Pivotal Altitude

AuntPeggy said:
This was sent to Hubby by his CFI. Can someone here decode this and explain in terms I can understand?
http://www.auf.asn.au/magazine/pivotal_altitude.html
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Don't sweat the math. All they are saying is that for a given airspeed there is a particular height above ground at which it will appear that the extended lateral axis of the aircraft actually pivots exactly on a point on the ground. Key assumptions are calm wind, constant airspeed, level turn, and coordinated flight.

If the aircraft height is varied such that is crosses the particular height then there can be optical illusions with respect to the apparent movement of a wingtip relative to a ground reference point. Other violations of the assumptions will cause similar optical illusions.

Maybe some folks can sense all the parameters and calculate the proper aircraft control inputs. I think most of us just make the aircraft go where we want in order to pass the check ride.
 
Clark1961 said:
Maybe some folks can sense all the parameters and calculate the proper aircraft control inputs. I think most of us just make the aircraft go where we want in order to pass the check ride.
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I'm pretty good at doing math in my head, but I still do Pylon-8's strictly by moving the stick as cued by movement of the point fore/aft of the wingtip.
 
When I was working on my commercial, the concept of pivotal altitude came into play as the ideal altitude for performing eights on pylons. The advantage of the pivotal altitude is that, in a no-wind condition, you'd theoretically not have to climb or descend to keep the position of your reference point off your wing.

It just comes down to an altitude that, for a particular speed and bank, a spot will appear to remain off your wing during a turn. My commercial book (which I've since loaned to a friend who's working on his commercial) had some simplified math formula to determine your pivotal altitude for eights on pylons. I did find that it made them easier to do, and then would climb/descend as the point moved fore/aft. Just remember that on windy days you're going to have to correct no matter what.

Are you working on your commercial, or just following along?

By the way, I was at BDR the other day and taxiied by your plane. :)
 
Ron Levy said:
I'm pretty good at doing math in my head, but I still do Pylon-8's strictly by moving the stick as cued by movement of the point fore/aft of the wingtip.
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+1. I did the math before the flight so I had a rough idea where to start, but there's ALWAYS some wind, so the actual manuever was always done by "freezing" the pylon.
 
Ron Levy said:
I'm pretty good at doing math in my head, but I still do Pylon-8's strictly by moving the stick as cued by movement of the point fore/aft of the wingtip.
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Me too. Too much thinking creates smoke in the cockpit. The fore/aft trick is foolproof. Well, nothing in aviation is foolproof......
 
TMetzinger said:
+1. I did the math before the flight so I had a rough idea where to start, but there's ALWAYS some wind, so the actual manuever was always done by "freezing" the pylon.
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I suspect that the important thing is that it looks right to the person in the right seat - not for it to be actually, mathmatically, correct.
 
Capt. Geoffrey Thorpe said:
I suspect that the important thing is that it looks right to the person in the right seat - not for it to be actually, mathmatically, correct.
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The math explains why the pivotal altitude is constant for a given groundspeed and independent of bank angle. To fly pylon 8's all you need to know about the math is:

"The pivotal height in nil wind conditions is readily calculated by squaring the TAS in knots and dividing by 11.3"

This formula gives you a place to start altitude wise and the relationship between groundspeed and altitude provides the clue for the following procedure:

When the groundspeed increases you will need more altitude and since pulling up will usually provide both a reduction in GS and an increase in altitude you can maintain the varying pivotal altitude when wind is present by climbing (and increasing bank) whenever you are too fast/low (pylon has moved to the rear) and vice versa.
 
gismo said:
The math explains why the pivotal altitude is constant for a given groundspeed and independent of bank angle. To fly pylon 8's all you need to know about the math is:

"The pivotal height in nil wind conditions is readily calculated by squaring the TAS in knots and dividing by 11.3"

This formula gives you a place to start altitude wise and the relationship between groundspeed and altitude provides the clue for the following procedure:

