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01-08-2019, 09:22 PM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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Quote:
Originally Posted by snopercod
Maybe Chris can adjust his gray Lancair 235 line accordingly to show it a little flatter on the bottom end.
Everyone should do this stuff to learn the numbers for their own plane.
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John,
The best thing to do is to generate a completely new plot from max flap extended speed all the way down to just above stall. These are qualitative tests to simplify things, but that means we need to gather all the data points in the same flight to eliminate variables.
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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01-09-2019, 05:34 AM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,106
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Quote:
Originally Posted by N91CZ
The best thing to do is to generate a completely new plot from max flap extended speed all the way down to just above stall.
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I'll do that when the weather is right and I get a round tuit.
__________________
(2020 dues paid)
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01-09-2019, 05:40 AM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,106
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Quote:
Originally Posted by YYC650
Brice: Can you explain what you are doing for this test?
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Not Brice, but I can explain (I posted a short video, above). At idle power, you descend at a series of constant airspeeds and document what your rate of descent is at each different airspeed. You can use your VSI but, for best accuracy, it's better to time your descent through 1,000' of altitude.
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(2020 dues paid)
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01-09-2019, 08:41 AM
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Join Date: Jun 2005
Location: Federal Way, Wa
Posts: 264
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Sure
Quote:
Originally Posted by YYC650
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.
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Scott, each data point is a constant IAS glide test where the sink rate is measured. Several data points were collected during a single glide by varying the airspeed and holding. As Snopercod mentioned, it would be best to be established at the airspeed and measure at the same altitude each time. The last data point is very near stall speed and something interesting is happening there. The blue line is data collected by hand while flying. the orange line is extracted from the data log.
__________________
Brice
RV-9A 90897 FLYING
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01-09-2019, 08:53 AM
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Join Date: Jun 2005
Location: Federal Way, Wa
Posts: 264
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RV-9A
Quote:
Originally Posted by scsmith
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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I found that with the RV-9A, there is a very small range of airspeed where the power increase is substantial to fly on the backside of the curve. I was expecting a much larger airspeed region where this occurs, but it occurs in the last 3 knots prior to stall. I tested 45 knots and 40 knots, and normal stall speed is 42 knots. I was definitely able to fly slower than 42 knots with added power. That day was a little gusty so it was hard to tell where any stall buffet might have been.
This is not a flight regime I would ever expect to fly the plane. The pitch angle is high, and the plane is on the verge of stalling.
The -9 flaps do not generate much drag. With full flaps (30 degrees) and a fixed pitch prop, I need to be at idle from midfield down wind to touch down at 65 knots or slower to keep from floating too much.
__________________
Brice
RV-9A 90897 FLYING
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01-09-2019, 06:34 PM
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Join Date: Apr 2017
Location: Calgary, Alberta
Posts: 16
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Quote:
Originally Posted by BJohnson
Scott, each data point is a constant IAS glide test where the sink rate is measured. Several data points were collected during a single glide by varying the airspeed and holding. As Snopercod mentioned, it would be best to be established at the airspeed and measure at the same altitude each time. The last data point is very near stall speed and something interesting is happening there. The blue line is data collected by hand while flying. the orange line is extracted from the data log.
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Thanks Snopercod and Brice. Now I understand.
Scott
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RV-7a @ CYBW
2020 dues paid
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01-09-2019, 10:14 PM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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Quote:
Originally Posted by BJohnson
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The inflection in the plot is very curious. Any idea what was causing that?
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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01-09-2019, 10:30 PM
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Join Date: May 2006
Location: Woodland, CA
Posts: 104
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Quote:
Originally Posted by BJohnson
I found that with the RV-9A, there is a very small range of airspeed where the power increase is substantial to fly on the backside of the curve. I was expecting a much larger airspeed region where this occurs, but it occurs in the last 3 knots prior to stall. .
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Brice,
Did you get the sense some flow was already separating? Any buffet?
The typical drag and power curves, especially the theoretical ones, are nice and smooth and predictable because they assume no discontinuities along the way. If you can get part of the inboard wing to stall, as in the buffet region, you are effectively making the flying portion smaller while still dragging the stalled portion along for the ride. Given enough power one could transition into a vertical hover. This would explain the very narrow speed band. Once part wing is separating, the rest has to make up the difference and it doesn't take long for the separated region to grow rapidly.
__________________
Chris Zavatson
N91CZ
Lancair 360
http://www.n91cz.com/
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01-13-2019, 09:06 PM
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Join Date: Jun 2005
Location: Federal Way, Wa
Posts: 264
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One last datapoint
Quote:
Originally Posted by N91CZ
Did you get the sense some flow was already separating?
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Weather cleared and I had another attempt at collecting data at high power and slow speed. I updated the plot below with one other data point at 41 knots. The air was very smooth and I was able to maintain a stable condition for about 10 seconds. There were no signs of buffeting that I could tell. In an attempt to get this number, I did stall it multiple times and it did buffet just before the nose dropped.
From earlier plots, the "backside" flight envelope is marked as beginning at the minimum of the curve, so that puts the region starting around 60 knots for my plane. But nothing dramatic occurs until less than 45 knots or 3 knots above the no-power level stall speed. I did get the plane down to 39 knots IAS again, but with 126 HP it was climbing at 425 ipm still.

__________________
Brice
RV-9A 90897 FLYING
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01-14-2019, 09:37 AM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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Quote:
Originally Posted by N91CZ
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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It's a 1955 Cessna 180, I still have it, and finding both time to do it and still air is iffy. Here on the lee of the Rockies, virtually any wind includes up and down flowing air.
It would be interesting to do it with different flap configurations. The info I included was based on 40 degrees flap, and flaps are very powerful on that airplane.
Dave
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