Pivotal ALtitude

[descout]

Any ideads on how to explain pivotal altitude to a commercial student. I have a commercial student who just doesn't get where it comes from. He can do the maneuver but wants to know the why part.
Help would be appreciated.

 

[Toonces]

8's on pylons

Pivotal altitude is the altitude at wich the airplane apears to pivot around a pylon.

 

[jeskoe]

I'm assuming they have already read the definition in the book, and you've explained it the best you could, but they still can't get it. I would try actually taking them up in an airplane. First fly 500 feet higher than the pivotal altitude and put the wing on a pylon and have them see what happens. Then try going 500 feet (or as low as you can) below the pylon, and have them see what happens. Then, go to the correct pivotal altitude, and they should see what pivotal altitude is.

 

[moxiepilot]

Pivotal altitude is the altitude at wich the airplane apears to pivot around a pylon.

This is where I start my presentation too.

Try putting a trash can in the middle of the room, have them stick their arm out pointing at it and walk around it keeping the same reference line. Now move them closer to it haaving them do the same. In order to keep the same perspective, their arm angle chages (similar to pivotal altitude where altitude is still the same but bank angle changes).

There are a few other tricks too. If this doesn't work, ask again & I'll give you some more.

 

[descout]

I have shown him the above and below altitude and also right on pivotal altitude in the aircraft. He can do the maneuver and hold the pylon but he dosen't get why and how it changes with groundspeed.

 

[moxiepilot]

I have shown him the above and below altitude and also right on pivotal altitude in the aircraft. He can do the maneuver and hold the pylon but he dosen't get why and how it changes with groundspeed.

have you tried explaining turns around a point in a no wind condition vs. a wind condition and how holding the same bank angle will vary the groundtrack?

although 8's on are not based on keeping a constant radius like turns around a point, using TAAP can show how wind will blow you around. Let me explain...

If we are flying along in a no wind condition and pick a point to fly around, let's say a house, do you agree the airplane would appear to pivot around the house? (Answer is yes)

Now, if flying around the house and our airspeed decreased do you agree that it would appear that the house would move forward because our goundspeed has decreased? (Answer is yes)

Why will the house appear to move forward if the ground speed decreases? (The radius changes: with a lower groundspeed the aircraft will have a smaller turning radius; in order to hold the same point with that smaller radius, the bank angle needs to change)

Okay, lets take a step back, taking out bank and talk about altitude...

Above we just mentioned that if groundspeed decreases the pylon appears to move forward. How do we get that pylon back to its reference without changing the power? You need to increase your groundspeed. How do you do that? Decrease your altitude since you have the same power setting.

By increasing the speed you return to the larger radius/turn.

Well, what determines the initial change in groundspeed and how we need to react to that change? The wind.

If we use the typical 8's on topview diagram of the maneuver, we can see there are two points where the groundspeed will be higher because the aircraft is moving with the wind and two points where the groundspeed is lower because the wind is against us.

There are two things we can do to try to stay on that pylon -change our bank angle or change our speed. Since we are trying to hold the reference line & pylon we can change our ground speed by varying our altitude. Since we lower/raise our altitude to increase/decrease speed we also have to change our bank angle slightly to maintain our same reference.

Our lateral reference varies by groundspeed which we change by pitching up or down, our horizontal reference is maintained by bank which we much change with every change in speed.

Hope this helps a bit, if you need further explination I will try a different manner.

 

[BrettInLJ]

So summarizing the above, its basically a geometric relationship. At a given airspeed there is one altitude that will form the correct angle with the reference point on the wing.

Since your airspeed is constant, to turn on the pylon and maintain a certain radius, you can only change your bank. Since you have to hold a reference point, the only way to change the bank is to change your altitude. Higher altitude = steeper bank, lower altitude = shallower bank.

 

[screennamie]

ratio

I have shown him the above and below altitude and also right on pivotal altitude in the aircraft. He can do the maneuver and hold the pylon but he dosen't get why and how it changes with groundspeed.

HA we just went over this in our commercial class here at Western. We all tried the different methods, trash can, drawing and what not. It was a site to see. I picked it up after it was explained as a ratio. Do it mathematically if I remember right its gndspd squared/11.3 or MPH squared/15. If ground speed decreases then so does the pivotal altitude. They are directly proportional instead of inversly cause you divide sorry my spelling might not be to hot.

hope that helped

 

[moxiepilot]

HA we just went over this in our commercial class here at Western. We all tried the different methods, trash can, drawing and what not. It was a site to see. I picked it up after it was explained as a ratio. Do it mathematically if I remember right its gndspd squared/11.3 or MPH squared/15. If ground speed decreases then so does the pivotal altitude. They are directly proportional instead of inversly cause you divide sorry my spelling might not be to hot.

hope that helped

to add to the above, it makes sense because the formula is based on groundspeed. if your groundspeed is decreasing because of heading into the wind, your PA will therefore also have to change

 

[n2o2diver]

Long post to follow

to add to the above, it makes sense because the formula is based on groundspeed. if your groundspeed is decreasing because of heading into the wind, your PA will therefore also have to change

Doesn't the Airplane Flying Handbook say it is True Airspeed Squared / 11.3 vise Ground Speed Squared?

Think about this:

If you start the maneuver at say 95 knots and no wind, then our PA is 798 feet. If there is no wind our Ground Speed will not change and neither will our PA. The bank angle will remain constant. For arguments sake lets say we started the maneuver at 798 feet up and 798 feet laterally from the pylon. The angle of bank would be 45 degrees. Good old Pythagorean.

