Aircraft Design Tutorial: Fundamentals of CG Analysis

[@decodd1988]

Thank you very much for this video, my grandfather was mechanic engineer and produced airplanes.

[@dr.gudmundssonaircraftdesign]

You're welcome. Glad to read you enjoyed it. Best wishes.

[@solomonbeyene2088]

Dr. it is my first tutorial to watch this type of tutorial. it is short and well enough explained to grasp the complexity of the science. at 11:23 the third step 1400 used, I think, mistakenly instead of 140.

[@dr.gudmundssonaircraftdesign]

Yes, you're right. How 'bout that? Good catch. Thank you for pointing it out. Unfortunately, not easy to fix in the video. Will put it in the introduction. Thanks!

[@spextre7104teji]

This will probably help me a lot in engineering statics

[@moneyeye24]

YOUR VIDEO IS THE BEST!!! would you pls make more for internet watchers? suggestion 1: pls spend some time on basics like what x axis represent, what y axis represent, the purpose of this diagram, the meaning of each intersection point, what does the slope line ,vertical, horizontal line mean? These might be instinct and redundant to you but it will be extremely helpful for internet learners since we are not in your classroom and can not ask you.

[@dr.gudmundssonaircraftdesign]

@moneyeye24 Thank you for your suggestions. Best wishes.

[@abduahmad4167]

thanks a small typo error towards the end,140 Kg not 1400 Kg

[@wolfgangchen7287]

Thanks Dr Gudmundsson. This was a very informing and poggers video.

[@dr.gudmundssonaircraftdesign]

Thank you for your kind words, Wolfgang. :-)

[@aliptera]

Very nice explanation of mass, weights, moments and CG calculation. The exercise at the end of the presentation gives me 17.953 inches. Is this because of weight rounding?

[@dr.gudmundssonaircraftdesign]

Your answer is precise to the 5th significant digit. I'd call that spot on. (I call it correct if it is accurate to the 3rd sigfig). Well done. You get an A+. :-) Interesting aircraft configuration, by the way.

[@myvlogs9477]

Many thanks from my side as I am in 3rd year of Aeronautical Engg. As I was looking out for a project and across your video it gave me an idea. From Mumbai, India

[@dr.gudmundssonaircraftdesign]

Glad it was helpful! Best wishes.

[@azadkhandoker]

Dear Prof. a small error on the video @11:22 seconds, 720 suppose to be divided by 140, instead of 1400, one zero extra. Hope it would be modified. Thanks for the video. Anyway your book is one of the best book ever I have read on a/c design.

[@dr.gudmundssonaircraftdesign]

Hi Azad. Thank you for your comment and very kind words about my book. I appreciate it. :-) The bug you speak of was discovered some time ago (please look into the Description), but thanks anyway. Best wishes.

[@jaggillarintefisk]

If the airplane weights 100 kg and the wing area is 1x10m, how fast do I approximately have to go to get airborne?

[@dr.gudmundssonaircraftdesign]

Robert. It depends on several things; first and foremost on the shape of your wing and airfoil selection. If we assume a poorly designed wing (CLmax = 1.0) , then your stalling speed (or minimum flying speed) would be approximately 46 km/h at sea-level. A better wing (CLmax = 1.5+) would require some 37 km/h . Of course, getting airborne is one thing, having a configuration that is also "stable" and safe to fly, is another. Best wishes.

[@skypiIotken]

Hello in example 2 at the end of your video, for #4 the fuselage shell my calculator said 3,780 not 3,785.1 and for #6 the main gear it came out to 455 not 455.8, were these errors on the spreadsheet or was my calculator wrong, with my calculations the cg still came out to 17.95 though, which was still slightly outside the rear cg, thank you I liked your video very much and it really helped me understand how to calculate cg from the datum line on any aircraft

[@dr.gudmundssonaircraftdesign]

@Skypilotken Greetings. The reason is that in my spreadsheet the actual weight of the fuselage shell and main gear is 84.113 and 91.165, respectively. However, since I'm showing the weight with no decimals (for clarity) this leads to minor discrepancies. I suppose I should have clarified that. Regardless, your arithmetic is spot on. Thus, given the reduced number of significant digits I'd say your calculations are correct. Best wishes.

