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VNE / Velocity Never Exceed

Quelqu'un a-t-il une explication qui justifie l'augmentation de la VNE sur le XE285

XE Vitesse maximale : 110 km/h. VNE : 130 km/h Wink

XE285 Vitesse maximale : 130 km/h. VNE : 160 km/h :o

Quelle est la consommation horaire du XE 285, annoncée un temps pour 10 l/h mais mentionnée désormais à 23 l/h?

Alors que, sauf erreur de ma part, aucune modification structurelle n'a été apportée entre les deux modèles! Sad



Does anyone has an explanation that justifies the increase VNE on the XE285

XE: Maximum speed: 110 km / h. VNE: 130 km / h Wink

XE285: Maximum speed: 130 km / h. VNE: 160 km / h :o

So that, unless I am mistaken, no structural changes have been made ??between the two models!

What is the hourly consumption of XE 285, announced a time for 10 l / h but now referred to 23 l / h? Sad

Thank you
Jean-Rémy (XE 1077 - Sud Bretagne - France)
Rotor rpm is 590 instead of 540.
hello LORNE,

Cela ne justifie pas une augmentation de la VNE.
Seule une modification structurelle peu donner lieu à la modification de la VNE ou alors le XE était sous estimé?
Ou encore la cellule du XE est différente de celle du XE285!!!
Ce sont des données très importantes à connaître de façon précise car nécessaires à l'établissement de documents officiels pour l'identification de tout aéronef en France.

hello LORNE,

This does not justify an increase in the VNE.
Only a few structural changes lead to changes in the VNE and then the XE was underestimated?
Or the cell XE is different from the XE285!!!
These are very important data, to know precisely as necessary for the establishment of official documents for the identification of any aircraft in France.
Jean-Rémy (XE 1077 - Sud Bretagne - France)
The biggest factor affecting VNE for the XE is retreating blade stall. If you increase rotor RPM you increase the speed at which you will experience retreating blade stall. Same for the XET and XE3.

Also, structurally, I suspect that the XE285 will have the thicker windshield, which is required for the XE3 and XET.

I wouldn't presume to speak for John U or Dwight, so if you need official information, I'll defer this to someone official.
Correct on both accounts Lorne. The higher rotor rpm pulls us further away from blade stall and some additional structural changes were made to beef up the higher power machines including the thicker windshield. The structural changes were added to all machines but the windshield in the XEL is still standard due to weight restrictions. The top speeds have significant margin for safety (I've had both machines well over their VNE's as a part of testing).
Thank You John Wink
Jean-Rémy (XE 1077 - Sud Bretagne - France)
Is there any reduction in VNE for machines with doors and aux fuel tanks?
VNE is normally viewed as a definitive line that if crossed will mean doom and gloom. The truth of the matter is that most helicopter operator's manuals state the VNE speed but don't state that the limitation is due to reatreating blade stall, flight with doors open or removed, floats installed or some external equipment hanging off the helicopter. It is implied that the VNE is due to retreating blade stall (RTB) but whatever that published VNE speed is for any given helicopter it is usually somewhere short of when RTB will actually occur. Pressure altitude, temperature, humidity, density altitude, static weight (aircraft operating weight), dynamic weight (artificially increasing weight through g-load factor in turns), rotor rpm, collective setting, excess power available and aircraft speed all factor into the final solution that can induce RTB. All these factors can be simplified into airfoil performance based on air density and aircraft weight that is being supported. These two factors added together dictate the angle of attack required to stay in the air and esteblish the limitation of the critical angle of attack where stall occurs. Once either or both of these factors are exceeded the total airfoil angle of attack will exceed the critical angle and stall will occur. It also important to remember that stall is not a function of speed but rather a function of angle of attack.

There is no way to know exactly where your aircraft is in relation to the absolute limitation of RTB so typically the RTB limitation is a worst case scenario somewhere short of where it will actually occur in level forward flight. I will even go as far as to say that you would be hard pressed to get into RTB in level flight because most helicopters don't have the power to achieve their true VNE in level (no dive) flight. As soon as you dive the aircraft to achieve that speed you immediately (artificially) reduce the weight that is being suppported by the rotor system and therefore effectively lower the angle of attack moving it farther away from the critical angle. Simply said: "if you are in level flight you are in 1G flight and if you are in a dive you are in less than 1G flight".

You are much more likely to get into RTB while performing tight turns with high bank angles on a hot day with a full load of fuel at moderate speeds than you are to get into it in high speed level flight. All that being said, always, always, always abide by the VNE limitation for your helicopter in level flight and make sure that you are operating close to your max climb speed when maneuvering into high bank angles and tight turns.

Additionally be aware that in cold temperatures VNE can actually be due to compressibility, a phenomenon sometimes referred to as advancing blade stall. In cold air the speed of sound and therefore the sound barrier is achieved at slower true airspeeds due to the density or viscocity of the air. In this situation it is easy for the air molecules going over the airfoil to accelerate from subsonic to supersonic speeds (locally). When the airfoil goes supersonic the center of pressure shifts aft and makes the airfoil supercritical. Since the rotor blade is spinning about an arc, the local airspeed at any given point along the blade is faster or slower than the point inboard or outboard. This makes for a diagonal line of center of pressure displacement points that cause a twisting force on the blade and can damage or even catastrphically destroy the blade in flight. Typically compressibility is a lower VNE than RTB but it won't happen until the air is very cold. I don't think there has been any compressibility testing for the Mosquito but that doesn't mean that it can't be achieved especially in the higher RPM models (XET, XE3 & XE285). So think about that for all you guys that have doors and thermal underwear and are planning to take advantage of the increased performance that is offered by flying in the cold.

Un grand merci pour cette explication d'une grande qualité.
Maintenant je comprends mieux l'importance de certaines données et caractéristiques.
Dommage que la traduction me complique un peu les choses, mais je vais rechercher pour trouver le sujet en Français afin d’être certain de ne pas avoir d'erreur de traduction et de bien en appréhender l'intégralité.
C'est toujours plus compliqué quand comme moi on ne maitrise pas l'anglais, mais je m'accroche. Wink

A big thank you for this explanation of a high quality.
Now I understand better the importance of certain data and characteristics.
Pity me that the translation complicates things a bit, but I will seek to find the subject in French to be sure not to have mistranslation and well understand the whole.
It's always more complicated when like me you do not master English, but I cling. Wink
Jean-Rémy (XE 1077 - Sud Bretagne - France)
JRL I am happy to know that you understand what I wrote and your translation is very good. I always get the point of what you are saying (or at least I think I do). Retreating Blade Stall is something that everyone worries about at high speeds but it almost never occurs. The reality is that it usually occurs at low speeds while doing heavy maneuvering.

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