As someone pointed out to me today, care should be taken not to inadvertedly bust airspace by this method, particularly in the UK.
Interesting to see, that this discussion started again. In fact, I wasn’t aware about that drag bucket until now and am thinking about that, too.
However, to put it in a nutshell, it’s all about Angle of Attack (and corresponding thrust). On any plane you can set a climb AoA and a cruise AoA. In cruise AoA you take benefit of the lift produced by the wing. In climb AoA you use simply the fact, that the nose of the wing is higher than its backside. So flying the “climb AoA” can be done with anything having the shape of a barn door, by simply adding thrust to that barn door. But the cruise AoA is specific to the shape of the wing and the wing foil used.
By the way, this is also true for climb, not only for cruise, and maybe here even more people might have noticed it. Each plane has some sweet spots, where you have a good ratio in between speed and climb rate. You can typically achieve the same ROC (rate of climb) with at least two different speeds, maybe even three different speeds when taking very low speeds into account, in the regime of Vx.
An eye opener in this respect is climbing a cruise climb in a powered motorglider, which is best achieved with pitch more or less 0° and just adding thrust to achieve climb. You can have the same ROC with a typical pitch of, for example, 10° or more, but speed will be sad.
Two points, however, have to be realized. Flying “on the step” is a bit tricky and needs higher attention during flight, as it’s not as easy to precisely hold altitude than flying “in the bucket”. And it’s not intuitive to find it. Second is, it is only “available” in a medium power region, and/or at high altitudes.
gallois wrote:
This argument developed into some saying that it is better to climb to 100ft or so above cruising altitude,(the step) reduce power on levelling out then dive down the 100ft to your cruising altitude, thus getting to cruising speed and cruising attitude quicker.
You know, this was what we used to do on the good old Caravelle. It was even mentioned in the manual someplace that this was the most efficient way of doing it, and as something that they actually expected pilots to know anyway. One of our instructors claimed he had been instructed to this by Pierre Nardot himself and explained it as follows (and I am going from 30 year old memory here):
Airplanes can achieve level flight with different AOA and pitch. The fact alone that VS is zero does not necessarily mean that the airplane is in the ideal attitude to be most efficient. Consequently, if you level off from below the level and immediately reduce power to cruise power before speed has built up, you will hang in there with VS zero, often held by AP, the aircraft will need a higher pitch to maintain level. With cruise power set prematurely before cruise speed is achieved, the plane will fly slower and with higher drag until it eventually will achive proper level flight and cruise speed, if ever.
With the Caravelle, we could see this effect quite well. Leveling up from below, it would take about 10-15 minutes to achive cruise mach and often it would end up slightly below book figure. So we always took it about 100 ft above the level, let the power stand and, once it was stabilized, actively flew it back to the level before engaging ALT hold. Only when speed was slightly above calculated Mach would we reduce climb power to cruise. Levelling off like this would get us into level flight with the advertized Mach number much faster and more efficiently.
I told this “secret” to the crew of a Tupolev 154 I had the chance to accompany for a while and while the captain said, he learnt it from his instructors on the A310 he had previously flown. So it appears that this may well be “French School”. I’ve ever since done it on every airplane I got to fly. On the TU, this method did indeed yield unexpected results of a few hundred kg’s less consumption on a given 2 hour leg, when consequently applied. Consequently, I sometimes wonder whether today’s fully automated airplanes do not waste some performance with their total integration of level offs by AP only.
With the Mooney as well it works there as well, even though the effect is not as pronounced and less visible, probably because the airplane has much less mass to settle. Also quite a few instructors will give you heat for overshooting the level, even if briefed before. It certainly will improve level off when the airplane is heavy, at least that is my impression. Where the effect was more visible was with the C150 I used to own before the Mooney. Notoriously short on power, the C150 would attain level flight with it’s 91 kt cruise much easier with this method.
Airborne_Again wrote:
With the “laminar bucket”, there is an additional intersection at point “C”. At this point, we can indeed cruise faster than at point “D” so this is also a “step” but a quite different one from the one Rogers wrote about.
Addendum: It seem like this “laminar step” is noticeable only in a quite narrow L/D ratio band, and thus for quite a narrow thrust band. For a slightly higher or slightly lower L/D ratio the phenomenon would disappear. So my guess is it would be hard to notice in practise.
