8's on

[EnRoute]

So I'm having a hard time with understanding why pivotal altitude changes. I've done a little research and although I haven't read it in such a way, this is my interpretation. Please correct me if I'm wrong.

We enter the maneuver traveling down wind, which will be our highest ground speed and highest pivotal altitude. As the turn progresses and become into the wind, I'm using the opposite extreme for conversation sake, our ground speed decreases, which puts our wingtip reference behind the pylon. We now need to "catch" the pylon which we do by increasing our air/ground speed. This is accomplished by descending. Again as our ground speed increases on the downwind side, we become ahead of the pylon, slowing down to let it catch up requires a climb for the reduction in ground speed (higher pivial altitude) Yes?

Do I have this right. Can any one shed some light on this.

 

[at1024]

The formula is TAS^2/11.3 for knots. The FAA flying handbook even talks about how the only way to know the true pivotal altitude is to use groundspeed instead. It's hard to know your instantaneous GS though without GPS. So yes you are correct. The way I was taught as an instructor applicant was to show the student what it looks like to be above or below the pivotal altitude and how to correct for it. If the pylon moves ahead of the wingtip then push forward on the controls, and if it moves behind the wingtip just pull back. This keeps the lateral axis aligned with the pylon. Works good for me.

 

[ProudPilot]

The formula is TAS^2/11.3 for knots. The FAA flying handbook even talks about how the only way to know the true pivotal altitude is to use groundspeed instead. It's hard to know your instantaneous GS though without GPS. So yes you are correct. The way I was taught as an instructor applicant was to show the student what it looks like to be above or below the pivotal altitude and how to correct for it. If the pylon moves ahead of the wingtip then push forward on the controls, and if it moves behind the wingtip just pull back. This keeps the lateral axis aligned with the pylon. Works good for me.

That should be Groundspeed squared. Otherwise, correct and that's how I teach it. If it's wrong, well, the students understand it and can accomplish the maneuver, so for now that's what I'm going with.

 

[shdw]

Eights on pylons is old maneuver flown by pilots so the gunners could have a steady visual target to shoot at. It is a great maneuver to teach the student to stay ahead of the aircraft.

Enter the maneuver on the down wind side, at the highest altitude because you are at the fastest ground speed. Math is unimportant here, higher ground speed means a higher pivotal altitude and conversely for a lower ground speed.

The entire first 180 should be a slight descending turn, ignore the wing tip, look in front of you with periodic (10-15 seconds or more between) scans of the wing tip. Fly ahead of, not with the aircraft. The next 90 will be a slight climbing turn with the straight away climbing back to pivotal altitude. I challenge you to try it once and never look at the point and see how it goes. The climb and descent is roughly 100 feet for each 10 knots of wind. Oh and for christ sakes enjoy the scenery and stop staring at your wing the point won't run away, you have my word.


For CFI's:

I caution explaining this as pitching down to "increase" speed with rate/radius. If you have ever flown with a GPS and taken a look at an equal power setting descent, meaning just pitch the nose down (Note: Decreasing power as you descend if you have a non turbo engine), the airspeed may increase but the ground speed shows little change. In our Seminoles 145 pitched for 165, a 500 fpm descent with 146 GS. The pitching up and down without a change in power setting does very little to the ground speed.

In this maneuver you will get blown further and closer to the point, if you hold constant bank +/-5 as you should, this requires a change in altitude to change the visual view of the point (for your gunner). Increasing speed to increase radius wouldn't work for reasons explained in the previous paragraph, you will not keep a constant radius circle, it will always change. The maneuver is based on visual view angle, changing your altitude as you get further/closer to the point, it has nothing to do with rate and radius.

PS: On an interesting note, I wonder how the bomber pilots did this since they didn't have a wing tip to stare at.

 

[IslandFlyer]

That should be Groundspeed squared. Otherwise, correct and that's how I teach it. If it's wrong, well, the students understand it and can accomplish the maneuver, so for now that's what I'm going with.

Not exactly.

From FAA-H-8083-3A AFH p6-12/13

The altitude that is appropriate for the airplane being flown is called the pivotal altitude and is governed by groundspeed. A rule of thumb for estimating pivotal altitude in calm wind is the square the true airspeed and divide by 15 for miles per hour (m.p.h.) or 11.3 for knots.

