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Robinson helicopters added to safety watchlist

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Robinson helicopters added to safety watchlist

Old 16th Nov 2016, 11:15
  #141 (permalink)  
 
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rotorfossil
Whatever the reason, if you get an unexplained power loss in an R22, after the inevitable " this can't be happening to me", you have to slam the lever on the bottom as fast as you can move your hand.
I'm not sure that's a good idea. Do that at 80kts and you risk a negative G event! Surely better to flare a little as you gently lower the lever eh?
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Old 16th Nov 2016, 11:26
  #142 (permalink)  
 
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Originally Posted by chopjock View Post
rotorfossil


I'm not sure that's a good idea. Do that at 80kts and you risk a negative G event! Surely better to flare a little as you gently lower the lever eh?
Rapidly lowering the lever does not cause a dangerous low G event. From a pilot input perspective, low G is only of concern when induced by a cyclic pushover.

Am very much in agreement about the flare though. Some interesting work done by Dick Sanford (I think) that concludes (IIRC) - time available in cruise to fully lower lever to prevent catastrophic RRPM decay is approx 1s. Time available with no lowering of collective, but with progressive flare - circa 3s.

Haven't checked my facts on that, but he puts forward a strong argument that the flare is even more important during the 'Oh shit' moment than lowering collective. BTW, relates to R22.
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Old 16th Nov 2016, 19:14
  #143 (permalink)  
 
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Originally Posted by rotorfossil View Post
If there is an unexpected power loss, the first reaction is "what's happening"? Followed by either far too slow lowering of the lever or no reaction at all. Result- massive drop in rpm, rotor blow back and break up.

Among the tail chop incidents are surely some following this pattern. But from quite a number of descriptions it is very clear that there are other cases where droop of RRPM did not occur. Neither typical low G maneuvering with subsequent violent Right roll.
These 'unexplicable' ones are the real worrying ones since it is not clear how to avoid them. Due to rareity of events and the sudden progression once they occur I also slightly tend to assume something structural or combined aero- elastic + structural. Most of the totally inexplicable ones appear to have occured above 80kts. This also seems to point in the direction aerodynamic/aero-elastic.
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Old 17th Nov 2016, 08:47
  #144 (permalink)  
 
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Chopjock. I agree that a flare is good news in power loss in the cruise. Unfortunately there are two scenarios when this is not a good idea, namely engine failure in the climb out and on the approach. Here you need to retain the speed that you've got. Bearing in mind that the problem is people not doing anything in timely fashion, a one size fits all reaction is the best that can be expected. My teaching was to lower the lever as smartly as possible and maintain or achieve a level attitude, which always requires a bit of rear cyclic.
Incidentally the same techniques are just as valid in the high inertia types like the 44, 66 and 206. Although the rpm don't drop as quickly, they don't recover very quickly either. Important in the failure on climbout case.
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Old 17th Nov 2016, 10:07
  #145 (permalink)  
 
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Suggest you read http://www.ntsb.gov/safety/safety-re.../A08_25_29.pdf
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Old 17th Nov 2016, 10:41
  #146 (permalink)  
 
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You will find when you dump the lever in any helicopter the nose will immediately drop due to couple between weight and thrust, resulting in a natural tendency to apply aft cylic which then loads the disc.
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Old 17th Nov 2016, 13:10
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when you dump the lever in any helicopter the nose will immediately drop
You sure about that Hughesy?
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Old 17th Nov 2016, 13:28
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in 206,369,269,341,355,22,44 it seems to unless you are doing a quick stop or perfect zero wind hover if that is what you mean then stand corrected, but in forward flight they do
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Old 18th Nov 2016, 08:59
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Dump collective, machine drops, airflow from underside, horisontal stabiliser raises boom, ( nose drops)
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Old 18th Nov 2016, 14:31
  #150 (permalink)  
 
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No, in forward flight it is essentially flapback/forward.

If you add or reduce collective, you change the pitch by the same amount all round the disc.

In forward flight there is a difference in V squared between the advancing and retreating side so the increase/decrease in pitch has a bigger effect on the advancing side. This is the same effect as introducing forward speed to a helicopter in the still air hover without changing collective pitch.

Therefore when you lower the lever the disc flaps forward (usually giving a nose drop) and when you raise the lever the disc flaps back. There may be some roll effects as well if the phase lag of the rotor system isn't exactly 90 degrees.

It has a correct term which I can't remember but it is something like Pitch Instability caused by Collective application.

The horizontal stabilisers and other airframe bits only cause any changes when the rate of climb or descent is established.
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Old 19th Nov 2016, 03:52
  #151 (permalink)  
 
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But when you dump collective, you have very little MR thrust, so itll have negligable effect on the airframe, till autorotation restores thrust.
Meanwhile, youv dropped and the HS has lifted to boom.

Or should i just stay under my rock?
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Old 19th Nov 2016, 10:34
  #152 (permalink)  
 
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Crab has nailed it in one
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Old 19th Nov 2016, 14:13
  #153 (permalink)  
 
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Crab, Perhaps the collective to pitch attitude aerodynamic coupling is being underestimated, ( not to ignore the inherent rotor dynamics you mentioned ).

