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OmniAtlas

If pitch = speed, and throttle = altitude...

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If you're on a 3 degree glide slope on the approach and you're dropping/increasing speed, should you be using pitch (trimming) to control the speed? Doesn't that deviate you from your 3 degree glideslope? 

 

Cheers.

 

 


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Nope, it will be a combo of thrust and pitch. 3 degree should be speed times 5 for your vertical speed i.e if my approach speed is 75 knots my vertical speed should be about 375fpm.

 

-Ray

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That depends on what part of the flight envelope you are in. If you are flying fast enough to be on the frontside, then you can use pitch to make small and immediate corrections to vertical speed to correct for the glideslope and power changes to adjust speed. Flying on the frontside of the drag curve allows this because pulling back on the stick not only decreases speed, but also decreases drag, causing a climb. So it can be treated like cruise flight. Airliners with their slats and flaps extended and not 30 knots below their ref speed and GA aircraft flying ILS approaches at 90kts are examples of aircraft that make their landing approaches this way.

 

If your aircraft is making its approach while flying on the backside of the envelope, then you must use power for the glide slope and pitch for speed. Because if you pull back in this regime, the plane sinks faster. For example, the F-4 Phantom's automatic carrier landing system maintained the glide slope with power and angle of attack with pitch.

 

In either case, the aircraft must be at the right power setting for rate of descent and pitch angle for speed or else the pilot will find the aircraft constantly trying to deviate away from the desired descent or speed.

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Thank you for the explanation, but I have no idea what the frontside of the drag curve means. Perhaps I should do some more reading on aerodynamics. :)

 

Drag_Curve_2.jpg


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Thank you for the explanation, but I have no idea what the frontside of the drag curve means. Perhaps I should do some more reading on aerodynamics. :)

 

Drag_Curve_2.jpg

Then I will explain. What pilots call the 'frontside' is the right half of the blue U shaped total drag curve, where form drag is dominant. The 'backside' is the left half of the curve where induced drag is dominant. When you are in flight at a speed on the frontside, then any reduction of speed reduces your overall drag. While any increase of speed increases your overall drag. And when your drag changes, so does your power requirement for flight. So lets say you are established in level cruise flight on the frontside. You are sitting right there on the blue line on the frontside. Now when you pull back on the stick, your airspeed decreases. The movement of your spot on the graph leftwards along the x axis puts you above the blue curve representing total drag and thusly, power required for level flight. With a surplus of power, now the aircraft begins to ascend. Now let's say you push forward on the stick. You now move rightward along the x axis and move underneath the blue curve. This means the aircraft is now at a deficit of power since the drag has increased at this higher speed and something has to give, such as altitude and the aircraft descends. This is how an aircraft can pitch for altitude. Pitching for altitude can work as long as you always fly on the right half of that U shaped curve.

 

Now let's look at the left half, or backside, of that drag curve. From a point of equilibrium on the curve, if you pull back on the stick, the decrease in airspeed puts you below curve because the higher angle of attack increases induced drag. The higher drag means more power is required to stay on the curve and maintain altitude. So by pulling back on the stick, you end up at a power deficit and the aircraft descends. Conversely, by pushing forward while within the backside, you decrease drag and cause the aircraft to ascend. Those who were taught that pitch is for altitude need to remember this as 'reverse command'. It becomes an asterisk to how they fly and if they ever find themselves in a critical situation, it makes it difficult for them call upon the right reaction.

 

Approaching the graph from the standpoint of power, think of the y axis as power. Power required for level flight is directly related to drag. If you have more power than what your airspeed/drag requires for level flight, then you climb. So that is why we add power to climb. By adding power you move yourself vertically along the y axis. And if that puts you above the curve, then you start climbing. This is why power is for altitude. Using power to for speed requires coordinated action with the stick. To accelerate in level flight, you need to both add power and push forward. Adding power while in equilibrium on the curve, obviously puts you at a surplus power situation. You can either allow the speed to stay the same and accept the climb, or you can use that surplus to move yourself along the curve right wards in an acceleration by pushing forward on the stick until you settle to a new equilibrium point on that blue curve further to the right and higher up along that curve.

 

But you see in all cases, the power determined your ability to go up or down while the stick determined whether you went faster or slower. There are no exceptions to it. Those controls affect altitude and speed the same way no matter whether you are on the frontside or backside. But pitch cannot be said to control altitude the same way whether you are on the front or backside. That is why pitch for altitude is not a true statement. And when a pilot is taught pitch for altitude, they are safe and comfortable only on the right half of that graph. When they do slip into the left half, they have a tendency to stall and crash.

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My idealized explanation...

The backside is the left side and the front side is the right side. Drag increases as you move up and airspeed increases as you move right. The airplane feels the drag on the blue dotted line called "Total Drag" which is the sum of the Induced Drag and Form Drag. Induced drag is a by product of lift which increases as the angle of attack increases. Form drag is a by product of the interaction of the aircraft surfaces with the air and increases as airspeed increases.

If you pitch up and do nothing else, the aircraft will move to the left on the chart you posted. Pitching up increases the angle of attack, increasing lift, and therefore increasing induced drag and losing airspeed. Pitching down does the opposite of the above.

