How to do variable sweep?

[mf0u3091]

I am trying to make a solid blade by blending two aerofoils. I tried todo it using boundary blendbut Icant make it a solid. I am sure I have to use Varable sweep but I dont understand what origin and chain mean. Please could you advise me on how iwould do this. Thank you
Edited by: mf0u3091

 

[jeff4136]

I sorta doubt you want a VSS if you are blending two airoil profiles.


[url]http://www.mcadcentral.com/proe/forum/forum_posts.asp?TID=32 075[/url]


for an example VSS. Search the forum (there aremany examples)and the internet (there are many free VSS tutorials). Think there's one or two in the articless section here.
Edited by: jeff4136

 

[CPiotrowski]

I would use a swept blend for what you are doing but unfortunately that is more complicated than a VSS so I guess I didnt help you much. But, atleast you know not to do a VSS and to learn a swept blend. A boundary blend will work its just a tedious process to make sure your curves are closed. I may have read what you are trying to do wrong. I am telling you how I would make an aerofoil not to blend between the two. you should post an image of what you are trying to achieve.
Edited by: CPiotrowski

 

[mcgowanp]

I've done a good deal of Aerofoil sections. Mainly though it has been turbine blades, which have a lot of similarities with aerofoil sections.


The best way I find of doing this is through boundary blending, my designs previously have been concerend with accuracy and functionality. And I find that I get the best manipulation from boundary blend method.


I have attached a stripped down version of the blade and I have changed the blade sections, I obviously cannot give you an accurate or finalised design, but I think that this part will at least get you going in the right direction.


On second thoughts if you email me I will send it on to you, my email on my web site.
Edited by: mcgowanp

 

[SportsMan]

I know this is an old thread, but I'm very interested in being able to model aerofoils so I hope you don't mind if I pick it up again.


How do you tend to model aerofoil sections. I'm tending to trace a section within a style feature, using as few nodes as possible, to keep the quality of the curve and subsequent curves good.This doesnt produce the most accurate aerofoil, but does allow me C2 surface connections.


Cheers
Nick

 

[jeff4136]

I don't claim to know anything about it but thought I'd offer a little
food for thought and observations, maybe stir a little additional
discussion. Someone that knows somethin' might even join in ...
2008-02-29_063518_airfoil_profile--wf2--.zip
_ The equation driven datum curve generates with about 50 CVs at .001"
abs tol, about 25 at .01".
_ The overlaid degree 3 bezier curves have 4 CV's each and a max deviation
of about .025. That might get a little better with some tweaking.
_ The IGES is a degree 7 bezier with 8 CVs created in Rhino and fitted to
less than .01 deviation by manually moving CVs (easy to do with their
Nudge function).


I, too, would like to know how the 'real guys' do it. I'd be genuinely
surprised if the old 'fit point' generating routines haven't been replaced
by routines that will output the CV positions for degree 7, 9, 11 ...
curves (?). I'd also be a bit surprised if the surfaces weren't swept
with the trajectory of each CV defined by equation. (I think that could
be done with degree 3 curves similar to what's in the attached but it
would be a lot of work for dabbling and probably beyond my meager
computational / descriptive math skills.) I'd love to see a piece of a
wing surface model from one of the new generation heavies. Even more so
than the Alias IGES. ;^)

 

[wsylvester]

this might be pointless

http://www.ae.uiuc.edu/m-selig/ads/coord_database.html

 

[design-engine]

Airfoils...

Boundary blends are only for inlet areas and joining wings to fuselage. The National Advisory Committee for Aeronautics started in the late 1920's and basically set the groundwork for airfoil development. NACA came up with all the math for everything really. biplane development from two wings to one... all the way to supersonic flight. Before NASA there was NACA

The NACA duct is my favorite since I like to go fast on motorcycles and ... surfacing classes I can basally capture the attention of the young engineer with motorcycles fairings. Seams to me the big four motorcycle folks take liberties to modify the standard NACA duct but back to the topic.... These forms would not be good with the boundary blend tool since the math is so very specific for bring the air back into a chamber such as a fuselage or engine compartment for cooling.

