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Old 01-13-2019, 10:06 PM
BJohnson's Avatar
BJohnson BJohnson is offline
Join Date: Jun 2005
Location: Federal Way, Wa
Posts: 238
Default One last datapoint

Originally Posted by N91CZ View Post
Did you get the sense some flow was already separating?
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.

RV-9A 90897 FLYING
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Old 01-14-2019, 10:37 AM
David Paule David Paule is offline
Join Date: Dec 2009
Location: Boulder, CO
Posts: 3,698

Originally Posted by N91CZ View Post
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 donít still have it, what model Cessna was it?
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.

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Old 01-15-2019, 10:03 AM
sblack sblack is offline
Join Date: Dec 2009
Location: Montreal
Posts: 1,313

One day at a bbq I witnessed a perfect demonstration of the back side of the curve on an RV6. The pilot was a fireman who had worked late the previous night and later confided that he probably should not have flown. It was a tight grass strip with trees all around and he got behind. Of course everyone was watching and I was filming and he just fell out of the sky. The nose came way up and as it did the AOA and sink rate went way up. We heard the power come up just as he hit, but since he was probably 500ft away it took a bit of time for the sound to get to us, but either way he did not quite catch it in time. He hit hard, tailwheel first, then main gear which splayed out. I think there was some green on the prop tips but he got away with it. Scared the **** out of himself and did it in front of 50 people which is always good for the ego.

Of course the Vans wing design will be susceptible to this. It is a short stubbing wing. High induced drag is desirable on approach if you are at the right speed, short wings make for low roll damping so sporty handling, and it makes for lower wing bending moments so a lighter wing than a high aspect ratio wing and the straight planform makes it easy to build. It is a great design, but every design has its particular characteristics. This one is well known and if you are familiar with it it is a non issue. But fall asleep on final and react with stick instead of power and things will get exciting very quickly. This event really stuck in my head. I think most airplanes will do this to a point but I think an RV will do it in a bit more pronounced way. It's a "feature"
Scott Black
RV 4, with an engine...and other stuff
VAF dues 2018
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Old 01-18-2019, 06:15 PM
N91CZ N91CZ is offline
Join Date: May 2006
Location: Woodland, CA
Posts: 100

It looks like you were able to make it to the back side. It is great to see folks out there exploring the slow end of the envelope. Too many (especially Lancair) pilots are scared to death by it.

Looking at your data and weight shown (didnít change between flights), the lift coefficient at 40 KIAS is a way up there ~2.3. There appears to be a little bit of a contradiction between the glide test and the level test at the extreme low end. In the glide, the left most point had power required going down, whereas in level flight it went up. The high power to maintain level flight may be enabling these lower flight speeds Ė airflow and direct lift (deck angle is probably ~20 deg).

It also struck me that the low end points could only be maintained for a few seconds? It should be a stable condition. In general it is really difficult to determine if one has a steady state level flight condition in just a few seconds. You are juggling power and trying to get altitude and airspeed to remain constant.

I wanted to rewind real quick back to the initial post about drag vs power curves and their differences and relate back to what you are seeing. I also found some additional theoretical curves on-line that show a good side-by-side comparisons of drag vs power.

The drag curve minimum is somewhere in the middle of approach speed region. It is not immediately apparent where this point is in that you would need to plot out sink rate vs forward speed or descent angle to find it. Sink rate doesnít start climbing when passing this point. It is the ratio of D/L that starts climbing.

The power curve is obtained by multiplying the drag curve by velocity. This pulls the left end of the drag curve down dramatically. The minimum of this curve occurs at a much lower speed than minimum drag. The onset of any significant rise in power required in your plot showed how close to stall you had to get.

I think what confuses many is this. Having read about the back-side concern when on approach, they pull the nose up and see the speed decay. The initial thought is that it must be the back-side of the power curve when in fact this is a normal response even on the front side of the power curve. A flatter descent angle requires more power to maintain speed. If power is not added during the angle change, speed decays. The only difference is that it will decay faster on the back-side because you now have an additive effect. What your exercise showed is how close to stall your aircraft has to be before the induced drag portion kicks in as a significant contributor. Normal approach speeds should keep you well clear.
Chris Zavatson
Lancair 360
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Old 01-28-2019, 02:09 PM
mbishop mbishop is offline
Join Date: Jan 2016
Location: Tampa
Posts: 112

Originally Posted by N91CZ View Post

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.
I've been at 25-30 knots in my Cessna at level flight. I'll get video of it next time I do it.
2019 Donation made.

Riveting left wing together (RV-10).
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