Go Back  PPRuNe Forums > Misc. Forums > Aviation History and Nostalgia
Reload this Page >

VC10 increasing Vmo above 20000 feet

Aviation History and Nostalgia Whether working in aviation, retired, wannabee or just plain fascinated this forum welcomes all with a love of flight.

VC10 increasing Vmo above 20000 feet

Old 21st Jan 2020, 12:57
  #1 (permalink)  
Thread Starter
 
Join Date: Jun 2001
Location: britain
Posts: 625
Likes: 0
Received 0 Likes on 0 Posts
VC10 increasing Vmo above 20000 feet

https://www.pprune.org/tech-log/629038-vc10-increasing-vmo-above-20000-feet.htm
Can one of our experienced VC10 pilots educate me?
bean is offline  
Old 21st Jan 2020, 15:57
  #2 (permalink)  
 
Join Date: Sep 2010
Location: by the seaside
Age: 74
Posts: 505
Received 9 Likes on 6 Posts
Might be wrong

But think it was to do with windshield resisting bird strikes...but then again could be a load of horlicks.
blind pew is online now  
Old 21st Jan 2020, 19:12
  #3 (permalink)  
 
Join Date: Mar 2002
Location: near an airplane
Posts: 2,540
Received 12 Likes on 12 Posts
I had a look at the Tech Log thread here: VC10 increasing Vmo above 20000 feet

I guess you found the BCal Sales Brochure on my site? Or was it a different document?

I don't have the answer for you, but I do have these graphs that illustrate the issue. In addition, I'd also like to know the reason for the changing Vmo.




Last edited by Jhieminga; 22nd Jan 2020 at 08:00. Reason: Dodgy typing
Jhieminga is offline  
Old 22nd Jan 2020, 07:05
  #4 (permalink)  
Thread Starter
 
Join Date: Jun 2001
Location: britain
Posts: 625
Likes: 0
Received 0 Likes on 0 Posts
Maybe it's because of compressability affecting ASI readings
I have been looking at the Caledonian/BUA brochure
It's fascinating



bean is offline  
Old 31st Jan 2020, 08:58
  #5 (permalink)  
 
Join Date: Mar 2002
Location: near an airplane
Posts: 2,540
Received 12 Likes on 12 Posts
I asked an ex-VC10 flight engineer who was involved with the test flying. His response:
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.
Jhieminga is offline  
Old 31st Jan 2020, 12:29
  #6 (permalink)  
 
Join Date: Jan 2006
Location: London
Posts: 184
Received 3 Likes on 2 Posts
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.
Be interesting to know how fast.
topgas is offline  
Old 31st Jan 2020, 13:26
  #7 (permalink)  
 
Join Date: Apr 2009
Location: A place in the sun
Age: 82
Posts: 1,208
Received 21 Likes on 7 Posts
But graphs I have seen for other jet aircraft transport types do not have that pronounced kink at 20,000ft. What was so different about the VC10?

And as for being outside the boundaries of the graph, remember it is drawn for 1.35g, I've been up to a little over 46,000ft, just between the high speed and low speed buffets in smooth air. Ease the stick forward - a gentle high speed rumble; ease it back a little - the onset of low speed buffet. This was with a certain ex-617 Squadron training captain.
Bergerie1 is offline  
Old 1st Feb 2020, 19:32
  #8 (permalink)  
 
Join Date: Mar 2002
Location: near an airplane
Posts: 2,540
Received 12 Likes on 12 Posts
The VC10 was the first supercritical wing to enter service on an airliner, as far as I know. With that in mind, a strangely shaped diagram doesn't surprise me as they must have encountered a few new things along the way. We also know that the wing leading edge extension on the Super (and all but the BOAC Standards) wing was a compromise between redesigning a whole wing structure and an economical solution. So aerodynamically, the wing was not ideal. Later supercritical wings will have learned from this process, most likely sorted it out at an earlier stage and didn't have to deal with four hungry engines behind that wing center section either.

Just a bit of guesswork on my end, but it fits....
Jhieminga is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell or Share My Personal Information

Copyright © 2023 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.