But the monotonic curve is based on a simplified theoretical model that takes only parasitic+induced drag into account.
So the monotic assumption may not be valid for a real world aircraft.
Of course the “traditional” power required curve represents the primary factor, but there are surely other secondary factors, and we are only talking about 1-2% variation in cruise speed for the effects that people observe and report.
Also, that curve only deals with power required. What’s to say that power available at the slightly slower speed, isn’t slightly less than at the higher speed? For example in Jason’s jet case where the aircraft gets “stuck” at a slightly slower speed, maybe the airflow to the engines is compromised by the higher AoA, and extra power is required to accelerate out of that regime? Also the thrust vector is different (worse) at the lower speed.
We did it in the Caravelle, every time. What it did is that it got the airplane to accelerate to designated cruise speed much faster than it would if you just level off and let it build the speed. At high weights and high altitude, it would sometimes just hang in there and never reach the cruise speed it should have without this technique.
I’ve used it in the C150 at the time for the same reason. Marginal performance, the altitude was reached in an attitude which, if you then pitched down a bit to accelerate, would let the airplane drop below the desired altitude again. So the trick was to climb 200-300 ft above and then fly it back to the altitude with full power still on (which is climb power with the fixed pitch most of the time). It would then settle at a higher speed coming down from that “descent” and keep that speed for quite a while before eventually settling back to cruise.
I’ve had it explained to me like this at the time:
In climb, your angle of attack is higher than in cruise. When you reach your altitude and you level off, the speed is still lower than your cruise speed, so the AOA to hold altitude is higher than with cruise speed. In theory, once you stay at that altitude with climb power still on to accelerate, AOA will diminish with the speed increase until you reach the cruise speed and it’s corresponding AOA for the power set. Higher AOA means higher drag. Depending on the power available, it may well be that with a level off at near the performance limit, the airplane may not be able to maintain altitude during this transition as drag is higher than the thrust available. Therefore, the airplane will hang in a higher AOA than actually necessary, resulting in lower speed. In other words, it will never really change to the actual cruise AOA but hang in a sort of “suspended climb” attitude where power holds it on altitude if you get what I mean. I’ve seen this with the Cessna 150 quite a lot.
If you level off from a descent, the AOA transition from descent to cruise is opposite. With our planes, descent is often done with higher speed than cruise. My normal cruise TAS is usually around 145-150 kts, however in descent, the plane can do more than that, I’ve seen over 170 kts TAS up high at just below red line. At level off, cruise power gets immediately re-applied (if ever reduced) and the plane will settle with an increase of AOA while it decelerates. I’ve never made a science out of it, but due to that, it is quite possible that the final TAS may be slightly higher just because the final AOA may be a tad below the nominal cruise AOA as it does not have to overcome the drag of the previously higher AOA you get in climb. More likely, it will eventually settle at nominal cruise TAS / AOA.
If you do the step, you climb above your altitude ever so slightly and then descend back to your altitude still at climb power, AOA will change to even below cruise AOA for a while. That means, the phase where you accelerate with decreasing AOA is eliminated as the AOA decreases during the pitch over to descend back. You arrive at your intended altitude with a lower than the cruise AOA and some excess speed over cruise speed. As you level off, AOA will increase to the optimum setting and if power is then reduced to cruise, settle at the “ideal” AOA for the cruise/power combination.
It made sense to me at the time. Climbing to altitude in the Cessna 150 with it’s marginal power often happens at Vy just to get there at all. Once the altitude has been reached and you hold it by just pitching down a tad, you end up at a lower speed at least initially than you would if you use the step, therefore a higher AOA, more drag and it is questionable if the thrust available would be enough to actually get the airplane to it’s intended cruise speed. With the step, you overcome this.
Of course if you pull power back to cruise power right at level off, you’d aggravate that situation. So to stay at climb power until cruise speed is reached is quite logical. In a situation where there is barely enough power to make altitude in the first place and the AOA transition from climb to cruise is slow even at climb power, if power is reduced before the transition is fully over, it would definitly leave the airplane in a higher AOA with lower power which equals slower speed.
With the Caravelle at MTOW – climb fuel it was much the same. If we levelled off at FL310, which was most of the time the max level in this config, speed would creep up very slowly to cruise Mach. Using the step, this phase was much shorter.
So maybe it’s a combination of factors. Using the step will almost certainly result in an INITIAL higher TAS once level with speed DECREASING and AOA INCREASING until the equlibrium between power and AOA for cruise is reached. Using the conventional level off, the initial TAS will be lower during the transition from climb attitude to cruise attitude for the reasons given above and, in cases of airplanes at the edge of their performance envelope, may cause a condition where it will never reach designated cruise speed in the first place, as it can not overcome the transitional AOA/drag. Quite possibly, this is where the original theory comes from.