Increasing thrust and doing nothing else will move you right, while decreasing thrust will move you to the left. Struck this as it is clearly not correct. Thanks KevinAu

Most aircraft left to their own devices will eventually find equilibrium at different pitch and airspeed combinations. Note that equilibrium can be found while holding altitude or in a climb/descent at a constant rate. It all depends on the balance of lift and power vs weight and drag.

So as you asked, if you are descending on the glideslope at 3 degrees, but your airspeed is too high, you will need to pitch up and possibly decrease power a touch. You are very likely on the right side of the curve in this scenario, but there is a little bit more to consider.

Pitching up say another 5+ degrees to get that angle of attack up and letting lots of airspeed bleed off will shift you into the left (backside) side of the chart. Now it will require a whole lot of power to counter all that extra drag to maintain airspeed and lift.

Another scenario to consider is where you are at idle and stable at your best glide speed which happens to be at the minimum drag point on the chart. You will be descending toward the ground in this glide. This is something which all student pilots are very accustomed to establishing very quickly at seemingly random times. :)

Adding a touch of power and pitching up a little to maintain the same airspeed should arrest the descent altogether. You will still be close to the minimum drag point so it shouldn't take much of an increase in power and pitch to do so. This is the best way to maximize your time aloft because it uses the least power and lowest fuel consumption rate, but it won't get you anywhere particularly fast.

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Have you read the age old Stick and Rudder?  Another great reference is a book titled "As the Pro Flies".


Frank Patton

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Former USAF meteorologist and ground weather school instructor; AOPA Member #07379126

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My idealized explanation...

 

 

Increasing thrust and doing nothing else will move you right, while decreasing thrust will move you to the left.

.

Okay, you need to go up in an actual airplane in flight, take your hand off the stick, then push the throttle forward. Then come back and let me know what happens and whether or not you still believe that statement.

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Okay, you need to go up in an actual airplane in flight, take your hand off the stick, then push the throttle forward. Then come back and let me know what happens and whether or not you still believe that statement.

 

Yes, I do log several hours almost every weekend, weather permitting, my friend. :)

 

Thank you, I agree that my statement is incorrect. If I increase throttle, and do not touch the trim or stick, the airplane will change attitude and then stabilize at the same airspeed and likewise if were to I pull the throttle.

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But you see in all cases, the power determined your ability to go up or down while the stick determined whether you went faster or slower. There are no exceptions to it. Those controls affect altitude and speed the same way no matter whether you are on the frontside or backside. But pitch cannot be said to control altitude the same way whether you are on the front or backside. That is why pitch for altitude is not a true statement. And when a pilot is taught pitch for altitude, they are safe and comfortable only on the right half of that graph. When they do slip into the left half, they have a tendency to stall and crash.

As we know, this subject has been debated for a long, long time. IMO, just teach the whole picture from day one, instead of completely insisting on power = altitude, and pitch = airspeed, as some do. It's a combination, and better to be thought of that way. And of course, that particular line of thinking is always debated too. These are not the days of "Stick & Rudder", along with the general population of under performing aircraft from that era.

 

Just keep this in mind, if you're a student pilot, and get behind the power curve while trying to climb with elevator.....you will go down..

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As we know, this subject has been debated for a long, long time. IMO, just teach the whole picture from day one, instead of completely insisting on power = altitude, and pitch = airspeed, as some do. It's a combination, and better to be thought of that way. And of course, that particular line of thinking is always debated too. These are not the days of "Stick & Rudder", along with the general population of under performing aircraft from that era.

 

Just keep this in mind, if you're a student pilot, and get behind the power curve while trying to climb with elevator.....you will go down..

Pitch control of airspeed does describe the whole picture, as illustrated by the power curve. And in a desperate or chaotic situation, that primacy of thought will save lives. By teaching 'it's a combination' you give the student nothing definite to fall back on. While teaching 'pitch for altitude' in primary training causes accidents like Colgan, Air France and Asiana.

 

Planes still use stick and rudders. And it has nothing to do with the horsepower of the aircraft. Pilots land F-18s and C-17s by pitching for speed. They are certainly not underperforming aircraft.

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Pitch control of airspeed does describe the whole picture, as illustrated by the power curve. And in a desperate or chaotic situation, that primacy of thought will save lives. By teaching 'it's a combination' you give the student nothing definite to fall back on. While teaching 'pitch for altitude' in primary training causes accidents like Colgan, Air France and Asiana.

 

Planes still use stick and rudders. And it has nothing to do with the horsepower of the aircraft. Pilots land F-18s and C-17s by pitching for speed. They are certainly not underperforming aircraft.

 

 

You know, that I'll disagree. We know that we pitch to speed for landing. We know that if someone hit's the net to look up this subject, that reading could go on for days. It's a highly debated subject, that won't end. I just keep going back to my formation flight example. You're tucked into the flight leaders wings. You're slightly below, because if the leader suffers any kind of engine problem, you'll collide. At this point, to maintain your seperated position of just inches, you'll be thinking throttle for speed, and not elevator. Elevator will be secondary. It will be throttle, throttle, throttle!

