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Old 06-28-2007, 07:17 PM   #1  
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Default Power Curves?

Can any one help me to understand their purpose, how to understand the info, and any thing else important about them.
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Old 06-29-2007, 03:03 AM   #2  
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As far as piloting is concerned, there is a point called being behind the power curve in the landing phase where induced drag exceeds available power and you are going to continue descending no matter how much power is added. Induced drag rises by a power of four as speed decreases. In turbines the power curve is such that at certain speeds it would be impossible to go around if power was reduced to idle. That's why they usually carry power on approach. It is also the reason after putting in full power on a go around you raise the flaps a notch and throw out the dog (kidding). Climb is not possible with that much drag.

The subject of power, efficiency, engine type, and altitude is a complicated subject that does not lend itself to a forum, but a book on aircraft performance like Aircraft Performance and Design by John D. Anderson explains the subject pretty throughly. Each engine has a power curve. The main categories are piston, turboprop, turbojet, high bypass jet, turbojet/high bypass jet with afterburner, and ramjet. They all have their distinct performance patterns with changes of density altitude, forward speed, and rpm. For pilots The Airplane Flying Handbook (FAA 8083) is enough information. Hope this helps.

Last edited by Cubdriver; 06-29-2007 at 03:14 AM.
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Old 06-29-2007, 05:15 AM   #3  
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That's why they usually carry power on approach. It is also the reason after putting in full power on a go around you raise the flaps a notch and throw out the dog (kidding). Climb is not possible with that much drag.
Actually, the jets will climb with all the stuff hanging down, some with more exuberance than others but they will climb. And on heavy aircraft you had better not do anything too quickly on a Cat III approach or you may hear scraping noises on a bad day.

Also, many aircraft have flap settings with high drag for a number of reasons including having the engines will be above what used to be called 'critical rpm' or something like that, the point at which the jet engine offered almost instant response. (I remember as a kid watching the B-47s loaf around the pattern with a **drogue** chute trailing so they wouldn't have to be in idle power coming down final)( from web: Because the J47, like all early jet engines, was slow to accelerate, Boeing had devised a drogue chute that was deployed in the landing pattern and allowed you to maintain the engine at a relatively high power setting from which a go-around could easily be made. Once on the ground, the brake chute assisted the excellent anti-skid brake system to get you stopped. The brake chute had another use: if you hit front-wheels-first and bounced on landing, you could - if you knew just when to do it - deploy the brake chute and bring the airplane down to a perfect rear-wheel-first landing.)

Because many think the jet will not climb very well on a missed approach/go-around they rush. Just think, "Nose up, power up, flaps up, gear up" and it becomes a simple process.
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Old 06-29-2007, 07:08 AM   #4  
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[Aero Engineer]
I think the problem that was mentioned by Cubdriver with not climbing is related to how a turbojet engine produces powers vs thrust. By definition, you must have forward speed for a jet engine to produce power. Since a turbojet engine operates at a (mostly) constant thrust across the entire airspeed regime for a given density, when the airplane is slower, there is less power available. Rate of climb is directly related to power available(angle of climb is determined by thrust available). As you get to the backside of the power curve, your power available decreases with airspeed. I can see how you would get to a point where you would need to accelerate first before attaining sufficient available power to get a satisfactory rate of climb.
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Old 06-29-2007, 01:11 PM   #5  
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Man, we had an entire semester of this in Aircraft Performance....

Basically, you have a curve that determines how much power is available (PA) based on settings and what engine you have. Secondly you have a curve that shows the power required (PR) to maintain altitude. If you chart both on the same graph, you can measure climb performance. The larger the distance between the curves, the better your performance. PA decreases with altitude, and PR increases with altitude. So at some altitude, the graphs will meet, and that is your absolute ceiling.
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Old 08-15-2007, 10:21 PM   #6  
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The slower you get an airplane, the higher the angle of attack to hold altitude or sustain a given descent rate, the higher the angle of attack, the more drag. At a certain point if you get slow enough it requires an enormous amount of power to recover or even just to sustain altitude or keep a desired descent rate. It has nothing to do with spool times, it is simply drag vs. airspeed. You can even demo this in a 172. You get a 172 slow enough, you'll need full power to hold altitude. Each airspeed has an associated amount of thrust to maintain altitude, at some point down low towards the stall..the amount of power starts going up like crazy just to hold altitude, and in theory at zero airspeed would have the nose pointed at the sky to hold altitude. When you run out of enough excess power to initiate a go-around you are now below the power curve required to avoid hitting the ground, or arresting the descent.
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Old 08-16-2007, 09:47 PM   #7  
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The slower you get an airplane, the higher the angle of attack to hold altitude or sustain a given descent rate, the higher the angle of attack, the more drag. At a certain point if you get slow enough it requires an enormous amount of power to recover or even just to sustain altitude or keep a desired descent rate. It has nothing to do with spool times, it is simply drag vs. airspeed. You can even demo this in a 172. You get a 172 slow enough, you'll need full power to hold altitude. Each airspeed has an associated amount of thrust to maintain altitude, at some point down low towards the stall..the amount of power starts going up like crazy just to hold altitude, and in theory at zero airspeed would have the nose pointed at the sky to hold altitude. When you run out of enough excess power to initiate a go-around you are now below the power curve required to avoid hitting the ground, or arresting the descent.
In addition it doesnt matter how much excess power you have once you exceed the criticle angle of attack you are stalled.
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Old 08-16-2007, 11:57 PM   #8  
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In addition it doesnt matter how much excess power you have once you exceed the criticle angle of attack you are stalled.
Entirely NOT true. Some airplanes, a la' seriously overpowered aerobatic airplanes and some fighters can hover...and recover in the vertical. How critically stalled is your wing when it's stopped in flight

If you have enough power, you can maintain a 0 degree AOA while climbing straight up and continuing to accelerate...perfect example...a rocket...or an F-15..or anything else with a positive thrust to weight ratio.

There are several ways one can decrease AOA of the wing, accelerate without climbing, lower the nose and let gravity help, or have so much power gravity doesn't matter, no matter how stalled your wing is.


I dig your avatar.

Last edited by wickedsprint; 08-17-2007 at 12:12 AM.
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Old 08-17-2007, 12:22 AM   #9  
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Entirely NOT true. Some airplanes, a la' seriously overpowered aerobatic airplanes and some fighters can hover...and recover in the vertical. How critically stalled is your wing when it's stopped in flight

If you have enough power, you can maintain a 0 degree AOA while climbing straight up and continuing to accelerate...perfect example...a rocket...or an F-15..or anything else with a positive thrust to weight ratio.

There are several ways one can decrease AOA of the wing, accelerate without climbing, lower the nose and let gravity help, or have so much power gravity doesn't matter, no matter how stalled your wing is.


I dig your avatar.
True on the fighter jet. However, you were discussing a 172. That would be limited to a certain degree with regards to power available.
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Old 08-17-2007, 01:04 AM   #10  
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True on the fighter jet. However, you were discussing a 172. That would be limited to a certain degree with regards to power available.

Your response led me to believe you thought this the case for all airplanes. A lot of people don't understand AOA that well and act surprised when you show them you can stall a wing at cruise speed and full power too.
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