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  #51  
Old 01-05-2019, 10:51 PM
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BJohnson BJohnson is offline
 
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Default Found the backside

Quote:
Originally Posted by RV7A Flyer View Post
A couple of these tests seem to stop just when it's getting interesting...
Tested again today, but the winds were a bit gusty and flying at high power and high AOA takes some practice. The best I got at near zero sink was the new data point at 40 knots shown in the plot below. I don't know how accurate the IAS is at these pitch angles though, at 20 degrees. I did get as slow as 38 knots and 92 HP, but was climbing at 400 fpm so I did not count that data point. Normal Vso is 42 knots.

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  #52  
Old 01-07-2019, 11:09 PM
N91CZ N91CZ is offline
 
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Lots of good flying going on out there.

A few thoughts….
Many of the curves are indeed showing that flattish bottom where the increasing drag is almost identically canceled by increasing thrust. While not linear, when you zoom in to very small regions things can look much more linear considering the full range of speed and power.

While gathering data it is best to stack conditions in your favor, i.e. picking a good day with smooth air and light winds aloft. Then holding each data point long enough to become steady state. If the resulting curve is smooth and continuous then one can be reasonably sure points are good. If you see a single point that falls out of line, it is suspect. If it is the first or last point that jumps, more intermediate point are need to see if it was a trend or anomaly.

If you hit any stall buffet then you’ve gone too far – at least in terms of following your drag polar, you’ll start getting discontinuities. And full stall, of course falls off the chart completely unless flying an Extra 300 or the V-22.

Whether or not you can get to the back side or to the left of min power required is a function of your drag polar. Required CL=sqrt(3 x CD/k). If you are able to generate a lot lift for the drag of the flaps you might get there. Simple flaps don’t make this trade-off very well, so it is a struggle.

What is quite clear is that the charts typically used to describe the phenomenon show a large area we simply can’t reach.

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  #53  
Old 01-08-2019, 11:51 AM
N91CZ N91CZ is offline
 
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Quote:
Originally Posted by 506DC View Post
The important thing that Van is trying to point out is that you don?t want to unknowingly get behind the power curve. There is an airspeed that at full power you will not be able to climb and if you are low enough, you will most likely impact objects that are in your direction of flight
What folks are finding is that it is very difficult to get behind the power curve in the landing configuration. It is a very tiny sliver of airspeed.


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Originally Posted by 506DC View Post

If you have enough power available, you can actually climb at zero airspeed.
In this case you are no longer on the ?power curve?. Consider that if power is velocity x speed and speed is zero?..
Yes the engine is still making power and you are staying airborne, but you are no longer using the drag/thrust curves to derive how much power you need.
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  #54  
Old 01-08-2019, 12:02 PM
David Paule David Paule is offline
 
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Quote:
Originally Posted by N91CZ View Post
....I am looking for aircraft to document that can reach reversed command in the normal approach region.
No problem. My old Cessna exhibits greater sink at slower speeds, below something like 80 or 85 mph indicated. It's a good way to control the landing event. I routinely make my solo approach at 70 to 75 mph indicated.

Dave
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  #55  
Old 01-08-2019, 12:04 PM
N91CZ N91CZ is offline
 
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Quote:
Originally Posted by BJohnson View Post

Perhaps the root of the discussion in this thread is that the Lancair 360, I assume optimized for cruise, does not have enough lift to exhibit a "backside" behavior because it stalls at or near the minimum sink speed, as represented by the descent rate vs airspeed plots for that aircraft.
The Lancair is very clean in the cruise configuration and exceptionally dirty in the landing configuration. It can perform landing approaches with up to a 2,000 fpm descent rate.
Our maximum lift coefficients will all be in the same neighborhood. One difference worth noting is that the Lancair has retractable gear. The landing gear adds drag with no lift benefit. If you look at the relationship of CL to CD at minimum power you see that any additional drag pushes the airspeed at minimum power to a lower value. With just simple flaps our maximum lift coefficients are not overly impressive compared with multi-segment fowler flaps.
So the behavior is driven by the relationship of lift and drag coefficients, not just lift the coefficient.
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  #56  
Old 01-08-2019, 12:11 PM
N91CZ N91CZ is offline
 
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Quote:
Originally Posted by David Paule View Post
No problem. My old Cessna exhibits greater sink at slower speeds, below something like 80 or 85 mph indicated. It's a good way to control the landing event. I routinely make my solo approach at 70 to 75 mph indicated.

