Concorde VMO question

[Joeyc41]

Why does the VMO of the Concorde increase with altitude, especially after reaching the tropopause, at a rate of 10 kts per 1000 feet?

[pattern_is_full]

Hmmm - are you confusing Vmo (max. operating indicated airspeed) with TAS (true air speed)?

It is my impression that Concorde flew a fairly constant VMO IAS of around 430-440 knots indicated on the cockpit gauge (or knots equivalent once supersonic) in climb once going supersonic at ~28000-30000 feet. But because it was climbing into thinner and thinner air, that constant indicated speed did result in a continuously increasing TAS and Mach (but those are not Vmo).
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* the asterisk on Mach 2.02 is because Concorde also had a leading-edge temperature limit of 127°C, and on some flights the weather was just too warm to reach Mach 2.02 without violating the temperature limit. Concorde has to use M2.0 or M1.96 or some such on those days.

[Vessbot]

https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1114x1436/d11d8a27a6a67ca536f511527c9fa3924a316b45_4d9d917a117275fc50d 39506a47a860f59b82e1e.jpeg
looks like Vmo does increase (in indicated) getting from 400-ish to 530, from the mid 30's to the mid 40's... and hanging out there for a while until reaching the Mmo of 2.04.

Also note that this is the envelope of what's allowed, which is not necessarily the same what it actually did on a typical flight

[stilton]

I stand to be corrected by the real Concorde experts who flew it on this site


From what I’ve read Concorde would climb at VMO with afterburners engaged until reaching M1.7 at FL450 at which point the AB was turned off and VMO maintained until reaching M2 around FL500 then cruise climbing at M2
From the graph it looks like 530 KIAS would be on the VMO profile at FL450

[Joeyc41]

Thank you all for your replies. My confusion arises because I saw footage of the barber pole on the airspeed indicator increasing as the Concorde climbed. At first I thought the airspeed indicator was unusually showing TAS, which would’ve explained what I saw. However, I believe this was not the case.

I quote this from what I read on another forum: “Firstly, when below the tropopause, climb at a constant IAS and as the OAT fell her Mach number would increase. Most subsonic aircraft do this anyway, climbing at a constant IAS (though not generally VMO!) until reaching their desired Mach number, when they transition to a constant Mach number for the rest of the climb.

Secondly, at or above the tropopause, the OAT would (in theory) remain constant at -56.5°C and would not fall further as she climbed, so this first technique would no longer work. Now, in order to increase her Mach number, Concorde had to climb and utilise the fact that her VMO increased steadily, by 10kts per 1,000 ft above the tropopause, and so by climbing higher she could now gradually increase her IAS, and thus her Mach number.

Thirdly, at around FL440, Concorde’s VMO stopped increasing and stayed at 530 kts IAS for the rest of the cruise/climb. With no further IAS increase possible, and no OAT decrease likely, the only way now to increase her Mach number was for her to climb at 530 kts IAS and utilise the fact that during a climb at fixed IAS, as the ambient air density decreases, TAS, and hence Mach number, will both rise.”

I am confused on the fact how the VMO was able to increase as an IAS/CAS with altitude.

[Bellerophon]

Joeyc41

A few posts after the post you quoted from, the author was asked the same question:

... Could you explain the reason for those Vmo changes with altitude and weight?...

And his reply was, and still is:

..."I’m afraid not, as this is getting in to an area well beyond my own level of technical knowledge! However, there are one or two learned gentlemen on this forum who were heavily involved in the design of Concorde and who may come along to shed some light on the matter"...

Fortunately one of those learned gentlemen did:

Last time I logged in someone was asking why the Vmo/Mmo was the way it was, but it seems to have disappeared along with Bellerophon's suggestion that someone else might be able to throw some light on it. This might help.

http://i1080.photobucket.com/albums/j326/clivel1/ProductionVmo.jpg

To be honest I can't remember exactly why 530 kts was chosen for the supersonic Vmo, but it was probably the best climb speed.
Mmo/Tmo was limited by a combination of intake and structural temperature.
The 'cut-off' in the sloping/530 kts boundary was, if I remember correctly, to avoid a minor aeroelastic problem at the Vd/Md condition one arrived at from that corner.
The variation of Vmo with weight was a device which, when associated with the CG corridor, allowed the aircraft to meet the manoeuvre requirements when flying on half hydraulics.
400 kts CAS gave 0.93M at around 28000 ft if I recall correctly, which was just below drag rise and gave optimum subsonic cruise performance

If CliveL is not able to shed any further light on your question, beyond his answer already given, I suspect there may not be too many others who can!

Kind Regards

Bellerophon

[Vessbot]

If CliveL is not able to shed any further light on your question, beyond his answer already given, I suspect there may not be too many others who can!