[@chafaon]

I completed my studies last July and I am seeking to test my knowledge by designing and, henceforth, building a small GA aircraft. I have started by studying this textbook which I, unfortunately, only found after completing all the design-related units of the course. I really think the textbook is great. The purpose of this comment is to seek direct contact with Dr. Gudmundsson (couldn’t find his email anywhere on his channel) so as to establish if he is available for individual, private consultation on small projects such as mine. It would be a a great pleasure to here from him.

[@dr.gudmundssonaircraftdesign]

You can try to contact the university where he works!

[@ADAPTATION7]

One question: In your example, how do you determine that the fuselage shell is to be located at station 45 of your diagram? Since the shell is the whole length of the aircraft, I can't figure out why 45 inches was chosen.

[@dr.gudmundssonaircraftdesign]

Great question, François. First, the installed engine weight is assumed to contain everything forward of the firewall, including the cowling. Therefore, the "shell" is really the length of the fuselage starting at the firewall. This is approximately at Fuselage Station -30 (call it FS-30) and it extends to, say, FS170. Thus, I consider the CG of the fuselage shell being at FS45 to be a reasonable approximation. Yes, it might be slightly farther forward, but I hope this doesn't take much from the learning offered by the example. I hope this clarifies. Best wishes.

[@ADAPTATION7]

@@dr.gudmundssonaircraftdesign Thank you for your response. That has always perplexed me. I presume in real life situation, the shell would be weighed and balanced with the airframe.

[@dr.gudmundssonaircraftdesign]

@@ADAPTATION7 Yes, weighing and balancing the shell would be done, although this can only be done after it is built. Before that's done, math is used to estimate where its CG is located. Today, solid modeling software is frequently used to determine this. An educated guess supported by mathematics does pretty well at this. Best wishes.

[@bejuristinngnondete6780]

Thank you Doctor

[@dr.gudmundssonaircraftdesign]

You're welcome.

[@isabella2652]

I finally found this video!

[@bejuristinngnondete6780]

Me too

[@ayomiindika3507]

Meeeeeeeee

[@dr.gudmundssonaircraftdesign]

@isabella2652 Just about time!!!

[@aerospaceengineer8484]

hello, Dr. You have a typo error in step 3 CG calculation. It should be 720/140

[@dr.gudmundssonaircraftdesign]

I know. Since 2017. Please read the introduction to the video. (Press "Show more").Thanks anyway. Best wishes,

[@guillaumedeseze8866]

Merci, vraiment

[@dr.gudmundssonaircraftdesign]

De rien, mon ami

[@davidm2386]

Thank you

[@dr.gudmundssonaircraftdesign]

You're welcome.

[@rahulsingh-ho2lc]

Sir how to buy this book in India plz help

[@dr.gudmundssonaircraftdesign]

Sorry about not responding sooner. Anyway, if there's an amazon.com in India, then you should be able to buy it there. If not, I wonder if you can purchase through amazon.com in another country and have it sent internationally. You could aslo try and contact the Elsevier store for the same. Good luck and best wishes.

[@rahulsingh-ho2lc]

Thanks sir

[@edilbertosantana8307]

Many Thanks

[@dr.gudmundssonaircraftdesign]

You are welcome.

[@olajideoyekunle9044]

Thank you sir, as always.

[@dr.gudmundssonaircraftdesign]

You're welcome.

[@volkanaldar5065]

I want to build a P51 Mustang Replica for display, I have some sponsors here who help me with free materials and services. I want to use the drawings of mr. Marcel Jurca to do the parts and assembly, but the drawings are in PDF, and I want an assumed, responsible, serious volunteer (free of charge!!!) CAD engineer to help me redraw the drawings from the PDF and put them into DXF files, so it can be easily cut on CNC from plywood, aluminium, steel etc... I have some progress on the frames and formers (pictures attached). The Mustang will be displayed for a new aviation museum project I am currently developing. The CAD engineer will be mentioned on the sponsor's list on the project website.