Another bad place for boundary blends is ductwork. When you try to channel air thru a tube. If you don't want to disturb the tunnel of wind before it hits the turbine blades. With the right equation and trajpar you can obtain the optimum form for bring in air to the turbine engine. (now a days the duct work is more complicated that anything NACA could have dreamed with stealth technologies) But thats a different topic and a completely different 2 day class

http://history.nasa.gov/naca/ 1915! Wikipedia probably has more

Back to the sweep thing.... I do a one hour talk in a surfacing workshop that discusses 'everything' a user can do with a sweep tool from sweeps that follow a pull direction (plastics) to multi trajectory sweeps (named wrong in Pro/E for a funny reason - food for saladworks fans) Nowing that engineers are like humans that they only retain 20percent of everything they see (except engineers tend to have a high figure it out factor they tend to play with things.. like the sweep tool till they re- enact everything I did in the hour over the next few months.

Its a good hour! The model I use looks so not intimidating until your done with the hour and look at it. Anyway ... maybe we should make challenges to ourselves because after looking at that propeller on a different post I am convinced I can learn tools from pushing different techniques.



Edited by: design-engine

 

[mcgowanp]

Thats not true, I use boundary blends to great effect on foils, sometimes I use VSS as a base feature though. My ole man, who works with me, is the expert on this stuff, hes got nearly 35 years experience in turbines of all forms. But the method works for us.


To say you cant use use something for a specific task....... ... Well thats just a challenge!!


I agree though that themarine prop technique was very well done.

 

[design-engine]

My dad did pipe supports....

I am the guy one airplane manufacturer brings in to blend everything together with boundary blends. I just go on what those smarty paints aerodynamics guys says. hey.. if it passes CFD it must be ok.

mcgowanp

Do you read in the forms from points on multiple planes then blend thru those?

 

[jeff4136]

> boundary bends ... turbines ...


Some thoughts ---


Linear blends, including linear tapers, twists, and transitions between
profiles (though the distribution of element lines may be of concern) are
as 'precise' as swept features. They can also be as 'light' as swept
features and the opp; swept features can be as 'heavy' as blends, is also
true. Both ways are dependant on input curves and, in the case of the
Boundary Blend, Control Point options / definitions.


My suppositions are founded in wing (or any aero exterior surface) modeling
techniques. I doubt anyone cares how heavy or light a turbine model surface
is as long as it's accurate and, within the range of a percentage of that
accuracy, smooth. For aero downstream applications thousands of copies of
those surfaces, their offsets, intersection curves, etc. will be made for
interfacing structures and the, so I'd imagine anyway, original surface
definition becomes more critical making the difference between a trim edge
/ intersection curve with a few control vertices or hundreds. That also
leads into another item of (?) concern; offsets. The generic surface offset
is normal to surface within system accuracy. For nonprimitive surfaces that
means the offset can have many times (unless the original surface is
already 'heavy') the number of control vertices. There is another method of
defining the offset; offset the control polygon. The resulting surface is
just a light as the original but the accuracy may not be within system
tolerance. For most applications and the type shapes being discussed not (?)
a concern as they are a small percentage of manufacturing tolerance. To my
knowledge it cannot be done in Pro/E. (Occasionally I do wish tolerance
could be defined at the feature level. Wonder if that's possible with any
parametric modeling system?)


Inlets... The most critical portion of the typical airfoil is the leading
edge back to some (15 - 30?) percentage of chord. Inlets are airfoils.
Blends are not 'precise' (where is the min rad, highlight or above / below
it? is it soft or well defined?) without critical setup of boundary
conditions or over definition so I'd be mindful of their use.


Blending between airfoil profiles... I guess there are infinite variations
on the theme, most of which I'd assume would be, ideally, more precise than
whatever the blend function gives you unless carefully controlled or, again,
over defined with additional sections. I do have one particular wing in
mind (and the prototype had my blood and chunks of skin on it's interior
surfaces); thirty degree swept, tapered and twisted, high performance with
different root and tip profiles. The root section tapered more rapidly than
the tip section. The 'blend' definition at the intersection was an 80 or
100 inch Radius.


Keep the thoughts coming. Interesting topic for discussion.