# What happens to Lift and Drag on aircraft (and its components) when sideslip is increased gradually from 0 degrees to 90 degrees?

I possess an Aerospace Engineering background, and work as a Aerospace structural analyst at a local firm. A portion of my job is dedicated to determining the critical conditions that the aircraft and its component can undergo, and then conduct Finite Element Analysis for those components under these limiting loading conditions. Before jumping onto the CFD team (to acquire the loads), I first have to make up my mind what conditions I reckon should return me critical loads acting on the aircraft.

I usually make use of V-N diagrams to deduce which are the most critical loadings on my civil aircraft, beyond which the structure is deemed to have failed. However, the V-N diagrams for an aircraft don't include the Sideslip effect. I am not a CFD specialist, so I don't know what would be the response of the aircraft (and its components) when subjected to wind at a certain sideslip angle.

Assume, I am currently working on the fuselage component of my civil aircraft, and want to obtain the most critical loads which can possibly act on it while in flight. So how would the sideslip play a role in this scenerio? What happens to the lift and drag values as the sideslip angle keeps on increasing gradually from 0 degrees to 90 degrees, in flight? What pattern would the lift and drag follow during this period? Should this pattern be the same for all components of the aircraft (like fuselage, wing, tail, etc) or it can be different?

• That's a huge question. Mar 22 at 16:56
• Don't forget to consider "sideforce" as well-- "sideways" lift generated by sideslip-- acting orthogonal to the flight path and free-stream relative wind, and also orthogonal to the lift vector. Mar 22 at 16:58
• Honestly I think the problem has to be tacked by a CFD model including the entire aircraft. Probably shouldn't have tried to answer at all, just wanted to point out that yes definite elements of the aircraft will definitely respond differently. Mar 22 at 19:27
• @quietflyer, I totally agree with you. Conducting a CFD and then observing the results would be the most feasible approach here. But just picking those conditons from the CFD where the aircraft (or its individual components) are experiencing the highest or most critical loads and then directly interpolating those loads onto the concerned structure in my FEA model, is something which I don't prefer, unless I know why it is happening, so that it will save me some time in the future if I encounter similar problems. Mar 22 at 19:32
• Good question. Quick comment: the team that supplies pressures can be the Aero (CFD) folks, but the team that defines "critical load conditions" are the Flight Loads folks. FL team uses flight dynamics simulations (tons of them) to determine critical load conditions for the whole airframe (and its components). Simulations include on-ground side winds, all flight regime and failure cases. If an airplane has 90 deg sideslip "in flight" it's possibly hovering (or falling). Mar 23 at 17:56

I don't know what would be the response of the aircraft (and its components) when subjected to wind at a certain sideslip angle.

Obviously you understand that "sideslip" just means "sideways wind". The nose is pointing at non-zero yaw angle (or more properly "slip angle") relative to the direction of the flight path and the free-stream relative wind, so the airflow is hitting the side of the aircraft.

What pattern would the lift and drag follow during this period? Should this pattern be the same for all components of the aircraft (like fuselage, wing, tail, etc) or it can be different?

Well, that part's easy. It will be dramatically different. Say you start increasing sideslip angle more and more, so the nose is pointing to left of the flight path.

If aircraft has dihedral, the right wing will experience an increase in lift. The left wing will experience a decrease in lift. The overall total lift vector is bound to decrease to some degree, especially at extreme slip angles. The overall drag vector of the wing may not change hugely, but the more the dihedral (or anhedral), the more the sideways wind will generate some extra drag.

The fuselage (and vertical fin) will see a huge increase in drag which may reach maximum at 90 degrees slip angle. The fuselage (and especially vertical fin) will also generate a "sideforce" component-- "sideways lift", acting orthogonal to the flight path and free-stream relative wind and also orthogonal to the aircraft's "vertical" axis or yaw axis. In other, words also generally orthogonal to the overall lift vector of both wings combined. This "sideforce" from the fuselage and fin will peak long before the sideslip angle reaches 90 degrees-- likely well before the sideslip angle even reaches 45 degrees, and then will start to decrease again, and may be zero when the sideslip angle reaches 90 degrees.

Does that help get you started?

It sounds like you've got an enormous task ahead of yourself here.

• I am guessing you are aware of the role in V-N diagrams in aerospace structures. It guides and informs us the limiting loads (or to be more precise, the limiting aircraft lift) that it can see during flight operations. But it doesn't include the effect of sideslip angle, atleast I haven't seen a V-N diagram having so. So how does the aircraft manufacturing companies figure out the V-N diagrams (and the respective limiting alpha and sideslip angles of the aircraft, for example at a 4g maneuver) if sideslip effects are also taken into account? Mar 22 at 19:41
• Re -"So how does the aircraft manufacturing companies figure out the V-N diagrams (and the respective limiting alpha and sideslip angles of the aircraft, for example at a 4g maneuver) if sideslip effects are also taken into account? "-- honestly I would guess that they often don't. And certainly not at really extreme sideslip angles like approaching 90 degrees. But maybe someone better qualified will post an answer. Maybe consider posting another question simply asking if loading from sideslip is taken into account, and if so, typically up to what sorts of sideslip angles? Mar 23 at 14:18