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- Опубліковано 26 тра 2024
- Continuing in our series on experimental testbed aircraft, today we are taking a look at a popular 1980s concept, a forward swept wing design known as the X-29.
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Love this channel. This episode kinda reminds me of that Discovery Wings program from back in the day.
That program becoming its own channel in the early 2000s was the best thing Discovery ever did
Yes! Totally agree, I have that series on DVD. Might pull them out and give them a watch.
*This is the first UA-cam video I have seen anyone do on the X-29 where you actually describe the aerodynamics correctly.*
I'm Australian but was lucky enough to do a degree in Aerospace Engineering in America courtesy of a sports scholarship in the late 80s. During my final year we had a series of guest lectures usually from alumni who were doing this in industry. I distinctly remember 2 of those lectures. One was on terraforming mars and the other was from an aerodynamicist who worked on the X-29.
So I have heard directly from one of the guys who did the aerodynamics of this plane and everything you have said here about the positive feedback of the wingtips is 100% correct. Also my professors area was aircraft structures and he specialised in 2 things finite element analysis of axisymmetric structures and aeroelasticity. Unfortunately he didn't get to teach the class in aeroelasticity because there were never enough students signing up for it but he did explain the basics of it to me. I actually spent a summer working for him doing FEA on the APDS for the Abrams M1, but that's another story.
*Hers's a slightly more detailed description of wing divergence and flutter.*
Both wing divergence and flutter are structural effects caused by aerodynamic forces on the wing, hence the term aeroelasticity. They come from the basic fact that pressure is a force and the flow of air causes differential pressure across the surface of the wing. The first effect of that force is to bend the wing upwards. That's easily noticeable if you get a window seat on a commercial jet where you can see the wing. On the ground the wing tip will usually droop a little towards the ground but in the air the wing bends upwards. The second effect is less obvious but its a twisting effect on the wing because of the variations of the pressure across the wing.
If you go fast enough those aerodynamic loads can bend or twist the wing far enough that it just breaks. That is called wing divergence because the wing simply diverges from the rest of the plane. For anyone who's a pilot that's why there's a Yellow mark on your airspeed indicator for Vne (Velocity Never Exceed). Yes the wings wont come off at that speed as there's a margin of error, but if you ca past that point and the wings stay on the plane has to go to the mechanics to find out what you damage you did do.
The other effect what we call flutter is more complex and the videos below will show why it has that name. As the wing twists it also changes the angle of attack. In some wings that gets to a point where the wing starts to stall at which point in dumps the lift and untwists. It can then load up and twist again until it stalls again. Its a cyclic effect that can under certain conditions activate one of the wings dynamic characteristics (natural frequencies). There's a few good videos here on UA-cam that show this. 2 of my favorites are:
"How to break a glider´s wing" on the YT channel diegocodagnone
"Aeroelastic Experiments - Very High Aspect Ratio Wing" on the YT channel Dani Levin
About 2 minutes into the second one you can see both the wing tip twisting and the wing flexing.
If you have a look through this video on the X-29 you will see that it has an incredibly thin wing. The trailing edge flaps and ailerons on the wing had to be powered and if you look carefully you'll see there's small pods under the outer wing. Those pods are the aileron actuators. the aerodynamicist who gave us that lecture explained they had to be in pods because the wing was so thin they could NOT be inside the wing. They also had to be powerful and very fast to counter the aerodynamic effects.
LASTING EFFECTS of the X-29
Another thing not brought up in much detail here is that the triple redundant dynamic control system has had a massive effect on aircraft systems since. This was the first time an aircraft of any sort had a digital system where the stability of the aircraft was fully controlled by the computers. All other aircraft up to this point had their stability either designed into the airframe or was done by analog computers. This has had lasting effects on fighter design and is most notable in planes like the F-22. There's a great video on the F-22 flight control system titled "Special Lecture: F-22 Flight Controls" on the YT channel MIT OpenCourseWare, *which is totally worth the time to listen to.* Also on the YT Channel Ward Carroll he interviews an F-22 pilot in the video titled "Deep Intel on the F-22 Raptor" which if you watch AFTER the other video will make a lot more sense with some of what's said.
That all said, if it makes you wonder why the Boeing Max-8 MCAS system was NOT even double redundant let alone triple redundant. The only answer I have there is to take your pick of stupidity, insanity, greed or a combination of all 3.