I know we're splitting hairs here but if you were to use groundspeed, at which point would you make the calculation using groundspeed? Would you take the speed while flying into a 20 knot headwind or tailwind.

Indicated 100 kias with 20 knot headwind = 80 GS. 80squared/11.3= 566'
Indicated 100 kias with 20 knot tailwind =120 GS. 120squared/11.3=1274'

Use TAS and fly the circles.

 

[shdw]

A rule of thumb for estimating pivotal altitude in calm wind is the square the true airspeed and divide by 15 for miles per hour (m.p.h.) or 11.3 for knots.

Note the "calm wind," ground speed is what you should use. Otherwise your pivotal altitude wouldn't change ever, we know thats not true.

Indicated 100 kias with 20 knot headwind = 80 GS. 80squared/11.3= 566'
Indicated 100 kias with 20 knot tailwind =120 GS. 120squared/11.3=1274'

These are right, that is what you should be doing, about 100 feet per 10 knots I used to use, looks like reality is about 150 feet so I will be changing my rule of thumb.


Edit:

is governed by groundspeed.

They even say it is governed by groundspeed, interesting they don't use it in the formula, good ol FAA for ya.

 

[tgrayson]

So I'm having a hard time with understanding why pivotal altitude changes.

First, keep in mind that the pylon must stay positioned not only horizontally, but vertically, too. In this example, we'll assume we want it exactly on the wingtip leading edge.

Let's say you're flying 1500 feet above the ground when you reach the abeam position of a pylon, exactly 1/4 nm away, and you roll into a 30 degree bank. We'll also assume you have 100 TAS and there is no wind. There are two things to evaluate:

1. Does this bank put the pylon on your wingtip, and
2. Will the bank angle produce a radius of turn equal to your present distance from the pylon?

The first question is easy to answer: your bank angle needs to be the same as the angle from you to the pylon. You can calculate this by atan(altitude/distance), as shown in the figure below. For the current scenario, atan(1500/1519) = 44.6 degrees. Let's call it 45 degrees. So, at this altitude and this distance away from the pylon, only a 45 degree bank would put the pylon right on the wingtip.

The conclusions, based on the math above:

• For a given distance away from the pylon, you need a steeper bank the higher your altitude.
• Conversely, for a given altitude, you need a shallower bank the further you are from the pylon.

Now for the second question. The 45 degree bank, at a constant airspeed, will produce a turn of a specific radius. If this radius is exactly the same as the present distance from the pylon, then the aircraft's wingtip will remain on the pylon because the aircraft will fly a perfect circle around the pylon, with the pylon at the center. But if the resulting turn radius is different from the distance to the pylon, the aircraft will essentially be flying a circle around some other imaginary pylon.

From the figure below, you can see that at the moment of banking, the aircraft is positioned on the circumference of several potential flight paths. Which one it actually follows will depend on the bank angle chosen. For a given airspeed, there is only one bank angle that will produce a turn of the proper radius, a radius equal to the present distance from the pylon. When the bank angle is wrong, then the radius will be wrong, and the wing tip will not remain aligned with the real pylon.

In our current scenario, the 30 degree bank that the pilot initiated will not put the pylon on the wingtip, so that bank is a non-starter. However, if the aircraft rolls over into the required 45 degree bank, it will follow the flight path of the steep bank in the figure above. The pylon will move ahead of the wing tip.

One way to solve this problem: since lower altitudes require shallower bank angles to position the pylon vertically, you could descend and shallow out your bank. There must be some altitude at which the bank required to position the pylon vertically will be exactly the bank that produces a circle of the required radius. This is pivotal altitude.

How to Use This

Assuming the initial conditions given above, and tossing in a 45 degree bank, since the pylon is moving ahead of the wing, the pilot knows his turn radius is too tight, so he needs to descend to an altitude where he can apply a lower bank angle. Since power must remain constant, he initiates this descent by pushing forward on the yoke.

