Grandad Posted March 9, 2014 Share Posted March 9, 2014 Finally I began modelling a Wessex, and studying lots of images from the net I discovered something I was unaware of: the fact that one of the two adjacent pairs of rotor blades were not in line. At least one rotor blade seems to be askew. You're probably laughing at me for not knowing this, but I assumed it was important all blades were equally spaced. Just browsing through AirBritain Wessex images I found dozens of images similar to this one. http://www.abpic.co.uk/index.php Can anyone tell me whats up! /Grandad Link to comment Share on other sites More sharing options...
gingerbob Posted March 9, 2014 Share Posted March 9, 2014 Wild speculation: blades are able to "swing" fore and aft to a degree? Probably a load of you know what... bob Link to comment Share on other sites More sharing options...
Dave Swindell Posted March 9, 2014 Share Posted March 9, 2014 Basic Helicopter Rotor operation Slow motion film of a helicopter rotor blade More detailed explanation of rotor operation 1 Link to comment Share on other sites More sharing options...
Grandad Posted March 9, 2014 Author Share Posted March 9, 2014 Can't remember where I found this image, but look at the angle! Three blades looking all right and one pointing from the viewer and the off line angle is not only a few degrees, but much more. Thank you Dave for pointing me in the correct direction! This is from the aviastar.org page, Dave's link above: These are movements the blades make in response to the powerful natural forces acting on the rotor, for which the articulated hub provides the necessary mechanisms — specifically, hinges — which permit freedom of movement so the blades can "articulate," or flap up and down and move back and forth slightly in the horizontal plane. The word slightly (I changed it to bold) apparently has different meanings, and my interpretation is not really as much as in the image below. The Lynx rotor is rigid so no "dragging, hunting or lead-lag" around the hinge there, but on the Wessex which I assume is an articulated type rotor. But why does this show when the engine is shut off? It says there are hydraulic dampers which prevents the blades from moving too much. But, since there are som many images showing this one blade (or two as it's impossible to which one is off line) maybe the Wessex rotor blades are always like this when not in operation, and we should assemble the Wessex rotor with one, or two blades looking like this! What will the Wessex geeks at the club think about me if my new Wessex would look like the one below? /Grandad 2 Link to comment Share on other sites More sharing options...
alex Posted March 9, 2014 Share Posted March 9, 2014 What will the Wessex geeks at the club think about me if my new Wessex would look like the one below? You should give a try. Which proves, once more, a model shouldn't look like the real thing, but it should look... like... a model! Alex 2 Link to comment Share on other sites More sharing options...
NAVY870 Posted March 9, 2014 Share Posted March 9, 2014 The blades have sagged foward on the drag dampers, they will do it slowly after shutdown. The hydraulic reservoir for the dampers is the plastic dome on top of the rotor head. Utter pig of a thing to change out! 2 Link to comment Share on other sites More sharing options...
heloman1 Posted March 10, 2014 Share Posted March 10, 2014 (edited) If you are refering to the tail rotorblades, there is a flapping hinge which when the aircraft is static the blade flop to either side of the true line of rotstion and is most prominent on the blades in the top quadrant of the circle. During gusting winds and when parked on the ground a 'gust lock' is fitted, which pushes two blades against the stops and pulls the opposite two blades. Similarly tip socks are fitted to the main rotorblades and lashed by means of ropes to the lashing eyes or main oleos and tail yolk. In cold ie, arctic weather conditions it's a whole nother story, as there is a full set of silver covers to drape, pull, push over various parts of the airframe. The possiblities are endless. As to your Wessex geeks/nuts at the club ask them to prove your model is incorrect. Colin Wessex nut. Edited March 10, 2014 by heloman1 1 Link to comment Share on other sites More sharing options...
