Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: RonB29 on February 26, 2016, 10:33:22 PM
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Return from Nagasaki to Okinawa by B-29 “Bockcar.”
In his book, “War’s End,” the aircraft commander Major Sweeney writes:
“Paul Tibbets taught me a technique that caused a lot of controversy among pilots, some vehemently denying it existed and others vocal apostles that a skilled pilot could use it. It was called “flying on the step." In theory, it was very simple. You kept the power steady and took the aircraft into a gradual descent, the airplane would pick ups fraction more airspeed without using more power and fuel. The pilot would then level off. To retain the increased speed and perhaps even supplement it a bit more, you would start down another step and then another step, and so on.
You could milk only a little bit more speed and fly a little farther without consuming more fuel, but that was all I needed—a little bit more. I had the advantage of being at 30,000 feet, so I started my way down the staircase.
I also decided to add some insurance by throttling back the propellers to 1,800 rpm from the recommended setting of 2,000 rpm. Turning the numbers over in my mind, I knew this wasn’t going to be enough. So I throttled back to 1,600 rpm, well below the engine specifications for any circumstance. This could damage the engines, but balancing the risks against the benefits, I concluded that I’d rather replace the engines and get my crew safely tucked away for the evening than be bobbing in the Pacific aboard a life raft hoping that we’d be picked up.”
Lt. Olivi, the copilot in his book “Decision at Nagasaki,” writes:
“Sweeney throttled back and put Bockscar on the step— the maneuver we had practiced so many times before at Wendover——…”
The interesting thing to me is that Sweeney describes a dynamic maneuver to extend the range. The Boeing POH describes ‘Flying on the step' as just over-climbing and diving slightly into your cruise condition. I think that is the common definition. Using the altitude dynamically, and repeatedly, for extra airspeed seems to be a key issue. Is it more fuel efficient that just setting a shallow dive, considering the extra altitude and distance to go, and flying the best speed for that descent angle? I have used quasi-steady aerodynamics and cannot find that Sweeney’s technique, compared to a steady 0.012 radian descent angle gives any better fuel consumption. I use the approximation that fuel usage is proportional to the horsepower multiplied by time.
I’ll be interested if anyone can find an improvement in fuel consumption using a flight simulator.
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Imo the question is if the technuiqe is more effective than a plane flying at 30k until it runs out of fuel and then glides as long as it can.
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Yeah I thought the whole "getting on the step" thing was just climbing above and then descending to your altitude, the idea being that if you leveled off from a climb and accelerated you wouldn't get as fast as if you leveled off from a descent and decelerated. The notion being the drag from the elevator would be greater in the first case because it would have to be further up to achieve level flight while the decelerating aircraft's elevator would not have to pitch up and thus it could transition to a faster level flight with less pitch up because of less drag. I thought it was supposed to be bogus.
I would have thought that their would just be an altitude speed engine setting combo that would give max range and you would just set it up that way, descend at a best glide speed and then fly at whatever the best performance would be. Also wind, the jetstream was a mystery back then.
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Getting "on the step" is a myth unless you're talking about a boat.
https://books.google.com.ng/books?id=szhF30LW2wcC&pg=PA75&lpg=PA75&dq=peter+garrison+getting+on+the+step&source=bl&ots=eaZEL1PsR7&sig=-HTrktWQVKfji_Of2VfkAG4r9eY&hl=en&sa=X&redir_esc=y#v=onepage&q=peter%20garrison%20getting%20on%20the%20step&f=false
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Do you have a link to something more substantial than an old magazine article? Maybe a peer reviewed journal article?
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Getting "on the step" is a myth unless you're talking about a boat.
https://books.google.com.ng/books?id=szhF30LW2wcC&pg=PA75&lpg=PA75&dq=peter+garrison+getting+on+the+step&source=bl&ots=eaZEL1PsR7&sig=-HTrktWQVKfji_Of2VfkAG4r9eY&hl=en&sa=X&redir_esc=y#v=onepage&q=peter%20garrison%20getting%20on%20the%20step&f=false
Actually, I think that article makes a very poignant point in the case of the B-29, with regards to the power curve up at service ceiling. These B-29s, to my understanding WERE flying very close to their service ceiling, unlike the planes mentioned in that article, and even the author points out a way in which that high-altitude flight might result in a difference.
