Polish HEMS EC135 almost losing it on landing
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Fenestron technology has changed vastly since the design of the Gazelle. Modern fenestrons are linear in there power delivery and feel no different to a conventional tail rotor.
Airbus recently published two Information Notices about unanticipated yaw. One for the French product range and one for the German. It dispels a lot of commonly held misunderstandings of loss of yaw control and recovery.
FNW.
Airbus recently published two Information Notices about unanticipated yaw. One for the French product range and one for the German. It dispels a lot of commonly held misunderstandings of loss of yaw control and recovery.
FNW.
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I've flown 120,130 and 135 fenestron. The 135 is hugely different in it's handling and feel to the other two, a lot of this is probably due to the 135 being built around an autopilot. The 135 is wiggle the toes as compared to muscle up that leg with the 130 and less so 120. The 135 I have also found to be linear whereas the other two very non linear in the thrust available. All of them use a lot of torque though, especially near the limits.
All of them use a lot of torque though, especially near the limits.
Avoid imitations
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I used to instruct on the Gazelle, in the mid 1980s. No-one had ever heard of "Fenestron Stall" despite the aircraft having been in service for quite some years.
I went away on a different job for a year and then went back to refresh on it, at the same base, with CFS and an instructor I knew of old. No-one told me that "Fenestron Stall" had by then then been invented/discovered and handling restrictions imposed. on the first flight I tried to do what I thought was a normal pre-takeoff lookout turn (must have done thousands before in exactly the same conditions) and was immediately told off because I'd dared to put the wind on the "wrong" side.
I went away on a different job for a year and then went back to refresh on it, at the same base, with CFS and an instructor I knew of old. No-one told me that "Fenestron Stall" had by then then been invented/discovered and handling restrictions imposed. on the first flight I tried to do what I thought was a normal pre-takeoff lookout turn (must have done thousands before in exactly the same conditions) and was immediately told off because I'd dared to put the wind on the "wrong" side.
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Unanticipated yaw
I've flown 120,130 and 135 fenestron. The 135 is hugely different in it's handling and feel to the other two, a lot of this is probably due to the 135 being built around an autopilot. The 135 is wiggle the toes as compared to muscle up that leg with the 130 and less so 120. The 135 I have also found to be linear whereas the other two very non linear in the thrust available. All of them use a lot of torque though, especially near the limits.
If you are interested in the unanticipated yaw topic and if you have time to spend the attached paper was presented last year in Warsaw and might explain the Polish event.
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I spent all of last week in a B2 and flew our 135 today, I'd walk past the 135 to get in the B2.
Thread drift.
I was on the Gazelle IFTU at Middle Wallop and as far as I know nobody had any flying problems with the fenestron despite performing prolonged max speed spot turns in either direction. I also don't recall any SAS on the IFTU aircraft. If that was the case then why and when was SAS fitted, and could that have caused any of the subsequent problems? I thought the Gazelle was a joy to fly but after flying/instructing on Hillers and Whirlwind 7's (where controlling RRPM with the collective twistgrip required constant attention) I felt that the ease of flying the Gazelle could introduce accidents caused by overconfidence rather than mechanical problems as had been experienced in the previous generation of helicopters.
I was on the Gazelle IFTU at Middle Wallop and as far as I know nobody had any flying problems with the fenestron despite performing prolonged max speed spot turns in either direction. I also don't recall any SAS on the IFTU aircraft. If that was the case then why and when was SAS fitted, and could that have caused any of the subsequent problems? I thought the Gazelle was a joy to fly but after flying/instructing on Hillers and Whirlwind 7's (where controlling RRPM with the collective twistgrip required constant attention) I felt that the ease of flying the Gazelle could introduce accidents caused by overconfidence rather than mechanical problems as had been experienced in the previous generation of helicopters.
76Fan - the AH1 Gazelle for AAC use didn't have SAS fitted although the ones for 3BAS (as was) did. The RAF and RN ones HT2/3 did have SAS fitted.
Many of us believe the SAS in the yaw channel was one of the problems as the series actuator could move the pitch of the blades without corresponding pedal movement until it ran out of authority. When that happened, the actual pedal position was often not where it needed to be and as the yaw started, the first application of pedal seemed to have little effect - this was why pilots thought they had a yaw control problem when in fact they just didn't have enough right pedal in.
Many of us believe the SAS in the yaw channel was one of the problems as the series actuator could move the pitch of the blades without corresponding pedal movement until it ran out of authority. When that happened, the actual pedal position was often not where it needed to be and as the yaw started, the first application of pedal seemed to have little effect - this was why pilots thought they had a yaw control problem when in fact they just didn't have enough right pedal in.
AMDEC - its a very good paper but omits the effect or loss of effect of the vertical fin/vertical stabilisers as they gain or lose airspeed and reach critical AoA.