Jonners Posted March 10, 2014 Share Posted March 10, 2014 Grandad, the Wessex (like the Sea King) had a 'fully articulated' rotor head which means that it had 3 independent hinges between the rotor head and each blade. These hinges were/are generally known as the dragging, flapping and feathering hinges. Each hinge allowed the blade to move in a particular axis as the result of normal aerodynamic forces. You have noticed the effect of the dragging hinge, which allows the blade to move forwards and backwards slightly. As Navy870 has said, each dragging hinge had its own damper ('drag damper') to prevent the blade from banging from one extreme to the other too harshly. When stationary, the blades would normally settle at roughly equal angles relative to one another, but if a blade was pushed against its damper - eg by a strong wind or a maintainer - then it would stay in its new position until another force was applied to move it back again. For completeness, the flapping hinge allows the blade to 'flap' up and down (you could think of it as the Larry Grayson hinge...) and the feathering hinge allows the blade tochange its angle of incidence, like feathering a propellor. Try it with you hand held out in front of you, pslm downwards: flap your hand up and down for the flapping hinge, move it side to side as though you are waving for the dragging hinge, and twist your wrist (the 'so-so' gesture) for the feathering hinge. All helicopters must allow their blades to move in each of these axes, but in modern designs this is normally achieved using an elastomeric bearing (like a very strong chunk of rubber) instead of heavy and mechanically complicated hinges. Here endeth the lesson! Jon 5 Link to comment Share on other sites More sharing options...
Windy Posted March 10, 2014 Share Posted March 10, 2014 You can tell some people on here are ex-grubbers......... Link to comment Share on other sites More sharing options...
Grandad Posted March 10, 2014 Author Share Posted March 10, 2014 You're never too old to learn! I'm happy with all the answers. Cheers /Jan Link to comment Share on other sites More sharing options...
Jonners Posted March 10, 2014 Share Posted March 10, 2014 Not a grubber, Windy - a driver, and ex-crab at that! 3 Link to comment Share on other sites More sharing options...
Ex-FAAWAFU Posted March 11, 2014 Share Posted March 11, 2014 (edited) Jonners is completely right (not bad for a Cr*b!). For completeness, the reason that the drag damper shows uneven blade distribution when shut down, but the feathering and flapping do not, is because it is the only hinge that's allowed to. When you shut down any helo, unless you are completely bonkers you have the lever right down and the cyclic pretty central (otherwise the cab will be trying to go somewhere else). So that means all the blades will be roughly the same in pitch / feathering. As for flapping, I'm not 100% certain about the dear old Wessie, but I think it's the same as a Sea King, which has mechanical droop stops. These work on centripetal force, and as the rotor head slows down they drop into place to stop each blade from flapping down (drooping) too far; without them the blades would hit the deck (I have seen one fail, and most spectacular it was too). If you see film of a Sea King engaging or disengaging rotors, you will see the marshaller with his arms straight up and his thumbs extended sideways: thumbs inwards means stops are in (you can see them), and thumbs outward stops are out. It's one of the checks: try to fly with droop stops engaged and you'd be poorly placed; shut down completely without them and you might trash the blades, so you don't cut the throttle completely until you know the stops are in. That just leaves the drag (lead & lag) dimension, which is what you can see in your photos, because of the loss of hydraulic pressure. The blades might be perfectly aligned, or they might not; it doesn't reality matter. The Lynx is different, because of its semi-rigid head; the flapping stuff is taken up by the flexibility of the titanium rotor head (which is what makes the aircraft so manoeuvrable). Feathering and dragging are as normal. Edited March 11, 2014 by Ex-FAAWAFU 1 Link to comment Share on other sites More sharing options...
Windy Posted March 11, 2014 Share Posted March 11, 2014 Not a grubber, Windy - a driver, and ex-crab at that! I am sure I can forgive you for being a crab... Link to comment Share on other sites More sharing options...
moaning dolphin Posted March 11, 2014 Share Posted March 11, 2014 As for flapping, I'm not 100% certain about the dear old Wessie, but I think it's the same as a Sea King, which has mechanical droop stops. Yep, you are 100% correct, exactly the same as a Seaking in that respect, although it has only 4 per head unlike a Seakings 5, (I think the Seakings may have a spare! ). Reference Pre-Tiff course notes! Link to comment Share on other sites More sharing options...
abat Posted March 11, 2014 Share Posted March 11, 2014 What a fascinating discussion. Thanks to grandad for getting this going and for the interesting replies. Link to comment Share on other sites More sharing options...
Evalman Posted March 11, 2014 Share Posted March 11, 2014 Same with the Puma. I've never noticed it before: Link to comment Share on other sites More sharing options...
heloman1 Posted March 11, 2014 Share Posted March 11, 2014 Standing on the line, you woudn't notice as it's all part of the daily drudge. It's only when you stand back and take cognisence of what's going on that you notice the detail. I've learnt more about the Wessex in past four years than I did in the six that I worked on them. Colin 1 Link to comment Share on other sites More sharing options...
