Pitch & power debate

[Typhoon1244]

In other words, you're making this much too hard.

You've got people here saying that pitch "controls" airspeed, and you're telling me I'm making it too hard?

I'll keep chanting it until the day I die: pitch controls altitude and power controls airspeed when both are available and variable.

With the possible exception of midlifeflyer's "Pitch + Power = Performance," there's no easier way to state it!

 

[Timebuilder]

pitch controls altitude and power controls airspeed when both are available and variable.

Sound too much like legalese. Without your second phrase "when both are available and variable", it doesn't hold up.

"Pitch controls airspeed and power controld altitude" does hold up, and it does so because the only limitless power is God, and He's too busy trying to figure out which car to drive.

Since there is a limit on power, the old standby is still true.

To climb you must add power, if not immediately, then sometime soon. Even an F 15 pointed straight up will run out of fuel, and then pitch will control airspeed, and the lack of power will control altitude.

 

[Typhoon1244]

"Pitch controls airspeed and power controld altitude" does hold up...

Good god, Timebuilder, no it doesn't! When I'm in straight and level flight and, for example, ATC wants me to slow down, I pull back on the throttle. If they want me to descend, I push the nose over. If I'm coming down the ILS and I need to slow down to get the last notch of flaps out, I don't start by hauling the nose up! I pull back the power! If...

...no. Never mind. I forgot how much you like to argue for the sake of argument. Of course you don't really believe that "pitch controls airspeed and power controld altitude" all the time. That'd be silly.

You just wanted to see how far you could push me until I called you a dirty name. I get it.

 

[blott]

I don't have auto throttles, but I do have an auto pilot. When I push the thrust levers forward the autopilot trims the pitch down to maitain altitude because the airplane wants to climb.(so the aircraft pitches for speed and the thrust is trying to control altitude?) The aircraft continues to accelerate untill thrust = drag, and the auto pilot trims the pitch down the whole time to maitain altitude. ie. the faster I go(more power) the less aoa I need(reduces lift). If I just have the aircraft trimed for level flight and I increase thrust(power) the aircraft will initially go faster, but as it goes faster the lift increases and the aircraft starts to climb. more lift = more drag, and the aircraft starts to slow down, over time when everything has stabilized the aircraft will be going the same speed it was first trimmed at, but it will be climbing-why? cause the extra thrust(power) was canceled out by the extra lift in the form of drag. now during this whole process pitch and speed are constantly changing to try to balance each other out-even though the whole time I haven't touched the controls(except to increas thrust.)

what does this all mean? I have no idea right now.

B

 

[Stifler's Mom]

Typhoon and Timebuilder

Please go outside and enjoy some fresh air.

 

[Timebuilder]

ATC wants me to slow down, I pull back on the throttle. If they want me to descend, I push the nose over. If I'm coming down the ILS and I need to slow down to get the last notch of flaps out, I don't start by hauling the nose up! I pull back the power! If...

These things that you are describing are what you would normally do while flying. I thought we had agreed that normally we use a combination of pitch and power.

The discussion as I see it, rightly or wrongly, is one of what is the ulitimate authority for these two aspects of flight, speed and altitude.

I'm not baiting you at all. If you want to descend without busting 250, you don't put the nose over, you reduce power and then trim pitch for descent. If you don't, you speed up. Why? You pitched down, which will cause your airspeed to increase. That's a taste of what this ultimate control of altitude and speed is about. It doesn't mean that every time you want to slow down, using pitch will be your best answer. However, if you want to climb, you must add power and if you want to land you must reduce power.

It is a foundational idea that we modify as necessary for normal operations.

 

[blott]

ok, thought about it some more. Now I'm a die hard pitch + power = performance guy.

try looking at this in a different way, not what you do when you are flying, but what the airflow over a airfoil is doing to lift-in terms of aoa(pitch) and windspeed(power).(break the lift down into horizontal and verticle components.)

If you look at it that way power controls lift and aoa controls speed.

thoughts?

 

[Timebuilder]

Yep.

When the Wright bothers first took off, it was POWER that made the thing fly, and they used PITCH to control the airspeed.

Still applies today.

 

[Typhoon1244]

This is going to seem a bit contorted, but bear with me:

Imagine we're going to fly an ILS. I will control only the throttle, and you will control only the elevator. (Forget about the course/localizer for now.) Each of us will only be allowed to look at one instrument: either the airspeed indicator, or the glide slope needle.

Remember: you can control only the elevator. Which instrument would you rather have in front of you? Airspeed or glide path?

Believe me, I know this is silly. Pitch and power are married to each other. Realistically the two can't be separated. But if you must build a foundation by stating which does what...

