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Blown tails saving fuel

Old 10-22-2014, 11:14 AM
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Default Blown tails saving fuel

This is pretty neat. Anyone who has experience with aircraft design knows that tails are heavily constrained by various takeoff and landing performance based requirements. Cruise is never a problem. VMC-induced limitations and 61 knots takeoff are some examples. So if you can remove these particular issues, the tail can be a lot smaller and more efficient.

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Sweeping air devices for greener planes

(10/21/14, J. Stoller-Conrad, Physdotorg) The large amount of jet fuel required to fly an airplane from point A to point B can have negative impacts on the environment and—as higher fuel costs contribute to rising ticket prices—a traveler's wallet. With funding from NASA and the Boeing Company, engineers from the Division of Engineering and Applied Science at Caltech and their collaborators from the University of Arizona have developed a device that lets planes fly with much smaller tails, reducing the planes' overall size and weight, thus increasing fuel efficiency. On October 8, the researchers—including Emilio Graff, research project manager in aerospace at Caltech and a leader on the project—were presented with a NASA Group Achievement Award "for exceptional achievement executing a full-scale wind-tunnel test, proving the flight feasibility of active flow control." An airplane's tail forms a critical part of the control system that helps steer the plane during flying. During flight, air rushes around the vertical tail with great force and is deflected by the tail's rudder—a moveable flap at the rear of the tail that can steer the plane by angling air to the left or right. By moving the rudder left or right, a pilot can move the air in one direction or the other, helping to keep the plane flying straight during a strong crosswind. During the high speeds of flight, the air flow around the tail is so strong that the rudder can control the plane's path with minimal movement. However, during the lower speeds of takeoff and landing, larger rudder deflections are required to maneuver the plane. And in the case of engine failure in a multiengine airplane, the vertical tail must generate enough force to keep the plane going straight by turning "against" the working engine. Airplane manufacturers deal with this challenge by fitting planes with very large vertical tails that can deflect enough air and generate enough force to control the plane—even at low speeds... Sweeping air devices for greener planes
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Old 10-22-2014, 12:10 PM
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Agree it's neat, though likely another old idea that someone is just getting around to acting on. Airframes/aircraft continue to be refined and some gains are seemingly impressive, even revolutionary. Similar progress has been made in the automotive industry. We need to make much bigger strides like replacing piston engines, as much as I love them. If the space aliens are flying around in their fancy ships, why aren't we? Are we really that stupid? I'm all for new technology though I think, as a people we are really just beginning to come out of the Stone Age.
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Old 10-22-2014, 04:03 PM
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That will work just fine, like Boundary-Layer Control (BLC) worked on F-104s and early F-4s...until they realized when you reduced power on approach, you lost the bleed air for the BLC. Pffft! There goes your liffffftttttttt.......

This probably wouldn't work in a total engine-out scenario, either. There the effectiveness of the empennage would be reduced, and it would be a wobbly-goblin.

There's another way: supersonic fighters use an all-moving horizontal stabilizer ("slab") to get the required pitch authority over a huge CG and speed range.

North American Aviation went one step further with the A-5 Vigilante. It had no rudder.

The whole vertical fin moved, just like a slab.

Lighter, smaller, less wetted area, more authority available when needed. Coupled with a modern FBW system? Easier than a blown-tail.

But, I guess they couldn't take a lesson from an "antique" 1950s jet....
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Old 10-22-2014, 04:26 PM
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I have not looked at this particular study project very closely, but Caltech has some smart researchers and I am sure they have an idea worth investigating. I did my senior design project on a clean sheet design using blown wings, and the idea works pretty well in all but the most demanding failure scenarios, and dual engine failures are generally not considered in aircraft design. Not to be trite, but some things are just too much to consider, like meteor showers and electromagnetic shock waves and so forth. We have to set reasonable design parameters or else nothing would ever be done. This blown tail the way I understand it uses APU air to bolster the boundary layers on TO and landing. There might be some remedy for an APU failure which in this case obviously is more significant than loss of cabin cooling. You could use one of the two remaining engines or there may be some other method that could be used.
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Old 10-23-2014, 07:56 AM
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[/QUOTE] This probably wouldn't work in a total engine-out scenario, either. There the effectiveness of the empennage would be reduced, and it would be a wobbly-goblin.[/QUOTE]

With total engine out, there's no need for any more rudder 'effectiveness' than is already designed in any airplane?


[/QUOTE] The whole vertical fin moved, just like a slab.

Lighter, smaller, less wetted area, more authority available when needed. Coupled with a modern FBW system? Easier than a blown-tail.

