kcaldwel's blog
Posted by kcaldwel | Sep 09, 2013 @ 12:10 AM | 1,551 Views
The short answer is, no. A conventional tail aft airplane actually has the lowest trim drag with a small positive static margin, usually around 5 to 7% depending on the geometry, and a small down load on the tail.

http://aero.stanford.edu/Reports/MultOp/multop.html

scherrer.pagesperso-orange.fr/matthieu/aero/papers/trimdrag.PDF

Kroo, I., "Trim Drag, Tail Sizing, and Soaring Performance," Technical Soaring, Vol 8, No. 4, pp 127-137, July 1984

I ran some XFLR analysis on a Supra to see what affect the CG location has on the aerodynamic performance.

Dr. Drela plan shows the CG at 94.5mm aft of the root LE, which I calculate gives a 7% static margin. I ran the CG at 7% SM (94.5mm, stab at -2.25 degrees for trim at best L/D), 0% SM (110mm, stab at -0.75 degrees), and 12% SM (85mm, stab at -3.15 degrees).

This is 25 mm of CG location change, or about 1" requiring a change in the stab angle of about 2.4 degrees to trim at the same wing AoA, 4.75 degrees, which is the best L/D wing AoA for all three cases.

CG (mm)____SM____Stab Angle________Best L/D at 4.75degrees wing AoA
85_________12%___-3.15 degrees______28.22
94.5________7%____-2.25 degrees______28.32
110________0.5%____-0.75 degrees_____28.28

Best L/D performance comes with about 7% static margin, and the stab at -2.25 degrees, with a small down load from the stab. Exactly where Dr. Drela's plan shows it.

XFLR5 shows about a 0.3% difference in best L/D over that CG range, without accounting...Continue Reading
Posted by kcaldwel | Sep 08, 2013 @ 11:44 AM | 2,224 Views
Below I have given a simplified description of aircraft stability and trim. So what can we do with these ideas?

One of the best uses is setting the CG location on airplanes. I have tried to describe how the CG location relative to the aircraft neutral point determines the stability of an an airplane. The amount of stability can be increased by moving the CG further forward from the neutral point, or decreased by moving it back towards the neutral point.

The amount of static stability an airplane has determines how it responds to gusts and to pitch control inputs. The further forward the CG is from the neutral point, the harder the glider will try to return to it's trim speed. Stable airplanes need larger elevator or flying stabilizer movements to change trim speeds, and less stable ones need smaller movements. It is always necessary to change the elevator throws when changing CG positions. Unless you increase the throw when moving the CG forward, the glider will feel increasingly dead on the elevator. Conversely, unless you decrease the elevator throw as you move the CG back, the airplane will feel twitchy and hard to control in pitch.

RC sailplanes can be successfully flown with the CG right at the neutral point, and even well behind it by good pilots. With the CG at the neutral point, the glider will stay in whatever attitude it is placed in, or disturbed to by a gust. The pilot must manually continually adjust the glider's attitude to maintain the desired speed. This...Continue Reading
Posted by kcaldwel | Sep 07, 2013 @ 02:40 PM | 1,762 Views
There are a lot of misconceptions amongst RC pilots about aircraft pitch stability and trim. Fortunately we have had one hundred years of smart people figuring out how airplanes work. I'm going to try to document a simplified version of aircraft static pitch stability and trim that I hope most people can follow.

Static stability and trim refers to 1G unaccelerated flight. In the case of a sailplane, this means a steady glide at a single speed in smooth air. This is the basis for understanding pitch stability and trim.

I will need to introduce the idea of a moment or torque. A moment is a force multiplied by a distance. This is what a torque wrench measures. If you can't get a bolt undone with a short wrench, you can apply a larger moment with the same force using a longer wrench. A small force at a large distance can apply the same moment as a larger force at a smaller distance.

The drawing below shows the main forces on a sailplane in a smooth glide. There are other forces, such as the fuselage drag and the moment of the tail airfoil, but these are usually quite small influences compared to the main forces I have shown. I am going to ignore the small forces and the small angles that exist between the main forces, and thrust and drag that must also be balanced, for the sake of simplicity.

The standard convention would be for upwards forces to be positive, and clockwise, nose up moments or torques to be positive. I have shown the tail lift as upward, but the amount of...Continue Reading