BigCol – do you know if it's possible to perhaps arrange a visit to the Rolls Royce factory? I'm qualified to fly the things, but would love to get a closer look at the manufacturing and development side.

My dad made many of the patterns for the RB211, so I've always had an interest in Rolls Royce engines. 🙂

Cheers,

Dave.

BigCol – do you know if it's possible to perhaps arrange a visit to the Rolls Royce factory? I'm qualified to fly the things, but would love to get a closer look at the manufacturing and development side.

My dad made many of the patterns for the RB211, so I've always had an interest in Rolls Royce engines.

Cheers,

Dave.

If you can get a visit sorted make sure you come to experimental, much more interesting than production 😉

Seem to remember Stephen Fry saying at the end of an episode of QI that there are several theories about how a plane manages to fly and they all contradict each other. Anyone know how true that is?

It might be true that there are various theories. Various theories of people who don't actually understand the things. I'm sure the engineers who design such things understand pretty well how they work (and there isn't much in the way of disagreement between them), given it's not that complicated physics, it's just the general public who tend not to.

How various insects fly is another matter – they tend to use very complex physics to maximise lift, and I'm not totally sure anybody understands completely!

How various insects fly is another matter

Yeah have also heard it's theoretically impossible for a bumble bee to fly – could just be one of those useless incorrect factoids that gets passed around tho.

Flaperon – I'll have an ask as to what the process is for visitors these days…..

hijack here, maybe some of the techies can answer this one

airbus 319's / 737's in fact most mid sized jets

whats the wierd noises just after they close the doors, still on GPU hook up, engines not started, usually just after they close the baggage hold

hard noise to describe, kind of like dodgy fan belt meets screetching , pump sort of noise, underneath the seats

and its not the leading edge / trailing edge stuff pre take off

any suggestions …that one's always got me stumped

Sounds like the cargo door closing – this can make a noise similar to what you describe, and would come from underneath you….

or to be a bit more descriptive kind of like the stop starting on the flat bit of a flume when the plastic goes eek eeek eeek eeek

is that really just a door closing ???

if so why does it make such a bloody racket

You can visit a small part of Rolls-Royce in Derby this Saturday

Open Day

Like dogs barking in the hold…?

Hydraulic power being transferred from one engine side to the other i believe

Andy B
bingo

cheers mate …you've answered a question thats bugged me for years

Ah yes – probably the PTU then.

I think the A320 family is different to other jets though?

yeah, I'm pretty sure it's unique to A320. I dont know for sure about A330/340 but I don't think it's on them.

A318/29/20/21 series only, I believe. 737 has electrical hydraulic pumps both sides.

So far then we have a fantastic array of amswers to how a jet engine works but no one has answered how jet airliners work.

Why can I see through the wings during takeoff and landing?
Why do the wings extend forwards and backwards before take off and landing?
Why do planes always feel like they have stalled as they are on their final approach?

I want to know how they fly

When a plane is flying slowly it needs a larger aerofoil (and a different profile and angle of attack) to generate sufficient lift than it does at cruising speed. Furthermore, all that surface area would cause excessive drag at higher speeds.

The wings have to be able to change their shape and size.

The stalling thing – I'm with you, but I've never been able to figure how a plane descends from 6 miles up whilst at the same time slowing down from 500mph to 150mph.

I've never been able to figure how a plane descends from 6 miles up whilst at the same time slowing down from 500mph to 150mph

Really big brake rotors. Probably.

Air resistance being a wonderful thing at times of course. Particularly when you change the shape of a wing, angle of attack etc. Possibly.

You can see through the wing because of the design of the flap. By leaving a slot, high energy air is able to pass through and prevent the air flowing over the wing detaching as it passes over the flap. Saves weight, too. The reason the flaps extend such a long way back is to create drag.

This is two-fold, it helps slow the aeroplane down as it descends towards the runway, and it also makes it speed stable. Unlike cars, the drag on an aeroplane reduces with increasing speed up to a point, and so reducing speed requires increased thrust.

