Power and Thrust
Join Date: May 1999
Location: UK
Posts: 35
Likes: 0
Received 0 Likes
on
0 Posts
Thrust is a force, a "push" or a "pull". If you push or pull something and it moves you are doing work. The rate at which you are doing work, ie how long it takes you to push or pull an object to it's new position, is power.
If you could stand an aeroplane on it's tail so it is pointing vertically upwards and generate enough thrust to overcome the weight (and any drag you encounter when moving) you will achieve the best angle of climb (Vx = 90 degrees). But the rate of climb might be very slow, ie only rising one foot every minute. To achieve the best rate of climb you need maximum power ie the best rate of doing work. It also helps if you are at a speed where the power required to move the aeroplane is at a minimum. So Vy, the best rate of climb, will be achieved when you are developing maximum power from the engine and flying at a speed (Vy) that requires the least force to move the aeroplane a set distance in a given time (minimum power). This will get you to altitude quickest.
If you could stand an aeroplane on it's tail so it is pointing vertically upwards and generate enough thrust to overcome the weight (and any drag you encounter when moving) you will achieve the best angle of climb (Vx = 90 degrees). But the rate of climb might be very slow, ie only rising one foot every minute. To achieve the best rate of climb you need maximum power ie the best rate of doing work. It also helps if you are at a speed where the power required to move the aeroplane is at a minimum. So Vy, the best rate of climb, will be achieved when you are developing maximum power from the engine and flying at a speed (Vy) that requires the least force to move the aeroplane a set distance in a given time (minimum power). This will get you to altitude quickest.
Join Date: Feb 2006
Location: Sweden
Posts: 42
Likes: 0
Received 0 Likes
on
0 Posts
Vx is simply the speed were you have maximum excess of thrust from the engine compared to the drag (thrust/drag)
Vy is the speed were there is maximum excess of power availible (mathematically: excess thrust*speed)
As you see an excess of thrust is an important factor in both as this is the force that makes you climb in the first place.
In Vy the speed also becomes important since you want to get up along your climbing path as quick as possible to reach your altitude in minimum time.
The fundamental difference is that power includes some kind of movement and the speed of the movement where thrust is just a force. In the previous explanation the 90 degree climb could be made at 1 feet / 100000 years and it would still be considered 90 degrees of climb = good
Vy is the speed were there is maximum excess of power availible (mathematically: excess thrust*speed)
As you see an excess of thrust is an important factor in both as this is the force that makes you climb in the first place.
In Vy the speed also becomes important since you want to get up along your climbing path as quick as possible to reach your altitude in minimum time.
The fundamental difference is that power includes some kind of movement and the speed of the movement where thrust is just a force. In the previous explanation the 90 degree climb could be made at 1 feet / 100000 years and it would still be considered 90 degrees of climb = good
Last edited by kme; 14th Oct 2007 at 13:28.
Join Date: Jan 2007
Location: Dunno ... what day is it?
Posts: 273
Likes: 0
Received 0 Likes
on
0 Posts
Thrust is a force. Power is the rate of conversion of energy.
To go further, since force x distance is energy, force x distance / time is rate of change of energy, and thus power. Since distance / time is speed, thrust x TAS is proportional to power.
To go further, since force x distance is energy, force x distance / time is rate of change of energy, and thus power. Since distance / time is speed, thrust x TAS is proportional to power.
Join Date: Jul 2007
Location: Italy
Posts: 17
Likes: 0
Received 0 Likes
on
0 Posts
Hi,
I guess this pic should tell you everything about the topic:
http://www.pilotsreference.com/Ablag...cessThrust.jpg
best download or print it to get proper resolution
I guess this pic should tell you everything about the topic:
http://www.pilotsreference.com/Ablag...cessThrust.jpg
best download or print it to get proper resolution
Last edited by transfer jack; 1st May 2008 at 17:13.
Join Date: Feb 2006
Location: Sweden
Posts: 42
Likes: 0
Received 0 Likes
on
0 Posts
To be specific, donīt mix the power-required graph with the thrust-required. They might look similar or even be wrongly simplified to be the same but it is a difference.
The thrust required is the same curve as total drag vs speed (where you find the speed for best glide in the bottom)
The power required curve looks similar but with the minimum point at a lower speed. It also appears to be somewhat "tilted" anticlockwise due to the influence of speed.
(To get a P-req diagram just take a T-req and multiply all the points of the curve with their own speed)
This makes the high speed part of the curve increase even faster than in the T-req but flattens out the penalty at lower speeds.
To find Vx you have to look in a Thrust-availible vs Thrust-required (Vx = max diff)
For Vy it have to be Power-availible vs Power-required ( Vy = max diff)
Its important that all comparisons is made in the same type (power or thrust).
Another trick is to keep the whole prop vs jet question out of the question until you understand this part of it since it is very easy to start mixing stuff up then.
Hope I didnīt confuse you more,
The thrust required is the same curve as total drag vs speed (where you find the speed for best glide in the bottom)
The power required curve looks similar but with the minimum point at a lower speed. It also appears to be somewhat "tilted" anticlockwise due to the influence of speed.
(To get a P-req diagram just take a T-req and multiply all the points of the curve with their own speed)
This makes the high speed part of the curve increase even faster than in the T-req but flattens out the penalty at lower speeds.
To find Vx you have to look in a Thrust-availible vs Thrust-required (Vx = max diff)
For Vy it have to be Power-availible vs Power-required ( Vy = max diff)
Its important that all comparisons is made in the same type (power or thrust).
Another trick is to keep the whole prop vs jet question out of the question until you understand this part of it since it is very easy to start mixing stuff up then.
Hope I didnīt confuse you more,
