Why Fighter Jets Can Be Too Unstable

this episode of real engineering is brought to you by my new moleskin style graph paper notebook on sale now at the link below flying on a plane through turbulence can be a bit of a nerve-wracking experience for some hearing that announcement bell ring followed by the pilot calmly saying that you are approaching rough air you fasten your seat belt the plane starts shaking and to those who have a fear of flying this can be terrifying knowing that the plane is specifically designed to deal with these disturbances without any input from the pilot may or may not ease your fears but that is exactly what they do every passenger plane is designed with something called static stability static stability essentially means that an aircraft left to its own devices flying in a straight level flight will return to straight level flight even when it is knocked off course this makes them much easier and safer to fly without having to constantly adjust the control surfaces to balance the plane yet the exact opposite is true for fighter aircraft any aircraft designed for air-to-air battles are designed to be capable of out maneuvering their opponent and one of the factors that affects a plane's ease of movement is how stable it is or in other words how unstable it is and thus how ready it is to deviate from a straight and level flight with minimal force I noticed with my recent video detailing the physics behind the forward swept wing that there is a general misconception floating around the internet surrounding the notion of instability in fighter aircraft in my description of the x-29 I mentioned that the plane was too unstable which was meshed by a mountain of comments saying that this was a good thing the more unstable the aircraft is the more maneuverable it is surely that is a hot take from people who have not studied stability and control of aircraft in depth so let's see why this is a nonsensical approach to aircraft design let's think about this with a simple analogy here we have two situations a ball placed on top of a hill and a ball placed in a valley if we push the ball on top of the hill even a tiny bit it will begin to accelerate down the hill and will not stop until we put energy in to slow it down and we will need to put even more energy into it to the top of the hill this is an unstable system the opposite is true for the ball in the valley apply a force and the ball will roll uphill and gravity will now provide a restoring force to bring it back it may all eight back and forth a few times before coming to a stop but it will eventually return to its original position this is a stable system so how does this apply to planes planes have three rotational degrees of freedom pitch roll and yaw let's first approach this problem with regards to pitch stability which is where the x-29 was extremely unstable small general aviation planes like Cessnas are designed to be very stable and so return to level flight automatically after they are knocked up or down by a gust of wind but how do they manage this pitch stability is determined by three primary factors the location of the center of gravity the location and design of the wing and the location and design of the horizontal stabilizer the center of gravity for our cessna is located about here this is the point which all lift will act around it's like the fulcrum on a seesaw our wing is located slightly behind us and thus the lift it generates is slightly aft of the center of gravity this would force the plane to pitch downwards without a counter acting downwards force further back on the plane which is exactly what the horizontal stabilizer provides a downwards force this force does not need to be the same magnitude as it has a greater control authority as a result of its greater distance from the center of gravity once again just like a seesaw this is how a plane maintains pitch stability without any outside influences but what happens if turbulence knocks our plane off balance if the force has remained the same the plane would continue on in whatever orientation the cost knocked it into this is not what happens just like a ball in a valley example we have a restoring force to bring the plane back to its original position this is a result of how our horizontal stabilizers down force changes with the pitch of the plane if there are two primary factors that influence this the first is downwash when air passes over the wings it is deflected downwards this creates a Down of air behind the wing this downwash strikes the top of the horizontal stabilizer and this produces a downward pressure the magnitude of the downforce on this surface is dependent on downwash and the magnitude of the downwash is dependent on the speed of the aircraft the faster we go the more air is deflected downwards the slower we go the less air is deflected luckily our speed is also dependent on our pitch if the plane pitches upwards it will lose air speed and the downforce on the horizontal stabilizer decreases as a result the weight of the plane acting through the center of gravity forward of the center of lift now wants to move the nose down again the opposite happens when we pitched the nose down here we gain airspeed and the downwash on the horizontal stabilizer increases causing the downforce to increase forcing the plane to nose up again this explanation is often provided as a complete explanation but it falls apart when you consider a t-tail configured plane where the horizontal stabilizer is lifted out of the downstream airflow of the wing here our other factor comes into play as a result of the angle of attack on the horizontal stabilizer here the horizontal stabilizer has a negative angle of attack this angle of attack changes as the plane pitches up or down if we pitch it up the angle of attack decreases and thus the downforce decreases allowing the weight of the nose to pull it back down if the plane pitches down the angle of attack increases and increases the downforce which forces the tail of the plane back down this is an elegant solution to the problem which is thrown out the window for planes like the X 29 here we have the forces acting upon the X 29 in the long eternal plane the center of gravity and the center of lift have shifted backwards as a result of the Ford swept design and thus in order to stabilize the plane the canards need to produce lift ahead of the center of gravity this is fine and level flight with no disturbances but what happens if we pitch upwards here there is no downwash that shifts to increase downforce on the stabilizer the angle of attack on the canards also increases with increased pitch which increases the lift forward of the center of gravity which pitch the nose of even more and thus for a very small energy input we could pitch the plane a tremendous amount just like giving that ball on the hill a little nudge this is great when you want to pitch the plane wildly but that's not always the case to achieve level flight the x-29 control computers had to be constantly adjusting to compensate for little disturbances up to 40 times a second this isn't a huge deal especially with today's computers that is not why the x29 was too unstable let's take it back to our ball and hill analogy and think about this as an energy problem if we push the ball and it begins to fall the steepness of the hill will determine not only how quickly it deviates away from its original position which is our analogous for maneuverability but it also affects how much energy we have to put in to roll it back up the hill to return it to its original position this is a problem because our original position is a straight and level flight and we are going to want to return to it at some point so if we make this hill too steep we have to apply an excessive amount of force to get back to our original position exactly the problem we are trying to solve with introducing instability where we have to apply energy to push the ball up the valley walls we introduce instability to reduce the energy and time required to maneuver not increase it and in a worst-case scenario we won't have the energy required to return to a straight and level flight and end up in an unrecoverable situation in an air-to-air battle energy isn't just a fuel burning problem it's a speed problem our energy source for maneuvering is our kinetic energy our speed to change our orientation we have to extend control surfaces into the free stream which creates drag which saps our speed fighter pilots have a saying speed is life speed and maneuverability is what wins a dogfight in reality we want to achieve something between a nice level field where the energy to shift the ball is the same in all directions and the ball on the hill scenario where we don't have to apply a huge amount of energy to get the ball to move visualizing that with a plane would look something like this this line would be statically stable where the plane naturally wants to return to its level flight this is statically neutral and this is statically unstable the f16 was the first plane to enter white service that was deliberately designed to be unstable departing from many of the design principles that influence planes like the f4 phantom it's older brother the f4 was found to have role instability during Windtunnel testing so the engineers added a 12 degree dihedral to the wing tips to increase its role stability and the f4 horizontal stabilizer generate downforce in a similar way to our example earlier creating a long incidentally stable plane the f16 in comparison had a noticeably straight wing making it more or less statically neutral in roll like our ball in the flat field the f16 also shifted its center of gravity rearward behind the center of lift and necessitated a horizontal stabilizer that produced lift making it unstable in pitch albeit nowhere near as unstable as the x-29 producing a plane that is unstable enough to allow for energy efficient maneuvering this is a complicated topic with a huge number of variables that I haven't mentioned here for example the center of lift per wing tends to shift forward with an increased angle of attack and with supersonic planes the center of lift shifts as it goes from subsonic to supersonic flight there is a lot more to this problem than you are going to get from a YouTube video and many of my viewers are actually students and practicing engineers who are likely to use this video as inspiration to go and learn more about the subject and to keep track of all the variables you will probably need a notebook while I was studying and working as a research and development engineer I always wanted nice moleskin style notebooks that had graph paper instead of lined pages I could never find ones I liked so I decided to just make my own the only way I could do this without spending an absurd amount of money was to buy in bulk I've already sold about half of them from a community post so if you act quickly you can buy some of these limited availability notebooks for yourself I wanted to keep these a reasonable price in relation to normal moleskin notebooks so the margin on these are small so we may or may not do another printing run but if people like them and we can afford to print a couple of thousand of them off we can look into it and just let me know on Twitter or Instagram which you can find the links to below [Music] you

