Posts about: "Vref" [Posts: 13 Page: 1 of 1]¶

Correct, Nick. More power was needed to fly the ILS at 160kts than at 190kts. So they kept 190kts and reduced to Vref+7 at 800 ft, IIRC (somebody wrote about it on the previous pages).

You're about right with the downwind speed - 250kts was standard.

Speed would be reduced to 190kts at the end of the leg, and then back to final speed on final approach.

Final speed would be one of the following:

Vref (not that often used, and not the nicest speed)
Vref+5 (one engine out)
Vref+7 (at the end of a fuel-saving ILS approach. Nicer to land off than Vref and the most common speed)
Vref+10 (as above in winds above 25kts?? Minimal flare off this one)

Round about the 155-165kt mark in normal ops.

Health warning - all the above from memory, NW1 will correct me if I'm wrong. (My manuals are stashed in the loft).

The pattern was flown at 250kts and 1500ft. The trouble is, you lift off at 210ish kts, not climbing that fast as you're way down the drag curve. Over the next thousand feet you steadily accelerate, and at the same time the RoC goes waaaay up as the drag reduces. This is fine - so long as you spot it and deal with it pronto.

It was quite easy to find oneself at 1000' flying at, say, 260kts. So you raise the nose a bit, to find you're still just creeping above 260kts passing 1300' (drag still reducing)....... and climbing at 5000fpm. And accelerating.

I'm told the record was 300kts and 3000' and I believe it!

Luckily we arrived on the scene armed with this story. I have to say the first thing I did passing 800' was roll on 30 degs of bank which calmed things down nicely.

Awesome fun.

Notfred -

There was no real difference between a touch and go and a normal take off/landing. As already stated earlier in the thread bucket contact was always a possibility if the landing was a little high-pitched, especially while the buckets translated. Not having the wings perfectly level reduced bucket clearance significantly. Not much of an issue on take-off.

As for checking runways - there was a lot of that done for this aeroplane, but nowt to do with clearances. Runway roughness was a potential issue on take-off, it was to do with the structural dynamics. No time to explain now, though, but I'll revisit it tomorrow if no-one else does.

In short - bucket contact would be the result of mishandling of some sort (e.g. incorrect Vref, speed decay, overflare, wings not level) not runway roughness.

[quote=ChristiaanJ] VLA (lowest admissible speed)
One would expect a curve for constant alpha max against IAS and altitude, not the staircase in the diagram.
Was this for simplicity of use of the diagram?[quote]

I don't have a complete explanation for all the regions - it was a long time ago and I'll need to dig, but:

Below 16000 ft Vla obviously needs to go as low as Vref to cover landing at elevated airfield altitudes. At present I don't have a satisfactory explanation for 250 kts between 16000 ft and about 45000 ft (250kts/Mach 1.0) A constant value in IAS is what you would get for a constant CLmax (the alphamax is not really the driver). Vla should give a margin above stall, and a quick sum suggests that 250 kts would be consistent with a 1.3Vs condition and a CLmax of about 0.8 up to Mach 1.0, which is not unreasonable, but I am not saying that is the correct interpretation.

From 45000 ft to 60,000 ft I think Vla may be set by manoeuvre requirements. Certainly the forward CG envelope boundary between 1.0M and 1.5M discussed in earlier posts is very close to the requirement to be able to pull 1.2g with half hinge moment available at Vla and heavy weights. Again not certain, but best guess at the moment.

Yes

[quote ]Mmo (max operating Mach number)
Mach 2.04 is usually quoted as having been chosen to assure an adequate life of the airframe.
But what effect does a higher Mach number as such have?
Or are Mmo and Tmo (127\xb0C) directly related?[quote]

I have always been puzzled by this statement as one does not normally associate Mach Number with a life limit. Going through my collection of lectures I found another, more plausible explanation:

quote" The scheduled cruise mach Number was 2.0. associated with a structural total temperature of 400 degK. Above ISA +5 Mc was cut back to maintain 400 degK.
To cope with variations of flight mach Number about Mc associated with often rapid and significant changes in wind and temperature which occur particularly in the vicinity of the tropopause (which can of course be as high as 60,000 ft in the tropics) a maximum operating Mach Number (Mmo) of 2.04 is selected" unquote [Leynaert, Collard and Brown, AGARD Flight Mechanics Symposium October 1983]

This is much more in line with my memory on this subject.

Yes in principle, but it is a bit chicken and egg, since 530 kts also represents a very good choice for best performance, and I am sure that the stucture would have been built to cope if a higher speed was needed for performance reasons. To the best of my knowledge there is no structural design case that would be critical in this flight regime (other than flutter of course)

Same as earlier - transonic manoeuvre requirements with failed hydraulics, matched to aircraft weight and CG envelope possibilities.

