When you picture an airplane, a pretty familiar shape comes to mind.
A long narrow body with two main wings sticking out from the middle, a pair of smaller wings
on the back end, and a tail fin in the rear sticking straight up.
The wings may be straight or swept to the back, but you wouldn't picture them sweeping
forward, would you?
Sure, some fighter jets have wings that change position, and older planes might have an extra
pair in the front, but you wouldn't have a backward wing.
Well, tell that to the X-29.
This experimental aircraft was developed by NASA and the US Air Force back in 1984.
That’s right, this strange little plane isn’t a left-over Star Wars prop, but an
actual prototype developed by the Grumman Aerospace Corporation for the US military
and space program.
Only two were ever built, but the X-29 wasn’t the first, or only, plane to look like someone
misread the instruction on their Lego set.
The concept of forward-swept wings dates all the way back to the JU-287, developed in the
1940s, and many countries have experimented with this format over the years.
The flaw that kept it from mainstream use after decades of trying is that a plane with
forward-swept wings can be a real nightmare for the person flying it.
This was such a problem that the X-29 needed to be made from state of the art composite
materials just to somewhat mitigate that problem.
I mean, just look at this thing.
I'm surprised anyone was able to fly it at all.
With such an obvious design flaw, why have so many people tried for so long to make it
work?
Well, before I can really answer that, we need to understand what the designers were
trying to achieve in the first place.
Look at any plane developed before the advent of jet engines, and you’ll notice that almost
all of them position their wings at a right angle to the body of the aircraft.
This works great for any aircraft flying less than 460 mph, but causes trouble for anything
moving faster than that.
You see, an aircraft's wings are shaped so that there’s higher air pressure on the
underside than above.
This is one of the mechanisms that contribute to a plane's ability to produce lift.
As a side effect of this phenomenon, the air moves across the wing faster than the plane
itself moves through the air.
Build up enough speed, and the top of the wings can break the sound barrier even if
the aircraft as a whole wasn't designed to.
This creates a shockwave on the top of the wing, which disturbs the air behind it.
This reduces the amount of lift the forward wings can generate, while increasing the lift
produced by the rear wings.
As a result, the plane’s nose is forced down, causing it to lose altitude and gain
additional speed, making the problem even worse.
When this happens, even experienced test pilots have been unable to regain control.
The first aircraft to experience this problem was the excitingly named and strange-looking
P-38 Lightning.
Despite, or maybe because of, looking like some engineers got bored and bolted three
planes together, it was one of the fastest aircraft in the world when it debuted in 1939.
To compensate for the plane's wings occasionally going supersonic, engineers installed a unique
flap on the underside of the wings.
Known as a dive flap, this device could be deployed during a dive to generate additional
lift underneath the front wings.
The P-38 only encountered this particular problem when flying at maximum speed, but
as planes started moving faster and faster, a more permanent solution needed to be found.
Enter the swept wing.
Unlike straight wings, where one hundred percent of the airflow moves parallel to the body
of the plane, swept wings create a stream of air running lengthwise down the wing.
This is known to aircraft designers as spanwise flow, and unlike the air moving across the
top of the wings, it doesn’t accelerate beyond the plane's current speed.
Incidentally, the air moving across the top of the wing is called the chordwise flow,
named after the chord, an imaginary line that goes from the front to the rear edge of the
wings.
It’s not, as I expected, the direction your phone’s charging cable would fly if you
hurdled it out a window in frustration.
Yeah, I’ve been there….
Even if you manage to hold back any battery related tantrums, it can be pretty easy to
lose something on a flight.
Why don't you tell me in the comments what was the worst thing you ever lost on a flight?
It doesn't even need to have been on a plane.
Drop your phone while taking a selfie in a hot air balloon?
I definitely want to hear about that.
Now, where was I?
Oh yeah.
At low speeds, aircraft want as much chord flow as possible to maximize lift.
But as they accelerate closer and closer to the speed of sound, a little over 767 mph
if you're keeping score, the spanwise flow becomes more and more desirable.
Some military jets, such as the American F-14 Tomcat, and Russian MiG-23, can change the
angle of their wings to take advantage of this effect.
Other air and spacecraft, such as the Avro Vulcan and NASA’s Space Shuttle, use triangular
delta wings instead to solve the same problem.
These are more reliable, but come at the cost of adding drag.
The issue that has yet to be resolved is that the further back the wings of a plane are
swept, the more susceptible it is to a stall.
When pilots use the word stall, they aren’t using it in the same way you would if your
car’s engine seizes up.
In aviation, a stall is when the amount of air moving across the wing is less than needed
to produce the necessary amount of lift.
Since the entire point of swept wings is to reduce the amount of airflow, you can see
why this might become a problem.
A stall can result in the pilot losing control of the plane.
This brings us to the reason so many people have tried so hard and for so long to make
forward-swept wings a viable option.
In rear-swept wings, the spanwise flow carries air toward the tip of the wings, away from
the body of the plane.
As a result, when a stall occurs, it begins at the tip of the wings and moves inward,
making it difficult for the pilot to maintain control.
However, if the wings were shaped in a way that carried air toward the fuselage, the
stall would begin closer to the body of the plane.
As a result, air would still be moving across the flaps positioned further down the wing,
allowing the pilot to remain in control.
This wouldn’t eliminate the risk presented by a stall, but it would still go a long way
to improving aircraft safety.
Think of forward-swept wings as training wheels on a bicycle.
They aren’t guaranteed to keep you from falling, but still make it easier to stay
in control if you start to get off balance.
Where that analogy breaks down is when we run into the stability issues I mentioned
way back at the beginning of this video.
While forward-swept wings reduce drag quite significantly, their unusual aerodynamic properties
subject them to a twisting motion as they move through the air.
It wasn't until the 1980s that aircraft technology advanced to the point that we could construct
wings able to endure this kind of pressure reliably.
This brings us back to the X-29, the shining pinnacle of early eighties aircraft technology.
Using state-of-the-art materials designed by NASA, and the most sophisticated flight
computers money could buy, the Air Force hoped they might finally have cracked the code to
stable forward-swept wings.
And they were entirely right, which is why all planes today use forward-swept wings.
Did that work?
I’m trying to see if believing hard enough will make it true.
In the Air Force's defense, they came closer than anyone else to producing a workable version
of this concept.
The X-29's flight computer made flying possible, but just barely.
Even with the three onboard computers making constant adjustments to the plane's handling,
test pilots still struggled to keep the aircraft under control.
Making matters worse, shortly before a test flight, one of the prototypes malfunctioned,
causing all three computers to fail simultaneously.
Oops.
If that’d happened while in the air, there's little the pilot would’ve been able to do
to keep their plane flying.
Ultimately, the X-29 program came to an end in 1991.
While NASA and the US Air Force learned a great deal from the decade long cycle of tests
and tweaks, a practical design remained just out of reach.
Still, not everyone has given up on the concept, and the age of forward sweeping aircraft could
be just around the corner.
Hey maybe they should just, you know, wing it!
Ha.
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