? is this another vocabulary lesson for me ?Umm, intersection drag? I am guessing from the back and forth that this model assumes totally attached flow.
? is this another vocabulary lesson for me ?Umm, intersection drag? I am guessing from the back and forth that this model assumes totally attached flow.
+1 it looks like a lesson is in order!? is this another vocabulary lesson for me ?
Maybe so.+1 it looks like a lesson is in order!
so why doesnt the other side of the fin count? air flows on both sides?Aerodynamic drag, and corresponding coefficient of drag, is only calculated by the frontal area (reference area). TR11 is correct.
Looking very briefly, especially earlier on page 21, it appears he is considering the planform area, not the surface area. If the coefficients are twice as large, using a measure that's half the area works fine. I'd have to spend more time than I have available to be sure, but i think the TR is correct.ok, i have a question. I think this is a typo in the tr11 report, but im not sure. It says that SF is the surface area of all the fins. So they calculated the area to be 4.5 on page 51 of the PDF but page 47 of the actual document. They got 4.5 by doing the area of a triangle and multiplying by 3. BUT that would only account for ONE side of the fin and not BOTH sides of the fin. Air flows on BOTH sides, so it should be 6(bh/2) to account for both sides of the fin, and not just one side.
Cd is referenced to final area, but surface area (or see my response a minute ago, planform area) is absolutely used in calculating it.Aerodynamic drag, and corresponding coefficient of drag, is only calculated by the frontal area (reference area). TR11 is correct.
it probably is correct, im just trying to see why its only the one side. I did see the section on planform area, but it didnt state why its just one side. I may have to start doing some extra digging to get this answered haha.Looking very briefly, especially earlier on page 21, it appears he is considering the planform area, not the surface area. If the coefficients are twice as large, using a measure that's half the area works fine. I'd have to spend more time than I have available to be sure, but i think the TR is correct.
ok, looks like i need to start looking up the planform area nad drag and see what i find. thanks for that. I have learned a lot of information with this thread.The fin is a wing. The fin [wing] imparts a force when deflected from the free stream airflow that depends on its coefficient of lift and area of the fin.
The program makes some simplifying assumptions about the lift coefficient and uses the distance from the center of area of the fin to the CG of the rocket to determine the stability contribution of the fin. The lift coefficient of airfoils was usually determined by wind tunnel testing in the era these equations [barrowman] were developed, today we have software that can predict the properties of airfoils pretty well.
Lift is denoted as a force per unit area. You do actually need both sides of the wing to generate the force but the convention is to use the planform area.
How lift is generated is a whole nother religious topic.
Convention.so why doesnt the other side of the fin count? air flows on both sides?
ohhhh, ok, i get it. it was setup as part of the metrics and convention then. Since the drag force does indeed include both sides of the fin, convention just made it easier to use planform. Perfect. Thank you very much for this information.Convention.
Cd (coefficient of drag) is usually determined experimentally.
View attachment 616954
Put an object, like a fin, into an air stream, measure the force on the object and the velocity of the fluid, choose a reference area and calculate Cd for that object.
For flat plate objects, the convention is to use surface area. For wings, planform area. Other objects may have different reference area, or something relevant. For a circular cylinder, it is typically diameter or Re (Reynolds number).
Take figure 40 in TR11. The drag coefficients were determined based on the thickness ratio (by cross section outline)
When you use a Cd to determine Drag Force, you need to use the Cd for the appropriate object and the appropriately associated A (reference area)...
In this case, the A (reference area) used to calculate Cd is the planform area.
Hope that helps!
Lots of ways to build drag models. In OR, use the "Component analysis" and see how they differ. "Component finish" also influences the drag.so far all my hand calculations match up to open rocket except 1. My velocity is 551 mph by hand, but OR reports 371.... so i messed up somewhere. Both include the 0.304 Cd etc etc.
likely...for it to be off by 200mph haha
Yup, the rabbit hole has been fun, but very deep and many caverns to explore.likely...
