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Happy Thanksgiving to ya'll,
This is related to my previous "overweight" post. I want to know if you agree with the following reasoning: Context: you take off over your maximum gross weight, and wonder how your climb speeds and performance are affected. Vy: The AOA corresponding to the best rate of climb remains the same. However, the airspeed at which the best rate of climb speed increases by the square root of the ratio of the current weight and maximum gross weight. The pilot then pitches to obtain that new, higher Vy airspeed. Even though you're flying at the Vy corresponding to your new weight, Q1: Is it correct to say that that your climb rate is now LOWER than your max. gross weight Vy, due to the increase in power required at the higher load factor? If so, you'd agree that you're flying at a "higher speed" but yet climbing slower? Is that "higher speed" the speed along the flight path line or the horizontal velocity that's parallel to the ground? Would clarifying this last question open another can of worms? The rate of climb is the vertical velocity with respect to the ground, that I am sure. I think I am confusing aircraft airspeed with velocity along the flight path vector or horizontal velocity with respect to the the inertial frame. Q2: Further, is it correct to write that the thrust required to counter the drag is higher at that moment? As such, is the angle of climb at Vy is also lessened? Vx: Again, the AOA corresponding to the best angle of climb for obstacle clearance remains the same. However, the airspeed to now achieve the best angle of climb at the current weight increases by the same square root of the ratio of the current weight and maximum gross weight. The pilot then pitches to obtain that new, higher Vx speed. Q3: Is it correct to infer that your thrust required to counter the drag is higher at that moment? Hence, is the maximum climb angle less that what it would be at max. gross weight? Q4: Further, is it correct to write that your power required at that moment is higher than at max. gross weight and as such, the rate of climb at the maximum climb angle is reduced? Best Glide Speed (best range): From a simple FBD with no thrust vector, one can find that the best angle of glide is only dependent upon the inverse tangent of the reciprocal of the lift to drag ratio or: Tan(glide angle) = 1 / (L/D), assuming small angles, glide angle ~ (L/D)^(-1) L/D is purely angle of attack driven. Therefore, the glide angle does not change with respect to weight. Here's question 5: I would make the risky proposition of stating that drag and power required ALWAYS increase with increasing weight. To me, when you have neither thrust nor power available during gliding flight, increasing weight/load factor STILL increases your drag and therefore results in a higher descent / glide angle. Likewise, the increase in weight/load factor under no power STILL triggers an increase in power required and therefore higher sink rate. Right now, I am seeing a contradiction between the 1/(L/D) equation and what I've described above. I can't seem to figure out why. Referring back to the best angle of climb question above, it appears that increasing your weight decreases your max. climb angle. I just don't see a difference between climb and descent angle insofar as both being excess thrust or drag phenomena. Why would increasing your weight decrease your max. climb angle but not affect your glide angle? In both cases, I see an increase in drag with higher load factor resulting in a decrease in max. climb angle and an increase in glide angle. Max. climb angle is very much defined by best L/D (i.e. min. drag) and excess thrust while min. glide angle (best glide angle) is defined by best L/D alone. That best L/D is still analogous to its "powered counterpart", as it still represents the point of minimum drag. However, that drag has increased with the higher weight, lessening the L/D ratio. As a result, without an engine, I expect that the glide angle to increase (reducing range). Perhaps the fact that there's no thrust vector has something to do with this fiasco. Ok, I'll stop rambling. I hope I conveyed my thoughts clearly enough. Happy Thanksgiving, Alex |
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