Hi Badboy,
>If you accept that the default speeds are the best climb speeds at sea level then you already know everything you need to know. By staying at the default speed, you get the best climb all the way up.
Hm, as a first approximation that's correct, but only as an approximation.
Here is a discussion on this topic copied from an earlier thread:
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>will speed of climb and rate of climb be constant with a constant MF pres?
First, let me modify the question to "... at constant power" to make things a little easier :-)
With constant power over altitude, the rate of climb will drop. If you'd climb at constant true air speed, this drop would be pretty bad, since the thinner air would require you to increase the angle of attack quickly, making the wing more and more inefficient, until you'd stall out.
To climb more efficiently, you have to increase the true air speed to keep the angle of attack shallow (thereby avoiding the stall).
Now you could try and keep the angle of attack at the optimum for climb at sea level, which would mean keeping the indicated air speed constant. You'd still lose climb rate over altitude since the higher resulting true air speed in the thinner air at altitude would mean that the thrust produced by the propeller would decrease, leaving you with a decreased climb rate.
The solution is to find the optimum speed where the angle of attack is still pretty shallow, and yet the thrust is still pretty good. This solution involves true air speed increasing with altitude, but indicated air speed actually decreasing.
In other words, the climbing aircraft would fly with a greater angle of attack, but nevertheless climb on a shallower flight path for the optimum rate of climb.
However, even flying at the optimum speed, the climb rate would still drop with altitude. There's no way around that :-)
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I'd add that once the propeller tips start going (close to) supersonic, a reduction in forward speed to reduce propeller tip speeds helps to improve the climb rate. An opposite influence is exerted by the exhaust thrust which increases with altitude and is more useful at higher speeds.
Pilot's notes sometimes quotes statements like "Best climb speed 170 mph IAS, reduce by 3 mph for each 2000 ft over 15000 ft" (I made up this example) to account for the various effects I've described.
The well-known Fw 190A-8 performance chart provides another example for climb speed decreasing with altitude. According to my analysis, the optimum climb speed graph is actually more complex than given by that chart, but as there is a wide region of very good climb around the optimum, a simplified rule for the climb speed will be very close to the optimum even when it's simple enough for the pilot to actually remember it :-)
Regards,
Henning (HoHun)