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Actually, I think that article makes a very poignant point in the case of the B-29, with regards to the power curve up at service ceiling. These B-29s, to my understanding WERE flying very close to their service ceiling, unlike the planes mentioned in that article, and even the author points out a way in which that high-altitude flight might result in a difference.
All things being equal for a given power setting there is only one cruise speed. Period. Whether you level off and accelerate up to it or dive past it and decelerate back to it.
The step is a myth.
(Do not confuse the backside of the power curve vs cruise speed for the same power setting as evidence of a step. This is just total drag being achieved by differing factors, namely the ratio of induced vs. parasitic drag.)
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Is it possible that the drag from angle of attack to maintain that altitude prevents an aircraft to accelerating to a speed to which is can lower its AoA and thus cruise and a lower drag?
Basically, if the manuals says that at 30,000 ft that the optimum cruise settings are 30" manifold pressure and 2000RPM, that climbing to that altitude and setting the engines to that power setting could result in an excessive nose high attitide due to lower than optimum airspeed? But if the optimum airspeed is reached (such as from diving from a higher altitude), the settings would work?
Anecdotal evidence from B-24 pilots (with the Davis wing) mean there has to be a kernal of truth to this. Or some other mechanism that is not being explained.
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Is it possible that the drag from angle of attack to maintain that altitude prevents an aircraft to accelerating to a speed to which is can lower its AoA and thus cruise and a lower drag?
Basically, if the manuals says that at 30,000 ft that the optimum cruise settings are 30" manifold pressure and 2000RPM, that climbing to that altitude and setting the engines to that power setting could result in an excessive nose high attitide due to lower than optimum airspeed? But if the optimum airspeed is reached (such as from diving from a higher altitude), the settings would work?
Anecdotal evidence from B-24 pilots (with the Davis wing) mean there has to be a kernal of truth to this. Or some other mechanism that is not being explained.
Slow flight near the stall can take as much power as max cruise. This is due to induced drag being high and parasite drag being low at slow speeds. The opposite is true at high speed.
If these points are close together one can wind up on the backside of the power curve and mush along while the airplane accelerates. But that is not evidence of a step. The step implies a magical phenomenon where a plane has a higher cruise speed for a given power setting because of how one gets there. It isn't true.
Also, let's be honest. Many of these guys had pretty limited aerodynamic knowledge. People were flying B-17s over Germany at 18 years old with less than 200 hours total time.
In any event, the slightest bounce of turbulence would knock you off this mythological step any way.
Cruise speed (with all else being equal) is determined by total thrust available vs total drag.
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And range decrease significantly at lower altitudes due to higher fuel consumption and more drag from air resistance and thus a lower cruise speed...
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And range decrease significantly at lower altitudes due to higher fuel consumption and more drag from air resistance and thus a lower cruise speed...
Depends on the airplane. Fuel consumption for a jet decreases rapidly with altitude. Not so much for a piston.
Generally speaking higher altitude results in a higher true air speed for a given indicated air speed. My airplane has its highest TAS for IAS in the mid-20s, then as I climb above that altitude my TAS falls off a bit. Every jet I have flown has behaved similarly.
For a normally aspirated piston engined airplane at some point altitude starts to hurt you due to a decrease in power. There are many factors that determine what's optimal.
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Here's another one. I can't find the article I really want to post...
https://books.google.com.ng/books?id=Db8OO6WFEhAC&pg=PA103&lpg=PA103&dq=peter+garrison+column+flying+the+step+b-29&source=bl&ots=nBBBZ7ENfl&sig=0eGyEoizS3cCcLQv3f4ct3RqzwA&hl=en&sa=X&redir_esc=y#v=onepage&q=peter%20garrison%20column%20flying%20the%20step%20b-29&f=false
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Depends on the airplane. Fuel consumption for a jet decreases rapidly with altitude. Not so much for a piston.
