CT Modeller Posted August 29, 2012 Share Posted August 29, 2012 Rotor Droop I felt moved to post this after seeing some photos of the new Italeri Wessex 5. A very nice kit indeed, but the moulded-in rotor droop just isn’t right. The 1/72 Italeri Wessex also suffers from the same issue, as do a number of their helicopter kits. If you try to imagine the bending moment caused by the weight of the blade, this will be highest at the blade root, where the total weight of the blade is acting. As you move out from the rotor hub, the bending moment gradually reduces to zero at the tip. This means that the blade droop will be greatest at the hub, and gradually reduce as you go towards the tip. So the ‘bend’ in the blade should be mainly at the root end, gradually straightening out such that there is very little bend towards the tip. Here are a couple of real Wessex pics to show what how it is: These are a couple of photos of Italeri Wessex blades taken from a number of posts to this forum (sorry if you recognise your Wessex here – I’m not having a go at anybody – it’s just to illustrate what the kit provides!). These blades have a drooping curve towards the blade tip end, which would simply defy the laws of physics in the real world. So – out with the warm water and bring on careful bending of plastic! 2 Link to comment Share on other sites More sharing options...
Bri48 Posted August 29, 2012 Share Posted August 29, 2012 Ah, those Bending Moments or CWM=ACWM, the memories flood back! Link to comment Share on other sites More sharing options...
Bertie McBoatface Posted August 30, 2012 Share Posted August 30, 2012 Good observation. I knew there was something wrong with the droop but I couldn't figure out what it was! Now all we have to get right is the blades' forward sweep about the drag hinges when parked. I've yet to see this modelled accurately, even by helicopter enthusiasts. Link to comment Share on other sites More sharing options...
heloman1 Posted August 30, 2012 Share Posted August 30, 2012 Well spotted, I have another problem, I need them straight all the way along! I started the first buy teasing it gently through my fingers with a little upwards pressure.Watch out for the mould gate at mid way down the blade it makes them fragile in that area. Colin on the Africa Station Link to comment Share on other sites More sharing options...
sammy da fish Posted August 30, 2012 Share Posted August 30, 2012 Ah the old lateral drag damper movement. Yes if you look at most older helicopters,(newer types tend to have what's called a rigid head and therefore would be approximatly 90oapart) but not all from above, as most of us will do, then the blades are not set apart from each other at 90o. The forward two move on the damper travel, so go to about 80oapart over the cockpit area. Whilst the rear two rest on the forward stops of the drag dampers so the angle bewteen them would be around 110o measured over the spine of the helicopter. SF Link to comment Share on other sites More sharing options...
Alex Gordon Posted August 30, 2012 Share Posted August 30, 2012 Thanks Sammy. It is still a mystery to me how four little tiny thin things like those can do that amount of work. Link to comment Share on other sites More sharing options...
Julien Posted August 30, 2012 Share Posted August 30, 2012 Just dont droop your Lynx baldes like I saw on an otherwise nice build at Telford! Julien Link to comment Share on other sites More sharing options...
sinclair1965 Posted August 30, 2012 Share Posted August 30, 2012 where's that uri geller (spoon bender) bloke when you need him,,, hey ? Link to comment Share on other sites More sharing options...
Andrew Perren Posted August 31, 2012 Share Posted August 31, 2012 I agree with you that the droop as molded is a little too extreme, although many modellers including myself will probably choose to ignore it. I do have one question for you though. If you are factoring in all the forces that will apply to the blade and thus affect droop, then what about the fact there is a built in rigidity in the blade? This will result in some resistance to sag closest to the root and decrease as you move out along the blade. Result is that the point of greatest "droop" is probably 1/4 - 1/3 way out along the blade where there is still a strong downward force as you mention but the rigidity has lost its ability to hold the blade horizontal. In your image of "H" XS674 draw a straight line from root to tip and see where the greatest chord is and you'll see what I mean. Does that make sense ? I'm not trying to be argumentative. What I find odd is models with blades that seem to point upwards near the hub then dive downwards towards the tip - like some sort of spider. Link to comment Share on other sites More sharing options...
Bertie McBoatface Posted August 31, 2012 Share Posted August 31, 2012 If you build the Italeri model with tip socks properly tied down, I reckon the curve would be just about perfect. Hmm... Well spotted, I have another problem, I need them straight all the way along! I started the first buy teasing it gently through my fingers with a little upwards pressure.Watch out for the mould gate at mid way down the blade it makes them fragile in that area. Colin on the Africa Station Showing them in a maintenance rack? Link to comment Share on other sites More sharing options...
heloman1 Posted August 31, 2012 Share Posted August 31, 2012 If you build the Italeri model with tip socks properly tied down, I reckon the curve would be just about perfect. Hmm... Showing them in a maintenance rack? Much more interesting than that! Colin on the Africa Station Much more interesting than that! Colin on the Africa Station Sorry missed your comment re the tip socks, that would be just about right. Better make a set for my second one. Link to comment Share on other sites More sharing options...
