View Full Version : Stall speed of 747 ????

14th Nov 2000, 09:50
Does anybody know ?

Another question - are they able to land this giant without the flaps and spoilers without actually putting a scratch on it?

14th Nov 2000, 12:50
B747-400 Initial Buffet Speed 100-230 kts depending on weight, configuration and pressure altitude (0 – 10,000 feet). Based on 1G speeds, Forward CG limits and Idle Thrust.

Max Landing Weight, SL, Clean = 198 kts. The stick shaker speed is a couple of knots higher.

B747-400 Non-normal configuration landing distance, Dry Runway.

Flaps UP – VREF30 +70 Landing Distance 6100 feet based on 250,000 kgs

I’m presuming that the Flaps UP description refers to the Flaps lever position and therefore accounts for Slats and Flaps UP. I’m sure that someone who actually flies them will correct me if I am wrong.

Mutt :)

15th Nov 2000, 00:08
your figures are pretty much correct, although I personally would doubt your landing distance figure of 6100ft. That might be landing ground run (not landing distance) with maximum braking and full reverse, but I wouldn't rate my chances of stopping a Classic (my jet - smaller brakes than the -400) from a 210kt Vref in much less than 9000ft! Boeing does promulgate a flapless landing procedure, but states that the chances of being in that situation are very, very remote - there is so much redundancy in the hydraulic, pneumatic and electrical flap operating systems. Incidentally, the leading-edge devices on Boeings are referred to as flaps, not slats.

15th Nov 2000, 11:57

Thanks for the feedback, the figures which I quoted are from the B747-400 Operations Manual page PI.10.28 dated October 01,1999.
The data is based on:
1: Actual (unfactored) landing distance.
2: Includes distances from 50 feet above threshold.
3: Assumes max manual braking.

I wouldn’t expect to see a B747 of any model operating off runways of that length, but the figures in the QRH are there for when you HAVE to land.

Thanks for setting me straight on the leading edge flaps. With leading edge flaps on the 747, leading edge slats on the 757 / 777 and both leading edge flaps and slats on the 737-200, I’m starting to get confused!!!

Mutt :)

16th Nov 2000, 15:36
yes, you're right that the figures are there for when it's just got to be done. Still wouldn't fancy my chances, though!
The slats/flaps question depends on the construction and operation of the devices. On the 747, the leading edge flaps ('Kruger' flaps) are panels that are hinged at the forward edge, and stow flush to the lower surface of the wing. When deployed, they rotate about the hinge and end up sticking out forward of the wing leading edge. In this position they effectively increase the wing's surface area and camber. This produces more lift (increased CL max) and a higher stalling angle.
Slats, on the other hand, are a method of moving the wing's leading edge forward to both increase the wing's effective area and produce a 'slot' behind the displaced leading edge. This slot is configured to allow air from below the leading edge to flow over the upper surface of the wing, thus 're-energising' the airflow over the top of the wing. This has the same overall effect as the Kruger flap, but can be more effective in increasing the stalling angle. As you say, you see both types on airliners, but as a generalisation Kruger flaps are better for heavy jets trying to obtain the lowest possible speed on approach, while slats tend to be better at providing increased manoeuvrability.
Hope that's not too confusing!

[This message has been edited by scroggs (edited 16 November 2000).]

John Farley
17th Nov 2000, 03:17
Anthony 2107

Perhaps a bit off topic, but I am genuinely curious to know why you were so interested in “stalling speed” because it is a number that has no significance unless you also know many other things about the aircraft’s circumstances.

Forgive me if I seem to be nitpicking, but an aircraft “stalls” only because its wing is flown at an angle of attack (AOA) that exceeds a certain number. This ANGLE is broadly the difference between where the nose seems to be pointed - looking side on - and the direction in which the aircraft is travelling. That ANGLE, which is unique for any aircraft type but does vary slightly between types, will usually be in the range 14 to 18 degrees for “ordinary” looking aircraft. Some slender deltas and other special configurations might be happy to go into the mid 20s. The stalling angle will only change slightly to another fixed value when leading or trailing edge flap positions are changed.

