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VC10 increasing Vmo above 20000 feet

According to charts I have recently downloaded the VC10s vmo was 317 knots at 20000 feet, thereafter increasing to 329 kts 11000 feey later
Why would the vmo increase when before it was decreasing.
Opinions and education welcome
Thanks

To illustrate the question, see graphs here: https://www.pprune.org/aviation-hist...0000-feet.html

The charts are indeed correct. I knew the reason once, but unfortunately have long forgotten it! I now seek enlightenment too.

I think you might find it is something to do with birdstrike tolerance of the windscreen. Birds don’t fly above 11000’ do they ???
From my past the 757 had the same limitation 313 kts below fl100

I thought I'd read / been told once that 250 kts below 10 K ft was mandated for windscreen bird strike protection, as that was all the turkey cannon testing device could muster. However as I've been on lots of aeroplanes doing a good deal more that 250 kts below 10 K, it's clearly changed.

When did it change and how does it work now ?

Was for collision avoidance although in the good old days it didn't exist and once saw 365 knots flying up the Thames estuary on the extended ils for 27 right at heathrow.
Most I've done is 300 knots below 3,000ft coming out of Tripoli.
Regularly asked heathrow departures if they had speed control on departure and accelerated to VNE under the Biggin pattern which restricted our climb clearance.
As for birds they fly above 25,000ft.

Maybe I wasn'y quite clear enough im my question.
Vmo was 337kts at sea level reducing to 317 at 20000 then incrasing again

Possibly an intelligent guess at that time

I asked my VC10 base trainer and he seemed to think we were told that it was to do with windshield heat and strength which could be the answer as the heating capacity was probably limited and with decreasing oat the screens became slightly less impact resistant, he quoted the adverse with Concorde which had a vne of 300 knots at sea level which increased to 400 knots BUT
one needs to realise the experimental nature of designing post war aircraft and old wives tales.
My father in law was stationed at Luftfahrtforschungsanstalt Volkenrode after liberating Belsen.
The allies divied up the German wind tunnels which were far in advance of ours. I met a guy last year who worked on the supersonic one designing the inlet doors of concorde. (RAS Toulouse.)
In my gliding days we reduced VNE with altitude due to reducing flutter speed wrt IAS. Imho that could be the reason for the reduction up to 20,000ft but it could still be a risk assessment of bird strike with altitude.
Even posters here have no realistic idea of how high birds fly and there was an accident with a bird strike in cruise above 30,000ft over Africa iirc.
Typical misinformation is on a thread concerning aquaplaning speeds calculated with tyre pressure.
I was on the BALPA tech committee in 1974, it was obvious that the pressure theory was relevant to smooth tyres and the speed that we slammed Tridents onto a wet runway was way above the then theoretical aquaplaning speed. Hence the procedure was a waste of time but 20 years on the myth had been adopted by Big Airlines. I don't know whether they still do it but its a waste. Proof is that I did a bike track day in torrential rain near Marseilles with less than 30psi and chickened out at 200+kph - without aquaplaning. (The technique was unheard of in Swissair).
On a sideways note a VC10 went way above VNE after a badly handled smoke incident.

I asked an ex-VC10 flight engineer who was involved with the test flying. His response:

Quote:

You will notice that the slope of the high speed part of the diagram is closely related to true air speed, and that is the clue.

The force on the air being driven upward by the leading edge of the wing is directly related to the air density and the acceleration that is being applied to that mass of air so that it can pass over the wing.

As the true air speed increases the acceleration required of the mass of air increases and it is eventually reluctant to follow the wing shape.

The reducing Vmo shows how this comes into effect.

However with increasing altitude, above approximately 20,000ft, the density of the air reduces to the extent that the mass of the air to be accelerated also reduces such that its reluctance to move is reduced.

It will therefore follow the shape of the wing more willingly.

The graph shows rigid straight lines but in truth there is a slight curve, particularly at about 20,000ft.

There is a slight difference between the standard and super high speed buffet but it was decided to leave the diagram the same and therefore not have to modify the path of the Vmo pointer in the ASI.

The top end of the diagram is fairly conventional and shows when shock waves are likely to significantly disrupt the air flow.

Just a reminder that the diagram is drawn, as required by the the Air Registration Board (ARB), for a situation of the aircraft manoeuvring and pulling a force of 1.35G. (A bank angle of about 42 deg.)

You can see how close the 290kt climb speed comes to the low speed buffet boundary. I normally advised my pilots to climb a bit faster at high weight, normally I suggested 300kt.

In straight and level flight, with no disruption, the aircraft could fly well outside the low and high limits, of the diagram as we proved during certification flying, but not approved for in service.

Going too fast produced a bit of a rumble.

I hope that helps to explain both side of the diagram.