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Inertia, Baby!

Definition: inertia is the force that keeps my butt connected to the couch when my wife points out that I need to cut the grass. This is a serious force, well understood by physicists. Unfortunately, I'm not a physicist.

One of the things we are currently trying to quantify and compensate for in our analysis is inertial effect, primarily the difference between the behavior of the wing and the angle of attack vane on the air data test boom under load. At 1G (those stable, trim shots I write so much about), things marry up nicely. And, for the most part, when we takeoff and land, we don't pull much more than a G anyway; so we don't have to concern ourselves too much with load effect during day to day flying. But as I recently wrote, we are chasing performance under G.

I also wrote that since we are operating below .3 Mach, aerodynamic effects can be disregarded. This is not correct. Being not correct is the story of my career; and my wife says I'm excellent at it. Turns out when we are maneuvering, both inertia and aerodynamic effects come into play. A great way to see this is the plot in Figures 1. This is a 4G accelerated stall. You can watch me fly the maneuver in the video:

To fly high G stalls in this manner, I target 5 to 5 1/2G's in the break turn to avoid over G'ing the airplane and force a stall at about 4G's. It's an unloaded roll and straight pull to the bucket to exceed 2G's per second onset rate. Energy will bleed in the turn, as evidenced by decreasing airspeed and G prior to stall. The stall is very well defined: the nose simply stops tracking across the horizon and a distinct "thud" is felt. The airplane structure is supporting an effective weight in excess of 6000 pounds at the break and 4800 pounds at the stall. Thanks Van! In the video, the OWS is in training mode so the "G Limit" voice warning occurs at 2 1/2 G's. Since I'm concerned about over G'ing the airplane in a maneuver like this, it would have been smarter to set that to the 6 G aerobatic limit so it could perform its duty and warning me of an actual over-G. No excuse. Note also that there is solid stall warning present prior to the actual stall--the Gen 2 system has no trouble keeping up with the stick monkey's maximum performance pull. This is a huge improvement over our first generation system, where I could always "beat the system" if I pulled hard enough.

Fig 1. 4G Acellerated Stall

Figure 1 shows us three different versions of AOA and G throughout the maneuver. The stall itself is clearly defined by the AOA peaks. And although not shown in this plot, there is also a distinct wing-rock signature that shows up in lateral G during the stall (as directional stability of the airplane breaks down). The blue line is the instantaneous Gen 2 pressure-derived alpha solution. The yellow line is the smoothed version I'm listening to in the cockpit. Since we are currently using a moving average technique, the "smoothed" version lags instantaneous alpha by a bit. The grey line is the alpha plot from the test boom. The difference between that line and the Gen 2 lines is due to inertial and aerodynamic effects. Because of inertia, the airflow on the upper surface of the wing separates at a higher AOA. The AOA device on the boom does not see this effect. Further, there are also aerodynamic effects associated with the fact that the differential pressure sensor that drives the V3 AOA solution is mounted on the bottom of the wing, where the flow lines are "squeezed" together under G.

We can see the same effects (although less pronounced) in the 1G stall in Figure 2, and the wind shear induced stall in Figure 3. Note the same "cross over" delta behavior between the boom alpha and instantaneous V3 alpha during the deceleration to the 1G stall. In Figure 3, during the wind shear induced stall, we see the V2 alpha spike as separation occurs and there is compression of stream lines below the wing. The reason the chart legends use "Instantaneous V2 Alpha" is because Lenny and I have V2 versions installed in our airplanes. There is no difference in AOA performance between the V2 and V3. The V3 is simply a more mature hardware design with integrated serial input.

Fig 2. 1G Stall
Fig 3. Wind Shear Induced Stall

Always learning, and never mowing the lawn!

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