If I am flying at absolute ceiling, can I still climb by trading airspeed for altitude?
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2$\begingroup$ How could that be, please? What is 'absolute ceiling' meant to mean? $\endgroup$– Robbie GoodwinSep 15 at 19:36
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4$\begingroup$ @RobbieGoodwin: According to the wikipedia link someone added to the question, The absolute ceiling is the highest altitude at which an aircraft can sustain level flight. The OP is asking about a non-sustained non-level-flight maneuver. $\endgroup$– Peter CordesSep 15 at 21:20
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2$\begingroup$ @PeterCordes Both your an Wikipedia's interpretations might be correct and still, it would behove Hitomhi to explain that in the exposition, and that in detail. $\endgroup$– Robbie GoodwinSep 15 at 21:24
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$\begingroup$ @RobbieGoodwin: Yes, agreed. Lack of detail / explanation like that is why I didn't upvote the question. $\endgroup$– Peter CordesSep 15 at 21:26
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1$\begingroup$ Purely FWIW for a general reader the definitions seem perfectly obvious and inevitable and the question seems quite clear. $\endgroup$– FattieSep 17 at 22:51
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4 Answers
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Yes, you can zoom climb. While you have no excess power to sustain a climb, if you just pull back you can gain altitude briefly. You will rapidly lose airspeed and will stall if you attempt to keep climbing for long.
Some airplanes can make it higher overall by accelerating to high airspeeds at lower altitudes and then zoom climbing all the way up. This has been used in fighter jets to reach over 120,000 feet, which is far higher than the altitude the same fighter jet can reach in a sustained climb.
Near the apex of such a climb, the airplane may be essentially a projectile- the thrust is minimal if it exists at all and the airspeed can be well below the 1g level stall speed.
If you want to try this yourself without spending ages climbing, you can basically simulate it by just using less throttle. For instance, you can figure out the would-be absolute altitude if your airplane had 80% as much power as it did, then climb there at full power, dial back to 80%, find what airspeed allows you to stay level, then pull back and see what happens.
This essentially simulates the exact same thing for an identical airframe with a smaller engine. It also emphasizes that nothing special is happening aerodynamically at the absolute ceiling- it's just the altitude where the engine can't work any harder to keep you climbing.
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$\begingroup$ When flying at a susstained flight at the absolute ceiling, do you have exess kinetic energy & aerodynamic forecs to pull higher? $\endgroup$ Sep 15 at 12:54
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5$\begingroup$ @Apfelsaft Yes, whatever margin you have over the stall speed is excess that allows you to pull higher. $\endgroup$– ChrisSep 15 at 13:51
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$\begingroup$ @Apfelsaft: If you were right at stall already, drag would be pretty high. Presumably absolute max altitude is only achievable at a better lift/drag ratio, which means a higher speed and lower angle of attack. en.wikipedia.org/wiki/Lift-to-drag_ratio#/media/… . (Drag = thrust since you're in sustained level flight, so better L/D means more lift from the same thrust. Or the same lift in thinner air to balance your weight?) So you'll have some kinetic energy to trade for altitude before stalling. $\endgroup$ Sep 15 at 21:32
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3$\begingroup$ @sdenham The amount of power the engine can output certainly does vary with airspeed, which is why Vy (maximum excess power) and Vms (minimum power lost) are different. Up to a point, available power increases with airspeed, so Vy is greater than Vms, so the AoA at the absolute ceiling is lower than the one that gives maximum Cl over Cd. $\endgroup$– ChrisSep 15 at 22:48
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2$\begingroup$ For some numbers- a C152 has Vx of 55 KIAS and Vy of 61 KIAS at 10000 feet. The IAS at the absolute ceiling is somewhere in the middle, perhaps 56 KIAS (Vy changes much more quickly with altitude than Vx does). The stall speed is between 36 and 40 KIAS depending on loading. So there is not a large excess of kinetic energy, but there is some. $\endgroup$– ChrisSep 15 at 22:57
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Vx and Vy converge at the absolute ceiling, at an airspeed that is higher than stall speed. So yes, you can still pull back on the stick or yoke and temporarily climb a bit. Of course, you will soon be coming back down.