Generally speaking higher altitude results in a higher true air speed for a given indicated air speed. My airplane has its highest TAS for IAS in the mid-20s, then as I climb above that altitude my TAS falls off a bit. Every jet I have flown has behaved similarly.
For a normally aspirated piston engined airplane at some point altitude starts to hurt you due to a decrease in power. There are many factors that determine what's optimal.
Yep, but i was just lazy and took the short version. For pistons its prob the superchargers that is the most important factor. But the B-29 has its peak at around 30k before starting to drop off so in this case it works.
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Depends on the airplane. Fuel consumption for a jet decreases rapidly with altitude. Not so much for a piston.
Generally speaking higher altitude results in a higher true air speed for a given indicated air speed. My airplane has its highest TAS for IAS in the mid-20s, then as I climb above that altitude my TAS falls off a bit. Every jet I have flown has behaved similarly.
For a normally aspirated piston engined airplane at some point altitude starts to hurt you due to a decrease in power. There are many factors that determine what's optimal.
What jets have you flown? Still fly? I was checked out in the Cessna 152 and Piper Warrior.
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All things being equal for a given power setting there is only one cruise speed. Period. Whether you level off and accelerate up to it or dive past it and decelerate back to it.
The step is a myth.
I agree that being "on step" is a myth. However I've found that your first statement might not be true in all cases.
The B-24 is a quirky airplane. It "seems" to have a step. We used 30 inches and 2000 RPM for cruise which gets you about 180 IAS at around 1500 MSL. If you get a bit ham-fisted with the elevator you can get the airplane slowed up a bit (5-10 mph) and it just doesn't want to get back up to the 180 you had before unless you bring the power up or descend a bit. You learn to be smooth.
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I agree that being "on step" is a myth. However I've found that your first statement might not be true in all cases.
The B-24 is a quirky airplane. It "seems" to have a step. We used 30 inches and 2000 RPM for cruise which gets you about 180 IAS at around 1500 MSL. If you get a bit ham-fisted with the elevator you can get the airplane slowed up a bit (5-10 mph) and it just doesn't want to get back up to the 180 you had before unless you bring the power up or descend a bit. You learn to be smooth.
We both agree that's not a step. That's just normal cruise. I don't have any data for the airplane but you might just be experiencing the result of a narrow range between front side and back side of the power curve. Either that or it just takes a long time to accelerate back to 180 because there is so little relative power available for the condition.
Jets will do this, too. An OAT spike and it falls on its butt. You can ride it out and slowly accelerate as you burn fuel or descend to a lower altitude. But cruise speed is cruise speed. You can dive to it or climb to it--but it always ends up at the same spot when everything stabilizes.
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What jets have you flown? Still fly? I was checked out in the Cessna 152 and Piper Warrior.
Everything except McDD, Boeing and Canadair (although I have plenty of hours in their jump seats).
Flown a few warbirds here and there, as well. Nothing special. A-4, Strikemaster, T-28, PBJ. Gonna try to get a ride with one of my buds who flies a P-51 for a foundation that tours the country every year.
Still fly. Still instruct. Am in Africa right now flying executive jets for high rollers. Been a fun adventure so far. :salute
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"Nothing special" I wish I could say that!
The a-4 is of one of my favorite hairdryers.
Thanks
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Also, I'd like to correct the name of this thread. I've seen "Bock's Car" at Wright Patterson AFB Museum.
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The name written on the nose of the plane is one word "BOCKSCAR." In Sweeney's and Tibetts' biographies, but not in Olivi's, the writers/editors changed it to the correct English possessive form "Bock's Car." I prefer the name written on the airplane.
The interesting question to me is; does a flight path of repeated diving and leveling out, as Sweeney describes, give better fuel economy that a steady descending flight path at constant airspeed? I don't think that it does, but a simulator might prove me wrong.
