Eights On Pylons
06-03-2008, 05:38 AM
#12
Blue Light Special
Joined APC: Jan 2007
Position: Learjet 45 & G-IV
Posts: 637
the funny thing is... (I haven't looked in awhile) but I'm pretty sure that 8's on is the only *required* maneuver on the commercial and cfi practical... everything else is an optional task at the mercy of the DE.
06-03-2008, 12:09 PM
#13
Gets Weekends Off
Thread Starter
Joined APC: Jul 2007
Position: Lear 55 CA
Posts: 244
Originally Posted by Pilotpip
How did you become an instructor without knowing how to compute this and teaching it to your students when you introduce Eights-on? That formula is first and foremost in nearly every aviation text out there.
I am a CFI just like any other CFI out there and worked hard for it. you dont even know me so go somewhere else with that stuff.....
06-03-2008, 12:34 PM
#15
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Position: ATP, CFI etc.
Posts: 6,056
Originally Posted by wmuflyboy
Had a student just take his commercial ride but he told me the DE was not to happy with the explanation of how my student found pivotal altitude in the airplane. We have always done them at 1,200 MSL and they work out nice. He passed and everything but I was just curious if any of you knew a simple way to figure PA out in the airplane. The GS is right there on the GPS but obviously you wont know your GS if your on the ground before the flight. Should he have broke out a calculator or something??
Someone posted a table [edit: table uses ground speed which is incorrect, it should be airspeed. GS works but it is better to use airspeed because that is what the pilot sees.] which would work if he didn't know what airspeed to use for the manuever but since airspeed is voluntary you can choose that. Groundspeed will be an average of this figure no matter what the winds are, because it is a closed loop (two loops actually, but Kelvin's Theorem applies). So is not required to know what the gps says about anything. The other variable is local ground elevation, and the only way to know that is with a radar altimeter, but just estimate it.
I do not know what his examiner said, but I doubt he wanted anything besides proper use of the formula on the ground (with calculator) before leaving. If he wanted something more in depth then I would be very surprised, but I posted a link to something on the actual physics and you could bring a printout of that along, but I seriously doubt he wants anything that deep. He probably saw the airspeed your student was using and it did not match the proper altitude for that speed. I use 100 IAS in the 172RG, and use about 900 AGL for pivotal altitude. Around here ground is an average 1400 MSL, so 1400' + 900' = 2300' MSL. It is not an exact kind of thing, because you don't know the precise ground elevation.
Whatever the case, it's nice you care enough to share your issue with the other pilots here on the subject. Discussions like these can help all of us in one way or another.
Last edited by Cubdriver; 06-04-2008 at 06:12 AM.
06-03-2008, 05:43 PM
#16
Line Holder
Joined APC: Aug 2007
Posts: 86
As your start the 8's on, your Ground Speed is at the highest since you always start on the downwind. When turning from downwind to upwind, your actual ground speed will decrease due to the headwind. Therefore to track a radius around the "pylon", you will have to descend the aircraft to a lower altitude to match the PA at that given lower GS. Were expected to hold a constant bank angle and keep a line from the wing to the pylon during this move, so the only way to keep a line on the pylon is to either "dive for it" or "climb to catch it" However, the PA is always changing because as you move from upwind back to downwind, your GS increases and therefore your PA increases. When moving from one pylon to the next, you will be at your entry airspeed and therefore your entry altitude.
The Pivotal Altitude is based on your ground speed. The best way I can explain it is to picture a funnel coming up from the pylon. Now, as your airplane enters the maneuver, it is at it's fastest speed. For a given unit of time, the plane will travel so much distance. As the plane slows down due to wind conditions, then it will travel less of a distance per given unit of time.
Picture the funnel now. Cut it off into five different portions, with the wide mouth at top and the narrow part at the bottom. Now the airplane is "tracking" the funnel at a required bank angle. The goal of the pilot is to "ride" the wall of the funnel and keep the wing on the pylon. Now, depending on what speed the plane is going at is going to determine where on the funnel the plane is. A plane going faster is going to make a wider radius turn, hence needing to be higher on the funnel. A slower plane is going to make a tighter radius turn, so it will need to be lower on the funnel. It is important to remember that the goal of this is to keep your wing on the pylon, and not so much an "equidistant" path around the pylon.
One thing people get wrong is that they underestimate the importance of PA. Pivotal Altitude is based on the SQUARE of your ground speed, so any slight changes in Ground speed is going to required NOTICEABLE changes in your pivotal altitude.
Above all though, once somebody wraps the mind around the concept of 8's on and what they are supposed to accomplish, they will become monsters with it. I myself just fumbled my way through through it during my commercial, entering, climbing and descending and completing it without a real understanding why I was doing it. Once you understand why you learn the move and the science behind it, you can nail it every single time.
The Pivotal Altitude is based on your ground speed. The best way I can explain it is to picture a funnel coming up from the pylon. Now, as your airplane enters the maneuver, it is at it's fastest speed. For a given unit of time, the plane will travel so much distance. As the plane slows down due to wind conditions, then it will travel less of a distance per given unit of time.
