Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!sdd.hp.com!zaphod.mps.ohio-state.edu!pacific.mps.ohio-state.edu!linac!att!cbnewse!cbnewsd!cbfsb!cbnewsc!cbnews!cbnews!military From: dnwiebe@cis.ohio-state.edu (Dan N Wiebe) Newsgroups: sci.military Subject: Re: Soviet helicopters Message-ID: <1991Jan25.032206.11434@cbnews.att.com> Date: 25 Jan 91 03:22:06 GMT Sender: military@cbnews.att.com (William B. Thacker) Organization: AT&T Bell Laboratories Lines: 56 Approved: military@att.att.com From: dnwiebe@cis.ohio-state.edu (Dan N Wiebe) Anton S|rensen writes: > What are the pros and cons of the counter-rotating main rotor & no tail rotor >principle? Counter-rotating dual main rotors have at least one big advantage: a higher maximum airspeed. Possibly the biggest aero- dynamic problem with helicopters is that when they're moving forward (or in any other lateral direction), blades on one side of the rotor disk (the forward-moving side) encounter the air at a higher speed than those on the other (rearward-moving) side. Therefore, there is a lift imbalance that tends to flip the chopper over. This has classically been dealt with by increasing the angle of attack of a blade when it's moving backwards, and decrease it when it's moving forwards. However, as the helicopter's forward airspeed increases, this solution becomes less and less effective--especially when you are moving fast enough that the blade tips are entering the transonic region on one side and falling to near zero airspeed on the other. Counter-rotating main rotors provide an elegant solution to this problem--there are forward-moving blades on both sides of the heli- copter, rather than just one side, which eliminates the lift imbalance. Coincidentally, the net torque is also zero, which obviates the need for a tail rotor (although it seems that a directionally-adjustable ducted tail fan, or even exhaust pipe, would be nice to provide yaw control at low speed or hover). What's the advantage of not having a tail rotor? Dono--except that if you don't have it you can't get it shot out. Now I have a question. I don't know a whole lot about real military helicopters, but I have a couple of attack-helicopter simulators for my computer--Gunship and LHX Attack Chopper. One of the features of both of these simulators is a pair of EWRs (one for radar, one for IR) each with three warning lights--search, targeting, and guidance. The search warning comes on when you appear on somebody's screen, the targeting warning comes on when he paints you with target-acquisition, and the guidance warning comes on when a missile is launched and begins homing. I understand basically how an EWR for radar would work; obviously, the signals emitted for different purposes would have different characteristics. But how does the IR thingy work? I was under the impression that IR homing devices are passive, rather than active. How do you tell when IR energy from your aircraft is being absorbed by a missile's seeker head, rather than by the ground or air? Beyond that, how do you tell what it's being used for once it's absorbed (search, targeting, guidance)? My roommate used to fly as an observer for scouts for AH-1 Cobra attack choppers in the National Guard, and he says that these IR warning devices really do exist, and are not merely an invention of Electronic Arts. But he doesn't know any better than I do how they work. Help! Shalom, Dan Wiebe dnw@rsch.oclc.org