[Post] [Email Reply] _________________________________________________________________ Re: Helicopter From: Steven_Hall@mit.edu (Steven Hall) Date: 1996/02/06 MessageID: Steven_Hall-0602960918080001@nyquist.mit.edu#1/2 Segment 1 of 2 (Get All 2 Article Segments) _________________________________________________________________ references: <4f6aa7$d11@pipe11.ny c.pipeline.com> organization: MIT newsgroups: sci.physics In article <4f6aa7$d11@pipe11.nyc.pipeline.com>, egreen@nyc.pipeline.com (Edward Green) wrote: > On Feb 01, 1996 17:49:45 in article , 'bwoerner@ilhawaii.net > (Bill Woerner)' wrote: > > >I'm doing a project for school and was wondering if anyone knows the > >equation for the force of lift a helicopter's rotor blade gets? I'm not > >looking for an answer to this, I need the equation so I can experiment > >with variable sized blades. Also, what is the average speed a rotor blade The tips of most rotor blades move at about 0.7 of the speed of sound, or about 700 ft/sec! > >on a helicopter spins? > > The lift is going to depend on the rotor speed (obviously), the angle of > attack, and to a lesser extend, the cross-sectional shape of the blade. > > On real helicopters the angle of attack can be varied while the rotor is in > motion, and even within a single cycle in a repetitive fashion, so that > the lift developed has a thrust component; that's how helicopters fly > forward. I want to say that helicopter pilots have two hand controls > called the throttle and the collective, but I am frankly fishing now. You > can easily find this info. The angle of attack of the blades is also > called the pitch. Helicopters have more controls than you state. One hand controls both the throttle and the collective. Roughly, the throttle controls the speed of the rotor, which must usually be held constant, and the collective controls the net lift, by changing the pitch of the blades. (Some helicopters have control systems which control the throttle.) The other hand controls the "cyclic", which controls the sinusoidal variation of blade pitch as each blade moves around. Move the stick left or right, and the helicopter banks. Move the stick fore or aft, and the helicopter pitches up or down. The feet controls the pedals, which change the pitch of the blades on the tail rotor. This cause a yawing motion of the aircraft. So there are really 5 separate controls (throttle, collective, two cyclic degrees of freedom, and the pedals.) Hecliopter pilots can be very busy people. > Anyway, you may be interested to know that it used to be one of those > popular science myths that helicoptor blades do *not* develop their lift > like a fan, but like an aircraft's wing, via the effect of the rotating > airfoil. It was one of those "Oh, you think a helicopter rotor is a > horizontally mounted fan? How niave " things. But I think > aerodynamicists changed their tune later, and decided the simple idea of > the blades pushing the air down, and receiving a reactive force up, was > not so far from the mark. I believe their has also been some backpedaling > in this regard on aircraft wings... certainly modern jet fighters can have > wings with little or no discernible "airfoil". Possibly the utility of an > airfoil turns out to be more a matter of stability than lift. To put this politely as possible, that is nonsense. Aerodynamicists have always understood that 1. an airfoil may be cambered or uncambered 2. an airfoil may be thin, or may have a thickness distribution. That is, a fighter certainly has an airfoil --- it's just thin and flat. Indeed, many of the classical results in aerodynamics start with a "flat plate" airfoil. The best shape for an airfoil depends on the flight regime. Supersonic or transonic aircraft tend to have very thin, flat wings to reduce drag due to shock waves. Slower aircraft have thicker wings, both so that they perform well over a range of angles of attack, and to allow a more efficient structure. Your statement is akin to saying that tire designers have "backpedalled" because they once thought that tires needed tread, but that modern Indy cars have no tread, so they must have been mistaken. (Get Next Article Segment) _________________________________________________________________ [ HOME ] - [ HELP ] - [ CONTACTS ] - [ SEARCH ]