Picture the funnel now. Cut it off into five different portions, with the wide mouth at top and the narrow part at the bottom. Now the airplane is "tracking" the funnel at a required bank angle. The goal of the pilot is to "ride" the wall of the funnel and keep the wing on the pylon. Now, depending on what speed the plane is going at is going to determine where on the funnel the plane is. A plane going faster is going to make a wider radius turn, hence needing to be higher on the funnel. A slower plane is going to make a tighter radius turn, so it will need to be lower on the funnel. It is important to remember that the goal of this is to keep your wing on the pylon, and not so much an "equidistant" path around the pylon.
One thing people get wrong is that they underestimate the importance of PA. Pivotal Altitude is based on the SQUARE of your ground speed, so any slight changes in Ground speed is going to required NOTICEABLE changes in your pivotal altitude.
Above all though, once somebody wraps the mind around the concept of 8's on and what they are supposed to accomplish, they will become monsters with it. I myself just fumbled my way through through it during my commercial, entering, climbing and descending and completing it without a real understanding why I was doing it. Once you understand why you learn the move and the science behind it, you can nail it every single time.
06-04-2008, 03:25 AM
#17
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Originally Posted by POPA
Yes - before even meeting with the examiner.
Prior to the checkride, get a copy of the local weather and use the winds to find your [student's] groundspeed. Using that groundspeed, calculate pivotal altitude and write that on the same paper that will be used for writing down the WX report in the plane.
Using the same pivotal altitude regardless of surface winds is indicative of a serious training deficiency.
Prior to the checkride, get a copy of the local weather and use the winds to find your [student's] groundspeed. Using that groundspeed, calculate pivotal altitude and write that on the same paper that will be used for writing down the WX report in the plane.
Using the same pivotal altitude regardless of surface winds is indicative of a serious training deficiency.
Let me try again. 1) ground speed is not the quantity used in the formula for pivotal altitude calculation, it is airspeed in either mph or knots. Square the airspeed and divide by either 11.3 if using knots, or 15 if using mph on the airspeed indicator. I just noticed most of the posters on this thread use ground speed, which is incorrect. See Airplane Flying Handbook FAA-H-8083-3A pg. 6-14 for a reference on this if in doubt. I realize some schools use ground speed in the formula and it works, but the best thing is to use airspeed because it is what the pilot sees.
2) the only need for info on local prevailing winds is so you can enter the maneuver going down wind and set up the course perpendicular to prevailing winds. It also helps to know the wind factor so you know what to expect more or less- and that's all. You do not have to calculate anything using ground speed or prevailing winds, period. The change in ground speed is taken care of by dipping and climbing as necessary. In strong winds you will do a lot of this, but the point is you do not need to be thinking about the wind speed or even ground speed.
In my last post I tried to clarify why this is so. In calculus there is something called Kelvin's Theorem, which says that the sum of all vectors- in this case prevailing winds- around any closed path is zero, meaning that the total is zero for wind effects on the airplane if taken in sum around the course. What is not even close to constant, is the ground speed the airplane has at any given time around the course, thus the need to dip or climb to remedy how changing ground speed affects the pivotal altitude. GS is a key player in the maneuver, true- but the pilot need only know his pivotal altitude, which is a single figure calculated on the ground before leaving, and then dip or climb as necessary to keep the pylon on the reference line of the airplane.
Here is the link again on theory of pivotal altitude. I put this to stimulate interest, not to divert this to a discussion of physics. Here another one that looks pretty good- theory link 2.
Indicated airspeed is a voluntary number, and is chosen before starting the maneuver. It will vary somewhat due to climbing and dipping, but keep the power set for the duration of the course based on your selected airspeed (Va).
Days or weeks ahead of time one can decide an airspeed (Va), put this number into a calculator, square it, and divide the result by either 11.3 or 15 and the result is the correct pivotal altitude in feet above ground.
If you want to be a tad more accurate you can use true airspeed, but this is a very minor difference and is not worth the trouble at such a low altitude.
Last edited by Cubdriver; 06-04-2008 at 06:10 AM.
06-04-2008, 05:33 AM
#18
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Position: ATP, CFI etc.
Posts: 6,056
Originally Posted by MobiusOne
As your start the 8's on, your Ground Speed is at the highest since you always start on the downwind. When turning from downwind to upwind, your actual ground speed will decrease due to the headwind. Therefore to track a radius around the "pylon", you will have to descend the aircraft to a lower altitude to match the PA at that given lower GS. Were expected to hold a constant bank angle and keep a line from the wing to the pylon during this move, so the only way to keep a line on the pylon is to either "dive for it" or "climb to catch it" However, the PA is always changing because as you move from upwind back to downwind, your GS increases and therefore your PA increases. When moving from one pylon to the next, you will be at your entry airspeed and therefore your entry altitude.
The Pivotal Altitude is based on your ground speed. The best way I can explain it is to picture a funnel coming up from the pylon. Now, as your airplane enters the maneuver, it is at it's fastest speed. For a given unit of time, the plane will travel so much distance. As the plane slows down due to wind conditions, then it will travel less of a distance per given unit of time.
