How to decide on a stall margin

[Devin Gibson]

Question from one of our users:

"We are aware the values in the Aero Lab's course are arbitrary, but we are looking to see how to decide the stall margin so that we can refine and test our initial aircraft design in MachUp."

[Aero Lab Team]

From Nathan Hoffer,

Devin,

Not sure how this works but I can give you my perspective on how I think about stall margin. The stall margin is the cruse airspeed subtracted by the minimum airspeed (onset of stall). But how do you set the margin? Here are the things I would take into consideration: 

• Max wind components the aircraft should be able to handle 
  
• If you expect to fly in 10m/s gust then the stall margin should greater than 10m/s 
• How recoverable is the aircraft from stall
• If the aircraft has very gentle stall characteristics then maybe it is not that big of a deal if it stalls and the margin can be relatively small. If the aircraft goes in to a spin upon stall that is very difficult to recover from then the stall margin should be large. This also depends on if it is manned, RC, or autopilot controlled because of the response time of the pilot is slower than the auto pilot. Also some autopilots are not capable of correcting for stall.
• Also consider the mission of the aircraft. If it is for data collection and needs to be very reliable then you probably want a large stall margin. Same if you are doing a competition and to ensure success. If it is just for research and you only fly when weather is good then maybe you can handle a small stall margin. 
• The stall margin will also effect  the design of the aircraft since it sets the cruse speed. Generally you design the aircraft configuration to be as efficient as possible at cruise for endurance purposes.  

Hope that helps.