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Flying on the 'step'

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Flying on the 'step'

Old 20th Nov 2004, 14:04
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Gizajob
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Flying on the 'step'

Reading 'Fate is the Hunter' - awesome book (where can I get a job flying stuff like that?)

In one chapter, Gann mentions climbing marginally above target alt (no such thing as a level bust then I guess), then descending to target in order to hit cruise speed at a more economical power setting. He calls it flying on the 'step' and claims it gives better performance than if you just level out at target and set cruise power.

I have previously heard of this technique and tried it in a DR300 - it does seem to work, giving us around 5kt more for a given cruise power setting.

Not sure how this works - can anyone shed any light?
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Old 20th Nov 2004, 14:45
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Most transport type aircraft (as opposed to fighter or bomber type aircraft) have an Operating Data Manual with a specific target speed for a given FL and ISA deviation. You climb at the recommended power settings or IAS or whatever (as specified by the ODM) until your planned route FL. You are taught by your primary flying instructor as a basic flying technique to anticipate the level off, so these lessons continue in later life and you reduce the power and start the forward movement of the stick shortly before the levelling altitude (usually only about 50 - 100 feet before, depending on the climb rate of the aircraft). and so you arrive, with the altitude absolutely hacked, but still at the climb IAS, not the route IAS.

Once levelled, you now wait for the engines (at the cruise power setting) to do their job. Sooner or later, they do. Eventually, because you are level, not climbing, the speed increases and you arrive at the target speed.

But all of this takes time. Some people believe that if you climb to a couple of hundred feet above the cruise altitude, still at the climb IAS/power settings or whatever, and then convert the excess altitude into speed by diving, you will arrive at the target IAS a bit more quickly, which means that you can then settle down into the cruise a bit earlier.

For me, the jury is still out on this one. Looking at it theoretically, it seems to me that it is a transfer between kinetic energy (speed and engine power) and potential energy (altitude). As energy can be neither created nor destroyed, it shouldn't make any difference which way you do it. However, practically, whenever I've done it, it always does actually seem to me to appear to be working, and you do seem to arrive at the target cruise IAS a bit more quickly if you do climb above the target altitude and then descend to pick up the target speed. I'm sure it can't be true, but it just always seems that way when you're doing it.
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Old 20th Nov 2004, 14:48
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Yes exactly! I too had been through the 'maths' of it and couldn't see how it works. However, like you, I found it does seem to somehow... hence the question!
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Old 20th Nov 2004, 15:20
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The referenced technique was especially useful in large propeller driven aircraft...specifically DC-6/7, 1649 Constellation, and F.27...the types with which I am most familar.

Climbing several hundred feet above the target cruise altitude, with climb power still set, then beginning a slow descent, to the desired altitude, resulted in a more rapid acceleration to the desired cruise airspeed.
This was especially useful with the F.27/FH227B, wherein the climb power setting was also the cruise power used.

Having said all this, when it is all averaged out, the resultant cruise speed achieved using the above referenced technique, will not be much greater than otherwise, using the AOM method, over the long haul.
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Old 20th Nov 2004, 17:55
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For me, the jury is still out on this one. Looking at it theoretically, it seems to me that it is a transfer between kinetic energy (speed and engine power) and potential energy (altitude). As energy can be neither created nor destroyed, it shouldn't make any difference which way you do it. However, practically, whenever I've done it, it always does actually seem to me to appear to be working, and you do seem to arrive at the target cruise IAS a bit more quickly if you do climb above the target altitude and then descend to pick up the target speed. I'm sure it can't be true, but it just always seems that way when you're doing it.
I think you've hit the nail on the head. When this came up elsewhere I did some very simple numerical integrations in the form of a spreadsheet...

Two aircraft with identical aerodynamics. Both aircraft climb at 55 m/s (just faster than min drag speed of 50 m/s) at an angle of about 2.5 degrees.

One aircraft gets to its chosen altitude and levels off. It accelerates to its cruise speed of 77 m/s. It reaches 65 m/s after about 30 s, 70 m/s after about a minute and is within 1 m/s of its cruise speed in a little over 2 mins.

The other continues the climb at 55 m/s for 20 s, so it gets about 50 m (150 ft) higher. It then pitches down and flies the reverse (2.5 deg downwards) for 20 s before levelling off. At that 40 s point, it's going about 1.8 m/s faster than the other aircraft. But it accelerates more slowly. After about 90 s, there's still a 0.6 m/s advantage in speed (about a knot and a half, probably the smallest noticeable increment that the step-believers would demonstrate).