But, I guess they couldn't take a lesson from an "antique" 1950s jet....[/QUOTE]

I would really doubt that any manufacturer could modify say a Boeing 777 with an all moving vertical stab that would be lighter and easier than a smaller vertical stab designed with Active Flow Control. The entire tail section would require modifications and huge structural pivot pin beams plus the actuation system required to move it? nope...
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Old 10-23-2014, 08:50 AM
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Technologies like boundary layer control are actually better in my view than conventional aerodynamic technologies on the whole. When I studied this years ago I came away with the view they should be implemented in not only military transport designs as they are occasionally now (C-17), but also in commercial airliner applications for all transport aircraft. In fighters it does not make as much sense because those aircraft tend to consider per mile operational costs beneath other objectives. The question is, can the reliability level be increased enough for blown tails and wings to be implemented on commercial transport aircraft(?). Dual engine failures are almost unheard of, although we do see an occasional event over enormous operating hours. We need to define a statistical occurrence which is acceptable, and if the number is achievable, then we can switch to blown lifting surfaces and reap an enormous bounty in fuel savings, shorter runways, and increased useful loading for any aircraft that use it. Boundary layer technology is the holy grail of aerodynamics and as supporting technologies progress, we should be able to meet the reliability goal. I am glad research is still going on in this area.
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Old 10-23-2014, 06:20 PM
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Doubler:

My premise was, BLC can make the empennage more effective. It artificially inflates tail-volume, and leads to efficiencies with less weight and drag.

However, if that BLC is reduced, or negated, you now have a conventional aircraft with very low tail-volume. That could make it a handful in an emergency due to phugoid oscillations/dutch roll/roll-coupling.

Some (a couple of friends of mine who have flown it) say the MD-11 has this issue for pitch control under normal circumstances. Adequate at cruise speed, but difficult at approach speeds.

An all-moving slab with FBW would probably be no heavier than a conventional Horizontal stab and elevator that has a trimmable jackscrew.

Vertical fin...maybe what you said. The practical side of BLC is how much does the ducting weigh and add to a conventional vertical stab?
And, through FBW, you could build a conventional fin where the rudder is a much bigger percentage of the total surface. Kind of achieving the same thing..don't make all of it move....just more of it.

And, I wasn't suggesting a retrofit to an existing design. It would have to be incorporated from the start.

Lockheed, McDonnell, Douglas, and Dornier all did major BLC tests or made actual production aircraft in the late 50s/early 60s. Some used blown upper surfaces, and at least one tried suction. Maintenance ended up being a major factor in not seeing widespread use. In particular, keeping tiny bleed pinholes free of dust, water, or ice was an unending challenge.

It's an interesting idea, but the pessimist in me says it probably won't work as well in actuality as it does on paper. Swing-wings suffered the same fate, and more recently, I was told the maintenance costs for fixing winglets on the 747-400 (cracks) cost more than the fuel they saved, and the little winglets on the A-320 had no effect at all on fuel efficiency!
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Old 10-23-2014, 07:42 PM
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I do find this subject interesting as well, however it all comes down to cost and practicality. We know all sorts of Gee Whiz stuff can be done in controlled conditions, though it's often not practical or features an Achilles Heal or some other gotcha that is not practical to overcome. So instead we settle/compromise with more traditional/simpler solutions and aerodynamic palliatives. How much more efficient can we make say a Citation X for example [Although far removed from its predecessor, the name that originally got chided about getting bird strikes in the rear] We can keep flogging these things forever I guess. We're still in the Stone Age and for no good reason. We might not need to reinvent the wheel, but we surely can build a much better mousetrap...

Last edited by Yoda2; 10-23-2014 at 07:55 PM.
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Old 10-24-2014, 03:29 AM
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UAL, I agree with you, and yes, it would be cost prohibitive to modify most if not all existing jets with an active flow control vertical since it would also require a whole new, (shorter span/length) vertical anyway in addition to the needed ducting and nozzles. I've studied this a bit and it really isn't that much of a weight penalty in terms of additional hoses, nozzles and ducting since most of the parts would actually be replacing and built into the vert stab's traling edge panels. $ is the main issue and it goes back to the winglet mods, is it worth it to mod the vert of older jets? Agreed also that either the blown tail or full flying vertical stab would have to be part of the new or original design.
Also ironically, new design jets, (non-legacy) are doing away with most of not all bleed air systems to include the APU.
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Old 10-28-2014, 06:27 AM
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I've seen some recent discussion where the BL air supply is not taken from the bleeds, but from the aerodynamic flow stream via ducts in or near the leading edge. This way the BL flow, and thus tail authority, would be proportional to airspeed.

But even with bleed air, you can design it to have enough authority in the needed regimes. Worst case, start the APU for T/O and LDG.

Two engine out is probably not too big a deal, since you don't need much rudder authority at that point, although you do still need the yaw stability.
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