pigsty Posted August 31, 2012 Share Posted August 31, 2012 (edited) I agree with you that the droop as molded is a little too extreme, although many modellers including myself will probably choose to ignore it. I do have one question for you though. If you are factoring in all the forces that will apply to the blade and thus affect droop, then what about the fact there is a built in rigidity in the blade? This will result in some resistance to sag closest to the root and decrease as you move out along the blade. Result is that the point of greatest "droop" is probably 1/4 - 1/3 way out along the blade where there is still a strong downward force as you mention but the rigidity has lost its ability to hold the blade horizontal. In your image of "H" XS674 draw a straight line from root to tip and see where the greatest chord is and you'll see what I mean. Does that make sense ? I'm not trying to be argumentative. What I find odd is models with blades that seem to point upwards near the hub then dive downwards towards the tip - like some sort of spider. I'm not sure it's like that. The blade is essentially made the same way all along so its resistance to bending is the same at all points. The greatest bend comes close to the hub because, while the moment arm is short, the weight acting on it is greatest. However, in those photos of real Wessex*, you can also see that there's a short, nearly straight section right next to the hub, with the bending beginning after that. This is there not because of the physical properties of the blade but because of the blade root attachments. Quite hefty chunks of metal are connecting the hub to the blade by gripping it on its leading edge. These are much stiffer than the blade itself, and the weight of the blade isn't enough to bend them, although it is enough to bend itself. I do agree with that last point. It looks as though the kit has a slight upward angle built in, instead of a dead flat root section. Glad it wasn't just me! * note plural! Or it might be Wesseces or even Wessexen. Certainly not Wessi, though, that would be Latin Edited August 31, 2012 by pigsty Link to comment Share on other sites More sharing options...
Scimitar F1 Posted August 31, 2012 Share Posted August 31, 2012 Excellent thread. This has answered a question that had been in the back of my mind since I saw the Italeri Wessex. The weight of the blade can be said to act around the centre of gravity which will be roughly midway along the blade so the moment arm is effectivel greatest at the hub. Will Link to comment Share on other sites More sharing options...
SkippyBing Posted August 31, 2012 Share Posted August 31, 2012 (edited) What I find odd is models with blades that seem to point upwards near the hub then dive downwards towards the tip - like some sort of spider. Depending on the type that may be an attempt to show the pre-coning in the hub. Basically as the helicopter hangs off the blades they'll cone upwards to a degree, to reduce the stress in the head it's designed for this having the blades attached pre-coned rather than straight out. If you look at the second picture, i.e. 'H', the blade pointing to the right of shot demonstrates this quite nicely. Obviously it varies by type being quite obvious on a Lynx and less so on your fully articulated head types, like the Wessex. As to whether the model kits get this right is another matter entirely... Edited August 31, 2012 by SkippyBing Link to comment Share on other sites More sharing options...
ErikB Posted October 8, 2012 Share Posted October 8, 2012 Ah the old lateral drag damper movement. Yes if you look at most older helicopters,(newer types tend to have what's called a rigid head and therefore would be approximatly 90oapart) but not all from above, as most of us will do, then the blades are not set apart from each other at 90o. The forward two move on the damper travel, so go to about 80oapart over the cockpit area. Whilst the rear two rest on the forward stops of the drag dampers so the angle bewteen them would be around 110o measured over the spine of the helicopter. SF Hi Sam, The topic is over a month old, I know, but since I got here using the "Search"-function, chances are that someone else looking for Wessex-rotorblades will too, in the future. On the position of the blades on the drag-hinges: as you describe it is of course when displaying the blades as an X on the helicopter. But for the blades to all hang forward on the drag hinges would require the rotorhead as a whole to be tilted forward. As far as I have understood, the fully articulated rotorhead on the Wessex doesn't do that? The "forward tilted" rotordisc for forward movement is created by having less AoA on the front blade and more on the aft blade. Because of the flapping hinges the aft blade will ride higher than the forward blade, in effect creating a forward tilted rotordisc. Therefore the blades at rest should be in the neutral position on their drag hinges since the dampers would push them to that position when parked? The blades at rest in their flapping-hinges is another case, although the effect on the Wessex is much less obvious than on the Chinook for example. What is very obvious is the top two blades of the tail rotor when parked in an X-position. They certainly hang forward on their flapping hinges! I don't really know very much about helicopters, so if I'm wrong please don't hesitate to correct me. If it does actually make any sense, please let me know too! I based my train-of-thought (or brain-fart) on the following: http://www.aviastar.org/theory/rotor.html Cheers, Erik. Link to comment Share on other sites More sharing options...
ErikB Posted October 9, 2012 Share Posted October 9, 2012 Unless, the whole gearbox with rotorhead is mounted with a few degrees forward tilt, of course... Hmmmmmmm, does anybody know? Cheers, Erik Link to comment Share on other sites More sharing options...
swordfish fairey Posted October 9, 2012 Share Posted October 9, 2012 (edited) Have a look at this, the orientation is wrong but you can get some idea of the forces acting on the blades.It shows how flexible rotor blades can be......Smudge Edited October 9, 2012 by swordfish fairey Link to comment Share on other sites More sharing options...
perdu Posted October 9, 2012 Share Posted October 9, 2012 (edited) I'm disappointed that Don doesn't think I got it right with my HAR2 model, seen here before unmasking and with rotors fitted to test the angles And this is the droop I usually try to get for a Wessex Edited October 9, 2012 by perdu Link to comment Share on other sites More sharing options...
