If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
I also liked the comment to fly a certain airspeed.
Our local CFI at Avenal pointed out that if one hits a gust up, the glider will speed up, because the C.G. is forward of the center of lift, so the nose drops. So he suggests quick back stick pressure to prevent the speedup and translate it into lift instead. He calls this "porpising" since that's what the G forces feel like. This same effect is subtle during landing. In ground effect, lift increases, and the nose drops and the glider speeds up. This is why transitioning to ground effect I need more back pressure. I learned how to take off and land a Cessna 172 using ONLY rudders and throttle, but was having trouble because it landed flat all the time when I couldn't use flaps. I couldn't set trim for further pitch because it would stall when out of ground effect. Instead I loaded for aft legal C.G., and the nose down pitch during transition to landing was much less. Oscillation was also obvious and needed throttle adjustments to dampen. I suspect competition glider pilots anticipate oscillations when encountering vertical gusts and counter them instantly with the stick. I've noticed myself getting wild pitch oscillations during the first turn when entering a thermal when I don't anticipate the oscillation... |
#12
|
|||
|
|||
|
#13
|
|||
|
|||
|
#14
|
|||
|
|||
With regards to vertical gusts...
A stable aircraft will react to this by pitching down and increasing its airspeed I thought a stable aircraft has the C.G. forward of the center of lift. If this is so, and this effect only happens if the aircraft is stable, then C.G. is important, right? If the C.G. and center of lift coincide, does this effect still occur? If the C.G. is behind the center of lift (my understanding of "unstable") does this occur? Martin has interesting points, but I'm not understanding them just yet (it may be I don't understand the terminology quite yet...) |
#15
|
|||
|
|||
On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:
On 15 Aug 2003 12:36:05 -0800, (Mark James Boyd) wrote: With regards to vertical gusts... A stable aircraft will react to this by pitching down and increasing its airspeed I thought a stable aircraft has the C.G. forward of the center of lift. If this is so, and this effect only happens if the aircraft is stable, then C.G. is important, right? If the C.G. and center of lift coincide, does this effect still occur? If the C.G. is behind the center of lift (my understanding of "unstable") does this occur? My guess, and it sure is only a guess, is that the changes in the indicated airspeed as a result of the glider flying into lift or sink WOULD occur regardless of the stability or instability of the aircraft. I'm guessing this is so because I'm also guessing that THESE changes in the indicated airspeed are not the result of instaneous pitch changes in the aircraft's attitude, but rather are changes in dynamic and/or static pressure directly created by the changes in lift and sink themselves. I suppose another way to say this is that the changes in indicated airspeed may be due to angle of attack changes that are not due to changes in the aircraft's attitude, but rather due to changes to the direction of the airflow (which are felt as changes in lift and sink. I dunno. This is absolutely wonderful stuff, but it leaves me really wanting a wind tunnel so I could test these things. I think I only further muddled this by my saying "actual airspeed" may not be changing. This is not at all the way to look at things. Indicated airspeed DOES change as a glider flies into lift and sink. Period. What I wanted to describe is a situation in which the changes in indicated airspeed are reflective of changes in the airflow over the glider created by the changed lift and sink, not of accelerations of the glider itself. Phooey. This probably only made it worse. I know what I want to say, I just can't find the right way to say it. |
#16
|
|||
|
|||
I am looking for a radio for my LS-4A and would like some
recommendations. Reliability, good features, power consumption, etc. I might also be interested in a used radio if available. A 2.25" Ø would be preferred. Thanks |
#17
|
|||
|
|||
What you guys are discussing is the 'Yates effect'
as described by Derek Piggot in 'Understanding Gliding' Appendix A and also published in Gliding magazine in 1951 by Dr A.H. Yates. John Galloway At 21:42 15 August 2003, Jim wrote: On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote: On 15 Aug 2003 12:36:05 -0800, (Mark James Boyd) wrote: With regards to vertical gusts... A stable aircraft will react to this by pitching down and increasing its airspeed I thought a stable aircraft has the C.G. forward of the center of lift. If this is so, and this effect only happens if the aircraft is stable, then C.G. is important, right? If the C.G. and center of lift coincide, does this effect still occur? If the C.G. is behind the center of lift (my understanding of 'unstable') does this occur? My guess, and it sure is only a guess, is that the changes in the indicated airspeed as a result of the glider flying into lift or sink WOULD occur regardless of the stability or instability of the aircraft. I'm guessing this is so because I'm also guessing that THESE changes in the indicated airspeed are not the result of instaneous pitch changes in the aircraft's attitude, but rather are changes in dynamic and/or static pressure directly created by the changes in lift and sink themselves. I suppose another way to say this is that the changes in indicated airspeed may be due to angle of attack changes that are not due to changes in the aircraft's attitude, but rather due to changes to the direction of the airflow (which are felt as changes in lift and sink. I dunno. This is absolutely wonderful stuff, but it leaves me really wanting a wind tunnel so I could test these things. I think I only further muddled this by my saying 'actual airspeed' may not be changing. This is not at all the way to look at things. Indicated airspeed DOES change as a glider flies into lift and sink. Period. What I wanted to describe is a situation in which the changes in indicated airspeed are reflective of changes in the airflow over the glider created by the changed lift and sink, not of accelerations of the glider itself. Phooey. This probably only made it worse. I know what I want to say, I just can't find the right way to say it. |
