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

rvbuilder2002 12-29-2018 03:40 PM

Quote:

Originally Posted by N91CZ (Post 1312557)

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.

Perhaps none of your test data subjects were comfortable getting very close to stall which would make your data invalid.

I have personally flown many aircraft that do exhibit an increase in decent rate with a decrease in IAS, and I imagine Van (author of the article you posted) has as well.

snopercod 12-29-2018 03:51 PM

Good post, Chris. I was one of the ones who originally thought that sink rate increased at lower speeds in my Lancair, but you (and 200 hrs. of flying) have convinced me otherwise. I'm glad you're here.

BobTurner 12-29-2018 04:12 PM

Quote:

Originally Posted by rocketman1988 (Post 1312568)
"...Stall depends on angle of attack, not engine power..."

Never said it didn't. The generic power available/power required chart has two intersections. The low speed side defines the point at which the power on stall occurs. t.

I don?t think so. Repeat the experiment, but limit the power to 60%. The power available will go down, the intersection with the power required (low speed end) will move to higher airspeed - but the stall speed will remain unchanged. This intersection is just the slowest airspeed where level flight can be maintained. (Assuming the wing hasn?t stalled).

rocketman1988 12-29-2018 04:41 PM

yep
 
You are correct, it should be the minimum speed for level flight...looking at too many charts tonight...

N91CZ 12-29-2018 05:02 PM

Quote:

Originally Posted by rocketman1988 (Post 1312563)
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...

Have you demonstrated reversed command in the landing configuration?
Not all data that comes back from folks is pristine, for sure. They are not all flight test engineers. For the PA28 it was the owner and his CFI/aerospace engineer son that gathered the data, so I am resaonably confident they followed instructions.

Here are some of the curves. They still show a significant downward slope at the last point taken.


N91CZ 12-29-2018 05:16 PM

Quote:

Originally Posted by rvbuilder2002 (Post 1312566)
Perhaps none of your test data subjects were comfortable getting very close to stall which would make your data invalid.

I have personally flown many aircraft that do exhibit an increase in decent rate with a decrease in IAS, and I imagine Van (author of the article you posted) has as well.

Scott,
The popular GA media/authors would have you believe that anything under approach speed gets you to reversed command. I just posted a plot of curves from different aircraft. The margin to stall varies, but are generally just a few knots away. Does the trend look like any of the curves are about to reverse and increase?

In my plane if you actually compute the CL required for min power in the landing config, you see that it is unachievable. Others may vary.

I am a little surprised that Van himself wrote the article. He should no better than to mix drag and power, jets and props.

N91CZ 12-30-2018 11:08 AM

Quote:

Originally Posted by BobTurner (Post 1312544)
Stall depends on angle of attack, not engine power.

Bob,
Here is an interesting segue on the topic of power vs stall AoA.

While mapping out Pitot-Static errors and the extreme low end of the speed envelope, I spent a lot of time in the buffet region at different flap and power settings. What emerged was a trend that showed how stall speed and stall AoA were affected by the prop wash as it transitioned from driving to back-driving. If the prop is driving it is adding energy to the air stream and it is relatively well behaved. If back-driving the air coming off the prop is low energy very turbulent. This affected the inboard section of wing. One could actually feel the difference sitting in the plane.

Note the units are KIAS and not KCAS





N91CZ 12-30-2018 11:11 AM

Quote:

Originally Posted by rocketman1988 (Post 1312519)
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...

Bob,
The original drag and power curves were actually obtained in this way. The aircraft was instrumented with data acquisition and calibrated test equipment. With help from Lycoming and Hartzell we were able to get shaft power and then thrust values at each data point. Engine and prop efficiencies really start to fall off at these extreme low power settings.

snopercod 12-30-2018 06:05 PM

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?

RV7A Flyer 12-30-2018 09:26 PM

Quote:

Originally Posted by N91CZ (Post 1312590)
Have you demonstrated reversed command in the landing configuration?
Not all data that comes back from folks is pristine, for sure. They are not all flight test engineers. For the PA28 it was the owner and his CFI/aerospace engineer son that gathered the data, so I am resaonably confident they followed instructions.

Here are some of the curves. They still show a significant downward slope at the last point taken.


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.


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