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#11
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PW-5 longitudinal pitch oscillation
Hi Chris -
61 flights and 96 hours in my PW-5 - mostly in MN. I can only remember once that the tail lifted in a thermal due to a vertical gust. There were no issues at all. The PW-5 just flew itself out of the gust and continued circling. The PW-5 is the easiest theramlling glider I've flown. Lou |
#12
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PW-5 longitudinal pitch oscillation
I didn't mention gust direction. I have been assuming it was a vertical gust lifting the tail, but I have no direct evidence for that.
In a nutshell, the comments offered seem to boil down to the following: a) check flight habits for unintended movements b) pull stick back to reduce tail-up pitching and roll into the thermal so indicated c) ignore it d) haven't seen/experienced it e) moving CG forward would exacerbate the rocking effect of the tail-up pitching I think about the rocking of a sailboat-- the retired coach says to move everyone toward the keel when dealing with waves so that the boat can rock fore and aft more easily (and thus lose less speed by bobbing a little more with the waves instead of slamming hard through the waves). From there I extrapolate that a wider weight spread toward the edges (fore and aft) can increase inertia and slow down oscillation about a central point. Seems the thing to do is to add ballast on the next flight, hope for strong thermal activity, and report back to the thread. More weight will inevitably move the CG forward; can't move myself backward or I wouldn't be able to reach the controls! Thanks, Chris |
#13
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PW-5 longitudinal pitch oscillation
Chris,
Some posters were confused over the title you used......believe you made a common mistake! An aircraft pitches around its lateral axis, rolls around its longitudinal axis and yaws around its vertical axis. Hope this helps, JJ |
#14
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PW-5 longitudinal pitch oscillation
Perhaps! What I describe is a pitch of the nose down and the tail up. Longitudinal seesawing (pitching) at the lateral axis.
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#15
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PW-5 longitudinal pitch oscillation
On Fri, 08 Sep 2017 21:08:42 -0700, larsonchristina wrote:
Perhaps! What I describe is a pitch of the nose down and the tail up. Longitudinal seesawing (pitching) at the lateral axis. All aircraft entering a thermal will do this, some more noticeably than others. Reasoning: as the glider flies into the thermal it enters a rising air mass and the further it moves into this air mass, the faster the vertical movement becomes. The effect on the glider is that its effective AOA is reduced by the air's vertical velocity (draw a vector diagram and this becomes obvious), which in turn means that the wing no longer supports all the glider's weight. The effect of trim means that the glider will automatically pitch down and speed up to regain lift. The combination of this small forward acceleration plus the upward acceleration as to enters progressively faster air is what you feel as "the surge" when you enter a thermal. What you see and feel depends on what you're flying. In my Libelle I don't particularly notice the pitch down, and anyway it may be masked as I'm pitching up to slow down from inter-thermal cruise speed or even faster flight through the sink surrounding the thermal, but the surge forward and up is quite easy to feel. The most obvious demonstration of this that I've seen was shown by an A/1 competition model glider of around 4 feet span, even when it was 50-60 m overhead. When entering a strong thermal it would pitch down quite rapidly and about as steeply as a helicopter does after takeoff when its pilot is in a hurry to get moving before returning to its normal trimmed attitude as it entered the core of the thermal. Like all free flight models of its era, its trim was set before launch and remained so throughout the flight, so this strong pitch down was not due to trim changes. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#16
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PW-5 longitudinal pitch oscillation
On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie wrote:
Reasoning: as the glider flies into the thermal it enters a rising air mass and the further it moves into this air mass, the faster the vertical movement becomes. The effect on the glider is that its effective AOA is reduced by the air's vertical velocity (draw a vector diagram and this becomes obvious)... Seems the opposite to me. The rising air hits the wings from below, and thus at a higher AOA. This increases the lift on both the wing and the tail, but more so at the tail. This is what positive stability means. The glider then behaves just like after raising the nose momentarily and then letting go of the stick: it pitches nose down. It's not the trimmed speed it is seeking per se, it is the trimmed AOA. Thus in the thermal entry it pitches nose-down despite the speed being normal. Once the glider accelerates upwards due to the rising air, the vertical motion relative to the air returns to normal, and the normal pitch attitude relative to the horizon will be restored on its own. The more pitch-stable the glider (and yes this is called "longitudinal stability" IIRC?), the stronger this pitching effect should be. Maybe that's why I haven't noticed it in my gliders, since I have the CG near the rear end of the range, i.e., weak stability. |
#17
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PW-5 longitudinal pitch oscillation
On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie wrote:
