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