Mir From United States of America, joined Jan 2004, 22224 posts, RR: 55
Reply 1, posted (2 years 8 months 4 days 22 hours ago) and read 5714 times:
Quoting mawingho (Thread starter): When the wind gradient is such that headwind component is decreasing with height, climb performance will be reduced. Why this is true?
A headwind helps climb performance by slowing the groundspeed so that the angle of climb is steeper. If the headwind reduces as you climb higher, then the benefit you get from it will be reduced as well.
Note that any headwind is better than none. So even if you take off with a 20 knot headwind and it slows to 5 knots as you climb, that's still better for climb performance than taking off with no headwind at all. What the real danger is here is thinking that you can achieve a certain climb gradient because you have a certain amount of headwind, and then finding out once you climb a bit that you no longer have that headwind, and thus are unable to maintain that climb gradient.
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FredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 2, posted (2 years 8 months 2 days 22 hours ago) and read 5489 times:
A headwind component means that you are going slower over the ground.
If the headwind component reduces, your airspeed decreases. To maintain airspeed, you have to accelerate.
You use thrust for acceleration and for climb rate. More acceleration = less climb rate.
That's why you always do two climbs when doing climb performance testing, the second one along the same track as the first one but on the opposite (reciprocal) heading. With those two sets of data, you can eliminate most of the effects of any wind gradients (i e wind changes with altitude) encountered. With just the one climb, you will have no way of deducing whether any perceived climb rate deviations were due to wind or due to the actual performance of the aircraft. Been there, done that, been saved by having the reciprocal run data. The effect of a wind gradient can be very significant.
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