Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 exptools 1/6/84; site ihnp4.UUCP Path: utzoo!watmath!clyde!cbosgd!ihnp4!cfiaime From: cfiaime@ihnp4.UUCP (Jeff Williams) Newsgroups: net.aviation Subject: To Flap or Not to Flap Message-ID: <728@ihnp4.UUCP> Date: Sun, 17-Feb-85 23:59:49 EST Article-I.D.: ihnp4.728 Posted: Sun Feb 17 23:59:49 1985 Date-Received: Tue, 19-Feb-85 07:02:09 EST Organization: AT&T Bell Labs, Naperville, IL Lines: 86 This question of flaps or not is about as personal a question as possible in aviation. However, I am quite willing to put my two cents worth into the discussion. I teach my students the following procedure in a Cessna 152, 172, PA-28 series, Grumman AA-5 series, and the like. 1. Downwind is flown about 3/4 of a mile out from the runway, at a speed of Vfe-5 knots (80 knots in a C-152/C-172). 2. Halfway down the downwind leg, add carb heat if the aircraft manual calls for it. 3. At the point on downwind abeam of the touchdown point, add the first increment of flaps, power at 1700 to 1500 depending on the temperature, ect. 4. A nice, square pattern, with base about 1/2 mile from the threshold of the runway. 5. Second notch of flaps on base leg. Still keep 1500 - 1700 rpm. Speed is touchdown speed +10 to 15 (70 to 75 knots in a 152 or 172.) 6. Full flaps on short final when the runway is made. Power off at a point where a comfortable landing will be made 500 feet down the runway (big runways) or 100 feet down (short runways). In a Cessna, it is either FULL flaps or NO FLAPS depending on the wind. If the wind is over 25 knots, or over a 15 knot crosswind, the flaps are up. Otherwise, the flaps are down all of the way. Touchdown is at Vso*1.1 or at the recommended speed. The stall warning horn is just starting to make it's mournful bleat. So, why this technique? Why not power off like one would fly a taildragger? Simple. First, this is a predictable pattern, both for the student and for following traffic. Next, it is possible from any point in the pattern to make the runway if the engine quits (after turning crosswind, natch). Third, we don't shock cool the engine, which causes a LOT of EXPENSIVE wear. By keeping the touchdown point somewhat down the runway, there is a built in "ooops factor" for an undershoot, especially if the engine were to die in the pattern. As for the flaps, take a look in a Cessna owner's manual at the stall speeds with and without flaps. You will notice that the first 20 degrees give a large amount of lift, while the last 10 or 20 degrees give mostly drag. As an example, lets look at the book figures for a 1971 Cessna 182. (I could find this manual, my wife filed some of the newer manuals when she moved my library to make a bedroom for our new kid. However, this will illustrate what I am talking about.) Power off stall, 0 degrees of bank = 66 mph CAS. Power off stall, 20 degrees of bank = 59 mph CAS. Power off stall, 40 degrees of bank = 57 mph CAS. Note, for the first 20 degrees of bank, the stall speed decreases 7 mph, a bit better than 10% decrease in stall speed. In the last 20 degrees, the stall speed decreases only 2 mph, under 5% stall speed decrease. So, picture this. The 182 pilot comes in with 10 knots extra for wife and kids, and partial flaps. The 182 pilot will float into the next county because the wing is much more efficient in ground effect, the 20 degrees of flap are providing more lift than a clean wing, but not a large amount of drag. By the way, a 182 is an acceptible airplane for the illustration by virtue of the wing being the same airfoil as the 150/152/172 series, that is a NACA 2412. Can pilot technique with full flaps get an airplane in trouble? Sure, if the pilot does not fly the attitude of the airplane and watch what the airplane is doing. With full flaps, the airplane wants to slow up much more quickly than when flown with no flaps. Pitch attitude is important, just as power setting is important. But, as an instructor, I have seen more pilots get into trouble by not using flaps than by using flaps. I, however, will not use full flaps in a Cessna O-1 Birddog, because the flap at 60 degrees can cause some problems. I also had problems getting full flaps on in a Cessna 180/185 because of where the flap handle was located, and where I have to sit to reach the rudder pedals in that airplane. After the first 20 degrees, I couldn't get my arm moved around to get the last 20 degrees. But, that's ok, because I would wheel land the thing and not play with the flaps until it was totally stopped. (By the way, the 185 is a blast to fly!) Oh well, the question is non-existant in a REAL airplane anyway. I mean, who ever wanted flaps in a Champ, Cub, or Taylorcraft? Jeff Williams AT&T Bell Laboratories - Naperville, Ill ihnp4!cfiaime