lml@lancaironline.net Mailing List Archive

I get a little worried when I hear “center of pressure” or "center of lift" in the same sentence as “stability”. One cannot really look at movement of center of pressure/lift and draw any useful conclusions regarding stability. There are three sub-parts of stability where terms and concepts routinely get mixed up in discussion: The trim condition, static stability, and dynamic stability. For static stability the primary drivers are the rates of change of lift with respect to angle of attack, or lift curve slopes, for the wing and the tail, the geometry, downwash and CG location. (see Equ. 1&2, http://www.n91cz.com/Stability/Lancair360_Static_Stability.pdf) These will locate your neutral point and determine your static margin. For the 360, I measured stick fixed-stability at -7 and 40 degree flap deflections. The static margin was virtually unchanged. Dynamic stability is affected by full flap deployment. There are changes in period and damping ratio. Also, the slope of the total aircraft pitching moment curve was reduced (Cmalpha in Table 1, http://www.n91cz.com/Stability/Lancair360_Stability_and_Control_Evaluation.pdf). Of practical interest to pilots is that it takes less stick force to get a response in this configuration. All configurations: Flaps-up, flaps down, CG forward, CG aft, all remained statically and dynamically stable. There is no question that changing flap setting does indeed move the center of pressure. Mathematically this is represented by a change in pitching moment coefficient (see TP-1865, http://www.n91cz.com/Interesting_Technical_Reports/NASA-81-tp1865.pdf). Note the 5 fold increase in pitching moment coefficient between -7 to +10 degrees flap deployment. This affects the trim condition. In other words, the tail is adjusted to counter the new pitching moment. It is very much possible to run out of trim, leading to some excitement in the cockpit. The MKII tail has more than enough trim to account for reflex at very aft CG conditions. I do not know the trimable range of the original small tail. TP-1865 (Figures 6-8) has the plots of pressure distribution for every condition tested. Theoretically each of these could be integrated to get center of pressure. You’ll note however that the NASA report never even mentions center of pressure. It simply isn’t a very useful parameter when analyzing stability. Chris Zavatson N91CZ 360std www.N91CZ.net -------------------------------------------- On Wed, 9/25/13, Wolfgang <Wolfgang@MiCom.net> wrote: Subject: [LML] Fw: [LML] Lancair 320/360 performance and stability To: lml@lancaironline.net Date: Wednesday, September 25, 2013, 8:01 AM Or move the CG forward. The further the CG gets behind the center of lift, . . or conversly . . the further the center of lift gets in front of the CG (reflex flaps) . . . . the less longitudinal stability you have . . . . or even goes negative. Check it out, trim for cruise and hold the elevator still. . . . . see if porpoising starts and amplifies. . . . . if so . . . your dynamic stability is negative. Wolfgang ----- Original Message ----- From: Christian Meier To: lml@lancaironline.net Sent: Tuesday, September 24, 2013 6:19 AM Subject: Re: [LML] Lancair 320/360 performance and stability Chris, today I made a picture during Cruise with Autopilot at 7500ft with following configuration: 770 kg 40l in header, 20l in each wing (80l total), 75kg and 83kg for pilot and co. Flap was on 7° reflex CG 26,20" My design CG is 22,8 - 30,3 from firewall back, horizontal was installed - 0.6° So it looks like if I would add more reflex than 7°, I would need more down elevator. So the gain with the higher reflex would be lost with the down elevator.... Christian Am 17.09.2013 um 21:18 schrieb Chris Zavatson <chris_zavatson@yahoo.com>: Scott, Thanks. Examining the 360 (MkII) performance and characteristics in greater detail as been very interesting. The small tail has a very low aspect ratio and may indeed be subject to higher drag if the stabilizer incidence requires significant elevator input to trim. The MkII tail adds about 2 sqft, but more significantly has a much greater aspect ratio. My stab was well aligned for the sweep of flap settings as the elevator deflection was about 0.5 degrees TE down. In fact, all of the points were inside of 0.1 degrees of elevator movement. The concept of aft CG being more efficient is by reducing trim drag. It is used quite successfully in aircraft that adjust the entire stabilizer for trim. A fixed stab angle that is too far from neutral in the aft CG or in the 'super-reflexed' cruise condition could negate any benefit. In my case the plot of flap setting vs. airspeed showed that I had not yet reached a peak. Extrapolating the curve gives me another 2 kts at 12 degrees reflex. Extrapolating is a bit dangerous with any polynomial curve, but on the other hand this one has an exceptionally well behaved 2nd order trend. -7 degrees certainly provides a large portion of the benefit. It would be very interesting to run through the same series of tests with a small tail at the same static margins for a side by side comparison. Chris Chris Zavatson N91CZ 360std http://www.n91cz.net/ From: "Sky2high@aol.com" <Sky2high@aol.com> To: lml@lancaironline.net Sent: Sunday, September 15, 2013 12:26 PM Subject: [LML] Re: Lancair 320/360 performance and stability Chris, Great research. In my small tailed 320, increased flap reflex experimentation did not result in increased top end speed. The nose up pitch was increased, requiring increased nose down trim - probably resulting in greater empennage drag negating any reduction in drag from the greater reflex. Of course, we would have to discuss the angle of incidence of the small tail and its relationship to the elevator correcting for nose down pitching ( my incidence was at -.9 degrees). By moving weights forward and aft in the same flight, forward CG was better for maximizing speed - unlike some aircraft that see max speed when the CG is at the neutral point, probably a consequence of more standard wing/tail design that saw drag from wing/horizontal +/- lift factors more balanced and minimized.