Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

Exceeding Vne

Rotorheads A haven for helicopter professionals to discuss the things that affect them

Exceeding Vne

Old 26th Nov 2018, 23:28
  #1 (permalink)  
Thread Starter
 
Join Date: Oct 2014
Location: Japan
Posts: 16
Likes: 0
Received 0 Likes on 0 Posts
Exceeding Vne

Hello all,

I’m trying to learn more about the background of Vne in helicopters. The only reasons I can find cited for observing Vne are retreating blade stall and compressibility (apart from the fact that it is an aircraft limitation).

It seems to be a fairly common belief among many I’ve flown with that Vne exists only to avoid the regime of flight where you may enter retreating blade stall. I’ve done some research and found references to mast bending, flight test cost, and aircraft-specific aerodynamic tendencies such as safe autorotation speeds. However, I can’t find anything concrete, or anything explaining these ideas in more detail.

Can anyone give me an idea, or point me in the right direction of where to find, potential reasons for Vne being defined where it is? This is mostly a curiosity question for me - but it’s also nice to have some concrete examples when “we exceeded Vne and nothing happened” comes up in conversation.

Thanks for the help!
Chucklehead is offline  
Old 27th Nov 2018, 00:02
  #2 (permalink)  
 
Join Date: Dec 2007
Location: Denver, Colorado, USA
Posts: 196
Received 10 Likes on 2 Posts
Some light reading for you
https://www.ecfr.gov/cgi-bin/text-id....1.27&rgn=div5
And search for VNE
mnttech is offline  
Old 27th Nov 2018, 00:12
  #3 (permalink)  
 
Join Date: Sep 2002
Location: Great South East, tired and retired
Posts: 4,349
Likes: 0
Received 193 Likes on 89 Posts
For starters, it ain't retreating blade stall.

You referred to mast bending - this is part of the overall stresses on the aircraft - the disc tilts forward to drag the fuselage through the air, and the horizontal stabiliser tries to pull the tail down to keep the cabin level and comfortable for the pax. In the middle is the transmission, its mounts, and the mast, which have to absorb these bending stresses.

The advancing blade tip speed starts to reach the mach drag rise, stressing the blade and making the Xmsn work harder for every advancing blade.

You will also start to run out of cyclic movement, or reach the place where cyclic displacement goes the wrong way, so it is unable to ensure stability as per the definitions.

Yes you can exceed Vne, but you are entering the area where things can be a little unpredictable, and the wear and tear will increase dramatically. I have been in an S76 (Vne 155kt) that hit 180kt when crossing a windshear, 35kt tailwind into a 40kt headwind. Massive balloon upwards despite dumping the collective.
Ascend Charlie is online now  
Old 27th Nov 2018, 01:00
  #4 (permalink)  
 
Join Date: Jul 2008
Location: New Zealand
Posts: 954
Likes: 0
Received 3 Likes on 1 Post
Also, control movements above VNE may cause important parts to depart from the aircraft with undesirable results...
krypton_john is offline  
Old 27th Nov 2018, 02:37
  #5 (permalink)  
 
Join Date: Mar 2007
Location: Hobe Sound, Florida
Posts: 949
Received 32 Likes on 26 Posts
KJ, the FAA may have changed the rules since I retired, but the test requirement in the advisory circular accompanying Part 29 used to require flight test at a stable point at 1.11 times the twin engine Vne. Losing parts would be considered bad form.
I’d imagine the Part 27 Advisory Circular reflects the same requirement.
JohnDixson is offline  
Old 27th Nov 2018, 03:04
  #6 (permalink)  
 
Join Date: Jul 2008
Location: New Zealand
Posts: 954
Likes: 0
Received 3 Likes on 1 Post
Originally Posted by JohnDixson
KJ, the FAA may have changed the rules since I retired, but the test requirement in the advisory circular accompanying Part 29 used to require flight test at a stable point at 1.11 times the twin engine Vne. Losing parts would be considered bad form.
I’d imagine the Part 27 Advisory Circular reflects the same requirement.
I knew you'd have the exact detail, Mr Dixson! Re: "stable point: is that straight and level and without control movements?
krypton_john is offline  
Old 27th Nov 2018, 03:13
  #7 (permalink)  
 
