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  #31  
Old 12-31-2018, 05:12 PM
Steve Melton's Avatar
Steve Melton Steve Melton is offline
 
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Default never seen that before - engine fell off

backside of the power curve, see at 3:35

https://youtu.be/fC5yscm9dsI
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  #32  
Old 12-31-2018, 11:14 PM
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BJohnson BJohnson is offline
 
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Default Other interesting bits from the Van's article

Interesting and thought provoking discussion. The OP's primary point is that Power is Thrust times velocity (N m/s) is worth noting. It will flatten the plot but does not eliminate the existence of the backside behaviors we all noticed early in our private training days, that to maintain altitude during slow flight with flaps extended, a significant amount of power had to be added and that increasing pitch caused a descent and decreasing pitch caused a climb.

Van goes on to say in the OP's first post:
?Back Side? operation is primarily for lower power aircraft of aircraft with high lift systems which reduce stall speeds but produce very high drag in the process. If the total thrust available is equal to or less than the drag of the airplane, it will be unable to climb or accelerate out of this condition. ? Most RVs have enough power that they, at normal operating altitudes, can ?power out? from behind the power curve.
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.

My 9A will drop out of the sky if I lower the speed below 60 knots, to 55 or even 50 knots on final, but I must confess, I don't know if if the sink rate increases. The glide slope certainly gets steeper as the speed decreases. This technique is as effective as a slip when high on the approach.

It's a good excuse to go flying and collect power vs airspeed curves for constant altitude in my 9A. It does take full power to stall the airplane during power on stalls. That, by definition, is a backside behavior.
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  #33  
Old 01-01-2019, 06:24 AM
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Default

Quote:
Originally Posted by BJohnson View Post
... increasing pitch caused a descent and decreasing pitch caused a climb...
Oh now you've gone and done it! I've never flown an RV but in every other plane I've flow in slow flight, pitch controls airspeed and power controls altitude (or sink rate). Here's the cockpit view of the same landing I posted earlier. You can see all the instruments and hear the power changes. I was using pitch to keep my airspeed at 75 Kts and power to control my rate of descent. As I pitched up in the flare, you can see my airspeed bleed off but my descent rate didn't increase until I eased out the power. Grabbing some popcorn...

https://www.dropbox.com/s/p14jwd07ns...ckpit.mp4?dl=0
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  #34  
Old 01-01-2019, 11:04 AM
andrewtac andrewtac is offline
 
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Default

You can fly front side and still use pitch to maintain airspeed and throttle/power for glide slope.

I flew some test points today. 30.17, OAT 16, 2300MSL. Wind was rather smooth. Fastback RV8, 180hp io360, constant speed blended airfoil prop. Fuel flow is broke. Mixture prop full forward for all points.

Technique for front side was set power let aircraft settle into speed maintaining zero on the VSI (looked outside for this mainly to see trends quicker), use feet to center the ball.

Backside, use slight G to increase drag to get slow and then set zero vsi and gradual power to maintain airspeed and zero sink/climb. Again feet for the ball.

KIAS/%power/mainfold/rpm
20 flaps
67/31/15.2/2030
65/30/14.9/1990
58/35/16.4/2060

Full flaps
67/37/16.8/2190
54/39/17.3/2110

Appears in both configurations I got backside. This isn?t professional quality data, just a quick eval. I probably got on conditions for about 20-30 seconds. I suspect I am close.
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  #35  
Old 01-01-2019, 01:39 PM
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Default Good data!

Quote:
Originally Posted by andrewtac View Post
I flew some test points today.
That was good data! We can all see that the slower you got, the more power you needed to hold altitude - especially with full flaps.

I went up today and did a brief flight test to confirm some of the Lancair 235 numbers I gave Chris a couple years ago (Gray line on Power Off Descent Rate - Landing Configuration chart) . Conditions were kind of gusty today so I was reluctant to go below 65 KIAS lest I fall out of the sky. At 90 KIAS my descent rate was still 1,200 fpm, but at 70 KIAS I was getting 950 fpm. Maybe Chris can adjust his gray Lancair 235 line accordingly to show it a little flatter on the bottom end.

https://www.dropbox.com/s/c3kginhloy...scent.mp4?dl=0

Everyone should do this stuff to learn the numbers for their own plane.
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  #36  
Old 01-01-2019, 01:51 PM
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Default Nice landing Snopercod!

