more deviating around thunderstorms'.

 

My questions are

 

1) If one was to try and simulate higher altitudes in the pmdg sim; what would happen?

 

2) Is the profile for PFPX encompassing of this lower cruise performance?

 

3) Could someone breath some more detail into the 'higher wing loading of the 300er'? (It's the same wing as the 77L etc, but why was it designed so?)

 

Awesome if anyone techy could answer these

 

 

Cheers and seasons greetings/

 

Alex

[ysfsim 22]

1) The plane simply won't go any higher because the air is too thin for the amount of weight it is carrying. That is the reason/ logic for the use of step climbs. If you try to force the plane to climb further.... remember Air France 447. They climbed too high when too heavy and stalled.

 

2)I can't answer that. I don't use PFPX

 

3)If you put a coupe's tires on a pick up truck,, isn't there more loading/ pressure on the tires? The same goes for wings. Because the -300er uses the same wing as the -200, there is more wing load due to the heavier weight of the -300. You can also say that there is more pressure on the wings to keep the -300 up in the air. So in essence, the wing on the -300 is not truly optimized for the type but it is used because it can save money and it works. If the -300 had new wings designed for it, it would be able to climb higher than it is now.

 

cheers!

[LeeL 272]

Doesn't the 77W have the same wings as the LR and not the 200/200ER?

 

Regards, Lee

 

 

[aceridgey 330]

Doesn't the 77W have the same wings as the LR and not the 200/200ER?

 

Regards, Lee

 

The 200/ER has doesn't have the raked wingtips

[LeeL 272]

The 200/ER has doesn't have the raked wingtips

Oh of course...haha never mind

 

Regards, Lee

 

 

[Flamin_Squirrel 71]

Hi guys,

 

The 777-300 is restricted in it's cruise altitude and isn't able to climb to the altitudes of it's smaller sister the -200.

 

Quoted from this months Airliner World:

 

'Even the best aicraft have some aspects of the flight envelope that aren't optimal, and when I pressed Bill he admitted that - due to the higher wing loading of the 300ER - the altitude capability is not as good as the -200's.'

 

"With so much more weight, we are usually limited to the mid and lower - 30,000ft flight levels. That reduces your choices if the rides are bad, and there's also a lot more deviating around thunderstorms'.

 

My questions are

 

1) If one was to try and simulate higher altitudes in the pmdg sim; what would happen?

 

2) Is the profile for PFPX encompassing of this lower cruise performance?

 

3) Could someone breath some more detail into the 'higher wing loading of the 300er'? (It's the same wing as the 77L etc, but why was it designed so?)

 

Awesome if anyone techy could answer these

 

 

Cheers and seasons greetings/

 

Alex

 

Airliners are horribly complex machines. The aim of the game isn't just to design them so all their constituent parts run as efficiently as possible, but that they do so all at once. You need to understand some of the factors involved otherwise the answers on their own might not help very much.

 

Lift/drag ratio: there will be an optimum AoA that provides the lift required for flight with the minimum drag.

High speed buffet: governed by mach no. while your aircraft might not be supersonic, localised airflow over the wing might be approaching it, and that's bad.

Low speed buffet: in short, the initial signs of a stall.

Engine RPM: turbo fan engines generally run most efficiently at high RPM (but you don't want to wear them out by bouncing off the rev limiter).

Fuel flow: for a given RPM, engines require less fuel at higher altitudes.

 

So, you want to set the highest practical engine RPM (climb power), pitch up for the IAS that provides the optimum AoA (giving the best lift/drag ratio) then climb as high as you can to reduce fuel flow. Mach no will increase as you climb at a constant IAS until this becomes the limiting factor due to high speed buffet, at which point you climb at constant mach. As the climb continues the air gets thinner and your climb rate slows until it's time to reduce engine RPM a little to level off. It's important to note than while climbing at constant IAS results in increasing mach, climbing at constant mach will result in decreasing IAS. This is an important interdependency; more on this later.

 

As fuel is burnt weight decreases. If you maintain speed then AoA will decrease (resulting in worse L/D ratio). If you slow down to maintain AoA you'll have to reduce engine power (less efficient), so you step climb to maintain engine RPM and AoA.

 

Now onto your questions:

 

1) During the climb your highest possible speed will eventually be mach limited. As you climb at constant mach, your IAS will be reducing; if you keep climbing beyond your optimum altitude you'll reach a point where you can't go any faster due to high speed buffet, but your IAS which has been reducing gets so low you'll enter low speed buffet (stall). This is known as 'coffin corner'.

 

2) Can't see any reason why not; no point having the profile otherwise!

 

3) Wing loading is simply weight divided by wing area. While the empty weight of the 300ER is greater than the 200LR, their max takeoff weights are very similar. I imagine in practice this means that a greater proportion of the 200LR's weight at takeoff will be fuel, which it'll burn off during the flight allowing it to step up to a greater range of levels.

 

I'll finish off with a caveat that I'm a lowly PPL holder that, although has an unnatural desire to learn this stuff, isn't qualified on these matters. Hopefully it's broadly correct.

[scandinavian13 5,192]

3) Could someone breath some more detail into the 'higher wing loading of the 300er'? (It's the same wing as the 77L etc, but why was it designed so?)

 

 

 

3) Wing loading is simply weight divided by wing area. While the empty weight of the 300ER is greater than the 200LR, their max takeoff weights are very similar. I imagine in practice this means that a greater proportion of the 200LR's weight at takeoff will be fuel, which it'll burn off during the flight allowing it to step up to a greater range of levels.

 

...to tack onto Jordan's 3) answer:

One thing to remember is that airports are designed to a particular standard.  Airport planners have templates with various aircraft so that they can efficiently plan gate space.  Of course, we're only able to find templates for aircraft that currently exist.  This is why you often hear manufacturers touting that their aircraft's performance is [insert marketing point here] while still maintaining the existing wingspan (/footprint/wheel loading/etc).  Anyone remember the industry counterpoints against the A380 regarding the airport modifications that would need to be done to accommodate it?  Who pays for that?  Usually a combination of the operating airline and the airport authority (depending on the agreement, usually more the airline than the authority).

 

So, if I were an airline with the option to:

1. Buy a plane that required me to pay for infrastructure modifications; or
2. Buy a plane that fit within a current profile (wingspan-wise)

I might actually choose the second option, even if it meant taking fewer passengers on a particular leg.

 

 

 

Those yellow lines on the ground have a lot more thought into them than most would think. :wink:

 

Source: I worked for a little while in airport planning.  Neat stuff.  If you're curious about all the thought that goes into runways, apron plans, and all that, take a look at an airport Master Plan Update.  This is also where a lot of your instrument approach planning comes in (though the FAA TERPS group actually writes the approaches, planners ensure the proper volumes are clear and within tolerances so that the approaches can be properly written).  Here's one that I helped to work on: http://www.vtmea.com/masterplan.html

[Flamin_Squirrel 71]

Having the same wing across different models is also good for the manufacturer too; less design/parts sourcing/tooling etc.

[scandinavian13 5,192]

Having the same wing across different models is also good for the manufacturer too; less design/parts sourcing/tooling etc.

 

Definitely.  I was surprised how astonished my friend was when I mentioned to her how it's pretty easy to retrofit some things from one car into another.  More specifically, I was explaining to a her how I helped a friend take heated seats from a salvaged MINI and put them into his car.  She thought the options were built into the car from the ground up.  Modular design wasn't apparently in her knowledge bank.  Oh well...learn new things every day, right?