Originally Posted by
TangoBar
I don't believe for a minute that Cessna designed, or allowed to enter into service, an aircraft whose horizontal stabilizer 'switches' from providing downforce at the tail to lift at the tail at any point during flight. Why? Because for that to happen, it would either result from or require a MASSIVE change in the CG.
Correct, CG location does not move very much if at all. The center of lift for the airplane does however and it moves forward in this case due to the shorter chordline created as flaps retract back into the wing. Caravan has oversize fowler flaps for meeting the 61-kt stall speed requirement. As a result, spoilers are required to augment the undersized ailerons. Retraction of the flaps may put the location of the center of lift forward enough to be forward of the CG location and an upward tail force would be required to counterbalance it.
FAA certifies an airplane for a type certificate only after rigorous theoretical and practical analyses have been performed. Aircraft manufacturers are held to a high standard in this country. That doesn't mean a problem cannot occur, but that in all the relevant areas of construction, operation, performance and theory the airplane has been examined and found able to pass by specific margins. Longitudinal stability is one of the areas it must pass, both in theory and in testing.
The downforce required is the same at liftoff, climb, cruise, and approach speed- remember that the fuselage acts as a see-saw suspended from the Center of Lift. Downforce is not a function of airspeed, it's a function of masses and the Center of Lift. If you have to REMOVE downforce from the tail end, it means you've REMOVED the load at the other end. You can't just say that it switches without thinking about why you had the downforce in the first place.
It's not so simple. Dynamic pressure on the tail always changes with changes in airspeed, prop wash, propeller slipstream, downwash from the main wing, and sideslip if there is some. The tail force increases and decreases and may change dramatically as the steady-state flight configuration goes from climb to level flight to descent. CG is only part of the total longitudinal static stability equation. The latter is a large equation with a ton of greek letters and is beyond the scope of our hangar-style discussion. CG may shift as fuel is burned off also, but not much in the Caravan.
If there's evidence from a deposition, can you post a link to the information, or at least a reference to the particular case where the engineer made this claim?
185flier posted some hand calcs he got somewhere, but they are a bit hard to use without clarification. The real thing to have would be the flight test data from original certification which is not public information.
Look, a Caravan is built like a big 172 or 182. Neither of those a/c allow the CG to get aft of the Center of Lift.
Center of lift can be summed across the entire airplane. If the addition of the main wing and the tail produces a
combined center of lift aft of the CG there is positive static stability. The main wing need not be designed with a center of lift aft of CG. Usually a tail will have a down force on it to create a nose up moment which opposes a nose down moment created by a wing, but not always.
NO Cessna, Piper, Beech, Cirrus, etc., is designed that way. If any a/c were designed to 'switch' in the manner suggested, it would be so sensitive to pitch force changes that it would be virtually uncontrollable.
Actually it would go to neutral stability before negative stability. The airplane will no longer correct back toward a moderate angle of attack, roll or yaw, and will stay wherever last put. This is not desirable for normal and utility category airplanes, because if it goes unnoticed it may end up somewhere away from what the pilot intended. It is not a recipe for disaster necessarily and aerobatic airplanes have neutral static stability on one or more axes. For example, aerobatic airplanes do not have dihedral usually so they tend to stay where placed in bank angle.
Now, as for the different procedure for the de-ice fluid? I'm willing to bet that rather than trying to blow the fluid off the plane, the revised procedures take into account the fact that a high-viscosity fluid coating the surface can disrupt airflow as it ripples and flows, and the revised procedures are designed to modify speeds/angles of attack to counteract the change in surface characteristics caused by the presence of the heavier fluids.
Some of the de-icing are fluids intended to stay stuck to the surface in flight. They have a negligible effect on boundary layer adhesion.