Pete in Lincs Posted March 11, 2014 Share Posted March 11, 2014 Automatic droop stops on the Puma. You can just see one on the picture above. Sticking out at a slight angle at the root of the left blade. On the HC 2's they had a circle of dayglo tape on them so you could just about see them in or 'flying' out as the rotor wound up or down. The two 'Loops' on the head that you can see above , Are two of the four hoses connecting the drag dampers to the reservoir. Imagine a circular plastic casserole dish with four compartments - The reservoir. (If memory serves, that is!) Link to comment Share on other sites More sharing options...
Wafu Posted March 11, 2014 Share Posted March 11, 2014 It's a blast down memory lane. Seems the drivers must have been awake during the tech part of their type training. BZ boys. Only thing to add is the droop stops whilst the rotorhead is 'running down' are pulled in by springs within each blades own stops. I have witnessed droop stops not going in and it ain't funny if your stood outside of the rotor disc, you as a marshaller do pay attention to the stops whilst the a/c is shutting down even going as far as indicating to the pilots to engage power to the rotors to free the mechanism so you can shut down safely. The Wessex pilots I came across both RN and RAF all displayed respect for this by not chopping the engines until the stops are in. With regard to the dampers, I have seen twice the effect of unserviceable dampers. Ground resonance is not an nice thing especially in a Wessex, if not corrected will cause for the aircraft to topple over, I'll leave the rest for you to imagine. By the way chaps what about a ridged head?? Nice thread. 1 Link to comment Share on other sites More sharing options...
Ex-FAAWAFU Posted March 11, 2014 Share Posted March 11, 2014 For some reason I never really understood (I never flew them except as a jolly with a mate) the Wessex was particularly prone to ground resonance, I believe. The Sea King also had dayglo markers on the droop stops so you could see when they were in and out - and we used to turn on the dome light (a light somewhere up near the head, whose true purpose I cannot now recall) so that the marshallers could see the stops in and out at night. Link to comment Share on other sites More sharing options...
Fritag Posted March 11, 2014 Share Posted March 11, 2014 Interesting stuff. Thanks for the discussion chaps - some of which I think I may even have understood....... 1 Link to comment Share on other sites More sharing options...
Jonners Posted March 11, 2014 Share Posted March 11, 2014 (edited) Blimey, I'm getting some stick here! (Crabstick???) Still, I rise above it! Jon PS Hey Fritag - just you wait until we start on helicopter Principles of Flight! Flapping to Equality, anyone? Edited March 11, 2014 by Jonners 4 Link to comment Share on other sites More sharing options...
Wafu Posted March 11, 2014 Share Posted March 11, 2014 The Wessex suffered due to the oleo's being so large, in addition with the possibility of a damper not being matched with the other three. The imbalance is amplified with rotor speed. 2 Link to comment Share on other sites More sharing options...
perdu Posted March 11, 2014 Share Posted March 11, 2014 Some of us have been modelling Wessexii like this for ages but I have heard gripes at shows, as you'd expect doit like it is or do it to suit 'The Suits'? Me I prefer the right way The green one represents XR525 (preserved at Cosford), XT601 has been parked for longer but both have different degrees of 'swing' built in. Photobucket seems to have mislaid the overhead shot I did of 525 3 Link to comment Share on other sites More sharing options...
Nobby57 Posted March 12, 2014 Share Posted March 12, 2014 (edited) The Wessex suffered due to the oleo's being so large, in addition with the possibility of a damper not being matched with the other three. The imbalance is amplified with rotor speed. Talking of weapon's and oleos Wafu; didn't they also trial a harpoon carrying version? You know...looking at the oleos and tail-wheel, it gives the impression that it could have been used like an archaic projectile launcher. I'm certain I read somewhere that a giant elastic band attached to both wheel lashing points, and stretched back to the tail-wheel was proposed to solve the damping problem. But then of course someone must of realised it would have been pretty poor form to re-christen the Wessex 'The Crab-Catcher'! Edited March 12, 2014 by Nobby57 1 Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now