I think cross wind can be well taken care by controlling angle of attack, side slip angle, and so on. Those matters should be already taken care of, I suppose.

If you are looking for a stabilizing system for a bigger UAV then an autopilot board with sensors like accelerators, gyros etc programmed properly will do the job for any unstable design and if its micro UAVs then the flapping wing mechanisms derived from nature are given top priority as they are self-stabilizing.  

The best answer is an aircraft design without vertical fin or tailless configuration. We have two such UAV design which have improved stability within a higher crosswind.

The first thing you have to check is the design of the control surfaces and specifically the rudder's design

Thank you Professor Park. Thank you all for your reply. 

@Simion:: Tailless configuration is something that I am considering as well. This UAV is estimated to be over 25kg in weight with a wingspan of about 2.5~3m and is expected to cruise at about 20~25m/s. And one of the most important requirement is that it should not flip or roll over at a strong crosswind or gust of 13m/s. I'm trying to come up with some concepts which can fulfill this criteria. 

When in flight  the crosswind is irrelevant as the aircraft will weather cock and behaves as if there is no crosswind.  It is only when landing does the cross wind need to be considered.  A strong landing gear is helpful as the landing might be a bit sideways.  Good aileron authority will also be of benefit as this is an important control input during the landing roll to maintain the aircraft level at close to stall speed.  Rudder is also used just before touch down to straighten the aircraft over the runway.  So basically good rudder and aileron control near the stall speed is what is required.

Thanks Dean!

Lets say we are dealing with a UAV cruising at a speed of 20~25m/s. Suddenly gust wind of about 13m/s attacks the a/c at a certain angle. The gust wind is very high. I just want to make sure that it does not completely flip or roll the aircraft over. Consider landing is not a concern at this point. 

A sudden crosswind gust would be considered a horizontal wind shear.  So you would want to lessen the reaction by having a reduced tail area and no dihedral on the wing.  You could add tail area below the fuselage to counter any rolling action by the conventional upper tail area.

Dear Nikesh Bhattarai 

Without changing the design of your UAV, you can develop a control law to stabilize and control it in worst weather conditions.

First, you have to develop a dynamic model with consideration of the weather inputs such as windshear, turbulence, etc. Then you build a controller based on this model.

Good luck!

Thanks Dean Soccol. 

Thanks Mr. Belkacem, But for now I'm more concerned about finding some fix in the design to counter the gust wind, more about aerodynamics. Developing a dynamic model like you said will be performed at a latter stage. 

Dear Nikesh Bhattarai

I am working on the control of UAVs in worst environments. I submitted a paper for publication, I will send you a copy when approved.

Concerning the design, only the map of poles in the complex plane can give you an idea about the stability of your UAV. Poles are function of the design parameters.

Ofcourse in crosswind , the vertical plane arae is off important, such as the the tail fin and fuselage. You can not eliminate the tail fin(rudder ) as you need it for directional control. So keep the tail fin area to the optimum for dirctional control. As for the fuselage , most UAV design fuselage as simple frames only, unless your propulsion force is reseoanably large.

Cross wind consideration is required at the low speed operation such as take off and landing.At the conceptual and preliminary design stage, this effect needs to be considered to check the size of vertical tail

It depends on how big your aircraft is, there are the specified regulations on cross wind for UAV, LSA, VLA, FAR23, FAR 25 etc...Normally in Subpart B