The speed at which this stalling angle is reached (your stalling speed) will depend (to varying degrees) on the weight, the bank angle being flown, the g being applied, the atmosphere (density/temp) the thrust in use and the position of the centre of gravity to mention some of the more obvious factors.

Indeed from a pilots point of view it would be harder to pick a single bit of data that had a more tenuous connection with when a wing actually stalls than the airspeed that happens to be on the airspeed indicator at the time.

From which you will see I think the term “stalling speed” is a rather meaningless abomination.
Only human pilots still use the idea of a stalling speed. All automatics like the autoplilot, the autoland system, fly by wire control systems and (yes!) the stall warning systems and stall recovery systems all measure the AOA to decide how to go about their business. Quaint eh?

But then I expect you knew all that – in which case sorry!


John Farley
18th Nov 2000, 14:40
Its gone very quiet. Did I get my choice of bait wrong again?

19th Nov 2000, 22:19

I guess that the original question had to do with a FS2000 operator......

I enjoyed your response, thanks.


John Farley
20th Nov 2000, 00:06

Thanks, silly me, I should have realised


20th Nov 2000, 07:19

What is it about this question that bugs you ? By the way I have never used FS2000,
maybe you have.

Every aircraft has a stalling speed in different circumstances and by the way I don't profess to know everything like you - or don't you know that with all your "mumbo-jumbo" about aerodynamics. All I'm asking is the stalling speed in the landing configuration - not a whole lecture about it because I think I remember hearing about it in class and reading about in a book called "Aerodynamics for Naval Aviators" and others so please don't give me a nitpicking comment because it is a legitimate question. The first answers suited me just fine without your off-tangent comment.
Thanks for nothing pal.......

20th Nov 2000, 11:12

Politeness dictates that after asking the original question you at least acknowledge the answers.

Mutt :rolleyes:

20th Nov 2000, 14:49

Thanks for the answer then.

I was away on a flight for almost a week,
just found out about your answers today because my brother has been using my log-on name.

By the way..... I'll look up that FS2000 you mentioned ( If I have the time ).

John Farley
20th Nov 2000, 17:11
Hi Anthony

Good to hear from you. Sorry I got up your nose. It’s the notion of stalling speed that I find so unhelpful - not you! I can only regret that my English lit skills were not up to explaining myself properly.


PS As to me thinking I know all about aviation – I wish! Fact is I give daily thanks that I managed enough to reach retirement.

Old Dog
22nd Nov 2000, 21:37

You seem to be quite upset by John Farley's post, but I don't see why you should. You asked:
Another question - are they able to land this giant without the flaps and spoilers without actually putting a scratch on it?

The way this question is being asked, implied your lack of knowledge about flying the B747-400, and probably other airplanes too. If you were a professional pilot, you would already have the answers without having to ask the more knowledgeble pilots. If you are not yet a professional pilot, you should be more polite when the more experienced and knowledgeble ones took the trouble to reply to your queries, regardless whether the reply is to your personal taste. As I see it, JF did not make fun of you. He is quite right to say that stalling speeds by itself is not very meaningful. His explicit explanation should be taken to heart by all junior aviators like you. Whether you like it or not, in aviation, seniority (=experience) does carry a lot of weight.

Flying the B747 is no different from flying other airplanes. Only the details differ.

You posted: I was away on a flight for almost a week, but your profile says you were a software consultant. You didn't say you were also a pilot.

[This message has been edited by Old Dog (edited 22 November 2000).]

Old Dog
3rd Dec 2000, 22:51
John, were you involved with the Harrier jump-jet? How on earth did you make that plane fly without the help of modern days artificial-stabilisation?

John Farley
4th Dec 2000, 01:31
Old Dog

Quite easily actually, because the aircraft had a very good reaction control system!

Give pilots good controls and they can cope with very little stability, or even in the case of the Harrier some instability in the circuit below wingborne speeds. You can’t expect to take your hands off the stick and have it stay steady, but hey, its not the cruise it’s the circuit.