If all I've got is the throttle while we fly this ILS, I'd sure as heck want the airspeed indicator in front of me!

 

[blott]

why? you already have the plane trimed for an A/S say 140. now you notice you are low-why cause you don't have enough power to maitian the fixed glideslope and your aircraft is descending. you add a little power and the aircraft will start to climb(still trimed at 140) untill you recapture the glideslope, now if you leave the power alone you will go above glide slope, so you pull back the power to maitain this new altitude(descent). still at 140 the whole time.

 

[blott]

If you are fast and low on the ils what do you do, pitch up..lol

the pitch slows the plane down, and the extra A/S(same as power) climbs the plane....lol...round and round we go

 

[Typhoon1244]

So if you're on glide path and slow...you pitch down???

Who way your CFI-I???

 

[blott]

technically yes, pitch down to increase airspeed then increase power to increase lift and go back up. but if you do both- pitch and power -at the same time, you skip the whole go down for A/S first then power up for altitude and stay on glide path, and just speed up....

 

[blott]

If you just want to speed up then just pitch down. (but you will loose altitude cause you didn't add power) is the fog lifting yet?

 

[blott]

try this one out.

get a piece of paper and draw a airfoil with some AOA on it(and give it some camber). Now draw how the air flows over it.

this is the most basic way to demonstrate that power controls altitude and pitch controls airspeed that I can think of.

back to basics: lift acts perpendicular to the airflow over the wing. this doesn't happen at one spot, it happens all along the airflow an infinite amount of times. so draw a few lift arrows perpendicular to the airflow along the top of the whole wing. (note: some of the arrows will actually be pointing forward while others are pointing up and even more are piinting back a little.(the ones pointing back are the induced drag-but that comes later)

TOTAL lift is the sum of all those lift arrows you just drew. If you have a book that shows these arrows it will really help cause you will see that the arrows are different lengths at different parts of the airfoil...IE... some areas produce more lift than others. If you add all them together for total lift you will get one big arrow that is pointing slightly backwards(induced drag).

Now we will only change aoa and speed.

Power(speed): add more speed(more lift) but don't change the aoa the line getts longer. look at how the verticle and horizontal components of lift change too. you get more verticle(go up) and a little more horizontal(drag) and end up with more total up than total back so you end up climbing with a little more drag that keeps you at the same speed.

now aoa: turn the paper to give it some more aoa. More AOA = more lift = both vertical and horizontal(you have to draw a longer line, that tilts back more- with more aoa) so you'd think the airplane will climb right? but wait you have more horizontal lift(drag) and this slows the plane down more. as the plane slows down both vectors get smaller-but lift ends up smaller than drag so the plane ends up slower with the same amount of lift it originally had.(cause of the tilt back of the arrow)

did that all come out right?

easy verson: It's all about the induced drag. you add power you get more vertical lift than induced = go up at same speed.

change pitch(aoa) get alot more induced than vertical so you slow down and get same lift...

 

[Typhoon1244]

technically yes, pitch down to increase airspeed then increase power to increase lift and go back up.

With all due respect, baloney. I don't fly an ILS this way...and I suspect you don't either.

 

[Super 80]

This is going to seem a bit contorted, but bear with me:

Imagine we're going to fly an ILS. I will control only the throttle, and you will control only the elevator. (Forget about the course/localizer for now.) Each of us will only be allowed to look at one instrument: either the airspeed indicator, or the glide slope needle.

Remember: you can control only the elevator. Which instrument would you rather have in front of you? Airspeed or glide path?

Believe me, I know this is silly. Pitch and power are married to each other. Realistically the two can't be separated. But if you must build a foundation by stating which does what...

If all I've got is the throttle while we fly this ILS, I'd sure as heck want the airspeed indicator in front of me!

Typhoon,

I know you're a liberal, so it's not surprising we see this differently, but I agree with blott.

I'll give you the throttle and the A/S indicator. Now I as the evil instructor will pull the stick or yoke back.

To maintain your airspeed, you'll have to push it up.

Since the configuration hasn't changed, and you're maintaining airspeed with power, the only variable is how I position the deck angle.

Within normal limits (+10/-5 degrees of deck angle) on most airplanes you ought to have no problem keeping your airspeed within 5 knots of your target.

So you say, pitch determines altitute because I set the attitude with the stick. Yes, that is a fundamental part of flying an airplane. You can set the pitch for what you need, and set the power to maintain your desired speed.