Best example I can recollect:

Original UH-60 prototype flew with a 60 sq ft. horizontal tail. The strategy during initial design was to meet US Army dynamic stability req'ts with only a pitch and yaw fluidic SAS. That later changed for the test machines, but the tail was designed around that idea.

Data point: in steady state autorotation at 120 KIAS with that tail ( fixed of course at that point ), the cyclic was almost on the aft stop, and the aft flapping was such as to result in contact with the aft damper stops. Has the feel of taking 51 cal hits. ( Just to be straight with this discussion: the inboard damper attachment point was the preceding hub arm back then, so the damper saw, and was exercised by, any steady atate flapping present. As a result, a damper attachment standoff was incorporated into the head, and that remains to this day ).

Couple of months later, the FBW stabilator was installed, and one of* the control laws integrated into the stabilator from the start was to uncouple the machine, collective-wise. This worked perfectly and resulted in no or minimal longitudinal trim shifts whether the collective was moved fast or slow, large trim shift or small.

* Once the decision was made to use a stabilator, all of the HQ and performance engineers jumped on those opportunities, as you can imagine, but that is another story.
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Old 19th Nov 2016, 16:54
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Thanks John, it's good to clarify the difference between the initial rotor response to collective movement and the subsequent fuselage attitude and control positions once the climb/descent has been established.
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Old 19th Nov 2016, 21:35
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I have occasionally pondered the potential role that unreported rotor head overspeed events could play in setting the stage for these these mishaps. Given that there are no readily identifiable witness marks from such rotor RPM only events and likely some considerable residual stress fatigue, multiple events could be absorbed unbeknownst to mx and later operators until the catastrophic unexplained departure from controlled flight occurs. There no databank of supporting evidence and its not statistically quantifiable so its just conjecture. Thoughts?
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Old 19th Nov 2016, 22:49
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givdrvr, in the days of the Bell 47 a rather common cause of engine failure was the dropping of valves. Cause given was unreported overspeeds inducing stresses which some time (hours & hours) later resulted in failure. Your theory is as good as anybodies at this stage I would say.
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Old 20th Nov 2016, 10:03
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only events and likely some considerable residual stress fatigue
See my previous posting.
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Old 20th Nov 2016, 22:08
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An extract from Blakmax's posted link
In addition, the bond joints between the spar and skin of the fractured main rotor blade
from the Fiji accident helicopter contained adhesive fracture features in many isolated areas of
the leading edge of the skin, indicating a weak bond. A weak bond in this area could allow the
leading edge of the skin to lift above the blade surface so that airflow during main rotor rotation
could peel the skin back further. If peel damage to the skin is minor, the main rotor blade will
vibrate irregularly, indicating that the helicopter must be landed immediately. However, if the
degraded bond on the main rotor blade is significant when the skin begins to peel, a large portion
of the skin may peel back suddenly, resulting in catastrophic fracture of the blade and complete
loss of control of the helicopter.

The Safety Board has determined that the adhesive fractures in the main rotor blade from
the Fiji accident helicopter propagated from the blade tip and leading edges and cannot rule out
the possibility that the in-flight breakup was initiated by a bond failure at the tip of the blade.
The indications of deteriorated bond strength uncovered in the main rotor blades highlight the
need for RHC and the Federal Aviation Administration (FAA) to address durability testing and
inspection of adhesive bonds in RHC helicopter blades.
So in addition to the suspect head design, you have suspect blade manufacture. when the crucial parts of your helicopter are held together by glue, you have to be bloody sure you have got the glue process 100% right.
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Old 21st Nov 2016, 00:40
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Crab, you said,
In forward flight there is a difference in V squared between the advancing and retreating side so the increase/decrease in pitch has a bigger effect on the advancing side
Not being argumentative, just need to clear this in my head.
Would it also be correct to say that increaseing collective also increases the teetering angle for a given airspeed?
This changes the tip plane path, and so, the RTV, hence the need to pole forward with increased collective to maintain attitude.
Or do i still not get it?

Last edited by Birdy2; 21st Nov 2016 at 03:24.
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Old 21st Nov 2016, 03:37
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Sorry to jump in - I think Crab's saying that any change in collective pitch applies immediately and equally to all blades in terms of pitch angle.

Coefficient of lift is basically directly proportional to angle of attack anywhere below the onset of the stall, so with all other things being equal except speed (the advancing blade being quicker than the retreating one in forward flight of course), a given pitch angle increase will give you more of a lift increase on the advancing side than on the retreating side. Due to phase lag that will manifest itself as a pitch up (or conversely a pitch down if you're dumping the collective).

Once the rate of climb or descent builds up, then the effect of aerodynamic surfaces behind the center of gravity (e.g. stab / elevator, and tail boom itself) will act in the same way - pitch down for a descent, pitch up for a climb.
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