The Fenestron

Post Reply
Message
Author
User avatar
TheGreenGoblin
Chief Pilot
Chief Pilot
Posts: 17596
Joined: Thu Aug 08, 2019 11:02 pm
Location: With the Water People near Trappist-1

The Fenestron

#1 Post by TheGreenGoblin » Wed Dec 16, 2020 11:53 am

I was prompted to start this thread after making a comment about "fenestron stall" with respect to a possible cause of an EC120 accident in NZ (see incidents thread). I have never flown, or in, a helicopter with a fenestron and had only heard the term fenestron stall term used by an experienced ex-army/police Gazelle pilot of my acquaintance who used the term with respect to this accident.

https://www.gov.uk/aaib-reports/aerospa ... nuary-2008

I note from TOP that some seasoned veterans deny the existence of "fenestron stall" with respect to the Gazelle or any other helicopter with a fenestron and point to LTE or possible mishandling for this phenomenon but I have read material by others that says that this is a real aerodynamic phenomenon specific to the fenestron itself. What do the cognoscenti here think?

Gazelle - the undiscovered gem
The down side of the arrangement is that the fenestron has been known to ‘stall’ on rare occasions, an aerodynamic effect in which the airfoil breaks away from the fenestron blades and renders them useless, effectively mimicking the effects of a tail rotor failure. Very unpleasant for the pilot and passengers alike. Exactly what causes the fenestron to stall is still a bit of a mystery. It is much talked about by the powers that be in the helicopter wing of the armed services, but it is known that if you keep the wind within limits (especially from the right) and keep all spot turns nice and gentle then you will not have a problem. This is one of the reasons that the Gazelle is considered to be a big cat rather than a kitten: being a capable machine it does need to be treated with a healthy degree of respect. It’s a big boy’s toy.
Though you remain
Convinced
"To be alive
You must have somewhere
To go
Your destination remains
Elusive."

User avatar
TheGreenGoblin
Chief Pilot
Chief Pilot
Posts: 17596
Joined: Thu Aug 08, 2019 11:02 pm
Location: With the Water People near Trappist-1

Re: The Fenestron

#2 Post by TheGreenGoblin » Wed Dec 16, 2020 11:54 am

Though you remain
Convinced
"To be alive
You must have somewhere
To go
Your destination remains
Elusive."

User avatar
TheGreenGoblin
Chief Pilot
Chief Pilot
Posts: 17596
Joined: Thu Aug 08, 2019 11:02 pm
Location: With the Water People near Trappist-1

Re: The Fenestron

#3 Post by TheGreenGoblin » Wed Dec 16, 2020 11:54 am

Though you remain
Convinced
"To be alive
You must have somewhere
To go
Your destination remains
Elusive."

User avatar
TheGreenGoblin
Chief Pilot
Chief Pilot
Posts: 17596
Joined: Thu Aug 08, 2019 11:02 pm
Location: With the Water People near Trappist-1

Re: The Fenestron

#4 Post by TheGreenGoblin » Wed Dec 16, 2020 12:41 pm

Though you remain
Convinced
"To be alive
You must have somewhere
To go
Your destination remains
Elusive."

User avatar
TheGreenGoblin
Chief Pilot
Chief Pilot
Posts: 17596
Joined: Thu Aug 08, 2019 11:02 pm
Location: With the Water People near Trappist-1

Re: The Fenestron

#5 Post by TheGreenGoblin » Wed Dec 16, 2020 1:52 pm

Part 5



Part 6

Though you remain
Convinced
"To be alive
You must have somewhere
To go
Your destination remains
Elusive."

User avatar
FD2
Chief Pilot
Chief Pilot
Posts: 5110
Joined: Thu Sep 03, 2015 10:11 pm
Location: New Zealand
Gender:
Age: 76

Re: The Fenestron

#6 Post by FD2 » Thu Dec 17, 2020 4:40 am

Alarming but recoverable in the Gazelle:



Gazelle yaw characteristics

In the majority of civil and military cases, loss of yaw control occurred in the hover or at low forward speed in light winds from the right. A few occurred in stronger winds or with wind from the left.
Both inexperienced and highly experienced pilots were involved in the military accidents and loss of control of pitch and roll during the subsequent high rates of rotation was a common feature. An 'optimised fenestron' was fitted to military Gazelles in the early 1980s as part of a weight upgrade programme.
The optimised fenestron had revised duct and hub fairings but did not appear to improve the incidence of sudden loss of yaw control.

The sudden loss of yaw control was attributed to 'fenestron stall' and, in response to concern, the Ministry of Defence (MOD) sponsored a trial by the manufacturer, Eurocopter France, to investigate the phenomenon.

The trial took place in 1992/93 and demonstrated that, in conditions of low natural wind, a relatively small left pedal input of 5% (of total pedal travel) from the hover position can result in a yaw rate of 150°/sec being achieved in 10seconds.
It also showed that high yaw rates to the left (165°/sec)can be rapidly arrested by application of full right pedal without any tendency for aerodynamic stall of the fenestron.
The MOD advice included a statement that the extremely rapid build up of yaw rate in these circumstances was exacerbated if the SAS was not engaged.

The MOD trial did not establish why a small pedal input can result in the rapid build up of very high yaw rates. However, an earlier study, in 1991, by Westland Helicopters Limited had suggested that the trigger mechanism might involve a coupling of fenestron rotor induced swirl with the circulation contained in the main rotor tip vortices which may become aligned with the fenestron in certain flight conditions.
The study also suggested that consideration should be given to changing the direction of rotation of the fenestron to become top-blade-aft which would probably solve the interactional aerodynamic problem.
Subsequent fenestron-equipped helicopters such as the SA365 Dauphin, EC135 and EC120 have top-blade-aft fenestron rotation; they are not known to suffer from sudden loss of yaw control.


The 'fenstron stall' phenomenon seems to have been put down to heavy application of boot. If the EC120 is not known to suffer from rapid yaw with a small application of pedal, like the Gazelle, then it may be discounted from the Kaikoura crash. Whether 'fenestron stall' was ever a 'thing' seems to be hotly disputed as you say GG. I had one flight in a naval Gazelle with a chum back in 1977 and can't say I enjoyed the experience. I have forgotten anything he may have told me about the machine!

User avatar
TheGreenGoblin
Chief Pilot
Chief Pilot
Posts: 17596
Joined: Thu Aug 08, 2019 11:02 pm
Location: With the Water People near Trappist-1

Re: The Fenestron

#7 Post by TheGreenGoblin » Thu Dec 17, 2020 6:15 am

Appreciate your considered response, experience and thoughts on this matter FD2. I guess that helicoptering is not for the leaden footed, something that has taken me some time to correct.
Though you remain
Convinced
"To be alive
You must have somewhere
To go
Your destination remains
Elusive."

Post Reply