Also, there will be some airspeed / altitude profile that lets you do a "zoom climb" that temporarily puts you quite a bit higher than the altitude you would obtain simply by pulling back on the stick or yoke once you are in stabilized flight at the absolute ceiling.
You can read about the 91,249' altitude record set in 1958 in the F-104 Starfighter using this method at this link. (Caution, may include ads.) Note that engine flamed out well before the peak altitude was reached-- something to keep in mind when considering the flight dynamics involved.
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At the absolute ceiling, you have no extra engine power to push you through the air any faster, so you cannot add lift to climb by pushing the power lever further forward - you are already maxed out.
Note also that at that condition, you cannot establish a climb by trimming in a bit more more angle of attack to get more lift. If you try that, you just slow down and mush along at the verge of a stall.
As pointed out below, if you haul back suddenly on the stick, you will pitch up (into a stall) and although this will furnish you with a few more feet of altitude, it's a transient effect and you will soon come back down again.
Now, just for fun, let's calculate the maximum possible height gain you can get from a "zoom climb", as follows. We'll assume a small plane mushing along at at its absolute ceiling of 12,000 feet while going 100 feet per second. Its kinetic energy is KE = $ \frac{1}{2}mv^2 $ and we will equate this to its potential energy gain, PE = $ mgh $. The $m$'s cancel and we solve for $h$, the zoom climb gain. We get $ \frac{1}{2}v^2 = gh $ or $ h = \frac{v^2}{2g} $. For a plane going 100fps this yields a height gain of 156 feet.
This assumes that the plane's wing is capable of converting all of its kinetic energy into height gain, which will requires enough elevator authority to pitch the plane (and the wing) up into a vertical attitude.
This also assumes that a suddenly-applied pitch-up to vertical will not stall the wing.
answered Sep 14 at 16:47
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3$\begingroup$ This "few more feet" might be an understatement. Some experimental aircraft have briefly reached space along a suborbital trajectory using that. If your airspeed is high enough at your absolute ceiling, you can trade it for a lot of altitude before falling back down. $\endgroup$– vszSep 15 at 5:47
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1$\begingroup$ @vsz, I was thinking more along the lines of ordinary cessnas and pipers. Should I edit? $\endgroup$ Sep 15 at 6:01
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2$\begingroup$ @nielsnielsen i think you're right. the "absolute ceiling" is determiend by "best AOA" and max thrust" AND "const velocity". Hence.. you change anything, you move out of the optimum. $\endgroup$ Sep 15 at 12:59
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$\begingroup$ 100 ft/sec (59 kts) seems pretty slow for a true airspeed at 12 000 feet. (Kinetic energy scales with TAS). For example a Cessna 172 cruises at a TAS of about 110 to 140 kts depending on model, if I'm reading wikipedia correctly. 2x as fast is 4x the energy. At max absolute altitude in sustained level flight, would your indicated airspeed be just above stall, or would you be at a better L/D aspect ratio going significantly faster? I'm not sure about that part, and maybe it depends on prop (constant power) vs. jet (constant thrust). $\endgroup$ Sep 15 at 22:02
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2$\begingroup$ (For optimal "zoom climb" as @vsz mentions, I think you'd typically start at high speed at lower altitude, not after edging up to the point where you can't climb any higher. But even so, unless max absolute altitude is right at the edge of stalling, with a high AoA near your C_Lmax coefficient of lift, you'd still have speed you could trade for altitude until stall + dive.) $\endgroup$ Sep 15 at 22:06
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A more understandable approach may be to take out the engine for a moment and look at a Vbg glide. Pulling back on the stick briefly reduces the glide angle, but greater drag will eventually increase glide angle, reducing glide distance.
An airplane at absolute ceiling is flying at its very best combination of available thrust and minimum drag. Pulling the stick would only briefly increase altitude, followed by a descent below absolute ceiling.
Changing angle of attack increases lift in a generally linear fashion, but lift is lost proportionally to the square of airspeed decrease from increased drag.
answered Sep 14 at 23:41