Join Date: Dec 2007
Location: Denver, Colorado, USA
Posts: 196
Received 10 Likes on 2 Posts
Originally Posted by mnttech
Some light reading for you
https://www.ecfr.gov/cgi-bin/text-id....1.27&rgn=div5
And search for VNE
So those are the actual rules. You can go up a level and then find the rules for part 29.
JohnD is correct, they still make you go to 1.11 so that you have shown it will work at 1.10, or something like that, I don't remember.
On the FAA's rgl.faa.gov web site, under Advisory Circulars, search for either AC 27-1B or AC 29-2C, and they describe more of the reasoning behind each of the rules.
mnttech is offline  
Old 27th Nov 2018, 06:18
  #8 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 10,289
Received 608 Likes on 266 Posts
John, would it be correct to say that the manufacturers and designers work out and test the maximum stable speed first and then reduce that by 10% to get that VNE for certification?

That would seem to be a way of guaranteeing you can produce the 1.11 x VNE for certification.
crab@SAAvn.co.uk is offline  
Old 27th Nov 2018, 07:29
  #9 (permalink)  
 
Join Date: Feb 2003
Location: Somerset
Posts: 282
Likes: 0
Received 0 Likes on 0 Posts
Rather more to it than that

Starting with a clean sheet design you choose a target and and then see if you meet all the rules.

under JAA/CS there are the obvious handling and control margin ones, the roll authority and flutter demo ones generally handles the blade stall issue. One of the first things to do is see if the selected value is sensible and achievable (i.e are the loads OK to produce a usable fatigue life and to have adequate strength margins,, is the vibration tolerable, is there sufficient power margin to make the value usable? There is no point in being in a 2000 fpm ROD at Vne.)

Part of the original decision is structural strength to cover air loads and bird strike requirements, UK military rules base the operating speed (Vno, which is equivalent to civil Vne) as 0.9 x Vne, where Vne is 0.9 x max demonstrated speed, so a UK design demonstrates 123% of the operating speed, civil rules is 111%.

there is never just 1 reason why Vne is what it is.....HOWEVER NEVER EXCEED IT AS YOU HAVE NO IDEA WHAT SET IT AND SOMETHING YOU HAVEN'T CONSIDERED MIGHT BITE YOU, THE CLUE IS IN THE NAME.....NEVER EXCEED SPEED
DM​​​
dangermouse is offline  
Old 27th Nov 2018, 09:05
  #10 (permalink)  
 
Join Date: Mar 2007
Location: UK
Posts: 321
Likes: 0
Received 0 Likes on 0 Posts
VNE of light helicopter, altitude reduction ?

Post #24

This is probably the most accurate, concise, and easily understood explanation of Vne that I have seen.
Also, if you haven't already, read the relevant parts of Shawn Coyle's 'Cyclic and Collective' and Cooke & Fitzpatrick's "Helicopter Test and Evaluation".
nodrama is offline  
Old 27th Nov 2018, 09:10
  #11 (permalink)  
 
Join Date: Oct 1999
Location: Den Haag
Age: 57
Posts: 6,218
Received 317 Likes on 176 Posts
I knew you'd have the exact detail, Mr Dixson! Re: "stable point: is that straight and level and without control movements?
Can be in a descent.
212man is offline  
Old 27th Nov 2018, 10:33
  #12 (permalink)  
 
Join Date: May 2002
Location: Downeast
Age: 75
Posts: 18,267
Received 467 Likes on 191 Posts
Brother Dixson might be able to confirm my foggy memory of an account by another Sikorsky Notable that reported the S-76A VNE could have been higher but the expense to test/document/certify the additional speed was deemed un-economical and thus was not done.


Determination of Vne is controlled by Section G.

What is interesting is the FAA requires the data used to compute Vne displayed to the Pilot have no more than two Parameters for the Pilot to consider to reduce the Mental Effort required.

https://www.faa.gov/documentLibrary/...hru_Chg_6_.pdf

Last edited by SASless; 27th Nov 2018 at 10:45.
SASless is offline  
Old 27th Nov 2018, 10:35
  #13 (permalink)  
 
Join Date: Mar 2007
Location: Hobe Sound, Florida
Posts: 949
Received 32 Likes on 26 Posts
212 is correct ( as usual ). It winds up being a dive. Set max power in level flight then push the nose over and fly to the desired speed, take data and then recover back to whatever comes next on the flight test card for that flight. 76 was 185 and 92 was somewhere in that ballpark-just don’t recall the number.