When the groundspeed increases you will need more altitude and since pulling up will usually provide both a reduction in GS and an increase in altitude you can maintain the varying pivotal altitude when wind is present by climbing (and increasing bank) whenever you are too fast/low (pylon has moved to the rear) and vice versa.
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With practice you find the ideal airspeed range for each airplane that meet the following requirements:
  • < 30 degrees of bank
  • < Va
  • Maintains altitude with little trim or power adjustment
If you know the local winds, you will have a pretty good idea when to climb and when to descend, and the "pylon on the wingtip" task will be a simple matter of push forward when moving forward, pull back when moving back.
 
dmccormack said:
With practice you find the ideal airspeed range for each airplane that meet the following requirements:
  • < 30 degrees of bank
  • < Va
  • Maintains altitude with little trim or power adjustment
If you know the local winds, you will have a pretty good idea when to climb and when to descend, and the "pylon on the wingtip" task will be a simple matter of push forward when moving forward, pull back when moving back.
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The "< 30 degrees of bank" is not fixed by the airspeed and associated altitude selected. You are correct that it's a good idea to fly pylon eights with 30 degrees of bank or less since greater angles will increase the g loading and stall speeds enough to matter. But you can adjust the required bank angle by adjusting the radius of the turn and that's mostly a function of the distance between the pylons.
 
gismo said:
The "< 30 degrees of bank" is not fixed by the airspeed and associated altitude selected. You are correct that it's a good idea to fly pylon eights with 30 degrees of bank or less since greater angles will increase the g loading and stall speeds enough to matter. But you can adjust the required bank angle by adjusting the radius of the turn and that's mostly a function of the distance between the pylons.
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Absolutely!

But I found that the total load on most students becomes a bit overwhelming when he/she has to deal with overbanking tendency in addition to all the other forces at play during this manuever.
 
YEEGADS!

I find the pylon 8 to be the easiest comm maneuver. If the idea of the comm maneuvers is to fly the plane, look outside, and learn the feel instead of flying the instruments, well, I still struggle with that on Lazy 8s and Chandelles. But the pylon 8? I don't even worry about #'s. Get down kinda low, and let the wingtip tell me what to do. Piece of cake.
 
Eights on pylons are easy so long as there's no wind. ;)

Lazy 8s and chandelles are fun.
 
flyersfan31 said:
YEEGADS!

I find the pylon 8 to be the easiest comm maneuver. If the idea of the comm maneuvers is to fly the plane, look outside, and learn the feel instead of flying the instruments, well, I still struggle with that on Lazy 8s and Chandelles. But the pylon 8? I don't even worry about #'s. Get down kinda low, and let the wingtip tell me what to do. Piece of cake.
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Pylon 8s are definately fun.... but some new comm students are a bit anxious that low to the ground.

On my Comm checkride, the DE picked a couple of silos. The problem was they were in the center of a very narrow valley south of Elkins, WV.

The mountains there rise steeply and fairly high on either side of the valley, and are definately above 800' AGL or so, which was near my pivotal altitude.

So with that added pucker factor I flew a long entry, looking for towers and wires, then concentrated on the pylons, somewhat confident I had at least 300' above the highest tree on the one side of the circle, and maybe a bit less on the other.

Lesson: If you're gonna fly in West Virginia, ya gotta respect those mountains.
 
Ted DuPuis said:
Eights on pylons are easy so long as there's no wind. ;)

Lazy 8s and chandelles are fun.
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For whatever reason, Lazy 8s and Chandelles were like landings for me -- for a while I just didn't get them, then one day they clicked, now they are automatic.

It didn't help it was always hazy when I was learning them.

A clear horizon definately helps!
 
Ted DuPuis said:
Are you working on your commercial, or just following along?
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Just following along. Don't have IFR yet.
Ted DuPuis said:
By the way, I was at BDR the other day and taxiied by your plane. :)
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We heard you went to lunch with Julia. You could have invited us for a ride, too.:goofy:
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I assume pivotal altitude has an effect on turns about a point, and I should be concerned about the altitude chosen for those, too?
 
AuntPeggy said:
I assume pivotal altitude has an effect on turns about a point, and I should be concerned about the altitude chosen for those, too?
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No - Turns around a point (the Private maneuver), you're trying to make a circle on the ground and it doesn't matter where your wingtip is. Same goes for Steep Spirals on the commercial.
 
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