Lifting line theory [Aerodynamics #16]

[@windsurfingphd]

I am a retired computer engineering professor, and have been building and flying model aircraft since my childhood from kit plans. But, now I have time in my hands, I started learning the fundamentals of aerodynamics to be able to design my own model aircraft, and understand the concepts of AoA, lift and drag etc tied to a design. This is by far the best lecture I have seen on lifting line theory period. I have seen others in KZbin, and read articles and book chapters, but this is the best :-)) Can you please prepare another video (if you have time, of course), and give us a good example of how to design an entire aircraft design, including the design goals, airfoil selection, moments, angle of wing incidence, tail design etc because I believe this example will connect all the dots of your very articulate lectures. Thanks again.

[@tom_winguill]

I vote for this request. Please do a lecture series on aircraft design professor. It will really make this channel complete.

[@prof.vanburen]

Thank you for the kind words! I am certainly looking to expand the content of the channel, and will definitely consider something more geared towards flight mechanics/aircraft design. (I guess, first, I would have to learn how to design an aircraft!)

[@rolandotillit2867]

Fantastic explanation for the logic and how the logic determines the math. You sir, truly understand because of the clarity in your explanation.

[@prof.vanburen]

thank you so much!

[@rodrigotorresvillanueva4991]

Best video of lifting line theory

[@setsototitipana8433]

Loving the series with every video😍 but sometimes it's hard to see the numbers and some parameters used in equations😩 if only the notes were given in the description to make it easy to follow along. But the explanation in general is 👌👌 I am loving it

[@prof.vanburen]

Great suggestion! I've linked my website where the notes can be downloaded in each video description.

[@reubenmatthew3442]

This helped me a lot. Thank you so much!!

[@prof.vanburen]

No problem! Happy it helped

[@MohammadQoreishi-s3p]

Awesome! Thank you professor

[@flth1040]

There is an error in integral calculation in 15:28. Bounds of integral must be from pi to 0 otherwise lift force will be negative.

[@prof.vanburen]

Thanks for clarifying!

[@stasiekpiekarski]

Why in Biot-Savart Law (4:00 ) you divide by 4 not by 2? There is: v=Gamma/(4*pi*h) not v=Gamma/(2*pi*h)? Is it an error or I'm missing something? 2*pi*h is the circle length and multiplied by velocity gives circulation. Two my knowledge 4 in the denominator appears im the following form: dv=Gamma r x dl /(4*pi*r**2)

[@Mi-nr3wp]

For a semi infinite (0=>infinity), the induced velocity is half that of the infinite conditions (-infinity=>infinity)

[@prof.vanburen]

Sorry I was late to this and thanks to @Mi for covering it!

[@stasiekpiekarski]

@@prof.vanburen Thank you both for the answer! Great lecture!

[@jonasdondi698]

Thank you Sir for this. I have a question at 16:16. Where did the minus sign go for the final equation for induced angle of attack?

[@prof.vanburen]

Good eye Jonas! I should have dropped the minus sign slightly above this on the gamma term at around 15:26.

[@suheladesilva2933]

Hello professor, thanks a lot for your video. I have one question, how would you determine the value of Gamma nought for an elliptical lift distribution. Thanks in advance.

[@prof.vanburen]

No problem! When Gamma(z) is arbitrary, the way to find it is a guess and check method (see around 19:28 in the video). You would start with guessing a Gamma distribution, then through a series of steps you can solve for the lift distribution which gives you a new Gamma. Plug this Gamma back into the beginning, and do that iteratively until Gamma converges. The video might be more clear!

[@suheladesilva2933]

@@prof.vanburen thanks a lot for the reply.

[@osamaarefi6770]

Thnx prof you saved my day❤

[@prof.vanburen]

Glad to help!

[@davidbeckschulte8246]

6:40 why would gama(z) be an ellipse? love your content by the way, thanks for reading (sorry for my bad English)

[@prof.vanburen]

Hi and thanks for the kind words! At this point in the video, nothing forces gama(z) to be an ellipse. When you add up the contribution of an infinite number of horseshoe vortices along the lifting line, you consider the induced velocity at a point from all the neighbors, which results in a curve distribution of gama. However, assuming elliptical gama distributions is common, and that leads to elliptical lift distributions which then leads to the elliptical planform wing.