Same again.

I'm not entirely sure, but:
a) there is absolutely no advantage is having a high Vmo at low altitudes as it could not be exploited even if one wanted to because of ATC limitations to 250 kts below 10,000 ft (in the USA at least)
b) there are a lot of things that get rapidly worse if you encounter them at high speed and which are anyway more likely at low altitude - hail, birds etc.
So why store up trouble for yourself!

CliveL

What was the minimum maneuvering speed for Concorde
It was expressed in the flight manual as "Lowest Authorised" speed, Vla, and didn't depend on weight. 0-15,000' Vla=V2 or Vref as appropriate, 15,000'-41,000' Vla=250kias, 41,000'-60,000' Vla=300kias

Also what was the typical climb speed I'm guessing you mean rate of climb rather than IAS?
- At lift-off? From memory Vr was around 200kts, V2 around 220kts and if restricted to 250kts (way below min drag) you'd get pretty poor rates of climb - about 1000fpm if you were lucky and IIRC - you'd quickly want to lower the nose, just barely climb and get her up to 400kts when she'd really fly...
- Once 240 kts is achieved? see above - but once you got her up to min drag (about 400kts at MTOW) things went better - about 4000fpm without reheat
- At minimum maneuvering speed at typical takeoff weight? At V2 she staggered up due the the drag of the slender delta wing at low IAS - but climb performance on three engines (in contingency reheat) at V2/MTOW was better than a conventional subsonic jet on three / MTOW / V2 due to conservative certification requirements of the TSS
- At MTOGW? Does the above answer your Q? Happy to add more if you need...

Edited to add, most transatlantic takeoffs were at MTOW - around 185 tonnes - and due to the slender delta aerodynamics, weight didn't affect performance as much as a conventional wing anyway because induced drag was the bigger player at slow speeds - and I've just completely exhausted my very limited grasp of aerodynamic engineering!!

NW1

I assume in the US then you were restricted to 250 kts below 10,000 feet just like all other aircraft?

Why higher speed? That have to do with shockwaves and the resulting pressure distribution differences?

No, I meant the airspeed you'd be flying at while climbing (post takeoff)

Wow, that's pretty bad. You'd figure with a T/W ratio of around 0.40 you'd do far better than most other aircraft.

Were you allowed to get over 250 below 10,000 feet in the US, or UK? Regardless, what rate of climb would you get at that speed?

408,000 pounds?

I didn't find any problems in strong headwinds.

We used to use Vref+10 instead of Vref+7 if it was windy (which made a bigger difference than the numbers may suggest) and, if anything, this made it easier.

Many thanks for the answers folks. Can't beat getting it from the horses mouth. EXWOK, could you expand on the whys and what fores?

This may be overtaken by later postings, but a couple of reasons why n5692s's explanation might not work:-

Most of the lift is generated on the upper surface and is dominated by the vortex lift which is a product of vortex strength and airspeed. The vortex strength depends on the local aoa at the leading edge. As the aircraft enters ground effect the passage of air under the wing is restricted so more has to go over the top and the local LE aoa is increased along with vortex strength. The important bit of the wing for this bit of lift increase is the front half which is in the higher part of the wind profile. But in any case, following our old friend Bernoulli, the upper surface suction will depend on the resultant circumferential velocity as the vortex scrubs its way across the wing upper surface, and I can't see a knot or two of wind making a big difference to the circumferential velocities under those vortices.

The undersurface flow is of course restricted. and the lift is more Newtonian in character. A reduction in local airspeed because of the wind height profile could give a reduction in lift due to ground effect near the TE. However, in the normal course of events this additional lift is accompanied by a nose down pitch which is countered by a steadily increasing back stick movement as the pilor maintains the more or less constant pitch attitude "flare" manoeuvre. This up elevator gives an increasing negative lift to maintain pitch control which, since the effective cop of the elevator lift is at the elevon hinge line means that the net gain in overall lift from this part of the ground effect is quite small. If this undersurface TE lift were to be reduced by the wind gradient the effect would. be that the nose down pitch would be smaller than usual and the pilot would have to apply less back stick, but I doubt he would notice this in a dynamic situation (remembering that strong winds are usually accompanied by turbulence).

So I can't identify any gremlin job specification that might support n5296s's argument.

Kaypam: Remember the Concotrde was certificated to TSS Standards not JAR25. The certificated approach speed is Vref, Vref plus 7 if memory serves, was introduced as an approach noise reduction and became anaccepted norm so Vrefplus 10 should be OK for 20 kt winds?

stilton

... so you might use reverse on only one engine? any assymetric issues with that ?...