I forgot to add a thing that's hinted at in this video that was also described by that X-29 engineer.
As a normal wing approached supersonic speed the air over the wing can accelerate to the point where its supersonic. This is usually at around 1/3rd of the wing chord where its thickest. That leads to what is called a strong shock (see below). For the X-29 that shock appeared a lot further back on the wing and was a weak shock.
If you have ever seep the drag vs speed curve for something like a fighter jet there's a significant hump at transonic speeds followed by a significant dip before drag again starts to rapidly climb. That hump is because of these strong shocks.
Because the X-29 forms its transonic shocks further back and because they are weak shocks with less drag it crosses the transonic region much easier than other aircraft. That sort of thing eventually lead to planes like the F-22 that could go to supersonic speed without the massive fuel guzzling of an after-burner.
The simplest explanation of strong and weak shocks is that the air flow after a strong shock is subsonic while the air flow behind a weak shock is still supersonic. Consider if you have a Mach 2 airflow that hits an object causing a strong shock. In an instant you are taking more than 50% of the speed away and that's over 75% of the kinetic energy. Remember Ke is a squared law and twice the speed is 4 times the energy and that flips going the other way. Now consider that if you only reduce the air speed to Mach 1.5 (from Mach 2) that shock is removing a lot less energy. If you then have another shock after that that gets the air back to subsonic then that 2 has a lot less energy removal.
If you go and look at the nose spikes of either the Lockheed D-21 or Lockheed X-7 on Wikipedia you'll see they have noticeable curves or steps. That's so they create a series of weak shocks instead of a strong shock. The downside to such inlets is they are a bit like 2-stroke motor cycle engines. They have a very narrow power band and within that band they are great but outside that band they suck. That's part of why ramjets never got much use despite being very efficient at their design speed.
Countering that go look at the Concorde inlet. It has a series of adjustable ramps to create a series of weak shocks. That gave it better performance over a wider speed range.
Thanks Tony and thanks for your expanded description as well - simply awesome. We didn't want to get too technical with the physics around aeroelastic divergence and anisotropic elastic coupling as that's not the nature of our summary videos, and we certainly can't claim to be aeronautical engineers! But thanks for your input - it's constructive comments/input like yours that makes UA-cam satisfying and inspires us to keep going!
@@raafdocumentaries Yeah you really don't want to get into the issues of aeroelasticity. *I got that warning 37 years ago from my professor.*
It one of those areas that's incredibly important to all aircraft because of needing to know Vne, but its so damn hard to get into and get experience and get experience linking the aerodynamics to the structural dynamics that its near impossible to be active in.
Wing divergence isn't too hard to get to because its a fairly straight forward "how much will break the wings off" sort of issue. But flutter means getting into the dynamic modes of the wing and that's serious work at the best of times.
Anyway thanks for the reply. I have started back tracking through some of you other videos.
@@raafdocumentaries I'll give you something to think about.
*AND I AM WILLING TO DO AN INTERVIEW ON THIS*
To graduate we had to do a couple of high level aerospace classes. These are usually run in parallel with postgraduate classes with exactly the same classwork except the postgrads do term papers for their extra credit. Most students do Finite Elements because its fairly straight forward and orbital mechanics because that's what the comms satellites people want.
A few rare (unwise and possibly stupid) people like me do something like space craft dynamics because it sounded "cool." It was a class where you learned the math of space craft orientation which sounds neat but is in fact insanely hard. All space craft have their own local X,Y,Z,y,p,r coordinate system but are actually flying in a spherical coordinate system. So it involves not only solving the differential equations but translating them into the other coordinate system. PLUS it involves gyroscopes so nothing is straight forward as there's coupling between the axis.
Quiet simply its the HARDEST math in engineering across all fields. Sorry to any other opinions it just is the hardest. Your solving (as in integrating) multidimensional simultaneous differential equations while doing a coordinate system translation. Try saying that.
After the first week I tried to get out of it (and wasn't the first) but the professor kept the 4 or 5 of us undergrads trapped along with the 7 or 8 post grads.