[@dr.gudmundssonaircraftdesign]

I think you need to contact an aerospace or mechanical engineering professor at your local university and suggest this as a project for an undergraduate engineering student. They are usually eager to work on any projects they can place on their resumes (most likely for free). You should be able to find a design professor from the online faculty directory at your local university. Good luck!

[@quentinakridge9225]

At 11:25 on step three your mass has an extra 0

[@dr.gudmundssonaircraftdesign]

Yes, the mistake is already documented in the video Description. Thanks for pointing it out though.

[@WhiteLightning117]

i'm out here just trying to figure out how to use the FAR static analysis tool lmao

[@dr.gudmundssonaircraftdesign]

I hope you have found what you're looking for. Cheers.

[@seansoblixe9711]

ENOUGH OF CG INFO. I WANT TO PLACE TAIL FEATHERS ON TAIL GIVEN A CERTAIN WING LENGHT VS A GIVEN TAIL AREA ETC.

[@dr.gudmundssonaircraftdesign]

You must buy my book and do this per the procedure of Chapter 11, Section 11.5 Initial Tail Sizing Methods. It does precisely that.

[@dr.gudmundssonaircraftdesign]

Maybe my latest video will help you: kzbin.info/www/bejne/qH7Ei2SXnNhljcU

[@NYRB29]

I really enjoyed this video Dr. Gudmunsson. Looking forward to watching more of your channel!

[@dr.gudmundssonaircraftdesign]

Thank you, Jeff. Kind words like that encourage me to make more videos. Best regards.

[@AinaweeUAE]

Thank you so much Dr. For your work and videos. I am really amazed that you take out time to even respond to some of our queries. Eternally grateful. If you'd allow me, can I request for a mass estimation video? How to use the various mass estimation techniques during the initial designing phase and how to make proper adjustments in case of deviations/variations from the applicable category (such as a fighter jet/ transport aircraft / commercial aircraft)?

[@dr.gudmundssonaircraftdesign]

Thank you for your kind words, AinaweeUAE. I appreciate it. Also, thank you for your suggestion. I'll definitely consider it, but please keep in mind that I have several videos in a queue at the moment, so there's probably a little while until I can get to it. Best wishes.

[@AinaweeUAE]

@@dr.gudmundssonaircraftdesign That's great to hear! Eagerly waiting for your new videos, and thank you once again for all your work!

[@sedyild8945]

Hello, Dr Gudmundsson. I have a question about optimum tail arm length. I am little confused as some books give different formulas for working out tail arm length. In a book Mohammaqd sadraey uses l=K (sqrroot){(4CSVh)/(pie *Df)} K=1-1.4 He also notes that tail arm length shorter than 3 times wing chord is short coupled, which possesses longitudinal trim penalty. Can you help me with this dilemma please?

[@dr.gudmundssonaircraftdesign]

Hi Sed. My formula sqr(2∙Vht∙C∙S/(pi∙(R1+R2))) is fundamentally the same as Sadraey's. He uses Df = 2R in his presentation, while I use R, where R1 is the radius of the fuselage at the wing's Cmgc/4 and R2 the radius of the fuselage at the HT's Cmgc/4. This also explains the factor 2 that is different between the two formulas. Sadraey only accounts for the diameter of the fuselage at the wing's Cmgc/4 and clearly assumes the aft fuselage is a cone, whereas I use a frustum. He also uses fudge factors (1-1.4) - I don't use any. These fudge factors are necessitated because when you use this form of the optimization one tends to come up with a slightly shorter tail arm than you find among real aircraft. So, as you can see, there are slight differences but not significant ones. Regardless, I present two additional optimization approaches in my book and the third one optimizes the tail arm based on horizontal and vertical tail volumes and its output is pretty close to what you see in real airplanes that have the centroid of their HT and VT close to one another (which is the case for great many real aircraft). You should also keep in mind that sometimes other factors come into play that force designers to change the tail arm. Thus, the optimized tailarm estimation should be considered an initial tailarm. Things that might force the designer to shorten or lengthen the tail arm might include dynamic stability concerns, requirements for fuselage volume, to name a few. Also, it is true that a short tailarm may lead to longitudinal stability issues, but one must be careful with such assertions. For one, thrustline and overall geometry impacts this too. A case in point are airplanes like the Cessna 150/152 and Piper J-3 Cub, both which have tail arms shorter than 3 x Wing chord and I would not exactly say either has any serious longitudinal problems - quite the opposite. Both are excellent trainers. Also, consider flying wings. Some of those have excellent flying characteristics as well and they surely have short tail arms. It's easy to suggest rules-of-thumb, but they don't always apply. I hope this helps. Have fun designing!

