Flying "on the step" - anything in it?

I was once flying a seaplane into a 10 knot wind, and over a long lake. I found that I was able to fly, land, and takeoff all at the same power setting. Now of course “ground effect” was playing a role in this, but without changing the power, I could cruise at 10 feet above the water, lower the nose a bit and gain a few MPH, which accelerated me so I could climb, or I could raise the nose, which slowed me so I settled. From the settling, I could touch down on the water, and plane on the step. With the now improved ground effect, I could takeoff and climb up to ten feet again, and repeat as I desired – all without changing the power. Thus I can affirm that there is more than one speed which can be achieved with one power setting at low altitude.

I think the Rogers paper just confirms the “back of the drag curve” speed that has already been mentioned. As only the higher speed is stable, I think you would notice in practice if a higher cruise speed were available.

Pilot DAR, that sounds remarkably similar to the Dynamic soaring that birds use to cross oceans

DavidS wrote:

As only the higher speed is stable, I think you would notice in practice if a higher cruise speed were available.

The speed on the back side of the power curve is also stable, though in some types, overheating becomes a concern. This is not step flying though, as massive configuration change from cruise is required to achieve the slower speed.

I think you would notice in practice if a higher cruise speed were available Quote

I have.

Of course the “step” exist. but it depends what is meant. Sometimes nonlinear relations makes huge differences. The low drag bucket of the airfoils used on high performance planes (sailplanes and airliners) will drastically alter the cruising/gliding abilities. A C-172 with bugs and dirt have no such “bucket”, but a clean Cirrus might very well have one. I would believe this to have the opposite effect though, unless the climbing is done at cruise speed, where there should be no measurable difference.

I once flew an airplane with a distinct “step” for a different reason. It was a CH650 (or maybe 600?), a Zenair with a 80 HP Jabiru, With two on boards it was underpowered, and the (very) fat airfoil made a distinct “step”. To be able to cruise at normal speed, a disproportional amount of power would have to be used, or it would fall behind the “step”. Thus climbing above (at the same power used for cruising), and then “gliding” down “above” the step would perhaps not make you reach your destination faster, but would definitely safe huge amounts of fuel, and make you go longer.

Pilot DAR, I had not seen your post when I submitted mine – I am very happy to accept that you have noticed it, and that it is not the Rogers effect!

Does anybody have any physics for how it works though? Could it come from downwash off the wing onto the tailplane, so that the two speeds correspond to two slightly different main-wing angles of attack, and and two very different tail-plane angles of attack?

This was the speed stability I was referring to on the back side of the drag curve, once again by Rogers.

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I was experimenting today flying home in the 150. I was able to repeatedly reproduce about a 2 MPH difference in cruise speed, with no power nor altitude change, depending on the tend of the aircraft to climb or descend, without actually changing altitude over several minutes.

Pilot_DAR wrote:

The speed on the back side of the power curve is also stable

Of course, if you don’t touch the elevator and you reduce power a bit, the nose will just drop a bit and the aircraft will settle at some speed, so you are right.

But it isn’t if you want to maintain alititude (or constant sink rate in an approach), since this form of drag curve is given as drag in level flight.

Full mechanism: lower speed —> less lift —> need to increase angle of attack to maintain altitude —> even MORE drag —> even LOWER speed —> continue until you stall or add power.

Exercise 10a.

On the right side of the power curve: lower speed —> lower drag —> aircraft accelerates, and higher speed —> more drag —> aircraft decelerates.

And on the original question – maybe there is also an effect in fixed-pitch aircraft?

These have a prop that is most efficient at some airspeed, probably a compromise so below cruise airspeed. So if you are flying at a lower than cruise speed, the lower prop efficiency means it will take ages to accelerate to cruise speed, a lot longer than if you have a VP prop? (the equivalent of trying to accelerate from 20 to 50 mph in 5th gear – not great)

Just a thought…