Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!att!cbnews!military From: shafer@drynix (Mary Shafer) Newsgroups: sci.military Subject: Re: Dogfighting Message-ID: <7630@cbnews.ATT.COM> Date: 21 Jun 89 02:46:05 GMT Sender: military@cbnews.ATT.COM Lines: 78 Approved: military@att.att.com From: Mary Shafer Steve Hix writes: >> [mod.note: For those of us unfamiliar (me included)... I take it >> alpha == angle of attack. Now, what's angle of attack ? Does (I'm >> guessing from context) an alpha of 30 degrees mean the plane can >> fly level with its nose 30 degrees above level ? - Bill ] >No. Not quite. >Take a line between the leading and trailing edges of the wing. >That's a chord. Measure the angle between the chord line and >the direction the air going due to the aircraft's motion. >The angle is what you're looking for. Well, not quite. This is the wing angle of attack. The angle between the fuselage reference line and the relative wind is the aircraft angle of attack. >(Roughly, a wing's lift [and drag] increase with increasing angle of attack. >Up to a point...past that point, lift falls off, usually with a related >separation of airflow from the wing. At that point the wing stalls. In fighters, the aircraft angle of attack of much greater interest because stall/spin/departure behavior is almost entirely a function of the flow at the nose and forebody (about back to the canopy), rather than the wings. The opposite is, however, true for general aviation and transport aircraft. Departure resistance is also related to stores loading and thrust assymetries. For example, F-4s and F-15s with centerline drop tanks are more easily departed. Did you see the article about the Dryden F-18 High Alpha Research Vehicle (HARV) in a recent AvWeek (it's on the cover)? (I think the article also mentions Dryden's earlier stall/spin/departure research using a 3/8-scale model of the F-15. We think that we have the unofficial world record for number of turns in a spin--83.) We have done a lot of wind and water tunnel testing, computational aerodynamics, and flight test with this aircraft. We've put smokers in the nose and injected smoke into the nose and LEX vortices, as well as doing surface flow visualization on the nose and LEX. The AvWeek article has some very nice pictures and good explanations of all this. >The stall is dependant on the angle of attack, the attitude of the aircraft >in relation to the earth is essentially irrelevant. You can stall at any >attitude or speed.) If you pull the nose of your fighter up, bleed off the airspeed, and stall the aircraft, you may, if the a/c has a flat spin mode, find yourself spinning with the a/c quite level with respect to the ground. The aircraft is essentially falling out of the sky, so your angle of attack is 90 deg and your pitch angle is 0. a/c-- _____ | direction | of | flight| (velocity | vector)V ground _____ This can be a very comfortable ride, according to our pilots. The horizon just goes by, nice and level. Much better then oscillatory or inverted spins! The F/A-18, which is _not_ spin-prone, has three upright spin modes--one flat, smooth, and fast and the other two oscillatory and slower. The F-15 is also very spin-resistant, but it can be provoked into both upright and inverted, oscillatory and steady spins. It recovers by itself with neutral controls. -- M F Shafer|Ignore the reply-to address NASA Ames-Dryden Flight Research Facility |Use shafer@elxsi.dfrf.nasa.gov NASA management doesn't know what I'm doing and I don't know what they're doing, and everybody's happy this way.