Pitch & power debate

[Typhoon1244]

My first instructor--my dad--put it like this: pitch controls altitude and power controls airspeed if both are variable.

That "definition" has held rock-solid for me through all my instruction (received and given) and all my flying.

 

[Mickey]

So if we hold pitch and reduce power we can maintain altitude but not airspeed ... I don't agree.

We're in cruise flight and have control of pitch and power, (both being variable). If you reduce power and leave pitch alone you will descend at the same airspeed you were at in cruise. If you are in cruise and only add power you will start to climb at the cruise speed.

You are at cruise power and pitch the nose up the airspeed will decrease. But, you say...we are climbing. To this I say for how long?

We have the ability to think and that's what too many people are trying to avoid when they come up with hard rules. I think your father was on the right track but, we need to go further down that same track to find out it goes in different directions.

Take care.

 

[flywithastick]

Pitch, power, airspeed, altitude - they're all interellated and to say one particular relationship is right with all planes and under all conditions is too general the way I see it.

 

[Typhoon1244]

So if we hold pitch and reduce power we can maintain altitude but not airspeed...

No, because if you're holding pitch (for whatever strange reason) then it's not variable.

Why is that so hard to understand?

 

[Mr. Cole]

This is the kind of thread that makes coming here worthwhile, particularly the post by Super 80. Below is one of my posts from another board where we discussed this very subject and how it could be better understood by looking at the power curve.

Dave

Think of the power curve for a minute and you'll realize that for the most part it is a parabola, so if you draw a line through it that line will cut the power curve a two points, one on each side of the minimum. What the power curve is showing you typically is the power required to overcome drag, both parasitic and induced, at a given airspeed. So if I'm flying at 100 knots and I'm in the region of normal command, there is a power setting that will allow me to maintain level flight because it compensates for the drag. So imagine a straight line that cuts the parabola at the exact place on the curve corresponding to 100kts. Of course power is not a force, so required thrust is probably a better measure, but power is directly related to the thrust required.

Now if you reduce the power, you will have insufficient power(thrust) to overcome the drag produced at 100kts. You can do one of two things. You can try to hold altitude and the plane will slow down to a speed where the given power is now sufficient (move down the curve until power required equals power available), or you can hold the same airspeed and descend (power required is less than power available). If you add power you can either hold altitude and your speed will increase (move up the curve to where the power required is equal to the power available), or keep the same airspeed an climb.

What this also implies is that in the region of normal command the plane tends to return to a stable airspeed, which is usually the trim speed. So assume again your available power is equal to the power required for 100 knots. Then say you push the nose down but you don't add power. As gravity kicks in and increases your airspeed, the parasitic drag grows and because you haven't compensated by increasing power, you eventually slow back down to an airspeed where the power is sufficient. If you pull the nose up and the airspeed starts to drop initially, in the region of normal command your total drag will drop, and the excess power required for the lower drag will return you to your trimmed speed assuming you don't continue pulling back.

The problem is that in the region of reverse command the plane doesn't seek equilibrium. As you become slower you need more power. The trick is that if you try to use power for airspeed, you will eventually run out of available power. So let's say you pull the nose up, increasing AOA and induced drag and don't touch the power. Unlike in the region of normal command where slowing requires less power, now you require more power to arrest the impact of drag. If you didn't do anything else, the drag would slow you down more, which further increases the drag and the power required. Eventually you run out of available power so you have to use pitch to reduce the AOA and induced drag, which breaks the cycle and allows you to build airspeed.

 

[Mickey]

No, because if you're holding pitch (for whatever strange reason) then it's not variable.


It's variable because I still have the ability to change it. If your engine is dead, then your power is longer variable because it is now zero and you don't have the ability to change it.

If you are saying it is only variable if you are moving it then you are seeing the light that flywithastick is trying to shine and we can get away from trying to fly without thinking.

So you are flying your CRJ or J41 (which is it?) and you start a descent for ATC. ATC tells you to increase your descent rate and increase your speed. Do you add power and lower your pitch? I think reducing your power and lowering your pitch will work.

After takeoff I reduce thrust and accelerate by lowering my pitch. They are both being changed.

 

[badog]

Region of reverse command

There is no such thing. It's been said too many times, No power or full power, the power is fixed IE full throttle means - no more power. Power off or idle, means no thrust in this situation.
In either case you pitch for airspeed full power or no power
If you have control over the thrust IE power is not fixed
power for airspeed.
So when your instructor pulls the engine on final, you pitch for airspeed GET IT ???

 

[Typhoon1244]

Well, I guess you guys are right. I give up.

Next time I'm skimming into DFW at FL240 and 330 knots, and Center asks me to slow to 250 for spacing, I'll leave the power in and pull the nose up fifteen or twenty degrees since, you know, pitch controls airspeed.

 

[Typhoon1244]

Sorry. I guess that was a little surly. I'll just answer the question...

So you are flying your CRJ or J41 (which is it?)...

CRJ. Not sure where you got J41 from...

...and you start a descent for ATC. ATC tells you to increase your descent rate and increase your speed. Do you add power and lower your pitch? I think reducing your power and lowering your pitch will work.

First, I'd increase the rate of descent by increasing the pitch attitude slightly, referencing the VSI...because pitch controlls altitude/rate-of-descent. Then I'd see what effect the change had on airspeed. There will be an increase. If I'm satisfied with the increase, I'll leave the power alone. If it's not enough, I'll add power. If it's too much and we're flirting with the barber pole, I'll decrease power...because power controls airspeed if both pitch and power are available and variable.

