Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!rutgers!att!cbnews!military From: willner@cfa.harvard.edu (Steve Willner) Newsgroups: sci.military Subject: Re: Rotary engine airplane troubles Message-ID: <11223@cbnews.ATT.COM> Date: 8 Nov 89 04:40:31 GMT References: <11069@cbnews.ATT.COM> Sender: military@cbnews.ATT.COM Lines: 77 Approved: military@att.att.com From: willner@cfa.harvard.edu (Steve Willner) >From article <11069@cbnews.ATT.COM>, by pvo3366@sapphire.oce.orst.edu (Paul O'Neill), who was either quoting or following up a quote from _Fighter Combat Tactics and Maneuvering_ by Robert L. Shaw: > Torque may also have an effect on turn performance, particularly with > high-powered prop fighters at slow speed. The effects of engine torque > must generally be offset by rudder power to maintain balanced flight. The main point of the article seems right, but the terminology is a bit confused. "Torque" is a roll force, counteracted with aileron. The propellor spins one way, and air resistance imparts a force to the aircraft in the opposite direction. The magnitude of the torque force depends mostly on power, and at low airspeed control authority may be insufficient to counter the torque. > Normally under these conditions considerable right rudder will be required > to balance the torque of a prop turning clockwise (when viewed from behind), > and vice versa. Another consideration here is called "P-factor," which is the > tendency of a propeller to produce more thrust from one side of its disc than > from the other. The explanation of p-factor is simple enough: as _aircraft_ angle of attack increases, the _propellor_ angle of attack becomes greater on the descending than on the ascending side of the arc. This produces more thrust on the descending side, which produces a _yaw_ force to the opposite side. For conventional rotation, the descending side is the right, the yaw force is to the left, and right rudder is needed to counter the yaw. Another yaw force comes from the fact that the upwash from the prop is moving crosswise when it encounters the vertical stabilizer. (For conventional rotation, this is left-to-right.) This produces a yaw force in the same direction as p-factor. The force is generated because the vertical stabilizer extends above but not (much) below the engine centerline. This force is mostly related to engine power, while p-factor depends on both engine power and angle of attack. > there may be conditions under which sufficient rudder power is just not > available. The resulting unbalanced flight (slip) may cause loss of aircraft > control. Generally the high wing (i.e., the outside wing in a turn) will > stall, causing the aircraft to "depart" controlled flight with a rapid roll > toward the stalled wing. I must be missing something here. Flying with insufficient rudder relative to bank angle is indeed a slip, but I don't see why it should lead to loss of control. More dangerous is the _skid_, which is _too much_ rudder relative to bank. If a stall occurs while skidding, entry into a spin can be very rapid. Furthermore, why should the _outside_ wing stall first? I would think the _inside_ wing would be flying at higher angle of attack. The case should be analogous to trying to turn without using rudder at all, which normally just produces lots of adverse yaw and little turning. The penalty of a slip is generally just loss of performance because of greatly increased drag. > This phenomenon has been used to good effect in combat, [FW 190's in climbing right turn stall and roll left] I don't question the narrative, but I do wonder about the explanation. Perhaps what happened could have been that _both_ wings stalled (an "accelerated stall" probably), and then the torque _rolled_ the planes to the left. If this is the correct explanation, increased rudder authority would not have prevented the stall, though it would have allowed the 190's a higher climb rate, so the pilots might not have been tempted to reach a stall. There still would have been drag associated with the rudder, though, and the general point that the 190's would climb slower in a right turn than in a left turn is certainly valid. (Assuming they had the normal direction of propellor rotation, of course.) ------------------------------------------------------------------------- Steve Willner Phone 617-495-7123 Bitnet: willner@cfa 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA Internet: willner@cfa.harvard.edu