If the pilot were flying only 500 feet above the ground, the required bank angle to position the pylon vertically would be only 18 degrees, but this bank angle would produce a turn radius that is too large, and the pilot would see the pylon move behind the wing. Since higher altitudes allow steeper banks, the pilot needs to climb to a higher altitude where the bank angle required to vertically center the pylon would also produce a turn of the required radius.

Wind

Adding back the wind doesn't change the nature of the analysis, it just changes the values of some of the variables we've already discussed. Most importantly, the effective velocity of the aircraft changes due to the existence of a head or tailwind, which changes the radius of the circle being flown at every point around the pylon. And, the wind may blow the aircraft closer or further from the pylon, changing the required bank angle to keep the pylon centered vertically.

For instance, an increasing headwind slows down the velocity of the aircraft with respect to the ground and the turn radius with respect to the ground will get smaller, tending the aircraft to turn too tightly. This would cause the pylon to move ahead of the wingtip reference. The pilot must descend in order to shallow out the bank so that the turn radius will increase.

 

[lhornaday]

Forget everything about pivotal altitude and eights-on. Think about doing a turn around a point but you can't change the bank angle. How would you keep a constant radius? Keep a constant groundspeed. Entering downwind you are fast. Turning into the wind you start slowing down - better pitch down to keep the speed.

If your groundspeed does not change, the reference line you have from your wing to the pylon will not move. Constant bank angle and constant speed, nothing can change. Now just pay attention to the altitude you are at while flying this turn around a point and give it a name - pivotal altitude.

So to answer the question, why does pivotal altitdue change? Well, you had to make pitch adjustments to keep your groundspeed the same as the winds changed.

 

[tgrayson]

How would you keep a constant radius? Keep a constant groundspeed. Entering downwind you are fast. Turning into the wind you start slowing down - better pitch down to keep the speed.

Keeping a constant radius is not a goal of the maneuver. And while slowing down or speeding up can change the radius of the circle in the proper direction to keep the pylon in line with the wingtip, this effect is only temporary until the proper pivotal altitude is reached, where the aircraft will resume the trimmed airspeed.

 

[shdw]

Keeping a constant radius is not a goal of the maneuver.

It isn't even possible, unless there are no winds. Like I mentioned above, ground speed wouldn't increase appreciably, just airspeed, the speed on the hypotenuse. The ground speed or speed of the adjacent side of the triangle will sill remain unchanged.

PS That explanation was a little confusing to me, but I think a combo of that with what I said earlier might make it simpler. Maybe comparing a triangle where the distance from the point, adjacent, is closer with a certain height gives a certain angle. To keep that angle when blown further from the point would require a higher height above the ground to keep the same angle, do you follow? What do you think?

 

[tgrayson]

Like I mentioned above, ground speed wouldn't increase appreciably, just airspeed, the speed on the hypotenuse. The ground speed or speed of the adjacent side of the triangle will sill remain unchanged.

That can't be true. You'd have to have a really, really steep angle of descent before ground speed would be appreciably different from airspeed based purely on trigonometry. With a descent angle of 3 degrees, like what you'd have on an ILS, your groundspeed would be 99.9% of your airspeed, meaning less than a knot difference.

That explanation was a little confusing to me, but I think a combo of that with what I said earlier might make it simpler. Maybe comparing a triangle where the distance from the point, adjacent, is closer with a certain height gives a certain angle. To keep that angle when blown further from the point would require a higher height above the ground to keep the same angle, do you follow? What do you think?

I'm sure I will have to improve on the explanation, but it's rather complex to convey simply. Your suggested addition, if I understand it, only captures a small part of the problem. The bank not only has to be right to position the pylon vertically, but it has to be the proper bank to keep the pylon positioned horizontally. There is only one altitude at a particular airspeed where it's possible to have one bank that satisfies both conditions.

 

[beasly]

Eights on pylons is old maneuver flown by pilots so the gunners could have a steady visual target to shoot at. It is a great maneuver to teach the student to stay ahead of the aircraft.

That's my approach as well.

It's also a great stress relief for the student.

Here is how:

Have the student visualize the Designated Examiner as the pylon and visualize a machine guncross-hairs on the wing; then the maneuver transitions from being all the technical talk to a first person shootem in a real airlplane.

