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On 12/7/2007, Paul Waite wrote:
"The information about the horizontal stabilizers is correct; the larger surface is for supersonic flight. You would have to look at the development history of the original Bell X-1 flights from 1947, where they found that the ailerons at supersonic flight would do nothing, and the plane would have no control; this is due to the surface area of the wing being too small. So they decided to move the whole wing, which was able to move the air easier, and so increase control. "
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SS1 Horizontal Stabilizers

Here is a great photo of the horizontal stabilizers on the SS1. The entire assembly pivots from -5° to +13° and are used "for trim and supersonic flight control" according to the Scaled Composites website. The ailerons (the flap on the trailing edge of the elevons) are used "primarily for subsonic pitch and roll control".

It was discovered during NASA's the earliest experimental supersonic flights that the airflow over typical control surfaces tends to delaminate -- which simply means they don't work very well. So designers learned to rotate the entire surface, which is why the designers at Scaled decided to rotate the elevons for supersonic flight. We don't intend to fly the SS1 at supersonic speeds — so why include this function on the SS1?

The photo of the SpaceShipOne at landing (to the left) shows these stabilizers rotated the full +13°. So we plan to include this control in our design. The rotating elevons may not be needed at all, but we won't know if we need the additional control surface area until the SS1 is in the air and on its way down.

Tell me that isn't a sobering thought.

The real SpaceShipOne has rudders at the trailing ends of the vertical stabilizers. At the time of this writing we do not plan to include those rudders in our design -- but things can change. This will leave us without yaw control. (I'm told by RC pilots that they would likely not be used anyway, so we will eliminate the additional complexity of the model by leaving them off.)

The SS1 elevons started out looking like this. These are the styrofoam cores covered in balsa sheeting.

Here they are again, after the end pieces have been attached and a layer of 1/4 ounce glass cloth has been applied. They are so pretty they look like artwork.

The square hole on the underside of the elevon is the compartment for the servo that works the aileron.

Once we made the decision to make the horizontal stabilizers rotate, I had to rethink how these are made and how they will be mounted. The first modification is to trim 3/4" off of the base and add a strip of aircraft balsa. This photo shows the rough-cut of the balsa addition before final shaping.

After the balsa bases have been trimmed to shape, I bored a 5/8" hole the entire length of the stabilizer up to the balsa tip. I inserted a 5/8" aluminum sleeve into this channel and secured it in place.

There will be three nested aluminum tubes that form the pivot mechanism.

  • The 5/8" outer sleeve that I described above. Inside the 5/8" sleeve is a;
  • 1/2" aluminum tube that is secured to the mounting location on the vertical stabilizer. Inside that tube is a:
  • 5/16" aluminum tube that is secured to the outer end of the horizontal stabilizer and extends out through the opposite side of the vertical stabilizer.

It is on the opposite side of the vertical stabilzer where the assembly is pinned in place by the arm the connects to the servo.

The central pivot rod must be secured to the balsa at the outer tip of the horizontal stabilizer. I drilled a hole and tapped it out to take a piece of 6-32 threaded rod.

Here is the threaded anchor rod in place in the pivot rod. I cut a slot in one end to accept a standard screwdriver, and then shaped the other end into a point.

Here is the outer tip of the horizontal stabilizer (without the pivot rod in place). The 5/16" hole on the end is where the pivot rod is inserted. The smaller hole on the forward edge is where the anchor rod holds the pivot rod firmly in place.

The 1/2" tube is installed into place

The 5/16" pivot tube extends right through the center of the 1/2" tube and out through the opposite side of the horizontal stabilizer.

To see how these parts get assembled with all the others, click [HERE!].

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