Difference between revisions of "Deep stall"
(Created page with "Deep Stall is a aerodynamic stall condition that is difficult or impossible to correct. In some flying tail aircraft it is the blanketing of the rear control surfaces by a stall...") |
|||
| Line 2: | Line 2: | ||
| − | The revised designs ensure that deep stalls occur only if the canard aircraft is built or operated beyond its approved limits. The most likely cause of a deep stall today is flying with the CG beyond the aft limit, either knowingly or unknowingly. Building errors that result in incorrect incidence angles on the canard or main wing can also be the cause. These issues should be identified and resolved during the aircraft’s test phase. Despite the rarity of deep stalls, some pilots express concern that unusual attitudes could alter the normally benign canard stall behavior. | + | |
| + | [[Image:deepstall.png|thumb|right|Deep Stall]]The revised designs ensure that deep stalls occur only if the canard aircraft is built or operated beyond its approved limits. The most likely cause of a deep stall today is flying with the CG beyond the aft limit, either knowingly or unknowingly. Building errors that result in incorrect incidence angles on the canard or main wing can also be the cause. These issues should be identified and resolved during the aircraft’s test phase. Despite the rarity of deep stalls, some pilots express concern that unusual attitudes could alter the normally benign canard stall behavior. | ||
| + | |||
| + | |||
| + | After deep stall testing was completed on the Velocity and Cozy IV, there was evidence that the strakes contributed to unrecoverable deep stalls. The delta wing planform of the strake delays its stall and the strake’s center of lift is forward of the aircraft’s CG. This was confirmed during low speed testing of the Velocity (Sport Aviation, July 1991) which showed the wing and canard stalled at 18 and 20 degrees pitch angle while the strakes did not stall until 26 degrees. At aft CGs, lift from the strakes were pulling the nose higher even after the canard had stalled; whereas canard stall normally results in a nose-down pitch. Strakes are known to be destabilizing and this demonstrates one reason why. | ||
| + | |||
| + | |||
| + | |||
| + | All the reports state that the application of power had almost no effect or resulted in a slight nose-up pitch. Some pilots reported that the propeller was “blocked” or “cavitating”. What would cause the nose to pitch up when adding power? Was this the result of the strake unstalling itself or is there another reason? If we review the previous picture (above), we can see that the upper half of the propeller is operating in turbulent flow from the stalled wing/strake, whereas the lower half of the prop is exposed to relatively clean incoming air. The application of power may cause the lower half of the prop to produce more thrust than the upper half, which pushes the nose UP! Makes perfect sense once you think about it. | ||
Revision as of 14:43, 3 July 2012
Deep Stall is a aerodynamic stall condition that is difficult or impossible to correct. In some flying tail aircraft it is the blanketing of the rear control surfaces by a stalled main wing. In a canard it is typically a stalled main wing, which the canard has no aerodynamic ability to nose down and correct.
The revised designs ensure that deep stalls occur only if the canard aircraft is built or operated beyond its approved limits. The most likely cause of a deep stall today is flying with the CG beyond the aft limit, either knowingly or unknowingly. Building errors that result in incorrect incidence angles on the canard or main wing can also be the cause. These issues should be identified and resolved during the aircraft’s test phase. Despite the rarity of deep stalls, some pilots express concern that unusual attitudes could alter the normally benign canard stall behavior.
After deep stall testing was completed on the Velocity and Cozy IV, there was evidence that the strakes contributed to unrecoverable deep stalls. The delta wing planform of the strake delays its stall and the strake’s center of lift is forward of the aircraft’s CG. This was confirmed during low speed testing of the Velocity (Sport Aviation, July 1991) which showed the wing and canard stalled at 18 and 20 degrees pitch angle while the strakes did not stall until 26 degrees. At aft CGs, lift from the strakes were pulling the nose higher even after the canard had stalled; whereas canard stall normally results in a nose-down pitch. Strakes are known to be destabilizing and this demonstrates one reason why.
All the reports state that the application of power had almost no effect or resulted in a slight nose-up pitch. Some pilots reported that the propeller was “blocked” or “cavitating”. What would cause the nose to pitch up when adding power? Was this the result of the strake unstalling itself or is there another reason? If we review the previous picture (above), we can see that the upper half of the propeller is operating in turbulent flow from the stalled wing/strake, whereas the lower half of the prop is exposed to relatively clean incoming air. The application of power may cause the lower half of the prop to produce more thrust than the upper half, which pushes the nose UP! Makes perfect sense once you think about it.