Stalling is an aerodynamic thing; what leads you to think that it's happening on final approach?

Regarding the going down and slowing down:- it's actually very difficult to do both. Typically, a jet (737 in this case) descends from high altitude at a Mach number, then at about 28,000 feet this changes to an indicated air speed (about 280 kts). At 10,000 feet the descent is stopped and the speed allowed to fall back to 250 kts, the speed limit below 10,000ft.

With about 20 track miles to run, the speed is reduced further to 210 kts, flaps are extended, and the speed reduces to 160kts. At this point the aeroplane is flying level, so slows down easily. Once it's coming up on the glide slope, full (or nearly full) flap is extended and the gear is lowered. This creates a phenomenal amount of drag, and so power is added to maintain the rate of descent.

You're actually descending on the glide slope with the engines at about 60% of their maximum speed. This is mainly because it can take nearly 10 seconds to spool the engines up from idle thrust to full power, which is a wait you don't want near the ground. From 60% full power is available within a couple of seconds.

We worked on the principle that to lose 1000 ft meant 3 miles covered across the ground, and likewise to slow down 10 kts meant another 3 miles. So it's all down to careful descent planning. To come down from 36,000 ft to 3,000, and slow from 280 kts to 210 means a ground distance of nearly 120 miles; hence why planning ahead is important. 🙂

Obviously, there are speed brakes as well, but they aren't used all that often. The other slightly counter intuitive thing is that the faster the forward speed, the faster the rate of descent, which is why the slowing down is left as late as possible.

Unlike cars, the drag on an aeroplane reduces with increasing speed up to a point, and so reducing speed requires increased thrust.

?? Thats opposite to the laws of physics? Unless you mean while going slower you tend to require flaps and landing gear to be extended, causing more drag. Drag on anything a constant shape and incident angle will only increase with speed?

BiGCol – or other person of knowledge – dufus q. but – given your pretty picture and description am I right in thinking then (in simple terms) the various compressors and turbines inside the engine do (amongst other things..) drive the ruddy great fan on the front which is what generates the trust that drives me and my duty free booze onwards and upwards?

(For reasons unknown – i'd always though that big fan brings in the air which is compressed more and more as it passes through the engine, exiting out the back at high speed, hence propulsion…?)

edit – what none of the above answers is why the hell folk feel the need to applaud when the plane lands safely – though I appreciate this was not the q raised by the op…. 🙂

what none of the above answers is why the hell folk feel the need to applaud when the plane lands safely

It's only ever happened to me and, to be fair, it was actually the designer of the undercarriage that should have been applauded, not the 'pilot' who dropped the thing to flat from about half a mile up.

It's only ever happened to me and, to be fair, it was actually the designer of the undercarriage that should have been applauded, not the 'pilot' who dropped the thing to flat from about half a mile up.

LOL – remember almost literally dropping back into Luton on the way back from Italy a year or two ago – MrsMM was asleep, for some reason I decided to wake her up before we landed, glad I did cos, right about then something happened and we all but fell to the runway from, i guess 50 feet….. 'welcome home!…' ..

coffeeking – total drag on an aircraft is the sum of induced drag (a consequence of generating lift through the wings) and parasite drag (basically air resistance against the fuselage). Induced drag is greatest at low speed, high angles of attack, and a "clean" wing configuration.

This reduces quickly with increasing speed, and you can see at some point the induced drag and parasite drag are going to overlap, with one going down and the other going up. This is the minimum drag speed (about 210 kts on a 737), when going either slower or faster increases drag and therefore requires more thrust. Dropping below this speed will result in a continuous fall in speed unless a large quantity of thrust is applied.

Extending flaps reduces this minimum drag speed due to the extra drag, and so the aeroplane is "speed stable" at a lower approach speed.

Induced drag is greatest at low speed, high angles of attack, and a "clean" wing configuration.

Surely it's all about the required AOA? You get more drag because of the required higher AOA rather than anything particular to do with the speed. If you pop the flaps out you can decrease the AOA and so decrease the drag. The minimum drag speed is presumably somewhere close to where the AOA stops decreasing as you get faster.

Typically, a jet (737 in this case) descends from high altitude at a Mach number, then at about 28,000 feet this changes to an indicated air speed (about 280 kts)

I think I understand what you mean, but then I have a bit of a clue about this – that isn't actually explained very well! What you kind of gloss over there is the effect of altitude – you get less drag, but also less lift, so the stall speed is much higher. Effectively you have to go faster than the minimum drag speed mentioned above (for low altitude), but because of the decreased drag at high altitude you don't actually lose out much (if it all – is there actually less drag at 30kft and 500kts than at 10kft and 210kts?) The issue then becomes that as you're going faster and because the speed of sound is lower at altitude, you start to get quite close to the speed of sound, at which point aerodynamic stuff all changes – hence why pilots worry about Mach number rather than IAS up there – the limit on top speed is presumably the Critical Mach Number? BTW is 28kft really not high altitude?

One interesting side point concerning extreme aircraft, the U2 flew at such altitude that it's stall speed approached it's limiting Mach number – therefore it had a very small window of allowable speeds. Not too bad you might think until you try to turn, at which point you have the issue of one wing tip stalling whilst the other tip has problems with the Mach limit!

Nobody's yet mentioned slats – like flaps, but for the leading edge, and doing much the same job (IIRC they actually have more effect on lift than flaps).

aracer – no, induced drag is a consequence of the airflow on the wings. Effectively, it rotates the lift vector backwards slightly so a component of the lift is acting as drag. We don't have flaps so we can fly at a lower angle of attack at high speeds; we have them so we can maintain the same AoA over reducing speeds. The Wikipedia article on this is helpful to understanding it, the maths isn't complicated. Flaps decrease induced drag because the disrupt some of the airflow patterns on the wings.

You're absolutely right that as we get faster we can reduce the angle of attack, and this is why induced drag reduces with speed.

I appreciate the point you're making that at altitude, the air density is reduced, so the lift is less. This is negated by the fact that we fly with reference to indicated airspeed, which is really a dynamic pressure measurement.

28,000ft just happens to be about the point that 280kts indicated is equal to Mach .74. Depends on the temperature, obviously. As altitude increases, the speed of sound continues to decrease and our .74 Mach is, in terms of both true air speed and indicated air speed, reducing as well. Eventually, you get to such an altitude where going faster leads to a Mach stall, and going slower leads to an aerodynamic stall.

Interesting about the U2. 🙂

Agree about slats, and there are also leading edge flaps as well, some of which are designed to deliberately provoke a stall under the right circumstances…

We don't have flaps so we can fly at a lower angle of attack at high speeds; we have them so we can maintain the same AoA over reducing speeds.

Given I wasn't suggesting using flaps at high speeds, just to allow you to use lower AoA at low speeds, isn't that pretty much the same as what I said?

This is negated by the fact that we fly with reference to indicated airspeed, which is really a dynamic pressure measurement.

Good point – though again one which is I think lost on those who don't already understand this. I have to admit to also blithely mentioning IAS. The point being that there are 3 different speeds, ground speed, true air speed and indicated air speed. Ground speed is obviously how fast you're travelling over the ground – useful for nav, but not really much good to a pilot otherwise. True air speed is how fast you're actually travelling through the air, which is what we're quoting, and what you might think is important. Indicated air speed however is what a pilot really cares about, as it tells him whether he's about to fall out of the sky – is pressure (hence altitude) related, with the same true air speed resulting in a lower IAS at higher altitude. "Indicated" because it's what the pitot tube airspeed indicator shows.

Feel free to correct me, flaperon, I'm just an amateur (though follow the discussions on the aviation NG in work, where we have professionals discussing stuff like this).

Ask a simple question… 😯

Related question – do all the number 1's and 2's get launched into the sea as a frozen block or is that urban myth ?

The point being that there are 3 different speeds, ground speed, true air speed and indicated air speed.

You forgot to mention the 4th type of speed.
That's the one you claim to be flying when you've busted the speed control imposed by ATC.