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On Tue, 28 Dec 2004 at 16:55:05 in message
, Peter Duniho wrote: It does this as long as the lift is slightly less and the speed drops to produce _less_ drag and lift, leaving more engine power and thrust to climb. At an airspeed just above stall, a reduction in speed results in MORE drag. There is a reduction in parasitic drag, but there is a greater increase in induced drag, with a net increase in total drag (and that's ignoring drag caused by the rudder and any other control surfaces that require a change in position). HI Peter. How easy it is to get slightly confused on Usenet! What I was trying to say is that if you maintain the same AoA then the lift drag ratio remains the same, but because the lift required when climbing is less than when flying level you can climb at a reduced speed but with less drag. So under some conditions if you just raise the nose a little you can find a new steady state where speed is slightly reduced but with the same thrust you can climb at the same AoA.. There is a maximum lift drag ration at a modest angle of attack. Above _and_ below that angle of attack that ratio worsens. When climbing extra work must be done against gravity. That extra work can come from increasing power or from reducing speed and therefore drag. The extra work comes ONLY from a net surplus of power. I agree with that. However that net surplus can come from either more engine power or reduced drag. Because even in a modest climb the engine thrust vector plus the lift vector combine to match the weight. Put another way if you are flying below the maximum lift drag ratio and you increase the AoA to the optimum whilst keeping the same power the aircraft should climb. This is self evident if you are flying level at high speed and at a climbing power setting. Bring the nose up increasing the AoA and your aircraft will definitely climb. Agreed? A reduction in speed is only guaranteed to produce a net increase in power available if the new airspeed is higher than Vbg. It can sometimes also produce a net increase, if the old airspeed was sufficiently higher than Vbg, and the new airspeed is close enough to Vbg, even if less than, but you need to know more about the old and new airspeeds in that case to say for sure what happens. More importantly, a reduction in speed is guaranteed to produce a net decrease of power available if the OLD airspeed is lower than Vbg (as it is when just above stall speed). I think that is another way of saying what I have just said? I cannot remember if we started off with an assumption that the aircraft was only just above stall speed? If so then you are correct of course. Even in a steady glide the required lift is less than that needed in level flight! That is easier to see because the drag vector helps the lift match the gravity vector. -- David CL Francis |
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