This article popped up in a Google search and was ironically linked back to this site. There are some fundamental errors I wanted to point out. These errors are not unique to this article, but have been around for a long time and unfortunately keep getting repeated.
The article describes the make-up of the drag curves and shows a plot of these. The error comes in when these curves are equated with power curves. These are not at all the same. In the text it states: ?We know that in steady, non-accelerated flight, Thrust equals Drag, so the Thrust (power) curves must be the same.? This is not true. While drag does equal thrust, it does not equal power. Power is obtained by multiplying the drag curve by velocity. This has a dramatic effect on the curves. It pulls down the left side of the curve. The left side can be pulled down so far that the upward hook disappears altogether. The consequence is that there is no region of reversed command in the power curve for the aircraft.
Now if we were flying jets and we had thrust levers, then the thrust/drag curves are the ones to use. A piston/prop aircraft, on the other hand, has a power lever and so we need to look at power curves. Below is an example of an actual thrust and power curve (my apologies, happens to be a Lancair) both in the clean and landing configurations. You?ll see the hook on the left completely disappears. What this means is that there is no back side of the power curve for this aircraft.
What drives this phenomenon? For a constant power input, propellers produce higher thrust as velocity is reduced. This automatically compensates to the increase in drag shown in the drag curve. If the increase in thrust keeps up with the increase in drag, the power required curves does not climb.
It is absolutely critical to separate jets (thrust lever) and props (power lever) because you need to look at different curves to determine if there is a region of reversed command.
Here is a link to a video showing the lack of a back-side in the Lancair. If power required was on the rise as the aircraft is slowed, sink rate would increase. Instead it either decreases (landing config) or remains the same (clean config) as stall speed is approached.
https://www.youtube.com/watch?v=RfEv4EiIA4c&t=338s
I have talked to many pilots who thought they had experienced the back side of the power curve in aircraft that did not have one. There are dynamic events (not steady state) that can easily lead one to falsely believe one was on the back-side of the power curve.
It is a simple thing to check in any aircraft. Replicate what was done in the video and see if sink rate starts climbing.
The article describes the make-up of the drag curves and shows a plot of these. The error comes in when these curves are equated with power curves. These are not at all the same. In the text it states: ?We know that in steady, non-accelerated flight, Thrust equals Drag, so the Thrust (power) curves must be the same.? This is not true. While drag does equal thrust, it does not equal power. Power is obtained by multiplying the drag curve by velocity. This has a dramatic effect on the curves. It pulls down the left side of the curve. The left side can be pulled down so far that the upward hook disappears altogether. The consequence is that there is no region of reversed command in the power curve for the aircraft.
Now if we were flying jets and we had thrust levers, then the thrust/drag curves are the ones to use. A piston/prop aircraft, on the other hand, has a power lever and so we need to look at power curves. Below is an example of an actual thrust and power curve (my apologies, happens to be a Lancair) both in the clean and landing configurations. You?ll see the hook on the left completely disappears. What this means is that there is no back side of the power curve for this aircraft.
What drives this phenomenon? For a constant power input, propellers produce higher thrust as velocity is reduced. This automatically compensates to the increase in drag shown in the drag curve. If the increase in thrust keeps up with the increase in drag, the power required curves does not climb.
It is absolutely critical to separate jets (thrust lever) and props (power lever) because you need to look at different curves to determine if there is a region of reversed command.
Here is a link to a video showing the lack of a back-side in the Lancair. If power required was on the rise as the aircraft is slowed, sink rate would increase. Instead it either decreases (landing config) or remains the same (clean config) as stall speed is approached.
https://www.youtube.com/watch?v=RfEv4EiIA4c&t=338s
I have talked to many pilots who thought they had experienced the back side of the power curve in aircraft that did not have one. There are dynamic events (not steady state) that can easily lead one to falsely believe one was on the back-side of the power curve.
It is a simple thing to check in any aircraft. Replicate what was done in the video and see if sink rate starts climbing.