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"Nothing special" I wish I could say that!
The a-4 is of one of my favorite hairdryers.
Thanks
Great machine. The first three times we rolled it I whacked my head against the canopy. It is eye watering.
:salute
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The big problem at 30k+ up is the jetstream that either help you where you're going, or it's slowing you down.
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The big problem at 30k+ up is the jetstream that either help you where you're going, or it's slowing you down.
That's why LeMay went to 5000 feet with firebombs and burned Japan to ashes.
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That's why LeMay went to 5000 feet with firebombs and burned Japan to ashes.
:airplane: For what ever it is worth to this discussion, this is how we did it! Call it a step or whatever, but when sitting up cruise, we would normally level off about 500 feet above cruise altitude and accerate as we descended back to assigned altitude. Then, as the fuel burned off, lowering our weight, which moved the C.G. aft a bit, we had to retrim and descend again another 3 to 500 feet to get back on "step". This gave us about 4 to 6 knots increase airspeed and didn't hurt the fuel burn. Once we got down to 15,000, it was pretty much 30 X 2,000 RPM back to home plate!
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I have hard to believe that it would increase the range in any significant way. It is just trading altitude for speed. Its a good way to save some fuel if you are at a certain distance from the field were you have to decend anyway but other than that i dont see any benefits from doing it (Unless you can catch better winds at a lower altitude of course).
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:airplane: For what ever it is worth to this discussion, this is how we did it! Call it a step or whatever, but when sitting up cruise, we would normally level off about 500 feet above cruise altitude and accerate as we descended back to assigned altitude. Then, as the fuel burned off, lowering our weight, which moved the C.G. aft a bit, we had to retrim and descend again another 3 to 500 feet to get back on "step". This gave us about 4 to 6 knots increase airspeed and didn't hurt the fuel burn. Once we got down to 15,000, it was pretty much 30 X 2,000 RPM back to home plate!
I think you have a very good point. Maj. Sweeney is just describing re-trimming for the proper descent rate as the weight decreases. My computer program shows that at 30,000ft and a glide angle of -0.012 rads, the speed should increase by 1.4 mph as the weight decreases from 85,000 to 84,500 lbs.
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:airplane: For what ever it is worth to this discussion, this is how we did it! Call it a step or whatever, but when sitting up cruise, we would normally level off about 500 feet above cruise altitude and accerate as we descended back to assigned altitude. Then, as the fuel burned off, lowering our weight, which moved the C.G. aft a bit, we had to retrim and descend again another 3 to 500 feet to get back on "step". This gave us about 4 to 6 knots increase airspeed and didn't hurt the fuel burn. Once we got down to 15,000, it was pretty much 30 X 2,000 RPM back to home plate!
That's not a step.
There is no step.
For a given power setting there is only one cruise speed. You can dive down to it or climb up to it. You will always stabilize at the exact same cruise speed for a given configuration.
Even B-29 instructors who taught this method testify to the fact that the stabilized cruise speed always wound up the same.
There is no magic way to gain knots in level flight.
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I have hard to believe that it would increase the range in any significant way. It is just trading altitude for speed. Its a good way to save some fuel if you are at a certain distance from the field were you have to decend anyway but other than that i dont see any benefits from doing it (Unless you can catch better winds at a lower altitude of course).
Exactly. And you are losing True Airpseed at a rate of two percent per thousand feet (standard day).
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I think you have a very good point. Maj. Sweeney is just describing re-trimming for the proper descent rate as the weight decreases. My computer program shows that at 30,000ft and a glide angle of -0.012 rads, the speed should increase by 1.4 mph as the weight decreases from 85,000 to 84,500 lbs.
Which is not a step.
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Unfortunately, “getting on the step,” seems to have two distinct aspects. The first is a process or flight path maneuver to get to the best-range cruise condition. If the plane has only modest extra power for acceleration, it avoids the long time-constant of level-flight acceleration to best cruise speed. The second aspect is the idea that the cruise state reached by this maneuver, the final “step” airspeed, is extra fast; a hysteresis situation that requires a certain approach path. Many (most?) pilots don’t believe that the final speed is anything unusual.