Airborne_Again wrote:
With the “laminar bucket”, there is an additional intersection at point “C”. At this point, we can indeed cruise faster than at point “D” so this is also a “step” but a quite different one from the one Rogers wrote about.
Well explained, thanks!
Airborne_Again wrote:
Yes, but in aviation it is a figure of speech
I don’t think all float plane pilots would agree that it’s just a figure of speech 
IMO you are overthinking this. Climbing the step is physically and literally exactly that, period You will cruise at Cl,opt in the figure, which corresponds to a Cl/Cd ratio where the dotted line crosses the “bucket”. But, this is only for high Re. When starting at lower Re (lower speed), you can still fly with the same Cl/Cd, but with a higher Cl, and also higher Cd. When speed increases, the drag reduces. That is the only real analogy I can think of regarding “the step”. The physics changes character, and you have a design (an airfoil), that takes advantage of this (and preferably to the max). If you didn’t have this design feature, you would never be able to get the low drag. Either you would have to fly slower for the same fuel usage per hour, or you would use more fuel per hour at the same speed.
LeSving wrote:
Climbing the step is clearly an expression from fast going hulls in water. The step itself is a tangible, real thing.
Yes, but in aviation it is a figure of speech. The analogy to hulls in water is only that there is lower drag at higher speeds.
But I’ve figured it out now. For simplicity, I’ll talk about thrust, not power.
A fixed thrust at a given mass corresponds to a particular L/D ratio. (I’ll disregard the effects of the vertical component of the thrust vector.)
Consider the blue L/D ratio line. There are two places on the polar curve with that L/D ratio corresponding to an equilibrium. These are both close to the minimum thrust point and thus in the low-speed regime. Point “A” corresponds to “being on the step”.
The red L/D line corresponds to the high speed regime. With a non-laminar airfoil, the line intersects the polar curve only at point “D” – thus there is no “step”. As Rogers explained, if we extend the polar curve there would be a second intersection at point “E”, but that is in the stall region and thus not noticeable.
With the “laminar bucket”, there is an additional intersection at point “C”. At this point, we can indeed cruise faster than at point “D” so this is also a “step” but a quite different one from the one Rogers wrote about.
And – the reason there can be an additional intersection between the red line and the polar is that a section of the polar is concave between “C” and “D”.
T28 wrote:
These airfoils aren’t necessarily designed to be laminar, they are typically designed to have low drag at high MACH numbersHow do you design a wing with low drag at high mach without laminar flow?
:) Ask swordfish!
These airfoils aren’t necessarily designed to be laminar, they are typically designed to have low drag at high MACH numbers
How do you design a wing with low drag at high mach without laminar flow?
Airborne_Again wrote:
I asked you previously if it was the concave parts of the L/D curve that produced this effect.
I said it’s a function of Re and the airfoil. Re is a function of speed and cord length (when it comes to airfoils). These airfoils aren’t necessarily designed to be laminar, they are typically designed to have low drag at high MACH numbers. Take a look at the wing of a Boeing. It isn’t exactly smooth, as in laminar flow glider wing smooth. This effect does not exist at low TAS, only above a certain Re. Everything else equal, just by increasing the speed, the Cl decreases. Lets say from 0.01 to 0.006. Clearly a huge reduction is drag, and therefore a huge reduction in required power, What I said was that this is the only thing I can think of that is something similar to “the step”, and the effect is large.
Anyway, I think you are overhinking this. Climbing the step is clearly an expression from fast going hulls in water. The step itself is a tangible, real thing.
From this we can derive a physics effect. Utilizing the change of physical character to our advantage by incorporating a step (a real tangible one). We can also derive a performance effect. We do something to improve perfomance for the same power for instance. We have:
When you say “the step”, what exactly are you talking about? I agree that the performance effect is similar to a planing boat, but to call it a “step” is IMO way beyond reason because:
It is purely an effect of two processes, the power available in the engine/propeller and the total powered required to overcome the drag. Depending on who you are talking to, you will get different answers. A person used to boats (me for instance) would most likely not think of “the step” in aircraft as a real step, but purely a slightly similar performance effect. Saying you can “climb the step” in an aircraft is complete nonsense IMO, because there is no step to climb, and no physics to exploit.
