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12-29-2018, 11:18 AM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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The Back-Side of the Power Curve (or lack thereof)
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.
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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12-29-2018, 11:49 AM
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Join Date: Mar 2006
Location: Newport, TN
Posts: 7,496
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Slowing my 7 down on final has a noticable effect on sink rate. Nose up to go down faster works every time.
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12-29-2018, 11:53 AM
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Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,189
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what you are looking for
This is the chart you are looking for:
[IMG]  [/IMG]
As you can see from the generic chart, the bottom of the power required curve is relatively flat. This explains your empirical result in the video.
You will find that if you go even slower than the 65 Kt in the video, you WILL begin to see the region of reversed command, as the increase in power required is exponential in nature. The question then becomes, "Will you perceive the rise in power required prior to the occurrence of the stall?" That will be airframe dependent, and with a little speedster like the Lancair, it is likely possible to reach the stall speed while still on the flat portion of the power required curve...
__________________
Bob
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12-29-2018, 11:57 AM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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Quote:
Originally Posted by Brantel
Slowing my 7 down on final has a noticable effect on sink rate. Nose up to go down faster works every time.
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Would you mind generating a plot of speed vs sink rate? Power off, flaps down. I have data for five different aircraft models that all show no back-side. -am looking for one that does.
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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12-29-2018, 12:16 PM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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Quote:
Originally Posted by rocketman1988
This is the chart you are looking for:
As you can see from the generic chart, the bottom of the power required curve is relatively flat. This explains your empirical result in the video.
You will find that if you go even slower than the 65 Kt in the video, you WILL begin to see the region of reversed command, as the increase in power required is exponential in nature. The question then becomes, "Will you perceive the rise in power required prior to the occurrence of the stall?" That will be airframe dependent, and with a little speedster like the Lancair, it is likely possible to reach the stall speed while still on the flat portion of the power required curve...
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If a particular aircraft can't generate the CL required to reach the left end of the curve, then the region of reversed command is just theoretical and of no consequence. These generic chart would leave you to believe the bottom will fall out from under you.
Perhaps the video could have nibbled a little closer to stall (it was within 4kts). It was certainly far below normal approach speed.
I am looking for aircraft to document that can reach reversed command in the normal approach region.
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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12-29-2018, 12:40 PM
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Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,189
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Curious
I would be curious to know what five different models have no backside, and your testing method...
The speed at which the power available and power required cross in the low speed regime is the minimum level flight speed, where they cross in the high speed regime is the maximum speed in level flight. I would suggest running your experiment at a zero rate of descent and monitor your power setting for each airspeed, up to and including the power on stall. You will likely find that the generic curve is accurate...
It is also a false assumption that an "increase in thrust as velocity decreases" compensates for the drag increase equally...
It is very easy to demonstrate in most training aircraft, however, in order to get a good demonstration, you must be well below the normal approach speeds
__________________
Bob
Aerospace Engineer '88
RV-10
Structure - 90% Done
Cabin Top - Aaarrghhh...
EFII System 32 - Done
297 HP Barrett Hung
ShowPlanes Cowl with Skybolts Fitted - Beautiful
Wiring...
Dues+ Paid 2019,...Thanks DR+
Last edited by rocketman1988 : 12-29-2018 at 04:43 PM.
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12-29-2018, 02:31 PM
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Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
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Quote:
Originally Posted by rocketman1988
I would be curious to know what five different models have no backside, and your testing method...
The speed at which the power available and power required cross in the low speed regime is the power on stall speed, where they cross in the high speed regime is the maximum speed in level flight. I would suggest running your experiment at a zero rate of descent and monitor your power setting for each airspeed, up to and including the power on stall. You will likely find that the generic curve is accurate...
It is also a false assumption that an "increase in thrust as velocity decreases" compensates for the drag increase equally...
It is very easy to demonstrate in most training aircraft, however, in order to get a good demonstration, you must be well below the normal approach speeds
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Stall depends on angle of attack, not engine power. But otherwise I agree with above. At low enough speeds, drag scales as 1/V^2, and power required scales as 1/V. So at low enough airspeed, the curve will turn up. But most if not all of this region will be below stall speed.
Cessna 152s can demonstrate this region of reversed comand: Carefully adjust power and trim for level flight, at a speed just above where the stall horn comes on. Remove hands from yoke, use rudder only to keep wings level. Do not touch power. Apply a small amount of nose up trim. After an initial pitch up, the plane will settle into a steady state descent. (The nose up trim trims the airplane for a lower airspeed. But the power required for level flight increases, not decreases(?reverse command?). Since you left the power unchanged, the plane will descend.)
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12-29-2018, 02:56 PM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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Quote:
Originally Posted by BobTurner
Cessna 152s can demonstrate this region of reversed comand: Carefully adjust power and trim for level flight, at a speed just above where the stall horn comes on. Remove hands from yoke, use rudder only to keep wings level. Do not touch power. Apply a small amount of nose up trim. After an initial pitch up, the plane will settle into a steady state descent. (The nose up trim trims the airplane for a lower airspeed. But the power required for level flight increases, not decreases(?reverse command?). Since you left the power unchanged, the plane will descend.)
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Bob,
Was this clean of flaps down?
Higher drag configs tend to push the min power point to the left.
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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12-29-2018, 03:13 PM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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other models
Quote:
Originally Posted by rocketman1988
I would be curious to know what five different models have no backside, and your testing method...
.....
It is also a false assumption that an "increase in thrust as velocity decreases" compensates for the drag increase equally...
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I didn't mean to imply compensation was -or remains- equal, just that in the region near the bottom of a very flat power curve the two effects can cancel each other out within the range of interest. Yes, many non-linear things working together, but the end result is a nearly flat power required curve from ~100KIAS down to stall (clean config).
Aircraft recorded were: Lancair 235, 360, Legacy, IV, and PA28-180
When others are flying I simply have them record steady state descent rate at idle and full flaps across the entire flap extended speed envelope - down to as close to stall that individuals feel comfortable. Increments vary by aircraft, but enough points to generate a good trend line. What you end up seeing is that the slope at the slowest point is still rather steep indicating the aircraft will stall before reaching minimum power speed.
For the 360, I have full drag polars, a propeller map, and engine map. Those are from a prior airframe characterization project.
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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12-29-2018, 03:37 PM
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Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,189
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more
I have no experience with the Lancair series of aircraft, however, I have many, many hours in the PA28 series...and demonstration of reverse command in this series of aircraft is easily accomplished.
If your data indicates that the PA28 does not indicate an area of reverse command, I submit that your method or assumptions are flawed...
__________________
Bob
Aerospace Engineer '88
RV-10
Structure - 90% Done
Cabin Top - Aaarrghhh...
EFII System 32 - Done
297 HP Barrett Hung
ShowPlanes Cowl with Skybolts Fitted - Beautiful
Wiring...
Dues+ Paid 2019,...Thanks DR+
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