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Flying on the 'step'

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Flying on the 'step'

Old 26th Nov 2004, 16:38
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Gizajob
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Doh! Cheers Yakker - worked it out meself in the meantime. Must be these long shifts...
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Old 26th Nov 2004, 17:03
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I personally think this whole business of flying on the step is a load of c**p.

What decides a final cruise TAS is an equilibrium between thrust and drag, remember, excess thrust can either make an aircraft accelerate or climb, once the total thrust is equaled by the total drag you will have level, unaccelerated flight.

I can appreciate that climbing above and then diving down will seem to yeild a higher speed or the same speed in less time than a normal level off and acceleration, but unless the two methods can be made under the exact same thrust / weight / ambient conditions any conclusion that is derived is purely subjective.

If anyone can post scientific data derived from proper flight testing that shows existence of the mythical "Step" I would be very intersted.
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Old 26th Nov 2004, 17:14
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What about the Concorde on final approach, using ~60% power (not completely sure about this though) to fly at 15% of cruising speed?
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Old 26th Nov 2004, 20:36
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Dufo,

All aircraft will require more power to fly at slow airpeeds such as on approach vs higher airspeeds such as LRC or max endurance, this is due to the effects of the two primary types of drag an aircraft has to overcome; Induced drag, which is greatest at higher AOA's and low airpeeds and gets less the faster you go, and Parasite drag which is proportional to airspeed and gets greater the faster you go. The aircraft has to overcome the combined effect of these and hence if you were to draw a graph of TAS vs total drag you would get a U shaped curve. From this it is possible to see that an aircraft can actually be flown at two different airspeeds with the same power setting, albeit the lower of the two speeds is substantially less than the minimum speeds typically used on approach. This is one of the reasons that primary students are taught slow flight.

The other reason an aircraft will use more power on approach is simply to overcome the drag of the landing gear and lift enhacing devices such as slats and flaps, and lets not forget in the case of the concorde that drooped nose has to add quite a bit of a drag.

And before anyone jumps on board and says "See, you just said it, you can go faster with less power!" just remember, the minimum power required occurs at L/D max for any aircraft which is typically around the airspeeds required for max rate of climb (since max ROC occurs at a speed where there is the greatest excess of thrust) as you go any faster than this speed (typical cruise speeds are substantially faster) you will require more power, period.

Diving down to an altitude may get you a higher TAS initially but sooner or later the laws of physics will prevail and you will get the same airspeed. In fact, if you think about it, the diving down method will be less efficient since you will have climbed higher than necessary and wasted fuel in the process.
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Old 26th Nov 2004, 21:05
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Calypso,
If you look at your power tables for a single engine piston you will be able to find values for TAS and power required to achieve this. These are the required power settings to maintain the speeds not the required power (energy) to reach them because to accelerate a body requires more energy.
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Old 27th Nov 2004, 23:47
  #26 (permalink)  
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If you use the technique stated by firefly bob you will also reduce the shock cooling effect on the engine.
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Old 28th Nov 2004, 03:28
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Strange no-one has considered the total drag curve in this thread.

Recall that it starts high at low speed and then reduces to min drag speed from which point up is the only way it goes. I know there may be a little excursion around transonic.

So for a fairly wide range of power settings we have two speeds having equal total drag.

A delta wing has a broad range of speed around min drag where there is little variation so if we use that as an example the effects are magnified.

Assume a flat bottom of the total drag curve over about 30 Kts as with the Vulcan. Now you have a 30 Kt range of speeds at which you can stabilise for a particular power setting.

If you want to cruise within that 30 Kt band of speed then you would be foolish not to accelerate beyond the speed band, set the power and let the speed stabilise back to the high speed end. Otherwise you could be stuck all day at the low end.

Most non deltas will also have a small band of speeds at the bottom of the total drag curve.

So - best method is to hold climb power after levelling, overshoot the expected cruise speed, reduce to cruising power and you should stabilise at the higher value.

Sometimes turbulence or other disturbances will cause you to slip back to the lower speed. More power will be needed for a while to regain the faster speed.

All this is irrelevent if your speeds are further up the total drag curve.
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Old 28th Nov 2004, 21:48
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Milt:

If you had checked back a couple of posts you will see that I had mentioned the total drag curve. The problem that exists with flying at the bottom of the curve as you point out, is that these speeds are far slower than what we usually cruise at, as I mentioned before, the TAS that equates to minimum drag (L/D max) is in the same vicinity as the speed for max ROC

Cheers!
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Old 29th Nov 2004, 00:48
  #29 (permalink)  
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This is what I love about flying - always something to learn and there are still some things that aren't fully understood!

Thanks for all your input folks - really interesting and educational for this newbie...
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Old 29th Nov 2004, 11:29
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Yakker and EGBKFLYER,
I think the logic is a little incorrect regarding the cranked wings: there will simply be a differing degree of lift depending on the AoA, not lift and no lift. I don't think aeroplane designers are in the business of adding bits that 'just travel along for the ride' while adding drag!
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Old 29th Nov 2004, 15:06
  #31 (permalink)  
 
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212man

It was my understanding the Jodel/Robin wing was efficient due to the tips being at a lower angle of incidence than the rest of the wing to reduce drag at cruise attitudes. The tips only providing significant lift at take-off and landing. The tips are also angled upwards to provide dihedral for stability and to reduce wingtip vortex drag.
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Old 29th Nov 2004, 15:55
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Why would you bolt on a section of wing that rides along at a less efficient AoA unless there was an overriding necessesity? I suggest the washout is for stall development control ie to induce a root stall first.
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Old 30th Nov 2004, 07:10
  #33 (permalink)  
 
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More or less what I thought!

