lml@lancaironline.net Mailing List Archive

<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> In a message dated 8/15/00 0:29:11 AM EST, Brent Regan writes: << The units displayed are NOT degrees but rather AOA "Units" of non specific magnitude. >> I thank Brent for his insight into the two AoA systems. I have excerpted the above comment to amplify slightly. My insight comes from my experience as a military flight test engineer. During that portion of my career, I was involved in a high-AoA test program on the F-5F. Like most military aircraft, this one had an AoA vane on the side of the fuselage, almost identical to the big ones you can usually see on an airliner as you walk through the door. These are NOT delicate flight-test AoA vanes (which are located far enough away from the fuselage to give a fairly accurate read out in degrees) but rugged sensors that are stuck on the side of the fuselage (that don't have a prayer of sensing actual degrees due to the effect of the fuselage on the free-stream airflow). The fuselage-mounted sensors do detect an angle. I took one apart one day and saw that it was merely a big potentiometer with a wedge-shaped air vane attached. The angle measured is NOT a true angle of attack, but rather the direction of the local airstream at that point on the fuselage. This measurement is expressed as "units" of AoA, and that's what it says on the gauge, precisely because it is not the actual AoA. It IS, however, correlated to angle of attack, and is consistent with respect to stall warning. That's the major lesson here -- even though it's not a real AoA, it is correlated and repeatable, and that's what counts. You can identify the critical angles of attack for stall or best glide, even though the gauge won't tell you whether it's 15 degrees or 17 degrees or something in between. But then, do you really care? Not really. You want to BE at that AoA even if you don't know the precise value. To finish the story, the single-seat F-5E had a not-very-restricted AoA envelope. The pilot were told to keep it below 29 "units" because the needle pegged at 29 "units" and they wanted to make sure the needle wasn't up against the stops. (It actually stalled well above 30 units) The two-seat F-5F with the longer nose departed controlled flight (the military term for "stall") at 29 "units" AoA, so the same limit was applied. I'm not sure the pilots were told the reasons -- they were just told to keep it below 29 "units" in both aircraft. With an in-flight refueling probe attached, the F-5F departed at 24 "units". What a surprise! By the way, I especially liked the Ferrari radio reference. I suppose the SFS EFIS is sophisticated enough that AoA and stall warning is an "oh, by the way" capability.... - Rob Wolf LML website: http://www.olsusa.com/Users/Mkaye/maillist.html LML Builders' Bookstore: http://www.buildersbooks.com/lancair Please send your photos and drawings to marvkaye@olsusa.com.