What Makes an Aircraft Stable? | Aerodynamics in X-Plane 11 (3/4)

[@FlyByMax]

8:46 I knew I must have made a mistake somewhere. The real life engineering definition of C_m_alpha is defined as d(T) / d(alpha), not the other way around (although the equation I used at 8:14 is still true). If C_m_alpha would then be equal to zero, then that would mean that for a change in angle of attack, T would not change. This means that the aircraft would be 'neutrally' stable, i.e. not stable or unstable. It's also important to note that I simplified some things for the video to make it digestible and offer an introduction into a world that is often dominated by abstract maths. For example, there is a difference between the pitching moment (or torque) and the dimensionless pitching moment coefficient, C_m. The pitching moment coefficient (C_m) is very similar to the total moment (T in the video), except that it's been made dimensionless. Making things 'dimensionless' sounds complicated (which is why I didn't address it in the video), but is super useful since it allows us to do make reasonable comparisons of flight dynamics regardless of altitude, speed or aircraft size. This is very similar to Lift (L) compared to the Lift coefficient (C_L), with C_L being dimensionless but Lift being expressed in Newtons (N). The complete true-to-life definition of C_m_alpha is then "the derivative of the pitching moment coefficient with respect to angle-of-attack" (imagine using that as a video title lol). But the meaning is again still the same, positive change in alpha -> negative change in moment (or vice versa). One final thing to mention is that at 7:26 I assumed the aerodynamic pitching moment of the wing to be independent of angle of attack. This is only true if you're considering the aerodynamic center of the wing, which is defined as the location where the aerodynamic moment is constant. By using this location, the maths becomes WAY simpler since then you only need to look at the lift of the wing and the tail (which DO change with angle of attack) to calculate C_m_alpha. There is so much more cool stuff that I had to leave out in the video. If you're interested in stability, check out this brilliant paper that I once used to first learn about static stability: ocw.tudelft.nl/wp-content/uploads/Hand-out-Stability_01.pdf

[@ssopkc_venom]

Good video, but i'd like to add that most aircraft have the CoG in front of the wing/lift for better stability, as you've shown when a gust of wind adds more AoA, in reality, the CoG forward of the lift will cause a downward pitching moment, and the H-Stabilizer adds negative lift to keep the aircraft stable in level flight, and thus avoid high AoA and ultimately stalling in high gust situations.

[@tarikporter4168]

The quality of your content is outstanding, keep it up.

[@FlyByMax]

Thank you!

[@Neptunefrog]

Hey i think you deserve hundreds of thousands of subs. As an av-geek and physics enthusiast this channel is so great for me. Please continue what you are doing i really like the videos

[@FlyByMax]

Thanks! Got something coming soon that I think you'll like :)

[@demosteneneacsiu6144]

Brilliant content, as an ATPL student and x-plane player, your videos make the concepts very easy to understand

[@nathanmorgan9807]

C_m_alpha=0 is where the plane will naturally want to go if no inputs are made by the pilots or any external forces. Very similar to taking your hands off the steering wheel of a car and watching it rotate itself as the tires find their own way to strait.

[@PilotStudd]

Wow! One of the best videos I have ever seem produced on a flight simulator. Truly impressive and a great help to me explaining the basics of flight in a truly visual format, helps with my PPL exam(s) revision, very good job!

[@FlyByMax]

Thank you, means a lot!

[@simulationflow603]

this is amazing.. with each video the quality gets better . KEEP IT UP!

[@himneeshchowdhary820]

9:09 if Cma is equal to zero, the conter-force will neither will be a pitch-up force nor a pitch-down force. so we can say the plane becomes too stable to do anything at the first place. @FlyByMax

[@FlyByMax]

You're absolutely right about the counter force neither being a pitch up or a pitch down! I made a small mistake in the video, Cma is actually equal to Delta T / Delta alpha, so if Cma is zero then it means that T will not increase or decrease for a change in alpha (exactly as you got right). This doesn't make the aircraft too stable though, it actually makes it neutrally stable. An aircraft that is too stable would have an infinite (negative) Cma, since then even a small alpha would cause a huge counter-reaction. Hope this makes sense, and great job on your reasoning!

[@himneeshchowdhary820]

@@FlyByMax i am a high-school student (in class 10) and i am interested a lot in maths and physics. your explanation was great, even a class 10 student could easily understand! great job, i know you will succeed!