 

We're still on the front side of the power curve, and the plan is to pitch to near vertical upwards. We now have to think of elevator for pitch, and throttle is the secondary thought. Settled into the near vertical climb, we're coming back to the backside of the power curve, unless our aircraft has more than a one to one power ratio. At this point, it's again throttle for climb. If you just think pitch for climb, you won't make it. At the top of the arc, and heading back down hill, it's once again pulling back on the throttle to maintain airspeed, and not pitch.

 

Combat is the same way. You constantly have to be thinking of both. You always have to think of the aircraft's power to weight ratio, and the power curves. This is why the FAA, has changed some of their litature, and why the Army & Navy have somewhat different views.

 

Besides, I still like that comical picture, where the student pilot is sitting "still" on the runway, vigoursly pumping the elevator up & down for takeoff speed.

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You know, that I'll disagree. We know that we pitch to speed for landing. We know that if someone hit's the net to look up this subject, that reading could go on for days. It's a highly debated subject, that won't end. I just keep going back to my formation flight example. You're tucked into the flight leaders wings. You're slightly below, because if the leader suffers any kind of engine problem, you'll collide. At this point, to maintain your seperated position of just inches, you'll be thinking throttle for speed, and not elevator. Elevator will be secondary. It will be throttle, throttle, throttle!

 

We're still on the front side of the power curve, and the plan is to pitch to near vertical upwards. We now have to think of elevator for pitch, and throttle is the secondary thought. Settled into the near vertical climb, we're coming back to the backside of the power curve, unless our aircraft has more than a one to one power ratio. At this point, it's again throttle for climb. If you just think pitch for climb, you won't make it. At the top of the arc, and heading back down hill, it's once again pulling back on the throttle to maintain airspeed, and not pitch.

 

Combat is the same way. You constantly have to be thinking of both. You always have to think of the aircraft's power to weight ratio, and the power curves. This is why the FAA, has changed some of their litature, and why the Army & Navy have somewhat different views.

 

Besides, I still like that comical picture, where the student pilot is sitting "still" on the runway, vigoursly pumping the elevator up & down for takeoff speed.

And I the comical picture of you sitting still on the runway pumping the elevator up and down for altitude.

 

When you add power to stay with your leader, you also need to push and trim foreward or else the aircraft climbs. Though your personal mindset is not pitch for speed, the aircraft still forces you to pitch for a faster speed. Because that is just how airplanes work, whether you believe it or not.

 

Of course you can throw any airplane around the sky in a wannabe dogfight, but it is only going to go straight up for a short time until it is no longer on the frontside, and as you say, you hzve to power for climb. By the way guess what happened to the airspeed as you pulled up into the vertical? It decreased. Yes.

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LAdamson, on 13 Jul 2013 - 11:02 AM, said:

It's a combination, and better to be thought of that way.

 

These are not the days of "Stick & Rudder", along with the general population of under performing aircraft from that era.

These are the days of Stick and Rudder and the discussions in the two books I referenced clearly make your first point (combination), as did every quality flight instructor I ever worked with. Power, lift, and drag have not aged. At a constant power setting, if lift is increased, drag increases. As drag increases, speed decreases. As speed decreases, lift decreases. And so on. Similar characteristics of a wing develop if control surfaces are kept constant and power is adjusted. It has not changed. Power and pitch. If you use only one you "chase the needle".

Frank Patton

Gigabyte GA-Z97X-Gaming 7 MOB; i7 4790 4.4Ghz; 16gb DDR3 2300; MSI GTX970 4gb Gaming 100ME, ASUS GTX 1070 TI Turbo 8GB, Coolermaster Pro 5 case, Corsair H100i

Former USAF meteorologist and ground weather school instructor; AOPA Member #07379126

There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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And now we move on to the non-idealized discussion where all the vectors are not found in their usually depicted locations. :)

 

 

 


At this point, to maintain your seperated position of just inches, you'll be thinking throttle for speed, and not elevator. Elevator will be secondary. It will be throttle, throttle, throttle!

 

In this scenario one would also need to simultaneously add corresponding forward pressure to maintain position as you gain airspeed. This is not a sequence of events with each control applied independently. While flying at a low angle of attack if one used pitch to lower the angle of attack (less induced drag) and waited for the speed to come in (more form drag), they may need to trade a significant amount of altitude for any significant change in airspeed.

 

In the next scenario for the loop there are a few more events at play to consider how the controls should be manipulated. As one goes into a vertical climb, the thrust component is a vertical vector and the weight component combines with drag in the same vertical plane, unfortunately, lift will now be mostly horizontal and not offsetting weight nor drag. Thrust is effectively your only source of "lift".

 

As one transitions to a vertical dive, thrust and weight are now working together against drag while lift again pulls perpendicularly towards the horizon. Closing the throttle allows only the weight to act as "thrust" and therefore prevent an excessive gain of airspeed. Then one can also use pitch to manage the angle of attack to increase induced drag to further maintain airspeed control while adding in throttle as the aircraft returns to level flight.

 

The way I look at it is if you act mechanically by rote, or have to think about all these concepts while actually flying, you might be better off as an engineer than a pilot! :lol:

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