Now let's try something a little different. Let's trim the aircraft for that speed going down the G/S and weld the yoke in place. You are now asking for a certain angle of attack against the relative wind. On my airplane at 140 knots, 28 flaps, and typical landing weight 120-125,000 pounds, that's about +2 degrees of deck angle with a 2.5 degree glide slope. It takes about 1.18 EPR to do that too with about a -700 fpm VVI. (All numbers are ballpark.)

That means I'm trimmed to take a 4.5 degree bite out of the air in order to fly at that speed and the stablizer is trimmed with no elevator deflection and the whole rig is welded in place.

Now let's push the power up again and set it about 1.50 EPR. The aircraft will initially accelerate, but the increased airspeed will also create more lift but the airplane still wants to take that 4.5 degree bite out of the air, so the nose will rise. After a couple of oscillations in the vertical, you'll find the airplane will settle down and hold level flight with a 4.5 degree deck angle.

Push it up again, and the airplane will momentarily accelerate, then immediately pitch up, overshoot in the vertical, oscillate and finally settle down in some climb attitude will still taking that 4.5 degree bite out of the air (angle of attack).

Angle of attack is also why my initial climb pitch of +17 degrees (15-20 with 20 being the max allowed) does not mean I'm actually climbing on a 17 degree incline. I'm actually doing something less because part of that deck angle is my angle of attack. My actual climb gradient will be more like 12-14 degrees in the first stage climb.

We can also chop the power to idle and the airplane will momentarily deaccelerate, then immediately pitch down, overshoot, oscillate and finally drop around 2000 feet per minute which would be roughly -3 degrees of deck angle. Of course, you're going to get a WHOOP-WHOOP PULL UP somewhere in there but the yoke is welded in place. The only way to put out the GPWS warning is to push it up.

So when I'm at +2 degrees of deck angle and 1.18 EPR (the control instruments) fully configured (I like 28 flaps, and only use 40 on runways less than 8000 feet), I'd expect (finally) to see my performance instruments registering 140 knots and -700 fpm. If also navigating, then I ought to hold the G/S.

If I fall under the G/S from a TRK-TRK condition (S80 lingo for on course, on glide path) I will add power to a buck twenty three to five (1.23-1.25 EPR) while controlling the pitch up force and oscillations by increasing my attitude an RCH (red pubic hair) on the ADI. Once I've centered the G/S I'll pull a little power and set a buck twenty or so while releasing some back pressure to control the oscillations and then come back to that same yoke position I had before. All the while though, I will maintain my airspeed and try to keep the bite I take out of the air constant so I'm not changing too many things at once. This way of thinking actually helps my cross check in the weather coming down final.

 

[bobbysamd]

Pitch v. Power

Alitalia had us teach our students during transitions from stable situations, e.g., approach to missed approach to go-around, not to add power first as is usually taught. Instead, Alitalia's philisophy was to make pitch the initial change and then follow with power; the idea being that if you add power first during a go-around in a turbojet you will fly the airplane in the ground. By raising the nose first, you are arresting sink and starting the climb because to make an airplane climb you have to point the nose up. From level flight to decent, you push over the nose first because that's how you make the airplane go down. Perhaps this is more of a technique issue than aerodynamic theory, but I believe the two are intertwined.

I will say that Super 80's discussions are very persuasive. I wish he would have been my instructor!

 

[blott]

Your right about me not actually flying the ils like that. I'm usually going about 250 till the marker then idle thrust, dump out everything to get slowed down for ref speed. (but my pitch is increasing the whole time to slow down..lol..)

but a jet isn't a good example because everything is happening so fast. try it in a 172, going SLOW. if you are low on the glideslope, but trimed for the speed, just add power and watch what happens.

didn't your instructor, back in the day, trim the plane for level and a A/S and then show you how if you add power you go up at the same speed and then if you decrease power you go down at the same speed? Were you an instructor? If so, what were you teaching your students? I think that was lesson #2, lesson #1 was let them fly around and have fun.

B

 

[Caveman]

Typhoon may be a liberal but this neo-con agrees with him on this debate.

I can change my altitude by only changing thrust but that isn't the most efficient or direct way to do it. I can make my car change directions by stomping on the accelerator and sliding the rear end out. I could have also simply turned the steering wheel. Similarly I can change the speed of my car by slinging it sideways and skidding to a halt. I could also just lift my foot off of the accelerator and slow down. Just because I get a similar result doesn't mean I'm doing it the best way. Both work because we manipulate the laws of physics to make it happen. But which one was a direct result and which one was a byproduct? Same way with pitch and power. Pitch best controls altitude and power best controls airspeed. It can be done the other way around but it isn't the most efficient way to do it.