Loss of control surface

[pmccarthy]

I did one of those landings last week, no flare, ask for a demo.

 

 

[Oscar]

The trim is not separate on your jabiru, nor is it on a tiger moth. it's a spring applying a force to the ONLY mechanical linkage on the plane. There is no pitch control redundancy. Nev

Nev:, Bruce is correct, the trim cable IS attached to the elevator horn. In the fairly early days of Jabiru flight testing, the elevator cable became detached at the forward clamp (under the pilot's elbow), due to inattention in assembly and was landed without drama (apart from some excited chatter coming from the ground..) at Bundaberg on trim only.

 

 

[facthunter]

It is a spring applied to a part of the original control mechanism so it's not redundant in the true sense. A spring just relieves the control force for the pilot. A trim tab or moving stabiliser relieves the needed control force at the source and if you unhooked the mechanism would operate separately and maintain a set sitruation. Landing on trim is not always easy but maintaining flight without PITCH control is not possible, which is the original question. You CAN fly without rudder in most planes as well as without aileron (Much easier) The Flying Flea doesn't have them

 

You probably can't fly a Tiger Moth without rudder but it's unusual in that respect. Nev

 

 

[dutchroll]

I think for the sake of semantics, the original post was about "loss of control surface" and should be taken to mean the complete loss of use of that control surface.

 

Many types of trim systems will of course enable the deflection of a control surface through trimming even if the link to the pilot's control column or yoke is not operating. So in those cases you get some degree of controllability, which is not the point of the original question.

 

So back to where we were - the complete loss of elevator control makes for a very tough time controlling the aircraft (ie - one which has conventional flight controls). Far more so than loss of aileron or rudder (note: loss of rudder does not imply loss of the entire vertical stabiliser in aircraft of conventional design, which makes retaining control highly improbable).

 

 

[Admin]

I personally think this has been a great educational discussion especially for those new pilots and discussions like this is a great asset to this site.

 

 

[IBob]

I think for the sake of semantics, the original post was about "loss of control surface" and should be taken to mean the complete loss of use of that control surface.Many types of trim systems will of course enable the deflection of a control surface through trimming even if the link to the pilot's control column or yoke is not operating. So in those cases you get some degree of controllability, which is not the point of the original question.

 

So back to where we were - the complete loss of elevator control makes for a very tough time controlling the aircraft (ie - one which has conventional flight controls). Far more so than loss of aileron or rudder (note: loss of rudder does not imply loss of the entire vertical stabiliser in aircraft of conventional design, which makes retaining control highly improbable).

I hadn't thought about losing rudder and tail fin: something to be entirely avoided, if I imagine correctly.

The Savannah has an anti-balance tab. How would that behave on loss of elevator control? (I'd try it, but I'm still building her.)

 

 

[facthunter]

IF you stick to the loss of surface concept, it's a more individual plane specific consideration, and becomes quite complex to discuss. In principle, roll can be by aileron or spoilers and loss of one side surface won't be a big problem. Also you can use the further effect of rudder (yaw). Yaw can be achieved by banking and the plane will weathercock to the dropped wing. (secondary effect of roll) in a normal keel surface design consideration. Loss of rudder surface may still permit weather cocking. Control surfaces if they don't completely detach may create control problems of their own. beyond the fact it isn't operating.. Nev

 

 

[dutchroll]

I hadn't thought about losing rudder and tail fin: something to be entirely avoided, if I imagine correctly.

Some people use this B52 example to suggest loss of the vertical stab is not the end of the world (they landed successfully) but they neglect to point out that it does actually have just enough left to give it some longitudinal stability. Also there were several other cases of B52s losing their vertical stabs, all of which resulted in the aircraft crashing. Also the loss of American Airlines AA587 in New York some years back was directly caused by the complete loss of the vertical stab, making the aircraft uncontrollable. It broke off due to the First Officer's aggressive overreaction on the rudders after entering wake turbulence behind a B747 (to maintain control in wake turbulence you don't actually need to use the rudder at all - just the ailerons). His rapid alternating full rudder deflection left and right overstressed the vertical stab to double its design limit loading, causing it to completely separate.

 

 

[Bruce Tuncks]

I have seen an Astir glider fly with one aileron disconnected. The wingtips were moving up and down through about 40 cm at about 3 cycles per second. This gave the pilot a very rough ride but otherwise the glider was controllable .

 

Many control surfaces are not completely mass balanced so control surface-induced flutter will happen. The same thing happens with Libelles but the amplitude and frequency is different due to the difference in wing stiffness. The Astir has a much stiffer wing so the frequency is higher.

 

Sorry but no demo will be done. In recent years the GFA has introduced a second inspection of newly-rigged gliders to try to eliminate this happening.