And eventually, they reach the same speed of 77 m/s. The step is illusory, but the illusion persisted for a good while. The aircraft that performed the zoomy manoeuvre spent an extra 20 s in the climb at a lower speed. If you track the distances travelled, it never quite catches up with the one that simply levelled off.
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Old 20th Nov 2004, 22:37
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I remember that "on the step" story. It seemed so plausible, pariticularly in a prop aircraft. And so many other little anecdotes from Fate is the Hunter. He was a great writer, simple and to the point, and I was enchanted.

Then, many years later, I read a few of the books he wrote after retiring from flying, buoyed by what he'd earned writing Fate etc. They were about sailing and I was absolutely horrified at the risks he took. He was still very much to the point but what showed through - to my by then jaundiced and cautionary eye - an elemental carelesseness.
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Old 20th Nov 2004, 22:47
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The chief pilot at the last company I worked for used to do this. I thought he was retarded. He used to also remain 200 ft high to remain out of the tops of cloud, instead of requesting a higher altitude. The funny part was, he thought he was being sneaky by resetting his altimeter so that is still read the correct altitude. Unfortunatly, his pea brain didn' t know that encoding altimeters send pressure altitude to the ATC radars, so it doesn't matter what setting you have in the altimeter.
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Old 21st Nov 2004, 06:16
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Gann wouldn't have got caught out by an altimeter, he'dve dangled his HF antenna to feel the ground.
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Old 21st Nov 2004, 15:07
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I'm not sure if the following applies to larger aircraft but I have observed the effects in light a/c and gliders:

Some things I have flown (high performance gliders, especially) seem to have a 'bistable' effect in terms of the wing performance.

The theory is that at some airspeeds, there exist two stable states for the airflow over the wing. You can be in either state, depending on whether you have speeded up or slowed down to get to it. I have heard aerodynamicists mutter about laminar attachment, separation bubbles and other techno-gabble but the effects are real (in some particular cases).

A sailplane I used to own climbed noticeably better if you approached the optimum thermalling speed from above, rather than below. It had boundary layer control devices over most of the span and I reckoned there was a small range of speeds where there was definitely some hysteresis in the performance polar.

It's similar to 'planing' seacraft, I think. You can motor along just below the 'critical' speed and be using a lot of power. You can then accelerate over the 'step' to get the boat in a lower drag configuration, then slowly reduce the speed to what it was previously, finding much less power is required to do the same job.
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Old 21st Nov 2004, 15:33
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>You are taught by your primary flying instructor as a basic flying technique to anticipate the level off, so these lessons continue in later life and you reduce the power and start the forward movement of the stick shortly before the levelling altitude (usually only about 50 - 100 feet before, depending on the climb rate of the aircraft). <

Well if he taught you this, he taught you wrong, Oxford Blue!

Yes anticipate by, say, 50 - 100 feet (or perhaps 10% of the Rate of Climb) and then level off and accelerate to cruise speed WITH CLIMB POWER MAINTAINED until the target cruise speed is reached (or maybe one or two knots before because it takes a finite time to reduce to cruise power).

That said, the step technique does seem to have some credence.
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Old 21st Nov 2004, 16:01
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A sailplane I used to own climbed noticeably better if you approached the optimum thermalling speed from above, rather than below. It had boundary layer control devices over most of the span and I reckoned there was a small range of speeds where there was definitely some hysteresis in the performance polar.
A good point. However, the only polars I've seen with hysteresis were those depicted for low Reynolds number flow, aimed at model aircraft. And for those, the hysteresis was the wrong way round: approaching a particular lift coefficient from above resulted in lower drag than approaching it from below.
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Old 21st Nov 2004, 17:41
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A good point. However, the only polars I've seen with hysteresis were those depicted for low Reynolds number flow, aimed at model aircraft. And for those, the hysteresis was the wrong way round: approaching a particular lift coefficient from above resulted in lower drag than approaching it from below.
Interesting. I think it may have had something to do with 'turbulation' and this not being so effective when approaching from the low speed end. I remember talking to the designer about this particular glider when I went to pick up one of the very first ones from the factory. They had done extensive wind tunnel and free air testing and were sure that it would climb faster and glide better than anything else. In very smooth air this was true but 'microturbulence' was it's downfall. Unfortunately there's a lot of it about in you average thermal...

I don't think any manufacturers admit to the possibility of bistable polars, even though they talk about it privately. I think it happens over quite a narrow speed range and only in some implementations. Also Re. numbers are higher in this instance.
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Old 21st Nov 2004, 19:43
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Just as it requires more energy to disturb an object from rest (to set it in motion) than to maintain an object in motion, it requires more energy to reach the cruise speed and altitude than to maintain it.

Nowadays this is achieved by accelerating to the a higher speed than the cruise speed and then reducing to crusie power.

If your max power is your cruise power then you can achieve your desired cruise speed either by waiting for the weight to decrease (thereby reducing the energy required) or by climbing above the selected altitude and converting the potential energy.
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Old 22nd Nov 2004, 09:20
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I think we are mixing several different scenarios.

1. Cruise by cruise power setting (piston and turboprops?)

2. Cruise by target cruise speed (jets?)

In the first case you are adding a constant ammount of ernergy and the speed achieved will depend on the overall energy equation. By climbing 200 above your cruise speed and then descending and acelerating you are adding more energy than in a normal climb because you are using climb power for longer. I guess the same effect could be achieved if you maintain climb power for an equivalent amount of time while level at the cruise altitude.

In the second scenario you climb at climb power but you cruise at the target speed. This means that the energy added is the necesary to achive the selected speed. Climbing higher and descending to accelerate will only achieve a shorter time at max cruise power or a period of time at a reduced cruise power.
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Old 22nd Nov 2004, 20:31
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You are quite correct about constant cruise power settings in old 4 engine piston transports, calypso.
These were always cruised at a constant BHP/BMEP, and as the weight reduced, the TAS increased slightly.

Jets of course turned this scenario upside down...some old piston drivers simply could not adapt...or found themselves in a jet upset situation when they climbed too high (for their weight) and slowed down (sometimes way down) for turbulence.
Bad news if tried.
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Old 22nd Nov 2004, 22:06
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411A,

That's almost but not entirely accurate. I flew one of those old 4 engine piston transports (Wright R3350 turbo compounds), albeit a military one for which range was more important than schedule.

In that case, the optimum approach was to reduce RPM (with max BMEP) to cruise at the best range cruise speed for the weight. This meant that the last portion of the flight would be flown at some 15% slower than the initial cruise. (Made for a long night: I once flew a 20 hr flight, returning to Nova Scotia while retaining Jacksonville FLA as my alternate. Very glad wx above minima on arrival.)

For fast cruise, which commercial airliners used as you said,(unless on the hairy edge of range), indeed the TAS would increase by as much as 7-9% at max cruise power.

Was much more comfortable when I could get back to jets which have essentially no range penalty to fly at high cruise speed when at high flight levels.

Your coffin corner situation appears to have been found by a Pinnacle crew.
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Old 26th Nov 2004, 13:39
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EGBK, you say you tried this in a DR300, with the Robin cranked wing. If the DR300 is at S&L flight, the outer cranked part of the wing produces no lift, and hence reduced drag, the lift is therefore from the main part of the wing. But to get to the 120 kts required you climb above the required height and the dive, when you reach 120kts you find you can maintain that speed in level flight.

However if you pull back on the stick just slightly the speed reduces, and you come off the step. Lowering the nose again to S&L you wil find you are at 110kts S&L, unable to get back to 120kts.

How the step applies to aircraft without this cranked wing I dont know.
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Old 26th Nov 2004, 15:20
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Yakker - having a brain-dead moment here. How does a cranked wing produce no lift s&l? Surely it will develop a lift vector perpendicular to the surface?

I also dispute that by raising the nose and lowering it again you won't get back to 120. If you raise the nose, you'll slow down and probably climb. If you lower it, you'll speed up and probably descend. Once back in equilibrium (which may take some considerable time - see other posts), power+attitude = performance: the same as before, since nothing in the force equation has changed.

This step thing is fascinating isn't it!?
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Old 26th Nov 2004, 15:41
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Does lift not vary with angle of attack. At a certain angle of attack the wing produces no lift. The cranked outer part of the wing has a different AoA to the main part of the wing. Hence at S&L the main wing produces lift, but the outer part does not.

You will not get back to 120 flying S&L as the drag is a problem. By diving you reach the Step, and can maintain S&L at 120.
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Old 26th Nov 2004, 16:31
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Personally, I'm sceptical of the entire concept of "the step" in cruising. Whatever...

It should, of course, be pointed out that such a technique is a total no-no in RVSM.
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