The Wright brothers understood this. Their design deliberately gave priority to control rather than stability.

My definition of good controls is they should have light forces that build up steadily with deflection, be fairly sensitive (so you don’t have to move your hand far to get the response you are looking for) and of course have no rubbish like backlash or sticktion. It is also necessary that they offer a fairly linear response (move your hand twice as far and you get twice the effect not say three times or one and a half times)

In my view all the aviation text books on the Stability and Control of aircraft have got their priority wrong they should be about Control and Stability – first things first! Stability is nice to have once control is sorted but a stable aircraft that you can’t control is no use to woman, man or beast.



4th Dec 2000, 05:31
And so the definition of a helicopter is

"an unstable aircraft you can control".

[This message has been edited by mik (edited 04 December 2000).]

Old Dog
4th Dec 2000, 22:58
>> My definition of good controls is they should have light forces that build up steadily with deflection.... [unquote the Chief Test Pilot of Harriers]

John, thanks for your reply. Now I know I am not the only fellow on earth to complain about Boeing's design of the flight controls : they have a rather harsh centering mechanism. I missed the lighter and gentler centering on the Airbus. When I talked to my colleages, they would just shrug it off and said it is normal for powered controls to behave like that. I am of the opinion that a centering mechanism with much lower thresholds (lighter and less abrupt break-out force) allows for more precise control input.

Thanks John, your experience and knowledge on control systems is beyond challenge. You are the man.

PS: The Harrier has always intriqued me as an 'unflyable' plane for the lack of natural stability. Helicopters have their big rotors (=gyroscope) for stability.

[This message has been edited by Old Dog (edited 04 December 2000).]

5th Dec 2000, 05:53
Just as a random fact.
During flight testing in Seattle the 747's all performed stop and go landings at Boeing field 10,000ft

John Farley
5th Dec 2000, 20:02
Old Dog

Yes I agree that clunking in and out of centre is bad news – or as you so rightly call it breakout. The Hunter’s ailerons were like that. (Hence all the guys waving goodbye as they left the ground on their first Hunter take off). Fortunately, Hugh Merewether – Bill Bedford’s deputy during the early P1127 and Kestrel flying - understood these issues very well and got the message across to the designers at Kingston. Having got rid of backlash, and most of the friction, there just remained the matter of establishing the roll power needed to handle the trim changes close to the ground. It turned out that any thing less than 2 radians per second squared (reaction controls are of course acceleration controls not rate controls like ailerons) was asking for dinged outriggers. The last polish came with the Sea Harrier when we persuaded them to double the sensitivity of the roll control (move your hand half as far for the same effect) despite the fact that the NASA bible on hovering controls said that would lead to overcontrol. (I was pretty sure that bible was based on a false premise – and so it turned out). I had tried hovering with my hand halfway down the stick to achieve the same effect, without the overcontrol that was forecast – so armed with this got the other guys to try it and we were able to force the change through “off the drawing board” for the SHAR. This made it a really sweet aircraft in the hover. It was such an improvement that I managed to take a shot of a fireman with a camera during the first hover. Only the other guys who flew the previous set up appreciated just how much better things had to be to be able to trim it laterally at least well enough to let go for 2-3 secs!



PS talk about off topic but what the hell.

Old Dog
6th Dec 2000, 18:43
John, thanks again.

Ever since I started logging into PPRuNe, I have come across many interesting messages posted by this gentlemen named John Farley. I was wondering who this wise, knowledgeble and very polite guy is. Then the other day I was watching a documentary about the Harrier Jet. Your name rings a bell!

I am honoured to have learned from the foremost test pilot in the history of aviation. My time at PPRuNe has been very rewarding.

Thank you, John.

(Yes, we were off topic here, but so what)

[This message has been edited by Old Dog (edited 06 December 2000).]

7th Dec 2000, 06:17

--trim it up so you can fly hands off for 2 or 3 seconds? Sounds like my DC-8 on final approach! ;)

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