In that class however was the single smartest engineer for solving complex math I have ever met. To do what we did in that course we were all well above average IQ but this guy made us all feel dumb. He was not at the next level but several levels above us.
it was 1987 so almost all of the postgrads were sponsored by DARPA and working on Star Wars this guy was hyper-funded because he could do the math to make a satellite point with next level precision. One of the major issues with pointing in space is structural vibration caused by inertia as you start and stop orientation manoeuvring. That's no issue with radio signals but is utterly useless when your trying to zap something doing kilometers per second on another orbit or ballistic trajectory.
So this guy had the idea of putting in small opposite moves to cancel out the inertial and vibrations. So he was taking the hardest math in engineering and making it harder by adding into it structural dynamics. He actually made it work for the satellite in 2 dimensions, but the math for more dimensions as well as mapping into that problem the trajectory of the target was even beyond him at that time.
Then of course the cold war ended and Reagan's Star Wars with it.
So I know as only a VERY FEW PEOPEL EVER HAVE of why Star Wars failed to ever make space based lasers work.
Yeah I know there are people who are now saying they can put a laser in LEO and start a fire.
*BULLSHlT* I have been in class with the only guy who ever got close to solving the math.
In layman's terms there's an easy way to see the main issue.
Imagine you are in a car doing 27kmh and you are asked to reach out the window and shoot a stationary target 200m away with sub-millimeter precision. Now considering that precision Olympic class gold medalists are only just that kind of accurate at 50m its a hard task let alone adding the fact your moving.
NOW ADD 1 zero to each of those numbers so that is 270kmh and 2km (2,000m).
NOW ADD 2 MORE zeros to each of those numbers so that its 27,000 kmh and 200km BECAUSE that's what you need to start a fire from space with a laser.
NOW change that 200km to 2,000km and make that target a FAST moving target and then you'll really begin to get your head around the problem of shotting a ballistic missile with an orbital laser.
But then there's what actually killed Reagan's Star Wars lasers off.
Somebody asked what would happen if the Russians simply polished their warheads so they were nice and shiny, like mirror finish shiny!
Just yesterday I saw Scott Galloway interview and author David Ignatius who wrote a fictional story about space based weapons and he acted like he knew what he was talking about and I can guarantee GUARANTEE that David Ignatius has not got the first clue about space based warfare. NOT EVEN a tiny fraction of 1%.
But then not many people do because they were not in the position I was, in the late 80s.
If you want to do something let me know.
@@tonywilson4713 I'd say things are a bit easier these days with the maturing of computer modeling but I guess (not being in the game) that physical experimentation is extremely limited (unless you are NASA/DARPA). Thanks for following us!!
Excellent review of an almost forgotten and very successful flight test program. Thanks for posting and please keep up the good work!
I enjoying this channel. That's a really good information provided and very accurate
Fun fact: the Grumman X-29 first appeared in fiction via Kaoru Shintani's manga Area 88, flown by the main character Shin Kazama (originally an airline pilot student who was forced into mercenary service with the fictional Kingdom of Aslan)
Wrapping around the fuselage with Thai flag ribbon.
You mentioned that the base plane was the F-5 Freedom Fighter to convert to the X-29, yet those have two engines and the X-29 has one.
I thought the main plane was the F-20 Tiger Shark? Maybe I'm wrong?
Your right about the engines, but there's an explanation.
If you look on Wikipedia it says _"The X-29 design made use of the forward fuselage and nose landing gear from two existing F-5A Freedom Fighter airframes"_
HOWEVER if you scroll down tot he airframe details you'll see that the single engine is the General Electric F404-GE-400 afterburning turbofan. If you click the link for that engine and scroll down you'll find the F20 also used a single engine from the same type except is was a F404-GE-100 not the 400 variant in the X-29.
I also have a fairly lengthy comment on this video about flutter and wing divergence. In my last year of my degree we had a guest lecture on the X-29 from an aerodynamicist on the X-29 team.
It's a shame really that SR 10 never took off. It's whole point was that it was incredibly cheap (the cheapest training jet plane). It's plane price was to be around 1 million dollars, and it's flight hour cost at around 2k dollars which is incredibly cheap if you compare it to literally anything else... I don't think you can find a cheaper flight training jet plane, and it was unique and cool looking too.
2:53 i need someone to look at me the way she looks at SFW F16...
Great into music, where did you get it please?
All our music is licensed from epidemicsound.com
X-7 or X-13