Ben M Posted February 7, 2015 Share Posted February 7, 2015 Hi, I have some doubts about overproping on helicopters and I would appreciate if you could help me. To be honest I do not know if we can talk about overproping on helicopters. I have seen that this term is used for fix pitch propellers. For this reason I will explain what I understand as overproping on helicopters: When the pull provided by the rotor disk at a significant RPM doesn´t provide lift enough to hold or lift the helicopter. In other words, the rotor head´s load (helicopter weight) doesn’t allow the helicopter climb (or hover) as it should with a given rotor speed and angle of attack. Is my previous definition of “overproping on helicopters” correct? Can we talk about "overproping" on helicopters or is there another term for helicopters? Can we fix an overproping issue by using longer blades (assuming the motor supports it)? Is there an easy way to measure or calculate if there is overproping? Could we calculate the lift a given rotor disk should generate at hovering? On the following link we can see how to calculate if there is overproping in a fixed wing aircraft: **LINK** I wondered if there is a similar method to calculate the same concept swapping translational speed by vertical lift. On the following link there is a formula to calculate rotor blade lift. However it depends on a parameter called “lift coefficient at specific angle of attack” which is unknown so I cannot use it. **LINK** Thanks in advance for your comments. Ben Quote Link to comment Share on other sites More sharing options...
ted hughes Posted February 7, 2015 Share Posted February 7, 2015 Not directly related to your question, but a superb example of home-building:**LINK** Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 8, 2015 Share Posted February 8, 2015 I'm not sure what you are trying to say here Ben....on most helicopters the angle of attack of the blades is variable & is known as the collective pitch. The pitch of the blades varies from around -10 to +12 degrees or so. Negative pitch is used when the helicopter is upside down so the rotor disc still produces lift. Model helis usually hover at around 3-4 degrees.....less pitch & they sink...more pitch & they climb. Now..increasing pitch increases lift but also increases drag so when we increase pitch we need to supply more power otherwise the rotors will slow down & lift will be reduced. So to come back to your question can you "overprop" a helicopter...yes you certainly can ...if you add more pitch & hence create more drag than the engine can overcome then the rotor will slow down. To me, "overpropping" simply means you are asking the engine to work harder & supply more power than it is capable of. Also the answer to the question "Could we calculate the lift a given rotor disk should generate at hovering? " is a very simple calculation.....in the hover the lift provided by the rotor disk exactly matches the weight of the helicopter...... Quote Link to comment Share on other sites More sharing options...
Ben M Posted February 8, 2015 Author Share Posted February 8, 2015 Hi Steve, Thanks for your help. I am not sure if I understand the concept of "Overproping". On the one hand, as per your explanation "Overproping" is an issue with the motor not having enough power to keep the requested RPM. On the other hand, as per the explanation on this site, "Overproping" is a problem caused by the propeller not being able to handle its load at the correct RPM. I understood that "Cavitation" was related to "Overproping" and it has to do with the propeller characteristics (including pitch) not with the motor’s power. The helicopter’s rotor is a propeller which pitch can be changed on the go. However at the end of the day it is a propeller and we can study it as a fixed pitched for a given angle of attack. Regardless of the meaning of “Overproping” my interest is about how to know if the size of the blades are right for the heli's weight. Is there a way to calculate the ability of a given blades to generate lift (assuming the motor has enough power)?. I am assuming that not all blades are good for all helicopters due to their ability to push air down and generate lift. I guess that the bigger the blades are the more air can push down and thus more lift. Does it make sense? Thanks again, Ben Edited By Ben M on 08/02/2015 23:48:13 Quote Link to comment Share on other sites More sharing options...
Dave Hopkin Posted February 9, 2015 Share Posted February 9, 2015 Hi Ben "Cavitation" is frequently a term applied to ship propellors where it is critical as the water boils and causes erosion on the blade faces and a great deal of noise - cavitation in a water is caused by prop speed/water density/water temperature, in an air screw its caused when the blade enters a stalled state, ie the angle of attack increases to such an extent that the bounary layer of air flow over the blade ceases to be smooth transitions into a vortex - thus loosing the screws pull In a helo - that will result in a loss of lift Over-proping to me, means putting a propellor/blade set that is too big for the motor (be it in diameter or in pitch) thus demanding more power from the motor than it can deliver - obviously in a helo the pitch is variable so a over proping situation could be created if the collective allowed to great an angle of attack Equally fitting a blade set greater than the motor could handle would also be oper propped Quote Link to comment Share on other sites More sharing options...
Chris Barlow Posted February 9, 2015 Share Posted February 9, 2015 A bit of a mis-understanding here. A helicopters rotor is a "wing" not a propeller and as such it should be calculated by the "wing" area, weight of the airframe and therefore "wing" loading to determine the rotors suitability. Steve is partially correct in what he says about the pitch of the rotor being responsible for the amount of lift produced. There are other factors in producing lift though which are rotor speed (head speed) and airspeed. A faster head speed will require less pitch to maintain lift and additionally air speed, be it forwards, backwards, sideways or simply wind speed will add to the head speed and create more lift. Quote Link to comment Share on other sites More sharing options...
Ben M Posted February 9, 2015 Author Share Posted February 9, 2015 Very interesting. Thanks for the information. I think I understand it better now. Does anybody know how can we calculate the lift done by the blades while the rotor disk is horizontally and there is no cyclic control applied to it. I now that calculating it when the rotor disk is tilted or it has cyclic control is much more complicated. I would like to be able to calculate it in a simple scenario where the heli is hovering without moving and without wind. The formula should take into account (as per your comments): Wind speed: should be 0. Blades' size. Blades' pitch Rotor head's RPM Density of air The formula could be very interesting to play with. I wouldn't mind if the formula is complicated as long as the input parameters are well defined with the right measurement units to be used. Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted February 9, 2015 Share Posted February 9, 2015 There is a calculation here you can have a play with: L=CL ½R S V² **LINK** Edited By WolstonFlyer on 09/02/2015 21:29:16 Quote Link to comment Share on other sites More sharing options...
Ben M Posted February 9, 2015 Author Share Posted February 9, 2015 Hi WolstonFlyer, That formula is interesting. I have seen other versions. However the parameters are defined in a non-numerical way. For instance CL is related to the pitch however we do not know if we need to use an angle in grades, radians or an trigonometry expression such as sin, cos, tang. The speed is related to the RPM however it doesn't say if it is angular or linear speed (the last one may need an integral because the linear speed is different for each point along the blade depending on the distance to the rotor's center). To be able to calculate Lift in a numerical way we need to properly define each input parameters including their units. Quote Link to comment Share on other sites More sharing options...
WolstonFlyer Posted February 9, 2015 Share Posted February 9, 2015 Hi Ben Cl is measured in degrees.Also have a look at: http://www.aerospaceweb.org/question/aerodynamics/q0015b.shtml Quote Link to comment Share on other sites More sharing options...
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