[@davidbeckschulte8246]

@@prof.vanburen , thank you for the reply, hope you have a great day

[@ВадимСавенков-з2ю]

Is this scheme works well with tapered wings, or even wing with 0 taper ratio?

[@romboom2405]

Good morning sir ! Thanks for this amazing lecture Regarding the discrete solution of LLT do you know why control/collocation point are chosen to be at 3/4 chord. The reason is usually to ensure flow tangency and recover the 2D incompressible airfoil slope but I never found any mathematical answer about the second one. Thanks for your answer

[@prof.vanburen]

Hi! Sorry for the late reply. Can you expand on your question regarding the 3/4 chord? I am not sure I'm following.

[@shrishkumar6018]

Thank you so much sir!

[@prof.vanburen]

No problem!

[@gloxe1282]

Thank you so much for the videos Sir! this is very helpful. Do you have any recommended reference for me to see an example for the detailed explanation for the process of using Lifting Line Theory with a General Lift Distribution (in a tapered wing if possible). This would be very helpful. Again, Thank you soo much for your videos sir! This is very helpful.

[@prof.vanburen]

You're welcome and glad you enjoy them! A good place to start would be Anderson's Fundamentals of Aerodynamics book, it's really stellar.

[@gloxe1282]

@@prof.vanburen Thank you for the answer, Sir! I have another question, if i want to use LLT for a tapered wing with the steps that you showed in 19:26, do i have to calculate the Fourier Series for the specific wing shape? or the steps applies to all planform wing shape (the first step particularly, where we assume with an elliptical wing distribution)? (which means i can use the elliptical distribution as a start despite the planform wing difference), but from here i dont have a clear view on how it will end up to be the distribution of the tapered wing instead of the elliptical one. I am confused at this. Thank you so much for your hard work!

[@davidbeckschulte8246]

could you do a video on sweet wings and the "Mittelefekt"?

[@prof.vanburen]

Oh! To be honest, I had to look up specifically Mittelefekt, which I understand is a term regarding the loss of lift near the root due to spanwise flow on swept wings? I certainly can consider a video on three-dimensional flows over finite-span wings after I finish up my Fluid Mechanics series (it's a bit more of an advanced aerodynamics topic). The Airfoils and wings video has some of the flow control devices used to mitigate these effects, but you're right in that I don't ever get into three-dimensional flow details.

[@davidbeckschulte8246]

@@prof.vanburen , i don't know if the mittelefekt emerges due to the span wise flow, but as far as I understand it (not very far) it does describe the loss of lifting force near the Roth. thanks again for answering, have a good day :)

[@SaraKhan-uz1vc]

somebody make a pdf of this

[@prof.vanburen]

I did! You can download all the videos as PDF notes at my website: sites.udel.edu/vanburen/education

[@SaraKhan-uz1vc]

@@prof.vanburen oh great thank you!!! one question is it bad if i do the lifting line theory on a swept wing mini aircraft?

[@prof.vanburen]

@@SaraKhan-uz1vc It depends on how mini the aircraft is and the foil geometry. Embedded in lifting line theory is thin airfoil theory. Low Reynolds number foils tend to be thicker, and that moves further from being considered a "thin airfoil". Also, when viscosity becomes more important then we can no longer assume inviscid. I think it could be okay for a first estimate, but I would be a bit cautious until I knew more about the flyer itself.

[@SaraKhan-uz1vc]

@@prof.vanburen airfoil is the NACA 4415 with the Reynolds number of 500000. The aircraft is a flying wing the weight limit is 16 OZ!

[@prof.vanburen]

@@SaraKhan-uz1vc The NACA 4412 airfoil at that Reynolds certainly follows thin-airfoil-theory behavior (specifically the lift-curve slope of 2*pi). airfoiltools.com/airfoil/details?airfoil=naca4412-il. I think lifting line theory is a good start to estimating performance!

[@mickolesmana5899]

And generally, the performance is hindered by reality. Some mad purple titan: "Reality is often disappointing"

[@prof.vanburen]

When the tail perfectly balances the main wing moment...some big dude: "Perfectly balanced, as all things should be"