Now let’s say we have a 10 knot tail wind. Starting at 95 knots and 45 degrees to the downwind we will have an increase in ground speed. The trigonometry works out like this. The Cosine of 45 degrees is .707 (We are solving for the Adjacent / the Hypotenuse) or tailwind component (similar to solving for crosswind component on take off). So 10 knots multiplied by .707 gives us roughly 7 knots of tailwind component. We add that to the 95 knots of airspeed and we arrive at 102 knots Ground speed. So 102 squared / 11.3 gives us a new PA of 920 feet. As we progress through the maneuver the tailwind component changes from more of a tail wind, to a crosswind, then a head wind, then a crosswind, then a tail wind again. With each change of wind component we have a change in ground speed and a change in PA.

Herein lies a problem with figuring this with ground speed. The PTS says for this maneuver the altitude will be between 600-1000 feet. So if the winds are 30 knots can we do this maneuver? Because the math works like this; if we multiply 30 knots by .707 we get roughly 21. If we add 21 to 95 we get 116 (tailwind). Plugging 116 knots into the formula gives a PA of 1190 at the start. If we subtract 30 from 95 we get 65 (headwind). Plug 65 knots into the formula and we get a PA of roughly 373. Both of these are outside the PTS. Yet I am sure some of us have done this maneuver with these wind conditions with out going outside the PTS. How can that be?

Interesting problem? I’m not looking for an answer to the problem (I had to figure it out during and interview) but I though it might get some wheels turning in people’s heads. At least it should make for some good discussion. That’s if anyone can even follow my ramblings, it makes much more sense when I can brief this maneuver in person. I’d like to see what people come up with.

 

[moxiepilot]

Man, your posts are like mine

Interesting points though - let me try some counter points.

Eights on pylons are listed under ground reference maneuvers, not performance maneuvers. Since they are ground reference doesn't that in turn mean that they have to be performed based on how the wind affects the groundtrack as opposed to the airspeed at which the a/c is flying? The current publication (2004) goes into depth regarding how these are ground based maneuvers and how the wind is a factor (pgs. 6-1 through 6-4).

One of the important points comes on page 6-12 where the author states, "The altitude that is appropriate for the airplane being flown is called the pivotal altitude and is governed by groundspeed. (Bold used for emphasis not arrogance). While not truely a ground track maneuver as were the preceding maneuvers, the objective is similar..."

Continuing on page (6-14), "There is a specific altitude at which, when the airplane turns at a given groundspeed, a projection of the line of sighting reference line...the pivotal altitude does not vary with the angle of bank being used unless the bank is steep enough to vary the groundspeed."

It does say that the general rule of thumb is TAS BUT that is referenced in a calm wind.

I agree with your math when you use the groundspeed of 125kts and 65 - it just doesn't make sense. I will meditate on this....until I figure that out, keep the conversation going.

Moxie-

Doesn't the Airplane Flying Handbook say it is True Airspeed Squared / 11.3 vise Ground Speed Squared?

Think about this:

If you start the maneuver at say 95 knots and no wind, then our PA is 798 feet. If there is no wind our Ground Speed will not change and neither will our PA. The bank angle will remain constant. For arguments sake lets say we started the maneuver at 798 feet up and 798 feet laterally from the pylon. The angle of bank would be 45 degrees. Good old Pythagorean.

Now let’s say we have a 10 knot tail wind. Starting at 95 knots and 45 degrees to the downwind we will have an increase in ground speed. The trigonometry works out like this. The Cosine of 45 degrees is .707 (We are solving for the Adjacent / the Hypotenuse) or tailwind component (similar to solving for crosswind component on take off). So 10 knots multiplied by .707 gives us roughly 7 knots of tailwind component. We add that to the 95 knots of airspeed and we arrive at 102 knots Ground speed. So 102 squared / 11.3 gives us a new PA of 920 feet. As we progress through the maneuver the tailwind component changes from more of a tail wind, to a crosswind, then a head wind, then a crosswind, then a tail wind again. With each change of wind component we have a change in ground speed and a change in PA.

Herein lies a problem with figuring this with ground speed. The PTS says for this maneuver the altitude will be between 600-1000 feet. So if the winds are 30 knots can we do this maneuver? Because the math works like this; if we multiply 30 knots by .707 we get roughly 21. If we add 21 to 95 we get 116 (tailwind). Plugging 116 knots into the formula gives a PA of 1190 at the start. If we subtract 30 from 95 we get 65 (headwind). Plug 65 knots into the formula and we get a PA of roughly 373. Both of these are outside the PTS. Yet I am sure some of us have done this maneuver with these wind conditions with out going outside the PTS. How can that be?

Interesting problem? I’m not looking for an answer to the problem (I had to figure it out during and interview) but I though it might get some wheels turning in people’s heads. At least it should make for some good discussion. That’s if anyone can even follow my ramblings, it makes much more sense when I can brief this maneuver in person. I’d like to see what people come up with.

 

[casey]

http://www.flying.sc/learn/res/turns/index.html

 

[moxiepilot]

http://www.flying.sc/learn/res/turns/index.html

yeah that makes it real simple

 

[aloft]

Isn't it sad that I can hold a bachelor of science degree from an accredited university, having never been exposed to this geometry/trigonometry stuff? It's a foreign language to me!

 

[av8rmsu]

Anybody know where this procedure came from in the first place?