The 1945 Boeing POH recommends climbing 500 ft. above the intended cruise altitude for each 10,000 ft. of cruise altitude. They call the best-range speed ‘the step.” It must have been the common term at that time. They never claim that it is extra fast.
In Sweeney’s description he is repeating the maneuver process again and again. So the question is not; is the final speed is extra fast? But, is the average speed of the maneuver faster? Is it faster than what? I think that it should be compared to a flight path with a constant descent rate at the best-range speed. The descent rate allows the power and fuel flow to be reduced. As Earl1947 points out, as the weight decreases you must re-trim if you keep the same power, and the speed will increase. Is this what Sweeney was doing, or was the descent and flare more emphatic?
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Unfortunately, “getting on the step,” seems to have two distinct aspects. The first is a process or flight path maneuver to get to the best-range cruise condition. If the plane has only modest extra power for acceleration, it avoids the long time-constant of level-flight acceleration to best cruise speed. The second aspect is the idea that the cruise state reached by this maneuver, the final “step” airspeed, is extra fast; a hysteresis situation that requires a certain approach path. Many (most?) pilots don’t believe that the final speed is anything unusual.
The 1945 Boeing POH recommends climbing 500 ft. above the intended cruise altitude for each 10,000 ft. of cruise altitude. They call the best-range speed ‘the step.” It must have been the common term at that time. They never claim that it is extra fast.
In Sweeney’s description he is repeating the maneuver process again and again. So the question is not; is the final speed is extra fast? But, is the average speed of the maneuver faster? Is it faster than what? I think that it should be compared to a flight path with a constant descent rate at the best-range speed. The descent rate allows the power and fuel flow to be reduced. As Earl1947 points out, as the weight decreases you must re-trim if you keep the same power, and the speed will increase. Is this what Sweeney was doing, or was the descent and flare more emphatic?
Climb to your cruise altitude. Start a watch. Wait ten minutes. Note your speed.
Climb above your cruise altitude. Descend to that altitude. Start a watch. Wait ten minutes. Note your speed.
They will be equal. You can accelerate up to it or decelerate back to it.
The simple fact is the extra time spent climbing through your cruise alt and mushing along at a higher altitude with even less excess power will more than offset any time saved a accelerating in a descent to the cruise altitude.
There is no step.
The step is a mythological technique that gives one extra knots for the same power setting in cruise. It is an impossibility.
B-29 instructors have said it made no difference despite it being "official" technique. They taught it that way only because they were under orders to. It made no difference.
As for trimming, that's a given. What's so magical about that? Nothing. If you move the CG aft you decrease downforce from/bythe tail. That will gain some performance at the expense of stability. If there were such a thing as a step due to aft CG the decreased stability would offset the supposed gain. In any case, one bump and you have to repeat the whole process.
I burn my tanks from the front to push the CG aft. It helps a tiny bit and when I am over the Atlantic I want to save every pound and gain every advantage. But this is not the fictitious step.
Columbo and Garrison made the point that the area of reverse command and cruise can be close together. There are two speeds for every power setting, but usually they are dozens and dozens of knots apart. Even in a Cessna 152 they start 70+ knots apart. Modern jets can have a spread of 100+ knots at 41,000 feet depending on wing design and weight. If they're only ten knots apart in a B-29 or a B-24 one could get the false impression of a magical step.
Regarding descents... There is an optimal place to begin a descent. Wait too long you waste gas for staying in cruise thrust when you could have reduced. Descend too early you waste gas for various reasons including increased burn and loss of True Airspeed. For every 1000' of altitude you descend you lose two percent of your True Airspeed. If a descent is flown incorrectly your fuel mileage and range suffer.
Would a B-29 be slick enough to gain some advantage by accelerating in a descent then leveling off...decelerating...then repeating the process? Would a constant rate descent have the same result? I know what my gut tells me. :salute