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I fly an ASW27, which is a 15 meter glider with flaps. I am flying with the CG at 62% aft. I use down flaps while in a thermal and climbing; negative (up flaps) are used in cruise between thermals. typical thermals speed is 50 to 60 kts (with ballast) and 90 to 100 kts in cruise. This glider has a T tail and I need nose up elevator for thermal and nose down elevator for the cruise.

I notice that in cruise, the elevator is deflected down about 10 deg. Is that deflection creating a significant amount of drag that could reduce my glide performance? I have the ability to move the CG by dumping the ballast in the tail tank

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  • $\begingroup$ I would think that you could determine some of that empirically by trimming for your selected glide with full ballast and with the ballast partially dumped to determine which is giving you a better glide descent ratio, which is going to be related to the total drag on the airframe. Keep in mind that when dumping ballast out you are altering the total inertia of the glider which is also going to affect the glide ratio of the craft. It may be that the additional drag penalty from trimming nose down is offset by the added inertia of the ballast. $\endgroup$
    – Romeo_4808N
    Apr 29, 2017 at 2:58
  • $\begingroup$ An empirical determination can be done but such tests need still air. Such air can be found early in the morning, or very late in the afternoon and during the winter months. The effect of total ballast is well understood. My question centers on the effect of the elevator deflection during the cruise portion. The deflection is in effect "lifting the tail" despite the negative lift normally created by the horizontal tail. $\endgroup$
    – John Mittell
    Apr 29, 2017 at 12:43

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As I mentioned in my comments. You could test this empirically with the experiment described there. If you find the additional ballast improved the glide performance of the craft, is there an aftermarket STC kit available which installs new ballast tanks in a more optimum position for a more neutral cruise trim?

I am a little concerned with the idea of continuing to move the CG aft of its current position in order to optimize the performance at the expense of. Asking the craft less stable about the lateral axis. No increase in performance is worth your life.

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  • $\begingroup$ The full range of the anticipated CG movement is known to be well within acceptable (safe) design limits of the glider. The aft most CG position under consideration is still well forward of the aft design limit. $\endgroup$
    – John Mittell
    Apr 29, 2017 at 12:22
  • $\begingroup$ The glider has both wing tanks to carry water which is to increase the total weight as well as a tank in the tail fin which compensates for the forward shift in CG due to the wing tank ballast. All tanks can be emptied in flight, which is usually done prior to landing. The design provides acceptable flight characteristics with any level of ballast. $\endgroup$
    – John Mittell
    Apr 29, 2017 at 12:28

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