I guess it's probably time to reveal some more secrets. Tom Clarkson asks:
>Lastly on the topic of boom length, I am having a hard time making the >empirically observed behavior fit with the way things should work. We have >two identical fuses and tail groups except one has a 2" longer boom length. >The shorter one turns better and tighter using all of the test wings at >different EDAs. So, I am trying to figure out what could be going on >because it does not make sense given the previous comments on this topic. First of all, be careful here. Damping has to do with what happens when you're changing something, such as rolling into or out of a bank. Other factors come into dominance when you're in a steady state condition, such as straight and level flight, or a constant diameter, constant bank angle turn. The other thing that seems to be inadequately covered or even missing altogether from these discussions and analyses is the curvature of the airflow while in a turn. The airplane is flying in a curved path, and therefore the relative wind at the tail is not coming from the same direction that it's coming from at the wing, or at the nose. You can see this in a full-scale sailplane that has both a ball-slip indicator on the instrument panel, and a yaw string taped to the front of the canopy. In a really tight turn, particularly with a light wing loading, the tail end of the yaw string will point a little bit towards the outside of the turn when the ball-slip indicator is perfectly centered. This makes perfect sense if you consider that the ball tells when the turn is coordinated (and therefore the wings are lined up with the airflow at their location), but the yaw string, located out on the nose, well ahead of the wings, is seeing a distorted flow due to the curvature of the turn. Likewise, in a tight but coordinated turn with the local airflow perfectly aligned with the wing, the airflow at the tail will be blowing inward and upward. From a control standpoint, this has the same effect as holding some top rudder and down elevator. The tighter the turning radius in comparison to the size of the model, and the longer the tail moment, the more pronounced this effect becomes. On a good HLG design, it's not uncommon to see differences in the direction of the airflow at the wing vs. the direction at the tail of 10 to 15 degrees or more in both pitch and yaw. It's an important enough effect that I've included it in my own design programs as one of the major parameters I consider when designing the tail assembly. On airplanes that normally make big, wide turns, this effect is often negligible. Not so on a good HLG! Another consideration: 2-channel (non aileron) HLG's need a little top rudder in a turn to yaw the airplane a little to the outside. This increases the angle of attack of the inside wingtip, increasing its lift coefficient and counteracting the loss of lift it sees due to its lower airspeed in a turn. Since the length of the tail moment arm influences how much angular difference there is between the local flow at the wing vs. at the tail, the tail moment arm can be adjusted to give just the right amount of outward yaw necessary to keep the turn balanced. If you have too much tail moment, not only will the damping make it difficult to roll crisply in and out of a turn, but the curvature of the flow during the turn will constantly be trying to make the airplane level out again. Yes, you can add more rudder to counteract this, but eventually you will run out of rudder. The airplane with a shorter tail moment will get to a tighter circle before it runs out of rudder. However, since damping is proportional to the square of the tail moment, the loss of damping from the short tail moment will make it more difficult to keep the short-tailed airplane steady in the turn, even though it has the same tail volume coefficient as the long-tailed airplane. The same tail volume coefficient will give it about the same static stability, but the shorter tail moment will give it less dynamic stability. You need to have the right amount of BOTH of those if you want the airplane to handle well. Unfortunately, the amount of yaw required, and the amount of yaw caused by the airflow curvature both change as you change bank angle. Therefore, you will probably have to tune the tail moment for your typical turn, and accept the need for some pilot inputs for turns with less or with more bank angle than that. Airplane design almost inevitably involves some compromise. Don Stackhouse @ DJ Aerotech [EMAIL PROTECTED] http://www.djaerotech.com RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
