Airborne_Again wrote:
No, the “step” relates to the fact that a powered aircraft has a minimum power required speed
How do you know that everybody is talking about the same thing? 
The drag bucket is after all a real “step”.
LeSving wrote:
The drag bucket is after all a real “step”.
Do you mean the fact that there are two small concave parts on the graph?
In any case, the “step” idea is older than laminar flow airfoils.
Airborne_Again wrote:
Do you mean the fact that there are two small concave parts on the graph?
I mean the drag bucket at low Cl. All airline aircraft has had this since the 40s maybe, or at least since the first jets. It’s a matter of Re number just as much as the airfoil. It is a real step where physics change a bit (extended laminar flow), either you have entered the “step” (bucket), or you haven’t. The requirement is an appropriate airfoil and enough speed (high enough Re number, thus the sheer size also matters).
Anyway, I think what he means is what he explains. I remember I tried a homebuilt Zenair CH 600 or 601. It had a Jabiru 80 HP engine, and was very under powered with two 95 kg guys in it. The owner talked about this “step”. It’s the only time I have ever actually talked about it, but didn’t really understand why he called it a step. When cruising it was important to get on the “step” he said, or it would fall behind on the power curve (at a an initial rpm). With a fixed pitch, this isn’t always easy because as speed decreases, so does RPM and therefore power. It is therefore better to cruse substantially faster than best L/D, so you don’t easily fall behind. This requires an engine with enough HP and/or a good torque curve. The Jabiru has both low HP and an rather suboptimal torque curve (and the aircraft was heavy with us on board).
LeSving wrote:
Anyway, I think what he means is what he explains.
Ok, what do you mean with “step”, if you don’t mean that you can achieve a higher speed with the same power output.
The power/thrust curve is just that. With a step, the analogy is obviously boats planing, that’s where the word comes from. At a certain speed the physics changes character. To get to that speed however, you have to go through a “barrier” that requires more power than what is needed to stay there, hence a step.
The word “step” is not a good analogy for what is really happening because there is no step in a physical sense.. nothing has changed.
LeSving wrote:
The power/thrust curve is just that. With a step, the analogy is obviously boats planing, that’s where the word comes from. At a certain speed the physics changes character. To get to that speed however, you have to go through a “barrier” that requires more power than what is needed to stay there, hence a step.
You’re not answering my question.
Airborne_Again wrote:
You’re not answering my question.
LeSving wrote:
At a certain speed the physics changes character
LeSving wrote:
the physics changes character
To me that’s too vague to be an answer. Specifics, please.
Airborne_Again wrote:
To me that’s too vague to be an answer.
Good grief 
With a teaspoon this time. I assume you know what a planing boat is? Yes? The physics of it changes character because you are travelling faster than the bow wave. We have discussed this in detail somewhere else here on this board. The analogy is in fact more that of an aeroplane going faster than the speed of sound. Both are going faster than the wave propagation velocity they create in the media they are travelling. The difference is that a planing boat gets out of the media it is travelling in, and therefore this will reduce drag. An aircraft cannot do this.
Now, speed boats (as in competition speed boats), and floats for aircraft for that matter, have stepped hulls. Stepped hulls creates less drag than a normal deep V shaped (un-stepped) hulls when planing because of less wetted surface area. In addition, with a step an optimal aoa can be maintained over a wide speed range, reducing drag even more.
That’s where the “step” comes from. Getting on the step, quite literally means exactly that. Clearly there is no such step of any kind, physical or literal for a cruising aircraft. An aircraft cannot “get on the step” because:
- unless of course it is a float plane or a flying boat. They both have this step, and use it for take off and landing.
The best analogy I can find is the low drag bucket of some airfoils at high Re numbers. It is something similar because it reduces drag considerably due to a change of physical characteristics of the media in which you are travelling. It is no artefact of two curves crossing. I mean, you can fly with full power at high aoa, barely moving forward, or you can fly at full power at top speed. Would you call that a “step” as well? It is exactly the same thing as he explains, only exaggerated.
It comes back to what I said in the beginning. Everybody isn’t talking about the same thing here A step for one person is something different for another person.
LeSving wrote:
The best analogy I can find is the low drag bucket of some airfoils at high Re numbers.
Having a low drag regime does not in itself mean that there are two different speeds where there is an equilibrium between drag and thrust.
I asked you previously if it was the concave parts of the L/D curve that produced this effect. You answer implied that it was not. Then I can’t possibly see how the drag bucket can give that effect. (Actually I believe that you are wrong and that it is indeed the concave shape that makes the difference. Or put it another way, there are two places on the curve with the same L/D ratio. When I get some time I can think of this in more detail.)