Picture the funnel now. Cut it off into five different portions, with the wide mouth at top and the narrow part at the bottom. Now the airplane is "tracking" the funnel at a required bank angle. The goal of the pilot is to "ride" the wall of the funnel and keep the wing on the pylon. Now, depending on what speed the plane is going at is going to determine where on the funnel the plane is. A plane going faster is going to make a wider radius turn, hence needing to be higher on the funnel. A slower plane is going to make a tighter radius turn, so it will need to be lower on the funnel. It is important to remember that the goal of this is to keep your wing on the pylon, and not so much an "equidistant" path around the pylon.
One thing people get wrong is that they underestimate the importance of PA. Pivotal Altitude is based on the SQUARE of your ground speed, so any slight changes in Ground speed is going to required NOTICEABLE changes in your pivotal altitude.
Above all though, once somebody wraps the mind around the concept of 8's on and what they are supposed to accomplish, they will become monsters with it. I myself just fumbled my way through through it during my commercial, entering, climbing and descending and completing it without a real understanding why I was doing it. Once you understand why you learn the move and the science behind it, you can nail it every single time.
The Pivotal Altitude is based on your ground speed. The best way I can explain it is to picture a funnel coming up from the pylon. Now, as your airplane enters the maneuver, it is at it's fastest speed. For a given unit of time, the plane will travel so much distance. As the plane slows down due to wind conditions, then it will travel less of a distance per given unit of time.
Picture the funnel now. Cut it off into five different portions, with the wide mouth at top and the narrow part at the bottom. Now the airplane is "tracking" the funnel at a required bank angle. The goal of the pilot is to "ride" the wall of the funnel and keep the wing on the pylon. Now, depending on what speed the plane is going at is going to determine where on the funnel the plane is. A plane going faster is going to make a wider radius turn, hence needing to be higher on the funnel. A slower plane is going to make a tighter radius turn, so it will need to be lower on the funnel. It is important to remember that the goal of this is to keep your wing on the pylon, and not so much an "equidistant" path around the pylon.
One thing people get wrong is that they underestimate the importance of PA. Pivotal Altitude is based on the SQUARE of your ground speed, so any slight changes in Ground speed is going to required NOTICEABLE changes in your pivotal altitude.
Above all though, once somebody wraps the mind around the concept of 8's on and what they are supposed to accomplish, they will become monsters with it. I myself just fumbled my way through through it during my commercial, entering, climbing and descending and completing it without a real understanding why I was doing it. Once you understand why you learn the move and the science behind it, you can nail it every single time.
However, there is an important distinction to make: while the physics of the manuever are indeed based on the ground speed of the aircraft (see the link in my post), the determination of pivotal altitude by the pilot is done using an intended airspeed number (Va). This is due to Kelvin's Theorem but don't get caught up too much in theory. Just be sure and use IAS in knots or mph for pivotal altitude calculation.
06-04-2008, 01:13 PM
#19
Gets Weekends Off
Joined APC: Dec 2006
Position: Reclined seat
Posts: 629
Originally Posted by wmuflyboy
Had a student just take his commercial ride but he told me the DE was not to happy with the explanation of how my student found pivotal altitude in the airplane. We have always done them at 1,200 MSL and they work out nice. He passed and everything but I was just curious if any of you knew a simple way to figure PA out in the airplane. The GS is right there on the GPS but obviously you wont know your GS if your on the ground before the flight. Should he have broke out a calculator or something??
06-04-2008, 01:33 PM
#20
Gets Weekends Off
Joined APC: Jan 2008
Posts: 1,212
Originally Posted by Cubdriver
No no no. Let me try again.
1) ground speed is not the quantity used in the formula for pivotal altitude calculation, it is airspeed in either mph or knots. Square the airspeed and divide by either 11.3 if using knots, or 15 if using mph on the airspeed indicator. I just noticed most of the posters on this thread use ground speed, which is incorrect. See Airplane Flying Handbook FAA-H-8083-3A pg. 6-14 for a reference on this if in doubt. I realize some schools use ground speed in the formula and it works, but the best thing is to use airspeed because it is what the pilot sees.
Let me try again. 1) ground speed is not the quantity used in the formula for pivotal altitude calculation, it is airspeed in either mph or knots. Square the airspeed and divide by either 11.3 if using knots, or 15 if using mph on the airspeed indicator. I just noticed most of the posters on this thread use ground speed, which is incorrect. See Airplane Flying Handbook FAA-H-8083-3A pg. 6-14 for a reference on this if in doubt. I realize some schools use ground speed in the formula and it works, but the best thing is to use airspeed because it is what the pilot sees.
http://flight.pr.erau.edu/docs/ERAU%...anual_2007.pdf
If you go to page 52 (of the PDF) or 1-47 of the document, you will see that they used GS^2/11.3 .... not saying it's right but we've got a lot of people out of this (crappy) school that learned it that way, including me.
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