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-   -   The Back-Side of the Power Curve (or lack thereof) (https://vansairforce.net/community/showthread.php?t=167242)

N91CZ 12-30-2018 11:11 PM

Quote:

Originally Posted by RV7A Flyer (Post 1312880)
It's been ages and ages since I flew spam cans, but the lowest points on these graphs seem to be considerably higher (in IAS) than I recall being able to achieve in either Cessnas or Pipers.

Not sure what the debate is here...it's pretty easy to demonstrate this "back side of the power curve". Just slow down and hold altitude, reducing power, until you can't hold altitude anymore. Continue pulling back *and adding power back in* to hold altitude. Voila.

I've had 172s, e.g., practically hanging on the prop, showing something like 15 or 20 kias IIRC (but in any case WAY below stall speed) and full power, holding altitude.

If that isn't the back side of something, I don't know what is.

Come to think of it, I've only done it a time or two with an instructor on board in my RV, but guess what? Same thing happens.

There is only one spam can in the list and it is about 3 kts above stall. 52 kts flown 49 kts stall. The only curve with any significant margin is the 235 curve.

It really wasn?t so much a debate as just clarifying that there were errors in the article shown where drag curves where equated to power curves. Many articles in the likes of AOPA and Flying magazine warn against getting slow on final approach. The claim is that a high sink rate can develop as one is supposedly flying on the back-side of the power curve. The question posed is this: Is a piston/propeller aircraft susceptible to the back side of the power curve on final approach ? and this of course assumes configured for landing.

From the discussion it appears some have managed to fly on the back-side of the power curve in the clean configuration. While this is certainly possible it is not the configuration that is relevant to the warnings in those articles. The physics of the situation would tend to make it less likely to be achievable considering the relationship of CL and CD at min power. For many aircraft it places the required CL too high to be flyable.

A number of aircraft were qualitatively checked to see if this assertion is generally valid. During engine idle descents, gravity is the propulsion and descent rate is an indicator of magnitude. If power required was increasing at progressively lower airspeeds, one would expect sink rate to start increasing. Instead, in all cases the trend is that sink rate decreased throughout. The curves all begin to flatten but one would be hard pressed to argue that with another 3 kts they would reverse and look anything like the hypothetical curves being published.

With respect to the published articles, when you look at the author bios, you will see lots of jet time, typically retired airline pilots. I suspect they end up transferring the characteristics of jets onto GA aircraft without understanding the differences. They all describe the increase in drag at low speed correctly and then make the mistake of equating that with power.

I would love to see a video of a 172 in level flight indicating 15-20 KIAS. Unfortunately, once you hit buffet or stall you are not operating on the power curve being discussed.

N91CZ 12-30-2018 11:24 PM

Quote:

Originally Posted by snopercod (Post 1312826)
Chris, I think what may be confusing people is your use of the term "reversed command". I'm not sure what you mean by that. In my plane in the landing configuration, pitch controls airspeed and power controls sink rate. I demonstrate that on every landing. What am I missing?

John,
The Clift Notes version.
Generally when one wants to fly slower (assume no altitude change) a power reduction is needed. In the case of a jet, a thrust reduction.
Flying in the region of reversed command means that flying slower requires more power, or in the case of the jet, more thrust to maintain altitude.

This is a common event for jets. This is because jets have thrust levers and their Vref approach speed can be very close to minimum thrust required. i.e. going faster or slower requires more thrust.
In propeller aircraft we have power levers. Our reversed command region starts once we get below minimum power required speed. This occurs at a much lower airspeed than typical approach speed. In fact, it is close to or can even below stall speed.

andrewtac 12-31-2018 05:59 AM

I will check my 8 in 20 flaps next time I have a chance.

For the poster asking about backside, another myth commonly believed is you add power to slow down (or a region of reverse command). Typically most aircraft will have more thrust or power available to get out of this region. If you are straight and level at some point you’ll find the least power or thrust required setting. To go slower pull some power, add drag, or g and slow a little more. If now you want to maintain straight and level the power/thrust setting will be higher than the minimum power setting you found before that was at a faster airspeed. And again you have almost always you have more power available than required to fly at this condition, firewall it and you speed up; you don’t slow down with added power (not reverse command). I have found backside in turbo props, I know it exists; haven’t tried a piston GA aircraft yet.