All aircraft entering a thermal will do this, some more noticeably than others. Reasoning: as the glider flies into the thermal it enters a rising air mass and the further it moves into this air mass, the faster the vertical movement becomes. The effect on the glider is that its effective AOA is reduced by the air's vertical velocity ... Seems the opposite to me. The rising air hits the wings from below, and thus at a higher AOA. This increases the lift on both the wing and the tail, but more so at the tail. ("More so" in the sense of the strength of the resulting pitching moments around the CG.) This is what positive stability means. The glider then behaves just like after raising the nose momentarily and then letting go of the stick: it pitches nose down. It's not the trimmed speed it is seeking per se, it is the trimmed AOA. Thus in the thermal entry it pitches nose-down despite the speed being normal. Once the glider accelerates upwards due to the rising air, the vertical motion relative to the air returns to normal, and the normal pitch attitude relative to the horizon will be restored on its own. The more pitch-stable the glider (and yes this is called "longitudinal stability" IIRC?), the stronger this pitching effect should be. Maybe that's why I haven't noticed it in my gliders, since I have the CG near the rear end of the range, i.e., weak stability.. |
#18
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PW-5 longitudinal pitch oscillation
On Sat, 09 Sep 2017 10:25:20 -0700, moshe.braner wrote:
On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie wrote: All aircraft entering a thermal will do this, some more noticeably than others. Reasoning: as the glider flies into the thermal it enters a rising air mass and the further it moves into this air mass, the faster the vertical movement becomes. The effect on the glider is that its effective AOA is reduced by the air's vertical velocity ... Seems the opposite to me. The rising air hits the wings from below, and thus at a higher AOA. If it was a body of air all rising at the same speed I'd agree, but in as it enters a thermal the vertical velocity will be increasing as the glider heads for the core, IOW its facing a continous disturbance which has two components. One is the vertical velocity that the glider has acquired from the air its just passed through. The second is the effect of flying into air with a still higher vertical velocity. ("More so" in the sense of the strength of the resulting pitching moments around the CG.) This is what positive stability means. If it was in a parcel of air with the same vertical velocity I'd agree with you, but it ain't: its flying onto steadily increasing vertical velocity as it approaches the thermal core, so if things are as you describe, you'd expect the glider to be pitching up. As that's not what we see and feel, there must be something else acting on it, which I think is the effect of flying into an increasing velocity gradient is to reduce the effective AOA of the wing. Of course in reality it affects both wing and tail, but its effect on the wing is a little bigger because the wing gets to the higher velocity rising air before the tail does. Granted its a small effect, but I think its enough to cause the nose to drop/tail to rise. It's not the trimmed speed it is seeking per se, it is the trimmed AOA. Yes, you're right there. I should have made that clearer. the stronger this pitching effect should be. Maybe that's why I haven't noticed it in my gliders, since I have the CG near the rear end of the range, i.e., weak stability. Yes, that makes sense. My CG is pretty much in the middle of the permitted range. The force set up in the model will also be different: free flight gliders normally have the CG at 52-55% of the wing chord, use a lifting section on the tailplane and, yes, they are stable with a lifting tail. Almost all designs operate with the wing at a Cl of around 1.2 and the tail at a Cl of around 0.2. This puts the wing at an AOA of about 4 degrees more than the tail, which keeps this layout stable with fast vertical upset recovery. The tail is much bigger than on a sailplane - about 15-18% of the wing area and, as a nice to have, most of the favoured tail sections give minimum drag at a Cl of 0.2 - a nice bonus, since the tail is also doing work against gravity as well as keeping the glider stable. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
#19
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PW-5 longitudinal pitch oscillation
So I increased the ballast weight in the PW-5 from parachute + 20 pounds to parachute + 40 pounds. The pitch stability increased noticeably, i.e. uncommanded nose-down excursions diminished.
Thanks all; I've enjoyed reading about model gliders and changes upon entering thermals. |
#20
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PW-5 longitudinal pitch oscillation
On Wednesday, September 13, 2017 at 6:10:58 PM UTC-7, wrote:
So I increased the ballast weight in the PW-5 from parachute + 20 pounds to parachute + 40 pounds. The pitch stability increased noticeably, i.e. uncommanded nose-down excursions diminished. Thanks all; I've enjoyed reading about model gliders and changes upon entering thermals. In my only flight in the PeeWee had hoped to do spins. Couldn't get it to stall, even accelerated, so gave up on spins. That was with no nose weight. Perhaps it could use tail weight? Like the OP, with way forward C/G, didn't notice any tendency to oscillate in pitch. It still climbed well, don't ask about cruise. Did notice turbulence perhaps from the large access hole in the horizontal stab. Now there is no World Class and associated rules, that design problem could be fixed with mylar. Jim |
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