No issue at all, on the one occasion I can remember that it happened to me.


... why the 4 minute restriction ?...

I believe the correct answer has been given by CliveL, but, when asked during ground school, the BAe instructors’ traditional answer was “Noise Abatement”. (as in a Concorde hitting the ground makes a lot of noise!)


... BA or AF always had one of their captains as an observer in one of the cockpit jumpseats on these flights ?...

Before my time, so I can’t say if BA crew flew on the jump seat, or in what capacity they were acting if they did, but I believe there is at least one contributor to this thread who may yet post an answer.



CliveL

... Can any of our pilot contributors confirm n5296s's remarks re landing in a strong headwind?..

Speaking personally, I never noticed any problem, and as EXWOK has said, I found using VREF +10 made life a lot easier.

However, Mike Riley, a well respected base training instructor on the fleet (and a past British Aerobatics team member) discussed this point in his “The Concorde Stick and Rudder Book”, where he says that there was a greater incidence of hard landings when landing into a strong headwind and goes on to discuss some of the possible reasons why and what to do about it.

His main recommendation was to leave the auto throttle in later than usual, down to 20R instead of 40R, and maintain a constant attitude to touchdown.


... The certificated approach speed is Vref, Vref plus 7 if memory serves, was introduced as an approach noise reduction...

Yes, VREF +7 was used for Reduced Noise Approaches that were flown whenever possible, and which were generally considered easier to land from than VREF approaches.

I'm not sure the 'negative elevator effect' was ever a practical issue in flight (as opposed to on rotation) - the response was entirely normal and I reckon that the increase in lift is near enough instantaneous. Aircraft with tailplanes would have a similar theoretical effect.

I don't recall hard landings being an issue on windy days - quite the opposite.

If there was an influence due to wind I would say it's more likely to be that in the gusts one may be tempted to 'tweak' the attitude: Putting the nose down by half a degree at 50' would have very disappointing consequences....

From memory, we had 4 speeds available:

Vref 'Normal' final approach speed (actually not used that much)
Vref+5 Engine out speed
Vref+7 Noise reducing approach speed (used as often as possible)
Vref+10 If the headwind component was over 25(?)kts

Most were at +7.

Vref was least nice - you had a higher attitude to start with, and needed more flare, which meant tail clearance was tight. It also meant that if you picked up a high RoD at 50' or so, it was VERY difficult to catch. One *could* add a bit of thrust at 30' or not close the throttles at 15', but this was not without drawbacks.

Vref+5 was better - the reason it was used was to give better g/a performance on 3 engines (there are obviously a lot of square laws at play here, because it made a significant difference).

Vref+7 was used off pretty much every ILS approach to a decent length of runway, where we would carry out a noise-reducing approach. This is probably explained elsewhere in the thread. These days it would be called an unstable approach! It gave you more lift margin into the flare and also more room to make pitch inputs (i.e. space for another half- to one- degree of flare).

Vref+10 was for windy days. I liked it! If you arrived at 40' in the right place, you basically just held the attitude and the ground effect did the rest. It did still *feel* like you had flared, as the ground effect would push the nose down and so back stick was still required to hold the attitude.

I hope that has answered CliveL and Megan's questions somewhat?

It would appear so....but I wonder if Vref+10 came about as a result of earlier experience and that was the source of the story?

I get what you mean about the height response at the cockpit vs flightpath response....but given that we're talking about landing, then it's the height of the undercarriage that matters.

FWIW, like all long-bodied aircraft, a last-minute pull has the capacity to drive the u/c into the ground harder because the pitch change happens before the flightpath change. That could well be exaggerated by 'negative elevator' effect.

A video of a landing could be quite telling if you could see the elevons - there's a very marked and increasing 'up' input to counter the pitch down from ground effect. I suspect that, in ground effect, this negative elevator effect would be swamped by all the other things that are happening.

A slightly tongue-in-cheek comment, Megan.

The BA Ops Manual requires an approach to be fully stable at 1000'R. It used to be slightly more relaxed, but even then a 'reduced noise' approach was an exception to the policy.

Basically we flew at a higher speed (190kts) until 800', then reduced speed to achieve final approach speed at 300'.

This had two benefits - thrust required at 190kts was a lot less than 160kts so flew a quieter and less-thirsty approach, and the portion between 800' and 300' was quieter still.

Vref+7 gave better control and performance margins at the end of this manoeuvre - I'll try to find a reference as to why +7 came about (I recall there was a specific reason for that number, but not the detail).