Trouble with no lift at trim condition

[Simon Boudoux]

Good afternoon,

I am running some stability analysis on a model of a cessna182 and it appears when I run my stability analysis with mach 0.3 and the C.G also fixed it appears that i have no lift which is strange regarding the fact that in Roskam database for the cessna 182 at cruise I am suppose to have some lift. I have tried different approach and I don,t have any results. If anyone could look into it please that would be very helpful.

I have attached my model 

Sincerely yours,

Simon BOUDOUX

[Rob McDonald]

What do you mean you have no lift?

I ran your case and everything seemed to work as expected.  It is unusual (probably a bad idea) to run a sweep of alpha (5 points) and stability analysis (7 case) at the same time -- resulting in 35 cases.

However, it does work and it generates lots of results.

What did you see that you did not expect?

Rob

[Simon Boudoux]

Thanks for the answer and yeah maybe i should reduce the number of iteration and case but what i mean is that if i checked correctly the Roskam database is that at alpha = 0 I am suppose to have CL = 0,301 and when I run my model I have like CL = 0.04 with my mach Number at 0.3. I have put screenshot of the database. But yes i got results but it doesn't seems to fit what i should get so I was wondering if I forgot to do something 

Thanks again for the answer

Simon

[Rob McDonald]

How accurately have you matched the C182 geometry?

Did you match the airfoils (wing and horizontal tail)?  If the wing is twisted, did you match the twist?

Does the wing have any incidence relative to the coordinate system you used in OpenVSP?  Does the tail have any decalage?

All of these changes will affect the lift at a given angle of attack.

Quite frankly, Roskam using zero alpha exactly at cruise leads me to think he used that condition as a reference -- i.e. he defined alpha to be zero at cruise at his starting point and then defined everything else as perturbations from that point.

Rob

[Simon Boudoux]

I think I matched pretty accurately the design of the cessna 182 with every aspect of the wing. I am not sure to understand what you said about the incidence relative for the wing, what did you meant by that ? 

So you think that roskam meant that he started with alpha at 0 and then there is the perturbation that will modify alpha and everything else on the plane ? 

Simon 

[Brandon Litherland]

Have a look at the incidence parameter on the wing to get an idea of what it does.  Also, google wing incidence angle.

As Rob describes, there are a lot of things that go into aircraft component orientation to achieve relatively flat and level flight at cruise.  For example, the angle of attack could be in reference to the aircraft frame itself, meaning zero angle relative to the freestream of the aircraft fuselage.  An occupant would not feel any upward aircraft pitch at this condition. Wings will typically have some incidence at the root and "washout" or twist to lessen the angle at the wing tip.  It's also worth having a look at Roskam's equations for CL and CD, specifically what reference area was used.  The dynamic pressure is provided.

As a baseline, you should match at least the camber of the 182 airfoils for the thin mode to work in VSPAERO. Then, I'd recommend incrementally adjusting the root incidence until you get 2650 lb of lift for the aircraft.

Is any of this still unclear?

[Simon Boudoux]

Thanks for the answer it is really nice but i think some stuffs are a bit off for me I am not really familiar with CFD and all that in detail so I would have to check that, but what does the dynamic pressure can impact my analysis in VSPAERO, like is there somewhere where I can put it ? And is there somewhere where I can check the total lift developed by my model in VSPAERO ? And does that means I have to implement the mass of my aircraft also ? What is precisely the root incidence ? 

Thanks again for the answer