compressor stall

9th Apr 2002, 12:16

Short and sweet.

Why does the IAS/CAS best rate of climb speed (Single Engine) decrease with altitude?

Why does the IAS/CAS best rate of climb speed (Single Engine) decrease with altitude?

View Full Version : Single engine climb speed (piston) compressor stall 9th Apr 2002, 12:16 Short and sweet. Why does the IAS/CAS best rate of climb speed (Single Engine) decrease with altitude? Tinstaafl 9th Apr 2002, 14:09 Both Vx & Vy (single engine or all operating) are determined by the drag vs thrust curves for the number of engines operating. The power required curve changes with increasing altitude - as does the power available. This changes the relationship between these curves. Vx occurs where there is maximum excess thrust, Vy where there is maximum excess power. The speeds at which these two different excesss happen gradually closes as altitude is gained until they become equal ie Vx increases while Vy decreases. When they're equal then the a/c has reached its absolute ceiling. At this point there is no excess power OR thrust available to support further climb. If they didn't become equal then it would lead to the nonsensical prospect of flying at the absolute ceiling while maintaining either Vx or Vy, then when climb performance reduces to zero, changing speed to the other of Vy or VX & then continuing to climb - in which case you couldn't have been at the absolute ceiling because there is still some amount of climb gradient AND rate available. bookworm 9th Apr 2002, 15:09 Just to add to Tinstaafl's comment, power available tends to increase with speed, and it's that aspect that causes Vy to reduce as power generally decreases. If power were constant wtih airspeed, Vy, the speed for maximum excess power would be equal to the power-off minimum-sink-rate speed. Generally, it's much higher than that, so the power available must increase substantially with speed. If you scale down the power by a constant fraction (say to 75% by going to about 8000 ft in a normally aspirated engine, or by 50% by feathering one engine), the slope of the power available curve reduces in proportion. So the speed for maximum excess power will shift back towards minimum sink-rate speed. Thus Vy decreases with altitude, and Vyse is less than Vy. [Easiest if you draw the curves] john_tullamarine 9th Apr 2002, 15:46 Still another slight variation on the theme. Considering the Vy situation, which is the thrust (pun intended) of the question ... this relates to the maximum available excess power. As altitude increases for the typical small aircraft, the net available power decreases while the component curves of the power required curve vary to cause a slight skewing of the combined curve. The common result is that the speed (TAS as we are talking about power) for maximum excess power increases slightly with altitude. The corresponding CAS will vary as normal with respect to the density. The end result is that CAS may reduce slightly ... one would need to consider the situation in relation to the specific aircraft being looked at .... compressor stall 18th Apr 2002, 07:38 Chasing prawns around the Gulf for 5 hours a day has allowed time for word for word perusal of flight manuals, and subsequent contemplation of the above question. Infrequent rural internet access has prevented my earlier reply here. My thoughts agree with John_Tulla's. Some of what comes under repeats his statements above, but I am cutting and pasting what is on my laptop so forgeive me. First, the fact that the aircraft is asymmetric is irrelevant - it is basically equivalent to an inefficient underpowered single... As for aircraft specifics, both the AC50 and the C310 have airspeeds (CAS and IAS respectively) listed in the flight manual which decrease with altitude at the rough rate of a couple of knots for a couple of thousand feet Grab a pen and a blank piece of paper...and start with the power on the y axis and TAS (note TAS) on the horizontal. Best rate refers to excess power (as mentioned above). Draw the best power required graph (the flattened J shape) for sea level. Now an increase in altitude will require an increase in TAS for steady level flight. The effect of this will move the curve upwards and to the right but not change its shape. A point on the altitude curve (corresponding to best range, but that is irrelevant) is still a tangent from the SAME line from the origin. Draw in this curve (in red) upwards and to the right (imagine "sliding" it up along the tangent line. Draw in the power available graph (upside down flattened "n") for sea level. The effects of altitude will reduce the power available, pulling the graph vertically downwards. Draw this new position (in red). Now the maximum excess power at altitude (between the red curves has reduced significantly (to be expected and resulting in a reduced rate of climb) but it has shifted to the right - to a higher TAS. As altiude increases, the difference between TAS and IAS/CAS increases. On some aircraft, the TAS increase is so slight over the change in altitude, that the corresponding IAS is actually decreasing. In a climb at a constant TAS the IAS decreases by such a rate that it must be possible to have a slight increase in TAS and still maintain a (reduced) decrease in IAS. Apologies for the layout - it was a notebook document! Checkboard 19th Apr 2002, 00:31 CS, something more for you to contemplate while flying around looking for prawns! Piston Airplane Cruise Efficiency (http://www.db.erau.edu/research/cruise/) Take the test, and tell us how you do! :D Tinstaafl 19th Apr 2002, 23:21 ******! 80%! Question 2 was the problem. My understanding about max. range has always included gains in engine efficiency by flying at an altitude that requires full throttle to achieve the speed that corresponds to the best range AoA. This site's answers are based around no change in engine efficiencies. :confused: Damn. Wish I knew that before I took the test... :D john_tullamarine 20th Apr 2002, 01:16 Tinny, engine and prop efficiencies are presumed to be constant .. a fairly common assumption in books directed at pilot training requirements ... the questions appear to be emphasising the L/D consideration at the expense of the minor things ..... a standard problem with multi-guess questions ... if the questionee is not up with the assumptions of the questioner then it gets muddy ...... bookworm 20th Apr 2002, 12:22 Tinstaafl I think you're quite right. The assumption seems to be that fuel burn per unit power is independent of altitude. Consider a 360 cubic inch engine operating at 2000 RPM throttled to 15". If I've got my sums right, the pumping losses with an ambient pressure of 30" cost about 6 HP. Full throttle at 18,000 ft, you get 6 HP more for your fuel. [Using a real engine chart I get similar numbers. I only have an O-360 chart to hand but that suggests 18" 2000 RPM 6.4 USgph gives 90 HP at a full throttle altitude of 12800 ft but only 78 HP at sea level.] That doesn't sound like much, but if you consider a typical aircraft with a 360 cubic inch engine (say a 2740 lb Mooney with a 12:1 L/D max, corresponding to 90 KCAS) the power required at best L/D is only 60 HP anyway. 6 HP makes a huge difference. For 66 HP, you can do more than 110 KCAS at sea level, a little less at altitude. So who's going to tell Byington that his assumptions are shot to hell? :) |

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