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I was wondering that spoilers in most of the aircraft are always placed aft (towards the tail) the CG location. Why aren't they placed ahead of the wings or near the nose of the aircraft? The same is the case with parachutes. For small UAVs, the parachutes are either at the CG location or placed behind it. Is there a really strong design point to this method? Any suggestions/information would be appreciated?

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  • $\begingroup$ How would you place spoilers ahead of the wing? Are you proposing something like "spoiling slats" or "spoiler canards"? Their purpose is to disrupt the airflow over the top of the airfoil and stall it, which is why they're normally integrated into the top surface of the wing. $\endgroup$ – TooTea Jun 25 at 15:06
  • $\begingroup$ I think you may be confusing spoilers and air brakes. A spoiler is a movable control surface, typically on the upper surface of the wing, intended to disturb airflow over the wing in order to reduce the lift generated by the wing. An air brake (or speed break) is a device, often implemented as a section of the rear fuselage which can be pushed open by hydraulics to create drag in order to slow the aircraft in flight. $\endgroup$ – Bob Jarvis Jun 25 at 17:58
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Surfaces sticking out ahead of the Centre of Gravity act in an unstabilising manner: any dissymmetry will want to amplify itself. A dart thrown with the feathers first is in an unstable equilibrium and will very quickly turn around 180º.

Surfaces behind the CoG act as stabilisers, keeping the nose pointing forward. An aeroplane has vertical and horizontal tail surfaces at the back just for this purpose. When spoilers are deployed at low speeds, the aeroplane is in a situation where you don't want any destabilising moments.

Aerodynamic behaviour is stable if a disturbance auto-corrects: a deviation is automatically blown back. Since motion in flight is relative to the Centre of Gravity, it follows that the aerodynamic Centre of Pressure must be behind the CoG.

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  • $\begingroup$ Thanks for the input. Is there a way to numerically compute such effects for preliminary quantitative understanding? $\endgroup$ – CuthillMckee Jun 25 at 6:35
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    $\begingroup$ There is most definitely a way, but it's a multidimensional matrix with lots of inputs and outputs. In case of a dart or a lifting body without wings, the fuselage just wants to turn perpendicular to the airflow with a certain torque, and the stabilising surface must be large enough to compensate for this. Presence of a wing complicates the matrix: a roll attitude causes a yawing moment, a yawing moment causes a rolling moment and a whole lot more cross-couplings. $\endgroup$ – Koyovis Jun 25 at 9:37
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    $\begingroup$ For the spoiler, if it is placed behind the aerodynamic centre of the aeroplane it should usually be OK, but there does need to be an analysis to see if actuator failure on one side can still be compensated for by the vertical tail etc. $\endgroup$ – Koyovis Jun 25 at 9:38
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    $\begingroup$ @CuthillMckee - a crude way to compute the effect is to calculate the area of a surface, multiplied by it's distance from the centre of gravity. Generally there will be about 10 times as much behind the CG as in front. Unless you've got a computer model, draw a grid over the plan (or side) view of the plane and count squares in each column. You can assume that the CG is 25-30% from the front of the average wing chord. Also, are we talking about air-brakes? Spoilers are usually on the top surface of the wing. $\endgroup$ – Robin Bennett Jun 25 at 13:46
  • $\begingroup$ Same thing with both Arrows and firework rockets. The latter really emphasises that drag has a really big effect, without the apparently obvious effect of the fletch/vane of an arrow/dart. $\endgroup$ – Mike Brockington Jun 25 at 16:08

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