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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! |
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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. |
Also, control movements above VNE may cause important parts to depart from the aircraft with undesirable results...
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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. |
Originally Posted by JohnDixson
(Post 10321350)
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. |
Originally Posted by mnttech
(Post 10321268)
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. |
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. |
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 |
https://www.pprune.org/rotorheads/47...ml#post9813152
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". |
I knew you'd have the exact detail, Mr Dixson! Re: "stable point: is that straight and level and without control movements? |
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 |
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. |
....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. 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 |
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! |
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. |
, 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. |
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. |
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. |
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. |
Brother John can confirm this story as well....as it happened more than once with various pilots involved.
The Sikorsky Completion Center was well out into the middle of no where....in the area of West Palm Beach. As part of the Conversion Training we conducted for customer pilots we included some ILS Approaches to West Palm Beach Airport. One evening.....a Eastern Airlines 727 was ahead of us on approach and seeing an opportunity that was impossible to pass up.....we laid whip to the horses and began to overtake the 727. Approach Control (being well familiar with this) requested Eastern to increase speed due to overtaking traffic. Eastern complied. A minute or so later....ATC requested Eastern to increase speed again......Eastern agreed. Another minute and ATC yet again asked for a speed increase to be queried by Eastern as to what kind of traffic was overtaking them.....to be told by ATC....."A Helicopter!". There was a moment's pause....then Eastern with an odd sound to his voice asked....."Helicopter? What kind of Helicopter?". To be told by an anonymous helicopter pilot....."A damn fast one!"! Yes....the 155 number was just that! |
The London to Paris competition had GBHBF (The Spirit of Paris) taking part. After it was over Sikorsky advised us that they would have authorised a 100/100% cruise if we had asked them.
A bit late; it was all over. "Helicopter? What kind of Helicopter?". |
212 hello! AFCS hardcover testing was sometimes boring and once in awhile “ interesting “ . The guys doing the S-61 certification were having trouble getting th required delay times doing the forward hardover at most forward CG and at max climb. ( this was a year or so prior to me joining ), so the Ch Experimental Pilot ( Byron Graham ) and the Ch Pilot Dmitry ( Jimmy ) Viner went to fly the data point. Byron was beyond good, flying skill-wise and he was flying. Winter, with 3-4 inches of snow on the ground. Took off to the south, threw the hardover in just over the Merritt Pkwy bridge southbound. Byron took the delay time and then some, and when he brought the cyclic back he cut the tail drive. Made the neatest zero thrust landing and there is a widely circulated ( internally ) picture of the tail wheel path thru the snow-dead straight. Boring? Due to the S-92 AFCS architecture, it required 2000+ hardovers, none of which were particularly attention getting. With the advent of FBW controls, that sort of failure mode testing will largely, if not totally, be a thing of the past. |
Sas, to your first sentence in Post 18. So right. To meet that requirement for the UH-60 ( an area where the military requirement mirrored the FAA ) we had to install a pitch bias actuator. After a number of years in service and with a PBA whose reliability record was below par, the Army did an eval and came to the conclusion we had talked about at the very beginning: the attitude vs speed slope was such that if you put the ship on an attitude, it held speed...QED. So the PBA came out. Well, the 76 was not much different here, but the FAA.......what was the Sergeant’s name in Hogan’s Heroes again? |
I shall not make a confession here but we had the Puma speed beat....well beat! Like you would have stepped outside to see what had you bogged down....beat! |
Shawn Coyle's 'Cyclic and Collective |
During the ‘70s, we were occasionally asked to do an air test of some sort in the Puma HC1. Part of the test was to fly at VNE which was 167 knots which was only achievable in a dive. It was a white knuckle ride with the centre (plastic) windscreen panting. Seem to remember the ASI kissing 167 for just a fraction of a second before recovering to a more sensible flight regime. |
Thanks for the replies, discussion, resources, and stories! I appreciate it. To summarize what I’ve been reading, potential dangers of exceeding power-on Vne include: negative effects on controllability, reduced service life on aircraft components, and resultant inaccuracy of time-change component schedules. If it’s exceeded by 10%, you *may* run into problems with compressibility, excessive vibration/aircraft stress, blade stall, or some combination of all of the above. Did I miss anything big there? |
The Lynx Mk7/9 had a Vne of 156Kts yet it held the speed record of over 200kts (heavily modified aircraft) - it would exceed 156 (without the TOW booms) with ease in level flight and I was advised by an ex-TP that it only started to become a handful above 180 Kts.
Of course I never tried it............................................. I'm sure Shy will remember the dive to Vne (168Kts) in the Gazelle - and then the subsequent pull into jackstall |
Crab,
Nearly reliable sources suggest it became a handful for you when you first put your hand on the door handle! |
Sas - yes, it kept going upside down when I flew it:)
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Originally Posted by [email protected]
(Post 10322366)
I'm sure Shy will remember the dive to Vne (168Kts) in the Gazelle - and then the subsequent pull into jackstall
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Think about this: The manufacturer has to design the helicopter to be flown by pilots of all skill-level from Private up through ATP. Not every guy that flies their new machine is going to be Chuck Aaron. They have to assume that some dope will take the thing up with it loaded to max gross and with it also at one of the (longitudinal or lateral) c.g. limits. And maybe they'll fly it up at its limiting airspeed. Sooooo...let's see what the FAR's say about this.