Kind Regards

Bellerophon

Just taking this chance to thank both you and him, as well as all the other participants in The Big Thread, which is one of the internet's treasures.

[EXWOK]

I can add little to Bellerophon's post, except to expand on the fact that it's necessary to consider that the Vmo on Concorde did a little more than on a conventional aircraft, as evidenced by CliveL's earlier post.

You will also note that it starts at about 300kts before it increases to 380-400 (depending on current mass). I don't know this for sure, but it was explained that this was partly or wholly a 'bird speed', as some other aircraft have. That may be fiction.

It certainly didn't function as such at Nairobi, where one could happily encounter the sizeable local avian fauna at the 'full' 400kts owing to the elevation.

As a pilot this didn't consume much brainpower- you just flew exactly on the 'barber's pole' as much as you could while fitting in with ATC requirements.

[megan]

at around FL440, Concorde’s VMO stopped increasing and stayed at 530 kts IAS for the rest of the cruise/climbThe VMO begins decreasing at a little over 51,000 from 530k to 440k at 60,000. Obviously the Mach limit gets in the way. Lot of "why" questions we can ask

[john_tullamarine]

I've not corresponded with him lately but I would be surprised if Clive doesn't see this thread and offer comment. If he doesn't know, or the answer has been lost with the passage of time, then we are unlikely to get the answer.

[M2dude]

Where the limits went right out of the DV window was during the annual C of A renewal flight for each aircraft. (I was privaleged to have been presesnt as a technical observer on the vast majority of ALL Concorde test flights for the last 15 years of service with BA)..
For the specific test n question it was necessary to trip the overspeed warning C/B, which just might give you a clue for what was to come:: From an altitude of around 51-52,000' a 'flat acceleration' was carried out, instead of allowing the aircraft, which was of course relatively light, being allowed to cruise climb. WITHOUT of course the use of reheat, the aircraft would accelerate up to Mach 2.1, Mmo being Mach 2.05. NOW things get really interesting, the aircraft then commences a steep zoom climb, the Mach number falling no lower than 1.8, climbing like a dingbat all the way up to 63,000'. (As most of you know I'm sure, the maximum allowable altitude on a commmercial flight was 60,000'). At this point the throttles are retarded and the nose pushed downwards in a gentle 'bunt' manoeuvre, for a second or two you hit zero G, I always used to toss my pen into the air and watch it seemingly hang in mid air before gently falling back down.
And all of this in an AIRLINER.
Best regards to all
DUDE. :)

[CliveL]

Not logged into PPRune for some time John.
With caveats: a) memories blur after half a century, b) this was an area of Aerospatiale responsibility
Worth remembering Concorde instruments and autocontrols were analogue - simplicity rules!
Vmo is manufacturers choice subject to some conditions, principally that Vmo cannot be greater than the structural cruise design speed Vc
So:
The low speed cutback could well have been a birdstrike precaution. The relevant rule is "The empennage structure must be designed to assure capability of continued safe flight and landing of the airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=999&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:92:25.631) after impact with an 8-pound bird when the velocity of the airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=999&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:92:25.631) (relative to the bird along the airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=999&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:92:25.631)'s flight path) is equal to VC at sea level"
The low supersonic "sloping" Vmo/Mmo was derived from performance considerations. The performance obviously depends of the variation of drag with airspeed and Mach No, together with the thrust variation with altitude and Mach. The objective was to choose a flight path which maximised the increase in energy height per lb of fuel consumed. From these sums one could generate a nominal boundary which gave the basis for a scheduled Vmo. The upper limit of this was defined by possible aeroelastic problems at the Vd/Md arising from the statutory upset from that point

[john_tullamarine]

How wonderful to see you on the boards again, good sir. Indeed, this thread ought to be distilled and published in hard copy for posterity.

blur after half a century

Now that I contemplate your comment, it's probably just as well that many of my antics from a half century back are lost to my conscious recollection .... and, hopefully, the other players' recall from the time as well ....

[Bleve]

The low supersonic "sloping" Vmo/Mmo was derived from performance considerations. The performance obviously depends of the variation of drag with airspeed and Mach No, together with the thrust variation with altitude and Mach. The objective was to choose a flight path which maximised the increase in energy height per lb of fuel consumed. From these sums one could generate a nominal boundary which gave the basis for a scheduled Vm

Having flown the ‘baby Concorde’ (i.e. Mirage III), the slope of the low supersonic Mmo boundary immediately reminded me of the optimum energy climb profile. CliveL’s post above confirms that. So it’s not a physical limit of the aircraft, just a performance limit. There’s little point in going any faster as you would just be wasting fuel. EXWOK’s comment that pilots simply climbed on the barber’s pole suggest that the lower supersonic Mmo boundary is related to this optimum energy climb profile.

For more information on how the optimum energy climb is determined, web search the term ‘Rutowski Climb’.