4 Finding the stall speed (Vs0)

We noted the stall occurs at 13° of angle of attack with the flaps out. But at this angle of attack the pitching moment is negative. To compute the true stalling speed with a good accuracy, we must find an equilibrium position with the elevators that can maintain the 13° angle of attack. We must find an elevator position that produce a null pitching moment at a 13° angle of attack. This will surely be a elevator-up flight configuration, and the maximum CL will probably be slightly lower than in the preceding analysis.
 

So we need to find the elevator angle that will give a null pitching moment at 13° of angle of attack. To do that we will set up a custom analysis with elevators at 5 , 10 and 15°. So we must start by creating configurations in the geometry for these pitch command angles.
 

• In the navigation bar, click 'Geometries'
• In the tree on the left, choose the geometry corresponding to this tutorial, "With-controls".
• At the bottom of the geometry page, click the 'Add a new configuration' button.
• Name the new configuration "Elevator-5" and set the following angles: Pitch = 5° / Flaps = 30° / Air brake = 90 °
• Click the create button at the bottom of the page.

 

Proceed the same way to create the configurations "Elevator-10" and "Elevator-15".

Then go back to the "Projects" tab, selected your project in the tree view, and create a new analysis, selecting the type "Custom". Set it up as shown in the following image. There should be 3 runs with the three elevator positions as configuration.

 

Add the runs to the queue and wait to get the results. You should get something similar to the following picture:
 
From this table, we can make the following plot:
 
We can see from this chart that the null pitching moment would be obtained by putting the elevator deflection at 8°.
 

We can now create a last configuration from our geometry, with the pitch command angle set at 8°. Then create an 'Aerodynamics' analysis from this configuration, and this analysis should allow us to find the Vs0 with a good accuracy. Here is an image of this analysis creation, we are only looking for angles of attack near the stall 13°.
 

 

Again we push the new runs into the queue, and wait to get the results. Once they have been processed we can go to the "analysis results" tab. In this tab, below the second chart, there is a frame called "additional results", with a tool where we can edit the mass and altitude similated for the aircraft. We use that tool to update the mass to 730 kg.

 

Just below we can now see the velocity corresponding to the CL_max value. We obtain a stall speed just under 50 knots for the max take-off weight, our Vs0 specification is validated: