# How can I determine the amount of trim drag?

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

• 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. Apr 29, 2017 at 2:58
• 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. Apr 29, 2017 at 12:43