[@amirhouseingholinia2023]

Why the moment values in example 2 for some components are different (the one with decimal points)? I graduated from ERAU, Daytona Beach Florida in aerospace engineering back in 1985. This video brought back so much memories. Thanks

[@sedyild8945]

@@dr.gudmundssonaircraftdesignThank you for responding to my question. You have been extremely helpful. I've read your answer long time ago I just forgot to respond to it.

[@invadervim9037]

I'm here to get better at scrap mechanic.

[@dr.gudmundssonaircraftdesign]

What?

[@invadervim9037]

@dr.gudmundssonaircraftdesign it's just a game in which you can build stuff, but you can take it way too far in regards to complexity.

[@0623kaboom]

first assumption for CM and CG i dont agree with ... a plane with top wings like a pontoon plane such as a cessna has its CM BELOW the CG ... so they are NOT in the exact same place in all axis' ... they are always inline for best performance yes BUT the CM and CG are NOT in the same exact location ... on a cessna the cg is above the centerline of the plane while its CM is almost double the distance below the centerline BUT directly UNDER the CG in all axis' (at rest or levcel flight)... because of this difference the CM and CG cannot be interchangeable they will be CLOSE to each other but NOT exactly the same in all designs ...

[@dr.gudmundssonaircraftdesign]

Hi 0623kaboom. Sorry pal, wrong! Whether you (or any other person) agrees with the principles of physics is irrelevant. MOTHER NATURE IS OUR BOSS! WE ARE NOT HER BOSS. Do you know basic physics? If so: Weight W=mg, where m is mass and g is the gravitational constant (9.81 m/s²). Thus, the CG (center of gravity or center of weight) is precisely at the CM (center of mass). For instance, a solid sphere (e.g. a ball bearing) has its center of mass at the center of the ball. Since weight is nothing but the amount of matter (mass) times g, the CG is at its center as well, i.e. at the CM. The same applies to all other bodies. The only time this doesn't hold is if the body is in a non-uniform gravitational field (e.g. near a black hole, where the magnitude of gravity changes rapidly with distance from its center).

[@RickSoaring]

Easy, clear explanation. Using a spreadsheet to calculate CG makes it really easy.

[@dr.gudmundssonaircraftdesign]

Thank you Rick. Happy to clear it out.

[@sameersam2676]

thanq u very much its vey simple and nice understanding with ur class.

[@dr.gudmundssonaircraftdesign]

You're welcome.

[@mil-fpv4931]

11:20 Shouldnt the Step 3 be 720kgm/140kg, not 720kgm/1400kg?

[@dr.gudmundssonaircraftdesign]

I know. Since 2017. Please read the introduction to the video. (Press "Show more").Thanks anyway. Best wishes.

[@mil-fpv4931]

@@dr.gudmundssonaircraftdesign Great video btw. I learned something.

[@alfaisalwork504]

Thank you for this wonderful course...... Doctor, I hope to find a way to communicate with you

[@dr.gudmundssonaircraftdesign]

@alfaisal work What would you like to communicate with me about? (If you prefer not to state here, then please say so.)

[@alfaisalwork504]

@@dr.gudmundssonaircraftdesign I did not like to ask this question in front of the audience because you will see the question as trivial because of your experience in the field, but I will ask my questions

[@alfaisalwork504]

@@dr.gudmundssonaircraftdesign I am an ambitious young man I have nothing to do with aircraft, but I am researching how to design an aircraft. I aspire to build an airplane What are the books you recommend to me to learn the field of aircraft design? I have strong knowledge in Mathematics and Physics Thank you

[@alfaisalwork504]

@@dr.gudmundssonaircraftdesign What do you advise me to develop my knowledge in this field?