I am aware it is called a region of reverse command, but this is a misleading term. Command is something you do, like add power or pull aft on the stick (command more thrust or command more g/aoa). A region of reverse command should be used when it actually means what it says. For example the mig15 exhibits regions of reverse command at elevated g, you push on the stick at some point to get more g (and vice versa); or many none fly by wire aircraft when transonic exhibit reverse command in roll (stick left and aircraft rolls right; true also at very slow speeds in some due to adverse yaw).

N91CZ 12-31-2018 07:07 AM

Quote:

Originally Posted by andrewtac (Post 1312908)

I am aware it is called a region of reverse command, but this is a misleading term. Command is something you do, ....

The term has been hijacked for sure. Should be more like "reversed response"

snopercod 12-31-2018 07:47 AM

Quote:

Originally Posted by N91CZ (Post 1312892)
The only curve with any significant margin is the 235 curve.

Nice of you to say ;) My 235 stalls around 60 KIAS (depending upon weight) in the landing configuration. I guess I need to re-perform that sink rate test to get you another data point on the lower end.

The only time my 235 develops a high sink rate is when I am in the flare and pull the power. Unless I'm doing a short field landing, I usually leave a little power in and ease it out when stabilized 6" over the runway in order to gently set down. Like so (Note: There's a 1 second encoding lag in the displayed ground speeds):

https://www.dropbox.com/s/5kfrgmuds4...Flare.mp4?dl=0

PilotjohnS 12-31-2018 08:06 AM

Glider guider
 
In my sailplane days, I would thermal at minimum sink speed, which was just before the onset of the stall buffet. And as it sounds, this is the speed with the minimum sink. As Paul Mcready proved with the Gosemer Condor, this is also the speed of least power to maintain level flight.
I think there is a nomenclature issue here in that many power planes can fly straight and level below this minimum sink speed if they are hanging on the prop. Helicopters do this all the time. And even in the 172, this is easily demonstrated by maintaining level flight with full power in the stall buffet region. JMHO

rocketman1988 12-31-2018 08:40 AM

Video
 
"I would love to see a video of a 172 in level flight indicating 15-20 KIAS. Unfortunately, once you hit buffet or stall you are not operating on the power curve being discussed."

You are never going to get a video of a 172 flying at 15-20 knots.

If I get out to the airport this week, I will try and get a video of the 172 requiring more power to fly at a slower airspeed AND maintain level flight...

andrewtac 12-31-2018 10:09 AM

Quote:

Originally Posted by N91CZ (Post 1312922)
The term has been hijacked for sure. Should be more like "reversed response"

It isn?t even reverse response, if you are in the region and addpower you will accelerate to the other side of the region past the minimum required point. One of the reasons without DLC or low slung motors flying the ball is a difficult task.

RV7A Flyer 12-31-2018 04:21 PM

Quote:

Originally Posted by rocketman1988 (Post 1312947)
"I would love to see a video of a 172 in level flight indicating 15-20 KIAS. Unfortunately, once you hit buffet or stall you are not operating on the power curve being discussed."

You are never going to get a video of a 172 flying at 15-20 knots.

If I get out to the airport this week, I will try and get a video of the 172 requiring more power to fly at a slower airspeed AND maintain level flight...

I think y'all missed my point. I did say it's been years (at least 15) since I flew Cessnas, and maybe it was a 152, but the point was that you CAN get the airspeed WELL below stall speed (in clean or dirty configuration) and into the so-called "back side of the power curve). So much so that I remember being shocked at how slow we were actually flying the first time my instructor demonstrated it to me. Then it became a challenge...see how slowly you can get the plane to fly (and hold altitude).

rocketman1988 12-31-2018 04:43 PM

Yes
 
Yes, you can...and you can do it in a PA28, too...


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