 

 

[dutchroll]

I have seen an Astir glider fly with one aileron disconnected. The wingtips were moving up and down through about 40 cm at about 3 cycles per second. This gave the pilot a very rough ride but otherwise the glider was controllable .Many control surfaces are not completely mass balanced so control surface-induced flutter will happen. The same thing happens with Libelles but the amplitude and frequency is different due to the difference in wing stiffness. The Astir has a much stiffer wing so the frequency is higher.

Some years ago we had a RAAF C130H overseas which developed an aileron trim tab electrical fault. A miscommunication between base engineering here in Oz and the crew resulted in the aircraft taking off with the aileron tab physically disconnected from the motor shaft and drifting freely in the airflow (they were meant to disconnect the electrical side only, with the tab frozen in a neutral position and motor inoperative). Shortly after takeoff they experienced severe flutter and vibration through the whole aircraft, significantly hampering controllability. They slowed right down and returned for an immediate landing. Popular opinion is that they were very lucky.

You have to be real, real careful what you do with flight controls!

 

 

[Oscar]

I have seen an Astir glider fly with one aileron disconnected. The wingtips were moving up and down through about 40 cm at about 3 cycles per second. This gave the pilot a very rough ride but otherwise the glider was controllable .Many control surfaces are not completely mass balanced so control surface-induced flutter will happen. The same thing happens with Libelles but the amplitude and frequency is different due to the difference in wing stiffness. The Astir has a much stiffer wing so the frequency is higher.

Sorry but no demo will be done. In recent years the GFA has introduced a second inspection of newly-rigged gliders to try to eliminate this happening.

Mass balancing of a control surface rarely means that it is 100% statically balanced at the hinge line. A degree of 'out-of-balance' is in fact required to dampen the combined effects of kinetic energy and moment arm AND aerodynamic effect excitation of flutter - with regard, as you have said, to the stiffness (and therefore harmonic period) of the structure to which they are attached.

 

Recent developments in computer modelling may have made all of that predictable, but until fairly recently it has been ultimately a matter of test-flying. AFAIK ( but I am most happy to be better advised), it is still the 'rule of thumb' in the FARs that no more divergence than 5% to the demonstrated 'flutter-free' configuration to a control surface is allowed without full flight testing for any modification to the control surface.

 

By way of example: when I changed the fin and rudder of my LSA55 to use a UL450 ( also used on the J120) fin and rudder, I found the supplied rudder was a bad fit to the fin. To rectify that, I needed to split the rudder spar and widen it at the top, necessitating a new spar moulding and some additional insertion structural changes. (It was done in consultation with the aero-engineer who did the structural justification of the LSA55, Alan Kerr, and the aero-engineer who did the certification flight testing, Dafydd Llewellyn). When all the mods were done, the change in balance was less than 3% - and by way of explaining relevance: the additional spar moulding, which sits around 45mm behind the hinge line, was about 60 grammes!

 

Seriously: a heavy coat of new paint on something as large as a full-span elevator can upset the mass balance. This is absolutely NOT an area where one should just wade in with some well-intentioned changes, without expert advice.

 

 

[IBob]

I personally think this has been a great educational discussion especially for those new pilots and discussions like this is a great asset to this site.

I have certainly found it so. It has caused me to try and think through those various scenarios, what it might be like, and what I might try to do.

I have been, momentarily, in one of these situations. I am not at liberty to publish it here, but we were very lucky to come out of it alive.

 

And, while the situation was volatile at the time, and demanding action, it would have been far better had I done absolutely nothing until I had thought things through.

 

So, what I learnt from that (I hope), is not always to automatically leap into action, but to think, if time allows.

 

 

[aripilot]

1996 lost most of rudder and right elevator in a Zlin 126 Trener. Remember to try to keep ball centered.

 

 

[Bruce Tuncks]

Wow aripilot, how did that happen? I would have got a fright for sure. Please tell us the story.

 

 

[aripilot]

I hope Google can translate Finnish investigation report "Tutkintaselostus C 10/1996L". A bearing (ø 13 mm) disconnected from elevator actuator arm due to too small washers (ø 12.5 mm) and right elevator bearing pins slipped from their mountings.

 

Elevator trim cables prevented total separation of the right elevator from the plane and loose elevator smashed the rudder. Service Bulletin issued 22.8.1958 mandated use of ø 16 mm washers. This Service Bulletin was only for Zlin 226 which has similar elevator to Zlin 126. Interesting flight with my son.

 

 

[IBob]

I hope Google can translate Finnish investigation report "Tutkintaselostus C 10/1996L". A bearing (ø 13 mm) disconnected from elevator actuator arm due to too small washers (ø 12.5 mm) and right elevator bearing pins slipped from their mountings.Elevator trim cables prevented total separation of the right elevator from the plane and loose elevator smashed the rudder. Service Bulletin issued 22.8.1958 mandated use of ø 16 mm washers. This Service Bulletin was only for Zlin 226 which has similar elevator to Zlin 126. Interesting flight with my son.

Crikey...glad you're still here to write about it!