ErikB Posted October 9, 2012 Share Posted October 9, 2012 (edited) [media] Have a look at this, the orientation is wrong but you can get some idea of the forces acting on the blades.It shows how flexible rotor blades can be......Smudge Hi Smudge, You piloted / flew in the Wessex, didn't you? So you've seen it a lot more times up close than I did... What do you think on my reasoning regarding the blades with the helicopter parked? The second photo in the Opening Post would seem to indicate that the rotorhead tilts forward but I'm afraid it only seems that way due to lens distortion. Althou that is an assumption on my side. BTW, I read that helicopter blades have a fair amount of travel both horizontally and vertically to isolate vibrations and other forces. But that they move that much themseves!... I'm disappointed that Don doesn't think I got it right with my HAR2 model, seen here before unmasking and with rotors fitted to test the angles [Pic] And this is the droop I usually try to get for a Wessex [Pic] Ey Perdu, Who's Don? Nice Wessex', Wessexes, Wessii (or what the plural for Wessex is) you have there! I don't know for sure if the blades are on the neutral position or hang forward in their drag hinges. To be honest, I found it a brilliant idea. Then I read that to create forward movement, not the rotorhead tilts forward in most cases, but the same effect is created by having a greater Angle-of-attack on the blade when it crosses the 6 o' clock position than when it crosses the 12 o' clock position. The tail comes up, the nose dips and the helo starts to move forward. I'm assuming that the gearbox + rotorshaft + rotorhead is in a 90 degree position relative to the ground when parked. In that case I don't think that the blades sag forward. If the gearbox + rotorshaft + rotorhead is mounted with a forward tilt of a few degrees, you might very wel be right! Over some time the weight of the blades could pull them slowly but surely through the dampers so the sag forward somewhat in their drag hinges. A reason for mounting the gearbox etc. tilted forward can be so the nose doesn't have to dip too far down for forward flight. Then the question is how many degrees forward and backward travel of the blades do those hinges allow for and would it be noticeable in 1/72? BTW; I think you nailed it with the droop. Very good! Cheers, Erik Edited October 9, 2012 by ErikB Link to comment Share on other sites More sharing options...
ErikB Posted October 9, 2012 Share Posted October 9, 2012 The Mark-I drawings in the 4+ show a 1-degree forward tilt of the rotorhead. I don't know if that's correct, but if so, would it be enough for the blades to sag through the dampers? Link to comment Share on other sites More sharing options...
SkippyBing Posted October 9, 2012 Share Posted October 9, 2012 It's been a while since I covered it so this may not be 100% accurate, but helicopter gear boxes are often installed with an offset to allow for the various coupling effects experienced by the dynamic system. I can definitely remember the Squirrel's being offset to the left to allow for the right skid being low in the hover (due to the tail rotor being lower than the main rotor pivot point), if the Wessex was tail low in the hover it'd make sense for the gearbox to be tilted forwards as the rotor head would then be in a less stressed position. Link to comment Share on other sites More sharing options...
swordfish fairey Posted October 9, 2012 Share Posted October 9, 2012 (edited) If I can remember correctly the main rotor drive is canted forward from the vertical by about 5 degrees and the blades do move forward onto their stops depending on where the rotor head is parked. This also allows the a/c to move forward on the ground.... .....Smudge Edited October 9, 2012 by swordfish fairey Link to comment Share on other sites More sharing options...
perdu Posted October 9, 2012 Share Posted October 9, 2012 This is the picture I believe Don supplied to show the required blade drag, if not I apologise to whomever it was Don is also known by his shorter professional moniker of "Per Arduam Ad Ostentation" or something like that Sorry Don if I got that wrong too Just look at the amount of blade angle between the front most (this side) and its corresponding blade on t'other side This is an almost perfect example to use in my opinion (which is why I snaffled it for my Wessex files) You can also see in this picture that the gearbox does seem to have a forward slant in-built Some of you may remember that this is the airframe I chose to model earlier this year and one reason was the brilliant amount of detailing visible on the picture. Lookit that lovely beastie. I love love love it So not actually any more rational than any other bloke with a fancy piece... even if her name is WALTER Link to comment Share on other sites More sharing options...
perdu Posted October 9, 2012 Share Posted October 9, 2012 And it seems that this picture was at a time that grey/blackish upper blade colours was in vogue I have pictures in my archive of both styles Link to comment Share on other sites More sharing options...
ErikB Posted October 9, 2012 Share Posted October 9, 2012 Ah, okay, I'm convinced! I'll build my Wessex with some forward sag in it too. Thank you all for chiming in and helping me out! Cheers, Erik. Link to comment Share on other sites More sharing options...
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