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Let's go back to the beginning. The FAA likes to teach four =
balanced forces, lift exactly cancelling gravity [weight] and thrust = exactly cancelling drag. Unstated or widely ignored assumptions are a = powered aircraft, constant altitude, constant speed, stationary airmass. = This does not normally apply to a glider ! Starting with your Cessna flying per FAA model, take away the = engine's thrust. Now something has to give - you can fly slower at = constant altitude as drag exerts a decelerating force on you; lift would = be reduced as airspeed drops, so to maintain altitude you increase angle = of attack [until you stall, then you will descend]. Or you can choose = to hold airspeed and descend by nosing down a little; then both the lift = and drag vectors are tilted relative to gravity, and two things happen - = a component of your total lift will resolve in the 'forward' direction = 90 degrees from the gravity 'down' direction AND a component of your = total drag will resolve in the 'up' direction. Only three primary = forces [lift, drag weight], resolved and summed in four directions. You = stabilize at a new flightpath angle, same speed, same angle of attack = [because your nose down input changed both fuselage angle and flight = path angle by equal amounts after transients wash out] and therefore = same drag, constant descent rate. That's how a glider works, I believe. = The vectors can be drawn EITHER in space coordinates [true up and down] = OR flightpath coordinates but please don't mix 'em together. Note that of your three primary forces, weight is always there but = lift and drag only exist because you have airspeed to begin with. And = you need altitude as well, otherwise you can't fly down your slope. To get to this stabilized state in the first place, you must have = airspeed and altitude. Bungees, winches, towplanes, ramps off clifftops = all supply the initial energy to get this altitude/airspeed to start you = off. Now soaring, that's a different matter, and really starts when the = airmass motion comes into play.. Ian |
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