Ask yourself this: in an airplane with an autopilot and autothrottles, when you instruct the autopilot to increase speed, what variable does it change to do that? (Hint: it's not elevator/stabilizer angle!)

 

[Timebuilder]

CRJ. Not sure where you got J41 from

He probably saw it in your "signature".

Of course, we use both pitch AND power to fly.

However, we have a limit on available power, and then when we run out we are back to which functions have ultimate control, which is really what the pitch and power argument is all about. After all is said and done, pitch has the last word on airspeed, and power has the last word on altitude. We fly using both, when and where they are appropriate, but in the final analysis, these are the controlling factors.

 

[Typhoon1244]

After all is said and done, pitch has the last word on airspeed, and power has the last word on altitude.

Tell an F-15 driver that!

A more accturate statement would be: Pitch has the last word on airspeed if power is not available or variable, and power has the last word on altitude if pitch is not variable.

Neither condition is normal, however.

Now I know I don't always have the right answer, but this is not one of those times!

 

[Timebuilder]

Since I prefaced my statement with the idea that power is finite, and you reach a point when you have no more power, then what I have said is true, in the final analysis.

Tell an F-15 driver he has no power. Now, order him to take off. How high will he climb?

It still holds true.

 

[Stifler's Mom]

Thank you all for reminding me how much I don't miss instructing.

 

[Typhoon1244]

Tell an F-15 driver he has no power. Now, order him to take off. How high will he climb?

Well according to some, all he has to do is push the stick and he'll shoot forward like a bat out of hell.

You didn't answer my autopilot question. How does yours work?

 

[dash8driver]

Well according to some, all he has to do is push the stick and he'll shoot forward like a bat out of hell.

You didn't answer my autopilot question. How does yours work?

when all you have to do is just pull back on the stick and you'll shoot into orbit.

 

[Timebuilder]

If he has no power, all the stick movement in the world won't do a thing.

It is the power that the airplane has at it's disposal that moves the mass in a way that produces lift. Pitch only becomes an issue when power has been applied, either at the same time or at some previous, recent time.

An unpowered glider is towed to altitude. That's where the power comes into play. The gain in potential energy is becuase of its new altitude, whcih was solely the result of the tow plane's power.

During flight at altitude, when additional power is removed from the equation, no aditional climbing is possible. Even a thermal imparts power.

A climb to a higher altitude is always the result of power, brought into play at some time, in some way.

 

[midlifeflyer]

I was in another forum almost a month ago when someone asked what this pitch power debate was all about.

My answer was:

==============================
We all know that pitch + power = performance.

That wasn't good enough so people started arguing about whether power "controls" airspeed and pitch "controls" altitude or vice versa. Each side of the argument tends to view it's opinion as the "one true way", marshaling dubious evidence with idiotic examples ("Oh yeah? Sit in the runway with the power at idle, pull back on the yoke and see how long it takes for you to get in the air!" or "Well that's how it's done in a B-747, so you have to do it the same way in CE-152!") to support it's own religious viewpoint.

All they really come down to is a personal preference in teaching style.
==============================

In case I'm asked again, I'm keeping this thread as proof of my point.

 

[Timebuilder]

Sit in the runway with the power at idle, pull back on the yoke and see how long it takes for you to get in the air!"

It is always a matter of power being necessary to do the work over time to accomplish tasks. Climbing to altitude in an airplane is only one example of this. The way you characterized it in the quote is a good example of the necessity of power to gain altitude.

 

[Super 80]

Next time I'm skimming into DFW at FL240 and 330 knots, and Center asks me to slow to 250 for spacing, I'll leave the power in and pull the nose up fifteen or twenty degrees since, you know, pitch controls airspeed.

Well in this case, you're talking about changing an equilibrium here aren't you?

When your airplane is trimmed to 330 knots and the altitude control is holding FL240, and you slow to 250 knots, the autothrottles come back to some low limit function, and the pitch increases to make up for the lack of lift as a result of the decreased airflow across the mean aerodynamic chord. The stabilizer trim will also compensate for elevator displacement. At 250 knots, you or the autothrottles will input the new equilibrium thrust value that will hold the reduced airspeed with the new combined drag on the aircraft.

If you did not have an autopilot function that held altitude by increasing the pitch, then your airplane would descend unless you increased the pitch. In my aircraft, the difference is not 15-20 degrees, but 2-3 degrees of deck angle.

Of course, I could pitch to a 250 knot "picture" and pull the power to correspond to FL240 and 250 knots, and eventually, the airplane would settle there in equilibrium, but not before I busted my altitude.

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

 

[Super 80]

You know the funniest thing about engineers is the way they think. They design an airplane to hold airspeed with power. Of course, they always live at ground level.

So the first time in the Super-duper 80 (it really is a super airplane - just look at the Rube Goldberg device they call the side window) I was going into Chicago on the Bradford arrival and center asked me to push it up, the autothrottes went to IAS MAX. That's what happens when you turn the A/S knob up in a constant rate descent (VVI).

Now if you're in IAS for pitch, and you ask for more A/S, the aircraft pitches over, just like I would have done in the 72.

I refuse to push it up in the descent when God's G will provide me with more than enough acceleration to get the job done. Besides, it's a waste of gas, and you can never have too much gas unless you're on fire.