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IS THERE A STEP?

June 21, 2017

 

In his column this month, Peter Garrison has once again addressed the enduring delusion of flying ‘on the step’. No matter that those in the know say that I, and many others, are wrong, I still do it.

 

 

When I get to top of climb, I carry on to a hundred feet above cruise altitude and then let the Saratoga descend to the chosen altitude. I reckon the plane gets its nose down and goes faster. It’s known as getting ‘on the step’.

The idea is that if you climb to the selected altitude and then just push the nose over into cruising attitude, then the aircraft is always trying to accelerate up to its cruise speed and will still be flying nose high. However, if you climb above cruise altitude and then descend, the aircraft arrives at cruising altitude with its nose down and a speed greater than that achieved by just levelling out at top of climb. The lower nose attitude means less induced drag and hence you go faster.

It’s a theory with intuitive resonance and the results can be seen in the higher air speed – at least for a while.

The problem is many aerodynamicists and pilots smarter than me maintain that there is no step. Boats that plane on water may be able to get up on a step, but planes in the air can’t because they don’t have a step in the hull – and they’re not floating.

So I gave intellectual assent to the impossibility of a step. Then, one of the marshals at a recent President’s Trophy Air Race was Mary de Klerk. Mary is a genuine ‘uber pilot’, having won Springbok (okay Protea) Colours for Precision Flying and is a past winner of the Air Race. We got chatting about how to go faster and Mary in all earnestness said that the best way was to put the nose down so that it is just below the horizon on the AH. She was emphatic that the plane would not be descending so much as just having a lower nose attitude and thus going faster because its angle of attack was reduced. In essence she was describing the contentious ‘step’, and I was both surprised and relieved to find someone of her calibre who still believed it existed.

So I started researching the subject and found that none other than my favourite aerodynamicist, Peter Garrison, had thoroughly dealt with the subject, way back in August 1974. Peter very graciously sent me the article he had written with full approval to ‘pillage it’, which I have done by repeating a large chunk of it verbatim.

Peter writes: “Motorboaters are familiar with the phenomenon of planning: a more or less flat-bottom hull can move in the water much faster in the planing mode, wherein the hull is supported dynamically on the surface of the water like a skipping stone, than in the displacement mode, wherein it is supported statically, like a floating log.

Many planing hulls, notably those on seaplanes, have a sharp discontinuity called a step somewhat aft of the centre of gravity that makes it easier for the hull to switch from the displacement to the planing mode. When the hull starts to plane, it is said to ‘get up on the step,’ and the difference in performance is immediately obvious.

Now, it is believed by many pilots that some analogous characteristic is found in airplanes, and that it permits them to move at higher-than-usual speeds at a given power setting, if properly flown. Hardly any pilot, talking about the cruising performance of his airplane, will fail to say that if you get it on the step, it will fly at such and such a speed with so and so much power.

Approached from a purely theoretical point of view based on textbook descriptions of the behaviour of airplanes, this ‘step’ looks like a fallacy. The problem with justifying it is that in the standard texts, drag is directly related to speed, so for a given power output, it is obvious that there is only one speed at which drag will be precisely balanced by thrust.

Because aerodynamics is a fairly difficult and obscure subject, and because air is invisible and therefore denies pilots any chance to develop a familiarity with its behaviour, pilots are veritable treasuries of misinformation about airplanes and flight. Most of what they say about the theory of flight, as opposed to practical techniques, should be given polite assent and then forgotten. Like the hazards, or lack of them, of downwind turns, the existence or nonexistence of the ‘step’ has generated millions of words in hangars and flight schools all around the world, most of them assuredly gibberish.

People will argue about the pros and cons of things for a long time before putting them to practical test; wishing to avoid this unscientific pitfall, I went tight-lipped to my local FBO with plans for a practical test.

What we are looking for is a situation that would occur at normal cruising altitudes of 5,000 to 10,000 feet, where there would be no ambiguity about which side of the power curve you were cruising on. We are looking for a situation in which we can get our power and then maintain two or more speeds sufficiently different – four or five knots, say – for there to be no mistake about the difference, and maintain either or any of these speeds long enough for there to be no likelihood that any of them have arisen from a transient condition like an up-draft.

My practical experiment worked as follows: Another pilot and I took a series of airplanes with constant-speed props: a Mooney, Bonanza, Bellanca and Cessna 182, to 5,000 feet. When we arrived at that altitude, the rpm was set at 2,500 and the manifold pressure at 19 inches while in the climbing attitude. Then, while the pilot concentrated on maintaining altitude with extreme care, not deviating by so much as 10 feet, the speed was allowed to stabilise. The final speed was noted, and then power was increased to 20 inches, then to 21 inches, and the process was repeated until we had a series of speeds for increments of one inch of manifold pressure up to full throttle. Then, continuing to maintain altitude with fanatical care, we ran back down the series of power settings to 19 inches.

The point was to approach the trim speed for each power setting both from a higher and from a lower angle of attack. If the notion of a step held water, we would expect, at least in some cases, to see the speeds in the second column differ from those in the first by significant and consistent amounts.

The Bellanca offered no joy whatsoever, returning rapidly and solidly to the same speed at each power setting, no matter how approached.

The Bonanza at first appeared to show some hope, with a second-column IAS of 168 mph falling opposite 161 mph in the first column at one point. Just before noting the figure, however, we noted a sag in the altimeter and, that corrected, in the ASI. This minute sag was followed by others which eventually brought us, after a long wait, back to 161 mph.

The Cessna 182 acted like the Bellanca, except at a couple of the lowest speeds, where a five- to seven-mph speed increase was found in the second, the ‘on the step’ column.

Only the Mooney seemed to provide powerful favourable evidence, with the entire second column standing three to five mph higher than the first. In order to verify this result, we took the Mooney up again several days later and repeated the experiment, this time maintaining each speed for several minutes in order to see whether the apparent speed gain from being ‘on the step’ was durable.

It wasn’t. The second test put the Mooney into line with the other three test airplanes, with the additional observation that it seemed capable of maintaining a certain airspeed at a given power setting for a remarkably long time and with an appearance of perfect stability, until suddenly out of nowhere would come the sag and the drop in airspeed. The high indication lasted longer than most people checking airspeed would bother to wait; hence our impression, on the first test, that the higher speeds were valid.

These five tests were not exhaustive, nor were they as carefully controlled as scientific tests should be, though I believe they were probably more carefully controlled than most pilots’ observations about their cruising performance. They bore out my impression from years of flying many types of airplanes, however, that it may take a while to get an airplane to settle into its trim speed, but that eventually there is one speed, and only one, at which it can fly level with a given power setting and weight.

I wish I could tell you that we found that it was possible to exceed book figures by 15 mph by some handy trick or other, or that we discovered the fallacy in all the textbooks, or that certain airplanes have a ‘step’ and others don’t. But the truth is dreary, so far as my modest experiments were able to illuminate it. There is no step.”

 

 

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