Static margin of aircraft?

[RadicalDisconnect]

Based on my current understanding, the longitudinal static margin is the distance between the neutral point and the center of mass, divided by the mean aerodynamic chord. An aircraft with negative static margin is considered statically unstable, and would require the control surfaces to create a pitch down moment to ensure level flight. For a tailed aircraft, this means that the elevators need to be trimmed at a positive incidence with the free stream to provide this downward moment. Now, there was some discussion over the F-35's static stability. Pictures of the aircraft in cruise conditions show that the elevators are trimmed at a positive alpha, but there have been some claims that a strong downwash near the elevators means that the elevators are actually at a negative incidence with the free stream. The implication is that the F-35 has positive static margin.

So the question is, how strong is the downwash of the air right behind the wing, and in the case of the F-35, can we tell whether the tail is at an positive or negative incidence with the free stream?

EDIT: a picture to help illustrate what I'm talking about.

Perhaps as an additional note, do we know the static margin of various fighter aircraft?

F-16 has up to negative 5% in subsonic, and becomes positive in supersonic flight.

 

[AeroFranz]

Most negative recorded SM (as far as i know) was X-29, supposedly 35% in some configurations. Early F-16s might have been around 16%, but got progressively more stable with the larger tail. I don't think there are accurate rules of thumb to predict downwash at the tail, but if you want to give it a try i recommend books such as Nelson or Perkins and Hage (I like the latter), and look at the chapters on stick-fixed longitudinal stability. You could also make a model of the F-35 in AVL (which you should learn anyway if you're serious about aerodynamics or aircraft design). Problem with that, is that the Vortex-lattice method is not very good at modeling fuselages, and the F-35 derives a lot of lift from the wide fuse. Still, this might be the best solution. Good luck!

 

[Orphic]

A couple of oilflow & CFD examples of downwash behind the wing albeit at high CL's

 

[Sundog]

A Treftz plane analysis would also tell you a lot about the downwash, but you would need a high fidelity 3D model and some good CFD software, or access to wind tunnel results for the F-35, which I don't think LM will be handing out any time soon.


If you want a quick analysis of the instability margin, just look at the relationship between the wing's aerodynamic center and the center of mass. Both can be calculated based on the basic geometry from a three view drawing.

 

[RadicalDisconnect]

There's some uncertainty over the F-35's center of mass. It should be ahead of the main landing gear, but by how much is unknown.

I recall the wing's aerodynamic center to be approximately 1/4 of the root chord length behind the start of the chord?

 

[shedofdread]

I recall the wing's aerodynamic center to be approximately 1/4 of the root chord length behind the start of the chord?

No, 0.25*mean chord back from leading edge at the location of the mean chord. However, with a fuselage like the F35's, you really need to look at the aerodynamic centre of the whole aeroplane.

 

[RadicalDisconnect]

I recall the wing's aerodynamic center to be approximately 1/4 of the root chord length behind the start of the chord?

No, 0.25*mean chord back from leading edge at the location of the mean chord. However, with a fuselage like the F35's, you really need to look at the aerodynamic centre of the whole aeroplane.

I suppose there's no reliable way to estimate fuselage lift? Hopefully I'll gain a better understanding by the end of the next quarter, as that's when I'll finish my aerodynamics course.

 

[AeroFranz]

short answer, IMHO: no. Sadly we're only good at using low order methods with high aspect ratio wings. As soon as you get into low AR and bodies, you need to use higher-order methods. Maybe a panel code?

 

[redbaron]

Based on my current understanding, the longitudinal static margin is the distance between the neutral point and the center of mass, divided by the mean aerodynamic chord. An aircraft with negative static margin is considered statically unstable, and would require the control surfaces to create a pitch down moment to ensure level flight. For a tailed aircraft, this means that the elevators need to be trimmed at a positive incidence with the free stream to provide this downward moment. Now, there was some discussion over the F-35's static stability. Pictures of the aircraft in cruise conditions show that the elevators are trimmed at a positive alpha, but there have been some claims that a strong downwash near the elevators means that the elevators are actually at a negative incidence with the free stream. The implication is that the F-35 has positive static margin.

So the question is, how strong is the downwash of the air right behind the wing, and in the case of the F-35, can we tell whether the tail is at an positive or negative incidence with the free stream?

EDIT: a picture to help illustrate what I'm talking about.

Perhaps as an additional note, do we know the static margin of various fighter aircraft?

F-16 has up to negative 5% in subsonic, and becomes positive in supersonic flight.

Aside from downwash, positive trim angle at the tail does not necessarily mean that the stability margin is negative. It only means that the wing aerodynamic center AC is ahead of the CG. For negative SM, so called Neutral Point (or you may call it AC at aircraft level) must be ahead of the CG.