Military qual ( US ) is a bit different. Typically the Vne is selected as a percentage of the design weight Vh ( level flight max speed at design gross weight ), and thus it too, is a dive point ( thus similar in thinking to the 1.11 FAA point ). The difference is that the military qual requires maneuver data at that point, to include a pullout. Thus the UH-60 at design weight did 2.8 G at 193 KIAS, as well as a maneuver called a “ Dynamic Yaw “, which is described as follows: set up the Vne ( sometimes referred to as Vdive ) then apply pedal in one direction to the max steady state sideslip angle, and upon reaching that, apply full pedal at max rate in the opposite direction to achieve the transient overswing sideslip angle*. Those two numbers for the UH-60 were 15 and 30 degrees. If you ever noticed the small bayonet type device on the leading edge of the 60 door, about half-way up the window, it is there because when we did this maneuver to get the left overswing angle data point, the left door would pop open with a very loud bang, and accompanying air racket, and the pilot sitting in that seat didn’t much care for it.
*obviously this requires some judicious buildup, so this maneuver ( to be accomplished separately in both directions ) winds up being done a couple of times.

Last edited by JohnDixson; 27th Nov 2018 at 10:43. Reason: typo
JohnDixson is offline  
Old 27th Nov 2018, 10:43
  #14 (permalink)  
 
Join Date: Oct 1999
Location: Den Haag
Age: 57
Posts: 6,218
Received 317 Likes on 176 Posts
....as well as a maneuver called a “ Dynamic Yaw “, which is described as follows: set up the Vne ( sometimes referred to as Vdive ) then apply pedal at max rate in one direction to the max steady state sideslip angle, and upon reaching that, apply full pedal in the opposite direction to achieve the transient overswing sideslip angle. Those two numbers for the UH-60 were 15 and 30 degrees. If you ever noticed the small bayonet type device on the leading edge of the 60 door, about half-way up the window, it is there because when we did this maneuver to get the left overswing angle data point, the left door would pop open with a very loud bang, and accompanying air racket, and the pilot sitting in that seat didn’t much care for it.
Sounds like a joyful exercise I'm sure you all looked forward to! Bit like AFCS hardover testing and probably met with similar enthusiasm as flutter tests by FW TPs......

Brother Dixson might be able to confirm my foggy memory of an account by another Sikorsky Notable that reported the S-76A VNE could have been higher but the expense to test/document/certify the additional speed was deemed un-economical and thus was not done
My understanding was it was associated with the negative stick trim stability that necessitated the fitting of the Pitch Bias Actuator - and the handling characteristics the pilot would be faced with if the PBA failed above the actual Vne as set.
212man is offline  
Old 27th Nov 2018, 10:56
  #15 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 75
Posts: 4,378
Received 24 Likes on 14 Posts
I’m also sure that Brother Dixson will recall the Sikorsky pilot who came over to WA and ran endorsement training for the Okanagan Australia pilots when the first two S76As arrived in Exmouth.

Running through the local ravines at significantly above published Vne was proof that the flight envelope was well on the conservative side!
John Eacott is offline  
Old 27th Nov 2018, 10:58
  #16 (permalink)  
 
Join Date: Mar 2007
Location: Hobe Sound, Florida
Posts: 949
Received 32 Likes on 26 Posts
SAS, you are correct, but while I flew with Nick Lappos for a lot of the 76A development, I switched back to S-70 projects immediately thereafter. Nick will ( I trust ) correct my recollection of the 76A Vne point, which is that it was controlled not by structural considerations, blade stall, vibration, nor tip mach number or power, but by the fact that beyond the 156 ( 155? ) speed we could not demonstrate the classic fixed wing longitudinal static stability stick position slope. It was pretty flat. So we left it there.
And yes, you are right, the ship had additional level flight speed which was left on the table, so to speak.
JohnDixson is offline  
Old 27th Nov 2018, 11:02
  #17 (permalink)  
 
