[Music]
>>Over 41,000 airports are scattered across our planet, housing more than 23,000 airplanes.
Everyday, everywhere, air traffic controllers keep pilots and passengers safe from not only
collisions, but also wake encounters, which can be just as dangerous.
[Airplane flying]
A wake is created whenever a vehicle passes through a medium.
A common example are the waves created by a boat moving across a body of water.
Aircraft wakes are a bit different.
>>When a vehicle is flying, it generates high pressure underneath and low pressure on top.
So the high pressure wants to go to the low pressure, and so behind an aircraft, you have
these counter-rotating vortices.
There is this invisible wake with this upwash portion on the outboard part of that counter-rotating,
and then a downward on the inside.
Just think of them as horizontal tornadoes.
>>Wake vortices are invisible and can continue for several minutes before dissipating, so
air traffic controllers must keep several miles' distance between airplanes.
This can result in inefficient flight paths and delays, especially if portions of airspace
are shut down due to things like volcanic ash plumes or severe thunderstorms.
[Music]
Engineers at NASA's Armstrong Flight Research Center are testing new technologies and methods
to help aircraft identify where wakes are, and avoid them- or even ride them.
[Music/cockpit ambience]
>>NASA's been interested in improving the efficiency of flight from its initial inception.
Some conventional, and some unconventional; and this is really one of the unconventional
ways.
It's really biologically inspired.
If you've ever seen a flock of geese, the reason they fly in a 'v' is that the trailing
birds are really gaining efficiency from the lead bird by flying in the upwash caused by
vortices generated by the wing in flight.
Some of the early formation flights were military, in the early 1900s.
In 2000 and 2001, Armstrong flew F-18s in formation, but really close, like within 150
feet or so of one another, and showed efficiency that way.
We've also done it in partnership with the Air Force on large transports, the C-17.
>>In this experiment, it's one of the first times that this has ever been tried with a
civilian airplane.
[Music]
Civilian are typically not quite as structurally robust as military aircraft, and the military
aircraft tend to have their own specialized data links.
One of the the barriers to transitioning wake surfing technology into the commercial sector
is the cost that would be required to equip a lot of different airplanes with a specialized
data link.
Currently the air traffic control system uses ground based radar to track aircraft, and
ground based radar have a fairly slow sweep rate, so you only get a position update from
an aircraft about once every 12 seconds- you can imagine a typical airliner flies quite
a large distance in 12 seconds.
Automatic Dependent Surveillance Broadcast is an improvement upon that, where the airplane
transmits its position and velocity information twice per second and it can be very accurate
if the airplane is equipped with GPS.
So what we're interested in looking at is 'can we use this new data link, ADS-B, that
the FAA has mandated for all airplanes operating in Class A, B, and C airspace by 2020, can
we use that data link for an application such as wake surfing?'
>>We're using the data link ADS-B between the two vehicles, and predicting where that wake
would be, and then flying near that wake to try to gain efficiency.
In doing so, we're really trying to also prove that we can use ADS-B as a way to do coordinated
flight between multiple vehicles; if you wanted to, for instance, avoid the wake, you could
still fly closer than what is currently allowed by the FAA, so that you could have an increase
in aircraft density.
>>What we're doing is looking at taking the next step and rather than trying to avoid
the wake, we're trying to fly in a particular portion of the wake that has an updraft.
The wake consists of two parts- the outboard parts, the air is rising, and in the center
of the wake, the air is falling- so if you fly in the area where the air is rising, it's
like gliders that soar in thermal updrafts, you can extract energy from that.
Flying on the outboard part, the upwash, allows a trailing airplane to re-trim itself at a
lower angle of attack, which reduces its induced drag; the less drag you have, the less engine
thrust that you need, the less engine thrust you need, the less fuel you have to burn,
and of course, the less fuel you burn, the fewer emissions come out the back of the airplane,
so the ultimate objective of flying in the upwash portion of the wake is to reduce the
fuel burn and the emissions of the trailing airplane.
[Music/jet engine revving up]
>>Ok, it's pilot's airplane.
>>Copy that, and let's go ahead and start moving on to Card 9, and give 808 the lead.