As I mentioned, I am guessing that the Lancair does not have high enough lift in the wing to fly on the backside of the power curve, which is what I think is at the root of the discussion. It appears to be stalling near the minimum sink rate, of the bottom of the U shaped drag vs airspeed for constant altitude plot in the first post. Van's referenced article specifically mentioned that this was "primarily for lower power aircraft of aircraft with high lift systems", which I suspect does not describe the Lancair.

Time to go fly and test the -9A
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  #37  
Old 01-01-2019, 04:59 PM
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Default Well - I was not expecting this!

Just got back from flight testing on a perfectly still air day in the PNW. I got the same flat power curve that has been shown for the other aircraft.
1560 lbs, 40°F OAT, IO-320, FP Sensenich GA, Density altitude between 1350 and 2580 ft, full flaps

For constant altitude I got:
IAS HP RPM MP
80 67 1830 19.5
75 59 1750 18.8
70 52 1690 18.2
65 44 1620 17.5
60 40 1570 17.2
55 40 1560 17.3
50 41 1555 17.3
45 42 1560 17.4



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



For the constant altitude test I was sure I would have to add power to fly at slower speeds. The data shows a slight increase, but that is only 2 HP.

The power curve at high lift was nearly flat just as Chris originally posted. I do only get 30 degrees of flaps out on the -9A. But still it was pretty amazing to slow down the plane and essentially not have to add power between 60 and 45 knots.
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Last edited by BJohnson : 01-03-2019 at 07:32 AM. Reason: added flight and aircraft conditions - added graphs
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  #38  
Old 01-02-2019, 10:45 AM
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RV7A Flyer RV7A Flyer is offline
 
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Default

Quote:
Originally Posted by BJohnson View Post
For the constant altitude test I was sure I would have to add power to fly at slower speeds.
Precisely. You should have tried it out...it's an interesting flight regime.
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  #39  
Old 01-02-2019, 08:23 PM
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skylor skylor is offline
 
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Default RV-8 Data

I ran my own set of tests this afternoon in my RV-8.

Tests were run at 3500' +/- 30', 47F OAT, 30.15 in Hg Baro.
The tests were flown in extremely smooth air over the ocean, with very low winds aloft (< 8 kts).

The airplane configuration was:
Weight: 1610 lbs
Flaps: Full
Prop: Full Forward (Hartzell BA, 74")
Mixture: Full Forward (IO-360-A1B6 Angle Valve)


IAS MAP Eng. Spd. FF
KTS in Hg RPM GPH
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85 17.5 2490 10.4
80 17.1 2400 10
75 16.5 2300 9
70 16.3 2220 8.5
65 16.3 2150 8.3
60 16.5 2140 8.3
55 16.8 2140 8.5
51 18.1 2230 9.8

I ensured that the plane was completely stabilized in speed and altitude at each test point before recording the data. In my plane, at least, there is quite a noticeable increase in power required just above stall speed. I will try to run these tests again someday closer to gross weight. I won't be able to fly the 51 knot test point at heavier weight but I'm pretty certain that in my plane at gross the speed at which increased power is required moves up to somewhere around 60-65 kias.

Skylor

Last edited by skylor : 01-02-2019 at 11:33 PM. Reason: Corrected error in 55 kias data (IS:16.8 in Hg WAS: 16.1 in Hg)
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  #40  
Old 01-02-2019, 10:30 PM
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RV7A Flyer RV7A Flyer is offline
 
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Default

A couple of these tests seem to stop just when it's getting interesting...just a tiny bit below Vs0 or Vs1, when they had to add a touch of power to slow further. Why not keep going? Increase pitch further while adding power, and continue until either you're uncomfortable with the pitch attitude (it'll be very nose up) or run out of power to give.

It's an interesting flight regime, just keep your turns coordinated and don't stall a wing!
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