Can no longer trim down after brakes, slow speed - Page 4 - ED Forums
 


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Old 01-20-2019, 04:28 PM   #31
kontiuka
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On a related topic, I've been trying to figure out why the air brake causes a pitch up moment. It just seems intuitive to me that a backwards facing force on the air brake would cause a pitch down moment. I must be missing something. Can you give a brief explanation of what forces are acting on the aircraft to make it pitch up? Does the brake drive the air into the bottom of the fuselage causing an upwards force?
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Old 01-20-2019, 06:12 PM   #32
Vibora
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It all depends on how much aft is the center of mass of the aircraft. Let's consider a C-101 flying in level flight. If the center of mass of the aircraft is far enough to the rear, the vertical component of the aerodynamic pressure created by the airbrake surface will create a pitch up moment higher than the pitch down moment created by the horizontal component of the aerodynamic pressure, since the arm of the first will be longer than the later. The center of mass is rather near to the center of the air brake, so it's not easy to see why this effect is really happening in the direction it does. But this is how it behaves, it is written that way in the manual, it is confirmed by real pilots and, more importantly, it is the real aircraft aerodynamic data that we got.
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Old 01-20-2019, 06:21 PM   #33
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Quote:
Originally Posted by Vibora View Post
It all depends on how much aft is the center of mass of the aircraft. Let's consider a C-101 flying in level flight. If the center of mass of the aircraft is far enough to the rear, the vertical component of the aerodynamic pressure created by the airbrake surface will create a pitch up moment higher than the pitch down moment created by the horizontal component of the aerodynamic pressure, since the arm of the first will be longer than the later. The center of mass is rather near to the center of the air brake, so it's not easy to see why this effect is really happening in the direction it does. But this is how it behaves, it is written that way in the manual, it is confirmed by real pilots and, more importantly, it is the real aircraft aerodynamic data that we got.
Makes sense. Thanks for the explanation.
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