[@alvaromelo]

In regards to the 747 Cargo plane crash from National Air Cargo, flight 102 , in Afghanistan in 2013. What made things even worse, was the fact that when the cargo (extremely heavy armored vehicles) slid back, they destroyed the hydraulic lines the controls the pitch of the aircraft, rendering it uncontrollable. Ref: en.wikipedia.org/wiki/National_Airlines_Flight_102

[@FlyByMax]

I didn't know they destroyed the hydraulic lines as well, that's interesting. Yeah that definitely wouldn't have helped with the c.g. shift. Thanks!

[@vitormd1138]

Yes! A new video! As i always say, you have a bright future ahead on KZbin, keep it up! Not saying this just because it is a small channel, but because the quality is outstanding compared to some bigger channels.

[@FlyByMax]

Thanks for your kindness and honesty! Glad to have you on board :)

[@vitormd1138]

@@FlyByMax The pleasure is completely mine, your videos are well structored, the script is flawless and with no interruptions (like stutters or gaps of silence between sentences). Keep it up, you're certainly in the right way.

[@woodytaylor8451]

Yes he’s back with an even better produced video

[@A2KLAU]

It was by chance I stumbled upon your 2nd video on the channel, but I watched it all the way through because it was interesting content. I would not have guessed this is such a new channel. Very well done and I think you’ve found a niche. Your content is so good I’ve watched all 5 videos in 1 go and subscribed. Keep up the great work 👏

[@FlyByMax]

Thank you so much for this comment and your encouragement. Glad to have you onboard, have a good one!

[@topjet4433]

Outstanding quality of your videos. I am a commercial pilot and would have loved to watch a few videos during my training 20 years ago.

[@FlyByMax]

Thank you, means a lot!

[@raspect333]

ive said this before these videos are amazing, trust me your channel will grow if you keep it up

[@FlyByMax]

Thanks! This really helps to keep this up. Have a good one :)

[@woofvault]

Great video, just subscribed! Thumbs up, looking forward to more videos!

[@2kDVI]

if cma is 0, then it would basically be like not having dynamic stability, right? so the same thing at the start of the vid where small input leads to the chain would take place i think

[@pinethegangsta]

Your videos are all so wonderful, easy to understand and perhaps useful, even if I'm only a Junior High student. It would be great if you could explain what winglets are for, as I never actually understood

[@woodytaylor8451]

I know it seems that you may not have many views or subs, but you have some of the best videos I have ever even on KZbin. Keep it up! Also the aerodynamics teaching in your vid’s are really helpful for my glider training lol

[@FlyByMax]

Thanks so much! Good luck with your gliding training, flying gliders got me into engineering and flight simulators in the first place, I had so much fun and I'm sure you will too :)

[@woodytaylor8451]

@@FlyByMax thanks :)

[@Parkinwad]

Very in depth and well explained.

[@marcusiii892]

I have no words, just Amazing!. Keep it up!

[@rebecatimoteojb]

I love this channel so much 😭😍😍😍

[@Gulliver1]

mmmm, plugins and datarefs from X-Plane, tasty 😎 Great to see someone who knows what they're talking about doing vids like this, keep it up! Back in 2005 I wrote a flight sim to explore flight (some older vids on my channel). Being a microlight pilot but having no decent maths skills I wrote it from feel. Watching these vids is great to see how much of the limited understanding of the flight maths I used to write it, were actually understood by me. 😆

[@FlyByMax]

That sounds awesome! I thought about trying to program a flight simulator as well, it's a huge challenge. I'm still learning a lot about stability and control algorithms, so I know the feeling of being out of your depth :)

[@theflightwrights1902]

@@FlyByMax Oh my god that is amazing! I have also just begun writing a global flight simulator. I've almost finished the scenery engine. I came to your channel for understanding flight physics. Thank you so much for the videos.

[@janmeifarth9884]

another very good video! just one thing to mention: I never saw any picture diagram or whatever during my studies in which the center of gravity was behind the center of lift for commertial airplanes under normal operation...do you have any source for that (wasn't able to find anything on the spot). Normally the elevator produce a down force to rotate the nose up (making the aircraft more draggy -> trim tanks in the elevators...).

[@FlyByMax]

Hi! Thanks for your comment. I do have sources which show the possibility of the aerodynamic center of the wing being in front of the centre of gravity. I also have a reason for why I decided to show the forces as I did in the video. First and for all, something I did not mention in the video (since it would over-complicate things) is that in my diagrams I assume the lift to be acting through the aerodynamic center (AC) of the wing. This is useful, since you may know that the AC is defined as the location around which the aerodynamic moment does not change with respect to angle of attack. I assume that when you say "center of lift", you are talking about the centre of pressure (COP), which you probably also know moves forward with decreasing angle of attack. You are completely right that for the vast majority of the time, the tail does provide a downward force in most commercial aircraft. The reason I chose to display things as I did in the video, is because this way it becomes extremely clear that the horizontal tail is not optional, but a requirement for stability. Without the tail and if the c.g. was to be in front of the a.c. of the wing, the aircraft would technically be stable if its aerodynamic moment was positive, although marginally (neglecting the de-stabilizing effect of the fuselage). But this was all a bit much to show in the video and would only make things less easy to grasp, which I think is also the reason why these concepts are taught to students the way they are (see sources below). However, I believe that it is a common misconception that the c.g. needs to be in front of the COP/AC of the wing (meaning tail pulling down) for the aircraft to be trimmed and/or stable. It is possible to have a stable, trimmed aircraft where the c.g. is aft of the a.c. You can do the analysis yourself by computing the derivatives, but here are also multiple sources which show similar diagrams of the c.g. being behind the a.c: Cornell University, "Introduction to aircraft stability and control", see diagram on page 26: courses.cit.cornell.edu/mae5070/Caughey_2011_04.pdf TU Delft, part of the course "Introduction to Aerospace Engineering", see 1:18:07 in the lecture "Moments and Axes": ocw.tudelft.nl/course-lectures/1-4-moments-longitudinal-static-stability/ Also see the corresponding handout to the above course: ocw.tudelft.nl/wp-content/uploads/Hand-out-Stability_01.pdf Finally, good ol' wikipedia: en.wikipedia.org/wiki/Longitudinal_static_stability I hope this helps! It's a shame I wasn't able to explain some of this in the video, but unfortunately I had to leave some things out :)

[@janmeifarth9884]

@@FlyByMax wow! Thanks for this in depth answer - I will have a close look on the provided material :)

[@khalifstigler7493]

Hmm if CMA was 0 the plane wouldn't be pitching up or down so I think neither it would be in stable flight or not because it has to be equal to-1 so yeah just I don't know

[@FlyByMax]

You are absolutely right. It would be neither :) not stable or unstable, just neutral! Great job!

[@khalifstigler7493]

@@FlyByMax thanks and I had to think like this to the wing it wants to pitch up and pitch down the plane and you said that ' CMA was to be -1 not positive so it can't be 0 just I don't know

[@harveyrichard2007]

My apologies but, at 2:42, I really don't get it. Why exactly would the wind flow around the wing translate into a torque around the lateral axis ??

[@Tortilla196]

The lift vector acting upwards (center of pressure) is acting behind of the center of gravity. This creates a pitch down moment, counteracted by the tailplane.

[@harveyrichard2007]

@@Tortilla196 thanks for your response - you should say exactly that at 2:42. A step is clearly missing here.

[@excellenceinanimation960]

So if CmA is 0 would the aircraft be stalled? Or perhaps in level undisturbed flight?

[@ailo8964]

If Cma is 0 the plane keeps pitching up/down in case of turbulence.

[@onebigflappyboi8276]

Hello sir! Awesome video I am just having a little bit of a hard time understanding what Cma actually is. I will try to understand the video more as I will replay it but can you please explain it to me? Thanks so much! I love your videos.

[@FlyByMax]

Hi! Thanks for your comment :) Cma is equal to the change in T (delta T in the video) divided by the disturbance in angle of attack, delta alpha. To explain it in more simple terms, Cma is a number that tells you how stable (or unstable) the aircraft is, because Cma says something about how much stabilizing torque the aircraft generates (7:13). So if Cma is zero, that means almost no restoring torque (delta T) is being generated, meaning that the aircraft is not very stable. If Cma would be largely negative, then that would mean that the aircraft's design produces a big torque delta T, making it very stable. Hope this helps!

[@onebigflappyboi8276]

@@FlyByMax Thank you so much my man! your the best!

[@OninDynamics]

This channel is the opposite of what I am

[@himneeshchowdhary820]

just wow

[@flybyairplane3528]

CG, IS MOST IMPORTANT,,THE REAL classic,,was that B747 on climbout from BRAGRAM IN AFGANISTAM, THE VERY LAST PEICE , OF FREIGHT WAS SIMPLY NOT PROPERLY SECURE,, THEN THE CG SHIFTED SO MUCH TO,THE REAR, , BRINGING THE AIRCRAFT , BACKWARDS, , RIP,,,,,,,,;;🇺🇸🇺🇸🇺🇸🇺🇸

[@AmauryChihuahua]

Bruh

[@LOLLMFAOLOLLMFAOLOL]

Great vid 🎖 need to be clear that CG is always in front of CL = nose heavy setting for flight stability…stabilizer will be trimmed at -aoa…so need to fix your illustration which CG was shown behind CL.