Join Date: Oct 1999
Location: Den Haag
Age: 57
Posts: 6,218
Received 317 Likes on 176 Posts
, but by the fact that beyond the 156 ( 155? ) speed we could not demonstrate the classic fixed wing longitudinal static stability stick position slope. It was pretty flat. So we left it there.
Glad my recollection was correct. - I think the issue was that with the PBA working all was fine, but if you found yourself without it unexpectedly above 155 kts you would get into difficulties quickly.
212man is offline  
Old 27th Nov 2018, 11:50
  #18 (permalink)  
 
Join Date: May 2002
Location: Downeast
Age: 75
Posts: 18,267
Received 467 Likes on 191 Posts
Negative Stick Gradient is a problem only for certification standards and does not impose a handling problem for the Pilot.

Other aircraft have had the same kind of issues as the S-76....notably my favorite helicopter the Boeing-Vertol CH-47 beginning with the A Model and going right on up through later MK's.

A Test Flight Report resulting from the installation of Non-Metal Blades states the following:


Control Positions in Trimmed Forward Flight

!6. Trim control positions were evaluated in conjunction with level flight performance tests at the conditions listed in table 1, using the test techniques described inappendix D. All evatuaticns were conducted with pitch stability augmentation sVstem (PSAS) in the AUTO mode. A representative plot of control positions versus airspeed is included as figu1re 6, appendix E. The longitudinal control position gradient was not conventional, in that increased aft control position was required to trim at increased airspeed; however, this was not readily apparent to the pilot and therefore was not objectionable. Both lateral and directionali control trim changes were minimal (less than 314 inch) throughout the airspeed range evaluated. The pitch attitude change with airspeed was linear and varied from 2 degrees nose-up at 52 KCAS to 6 degrees nose-down at 150 KCAS, and provided the pilot with adequate cues to airspeed variations. The trim control position characteristics of theCH47C were essentially the same as the CH-47C with metal blades and are satis- factory.

Last edited by SASless; 27th Nov 2018 at 13:23. Reason: Coffee kicked in....replaced Bias with Gradient
SASless is offline  
Old 27th Nov 2018, 12:55
  #19 (permalink)  
 
Join Date: Oct 2006
Location: UK
Posts: 5,222
Likes: 0
Received 4 Likes on 3 Posts
Years ago during early trials of the 330 Puma they would wire the yaw channel with a switch that would cause it to go to open loop; i.e. throw the actuator full over. They would then load the aircraft so it was at the aft CofG, get airborne and dive it at VNE 310 k/h and throw the switch.

Aaaaaaaaand one, aaaaaaaaand two.

Then recover from the inverted.
Fareastdriver is offline  
Old 27th Nov 2018, 13:36
  #20 (permalink)  
 
Join Date: Mar 2007
Location: Hobe Sound, Florida
Posts: 949
Received 32 Likes on 26 Posts
Hello John Eacott. Sorry, but do not recall who that might have been. When Nick reads this thread, he’ll surely remember. As to the above Vne flight in the 76, there was some speed record flying done by Nick and friends. I have to retell one neat story here. The FAI rules for speed records provide for keeping the ship within a small altitude band. For one of the records, Bill Kramer flew the ship and was precise enough to enter the run at precisely the upper altitude limit and gradually lost altitude during the run, so as to exit the run at the lower limit, using therefore 100% of the potential energy transfer legally possible. Bill was an ex USAF 53 aviator who was also pretty good with cars. One weekend day he and Nick were scheduled to leave Sikorsky with a 76 for some NBAA event or such. Nick was waiting on our flight field with Gerry Tobias, who was the CEO and the person who had sold the 76 project to UTC leadership, and thus had really personal interest in seeing the 76 succeed. Kramer was late and then appeared driving his car across the flight field at rather too high a speed, going by Tobias and Nick, putting the car into a four wheel drift and perfectly stopping in his parking spot. Tobias turns to Nick and says, “ Nick, is he flying the 76? “. I wasn’t there, so this is another thing that Nick can clear up more factually.

Last edited by JohnDixson; 27th Nov 2018 at 13:37. Reason: typo
JohnDixson is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.