#18
|
|||
|
|||
On 15 Aug 2003 22:58:24 GMT, John Galloway
wrote: What you guys are discussing is the 'Yates effect' as described by Derek Piggot in 'Understanding Gliding' Appendix A and also published in Gliding magazine in 1951 by Dr A.H. Yates. John Galloway At least someone gets it. Also mentioned by Doug Haluza in an article in "Soaring" a few years ago. As you enter lift the glider accelerates forward due to the lift vector tilting forward in the flight direction. Entering sink the reverse effect occurs. This is a short lived effect for sharp edged gusts with time constants of the order of 0 .15 to 0.5 seconds for typical glider airspeeds and wing loadings. It also has interesting effects on TE varios and is one of the reasons that TE varios seem much quicker or more "nervous"in response than uncompensated varios connected to static sources. The other is the sensitivity of the TE vario to horizontal airmass changes"horizontal gusts". There is an article on our website about this. Mike Borgelt Borgelt Instruments www.borgeltinstruments.com |
#19
|
|||
|
|||
I On 15 Aug 2003 22:58:24 GMT, John Galloway wrote: What you guys are discussing is the 'Yates effect' as described by Derek Piggot in 'Understanding Gliding' Appendix A and also published in Gliding magazine in 1951 by Dr A.H. Yates. One day I'll get round to reading that. As you enter lift the glider accelerates forward due to the lift vector tilting forward in the flight direction. Entering sink the reverse effect occurs. This is a short lived effect for sharp edged gusts with time constants of the order of 0 .15 to 0.5 seconds for typical glider airspeeds and wing loadings. It also has interesting effects on TE varios and is one of the reasons that TE varios seem much quicker or more "nervous"in response than uncompensated varios connected to static sources. The other is the sensitivity of the TE vario to horizontal airmass changes"horizontal gusts". There is an article on our website about this. Mike, That is the clearest reason for it happening that I have ever seen. When you sketch out the lift and drag vectors and then see what happens when extra lift is added and removed it's obvious. Thanks Robin Mike Borgelt Borgelt Instruments www.borgeltinstruments.com -- Robin Birch |
#20
|
|||
|
|||
On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:
I suppose another way to say this is that the changes in indicated airspeed may be due to angle of attack changes that are not due to changes in the aircraft's attitude, but rather due to changes to the direction of the airflow (which are felt as changes in lift and sink. That's pretty much what I was trying to say. The change is AOA is instantaneous, but inertia effects will delay the change in attitude and (probably) this delay is responsible for quite a lot of the indicated airspeed increase on entering the thermal because it makes the required correction bigger than an instantaneous correction would require. I'm sorry that I can't easily diagram the velocity vector using only ASCII text! This was why I suggested you draw the still air vectors for forward speed (l-r horizontal), sink in still air (downward) and the resultant path (sloped down completing the triangle). There's a simplifying assumption that the wing's AOA is given by the angle of the resultant path. That's not strictly true, but doesn't affect the argument. Now draw the thermal velocity vector (upward, starting from the bottom of the sinking speed vector) and draw a new resultant slope. This will have a lesser slope than the still air situation and shows that the instantaneous AOA has been reduced, which reduces the wing's lift. This is an unstable situation which must be corrected and the normal reaction of a stable aircraft is to pitch down and accelerate to restore the lost lift. The attitude change in a free flight model is often quite obvious. Its pitching inertia is minimal by design: large efforts are made to concentrate its mass at the CG by shortening the nose as far as possible and reducing the weight of the tail group and boom. I've often seen them pitch down quite sharply on entering a thermal but not noticed a parallel speed increase. I dunno. This is absolutely wonderful stuff, but it leaves me really wanting a wind tunnel so I could test these things. This is actually quite difficult to show in a wind tunnel because it is a dynamic effect. Wind tunnels, OTOH generally show static effects. The best tools I know for showing dynamic effects are visualisation tools, vector diagrams and carefully watching free flight model planes. Martin has interesting points, but I'm not understanding them just yet (it may be I don't understand the terminology quite yet...) During a flight yesterday I realised that you can feel the pitch-up as you enter sink when dolphinning: as well as the sudden soggy feeling there is a distinct sensation that the rear of the glider is sinking fastest. I still can't say I saw a pitch up, just that the tail feels like its sinking faster. The resulting speed loss is almost certainly masked by pushing forward accelerate and the resulting acceleration is certainly slower than you can get by pushing over before leaving a thermal. -- martin@ : Martin Gregorie gregorie : Harlow, UK demon : co : Zappa fan & glider pilot uk : |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
PA28: Difference in constant speed prop vs fixed pitch | Nathan Young | Owning | 25 | October 10th 04 04:41 AM |
Constant speed props | GE | Piloting | 68 | July 3rd 04 04:08 AM |
Why do constant speed power setting charts limit RPM? | Ben Jackson | Piloting | 6 | April 16th 04 03:41 AM |
Practicing SFLs with a constant speed prop - how? | Ed | Piloting | 22 | April 16th 04 02:42 AM |
Constant Speed Prop vs Variable Engine Timing | Jay | Home Built | 44 | March 3rd 04 10:08 PM |