All of what you are doing can be a bit, or a bunch of a rabbit hole. Fun, but a rabbit hole. I spent the first week of November going down the drag functions rabbit hole for a particular question/interest...I started with the first large scale tests that Eiffel did in 1907 from the Eiffel Tower (French), then Wieselsberger's work in 1921 (German), then flowed through the NACA work, then Hoerner, then, then, then. Rabbit holes can be fun...I just prefer them in English. Translating/reading in French and German is a bitch.
The other aspect of my research that is interesting is A) references/citations early in the 20th century were shorthand...no APA, Chicago etc. There was sparce work, and everyone knew what everyone was doing, so shorthand worked fine. B) terminology, both across languages and within languages were different. Definitions may or may not be clear. This started to shift in the 1930s/1940s...at least for definitions within the work...less so for citations. Definitionally, what we might now know as planar flow and axial flow as standard terminology was not true then...words were different.
I'm still curious how information traveled back then. I grew up in the age of mail and written material. No computers, cell phones, internet. Between 1900 and about late 1920s, it seemed like communication and evolution took about 10 years. After about 1930, at least for aerodynamics research, what others had been doing elsewhere shifted to center around NACA. WWII certainly impacted the volume and source of research (shifted to NACA) for what is available readily today. Interesting stuff.
Eiffel in French.
https://histoire.ec-lyon.fr/docannexe/file/1870/01056v01.pdf
Wieselsberger in German.
https://babel.hathitrust.org/cgi/pt?id=uiug.30112008570167&seq=1
Eiffel's apparatus:
View attachment 617026
Thanks for the link @UVU_Team_Rocket !<<snip>>
So far i have found some very interesting equations for things that i have not seen. Such as this simplified approach. It looks like the "big 3" from physics with a little bit of manipulation.
https://www.nakka-rocketry.net/articles/altcalc.pdf
i also have some papers i found from NASA in the 50s from barrowman and he references some other works.
I wonder if that's what @JohnCoker is doing in his ThrustCurve.org - Match a Rocket - Motor Guide WebApp ?
Agreed. I see no merit to that simplified approach, other than just noodling around at random. There is much utility in the extended Feskins-Malewicki equations. They are analytic and exact within reasonable assumptions. Tom Keuchler even had some analytic solutions for variable thrust and exponential atmosphere, although he had to use special functions (Bessel functions?). You can't do as much with digital numerical solutions except endlessly run more of them.No, Thrustcurve is performing numerical integration.
https://www.thrustcurve.org/info/simulation.html
The "simplified approach" from Nakka might have been useful in 1960. It is strange that he would propose it in 2007, though. Numerical methods and software have been highly efficient and available to any hobbyist with a PC since the 1980's. I never understood why aerodynamicists like to play around with "zero drag" concepts.
analytic and exact within reasonable assumptions.
Agreed. I see no merit to that simplified approach, other than just noodling around at random. There is much utility in the extended Feskins-Malewicki equations. They are analytic and exact within reasonable assumptions. Tom Keuchler even had some analytic solutions for variable thrust and exponential atmosphere, although he had to use special functions (Bessel functions?). You can't do as much with digital numerical solutions except endlessly run more of them.
I could explain the utility and interest in "zero drag" CFD like aerodynamics, but you might have to buy a beer or two.
Nobody is arguing against the precision of properly done numerical integration. My point is that the F-M like equations are analytic, i.e., closed form algebraic equations, from which you can easily derive partial derivatives for creative use in other algorithms. For example, you could efficiently solve for the Cd that gets your rocket to a target altitude.Yes, F-M equations are exact and still offer some areas of improvement to play around. In addition to overcoming the constant thrust and density as you mentioned, I think Cd is also assumed constant.
I will argue that numerical solutions are essentially exact (something like errors of 1.0E-14 or machine precision) even with time steps as large as 0.01 or 0.001 seconds.
Nobody is arguing against the precision of properly done numerical integration. My point is that the F-M like equations are analytic, i.e., closed form algebraic equations, from which you can easily derive partial derivatives for creative use in other algorithms. For example, you could efficiently solve for the Cd that gets your rocket to a target altitude.
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