Discussion: Spoilers as Lateral Control

[Sparweb]

Hi everybody,

I've been wondering for a while about the benefits and trade-offs that are involved in using spoilers for lateral control of an aircraft. Although a lot of work was done a long time ago on the subject (by NACA and NASA), spoilers only seemed to show up on transport aircraft. On any other aircraft, spoilers were only used as speed brakes, and done mostly by modifying the basic aircraft (Cessna 40x comes to mind).

I balance the pros and cons this way:

Ailerons:
Even when retracted, there's a drag penalty due to the gap,
Take up valuable real-estate at the trailing edge that could otherwise be used for flaps,
Rotate the aircraft around its central axis,
Induce a yaw opposite to the bank,
Require balancing to prevent flutter,
Simple actuation,
Continuous effectiveness,
immediate responsiveness,
positive feedback,
Ineffective and sometimes spin-inducing in a stall,
Addition of weight compared to flap or just plain trailing edge is small.

Spoilers:
When retracted, do not intrude on the wing surface at all,
Allow flaps to use almost the whole span of the wings,
Rotate the aircraft around an axis offset from the central axis,
Induce a yaw following the bank,
No balancing required (I think),
Simple actuation (no big difference from ailerons, really),
Response delayed as spoiler extends thru boundary layer,
Feedback not necessarily positive but easily designed in,
Effective in the stall,
Maybe heavier than ailerons, depending on actuation system.

Obviously, I've had some time to ponder, but what do the experts out there think? Why aren't spoilers more common, particularly in GA aircraft?

STF

 

[corgum]

What's a Cessna 40x ? There are no spoilers on the Cesssna 400-series airplanes, as delivered by Cessna. Did someone sell an after-market spoiler mod?

What do you mean by central axis -- the longitudinal (fore-aft) axis?

Why would spoilers cause a rotation about a different axis than ailerons?

Spoilers have very low actuation loads, compared to ailerons. Either an artificial feel system or ailerons are added to give feel. Neither solution is particularly easy.

Relatively large spoilers often cause buffeting that can be felt in the airplane. Some people object to it.

Spoilers take up valuable real estate in the fuel tank area.

 

[Sparweb]

Yes, an aftermarket mod to the 400's is what I meant. The air-brakes are only a few square feet each, and used only briefly for slowing approaches/increasing ROD, in the Cessna's case. Pilots want them to get into short airstrips more easily.

In terms of rotation around the longitudinal axis, the ailerons produce a force-couple, opposite to each other on each wing. The spoilers, on the other hand, reduce the lift force on one wing, and there is no balancing force on the other. Hence, the wing with spoiler deflected goes down, and the other wing does not go up. This seems to have a lot of subtle implications. For example, since you've spoiled some of your lift every time you tip the wings, you have to pull up a bit, too, to stay level.

I always assumed that the extra little ailerons on spoiler-equipped aircraft were there as a backup, so you don't lose lateral control, or, to provide the balancing force-couple that I just mentioned is missing. Actuation loads may vary depending on the type of spoiler used - but if they're typically lower than ailerons, then I suppose that's why they show up more on larger aircraft.

I recall seeing spoilers used on the TBM 700 (currently checking this). If so, does anyone know if buffeting is present at large spoiler deflections? With flaps up/down?


STF

 

[Miper]

The Mitsubishi Diamond / Beechjet has noticeable buffeting when its spoilers are deployed.

In analyzing the effect of control deflections, there are both force and moment effects to consider. With ailerons, there is an increase of lift on one wing and a decrease in lift on the other. These two effects are almost never equal in magnitude; so the effect of deploying ailerons is to impart both a rolling moment and a moderate change in overall lift force. With spoilers, there is just a reduction of lift on one side, which results in a rolling moment and a net decrease in lift.

By the rules of Statics, the action of a moment on a body does not depend on its point of application, and moments are all assumed to act about the CG in flight.

The difference between ailerons and spoilers is in the change of lift force. Ailerons change it a little, but spoilers always change it more (for a given rolling moment).

Control deflections have interesting secondary effects on the flight path, which are usually very difficult to detect. In this case, an airplane with spoilers tends to drop a bit more when spoilers are deflected. This effect is masked by the bank effect, which reduces the vertical component of the lift vector, which also causes the airplane to descend.

Another example is the elevator. In a pull-up, the up elevator command causes a substantial tail download to occur, which pulls the whole airplane down as it rotates to a higher angle of attack. Very quickly, the airplane begins to climb and make up the small altitude loss at the beginning of the maneuver.

 

[Minifloresta]

Spoilers are different from ailerons in that ailerons generate roll moment that is pretty linear to their deflection; not so with spoilers. Spoilers generate very little roll moment if the deflection is small enough to keep the spoiler inside the boundary layer, but as soon as the deflection is big enough to have the spoiler panel protruding out of the boundary layer, a substantial roll moment appears. So developing a good mechanical spoiler system is not that easy. The Robertson STOL version of the Piper Seneca, for example, does not have ailerons and roll control relies on spoilers only. To make them acceptable, a complex cam system had to be designed - that is very difficult to adjust.
Other aircraft that have roll control based on spoilers only - such as the Mitsubishi MU2 - are known to be roll-unfriendly.
However spoilers have a positive advantage over the ailerons: they do not generate adverse yaw. On the contrary: they generate positive yaw. Larger jets (Boeing 737 and bigger) use roll spoilers combined with ailerons in such a way that adverse yaw is almost non existent.
I would say that the use of spoilers in light aircraft is not worth the extra weight and complexity, unless some special type of performance is sought such as STOL (where full span flaps can increase maximum CL). For normal, non-nonsense light aircraft, the good old pair of ailerons is still the simplest, lightest solution that produces good flight qualities.