AOA - IV
I would be glad to have a discussion on these topics. I don't know what the answer is, but I suspect I will learn something. To disclose my biases, I love AOA and use it, but didn't think it was valuable in the Cub. You got my wife and I pulling out our copy of "Aerodynamics for Naval Aviators," which is always a good thing. I think it is also true that the FAA would absolutely freak out if they thought pilots were using the LRI as a primary instrument in any of your examples, but let's put that aside.
[You are correct. They specifically say the LRI should not be used as a primary instrument. This is not a good forum to challenge them.]
1) Minimum turn radius. As I understand it, a minimum radius turn is executed by pulling at the load limit (3.8 for a Husky) of the aircraft at maneuvering speed. I would be curious how you use the LRI for a minimum radius turn.
[From my fighter days (admittedly pretty ancient, but aerodynamics are aerodynamics), min radius is obtained at the minimum speed at which you can pull the maximum allowable G. We have no way to figure that out in our cubs; no g meter, no test data, and for many of us no way to tell our angle of bank. You could figure these numbers if you did know (and could fly) the bank angle that equated to your G limit, and the minimum speed that equated to that. Given a poor playing hand, all we can say is whatever the angle of bank, the minimum turn radius will be at the speed that equates to optimum angle of attack. If this is important to us, the only way is via an AOA indicator of some sort.]
2) Best glide. Let's assume at Cub payload weights, best glide varies by a couple of knots. We still need to adjust that for wind, which means LRI in itself doesn't tell us the whole story. Even if you nail all that to the knot, isn't best glide to that theoretical max distance landing spot pretty unlikely? I think the real world in a Cub, with its slow landing speed, is that you pick a spot that you know you will make, maneuver to land into the wind, and show up with extra airspeed that you can dissipate with a slip before touchdown to make sure you have enough energy to flair? Would you be just as well off with a grease pencil mark on the airspeed indicator for best glide.
[You are correct, and as I relayed to you via PM, test data in my J-5 shows a min descent rate at 40, max range at 45 and a book number of 60 (the fat tires and bigger engine really hurt me here). I might use the former numbers if in deep do-do obstacle wise, but 60 is a better number since it allows maneuvering, a last minute stretch over the unforseen, and flare capability. (Again, the LRI will refine these numbers for optimum AOA.) I certainly agree that given the option, the answer is always higher-faster and live by the slip.]
3) Landing with turbulence. I don't know how stable the LRI is in turbulence, but in a Pilatus PC 12, with the AOA probe out on the wingtip, the AOA indexer bounces so much in turbulence that it is useless. Assuming it works better than the system in the PC 12, it may tell you AOA right now at this bump, but does nothing to predict what AOA will be in a minute. That is why we carry extra airspeed on approach in those conditions and I don't see how the LRI does anything more useful?
[The LRI is more stable than the IAS, but in no way is rock solid. It does, however, let you know if you are keeping an adequate energy margin. Turbulence related G forces are reflected in the angle of attack.]
[Please don't think I claim the ultimate answer here. The LRI is above "Rube Goldberg" science, but is not the best money can buy. For a few tanks of gas worth of $$, I think it is a good safety addition.]
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