Thanks for that Crab, however one of the IFTU Gazelles in 1973 was an HT2 for the RN, and I delivered and did the conversion of some of the 705 Sqdn instructors onto the Gazelle in 1974. None, either AAC or the RN had SAS fitted at that time, but I never flew the Gazelle again after September '74.
Ah OK, I didn't fly the Gazelle until 83 so it must have been an add-on sometime between 74 and early 80s.
The 135 has a few little "quirks" but all completely understandable if you stand back and take a look at what is going on behind you.
Relative airflow from the forward quarters either left or right will induce some yaw instability from the wake of the horizontal endplates. Nothing too obtrusive but may screw up your camera shot. You can fly around it if you appreciate
what is happening but nothing to get too concerned about. They removed them on the "3" and increased the height of the fin tip cap.
Another factor is the "A of A" of the fin. It can upset you a little more than a conventional aircraft but again not much to be too concerned about. Just be aware that as per the RFM a "run on" is NOT recommended with loss of TR drive.
Reason being you will feel fat dumb and happy until the fin stalls and loses effect which you probably won't recover from. Hence the recommendation to perform a full-on autorotation.
Authority - no issue - the thing will reach 65 knots or more sideways at the stops.
Power - yes it can use more in the hover. And conversely less in the cruise.
Like a lot of these things too much BS gets read into it. Your the "pilot in command" - just fly the thing - don't ever become a passenger!!
As alluded to in the report - "Light helicopters with low experience pilots are the preferred victims."
Relative airflow from the forward quarters either left or right will induce some yaw instability from the wake of the horizontal endplates. Nothing too obtrusive but may screw up your camera shot. You can fly around it if you appreciate
what is happening but nothing to get too concerned about. They removed them on the "3" and increased the height of the fin tip cap.
Another factor is the "A of A" of the fin. It can upset you a little more than a conventional aircraft but again not much to be too concerned about. Just be aware that as per the RFM a "run on" is NOT recommended with loss of TR drive.
Reason being you will feel fat dumb and happy until the fin stalls and loses effect which you probably won't recover from. Hence the recommendation to perform a full-on autorotation.
Authority - no issue - the thing will reach 65 knots or more sideways at the stops.
Power - yes it can use more in the hover. And conversely less in the cruise.
Like a lot of these things too much BS gets read into it. Your the "pilot in command" - just fly the thing - don't ever become a passenger!!
As alluded to in the report - "Light helicopters with low experience pilots are the preferred victims."
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Originally Posted by [email protected]
AMDEC - its a very good paper but omits the effect or loss of effect of the vertical fin/vertical stabilisers as they gain or lose airspeed and reach critical AoA.
That was sort of my point - the differing conditions of crosswind as IAS is reduced towards the hover, combined with any ingestion of the MR vortex by the TR can cause the vertical stabilisers to lose lift earlier than in a pure headwind and cause unexpected yaw variations.
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Tailfin, fenerstron and SAS all had a hand in Fenestron Stall in a Gazelle in which I was a passenger. We departed Shawbury in a 20kt wind and promptly had a generator failure, we therefore returned to the landing point and having lost the generator, we had also lost SAS/Heading Hold. This made the pedal inputs much more responsive in the hover. As we carried out a lookout turn left prior to taxiing back, we passed through the downwind and the wind caught the tail fin. The rate of yaw increase caught the pilot out and he went to full right pedal. We then continued to spin several times and thought we had a tail rotor failure.I could see his right pedal was fully forward and totally ineffective.His immediate reaction was to gain a few feet clear of the ground but as he applied collective we started to pitch nose up and he over-corrected with nose down, by this stage we had probably gone through 6 revolutions, he couldn't bring himself to release the right pedal and unstall the fenestron (partly because we had never heard of fenestron stall, let alone discussed a solution). He then put down the collective and accepted the ensuing hard landing. He did keep us as level as possible and the aircraft was a Cat3. The fenestron was fine. I am certain Fenestron Stall exists, even though the Aerospatiale test pilots were not able to re-create that situation. In any case, they would have been briefed and ready to respond....not quite the same in my opinion.
as far as I know nobody had any flying problems with the fenestron despite performing prolonged max speed spot turns in either direction
The other was me. Mutual tac sortie on the APC. Hover taxying across a field up on the ink pen ridge, flying from the lefthand seat. Saw what I thought was a line of beaters(pheasant shoot) ahead, so stuck in a boot full of left pedal to turn away. The aircraft spun rapidly to the left. Tulles power to get away from the ground which, of course didn't help. Thought I put in full right pedal, but maybe I didn't. My stick buddy was an experienced crewman, saw that I wasn't coping, took over and got things under control.
After we recovered our composure, we made our way home deciding not to tell anyone.
Oh, and I can't remember what they were, but they weren't beaters.
Below the Glidepath - not correcting
I've said it before here, "Fenestron Stall" is simply French for mishandling. There is another translation from ancient Greek which means "failure to anticipate". The aircraft spinning is very real, the Fenestron Stalling, not so much...