Dave
Dave,
Excellent. Do you still have it? If so could you replicate the engine out glide test in the landing configuration and plot descent rate vs airspeed. The closest thing in the chart above is the PA-28
If you don?t still have it, what model Cessna was it?
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  #57  
Old 01-08-2019, 01:48 PM
scsmith scsmith is offline
 
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Quote:
Originally Posted by N91CZ View Post
The Lancair is very clean in the cruise configuration and exceptionally dirty in the landing configuration. It can perform landing approaches with up to a 2,000 fpm descent rate.
Our maximum lift coefficients will all be in the same neighborhood. One difference worth noting is that the Lancair has retractable gear. The landing gear adds drag with no lift benefit. If you look at the relationship of CL to CD at minimum power you see that any additional drag pushes the airspeed at minimum power to a lower value. With just simple flaps our maximum lift coefficients are not overly impressive compared with multi-segment fowler flaps.
So the behavior is driven by the relationship of lift and drag coefficients, not just lift the coefficient.
This comment starts to get at the root of the issue. Adding parasite drag makes the "back side" smaller and smaller, in line with the OP's personal observations with his Lancair. The min-sink point moves closer to the stall.

On the other hand, adding induced drag makes the "back side" bigger. Our RVs with low aspect ratio wing should have more separation between min sink and stall, a bigger back-side region.
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  #58  
Old 01-08-2019, 05:03 PM
N91CZ N91CZ is offline
 
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Quote:
Originally Posted by scsmith View Post
This comment starts to get at the root of the issue. Adding parasite drag makes the "back side" smaller and smaller, in line with the OP's personal observations with his Lancair. The min-sink point moves closer to the stall.

On the other hand, adding induced drag makes the "back side" bigger. Our RVs with low aspect ratio wing should have more separation between min sink and stall, a bigger back-side region.
Steve,
There are a few drag contributors pushing in opposite directions.

A lower aspect ratio wing will indeed have higher induced drag- pushes min power to the right. On the other hand, the baseline drag of the RV is higher ? pushes min power to the left. These terms are all linear.

The bigger contributor is lift coefficient, a squared term in induced drag. Improve CL max and you can really drive up induced drag quickly.
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  #59  
Old 01-08-2019, 05:14 PM
scsmith scsmith is offline
 
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Quote:
Originally Posted by N91CZ View Post
Steve,
There are a few drag contributors pushing in opposite directions.

A lower aspect ratio wing will indeed have higher induced drag- pushes min power to the right. On the other hand, the baseline drag of the RV is higher ? pushes min power to the left. These terms are all linear.

The bigger contributor is lift coefficient, a squared term in induced drag. Improve CL max and you can really drive up induced drag quickly.
Well, yes of course, having a higher CL-max will allow you to fly farther into the back-side. But by the way, the wing span is also a squared term. The dimensional induced drag is Lift^2/(pi * q * b^2).

Reduce the wing span and you can really drive up induced drag quickly.
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  #60  
Old 01-08-2019, 07:29 PM
YYC650 YYC650 is offline
 
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Quote:
Originally Posted by BJohnson View Post
For constant airspeed with full flaps and 9 inHg MAP I got:
IAS Descent Rate
80 1400
75 1090
70 900
65 730
60 600
55 550
50 500
45 450
42 300* 1 knot above stall - Verified from Skyview data log


Brice:
Can you explain what you are doing for this test? You note Constant Airspeed in the text (Constant Velocity in the figure) but the table and graph both show a range of IAS. I must be missing something.

Thanks,
Scott
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