Spoilers...Why..Where..How
by Ray Hayes August, 2002
In the past, no one asked "how do you install spoilers in a sailplane wing," we all knew how because most all the kits in the earlier days of sailplanes included parts and instructions on spoiler construction. Today's new builder probably cut his building teeth on a Gentle Lady or similar kit that does not offer spoilers and so it is all new. This article hopes to address this builders gap.
I built my first spoiler sailplane in 1974 and many more since then, and from my experimentation's with location, size and deployment, I offer the following tips.
WHY SPOILERS
As the name implies, the function of spoilers, is to spoil the
lift generated by the wing and at the same time increase drag.
Spoilers are usually thought of as an aid in landing accuracy and
almost always on a sailplane limited to rudder and stab control.
Spoilers are used by some pilots to bring the plane down from great
heights in a safe manner, although, a plane with a strong wing like my
Bird Series, can be dived down from spec altitude with the spoilers
closed. Probably the most important function is after the plane has
entered the last minute to thirty seconds of the landing flight path,
they can be deployed on and off to drop the plane to the proper height
needed to hit the landing spot at the precise target time. The proper
use of spoilers requires pilot skill, which translated, means practice
and experimentation. There is no free lunch.
WHERE SHOULD SPOILERS BE LOCATED
This is easily the most discussed aspect of spoilers along with the size of spoilers. The first item on my list of this subject is to state that the greatest value of spoilers is on a plane with flaps. That is, if you prefer not to spear chuck your landings, but make a more full scale type landing, where if there were lives on board your plane, they would all survive. Today's programmable radios allow the spoilers to be coupled to the flaps, which produces a very realistic landing approach and touch down.
Tom Scully among others have coupled spoilers and flaps on their Sky Bench Challengers and will attest to the value this system produces. I am a realist and spear my RES contest landings very hard.
Dr Mark Drela has given permission to quote his thoughts on spoiler location that he sent to me in response to this question I posed to him recently.
Some of my friends are telling me that it doesn't reduce the airfoil's efficiency to have the spoilers taped on top rather than recessed. True or false?
That's quite an over-generalization. It completely depends on the chordwise position of the spoiler. Protruding can be benign or a disaster. Recessed is always safe. To be more specific, the step in the surface should be smaller than the boundary layer displacement thickness (delta-*) at that location if there is to be little or no penalty. The delta-* distribution can be easily calculated with Xfoil. The worst-case situation is at high speed, like a fast penetration glide, since delta-* will be the thinnest then. In general, the farther back on the chord, the less critical the fit. On my RES ships I put the spoiler as far back as the structure or the rules allow (within 2" of the TE). That way an ill-fitting spoiler is much less likely to cause problems.
Mark
The spoiler size is another debated issue among sailplaners. Most arguments in favor of long spoilers, anything exceeding 3 bays is long in my view, are based on the early kits of the seventies. What you see is what you copy unless you experiment like I have and learned that smaller spoilers work great because they mess up the airfoil boundary layer less than larger ones, drop the nose, but not dramatically and open up a wider envelope of performance parameters. I deploy my spoilers to full open with the landing gear switch, this means they are either open or shut.
It is likely, after seeing the small spoilers on my Big Bird, pictured here at the AMA LSF RES NATS, that many will say, there way to small, never try them and never learn just how good they work.
They are just two bays long and work like a charm.
HOW TO BUILD SPOILERS
This is a scan of my Big Bird construction plan showing the recommended spoiler location and installation. A frame is built around the spoiler blade to secure the covering material and provide a hinge surface to the front of the spoiler, make the frame as gap free as you can to insure less drag, the servo is located forward and requires building a mounting surface. You have the option of building a servo access hatch on the bottom side of the wing or just cover over the servo to save time and weight. Notice the spoiler horn push rod connector hole is well forward of the spoiler hinge line, this is necessary to make the spoilers open up wide. The hinge material can be iron on covering, but a tape works second best to recessed hinges. Both servos must be positioned facing the same direction if you use the landing gear switch or throttle stick to activate the spoilers.
An alternative spoiler installation is a single servo located in the fuse between the wings, with tubes in the wings to house radio dial chord or spider non stretch fishing line that pull the spoilers open. A long servo out put arm is necessary for a full up position. This installation requires an access hatch to the servo in the fuse to hook up the pull system. I like to make a loop in the string that will be attached to the two screws I place in the servo arm and crimping small aluminum tubing secures the size of the loop. Turn your RC system on to make the finale adjustments before crimping the tubes.
This system has been around since day one.