On Mon, 2002-03-18 at 14:41, Andy Ross wrote: > [[Whoops, my To: line wasn't, so the first time this got sent is went > into the moderator queue. Moderators, please ignore. My > apologies]] > > [This came in private mail, but it involves questions that I think the > rest of the list might want to weigh in on. Sent to -devel rather > than -flightmodel to get a broader selection of real-world pilots' > input.] > > Curtis L. Olson wrote: > > I think there may be a problem with flap modeling. Here's how I set > > up my experiment. > > > > Aircraft: Yasim C172 > > > > I took off and set the autopilot for 1000'. Zero flaps, full throttle > > climbed to 1000' leveled off and let the speed settle out. Then I > > added 10 degrees flaps and let the speed settle out, then 20, then > > 30. Here are the results (again full throttle, autopilot locked to > > 1000' ASL): > > > > Flaps Speed > > ===== ===== > > 0 deg 119 kts. > > 10 deg 123 kts. > > 20 deg 126 kts. > > 30 deg 128 kts. > > > > It appears that the flaps give a good lift boost, but perhaps the drag > > is going in the wrong direction??? (Or maybe it's more complicated > > than that ... are you modeling extra drag? maybe the extra lift means > > less work the engine has to do so it can go faster.) > > > > I see the same general trend with the DC3. > > You're not changing the trim settings when you do this, right?\
He had the autopilot on, maintaining 1000 ft. So, yes, the trim was changing. > > What you're seeing is the fact that, when you add flaps, you increase > the nose-down pitching moment of the wing. This happens, > qualitatively, because most of the lift you add gets added at the back > of the wing where the flap is. Most of the "regular" lift happens > toward the front of the wing, where the point of lowest pressure on > the upper surface is. > > So, since there is more nose-down moment due to AoA, and you haven't > changed the elevator force (trim) at all, the aircraft is now trimmed > to fly at a lower AoA. At the lower AoA, there is less lift and less > drag than at the higher AoA. All of this is physically correct, and > in fact is very poorly modeled by other consumer simulators -- I know > from personal experience that a Piper Cherokee 180 pitches down when > flaps are applied. (To be fair, high-wing aircraft like cessnas might > not show this at all -- they get a pitch-up moment due to the extra > drag that may offset the nose-down effect due to flap deflection). > > The question is: is the drag added due to flap deflection more or less > than the drag reduction due to lower AoA? I don't know. It's > probably aircraft-dependent, and I haven't seen any numbers anywhere > that talk about it in the context of a whole aircraft. Wing section > books, however, *do* note this effect. On a quantitative level, YASim > is roughtly in accord with published section data for typical > airfoils. The thing about drag due to lift (not AoA) is that it will be about the same for a change in flaps in trimmed flight and no change in speed. The amount of lift you need to make is unchanged. So, here, the 2D data is probably most telling. > > Right now, YASim applies all of the "extra" lift due to flap > deflection (including control surfaces -- they're flaps too) at a > point one third of the way up from the trailing edge. Perhaps this is > too far from the center? I'll investigate changing the point of > flap-force application to see if this helps, but I'd be really curious > as to what the "right" behavior should be. I suspect your intuition > is perhaps leading your to a wrong assumption. Flaps add drag, but > they do other things too. The change in trimmed airspeed is affected > by things other than drag. > > If you repeat your experiment, not with a constant trim but with a > constant AoA, you will see the speed reduction you are expecting. > Since POH books don't talk about trim, and only about level flight > numbers, neither one of us can be easily proven "wrong" without > reference to a real aircraft to experiment with. Thus, the cc: to the > list. For the real pilots out there, what does the airspeed and AoA > do when you apply flaps but leave the elevator trim alone? I think he's right. Here's some data that Alex obtained for me: //Alex Perry's descent data // Idle power, 70 KIAS // Flaps Up 700 FPM // Flaps 10 800 // Flaps 20 850 // Flaps 30 900 // Flaps Up, Full Rudder Sideslip 1000 FPM // Flaps 30, Full Rudder Sideslip 1200 Since these are all low power, the indicates in descent rate indicates an overall increase in drag. > > Andy > > -- > Andrew J. Ross NextBus Information Systems > Senior Software Engineer Emeryville, CA > [EMAIL PROTECTED] http://www.nextbus.com > "Men go crazy in conflagrations. They only get better one by one." > - Sting (misquoted) > > > _______________________________________________ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- Tony Peden [EMAIL PROTECTED] We all know Linux is great ... it does infinite loops in 5 seconds. -- attributed to Linus Torvalds _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
