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I've just watched a video of a small Cessna turning not with ailerons or rudder but by opening its doors. They said that the door acts like a rudder, but in the video one of the guys opens the left door and the airplane rolls right. That doesn't make too much sense to me...

I've been searching on the internet and I found a different explanation which stated that the airflow stacks up below the wing because of the door and then an over-pressure is exerted making the airplane roll into the opposite direction, in this case to the right (which is what can be seen on the video). Can anyone confirm this second explanation?

And if I wanted to roll a Piper or any other low-wing aircraft by using the doors, what would the airflow's behaviour be?

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    $\begingroup$ Can you put in a link to the video? $\endgroup$
    – Ben
    Nov 6, 2016 at 13:48
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    $\begingroup$ A lot of low wing Pipers, like the Cherokee, only have one door. $\endgroup$
    – jamesqf
    Nov 6, 2016 at 19:37
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    $\begingroup$ ProTip: I'd use the aileron to roll the piper. $\endgroup$
    – KorvinStarmast
    Nov 7, 2016 at 21:54

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The second explanation is right. The blocking effect of the door below the wing will force more air above it, so suction and lift above get higher. Of course, the blocking effect will also increase pressure locally on the lower side of the wing.

If you do the same on a low wing aircraft, it will roll to the opposite direction. Now the door acts like a spoiler and reduces lift.

Generally, such a one-sided creation of a disturbance will upset the aircraft and will make it roll and yaw in an uncoordinated manner. And the "rudder" explanation is wrong - a rudder creates a sideforce at a distance to the center of gravity, while the door is pretty much where the pilots and the center of gravity are. The better explanation would say that the door acts like a spoiler, and those are used routinely for roll control:

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Well kind of it acts like a rudder. It really is more of a differential airbrake, creating asymmetrical parasite drag about the vertical axis of the aircraft and inducing a yawing moment, turning the longitudinal direction of the aircraft to the left or the right of its current path of motion. The thrust from the propeller then creates a lateral component of thrust, turning the aircraft.

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  • $\begingroup$ very good answer! I suspect it may be both what you said and what @Tyler Durden said combined... $\endgroup$
    – dalearn
    Nov 8, 2016 at 0:38
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    $\begingroup$ Only small addition. I believe as KorvinStarmast has pointed out below, the door works as a spoiler. It creates form drag by separating the flow. (not parasite drag) and yaw-roll coupling via dihedral is the mechanism which should be responsible for creating rolling moment.( not propeller thrust) $\endgroup$
    – ABCD
    Nov 8, 2016 at 12:15
  • $\begingroup$ Form drag is parasite drag. $\endgroup$
    – Romeo_4808N
    Nov 8, 2016 at 14:14
  • $\begingroup$ Also, it might be because of the weight balance, allowing it to "tilt" to one side. $\endgroup$
    – mzdv
    Nov 22, 2016 at 11:57
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An open door in flight on a high-wing aircraft will act like a flap. It will increase the drag on that side, both vertical and horizontal. The aircraft will slow down and the increase in surface area on the left will cause the right side to drop. It depends of course on the type of door and aircraft. On a low-wing craft, opening a door will just increase the drag since the vertical presentation will be unchanged.

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    $\begingroup$ How does it act like a flap? Flaps deflect air downwards; doors deflect air sideways. Why does greater surface area on one side cause the other side to drop? $\endgroup$
    – David Richerby
    Nov 7, 2016 at 21:19
  • $\begingroup$ @Richerby Even though the door is mostly vertical, due to turbulence effects it increases the effective surface area on that side of the aircraft (assuming it is a high-wing aircraft). $\endgroup$
    – Tyler Durden
    Nov 7, 2016 at 21:26
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    $\begingroup$ "Acts like a spoiler" would be more accurate than "acts like a flap" since a flap gives you lift, and the door does not. $\endgroup$
    – KorvinStarmast
    Nov 7, 2016 at 21:53
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    $\begingroup$ please, show me how a door can increase the surface area of a wing, I'm intrigued. $\endgroup$
    – Federico
    Nov 8, 2016 at 14:04
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    $\begingroup$ Spoilers generally only increase the drag Likewise with doors. Heck, even the landing gear doors increase drag while they are open. I am trying to help you improve your answer, Tyler. $\endgroup$
    – KorvinStarmast
    Nov 8, 2016 at 15:05

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