This link may explain more about the original post topic:

http://web.usna.navy.mil/~dfr/flying/step_wide.pdf
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Old 1st Dec 2004, 08:24
  #34 (permalink)  
 
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In the P2V7 (aka SP2H or 4eng Neptune) you could engage the autopilot in baro hold and then quickly gain up to 12kts indicated (190 to 202 rings a bell) by trimming a load onto the elevators via the autopilot's electric trim-wheel (with auto-trim declutched).

My theory on that was that it was loading the control surface and streamlining the trim-tabs - thus reducing trim drag. It's also possible that the resulting airplane attitude was more streamlined (and thus benign) in terms of the Thrust-Weight/Lift-Drag couples (and their equilibrium position).

It wasn't an induced static error either, because the NAVS would always confirm the extra TAS. I mulled over this for ages way back then and tried to factor in what else might have been happening with the varicam (variable camber tailplane horizontal stabilizer). Never did find a bottom line there.

Trim tab position does make a difference. In an MB326H you could hold the nose off down to ultra low speeds (for aerody braking) if you ran the elevator trim full nose-down after touchdown. It was then acting as an elevator augment.....and boosted elevator effectiveness tremendously.
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Old 1st Dec 2004, 14:31
  #35 (permalink)  
 
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Danger

OTOH, overtalk, one could always fire up the two jets for that added few knots....while reducing the range ah...just a bit.
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Old 1st Dec 2004, 15:06
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As far as I can see, the only situation where an on the step level off would provide benefit is where there's a drag rise & then drop with incr. speed eg transonic to supersonic.

If the a/c has insufficient thrust to overcome the maximum drag, but sufficient to once the drag has reduced a bit then trading altitude for speed + max thrust could allow a final speed to be greater than without using a step. Effectively the additional height was used to store & then rapidly release a bit more energy to overcome the drag 'bump'.

Otherwise I can't see it. If you have thrust to climb then you have thrust to accelerate and eventually the steady state speed will occur when T=D.
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Old 1st Dec 2004, 18:43
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OVERTALK, how did the navs confirm the increase in TAS? If it was by applying density correction to the CAS, if the CAS was wrong, their calculation of TAS would be wrong.

Unless you had Doppler and they noticed the groundspeed increasing on the Doppler, their calculations were no better than you reading the raw IAS.
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Old 1st Dec 2004, 19:01
  #38 (permalink)  
 
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tinstaafl,

Absolutely right! with the possible exception of the transonic range, the existance of the step is a myth.
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Old 2nd Dec 2004, 08:32
  #39 (permalink)  
 
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We had Doppler and VOR/DME (and a very accurate radar - the APS20E).

It was a genuine TAS increase alright. Some Sqn pilots claimed up to 15kts extra IAS but I only ever saw a pretty standard 12kts.

We had the manual spark that meant that you had to go to climb power every hour (for a minute) and check the plugs out on the ignition analyzer. No matter how fatigued, you only ever got to get that wrong once.

Reclutch auto-trim
Mixtures auto-rich
Spark - from ADV to Retard
RPM 2600
50" dry Torque (cowl flaps cracked) & oil-coolers auto

Balfanging the donks and waking up the whole retinue of back-enders wasn't worth the grief (nor the half-eaten apples that eventually got to the sharp end from the galley together with the tangy coffee). Of course once they had us started into wearing chute harnesses you could almost guarantee that you'd be creating an emergency as you squeezed down back past the Tactical station and your harness tripped multiple breakers. So one always had to rely upon good relationships with your crew for sustenance.

Eventually I got so paranoid about the supercharger upshift I resolved never to use that again. Shifting both R3350-32WA's into high gear at once was the only option, you couldn't do it one at a time. Shearing a shaft was too easy.

After a few Neppies were lost due to high speed exhaust PRT breakups I was a tad paranoid about that accessory too. CSD disconnect was also a grim business if it didn't work. But by far the best trick I ever recall was the guy who lit off both AVGAS fuelled jets one dark night - without dumping them first (from standby) and forgot to turn boost pumps off and low. It's twice as impressive as the F-111 dump and burn when you see the flames from those Westinghouse J34-WE-36 dump-valves extending twice the length of the airplane.

And 411A is right. If you were prepared to go on oxygen and cruise at 30,000ft you could true out at over 330kts on all 4 - but the anmpg was down a bit.

What I miss most is arming the spoilers and making all the rear-enders sick as dogs in a MAD-trapping pattern. You could generate roll-rates as good as any fighter of the day. But once you shut down and left the airplane, you had to sprint for your life.
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Old 2nd Dec 2004, 09:02
  #40 (permalink)  

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I was going to say the same as Tinstaafl.

The transonic regime is analogous to the planing hull scenario.

From a mechanics of flight point of view, I can imagine some interesting things happen if you are climbing at a speed which corresponds to the neutrally speed stable point at the bottom of the drag polar i.e., V_md. In this case, how you reduce the power as you approach the target altitude is critical as you may or may not wind up on either side of V_md whereupon speed stability is qualitatively different. It tends not to be a problem in commercial jets because we don't climb at speeds approaching V_md. Au contraire, I suppose in gliding, its all about V_md!

From an aerodynamic point of view it doesn't get any more difficult than unsteady (transonic) aerodynamics and without doing the testing, I wouldn't like to comment about the potential hysteretic effects FullWings mentions but essentially unless you've somehow sustained a qualitatively different flowfield, in the steady state case, there should be a unique value of velocity for the given thrust. My summation would be that in the world of in-flight Reynolds numbers, turbulence will destroy any laminar type effects that you'd prefer to persist.

OVERTALK

The fact you only saw a miserable 12kts as opposed to 15kts, is surely a reflection on your good airmanship and your view on where its sensible to keep the CG.



My $0.02.


Last edited by SR71; 2nd Dec 2004 at 09:50.
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