If the DE gets ahead of the Cross Hairs, pitch down , if the DE gets behind the cross hairs pitch up (keep coordinated and watch airspeed, please)

When the student starts to get it right, I start making machine gun noises "ch-ch-ch-ch-ch-ch". When I don't have to interrupt my machine gun noises to correct the student, I figure they have it down pat.

How the bomber guys did it without visual reference to the wing is a very interesting question!

b.

 

[shdw]

How the bomber guys did it without visual reference to the wing is a very interesting question!

The point of it was, you don't need the wing if you fly the maneuver as it should be flown. I don't let my students use the wing other than to take a peek every 15-20 seconds to verify. I have taped my sectional to the left window on multiple occasions to make them do it without and lift it periodically to show them they can fly perfect w/o the wing.

 

[Douglas]

For CFI's:

I caution explaining this as pitching down to "increase" speed with rate/radius. If you have ever flown with a GPS and taken a look at an equal power setting descent, meaning just pitch the nose down (Note: Decreasing power as you descend if you have a non turbo engine), the airspeed may increase but the ground speed shows little change.

I don't agree with that at all. I love the extra ground speed picked up by pitching over for the descent.

My take:
If it is not an extremely windy day pivotal alt. will barely change and only small control inputs (pitch and bank) will be needed. Pitching for airspeed/GS in concert with the minimal bank change allowed by the PTS makes for one smooth sexy looking 8 on.

It takes two to tango well, pitch and bank.
Be fluid.

 

[pilot4higher]

Eights on Plyons....This post will start a holy war and run for days!

 

[Douglas]

Eights on Plyons....This post will start a holy war and run for days!

yo face

 

[MikeD]

The question is though, FBO or Academy?

 

[29.92inHg]

Not exactly.

From FAA-H-8083-3A AFH p6-12/13

Would you take the speed while flying into a 20 knot headwind or tailwind.

Indicated 100 kias with 20 knot headwind = 80 GS. 80squared/11.3= 566'
Indicated 100 kias with 20 knot tailwind =120 GS. 120squared/11.3=1274'

Use TAS and fly the circles.

I have them find the wind and turn so there is a quartering tailwind off the left side, and then I use the groundspeed while on the entry heading to calculate the pivotal altitude, that way you are starting at the proper altitude, from there its just a matter of adjusting based on the maneuver and the visual reference. I do try to keep them from staring down the wing too much and trust the mechanics of the maneuver, but I like for them to calculate the entry altitude by the groundspeed that they will be at when they enter. It seems to work well for me. I took the description in the AFH of calm wind and true airpeed for the altitude calculation as using groundspeed in other than calm winds for it to work( I know this is a very loose interperetation). Having said that all of our planes have a GPS for groundspeed. If I didn't have that, I would estimate the Altitude and go from there I guess.

That is interesting about the machine gunner though, I was not aware of that. This is a good thread, (that may create a holy war ), but I'm game to learn more...

 

[29.92inHg]

The question is though, FBO or Academy?

:insane::rotfl:

 

[Maurus]

but you can't change the bank angle

Since when?

VI. AREA OF OPERATION: GROUND REFERENCE
MANEUVER

TASK: EIGHTS ON PYLONS (ASEL and ASES)
REFERENCE: FAA-H-8083-3.

Objective. To determine that the applicant:

1. Exhibits knowledge of the elements related to eights on pylons.

2. Determines the approximate pivotal altitude.

3. Selects suitable pylons, that will permit straight and level flight,
between the pylons.

4. Enters the maneuver at the appropriate altitude and airspeed and
at a bank angle of approximately 30° to 40° at the steepest point.

5. Applies the necessary corrections so that the line-of-sight
reference line remains on the pylon.

6. Divides attention between accurate coordinated airplane control
and outside visual references.

7. Holds pylon using appropriate pivotal altitude avoiding slips and
skids.

This indicates you can change the bank. Although the maneuver is not to be exact on radius, you are not limited to a specific bank throughout the maneuver. Keep a bank angle that will allow you to maintain your sight picture. Something I find funny is the FAA's picture in the AFH shows a constant radius around the points with a wind from the north.