>>We would begin first by positioning ourselves about 4,000 feet back, and then to the side
of and above where we estimated where the wake would be.
And then we'd take what's called a tare, which is just a test point, it's vocabulary used
from the wind tunnel, where you take data with the wind off in a wind tunnel, and compare
that to the wind on.
We're gonna compare flight outside the wake and flight inside the wake, and then we would
step in and down to where we estimated where the wake was.
>>As we get closer to the wake, what we're doing is we're looking at the data from the instrumentation
that we have onboard, to try determine if we're getting into the wake's area of influence.
It's difficult to see that in fuel flow until you get deep into the wake, but what you can
start to see, because one area of the trailing airplane is experiencing more upwash than
the other wing, it tends to cause the airplane to roll.
And so our experimental autopilot was designed to trim that out, and so when we looked at
the roll command that our autopilot was having to put in, we could tell that it was starting
to sense where the wake was and starting to have to do some work in order to trim its
effects out.
>>PPA is engaged...
>>TCS released...
>>saw the engage...
roll command is climbing...
we're getting some long-term oscillations, so I think we're close...
>>we're gonna do a 5 foot step-in, to a cross-track
of minus 75...
>>ok...
>>activating new command, 3, 2, 1- activate...
>>we're getting a little bit of buffeting, very slight, can you guys feel that in back?
>>Just a little bit, but yeah.
>>So wake surfing, particularly for commercial aircraft, has a number of barriers to its
adoption.
One of the prime ones is passenger ride quality.
>>You can't see it, but the aircraft can feel it.
>>When you fly in the wake of another airplane, there's the potential that there are some
unsteady aerodynamics that cause some vibration in the wing and that gets transmitted to the
passenger's seat, and they'll feel some vibration, maybe something similar to light turbulence.
And so we have special instrumentation onboard to measure sound levels within the cabin of
the airplane, as well as vibration in the seat rails, so that we can use some metrics
on passenger ride quality to evaluate 'what would the comfort level be? or the degradation
in comfort level be? when you're doing wake surfing.'
So that's one of the concerns that we're trying to answer.
Another is 'what are the effects on the aircraft's structure?'
Typically, airplanes are designed for flying in a continuous airfield, not necessarily
flying in the vortex of another airplane for any long period of time.
It doesn't do any good to implement a technology that saves fuel if it requires the operator
to do more maintenance and to take the airplane out of service more frequently so that they
can inspect it and replace parts and things like that, so we want to make sure that this
technology doesn't wear the airplane out quicker than it would normally.
>>PPA is engaged...
>>TCS released...
>>ok, pilots, this is TC, as I imagine you can feel, we're right on the
wake, so we're going try to sit here as long as we can, hopefully we can do about 5 minutes
here.
>>What we're noticing about this flight, I think, is that the power required to maintain a constant
position when our wing tip is in the wake is less, than is required when we're not in
the wake, which means we're gonna save fuel, that's the idea.
>>For aircraft, that are flying 16-18 hours a day, it's a lot of fuel, even to save 1
or 2 percent.
So that's what we were looking at, was ways that you could improve the efficiency of vehicles by
not changing the vehicles at all, but by changing how we use them, or operational efficiencies.
>>Ok, and point complete...
>>Ok, pilots this is TC, I'm gonna do a 300 step out to minus 400.
[Airplane landing]
[Music]
>>I was even surprised by some of the flight dynamics, and things that we saw in flight.
There really is no substitute for taking an experiment to flight.
You learn things that sometimes you don't want to learn, but you also see the beauty
of the real atmosphere and the challenges that are associated with real vehicles being
flown by real systems and real pilots.
So you can do lots of simulation, and you can do lost of experiments, but you eventually
have to take it to flight, and that was really the focus of this experiment, was, even though
there were lots of predictions that were well sustained, we wanted to take it all the way
to flight to find out the things that we don't know, because you're often surprised by the
things you don't know.
>>So based on the results of these flight tests, we believe that we'll be able to show a significant
amount of drag reduction and also make a determination as to whether the ride quality associated
with that level of drag reduction degrades the passenger comfort level or not.
[Music]