It seems to me that a spoiler, as we commonly use them, tend to cause the airplane to loose altitude, but not necessarily speed. Flaps tend to slow the plane down.

Is this a common or correct observation?

From : Don Stackhouse

Pretty much. If you want to fly slower (i.e.: lower your stall speed), you need to find a way to increase the lift of the wing to compensate. Flaps increase the lift coefficient, and therefore lower the stall speed.

In general, and especially at large deflections, flaps of all types increase both lift and drag. The initial small deflections tend to mostly increase lift, while the lager deflections increase mostly drag. Flaps generally reduce the L/D, especially at larger deflections, because the cost in additional drag tends to be greater than the gain in lift.

Spoilers destroy the lift over a portion of the wing, and therefore reduce the wing's average lift coefficient, and the lift. This increases the stall speed.

However, flaps can be a trap for beginners. If you deploy flaps, slow the airplane down to take advantage of the reduced stall speed, then find you misjudged your approach and are now too low, you cannot just retract the flaps. To do so could cause a stall. You have to dive and regain some airspeed before you can safely retract the flaps and clean the airplane up. Since you wanted to clean the airplane up because the airplane was already too low, then diving to regain airspeed is probably not an attractive option!

At large deflections it is possible to reduce part of the flap deflection to reduce drag without significantly altering the stall speed. This can help avoid the too-low trap to a limited extent. For example, the Cessna 172 I flew for part of my flight training had available up to 40 degrees of flaps for landing. Most of the lift increase came from the first 15 to 20 degrees. In the case of a go-around from a full-flap landing it was OK to raise the flaps from 40 degrees up to the 20 degree setting immediately, but any additional flap retraction had to wait till you regained some airspeed.

With spoilers, if you find yourself too low you can just retract them. In fact, since the spoilers increase the stall speed, you might be carrying a little extra airspeed at that point, which is precisely what you need to get out of your too-low-on-final problem. For this reason, spoilers are more forgiving than flaps, and this is why we designed our Chrysalis 2-meter with spoilers, not flaps (it had nothing to do with RES, which had not been invented yet when I started on that design).

What if you put a moving surface on the *bottom* of a wing, in a position and of similar size to a common spoiler, sorta like a split flap, but moved forward of the trailing edge. If you deploy that will it function as a flap or a spoiler. That is, will it cause the plane to loose altitude without slowing down much, or will it cause the plane to slow down significantly. Is there any profound reason not to do that? Is there any potential advantage to doing that, aerodynamically? Has anyone ever done anything like that?

We covered this question a little while back when someone wanted to know if they could use "spoilers" on the lower surface for RES. The answer is that regardless of what you call them, they will act as split flaps, and varying their chordwise location on the lower surface will cause very little change in their lift-increasing ability (which is also why these "lower surface spoilers" are illegal for RES competition; a flap by any other name is still a flap). Putting them at 30% will have nearly the same effect on lift as putting them at the trailing edge.

However, putting them well forward on the lower surface will reduce their effect on the airfoil's moment coefficient, which is why the Me163 Komet used split flaps located at about 30% chordwise location on the lower surface. Tailless aircraft like the Komet tend to be very sensitive to changes in aerodynamic pitching moments.

Don Stackhouse
DJ Aerotech