§27.143 Controllability and maneuverability.(We'll disregard letters 'a' through 'd' for now so we can focus on the really meaty part of this standard, which is 'e' and especially 'f'.)(e) The rotorcraft, after (1) failure of one engine in the case of multiengine rotorcraft that meet Transport Category A engine isolation requirements, or (2) complete engine failure in the case of other rotorcraft, must be controllable over the range of speeds and altitudes for which certification is requested when such power failure occurs with maximum continuous power and critical weight. No corrective action time delay for any condition following power failure may be less than— (i) For the cruise condition, one second, or normal pilot reaction time (whichever is greater); and (ii) For any other condition, normal pilot reaction time. (f) For helicopters for which a VNE (power-off) is established under §27.1505(c), compliance must be demonstrated with the following requirements with critical weight, critical center of gravity, and critical rotor r.p.m.: (1) The helicopter must be safely slowed to VNE (power-off), without exceptional pilot skill, after the last operating engine is made inoperative at power-on VNE. (2) At a speed of 1.1 VNE (power-off), the margin of cyclic control must allow satisfactory roll and pitch control with power off. Yikes! Remember, these conditions must be met without "exceptional pilot skill." Can you imagine being a test-pilot for a manufacturer and having to bring it up to max gross, and then load it to a "critical" c.g. (i.e. one of the limits). Then you go out and set max cruise power, and dive to power-on VNE...and then cut the engine. I'm no test pilot, but I would assume that you'd have to run that test at all of the corners of the c.g. envelope. At "critical" MRRPM. And VNE. Scary stuff, when you think about it. Back when we were trying to get the FH1100 back into production, an operator called and asked how fast the ship would go? I said "127 mph" (which is its VNE). And he said, "No, seriously, how fast will it really go?" I guess he was comparing it to a Bell 206, which as a VNE of 150 mph and figured it would go just as fast. And maybe it would. But an 1100 is not a 206. I told him honestly that I never took one above published VNE. Because as I said, I'm no test-pilot. An FH1100 with a RR250-C20B would certainly go faster than 127 mph. But the mast on the 1100 is not tilted forward (as on a 206) and it has a tiny horizontal stab to help pull the tail down. Just doing 127 produces a pronounced nose-down cabin attitude of around -10 degrees. Making it go faster would make the nose drop even further. Now, can you imagine if the engine quit at "something above" VNE? Your Private Pilot natural instinct might be to quickly lower the collective. And if you did that before pulling back on the cyclic the nose would drop even further and the airspeed would increase. You'd probably cut the tailboom off or bump the mast off trying to get the nose up. I had talked with enough of the original FH1100 test pilots to know not to mess with the limits. They put the fear of God into me - and I'm an atheist! As people have said, VNE is about more than just retreating-blade stall. A lot more. |
Actually it's not
The rule is applicable to all phases of flight (you must be able to survive engine failures and safely fly into and in autorotation),
The second part (f) is specific in requiring satisfactory handling within a power off envelope, including getting there from a power on Vne failure ( which is a test case that focusses your mind) If your power on envelope is the same as power off , only (e) applies. as with defining any Vne there is more than one factor to take into account, but as before the clue is in the title: never exceed |
FH properly expands the considerations re the speed limit test requirements, but we need to also realize that the FAR’s recognize the establishment of an OEI Vne. Thus for the 92, the twin engine power on Vne is 165 and the OEI Vne is 120, which is also the power off Vne. So doing the second engine cut test is performed at the OEI Vne. Makes the second cut test a lot less eventful than implied above ( the collective position is far lower, thus the Nr decay rate is lower etc ). I can only comment re the 76 and 92 here, but the power on Vne for both was achievable in level flight, not a dive, so the cut of the first engine at Vne was also a bit less exciting than implied above. |
Originally Posted by [email protected]
(Post 10322366)
I'm sure Shy will remember the dive to Vne (168Kts) in the Gazelle - and then the subsequent pull into jackstall |
Originally Posted by [email protected]
(Post 10322366)
I'm sure Shy will remember the dive to Vne (168Kts) in the Gazelle - and then the subsequent pull into jackstall
These days I fly a heli that will easily exceed its 167 kts Vne in level flight, let alone in a descent - slippery little beast. It was quite amusing to once be asked to make "best speed" to fit in between two airliners in the instrument pattern - then to be asked to slow down because I was catching a preceding Boeing 767 on finals. |
Have a question for the Gazelle pilots here: did Eurocopter ever upgrade the hydraulics and servos to provide a force capability commensurate with that rotors’ control loads? |
Originally Posted by JohnDixson
(Post 10324021)
Have a question for the Gazelle pilots here: did Eurocopter ever upgrade the hydraulics and servos to provide a force capability commensurate with that rotors’ control loads? |
Thanks, Shy, but it seems I totally misunderstood the use of “ jackstall” in connection with several events involving Gazelles. I thought it signified a condition wherein the maneuver control loads exceeded the force capability of the servos, hence the cyclic became stalled or immovable. What did it in fact mean? |
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