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Forum Index : Windmills : Centrifugal force
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Downwind Guru  Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Out of curiosity i went looking for a calculator for centrifugal force and found this handy one. http://www.calctool.org/CALC/phys/newtonian/centrifugal I was interested in the force a set of blades applied to the mounting point of a windmill under centrifugal force at peek RPM. A set of 2.4m blades, with a weight of 1.5kg each, spun at 500rpm will exert a force of 1 ton to the blade mounting point of a windmill. No wonder things fail at times 
Or 
Pete. Sometimes it just works |
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Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks Pete, I will make good use of the info when I modify that whirly gig on my shed roof. Klaus |
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Perry Senior Member  Joined: 19/11/2009 Location: Posts: 190 |
Hey Pete, You need to apply the mass of the blade at the blades CG. You should have entered 1.2 (roughly where it may be) not 2.4m Still a lot though. Perry |
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MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Crew Remember back a while, when I posted a build on "Coroflute" blades? Here's a picture showing one blade on my scale. The blades are both robust as well as feather light and might be something of further interest now that Pets's discovered how much force gets exerted at the hub of our toys. 
That's 4.2 ounces for a 4" x 24" blade with spar. Due to different wood densities, the 3 blades weiged 4.1, 4.2 and 4.3 ounces each. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Pete Lots of force and imbalance should it get out of balance, but the one that is the main destructive force is the gyroscopic force, a combination of the centrifugal force's by the angular displacement of the mill rotating as in furling or tracking the wind, thats what does the damage to the mill head. In aircraft it causes the engine case to crack or the crankshaft to break, if the force is concentrated by rapid change of the propeller disk such as doing aerobatics. All the best Bob Foolin Around |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Hi Mac, As light as your blades are it still equates to 20kg per blade force at 500 rpm. It is rather surprising the mass force applied. 
Hey Perry, I did realize that and why i added the second picture, but as you said ...still a lot. Pete. Sometimes it just works |
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| neil0mac Senior Member Joined: 26/12/2009 Location: AustraliaPosts: 210 |
Question. Perhaps fatigue due to flexing at the blade root from irregular wind loading on the blades should be added to your stress calculations? (Presumably, on an aircraft prop the loading would be very close to 'constantly equal' as actual wind speed would be negligible?) |
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| MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
neilOmac That's a good point in my case especially, since I use a stand-off spar to gain swept area, while not having inboard blade drag near the hub. That type of stress has been the reason for at least one catastrophe (read that tower strike). I've considered going to a carbon-fibre spar, but they have 'other' problems, cost being right up there near the top of the list. . . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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Oscar4u Regular Member  Joined: 23/02/2011 Location: New ZealandPosts: 42 |
Hello Mac gyver I had thought of using a stand off spar as well. Figured it would give a much more powerfull machine for the same size blades (most other parts of the machime are not that difficult to make bigger and stronger). But usually ther is no such thing as a free lunch. Is it a recomended idea? Also the spar and hub are welded (stronger) so the join from steel to blade would not have as much centrifugal force, as if it was a long blade. look forward to your advice. Thankyou Oscar Oscar4u - for all your rotary cowshed repairs |
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Rastus Guru Joined: 29/10/2010 Location: AustraliaPosts: 301 |
Hi to all, Centrifugal forces seem immense,I exspect they are balanced at the dead centre of the drive shaft.As Bob pionted out,these forces multiply and twist out of balance when furling,making me feel that the drive shaft should be supported as close as possible to the hub.Then the shear strength of the shaft diameter is more effective having removed leverage.Is this point valid? cheers Rastus see Rastus graduate advise generously |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Rastus The closer to the support bearings the less twisting load on the main shaft, the ideal is as in my new mill where the centrifugal and gyroscopic forces are between the bearings as there is no overhung forces so lighter structure is suitable. In a aircraft propeller the forces are balanced in level flight, but come into play when turning left or right, aerobatic aircraft put enormous loads on the propeller, hub crankshaft and main bearings, this results in a shortened service life compared to normally operated aircraft, but out furling causes higher proportional loads than in an aerobatic aircraft, because of the short time frame for the action, in our furling it can happen in several seconds whereas in an aircraft the angular displacement could be over 5 to 10 seconds depending on the maneuver. Wild forces are in play as demonstrated by karlJ windmill a few months ago.This demonstrates the forces as transferred to the yaw axis bearing on top of the mast. All the best Bob Foolin Around |
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| Rastus Guru Joined: 29/10/2010 Location: AustraliaPosts: 301 |
Hi Bob, Thankyou for your prompt reply!So having a short shaft to hub distance,transfers the combined forces between the head and furling point onto the yaw bearing.Is that a correct assesment? cheers Rastus see Rastus graduate advise generously |
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| VK4AYQ Guru Joined: 02/12/2009 Location: AustraliaPosts: 2539 |
Hi Rastus The centrifugal force in a balanced propeller isn't an issue unless it is structurally deficient, The gyroscopic force is the danger to the structure as this is the force acting through the shaft and bearings all the way back to the yaw bearing and assembly. The shorter the moment arm from the spinning disk (propeller)the less will be the leverage force and the bearing load will be less, but in saying that, the same force is still transferred back to the yaw bearing, so the longer the moment arm to the yaw bearing the more leverage and load there. The gyroscopic force isn't diminished it must be dissipated by the head, the only way to diminish the force is to slow the yaw action of the mill and / or use a lighter propeller. The centrifugal force isn't transferred through the bearings as in a balanced propeller it is dissipated at the center of rotation. The gyroscopic force is a product of the centrifugal force times the rate of yaw, the faster the rate of yaw the higher the gyroscopic force, no yaw no force. Now that I have confused you completely, the other option is a feathering propeller that stays into wind and doesn't cause furling loads, but is still subject to gyroscopic forces as it tracks the wind, so needs to be as stable as possible to reduce forces. All the best Bob Foolin Around |
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| Rastus Guru Joined: 29/10/2010 Location: AustraliaPosts: 301 |
Hi Bob, I think the confusion set in from being dropped at birth  I've enjoyed the brief engineering lesson and being helped to understand sound design criterior.Smoothing out the furling action would be an ideal to aim for,probably not effectively achieved though on the scale of mill I'm considering.Feathering is being discussed in another thread, your input equally valued there!Thanks again!
cheers Rastus see Rastus graduate advise generously |
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| Rastus Guru Joined: 29/10/2010 Location: AustraliaPosts: 301 |
Hi Mac, Does having a nose cone reduce/effect the indoard blade drag near the hub? cheers Rastus see Rastus graduate advise generously |
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