In the latter case, the theory would be correct, the step would definitly produce a higher cruise speed with the lower AOA. In the “normal” case, where enough thrust is available, I would expect the INITIAL cruise speed to be higher and eventually settling in to the same amount that would be achieved also by a conventional level off albeit slowlier.
Not sure if it makes sense to you but it did to me at the time.
Ortac, maybe in a wind-tunnel or some other highly stable steady-state conditions…but I would postulate that any effect in the real world, in the normal operating envelope, the effect is very short-lived…minor turbulence, minor control inputs/responses etc. would destabilize the effect and the speed achieved would converge on the same average value no matter which technique was used.
[edit] As Monneydriver also speculates in his penultimate paragraph…
Hysteresis in the airspeed indicator. It’s just backlash in the pointer gearing and stiction in the bellows suspension – an instrumentation artefact and not a real effect. The reason some airframes are more “susceptible” to a higher airspeed on diving down to cruise is because they have slacker ASIs.
Jarvis,
Jarvis wrote:
an instrumentation artefact and not a real effect
For most conditions I’d say you are right.
I would say that the method does have it’s advantages still.
In NORMAL performance conditons, flying the step maneuver (within tolerances!) will get you to cruise speed faster and more economical than a normal level off.
In LOW performance conditions such as high weight/close to service ceiling or similar, I’d consider the step maneuver a pretty good idea.
In such a condition it will definitly get you to normal cruise attitude faster, will accelerate to cruise speed quicker and, in certain conditions, possibly have a noticeable effect on cruise performance.
If you reach an altitude close to the performance limits, what can happen is that if the airplane is not allowed to accelerate properly, it can hang in a higher pitch and be trimmed out to hold altitude against power with a higher AOA than what it would have at proper cruise attitude. If that happens and is not corrected, you can get a cruise speed significantly lower than you should, as you are not really flying cruise but some sort of a zero vertical speed profile. I’ve seen this a lot in the C150, not so much in the Mooney, but also in the Caravelle.
I have paid too much attention, but I could imagine a level off on AP could do this too, particularly if autotrim is involved. If the AP locks from VS to Alt hold, it lowers pitch and trims it until it attains 0 VS and captures the altitude. If in this phase power is reduced too early, it could well end up in a pitch high/speed low attitude and consequently with more drag and lower speed.
To avoid this, I think the step is a good method. By getting the airplane into a slight descent at climb power, it is almost sure to reach cruise speed and cruise AOA.
What is sure in my understanding is that even if the effect will eventually zero out, it is a much better way to fly a level off than the conventional way, particularly with regard to faster acceleration and quicker transition into cruise attitude.
Oh yes, I think diving to cruise is a good idea, it certainly gets you to cruise faster if you don’t have an excess of power to use. But I don’t think there’s any persistent effect beyond some tens of seconds.
There’s also backlash in the tachometer; so on a 150 I’d also want to be sure about the extent of that before considering precise power measurement by reading the RPM as reliable.
The speed/power curve gets very flat at high altitude when you’re approaching the best l/d so It makes sense that an autopilot in the wrong mode could “stick” in the wrong place. Doubly so for a jet where for a given fuel flow the power produced increases with airspeed – this would make the airspeed even more twitchy.
How do you climb above the cruising altitude if you do not have excess power?
we are talking 200 ft here maybe… or even less. If you can get to the cruise level, you can get a tad above it too.
Mooney_Driver’s AoA explanation above is valid but does not relate to induced drag changes with AoA, as these are factored in to the power required curve already.
I agree that in turbulence, an updraft may provide extra energy that would allow you to accelerate out of the slower speed to a new equilibrium, but in smooth air it would be possible to get “stuck” in equilibrium at the lower speed. Then you need either more power, a descent, or an updraft to break that equilibrium and accelerate to the “correct” cruise speed.
If the theory depends on the calculus of the power required curves, then it may not apply to turbofans?
On propeller aircraft the theory might be applicable when Vy (approximately Vmd on the drag curve for propeller aircraft) starts to converge towards Vx and therefore shift from the minimum drag condition at design angle of incidence. ie approaching service ceiling when Vy and Vx converge and you are on the backside of the drag curve. Not sure why the theory applies if you are on the front side of the drag curve. ie comfortably below service ceiling.
With a turbofan, Vy converges with Vx as you approach service ceiling, so in theory you never go on the backside of the drag curve as Vx is Vmd for a turbofan. However the increased AoA as you approach Vx may result in transonic, Mcrit effects, but not sure how descending from the step would help this. By definition you would need to be above service ceiling (possible with some residual zoom climb effect?), to then fly descent at a lower AoA than Vmd.
As my preferred test vehicle is the draggy Super Cub I can’t claim to have observed this and tend to like Jarvis’ suggestion. Blame it on hysteresis!