Channel: Real Engineering

Views: 890,376

Rating: 4.9210372 out of 5

Keywords: engineering, science, technology, education, history, real, aviation, x-29, instability, stability, control, maneuverability, pitch, roll, yaw

Id: h6NsYyAUOHE

Length: 11min 48sec (708 seconds)

Published: Sat Aug 17 2019

Reddit Comments

[u/AgAero]

I say this in response to most of this guy's videos, but I"ll say it again:

RealEngineering is a 'pop sci' channel covering engineering topics. He's an intelligent guy with an engineering degree, but he's not an aeronautical engineer to my knowledge and so he gets things wrong on occasion. Always be a little skeptical of 'informative' youtubers!

In this video for example, at about 4:45 he overstates the effect of downwash massively IMO. The argument he should be making is about how control effectiveness of the H-tail and elevator is affected primarily by changes in the AOA(mentioned) and the airspeed. Downash is an important consideration, but it is not a primary contributer to the aircraft's stability.

His arguments about what make the X-29 unstable would apply to any canard configuration aircraft, including those that are not actually unstable 'super maneuverable' fighters. There's something missing here... The location of the CG is the primary contributer to longitudinal stability. Statically stable canard planes like the Rutan Long EZ solve this problem by shifting their CG just aft of the wing's center of lift so that the canard pushes down.

Also, I wish he'd talked a bit about the Cooper Harper scale and the pilot effort needed to control the aircraft, but that's personal preference on my part.

[u/Lebrunski]

Neat video. Wished he also included quasi stability (like being in a small valley but once over that small hill up, there’s a bigger valley down another hill. Kind of like a local minimum vs a global minimum.)

Also, I highly recommend National Brand gridded computational notebooks.

[u/_Mountain_Water]

Because they’re can’t afford therapy. /s

[u/Matt17908992]

Stability is inversely proportional to maneuverability..I think. So fighter jets need to be very maneuverable but are not very stable while airlines need to be more stable and thus less maneuverable.